Diss Chen Yifei

MOLECULAR SIMULATIONS FOR CO2 CAPTURE IN

METAL-ORGANIC FRAMEWORKS

CHEN YIFEI

NATIONAL UNIVERSITY OF SINGAPORE

2012

MOLECULAR SIMULATIONS FOR CO2 CAPTURE IN

METAL-ORGANIC FRAMEWORKS

CHEN YIFEI (B.Eng., Hebei University of Technology,

M. Eng., Tianjin University, Tianjin, China)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF ENGINEERING

DEPARTMENT OF CHEMICAL AND BIOMOLECULAR

ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2012

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Acknowledgements

First of all, I would like to express my sincere gratitude to my supervisor Professor

Jiang Jianwen. His close guidance, suggestions, and discussions have helped me all

the time during my study and research at NUS. His patience and encouragement have

been of central importance for me to complete my PhD program. His immense

knowledge and enthusiasm in research have motivated me and will have substantial

impact on my future professional career.

I would like to thank my group members: Babarao Ravichandar, Hu Zhongqiao,

Anjaiah Nalaparaju, Luo Zhonglin, Fang Weijie, Zhang Liling, Liang Jianchao, Xu

Ying, Li Jianguo, Krishan Mohan Gupta, Naresh Thota, Zhang Kang and Huang

Zongjun for their interactions during my personal and professional time at NUS. I am

grateful for their suggestions, discussions, and comments on my research.

I would like to thank my family and friends for their support and encouragement. I am

also grateful to NUS for granting me the scholarship.

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Table of Contents

Acknowledgements ....................................................................................................... i

Table of Contents ......................................................................................................... ii

Summary ...................................................................................................................... vi

List of Tables ................................................................................................................ ix

List of Figures ............................................................................................................... x

List of Abbreviations .................................................................................................. xv

Chapter 1. Introduction ............................................................................................... 1

1.1 MOF Structures ................................................................................................ 3

1.2 MOF Synthesis ................................................................................................ 6

1.3 MOF Applications ............................................................................................ 7

1.4 Objective ........................................................................................................ 11

1.5 Thesis Outline ................................................................................................ 12

Chapter 2. Literature Review ................................................................................... 13

2.1 Experimental Studies ..................................................................................... 13

2.1.1 H2, CH4 and CO2 Storage .................................................................... 13

2.1.2 Water Adsorption ................................................................................ 18

2.1.3 Gas Separation .................................................................................... 19

2.1.4 Adsorption and Separation of Alkanes ................................................ 22

2.2 Simulation Studies ......................................................................................... 23

2.2.1 H2, CH4, CO2 Adsorption .................................................................... 23

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2.2.2 Water Adsorption ................................................................................ 30

2.2.3 Gas Separation .................................................................................... 30

2.2.4 Adsorption and Separation of Alkanes ................................................ 33

Chapter 3. Models and Methods ............................................................................... 35

3.1 Atomic Models ............................................................................................... 35

3.2 Computational Methods ................................................................................. 40

3.2.1 Density Functional Theory ................................................................. 40

3.2.2 Interaction Potential ............................................................................ 41

3.2.2 Molecular Dynamics Simulation ........................................................ 43

3.2.3 Monte Carlo Simulation ...................................................................... 43

3.3 Analysis Methods ........................................................................................... 45

3.3.1 Radial Distribution Functions ............................................................. 45

3.3.2 Adsorption Selectivity ......................................................................... 46

3.3.3 Mean-Squared Displacement .............................................................. 46

Chapter 4. Adsorption of CO2 and CH4 in MIL-101 .............................................. 47

4.1 Models and Methods ...................................................................................... 47

4.2 Results and Discussion .................................................................................. 53

4.2.1 Sensitivity of Framework Charges ...................................................... 53

4.2.2 United-Atom and Five-site Models of CH4 ........................................ 54

4.2.3 Adsorption of Pure CO2 and CH4........................................................ 55

4.2.4 Adsorption of CO2/CH4 Mixture ......................................................... 63

4.3 Summary ........................................................................................................ 64

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Chapter 5. Adsorption and Separation in Hydrophobic Zn(BDC)(TED)0.5 ......... 66



5.2.1 CH3OH/H2O ........................................................................................ 71

5.2.2 CO2/CH4 .............................................................................................. 75

5.2.3 Hexane ................................................................................................ 79

5.3 Summary ........................................................................................................ 81

Chapter 6. CO2 Capture in Bio-MOF-11 ................................................................. 83



6.2.1 Pure Gases ........................................................................................... 87

6.2.2 CO2/H2 Mixture .................................................................................. 91

6.2.3 CO2/N2 Mixture .................................................................................. 94

6.3 Summary ........................................................................................................ 97

Chapter 7. CO2 Adsorption in Cation-Exchanged MOFs ...................................... 99

7.1 Models............................................................................................................ 99

7.2 Methods........................................................................................................ 102

7.3 Results and Discussion ................................................................................ 104

7.3.1 Characterization of cations ............................................................... 105

7.3.2 Isosteric Heat and Henry’s Constant ................................................. 107

7.3.3 CO2/H2 Mixture ................................................................................ 112

7.4 Conclusions .................................................................................................. 115

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Chapter 8. Ionic Liquid/MOF Composite for CO2 Capture ................................ 116

8.1 Models and Methods .................................................................................... 116

8.1.1 [BMIM][PF6] .................................................................................... 116

8.1.2 IRMOF-1........................................................................................... 118

8.1.3 IL/IRMOF-1 Composite and Adsorption of CO2/N2 Mixture .......... 120

8.2 Results and Discussion ................................................................................ 122

8.2.1 Structure and Dynamics of IL in IL/IRMOF-1 ................................. 122

8.2.2 Separation of CO2/N2 Mixture in IL/IRMOF-1 ................................ 125

8.3 Conclusions .................................................................................................. 129

Chapter 9. Conclusions and Recommendation ..................................................... 131

9.1 Conclusions .................................................................................................. 131

9.2 Recommendation ......................................................................................... 135

References ................................................................................................................. 138

Appendix ................................................................................................................... 151

Publications .............................................................................................................. 162

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Summary

As a special class of hybrid nanoporous materials, metal-organic frameworks

(MOFs) have received considerable interest in the past decade. The achievable large

surface areas, high porosities, and tunable structures place them at the frontier for a

wide range of potential applications such as gas storage, separation, catalysis and drug

delivery. Since a vast variety of MOFs with different pore shapes and dimensions

have been synthesized and many more are possible, experimental screening of

appropriate MOFs for specific application is a formidable task. As an alternative,

molecular simulations can provide microscopic insights and quantitative guidelines

that otherwise are experimentally inaccessible or difficult to obtain, and thus assist in

the rational screening and design of novel MOFs. In this thesis, molecular simulations

have been performed primarily for CO2 capture in different MOFs with diverse

structures and functionalities.

Firstly, CO2 adsorption is investigated in a mesoporous MOF namely MIL-101,

which is one of the most porous materials reported to date. The simulation results

agree well with experimental data and the terminal water molecules play an

interesting role in adsorption. At low pressures, the terminal water molecules act as

additional adsorption site and enhance gas adsorption; however, they decrease the

available free volume and reduce adsorption at high pressures. The hydrated MIL-101

has a higher adsorption selectivity for CO2/CH4 mixture.

Secondly, the adsorption and separation of CO2/CH4, as well as methanol/water,

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in highly hydrophobic Zn(BDC)(TED)0.5 are examined. Good agreement is found

between simulation and experimental results and a large separation factor for

methanol/water is predicted. This reveals that Zn(BDC)(TED)0.5 could be a good

candidate for the purification of liquid fuel. The simulation results also imply that

water has a marginal effect on CO2/CH4 separation, thus pre-water treatment is not

required prior to separation.

Thirdly, CO2 capture is investigated in bio-MOF-11 consisting of biological

ligands. The simulation results are in accordance with experimental data. The

predicted adsorption selectivities of CO2/H2 and CO2/N2 mixtures in bio-MOF-11 are

higher than in many porous materials, which suggests bio-MOF-11 might be

interesting for pre- and post-combustion CO2 capture. In addition, water has a

negligible effect on the separation of these two CO2-containing mixtures.

Fourthly, CO2 adsorption is simulated in rho zeolite-like MOFs (rho-ZMOFs)

exchanged with a series of cations (Na+, K+, Rb+, Cs+, Mg2+, Ca2+ and Al3+). The

isosteric heat and Henry’s constant at infinite dilution increase monotonically with

increasing charge-to-diameter ratio of cation (Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ <

Al3+). The adsorption selectivity of CO2/H2 mixture increases as Cs+ < Rb+ < K+ <

Na+ < Ca2+ < Mg2+ Al3+. The simulation study provides microscopic insight into the

important role of cations in governing gas adsorption and separation, and suggests

that the performance of ionic rho-ZMOFs can be tailored by cations.

Finally, a new composite of ionic liquid (IL) [BMIM][PF6] supported on

IRMOF-1 is proposed for CO2 capture. The confinement effects of IRMOF-1 on the

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structure and mobility of cation and anion are examined. Ions in the composite

interact strongly with CO2, particularly [PF6]anion is the most favorable site for CO2

adsorption. The composite selectively adsorbs CO2 from CO2/N2 mixture, with

selectivity significantly higher than polymer-supported ILs. In addition, the selectivity

increases with increasing IL ratio in the composite.

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List of Tables

Table 3.1 The structure parameters of the five MOFs. ................................................ 35

Table 4.1. Potential parameters and atomic charges of CO2, CH4, and terminal H2O.52

Table 5.1. Potential parameters and atomic charges.................................................... 70

Table 6.1. Atomic charges in the fragmental cluster of bio-MOF-11. ......................... 85

Table 6.2. LJ Potential Parameters and Charges for CO2, H2, N2, and H2O. .............. 86

Table 6.3. Parameters in the dual-site Langmuir-Freundlich equation fitted to the adsorption of pure CO2, H2, and N2. ............................................................................ 88

Table 6.4. Selectivities and capacities for the adsorption of CO2/H2 mixture in porous materials. The capacities are for CO2 at a total pressure of 1 bar for mixture. ............ 93

Table 6.5. Selectivities and capacities for the adsorption of CO2/N2 mixture in porous materials. The capacities are for CO2 at a total pressure of 1 bar for mixture. ............ 96

Table 7.1. Charges Z, well depths /kB and collision diameters σ of cations. .......... 101

Table 7.2. Lennard-Jones parameters of framework atoms in rho-ZMOF. ............... 102

Table 7.3. Porosity, isosteric heat and Henry’s constant of CO2 adsorption in rho-ZMOFs. ............................................................................................................... 106

Table 8.1. Atomic charges in [BMIM]+ and [PF6]. .................................................. 117

Table 8.2. Simulated and experimental densities of [BMIM][PF6] at 1 atm. ............ 118

Table 9.1. CO2 selectivities at ambient conditions in different MOFs. ..................... 135

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List of Figures

Figure 1.1 Number of publications for MOFs. (Data from Scopus using “metal organic frameworks” as the topic on 10 November 2011) ............................................ 2

Figure 1.2 Examples of SBUs from carboxylate MOFs. Color scheme: O, red; N, green; C, black. In inorganic units, metal-oxygen polyhedra are blue, and the polygon or polyhedron defined by carboxylate carbon atoms (SBUs) are red. In organic SBUs, the polygons or polyhedrons to which linkers (all –C6H4– units in these examples) are attached are shown in green.4 ......................................................................................... 3

Figure 1.3 Single crystal structures of isoreticular MOFs (IRMOF-n, n = 1 to 16). Color code: Zn (blue polyhedra), O (red spheres), C (black spheres), Br (green spheres in 2), amino-groups (blue spheres in 3). The large yellow spheres represent the largest van der Waals spheres that would fit in the cavities without touching the frameworks. All hydrogen atoms have been omitted for clarity.5 .................................. 4

Figure 4.1. A unit cell of dehydrated MIL-101 constructed from experimental crystallographic data, energy minimization and density functional theory calculation (see the text for details). The pentagonal and hexagonal windows are enlarged for clarity. Color code: Cr, orange polyhedra; F, cyan; C, blue; O, red; H, white. ............ 48

Figure 4.2. Merz-Kollman charges of Cr3O trimer with terminal fluorine and water molecules in (a) dehydrated and (b) hydrated MIL-101. The cleaved bonds of Cr3O (indicated by the circles) were saturated by methyl group. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white. ................................................................................... 49

Figure 4.3. Mulliken charges of the Cr3O trimer with terminal fluorine and water molecules in (a) dehydrated and (b) hydrated MIL-101. The cleaved bonds of Cr3O (indicated by the circles) were saturated by methyl group. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white. ................................................................................... 49

Figure 4.4. Electrostatic potential maps around the Cr3O trimer in (a) dehydrated and (b) hydrated MIL-101. ................................................................................................. 51

Figure 4.5. CO2 adsorption in dehydrated MIL-101. The squares, diamonds and circles are experimental data133 in MIL-101a, MIL-101b and MIL-101c, respectively....................................................................................................................................... 54

Figure 4.6. CH4 adsorption in dehydrated MIL-101. The circles are the experimental data in MIL-101c.133 .................................................................................................... 55

Figure 4.7. Adsorption isotherms of CO2 and CH4 on a gravimetric basis. The squares, diamonds, and circles are the experimental data in MIL-101a (as-synthesized),

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MIL-101b (activated by hot ethanol) and MIL-101c (activated by hot ethanol and KF), respectively.133 ............................................................................................................. 56

Figure 4.8. Adsorption isotherms of CO2 and CH4 in MIL-101 (a) at low pressure and (b) high pressure based on the number of molecules per unit cell. .............................. 57

Figure 4.9. Snapshots of CO2 and CH4 in a pentagonal window in dehydrated MIL-101 at 10, 100, and 1000 kPa. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white; CO2, green; CH4, pink. ...................................................................................... 58

Figure 4.10. Radial distribution functions of CO2 and CH4 around Cr1 and Cr2 atoms in dehydrated MIL-101 at 10, 100, 1000, and 5000 kPa. ............................................ 59

Figure 4.11. Schematic locations of CO2 and CH4 near Cr3O trimer in dehydrated MIL-101. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white. ........................ 60

Figure 4.12. Radial distribution functions of CO2 and CH4 around Cr1 and Cr2 atoms in hydrated MIL-101 at 10, 100, 1000, and 5000 kPa. ................................................ 62

Figure 4.13. Radial distribution functions of CO2 and CH4 around the oxygen atoms of terminal water molecules in hydrated MIL-101 at 10, 100, 1000, and 5000 kPa. .. 63

Figure 4.14. Adsorption of equimolar CO2/CH4 mixture. (a) isotherm and (b) selectivity of CO2 over CH4. ........................................................................................ 64

Figure 5.1. A unit cell of Zn(BDC)(TED)0.5 constructed from the experimental crystallographic data and first-principles optimization. Color code: Zn, pink polyhedra; N, green; C, blue; O, red; H, white. ............................................................................. 67

Figure 5.2. Channels along the Z, X, and Y (from top to bottom) axes in Zn(BDC)(TED)0.5. The green regions denote the small windows. .............................. 68

Figure 5.3. Atomic charges in a fragmental cluster of Zn(BDC)(TED)0.5. The dangling bonds (indicated by circles) were terminated by hydrogen. Color code: Zn, pink polyhedra; N, green; C, blue; O, red; H, white. ................................................... 69

Figure 5.4. Isotherms of pure CH3OH and H2O at 303 K. The filled circles are experimental data. The upper and lower triangles are adsorption and desorption data from simulation. The insets show the isotherms a function of reduced pressure. The saturation pressure Po is 21.7 kPa for CH3OH and 4.2 kPa for H2O. .......................... 71

Figure 5.5. Density contours of CH3OH at 1 kPa (top) and 10 kPa (bottom). ............ 72

Figure 5.6. Radial distribution functions of CH3OH around Zn, N, C2, and C4 atoms of Zn(BDC)(TED)0.5 at 1 and 10 kPa. ......................................................................... 74

Figure 5.7. (a) Adsorption and (b) selectivity of CH3OH/H2O mixture at 303 K. ...... 74

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Figure 5.8. Adsorption of pure CO2 and CH4 at 298 K. The open symbols are simulation results and the filled symbols are experimental data.327,328 ........................ 76

Figure 5.9. Density contours of CO2 at 10, 100, and 3000 kPa (from top to bottom).76

Figure 5.10. Radial distribution functions of CO2 around Zn, N, C2, and C4 atoms of Zn(BDC)(TED)0.5 at 10, 100, and 3000 kPa. ............................................................... 77

Figure 5.11. (a) Adsorption and (b) selectivity of CO2/CH4 equimolar mixture at 298 K. The filled symbols refer to the CO2/CH4 mixture with 0.1% H2O. ........................ 78

Figure 5.12. Adsorption of hexane at 313 K. The open symbols are simulation results and the filled symbols are experimental data.150 .......................................................... 80

Figure 5.13. Density contours of hexane at 0.001 kPa (top) and 10 kPa (bottom). .... 81

Figure 6.1. (a) Cobalt-adeninate-acetate cluster. N1 and N6 are the Lewis basic pyrimidine and amino groups, while N3, N7, and N9 are bonded with cobalt. (b) A unit cell of bio-MOF-11. The cavities are indicated by the green circles. Co: pink, O: red, C: grey, H: white, N1: green, N6: blue, N3, N7, and N9: cyan. ........................... 84

Figure 6.2. A fragmental cluster of bio-MOF-11 used to calculate atomic charges. The dangling bonds (indicated by circles) were terminated by hydrogen atoms. Color code: Co, pink; O, red; N, cyan; C, grey; H, white. .............................................................. 85

Figure 6.3. Adsorption isotherms of pure CO2 and N2 at 298 K and of H2 at 77 K, respectively. The open symbols are from simulation and the filled symbols are from experiment. The lines are fits of the dual-site Langmuir-Freundlich equation to the simulation data. ............................................................................................................ 87

Figure 6.4. Radial distribution functions of CO2 around N1, N6, and Co atoms in bio-MOF-11 at 298 K and 10 kPa. N1 and N6 are in the pyrimidine and amino groups, respectively. ................................................................................................................. 89

Figure 6.5. (a) Simulation snapshot and (b) density contour of CO2 in bio-MOF-11 at 298 K and 10 kPa. CO2 molecules are represented by sticks. The density has a unit of 1/Å3 and brighter color indicates a higher density. Co: pink, O: red, C: grey, H: white, N1: green, N6: blue, N3, N7, and N9: cyan. ................................................................ 90

Figure 6.6. Density contours of CO2 and H2 for CO2/H2 mixture (15:85) in bio-MOF-11 at 298 K and 100 kPa. The density has a unit of 1/Å3. The density distributions are largely similar to Figure 5.5b for pure CO2. ..................................... 91

Figure 6.7. (a) Adsorption isotherm and (b) selectivity of CO2/H2 mixture (15:85) in bio-MOF-11 as a function of total pressure in the absence and presence of 0.1 % H2O....................................................................................................................................... 92

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Figure 6.8. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure. The open symbols are from simulation and the filled symbols are from IAST. ......................................................................... 94

Figure 6.9. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure in the absence and presence of 0.1 % H2O....................................................................................................................................... 95

Figure 6.10. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure at 298 K. The open symbols are from simulation and the filled symbols are from IAST. ....................................................... 97

Figure 7.1. Crystal structure of rho-ZMOF. Color code: In, cyan; N, blue; C, grey; O, red; and H, white. The a-cage, double eight-membered ring (D8MR), 6-membered ring (6MR) and 4-membered ring (4MR) are indicated. The yellow spheres in the D8MR represent inaccessible cages. .......................................................................... 100

Figure 7.2. Atomic charges in a fragmental cluster of rho-ZMOF. Color code: In, cyan; N, blue; C, grey; O, red; and H, white. ............................................................ 101

Figure 7.3. Equilibrium and initial locations of cations (a) K+ (b) Ca2+ (c) Al3+. The initial locations are indicated in pink. ........................................................................ 105

Figure 7.4. Porosity versus the packing fraction of cation in rho-ZMOFs. The solid line is a linear correlation between and . ..................................................... 107

Figure 7.5. (a) Isosteric heats and (b) Henry’s constants for CO2 adsorption in rho-ZMOFs versus the charge-to-diameter ratio of cation. The dotted lines are to guide the eye. ............................................................................................................. 108

Figure 7.6. Adsorption isotherms of CO2 in rho-ZMOFs (a) low-pressure regime and (b) high pressure regime. ........................................................................................... 109

Figure 7.7. Density contours of CO2 in Na-rho-ZMOF at 10, 100 and 1000 kPa. The locations of Na+ ions are indicted by the large spheres. The density scale is the number of CO2 molecules per Å3. .............................................................................. 110

Figure 7.8. Radial distribution functions (a) CO2 around Na+ ions, N and In atoms in Na-rho-ZMOF at 10 kPa (b) CO2 around Na+ ions in Na-rho-ZMOF at 10, 100 and 1000 kPa..................................................................................................................... 111

Figure 7.9. Selectivities of CO2/H2 mixture in rho-ZMOFs. The composition of CO2/H2 mixture is 15/85. ........................................................................................... 113

Figure 7.10. Selectivity of CO2/H2/H2O mixture in rho-ZMOFs. The mole fraction of H2O in the mixture is 0.1%. ....................................................................................... 114

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Figure 8.1. Atomic types in [BMIM]+ and [PF6]. .................................................... 116

Figure 8.2. IRMOF-1 structure. Color code: Zn, orange; O, red; C, grey; H, white. 119

Figure 8.3. Atomic charges in a fragmental cluster of IRMOF-1. The dangling bonds indicated by dashed circles are terminated by methyl groups. .................................. 119

Figure 8.4. Adsorption isotherm of CO2 in IRMOF-1 at 300 K.39 The filled symbols are from simulation and the open symbols are from experiment............................... 120

Figure 8.5. [BMIM][PF6]/IRMOF-1 composite at a weight ratio WIL/IRMOF-1 = 0.4. N: blue, C in [BMIM]+: green, P: pink, F: cyan; Zn: orange, O: red, C in IRMOF-1: grey, H: white. ..................................................................................................................... 121

Figure 8.6. Radial distribution functions of [BMIM]+ and [PF6] in IL/IRMOF-1 at

WIL/IRMOF-1 = 0.4 and in bulk phase, respectively. The solid lines are in IL/IRMOF-1 and the dash lines are in bulk phase. .......................................................................... 122

Figure 8.7. Radial distribution functions of (a) [BMIM]+ and (b) [PF6] around O1, O2,

Zn, and C3 atoms of IRMOF-1 at WIL/IRMOF-1 = 0.4. .................................................. 123

Figure 8.8. Mean-squared displacements of [BMIM]+ and [PF6] in IL/IRMOF-1at

WIL/IRMOF-1 = 0.4, 0.86 and 1.27. ................................................................................ 124

Figure 8.9. Reduced velocity correlation functions of [BMIM]+ and [PF6] in

IL/IRMOF-1 at WIL/IRMOF-1 = 0.4, 0.86, 1.27 and 1.5. ............................................... 125

Figure 8.10. Simulation snapshot of CO2/N2 mixture (Ptotal = 1000 kPa) in IL/IRMOF-1 at WIL/IRMOF-1 = 0.4. .............................................................................. 126

Figure 8.11. Radial distribution functions of CO2 (Ptotal = 10 kPa) around Zn, N1, N2, and P atoms in IL/IRMOF-1 at WIL/IRMOF-1 = 0.4. ...................................................... 127

Figure 8.12. Radial distribution functions of CO2 around P atom in IL/IRMOF-1 (a) Ptotal = 10, 100, 1000 kPa and WIL/IRMOF-1 = 0.4, (b) Ptotal = 100 kPa and WIL/IRMOF-1 = 0.4, 0.86, 1.27 and 1.5. ............................................................................................... 128

Figure 8.13. Selectivity of CO2/N2 mixture in IL/IRMOF-1 at WIL/IRMOF-1 = 0, 0.4, 0.86, 1.27 and 1.5. ...................................................................................................... 129

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List of Abbreviations

5-AT 5-aminottrazole

B3LYP Becke, three-parameter, Lee-Yang-Parr

BDC benzenedicarboxylate

bpy bipyridine

bipy 4,4’-bipyridine

BSSE basis set superposition error

BTC 1,3,5-benzenetricarboxylate

CCS carbon capture and sequestration

COF covalent organic frameworks

DFT density functional theory

dhtp 2,5-dihydroxyterephthalate

DMAz N,N’dimethylformamide-azine-dihydrochloride

DMF N,N’-dimethylformamide

DNP double-ξ numerical polarization

DOE Department of Energy

ESP Electrostatic Potentials

ETS-10 Engelhard Titano Silicate-10

FAU Faujasite

F-pymo 5-fluoropyrimidin-2-olate

GCMC Grand Canonical Monte Carlo

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HPP 1,3,4,6,7,8-hexahydro-2H- pyrimido[1,2-a] pyrimidine

IAST ideal adsorbed solution theory

IDC imidazole-4,5-dicarboxylate

ImDC 4,5-imidazoledicarboxylate

IL ionic liquid

IRMOF isoreticular metal-organic framework

LJ Lennard-Jones

MC Monte Carlo

MD Molecular Dynamics

MFI Mobil Five

MIL Material Institute Lavoisier

MK Mera-Kollman

MAMS mesh-adjustable molecular sieves

MOF Metal-organic Frame work

MP2 second order Møller–Plesset

MSD Mean Squared Displacement

MTV Multivariate

NDC 2,6-naphthalenedicarboxylate

PCN Porous Coordination Network

POM polyoxometalate-based

prz piperazine

PVDF polyvinylidene fluoride

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pyz pyrazine

pzdc 2,3-pyrazinedicarboxylate

RCSR Reticular Chemistry Structure Resource

RTILs room temperature ionic liquids

SBU secondary building block

ScD supercritical drying

SILMs supported ionic liquid membranes

tbip 5-tert-butyl isophthalate

TED triethylenediamine

TIP3P Three point transferable interaction potential

TraPPE Transferrable Potentials for Phase Equlibria

UFF universal force field

ZIF zeolitic imidazolate frameworks

ZMOF zeolitic-like MOF

[BMIM] 1-n-butyl-3-methylimidazolium

[PF6] hexafluorophosphate

[HMIM] 1-n-hexyl-3-methylimidazolium

[TF2N] bis(trifluoromethylsulfony)-imide

[SCN] thiocyanate

Chapter 1. Introduction

1


Human being has been using porous materials for centuries.1 Based on various

criteria (pore size, shape, arrangement, and chemical composition), porous materials

can be classified into different types. For example, they can be classified into three

types based on the pore size: microporous (pore size < 2 nm), mesoporous (pore size

between 2 and 50 nm), and macroporous (pore size > 50 nm). Porous materials with

pore size < 100 nm are usually termed as nanoporous materials. Traditional

nanoporous materials include inorganic (zeolites) and organic (activated carbons and

polymers). Although these materials have been utilized in many industrial processes

such as water purification, gas separation, and catalysis, they have certain limitations.

For example, highly porous activated carbons are not well ordered. On the other hand,

highly ordered zeolites lack diversity because only limited number of elements can be

used in tuning the tetrahedral building blocks.

Hybrid nanoporous materials consisting of both organic and inorganic moieties

possess unique features. They can have both highly porous and highly ordered

structures. Recently, a newly emerged class of hybrid materials named as

metal-organic frameworks (MOFs)2 or also called porous coordination polymers

(PCPs) have attracted a great deal of attention. MOFs are crystalline structures

assembled from organic linkers and metal oxides. Compared with traditional

nanoporous materials, almost all cations can participate in MOF frameworks. In

addition, the wide variety of organic linkers and linker functionalities leads to a vast


2

diversity of MOFs. In principle, MOFs could have infinitely number of different

structures. The controllable organic linkers allow MOFs with designed functionality

and tunable pore size, surface area, and porosity. Both hydrophobic and hydrophilic

groups can be present in the frameworks, and the pores can range from microporous

to mesoporous. Therefore, MOFs are considered versatile materials for many potential

applications.3 Over the past decade, a large number of MOFs with various topologies

and functionalities have been synthesized, and their applications in gas storage,

separation, catalysis and drug delivery have been explored. Figure 1.1 demonstrates

that the number of publications for MOFs increase rapidly in the recent years.

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mb

ers

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Years

Figure 1.1 Number of publications for MOFs. (Data from Scopus using “metal organic frameworks” as the topic on 10 November 2011)

In this thesis, CO2 capture by adsorption in different MOFs is investigated. The

subsequent sections provide an overview for the structures, synthesis and typical

applications of MOFs. A more detailed literature review for the specific applications

in gas adsorption and separation will be presented in Chapter 2.


3

1.1 MOF Structures

Crystalline MOFs can be conceptually designed and constructed directly from

molecular building blocks. This route was coined as reticular synthesis by Yaghi.4 The

molecular building blocks are linked by strong bonds and retain their structures

throughout synthesis process. The design strategy of reticular chemistry is based on

the direct expansion of secondary building units (SBUs). As shown in Figure 1.2 for

carboxylate MOFs, SBUs are the geometric units defined by the points of extension.4

Figure 1.2 Examples of SBUs from carboxylate MOFs. Color scheme: O, red; N, green; C, black. In inorganic units, metal-oxygen polyhedra are blue, and the polygon or polyhedron defined by carboxylate carbon atoms (SBUs) are red. In organic SBUs, the polygons or polyhedrons to which linkers (all –C6H4– units in these examples) are attached are shown in green.4


4

By extending different SBUs with a wide variety of organic linkers, various MOFs

can be produced. For example, Eddaoudi et al. developed a series of MOFs from the

prototype MOF-5 by using various functional organic linkers.5 Sixteen highly

crystalline isoreticular MOFs (IRMOFs) produced are shown in Figure 1.3.

Figure 1.3 Single crystal structures of isoreticular MOFs (IRMOF-n, n = 1 to 16). Color code: Zn (blue polyhedra), O (red spheres), C (black spheres), Br (green spheres in 2), amino-groups (blue spheres in 3). The large yellow spheres represent the largest van der Waals spheres that would fit in the cavities without touching the frameworks. All hydrogen atoms have been omitted for clarity.5

To date, tens of thousands of MOFs have been synthesized and characterized.

Based on the framework flexibility, MOFs can be categorized into rigid and flexible.

The former have rigid frameworks, largely similar to inorganic counterparts (e.g.

zeolites). In contrast, the latter can change frameworks at external stimuli like

pressure, temperature and accommodating of guest molecules.6-8 The change may

include stretching, rotational, breathing and scissoring, and induce various effects


5

crystalline structures. Based on the framework properties, there are chiral MOFs,9,10

magnetic MOFs,11 luminescence MOFs.12 The complex structures of MOFs can be

reduced to underlying nets13 and the important nets are collected in Reticular

Chemistry Structure Resource (RCSR) database. As there are a tremendously large

number of MOFs, here we specifically introduce typical examples of MOFs, such as

IRMOFs, zeolitic imidazolate frameworks (ZIFs), covalent organic frameworks

(COFs), zeolite-like MOFs (ZMOFs), and MILs (Materials of Institut Lavoisier).

Yaghi’s group pioneered the development of a series IRMOFs.5 The reticular

frameworks have a large open space up to 91.9% of crystal volume, and the free pore

diameter varies from 3.8 to 19.1 Å. They also synthesized ZIFs14 and COFs.15-17 ZIF

structures are based on the nets of aluminosilicate zeolites, in which oxygen atoms

and tetrahedral Si or Al atoms are substituted by imidazolate linkers and transition

metals, respectively. Two prototypical ZIFs (ZIF-8 and ZIF-11) exhibit permanent

porosity and high thermal/chemical stability.14 COFs consist of light elements (B, C,

N, O) via strong covalent bonds. The pores in COFs can run in 2D and 3D with a size

ranging from 6.4 to 34.1 Å. Because of the unique structures, COFs exhibit high

thermal stability, permanent porosity, low density, and high surface area. ZMOFs have

similar topologies and structural properties to inorganic zeolites.18 However, the

difference is oxygen atoms in zeolites are substituted by organic linkers, leading to

extra-large cavities and pores in ZMOFs. This edge expansion approach offers a great

potential towards the design and synthesis of widely open materials. In addition, some

ZMOFs possess ionic frameworks and contain charge-balancing nonframework ions.


6

For example, rho-ZMOF contains 48 extraframework ions per unit cell to neutralize

the anionic framework.18

Ferey and co-workers synthesized a series of 3D rare earth diphosphonates named

as MIL-n.19-21 They also extended to compounds containing transition metals (V, Fe,

Ti) and metallic dicarboxylates.22-24 The first synthesized MIL-53(Cr) can exist in two

forms, one is filled with water molecules at low temperatures and the other is

dehydrated at high temperatures.25 The transition between the hydrated and

dehydrated crystals is fully reversible and considered as breathing effect. Similar

breathing effect occurs when Cr metal is replaced by Al, Fe and Ga. This is due to the

presence of OH groups in one-dimensional channels that have strong interactions with

water molecules.26-28 In MIL-47(V), however, no breathing occurs because of the

absence of OH groups in the skeleton.29 Ferey et al. also synthesized chromium

terephthalate-based mesoscopic MIL-101,30 which is one of the most porous materials.

It is stable in air or boiling water and its structure is not altered in various organic

solvents or solvothermal conditions.

1.2 MOF Synthesis

MOFs are usually synthesized by self-assembly at a low temperature (below

300 ℃ ) using organic or inorganic solvent without additional template. The

traditional synthesis methods include classical coordination chemistry and

solvothermal syntheses. In the traditional synthesis, temperature is crucial because it

can change the properties of solution and hence the dimension and structure of a MOF.

In addition, pH value, solution concentration and the chemical nature of cations can


7

also influence crystal structure. Furthermore, the nature of initial metallic salts and

precursors also play an important role in synthesis.3

In recent years, new methods have been proposed including (1) hydrothermal

synthesis using immiscible solvents (2) electrochemical synthesis (3) microwave

synthesis3 (4) sonication synthesis31,32 (5) mechanochemical method33,34 and (6)

high-throughput method. These new methods are regarded as environmentally

friendly and can significantly reduce reaction time. Their combination can also been

used for MOF synthesis.35 In addition, the way to activate MOF samples is crucial to

determine the final crystal structure. Hupp’s group developed supercritical drying

(ScD) method, in which supercritical carbon dioxide is used to increase the accessible

surface area of MOF samples.36

1.3 MOF Applications

MOF are potential candidates in many important areas such as gas storage,5,37-39

separation,40-43 catalysis,44,45 sensing,46 drug storage and delivery,47-49 templates for

new materials synthesis,50 luminescent and fluorescent materials,51 magnetic

materials,52 proton conductors.53,54 Several reviews have summarized the potential

applications of MOFs.3,55,56 A brief introduction is presented here for the applications

of MOFs in storage, separation, and catalysis. More detailed discussions in these areas

are described in Chapter 2.

Storage

Gas storage in porous materials has become increasingly important as the

growing concerns for energy and environment. The most extensively studied gases in


8

storage are H2 and CH4 for clean energy as well as CO2 for environmental protection.

Particularly, H2 is regarded as an ideal energy carrier due to the absence of carbon and

zero emission. The development of safe, efficient and high-capacity storage system is

a key step for the practical utilization of H2. The U.S. Department of Energy (DOE)

has set the targets for H2 storage as of 7.5 wt% or 70 g/L for the “ultimate Full Fleet”

target.57

With high porosity and large surface area, MOFs have attracted considerable

attention for gas storage. Rosi et al. first measured H2 adsorption in MOF-5,

IRMOF-6 and IRMOF-8 and showed that MOFs have much larger capacity for H2

storage than traditional zeolites and active carbons.37 After this first experimental

measurement, numerous studies have been reported for H2 storage in MOFs over the

past few years, as summarized in several reviews.55,58-61 CH4 is the major component

of nature gas and an alternative fuel to fossil fuels. The storage target set by the U.S.

DOE is 180 v/v at 35 bar (the volume of gas adsorbed at standard temperature and

pressure per volume of the storage vessel). Kitagawa and coworkers reported the first

CH4 adsorption in M2(4,4’-bpy)3(NO3)4](H2O)x (M = Co, Ni, and Zn).62 CH4 capacity

in this MOF is about 71 v/v anhydrous sample at 30 atm. Düren et al.63 and Wang64

used simulations to examine CH4 storage in various MOFs. Furukawa et al.

investigated the adsorption of H2, CH4 and CO2 in seven COFs.65 Among many

reported studies, MOF-200 and MOF-210 exhibit the highest adsorption capacity for

CO2 (64.32 and 65.23 mmol/g, respectively, at 298 K and 50 bar).66

As a biologically important gas, NO storage been examined in MOFs. Morris’s


9

group measured the adsorption, storage and delivery of NO in MOFs with accessible

open metal sites.67,68 The porous MOFs showed exceptionally good performance for

the adsorption and water-triggered delivery of NO. In addition, drug storage and

delivery in MOFs have also been reported. Ferey’s group determined the adsorption

and delivery of ibuprofen in MIL-10147 and MIL-53.48 The loading was found to be

1.38 g/g in MIL-101 and 0.22 g/g in MIL-53. An et al. reported cation-triggered

procainamide release in a bio-MOF,69 which was constructed from biocompatible

linkers. They found that the drug was complete released after 72 hours and the

framework maintained crystalline structure in the whole process. The biomedical

applications in MOFs were recently summarized by Keskin et al.70

Separation

Porous materials are commonly used in industry adsorbents for gas separation. A

suitable material should have large capacity and high selectivity. MOFs have the

potential for gas separation due to the tunable pore sizes and surface properties.

Numerous studies have been reported for gas separation in different MOFs. As

demonstrated in a breakthrough experiment, CO2 can be separated completely from

CO2/CH4 mixture in Mg-MOF-74.71 ZIF-68, ZIF-69, and ZIF-70 exhibit large

capacity for CO2 and unusual selective adsorption for CO2/CO mixture.72 In a

breakthrough experiment for CO2/CO mixture, the complete retention of CO2 and

passage of CO were observed. Such a high selectivity is based on the difference

quadruple moments of CO2 and CO. ZIF-95 and ZIF-100 have the capability to

efficiently separate CO2 from CH4, CO and N2.73 This is attributed to the combined


10

effects of the appropriate aperture size and the strong quadrupolar interaction of CO2

with the N atoms on the framework surface.

Important factors influencing separation efficiency include the size/shape of gas,

and the interaction of gas with framework. With regard to the second factor, different

strategies have been proposed to improve separation by tailoring MOF structures,

such as the addition of open metal sites and functionalized groups. Recently, Zhou

and coworkers reviewed the selective adsorption and separation in MOFs.74,75

Catalysis

With high metal contents, well-defined pores and narrow pore distributions,

MOFs have potential application in heterogeneous catalysis. Fujita et al. first reported

MOF-based catalyst for the cyanosilylation of aldehydes and imines.76 Several studies

investigated the catalytic properties of MOFs with chiral porous.10,77-80 The catalytic

properties of common MOFs such as MOF-5,81,82 HKUST-1,44,83 MIL-10145,84,85 were

examined. It was found that the catalytic activity of MOF-5 is attributed to the

encapsulated zinc-hydroxide clusters or to the hydrolytically degraded form of the

parent framework. MIL-101 has a stronger catalytic activity than HKUST-1 for the

cyanosilylation of benzaldehyde because of the greater Lewis acidity of Cr(III) vs.

Cu(II). The modified MIL-101(Cr) was tested for its catalytic activity for

Knoevenagel condensation of benzaldehyde with nitriles.85

Hasegawa et al.86 synthesized a MOF and found it was able to catalyze the

Knoevenagel condensation reaction due to its selective heterogeneous base catalytic

properties. Alkordi and Eddaoudi et al.87 reported the catalytic properties of


11

rho-ZMOF with cationic porphyrins encapsulated in the framework. The encapsulated

porphyrin with Mn metallated showed catalytic activity towards the oxidation of

cyclohexane. Jiang et al.88 synthesized a Cu-MOF that has comparable catalytic

activity for the ring-opening reactions of epoxides in the presence of alcohols and

aniline under ambient, solvent-free conditions. A comprehensive review for MOFs

used in catalysis was recently presented by Corma et al.89

1.4 Objective

As a new class of nanoporous materials, MOFs are considered versatile

candidates for a wide variety of important applications. However, a very large number

of MOFs have been synthesized and many more are possible, experimental exploring

the performance of MOFs for specific application is not economically feasible. With

ever-increasing computational power, molecular simulations have become

increasingly important in materials science. In particular, microscopic insights gained

from simulations are indispensable in quantitatively elucidating the underlying

physics and subsequently provide guidelines for the rational design of new materials.

The objective of this thesis is to investigate CO2 capture using molecular

simulations in five unique MOFs with different pore sizes and functionalities. Firstly,

adsorption of CO2 and CH4 is simulated in mesoporous MIL-101 and compared with

experiment. The effect of the coordinated water molecules on adsorption is carefully

examined. Secondly, adsorption and separation of CO2/CH4 as well as CH3OH/H2O

are examined in highly hydrophobic Zn(BDC)(TED)0.5. Thirdly, CO2 capture from

syngas and flue gas is studied in a bio-metal organic framework. The effect of


12

humidity on separation is estimated. Fourthly, CO2 adsorption and CO2/H2 separation

in cations exchanged rho-ZMOFs are explored to study the influence of cations.

Finally, CO2 separation from flue gas in an ionic-liquid/MOF composite is

investigated. The effect of ionic liquid loading on separation is examined in detail.

1.5 Thesis Outline

There are eight chapters in this thesis and the outline is as follows. In Chapter 1,

the general background of MOFs is introduced, such as the structures, synthesis, and

applications of MOFs. Chapter 2 reviews the current state of both experimental and

simulation studies for adsorption and separation in MOFs, including gas storage and

separation, water adsorption, and adsorption of alkanes. In Chapter 3, the adsorption

of CO2 and CH4 in MIL-101 is examined. In Chapter 4, the separation of CH3OH/H2O

and CO2/CH4 in highly hydrophobic MOF Zn(BDC)(TED)0.5 are investigated.

Chapter 5 describes the CO2 capture in bio-MOF-11. Chapter 6 discusses CO2

adsorption in different cation-exchanged rho-ZMOFs. CO2 capture in an ionic

liquid/metal-organic framework composite is proposed in Chapter 7. Finally, the

conclusions and recommendation for future work are presented in Chapter 8.

Chapter 2. Literature Review

13


MOFs are considered versatile materials for a wide rang of applications.

Nevertheless, most current studies have been focused on gas storage and separation.

In this Chapter, a literature review is provided for the corresponding experimental and

simulation studies.

2.1 Experimental Studies

2.1.1 H2, CH4 and CO2 Storage

H2 Storage

H2 is regarded as an ideal energy carrier and its storage in MOFs has been

extensively studied. The first experimental measurement of H2 storage was conducted

in IRMOF-1, IRMOF-6, and IRMOF-8.37 In this study, the length of organic link was

found to play a dominate role in tuning H2 capacity. Therefore, H2 uptake in

IRMOF-6 and -8 are much higher than IRMOF-1. Compared with inorganic cluster,

organic linker has a lower binding energy. However, it governs the free volume and

surface area, thus determines the capacity for H2 storage. The predominant effect of

organic linkers on H2 storage capacity was also demonstrated by Rowsell et al.90,91 In

MIL-53(Al and Cr), Ferey et al. found H2 storage capacities are 3.8 wt% and 3.1 wt%

respectively at 77 K and 1.6 MPa.38 Zhao et al. prepared three MOFs with flexible

linkers and compared their H2 capacities with active carbons at a supercritical

temperature of H2.92 Surprisingly, a hysteresis of H2 adsorption was observed,


14

implying the ‘windows’ opening of the frameworks upon adsorption. Chen et al.

synthesized MOF-505 with open metal sites and found H2 adsorption in the fully

activated MOF-505 is 2.47 wt% at 77 K and 750 torr.93 In a MOF that has physical

characteristic similar to single walled carbon nanotubes but a stronger interaction with

H2, Pan et al. concluded that H2 capacity is affected not only by pore volume but also

by pore quality, and the ideal storage materials should have the largest possible pore

volume and the proper pore which can fit with H2 molecules.94

Rowsell et al. reported several strategies to enhance H2 storage in MOFs, such as

linker modification to optimize pore size and adsorption energy, impregnation,

catenation, and including of open metal sites and lighter metals.95 Following this, the

effects of catenation and open metal sites on H2 uptake have been extensively studied.

For example, Yaghi and coworkers96,97 measured H2 storage capacities in various

MOFs and investigated the effects of functionalization, catenation, metal oxide cluster,

and organic linker on H2 adsorption at low pressures. The saturation capacities in

MOF-177 and IRMOF-20 at 77 K and below 80 bar were determined to be 7.5 wt%

and 6.7 wt%, respectively, which had achieved the U.S. DOE target for H2 storage in

2010 (6wt%). Dinca et al. synthesized MOFs with exposed coordination sites (Mn2+,

Li+, Cu+, Fe+, Co2+, Ni2+, Cu2+, Zn2+) leading to high H2 storage capacity ranging from

2.00 to 2.29 wt% at 77 K and 900 torr.98,99 Particularly, the MOF with exposed Co2+

exhibits a high isosteric heat (10.5 kJ/mol) because of the strong interaction between

H2 and unsaturated Co2+. Ma et al. showed that both catenation and unsaturated

metals in PCN-6 can enhance H2 uptake.100 In PCN-10 and PCN-11 containing open


15

metal sites, the measured H2 uptakes at 77 K and 760 torr were 2.34 wt% and 2.55

wt%, respectively.101 With a high porosity and metal sites, PCN-12 exhibits high H2

uptake of 3.05 wt% at 77 K and 1 bar.102 Schroder’s group synthesized a series of

MOFs, which show high H2 storage capacity because of the exposed metal sites and

high surface areas.103-106 CPO-27-Ni with open metal sites was found to possess the

highest isosteric heat for H2 (-13.5 kJ/mol).107 Dinca and Long have reviewed the

experimental studies for H2 adsorption in various MOFs with open metal sites.59

In a separate study, a microporous copper MOF with polar network and narrow

pores was observed to exhibit the highest H2 capacity (3.07 wt%) at 77 K and ambient

pressure.108 H2 capacity can be also enhanced in MOFs doped with alkali metals (Li+,

Na+, and K+)109,110 and modified by ligands.111 New technique such as spillover has

been proposed to enhance H2 capacity in MOFs.112-114 Using this technique, the

capacities in MOF-5, IRMOF-8, and MOF-177 were enhanced by 3.3, 3.1, and 2.5

times, respectively. The capacity in IRMOF-8 was further increased to eight times

higher than that in pure IRMOF-8 using hydrogen spillover with bridges. The effect

of MOF structures on H2 storage by spillover has been investigated in detail.115 In

addition, MOF composites incorporated with carbon nanotube were examined for H2

storage116,117 More comprehensive review on H2 uptake in MOFs can refer to a recent

critical report by Long and co-workers.118

CH4 Storage

CH4 is also considered a clean energy carrier like H2. Although CH4 storage in

MOFs is far less studied, it is still an interesting research field. After the first


16

experimental measurement,62 Eddaoudi et al. synthesized a series of IRMOFs and

tested for CH4 storage.5 Based on the prototype IRMOF-1, they functionalized or

modified the organic linkers and synthesized 16 highly crystalline MOFs with open

space up to 91.1% and pore size from 3.8 to 28.8 Ǻ. One member of this series

exhibits a high capacity for CH4 storage of 155 v/ at 36 atm and ambient temperature.

Kim measured CH4 adsorption in Zn(BDC)(TED)0.5 up to 35 bar over temperature

range from 198 to 296 K. The adsorption capacity was 137 v/v at 35 bar and 296 K,

and three adsorption sites were indentified.119 Among CuBTC, Zn(BDC)(TED)0.5 and

MIL-101(Cr),120 Kaskel and coworkers found CuBTC has the highest excess CH4

adsorption of 228 v/v at 150 bar and 303 K. Wu et al. studied CH4 storage in five

MOFs (M2(dhtp), M = Mg, Mn, Co, Ni, Zn) with open metal sites.121 These MOFs

have large capacity ranging from 149 v/v to 190v/v at 298 K and 35 bar. Particularly,

Ni2(dhtp) shows the largest capacity of 200 v/v. They also found that the primary

adsorption sites are the open metal sites. In another study for CH4 adsorption in

MOF-5 and ZIF-8, they found the primary adsorption sites in ZIF-8 are associated

with the organic linkers, in contrast to the metal oxide clusters in MOF-5.122

Zhou and coworkers synthesized a series of PCNs and tested for CH4 storage. In

PCN-10 and PCN-11 with unsaturated metal sites, PCN-11 exhibits an excess CH4

uptake of 171 v/v at 298 K and 35 bar, approaching the U.S. DOE target.101 In

microporous PCN-14 based on anthracene derivative consisting of nanoscopic cages,

the excess CH4 capacity was reported to be 230 v/v (excess 220 v/v).123 This is the

highest CH4 capacity reported to date, higher than the U.S. DOE target at 290 K and


17

35 bar, and also higher than in MOF-200, -205 and -210 with ultrahigh porosity.66

CO2 Storage

CO2 emissions have caused detrimental effects on environment such as global

warming, sea-level rise, and an irreversible increase in the acidity level of oceans. The

removal of CO2 is thus practically important and MOFs have been extensively studied

for their application for CO2 storage.

Yaghi’s group first reported CO2 adsorption in MOF-2 to characterize the

microporosity.124 Later, they examined CO2 storage capacity in nine MOFs at ambient

temperature up to 42 bar.125 These selected MOFs represented different characteristics

such as square channels (MOF-2), open metal sites (MOF-505 and Cu3(BTC)2),

hexagonally packed cylindrical channels (MOF-74), interpenetrated (IRMOF-11),

amino- and alkyl-functionalized (IRMOF-3 and -6), and highly porous (IRMOF-1 and

MOF-177). Because of the high porosity, MOF-177 shows a higher CO2 capacity than

other MOFs studied. At 35 bar, CO2 loading in MOF-177 is 9 times higher than the

pressurized CO2. They also synthesized a series of ZIFs with high thermal and

chemical stability, and selective adsorption for CO2.72 Furthermore, they measured

CO2 adsorption in various 1D, 2D and 3D COFs and found 3D COFs have a better

performance.16,17,65 In MOFs of ultrahigh porosity they synthesized, MOF-210

exhibits the highest CO2 storage capacity of 2870 mg/g.66

CO2 capacity can be enhanced in functionalized MOFs. It was suggested that

varying amine substituent in the frameworks would affect CO2 adsorption.126-131

Recently, multivariate (MTV) MOF-5 frameworks decorated with different functional


18

groups were synthesized.132 The properties of MTV-MOFs with mixed functional

groups in one pore were found to outperform the simple combination of their

constituents. In MTV-MOF-5-EIH, CO2 capacity was determined to be approximately

4 times higher than MOF-5.

In a number of MILs produced by Ferey and coworkers, CO2 adsorption has

been examined. Specifically, Llewellyn et al. measured MIL-101 has a high CO2

capacity of 40 mmol/g (i.e. 390 v/v) at 303 K and 5 MPa.133 Similarly, Chowdhury et

al. determined CO2, CH4, C3H8, SF6 and Ar adsorption in MIL-101 at 283, 319 and

351 K.134 The preferential adsorption sites were found to be the bare metal sites inside

supertetrahedra cages. Strong interaction was also found between CO2 and the open

metal sites in MOF-74 that has an exceptionally high CO2 capacity.135,136 In addition,

CO2 adsorption in MOFs has also been investigated in the presence of water.137,138

2.1.2 Water Adsorption

Studies have shown that a number of MOFs are not stable in water.114,139-142

Understanding the properties of water in MOFs is crucial to identify and design

water-resistant MOFs for technological applications, e.g., waste water treatment and

biofuel purification. Wang et al. showed that Cu-BTC exhibits a high H2O adsorption

capacity and a reversible color change upon H2O adsorption.143 In three MOFs,

Kondo found the adsorption isotherms of H2O are all type Ι.144 This reveals H2O

adsorbs strongly on the hydrophilic sites of the three MOFs, which correspond to

crystalline water. Kitagawa and coworkers reported H2O and methanol adsorption in a

dynamic microporous MOF with 1D hydrophilic channels.145 In a 3D porous MOF


19

namely [Zn6(IDC)4(OH)2(Hprz)2]n, Gu et al. observed the selective adsorption of H2O

over organic solvents and the reversible formation of MOF channels upon H2O

adsorption/desorption.146 Similarly, Barcia et al. found the selective adsorption of H2O

over methanol in Cu(R-GLA-Me)(4,4’-bipy)0.5 due to size/shape discrimination.147

However, the selective adsorption of alcohols over H2O was observed in some MOFs

Cu(hfipbb)(H2hfipbb)0.5,148 Zn(tbip),149 Zn(BDC)(TED)0.5,

150 and ZIF-71.151

2.1.3 Gas Separation

Gas separation in MOFs has been widely studied, particularly for the mixtures of

light gases (CO2, H2, N2, CH4, etc). For example, size- or shape-selective adsorption

has been observed in several MOFs. Dybtsev et al. synthesized a MOF with high

thermal stability and permanent porosity, which selectively adsorbs H2 and CO2 over

N2 and other gases of large kinetic diameters.152 This is attributed to the small

apertures that block the adsorption of large molecules. Similarly, selective uptake of

H2 and O2 over N2 and CO was reported in the first magnesium-based MOF

Mg3(NDC)3.153 It was suggested that Mg3(NDC)3 could be potentially used for N2

separation from air, H2/CO separation for fuel cell, and enrichment of H2 from

ammonia synthesis. Selective adsorption of H2 and O2 over N2 and CO due to size

exclusion was also reported in PCN-13154 and PCN-17.155 In addition, Cu(F-pymo)2156

and ‘pillar-layer’ MOFs157 were found to selectively adsorb H2 over N2. The

mesh-adjustable molecular sieves (MAMS) possessing infinite numbers of mesh sizes

were proposed to have the ability to separate any two gases with different kinetic

diameters.158,159 This is because the mesh size increases linearly with temperature;


20

therefore, different gases can be separated by tuning temperature. For example, H2

exhibits a higher uptake over CO, N2 and O2 at 77 K; however, O2 has a much higher

uptake than CO and N2 at 87 K. Further increasing temperature, N2 is selectively

adsorbed over CO and CH4 at 113 K. CH4 can also be separated from C2H4 at 143 K.

In MIL-96, CO2 was found to has a shorter adsorption equilibrium time than CH4

because of the small apertures in MIL-96.158 CO2 selective adsorption over CH4 was

also found in PCN-5.159 Recently, two MOFs with the same static aperture size but

different effective aperture sizes were reported to exhibit different separation

properties for N2/Ar.160

Besides the size-/shape-based selective adsorption, interaction between gas and

MOF is also an important factor to determine separation selectivity. The interaction is

mainly governed by the nature of adsorbate and adsorbent. Both the polarity and

quadrupole moment of adsorbate and the functional group in MOF can affect the

interaction. In addition, the proper pore size may increase the interaction in the pore.

It was found Cu2(pzdc)2(pyz) selectively adsorbs C2H2 over CO2.161 The reason is that

the H atoms of C2H2 and the non-coordinated O atoms in the framework form

hydrogen bonds, thus C2H2 binds more strongly than CO2. Because of the different

gas-framework interactions, CO2 and H2 were found to be selectively adsorbed over

N2 in an interdigitated 3D MOF.162

MOFs with open metal sites can enhance gas separation. Britt et al. performed

dynamic separation experiment to measure the dynamic capacity of CO2 in

Mg-MOF-74 replete with open metal sites.71 Their breakthrough curves demonstrated


21

that CO2 can be separated completely from CH4. In addition, the isosteric heat of CO2

in Mg-MOF-74 is moderate, suggesting the energy for regeneration is not high.

Mn(NDC) with open metal sites exhibits a larger capacity for CO2 than CH4 at

ambient temperature.163 Snurr and coworkers reported the separation of CO2/CH4 in a

carborane-based MOF with and without open metal sites, and observed a higher

selectivity in the former.164 A 2D interpenetrating MOF with unsaturated metal sites

and uncoordinated carboxylic group was found to have high CO2/CH4 selectivity.165

Furthermore, Li-doped MOFs also show enhanced CO2/CH4 selectivity.166

Another strategy to improve gas separation is to functionalize MOFs. At 298 K

and 1 bar, the selectivity of CO2/N2 in Cu-BTTri increases from 21 to 25 by

functionalizing the framework with ethylenediamine.129 A 3D porous MOF with

tetrazole functionalized aromatic carboxylic acid was found to exhibit high selectivity

for CO2/CH4 at 195, 273, and 298 K.167 With both amino and pyrimidine groups

presented in the framework, bio-MOF-11 exhibits high CO2/N2 selectivity of 81 at

273 K and 75 at 298 K.168 Banerjee et al. synthesized a series of ZIFs and the

predicted selectivity of CO2/N2 was in the range between 17 and 50.169 Particularly,

ZIF-78 was found to have a higher selectivity because the presence of –NO2 enhances

interaction between CO2 and framework. Post-modified MOFs with polar group –CF3

were also observed to increase the selectivity of CO2/N2.170 In a rht-type MOF

decorated with acylamide (–CONH), Zheng et al. determined the selectivity of

CO2/N2 is 22 at 1 bar and 33 at 20 bar.171 The selectivity is enhanced upon

comparison with the non-decorated framework PCN-61. A MOF functionalized with


22

flexible alkyl ether chains was found to have high selective sorption of CO2 over N2

due to the gate effect of the side chains.172

2.1.4 Adsorption and Separation of Alkanes

Despite numerous studies reported for the adsorption and separation of light

gases in MOFs, there are few studies for alkanes. Pan et al. first experimentally

reported the separation of hydrocarbons in microporous MOFs built from

paddle-wheel metal clusters and dicarboxylate ligands.148 These MOFs possess

irregular-shaped microchannels with alternating large cages connected by small

windows, and the internal surfaces are highly hydrophobic. Selective adsorption in

these MOFs was observed, specifically nC2, C3 and C4 olefins and alkanes can be

separated from branched alkanes, and n-C4 from higher alkanes and olefins. Chen et

al. examined the chromatographic separation of linear and branched isomers of

pentane and hexane in microporous MOF-508 based on size- and shape-selectivity.173

The pores in MOF-508 may be tuned to match the sizes of alkanes. Later, their group

presented that Zn(BDC)(TED)0.5 with two types of intersecting pores can kinetically

separate hexane isomers by fixed-bed adsorption.174

Adsorption of a series of alkanes, along with alcohols and aromatics, was

determined in a multifunctional microporous MOF and the results indicated that the

MOF can be used for the separation of components with similar boiling point.175

Adsorption and diffusion of alkanes in Cu-BTC were studied by infrared microscopy,

and strong inflection in isotherms was observed due to the preferential location close

to the mouth of octahedral pockets.176 Luebbers et al. reported the adsorption of more


23

than 30 volatile organic compounds in IRMOF-1 to examine the effect of structural

degradation on adsorption.177 Recently, gas phase separation of ethane and ethene in

ZIF-7 was conducted and the results showed that gate-opening effect controls the

separation.178 Adsorption equilibria (as well as diffusivities) of ethane, ethylene,

propane, and propylene in Mg-MOF-74 were experimentally determined. The

relatively high adsorption selectivities suggested that this MOF could be potentially

used to separate C2H4/C2H6, C3H6/C3H8, and C3H6/C2H4.179

Trung et al. studied adsorption of alkanes in flexible MIL-53(Al, Cr).180 They

showed that the metal type and length of n-alkanes govern the shape of adsorption

curve. Llewellyn et al. investigated the adsorption of short linear alkanes in flexible

MIL-53(Fe) and observed multisteps in adsorption isotherms.181 This is because there

are four discrete pore openings in the whole adsorption process. They also performed

simulation and found various structural states of flexible MIL-53(Fe) in the presence

of short linear alkanes.181 By combining experiment and simulation, very recently

they found the adsorption process of n-alkanes in functionalized MIL-53(Fe)-X (X =

CH3, Cl, Br, NH2) is different from that in the original MIL-53(Fe).182

2.2 Simulation Studies

2.2.1 H2, CH4, CO2 Adsorption

H2 Adsorption

Numerous simulation studies have been reported for H2 adsorption in various

MOFs. Kawakami et al. reported in 2001 the first simulation study of H2 in

Zn(BDC)(H2O) and showed H2 has higher adsorption than Ar and N2 at their boiling


24

temperatures.183 Combining quantum chemical calculation and molecular simulation,

Sagara et al. examined H2 adsorption in IRMOFs.184-186 The zinc-oxide corners were

identified to have a higher binding energy compare with the organic linkers. The

binding energy increases with increasing size of organic linker or by adding NH2 or

CH3 group. In addition, they proposed new IRMOFs with a high H2 storage capacity

of 6.5 wt% at room temperature. Yang and Zhong simulated H2 adsorption in

IRMOF-1, IRMOF-8 and IRMOF-18 and concluded that metal-oxide clusters are the

preferential adsorption sites.187 In IRMOF-1, Zhang et al. suggested that the

adsorption sites are mostly determined by adsorbate-MOF interaction at a low

temperature; as temperature increases, however, the loading and diffusion ability of

adsorbed molecules would influence.188 Zhou et al. investigated H2 adsorption in

ZIF-8 at 77 K and pressure range from 10 to 8000 kPa.189 They identified the first

adsorption site is located at the both sides of imidazolate ring and close to imidazolate

C=C bond, and the secondary adsorption site is the pore channel. With ab initio

derived force field, Han et al. simulated H2 adsorption in 10 ZIFs and found the

adsorption sites are diverse with ZIF structures.190

At cryogenic temperatures, quantum effects might play a substantially important

role in determining H2 adsorption. To address this, Garberoglio et al. predicted H2

adsorption isotherms in various MOFs using classical and modified potential.191 From

both experiment and simulation, Liu et al. examined H2 adsorption in

Zn(BDC)(TED)0.5 at 77 K and 298 K up to 50 bar.192 The simulation results agree

well with experimental data when quantum effects were included. Zhong and


25

coworkers reported the influences of pore size, pressure and temperature on quantum

effects.193 Their results suggested that quantum effects increase with decreasing pore

size and are mainly determined by gas-gas and gas-MOFs interactions.

H2 adsorption in MOFs is governed by many factors. Frost et al. discussed the

effects of isosteric heat, surface area, and free volume.194,195 The required isosteric

heat for H2 storage to meet the U.S. DOE target was predicted to be 10~15 kJ/mol

with a free volume of 1.6~2.4 cm3/g and larger than 20 kJ/mol with a lower free

volume. They also proposed correlations for H2 uptake with isosteric heat, surface

area and free volume at 0.1, 30 and 120 bar, respectively. Assfour and Seifert

concluded that H2 uptake at 77 K is mainly determined by isosteric heat at low

pressures and by free volume at high pressures, while H2 uptake at 300 K is only

related to free volume.196 The effect of catenation on H2 adsorption was investigated

by Jung et al.197 and catenated MOFs were found to exhibit a higher capacity because

of the stronger potential overlap generated by catenation. Similar catenation effect

was also reported for improving H2 storage at cryogenic temperatures.198 H2 uptake

was found to increase at low pressures by the inclusion of C60 into MOF-177 at 77

and 300 K; however, the uptake at high pressures decreases due to the reduced pore

volume.199

The binding energy of H2 in MOFs can be enhanced by open metal sites. As

several theoretical studies indicated, it can be tuned up to 10-50 kJ/mol by using

different transition metals in MOFs.200-202 A new set of potential parameters used to

accurate describe H2 adsorption in MOFs with open metal site were derived from ab


26

initio calculations.203 On the other hand, doping alkali elements on the organic linkers

of MOFs is another strategy to improve H2 uptake.204-213 IRMOF-14 composed of

-SO3-1 group and Li cation was predicted to have a higher H2 binding energy and

hence a higher H2 capacity at ambient conditions.214 A simulation study of H2

adsorption in metal alkoxide (Li, Mg, Mn, Ni, Cu) modified MOFs (IRMOF-1, -10,

-16, UiO-68, and UMCM-150) revealed that magnesium alkoxide might be promising

to enhance H2 storage at room temperature.215 In ionic rht-MOF balanced by NO3-

ions, high H2 adsorption capacity was predicted (2.4 wt% at 1 bar and 6.2 wt% at 50

bar).216

CH4 Adsorption

In 2004, Düren et al. simulated CH4 adsorption of in several IRMOFs and

compared with traditional zeolites, MCM-41, carbon nanotubes, and molecular

squares.63 A complex interplay was found for CH4 adsorption with surface area, free

volume, interaction strength, and pore size. Later, they determined the effects of

organic linker on CH4 adsorption.217 Based on simulated CH4 isotherms in nine

IRMOFs, they correlated the adsorption amount to isosteric heat, surface area and free

volume at low, medium, and high loadings, respectively.218 Similar correlations were

also proposed by Wang from the simulation of CH4 in 10 MOFs and it was suggested

that the desired MOF for CH4 storage should have high isosteric heat, surface area,

free volume, and low density of framework.64

Different strategies such as MOFs with functionalized groups, open metal sites

and catenation have been proposed to enhance CH4 storage. Jhon et al. simulated CH4


27

adsorption as well as diffusion in alkoxy-functionalized IRMOFs.219 They found that

functionalization increases adsorption at low and moderate pressures (< 50 bar at 298

K), but reduces saturation capacity and diffusion. Based on IRMOF-1, Zeng and

Zhang designed 10 MOFs with different functional groups for CH4 storage.220 NO2

functionalized MOFs were demonstrated to have the highest adsorption at 298 K and

3.5 MPa. Specifically, the capacity of CH4 in a MOF substituted by four –NO2 groups

was predicted to be 228 v/v. From simulation on CH4 adsorption in six heterometallic

MOFs (MOF-Fe/AgBF4-1, MOF-Fe/AgPF6-1, MOF-Fe/AgSb6-1, MOF-Co/AgBF4-1,

MOF-Co/AgPF6-1, MOF-Co/AgSb6-1), Liu et al. suggested that anions are the

preferential adsorption sites for CH4 at low pressure.221 In five MOFs with large

surface area (MOF-5 and MOF-177), catenation (IRMOF-11 and MOF-14), and open

metal sites (MOF-74), Gallo et al. showed that CH4 storage capacity in MOF-74 is

170 v/v at 35 atm and 298 K, close to the U.S. DOE target.222 Thornton predicted CH4

adsorption in a MOF with magnesium-decorated fullerenes would reach 265 v/v,

higher than the DOE target.223 Zhong and coworkers designed a new MOF by

modifying the organic linkers of PCN-14 for CH4 storage.224 The new MOF was

predicted to have a very large CH4 capacity of 241 v/v at 298 K and 3.5 MPa, which

is 34% higher than the U.S. DOE target. They concluded that the rational

modification of organic linkers is a feasible way to improve CH4 storage capacity.

They also suggested that catenation225 and pore topology226 are important factors to

enhance CH4 adsorption capacity.

CO2 Adsorption


28

CO2 adsorption in MOFs has been also studied using molecular simulations.

Babarao et al. compared CO2 adsorption in IRMOF-1, silicalite, and C168

schwarzite.41 The simulated adsorption isotherms match well with experimental data,

and CO2 has a higher adsorption capacity in IRMOF-1 than the other two materials.

Later, they simulated CO2 storage in series of IRMOFs, UMCM-1, and COFs.39,227

CO2 affinity was found to be enhanced by functional groups or catenation. The

saturation CO2 capacities were correlated with free volume, porosity, surface area and

framework density. CO2 adsorption in Zn(BDC)(H2O) was theoretically studied by

Kawakami et al.183 Although the simulation overestimated experiments, the tendency

was correct. They attributed the deviations to the possible unsaturated adsorption in

experimental samples.

Good agreement was found by Snurr and coworkers between simulation and

experiment for CO2 adsorption in IRMOF-1 at different temperatures.228 It was

suggested that electrostatic interactions between CO2 and frameworks are responsible

for the observed stepped isotherms. They also simulated CO2 adsorption in MOF-177

and IRMOF-3 and the results agreed well with experimental data. Later, they

examined the effect of coordinated water molecules on CO2 adsorption in Cu-BTC,

and the presence of coordinated water molecules was found to increase CO2 uptake

and the selectivity over N2 and CH4.137 As electrostatic interactions are important to

mimic CO2 adsorption in MOFs, the calculation of atomic charges for MOF atoms is

always needed. However, quantum chemical calculation is time consuming. In order

to improve the efficiency, they introduced a rapid charge equilibration method and


29

demonstrated its accuracy for CO2 adsorption in 14 different MOFs.229

Zhong and coworkers predicted CO2 adsorption in various MOFs to evaluate the

effects of organic linker, pore size, and topology.230 They correlated CO2 uptake with

isosteric heat, free volume, and surface area; suggested that CO2-framework

electrostatic interactions would enhance adsorption amount. They also performed

simulation to investigate CO2, CH4 and H2 adsorption in 2D COFs.231 As attributed to

CO2-CO2 electrostatic interactions, stepped isotherms were observed and affected by

temperature, pore size, and CO2-COF interactions. From simulation in ZIF-68 and

ZIF-69, they found CO2 is preferred to adsorb in the small pores formed by

2-nitroimidazole linkers and the chlorine atoms in ZIF-69 increase the binding energy

between CO2 and framework.232 Later, they computationally examined the influences

of framework charges on CO2 adsorption in 20 MOFs with different topologies, pore

sizes, and chemical characteristics.233 The relationships between framework charges

with pressure and pore size were identified.

Ramsahye et al. explored the breathing effect of MIL-53 upon CO2 adsorption

from simulation.234,235 They also used first-principles calculation to probe different

adsorption sites for CO2 in MIL-53(Al, Cr) and MIL-47(V).236 By functionalizing

MIL-53(Al) with different groups -OH, -COOH, -NH2, and -CH3, Torrise et al.

examined the impact of these groups on CO2 adsorption at low pressures and room

temperature.237 It was suggested that -(OH)2 functionalized MIL-53 was an optimal

material for CO2 capture at low pressures.


30

2.2.2 Water Adsorption

Although the studies of water adsorption in MOFs are few, it is important for the

rational design of water-resistant MOFs and their applications in aqueous media.

Greathouse and Allendorf reported a molecular dynamics simulation study for water

in MOF-5.238 It was found MOF-5 structure collapsed upon the water content larger

than 3.9%. By fitting the charges of framework atoms to reproduce the experimental

data of water adsorption in Cu-BTC, Castillo et al. showed that water has a strong

affinity with the open metal sites and water adsorption is sensitive to the atomic

charges.239 Water adsorption on the open metal sites in Cu-BTC was further explored

by DFT calculations and the predicted adsorption enthalpy agreed well with

experimental data using DFT/CC correction scheme.240,241 Nalaparaju et al.

investigated water adsorption, diffusion and vibration in cation-exchanged

rho-ZMOFs and revealed the important interplay between water and nonframework

ions.242 Combining simulation and experimental methods, Maurin and coworkers

examined water adsorption and diffusion in MIL-53(Cr).243 The results suggested that

MIL-53(Cr) is mildly hydrophobic in the large pore form and hydrophilic in the

narrow pore form. Thus MIL-53(Cr) can be tuned to hydrophobic or hydrophilic

without functionalization.

2.2.3 Gas Separation

Gas separation based on adsorption in MOFs has been extensively studied using

molecular simulations. Düren and Snurr simulated the adsorption of CH4/nC4H10

mixture in IRMOFs to determined the influence of organic linkers on adsorption and


31

separation.217 Furthermore, they proposed a hypothetical IRMOF with a larger organic

linker, which exhibits a higher selectivity for C4H10. In a mixed-ligand MOF, Bae et al.

predicted the adsorption isotherms of CO2/CH4 using ideal-adsorbed solution theory

(IAST), which compared well with simulation results.244 Martin-Calvo et al.

examined the adsorption and separation of CO2, CH4 and N2 in IRMOF-1 and

Cu-BTC.245 They found the adsorption selectivity of CO2 over CH4 and N2 in

Cu-BTC was higher than in IRMOF-1, suggesting the efficiency largely depends on

the shape, composition and linker. Combining experiment and simulation, Yazaydin et

al. observed that CO2 selectivity over N2 and CH4 in CuBTC increases in the presence

of 4 wt% of H2O.137

Babarao et al. simulated the adsorption and separation of CO2 and CH4 in

IRMOF-1, silicalite, and C168 schwarzite.41 The predicted adsorption isotherms and

isosteric heats of pure gases agree well with available experimental data, and the

predictions of mixture adsorption from the IAST are in accordance with simulation

results. As the functionality of MOFs can be readily modified by tuning metal-oxide

and organic linker, Babarao et al. further explored a variety of MOFs with unique

characteristics such as open metal sites, interpenetration, and ionic framework for

CO2/CH4 separation.246 The selectivity was found to increase slightly in the presence

of open metal sites and interpenetration, but significantly in ionic framework. Jiang

and coworkers systematically investigated CO2/H2 separation in ionic MOFs

(soc-MOF, rht-MOF and rho-ZMOF).42,216,247 The predicted CO2/H2 selectivity is

significantly higher than in non-ionic MOFs. By switching off the charges on the


32

framework and ions, the selectivity was found to decrease substantially. This indicates

the important role of electrostatic interactions in the observed high selectivity in ionic

MOFs. Particularly noteworthy is the selectivity behaves differently in the three ionic

MOFs with different free volumes and charge densities. The fractional free volume

increases in the order of rho-ZMOF < soc-MOF < rht-MOF, but the charge density

increases in the opposite order. Consequently, rho-ZMOF has the smallest porosity

and largest charge density, and thus exhibits the highest selectivity, followed by

soc-MOF and rht-MOF.

Zhong and coworkers showed the separation selectivity of CO2/CH4/C2H6

mixture in manganese-formate MOFs is higher than in IRMOF-1, Cu-BTC, and most

carbon and zeolite materials248 and the selectivity can be enhanced when electrostatic

interactions are included.249 They also simulated in Cu-BTC for the separation of flue

gas (CO2/N2/O2),250 CH4/H2, CH4/N2, N2/H2, CO2/H2, CO2/N2, CO2/CH4,

251 and

CO2/CO, C2H4/CO2, and C2H4/CO mixtures.252 The side pockets of Cu-BTC were

found to have positive contributions to mixture separation and Cu-BTC was suggested

to be useful for the purification and capture of CO2. By comparing the separation of

CH4/CO2/H2 mixture in COFs and MOFs, they concluded COFs and MOFs have

similar separation performance.253 They predicted a higher CO2/CH4 selectivity in

metal-modified254,255 and organic-group functionalized256 MOF-5 from the prototype

MOF-5. Combining experimental and simulation methods, they examined the effect

of temperature on the separation of CO2, CH4, CO, and N2 binary mixtures in

ZIF-8.257 Furthermore, they computationally screened 105 MOFs for CH4/H2


33

separation in pressure swing adsorption based on high working capacity and

selectivity.258

Liu et al. investigated the effect of interpenetration on CH4/H2 separation in

MOFs and observed a higher selectivity in interpenetrated frameworks than the

non-interpenetrated counterparts due to the enhanced potential overlap in the

former.259 Liu and Smit simulated the separation of CO2/N2 and CH4/N2 mixtures in

three zeolites (MFI, LTA and DDR) and seven MOFs (Cu-BTC, MIL-47, IRMOF-1,

-12, -14, -11 and -13). The results showed that MOFs are better for gas storage but

their separation performance is comparable to zeolites.260 By matching experimental

data with adjustable force field parameters, they examined the separation of CO2/N2,

CO2/CH4, and CH4/N2 in ZIF-68 and -69. In the presence of –Cl atoms, ZIF-69 was

found to exhibit a higher selectivity.261. Recently, they investigated the effects of

interpenetration and mixed-ligand on CH4/H2 separation. Similar with their previous

study, interpenetration in mixed-ligand MOFs was found to enhance adsorption

selectivity.262

2.2.4 Adsorption and Separation of Alkanes

Jiang and Sandler conducted a simulation study for pure and mixed linear and

branched alkanes in IRMOF-1.263 A linear alkane is preferentially adsorbed over a

shorter one at low pressures, whereas the reverse is found at high pressures. In

addition, a linear isomer adsorbs more than its branched analogue as a result of

configurational entropy. This study also indicates that the extent of alkane adsorption

in IRMOF-1 is larger than in silicalite and carbon nanotube. Later, Jiang and


34

coworkers simulated the adsorption and diffusion of C4 and C5 isomer mixtures in

catenated and noncatenated MOFs.43 The catenated MOFs showed a larger separation

factor compared with the the non-catenated counterparts. From the adsorption of a

mixture of 13 alkanes in Zn(BDC)(TED)0.5,264 Dubbeldam et al. observed the

segregation of alkanes according to the degree of branching and suggested a new

possibility for the design MOFs. For propane, n-butane, n-pentane and n-hexane in

cobalt formate frameworks, Krishna and van Baten revealed the possibility for

separating C3/nC6, C3/nC4, nC4/nC6 and nC4/nC5 mixtures because the adsorbed phase

contains primarily short alkanes.265 Jorge et al. presented simulation for the adsorption

of pure and mixed propane and propylene in Cu-BTC.266 The predicted selectivity for

propylene over propane is approximately 4. Maurin and coworkers examined the

adsorption and diffusion of long n-alkanes (C5-C9) in MIL-47(V) using UFF-Dreiding

force fields, and the simulated isotherms and enthalpies agree fairly well with

experimental data.267

Chapter 3. Models and Methods

35


3.1 Atomic Models

In this thesis, five MOFs with unique structures were selected to study CO2

capture, namely MIL-101, Zn(BDC)(TED)0.5, bio-MOF-11, cations exchanged

rho-ZMOF, and IL/IRMOF-1 composite. The objective is to determine the factors that

influence CO2 capture and assist in future materials design. Table 3.1 listed the

structure parameters of the five MOFs.

Table 3.1 The structure parameters of the five MOFs. MOF Unit Cell (Å) Cell Angle

(degree)

Pore Volume

(cm3/g)

Pore Size

(Å)

Surface

Area (m2/g)

MIL-101133 a=b=c=

88.869

α=β=γ=90 2.15 12, 16 (29,

34)

4230

Zn(BDC)(TED)0.5150,268 a=b=10.9288,

c=9.6084

α=β=γ=90 0.65 3.2 (7.5) 1450

Bio-MOF-11168 a=b=15.4355,

c=22.775

α=β=γ=90 0.45 5.2 (5.8) 1040

rho-ZMOF18 a=b=c=

31.062

α=β=γ=90 0.48 5.6 (18.2) 1067

IRMOF-15 a=b=c=

25.832

α=β=γ=90 1.55 11.2 (18.6) 3800

MIL-101

MIL-101 is a chromium terephthalate-based mesoscopic MOF and one of the

most porous materials reported to date. It is stable under air atmosphere or boiling

water and its structure is not altered in organic solvents or solvothermal conditions.

These remarkable properties and its highly porous structure have resulted in


36

considerable interest in MIL-101 for gas storage, catalysis, drug delivery, etc. Despite

a number of experimental studies in this unique MOF,133,134,269-273 we are not aware of

molecular simulation study reported for gas adsorption in MIL-101. However, such

molecular information is important to understand the microscopic adsorption behavior

and facilitate the development of new super-adsorbents.

Zn(BDC)(TED)0.5

A novel MOF named Zn(BDC)(TED)0.5 (BDC = benzenedicarboxylate, TED =

triethylenediamine) was recently synthesized and characterized.150,268 It is thermally

stable up to 282 °C, readily activated and highly porous. H2 sorption capacity in

Zn(BDC)(TED)0.5 is 2.1 wt% at 78 K and 1 bar. In addition, Zn(BDC)(TED)0.5 is

highly hydrophobic and adsorbs a large amount of hydrocarbons giving the highest

capacities among all MOFs reported to date.150 Attributed to these salient features,

several studies have been carried out in Zn(BDC)(TED)0.5. Adsorption and diffusion

of H2 were examined using both experimental and simulation methods.192 Segregation

of complex alkane mixtures were observed from simulation, which suggested a new

possibility for the design and creation of highly selective adsorption sites in MOFs.264

Adsorption of hexane isomers was studied and compared with experimental results.265

Diffusion of binary mixtures was studied in Zn(BDC)(TED)0.5, other MOFs and

zeolites by a unified Maxwell-Stefan description.274

Bio-MOF-11

A unique set of bio-MOFs based on adenine have been developed recently by

Rosi and coworkers.79,195 As a natural nitrogen-based heterocycle, adenine consists of


37

four imino nitrogen atoms and one exocyclic amino nitrogen atom. Interestingly, all

the five nitrogen atoms can coordinate with metals. Adenine possesses multiple

binding modes and is an ideal biomolecular building block for bio-MOFs. Among

several synthesized bio-MOFs, bio-MOF-1 can serve as a host for adsorbing a

cationic drug,69 bio-MOF-11 shows exceptional ability to adsorb CO2,168 a

zinc-adeninate macrocycle has large cavities for gas sorption.275 For further

development of new bio-MOFs as ideal sorbents for CO2 capture, a fundamental

understanding of their adsorption properties from a molecular level is crucial.

rho-ZMOF

In the continuous quest for novel MOFs to achieve high-performance gas

adsorption and separation, ionic MOFs with charge-balancing nonframework ions

have been produced. The presence of nonframework ions in a nano-confined space

can enhance the interactions between adsorbate and framework, which in turn

increases adsorption capacities. Recently, Eddaoudi and coworkers synthesized rho

zeolite-like MOF (ZMOF),18 which is topologically relevant to rho-zeolite. The

substitution of oxygen atom in rho-zeolite by 4,5-imidazoledicarboxylic acid

(H3ImDC) leads to extra-large cavities. Our group has performed systematic

simulation studies on gas adsorption and separation in several ionic MOFs (soc-MOF,

rho-ZMOF, rht-MOF and Li-MOF).42,216,246,247,276 Good agreement was observed

between simulated and available experimental data. The nonframework ions in an

ionic MOF can be exchanged. For example, as-synthesized rho-ZMOF contains

doubly protonated 1,3,4,6,7,8-hexahydro-2H- pyrimido[1,2-a]pyrimidine (HPP). The


38

HPP cations in parent rho-ZMOF can be fully exchanged with other cations such as

Na+ ions.18 However, it remains unknown how different nonframework ions would

affect gas adsorption and separation.

IL/IRMOF composite

There has been considerable interest in using ionic liquids (ILs) for CO2

capture.277 As a unique class of green solvents, ILs are nonvolatile and nonflammable

with high thermal stability, and thus might potentially substitute traditional

solvents.278,279 Since the first observation of high CO2 dissolution in ILs,280 a large

number of experimental and theoretical studies have examined CO2 behavior in

various ILs.281-284 For example, Shiflett and Yokozeki measured the solubilities and

diffusivities of CO2 in 1-n-butyl-3-methylimidazolium hexafluorophosphate

[BMIM][PF6] and 1-n-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4].285

Bara et al. synthesized imidazolium-based ILs with one, two, or three oligo(ethylene

glycol) substituents and determined the solubilities of CO2, N2, and CH4.286 Compared

to corresponding alkyl analogues, these ILs were observed to have 30 – 75% higher

ideal solubility selectivities for CO2/N2 and CO2/CH4 mixtures. By measuring CO2

solubility in a range of ILs, Muldoon et al. found ILs containing more fluoroalkyl

chains on either cation or anion improve CO2 solubility when compared to less

fluorinated ILs.287 Shi and Maginn simulated the sorption of CO2 and gas mixtures in

1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide [HMIM]

[TF2N].288,289 Quantum chemical COSMO-RS method was applied by

Gonzalez-Miquel et al. to predict CO2/N2 selectivity in 224 ILs and they found


39

[SCN]-based ILs can enhance selective separation.290 On the other hand,

task-specific ILs have been investigated for CO2 capture, e.g. by functionalizing

ILs291 or tuning the basicity of ILs.292

There are two major issues in the use of ILs for CO2 capture, i.e., the cost and

high viscosity of ILs. As a new strategy, supported ILs (SILs) have been recently

proposed in which ILs impregnate the pores of a solid porous support.293 In addition

to enhanced mechanical strength, SILs reduce the amount needed and viscosity of ILs,

and thus increase separation efficiency. Scovazzo et al. first tested polyethersulfone

supported non-hexafluorophosphate ILs for gas separation and found the

ideal-selectivity versus permeability ratios in these SILs were above the Robeson

upper-bound for polymers.294 Brennecke and coworkers demonstrated the

high-temperature separation of CO2/H2 mixture using an amine-functionalized IL

encapsulated on a cross-linked nylon support.295 Using both hydrophilic and

hydrophobic polyvinylidene fluoride (PVDF) as supports, Neves et al. measured gas

permeation in 1-n-alkyl-3-methylimidazolium based ILs.296 Their results show that

the hydrophobic PVDF-SILs are more stable and have a higher affinity for CO2.

Most studies to date have used either organic polymers or inorganic zeolites as

supports to prepare SILs.297,298 Although numerous experimental and simulation

studies have been reported separately for ILs281-284 and MOFs,74,299,300 we are not

aware of any study for MOF-supported ILs.


40

3.2 Computational Methods

3.2.1 Density Functional Theory

In quantum chemistry, many approximate methods have been developed to solve

the Schrodinger equation with a low computational cost. Density functional theory

(DFT) is a robust method that is much more efficient than ab initio wave functional

methods. DFT is based on the theorems by Hohenberg and Kohn (1964) who proved

the ground state properties are directly related with electron density. For a system with

N electrons, the total ground state energy can be expressed as

ˆ ˆ ˆ ˆ ˆ ˆee eeE T V U T V U (3.1)

where is the electron wavefunction, T̂ is the kinetic energy, V̂ is the energy

from external fields, and ˆeeU is electron-electron interaction energy. For a

many-body electronic system, Uee can be written as

1 ( ) ( ')'

2 'ee xc

r rU drdr E

r r

(3.2)

where Exc is the exchange-correlation energy and ρ(r) is the electronic density.

Exc remains unknown and must be approximated. Local-density approximation

(LDA) is a simple widely used approximation based on exact exchange energy for a

uniform electron gas. In contrast, generalized gradient approximation (GGA)

considers the inhomogeneous electron gas in a natural molecular system. Commonly

used GGA functionals include BP, PW91, BLYP, and PBE. Hybrid functionals (e.g.

B3LYP) which combines the gradient-corrected functionals with exact HF-type

exchange is also a commonly used functional. The electron density ρ(r) is calculated


41

from molecular orbitals mathematically expressed by basis set. A basis set determines

the number and types of atomic orbitals used in the expansion of molecular orbitals.

There are numerous basis sets, for example, the minimal basis set STO-3G,

split-valence basis set 6-31G(p), and correlation-consistent basis set cc-pVTZ.

3.2.2 Interaction Potential

In classical simulation, interaction potential of a system is usually determined by

empirical force field without solving the Schrodinger equation. The interaction

potential can be subdivided into two contributions

total bonded non bondedU U U (3.3)

where bondedU is the intramolecular energy and non bondedU is the intermolecular energy.

The intramolecular energy bondedU consists of

stretchingbonded bend torsionU U U U (3.4)

where stretchingU is the stretching energy arising from the change of bond lengths,

bendU is the bending energy resulting from the change of angles between two

successive chemical bonds, and torsionU is the torsional energy arising from the change

of dihedral angles.

The non-bonded intermolecular energy has

non bonded vdW CoulombicU U U (3.5)

where vdWU is the van der Waals interaction energy and CoulombicU is the coulombic

electrostatic potential energy. The van der Waals interaction is usually mimicked by

Lennard-Jones (LJ) potential. For a system composed of different types of atoms, the

total LJ potential is


42

12 6

.

( ) 4 ij ijLJ ij

i j ij iji j

u rr r

(3.6)

where σij and εij are the collision diameter and well depth, respectively; The cross

parameters εij and σij were evaluated by combining rule, for example, Lorentz-

Berthelot rules

ij i j (3.7)

σij = (σi + σj)/2. (3.8)

The electrostatic interaction is modeled by the Coulomb’s law

0,

( )4

i jCoulomb

i j iji j

q qu r

r

(3.9)

where qi and qj are the charges on particle i and j, ε0 = 8.8542 10-12 C2 N-1 m-2 is the

permittivity of vacuum. The charges are usually estimated by fitting electrostatic

potential (ESP),301 which can be calculated by DFT.

Different force fields have been developed for simulations. For example,

Universal Force Field (UFF)302 is a general force field can be used for all the elements

in the periodic table. Dreiding force field303 can be used for organic, biological and

main-group inorganic systems. PCFF304 is developed for polymers and organic

materials. COMPASS305 is a force filed to predict both gas- and condensed-phase

properties for most common organics, polymers and small inorganic molecules.

AMBER306 is a force field widely used for nucleic acids and proteins. CVFF307,

CHARMM308, OPLS-AA309 and GROMOS310 are also common force fields for

organic and biomolecular systems.


43

3.2.2 Molecular Dynamics Simulation

Molecular dynamics (MD) simulation is a technique to compute equilibrium and

transport properties in a system in which the motion of particles obeys the classical

mechanics laws. The transport properties such as mass transfer (diffusion coefficients),

heat transfer (thermal conductivity) or momentum transfer (viscosity) can be

calculated from MD simulation performed at equilibrium ensembles (NVE, NVT,

NPT) or non-equilibrium ensembles (when perturbations or the specific boundary

conditions are imposed).

Classical MD simulation is based on the integration of Newton’s equation of

motion,

2

2, =1, 2, ,

iii

rm F i N

t

(3.10)

where mi is the mass of particle i, ir

is the position of the particle, iF

is the force

acting on the particle from the system. For a system with N interacting particles, the

force is derived from potential energy

( ( ))i iF U r

(3.11)

These equations are solved numerically using a small time step, usually in

femtosecond scale.

3.2.3 Monte Carlo Simulation

Monte Carlo (MC) simulation is a statistical method to generate the collection of

configurations with desired probability distribution at given conditions (statistical

ensembles) such as temperature, pressure, volume, or chemical potential. MC


44

simulation is efficient because only potential energy rather than force is evaluated in

configuration sampling. Moreover, MC can perform motions without physical mean,

for example, a jump from one position to another, insertion or deletion of a new

molecule. In its simplest form, MC method is an algorithm for numerical integration

in mathematics. A set of parameters are randomly selected or randomly perturbed, and

a function of these parameters is evaluated. Depending on the system of interest,

various types of trial moves can be attempted to reach equilibrium, for example,

translation, rotation, displacement, regrowth. After that, statistically averaged

properties can be obtained. The ensembles for MC simulation generally include

canonical (NVT, in which the number of particles, volume, and temperature T are

constant), microcanonical (NVE, in which number of particles, volume, energy are

constant), isobaric-isothermal (NPT, in which number of particles, pressure,

temperature are constant), grand-canonical (VT, in which the chemical potential,

volume and temperature are constant).

In this thesis, adsorption in MOFs was simulated by the grand-canonical Monte

Carlo (GCMC) method. As the chemical potentials of adsorbate in adsorbed and bulk

phases are identical at thermodynamic equilibrium, GCMC allows one to directly

relate the chemical potentials of adsorbate in both phases and has been widely used to

simulate adsorption. The framework atoms in MOFs were assumed to be rigid and the

potential energies between adsorbate atoms and framework were pre-tabulated. This is

because the low-energy equilibrium configurations are involved in adsorption and the

flexibility of framework has only a marginal effect. A recent simulation study on the


45

adsorption of noble gases in IRMOF-1 demonstrated that rigid and semi-flexible

frameworks gave close results at both low and room temperatures.311 For most cases,

the LJ interactions were evaluated with a spherical cutoff of 15 Å (13 Å for

Zn(BDC)(TED)0.5 and bio-MOF-11) with long-range corrections added. The

coulombic interactions were calculated using the Ewald sum method because it is the

best technique for electrostatic interactions in a periodic system. The real/reciprocal

space partition parameter and the cutoff for reciprocal lattice vectors were chosen to

be 0.2 Å-1 and 8 (10 for MIL-101), respectively, to ensure the convergence of Ewald

sum. The number of trial moves in a typical GCMC simulation was 2 107, though

additional trial moves were used at high pressures. The first 107 moves were used for

equilibration and the second 107 moves for ensemble averages. Five types of trial

moves were randomly attempted in the GCMC simulations: displacement, rotation,

and partial regrowth at a neighboring position; complete regrowth at a new position;

and swap with reservoir including creation and deletion with equal probability. For

mixtures, another type of trial move, the exchange of molecular identity, was also

included. Unless otherwise mentioned, the simulation uncertainties were smaller than

the symbol sizes presented below.

3.3 Analysis Methods

3.3.1 Radial Distribution Functions

Radial distribution function represents the probability of finding a particle at a

certain distance from another particle. To examine the structure of a system, g(r) is

calculated by


46

2

( )4

ij

iji j

N Vg r

r r N N (3.12)

where r is the distance between the geometric centers-of-mass of species i and j, ΔNij

is the number of species j around i within a shell from r to r + Δr, V is the volume, Ni

and Nj are the numbers of species i and j. Essentially, g(r) gives the ratio of local

density at position r to averaged overall density in the system.

3.3.2 Adsorption Selectivity

The separation factor of a binary mixture is usually quantified by adsorption

selectivity

/ ( / )( / )i j i j j iS x x y y (3.13)

where ix and iy are the mole fractions of component i in adsorbed and bulk phases,

respectively.

3.3.3 Mean-Squared Displacement

The dynamic properties of a system is evaluated by mean-squared displacement

2

1

1MSD ( ) ( )

N

ii

t tN

r (3.14)

where N is the number of ions and Δri(t) is the displacement of ith ion at time t.

Chapter 4. Adsorption of CO2 and CH4 in MIL-101

47


In this chapter, adsorption of CO2 and CH4 in MIL-101 is investigated at 303 K

combining quantum mechanics and molecular simulation. Both dehydrated and

hydrated MIL-101 are considered to examine the effect of terminal water molecules.

4.1 Models and Methods

MIL-101 is assembled by corner-sharing supertetrahedra, which consist of Cr3O

trimers and 1,4-benzenedicarboxylic acids.30 Connected by 8.6 Å apertures, the

supertetrahedra include 4 vertices and 6 edges that are occupied by the trimers and

organic linkers, respectively. MIL-10 has an augmented three-dimensional MTN

zeotype structure with a giant cell volume (~ 702,000 Å3). Two types of mesoporous

quasi-spherical cages exist in MIL-101: a small cage of 20 supertetrahedra and a free

diameter of 29 Å accessible through a pentagonal window with 12 Å aperture, and a

large one of 28 supertetrahedra and a free diameter of 34 Å accessible through both

hexagonal and pentagonal windows with 14.7 16 Å aperture. MIL-101 is highly

porous with BET and Langmuir surface areas of about 4100 ± 200 and 5900 ± 300

m2/g, respectively. Each octahedral Cr is bonded with four oxygen atoms from

carboxylates, one 3O oxygen atom, and one terminal site. The terminal site could be

occupied by a fluorine atom or a terminal water molecule. As a consequence, there are

three terminal sites in each Cr3O trimer and the ratio of fluorine to water is 1:2. The

terminal water molecules can be removed after dehydration, thus providing exposed

metallic Lewis acid sites in MIL-101.30


48

Figure 4.1. A unit cell of dehydrated MIL-101 constructed from experimental crystallographic data, energy minimization and density functional theory calculation (see the text for details). The pentagonal and hexagonal windows are enlarged for clarity. Color code: Cr, orange polyhedra; F, cyan; C, blue; O, red; H, white.

Figure 4.1 shows a unit cell of dehydrated MIL-101, in which the pentagonal and

hexagonal windows are enlarged for clarity. The unit cell structure was constructed by

three steps based on experimental crystallographic data and computational approaches.

In the first step, all the disordered and terminal water molecules in the

crystallographic data were removed. Indeed, only the oxygen atoms of water

molecules were removed because the hydrogen atoms were not in the crystallographic

data. To construct hydrated MIL-101, however, only one terminal water molecule in

hexagonal

pentagonal


49

Figure 4.2. Merz-Kollman charges of Cr3O trimer with terminal fluorine and water molecules in (a) dehydrated and (b) hydrated MIL-101. The cleaved bonds of Cr3O (indicated by the circles) were saturated by methyl group. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white.

Figure 4.3. Mulliken charges of the Cr3O trimer with terminal fluorine and water molecules in (a) dehydrated and (b) hydrated MIL-101. The cleaved bonds of Cr3O (indicated by the circles) were saturated by methyl group. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white.

each Cr3O trimer was removed. In the second step, hydrogen and fluorine atoms were

added respectively to the phenyl rings and the exposed Cr sites (one in each Cr3O

trimer). For hydrated MIL-101, hydrogen atoms were also added to the oxygen atoms

of terminal water molecules. The unit cell was then energetically minimized using

Materials Studio.312 The framework atoms were represented by the Universal Force

(b) (a)

C3 (-0.094)

H (0.154)

Cr1 (1.557)

O1 (-0.889)

C2 (-0.093) C1 (0.853)

Cr2 (1.886)

O2 (-0.766)

Cr2 (1.886)

F (-0.523)

Cr1 (1.448)

C3 (-0.120)

H (0.148)

C2 (-0.037) C1 (0.689)

Cr2 (1.747)

O2 (-0.654)

Cr2 (1.747)

F (-0.526)

OW (-0.758)

Cr1 (1.153) C3 (-0.043)

H (0.098)

O1 (-0.822)

C2 (-0.069)

C1 (0.631)

Cr2 (1.155)

O2 (-0.537)

Cr2 (1.155)

F (-0.512)

OW (-0.437)

(b)

C3 (-0.041)

H (0.105)

Cr1 (1.159)

O1 (-0.764)

C2 (-0.070)

C1 (0.641)

Cr2 (1.152)

O2 (-0.533)

Cr2 (1.152)

F (-0.505)

(a)


50

Field (UFF).302 The phenyl rings in the experimental crystallographic data were

slightly non-planar and transformed to planar after the energy minimization. In the

third step, geometry optimizations were conducted by density-functional theory (DFT)

to precisely determine the positions of the added hydrogen and fluorine atoms.

Because of the tremendously large number of atoms in a unit cell, the DFT

optimizations were performed on a Cr3O trimer as shown in Figure 4.2. To maintain

the original hybridization, the cleaved bonds of the trimer were saturated by methyl

groups. The Becke exchange plus Lee-Yang-Parr correlation functional and the

effective core potentials were adopted in the DFT calculations. In the latter,

pseudo-potentials are used to represent the potential of the nucleus and core electrons

experienced by the valence electrons. This allows only the softer valence electron

wave functions to be explicitly treated, which is usually the portion that controls the

chemistry and can significantly reduce computational cost. Double- numerical

polarization (DNP) basis set was used in the DFT calculations, which is comparable

to 6-31G(d,p) Gaussian-type basis set. DNP basis set incorporates p-type polarization

into hydrogen atoms and d-type polarization into heavier atoms. In both dehydrated

and hydrated MIL-101, the optimized distance from F to Cr is 1.86 Å. In hydrated

MIL-101, the oxygen atom (OW) of terminal water is about 2.26 Å from Cr.

The atomic charges of MIL-101 framework atoms were fitted to the electrostatic

potentials using the Merz-Kollman (MK) scheme.301,313 It is worthwhile to note that

the concept of atomic charges is solely an approximation, and no unique

straightforward method is currently available to determine them on a rigorous level.


51

To evaluate the sensitivity of framework charges, the Mulliken population analysis314

was also used to estimate the charges. The MK and Mulliken charges are listed in

Figure 4.2 and Figure 4.3, respectively.

The DFT calculations also gave the electrostatic potential maps around the trimer.

Figure 4.4 illustrates that the fluorine atom possesses large electronegativity in both

dehydrated and hydrated MIL-101. The significant difference is the electric fields

generated by the terminal H2O molecules in the hydrated MIL-101. As we shall see,

this has an intriguing effect on adsorption.

Figure 4.4. Electrostatic potential maps around the Cr3O trimer in (a) dehydrated and (b) hydrated MIL-101.

The free volumes of the dehydrated and hydrated MIL-101 structures were

estimated by Monte Carlo methods.315 A single helium atom was attempted to insert

into the host structure and He

ad Bexp[ ( ) / ] dr rV

u k T-ò gave the free volume, in which

He

adu is the interaction between helium and adsorbent. It should be noted that the free

volume estimated by helium adsorption is slightly greater than the actual geometrical

(b)(a)


52

volume.316 The calculated free volumes in the dehydrated and hydrated MIL-101 were

1.89 and 1.85 cm3/g, respectively; and the porosities were 0.83 and 0.81.

Table 4.1. Potential parameters and atomic charges of CO2, CH4, and terminal H2O.

Site (Adsorbate) (Å) /kB (K) Z(e)

C (CO2) 2.789 29.66 +0.576

O (CO2) 3.011 82.96 0.288

CH4 3.73 148.0 0

OW (H2O) 3.151 76.42 0.401

HW (H2O) 0 0 +0.758

For the adsorption of CO2 and CH4 in MIL-101, CO2 was represented as a

three-site rigid molecule and its intrinsic quadrupole moment was described by a

partial charge model.317 The CO bond length was 1.18 Å and the bond angle OCO

was 180. CO2CO2 interactions were modeled as a combination of Lennard-Jones

(LJ) and coulombic potentials. A united-atom model was used for CH4 with the LJ

potential parameters from the TraPPE force field that was developed to reproduce the

critical parameters and saturated liquid densities of alkanes.318 The LJ potential

parameters of the terminal water molecules were adopted from TIP3P (three-point

transferable interaction potential) model.319 Table 4.1 lists the potential parameters

and atomic charges of CO2, CH4 and terminal water. The dispersion interactions of the

framework atoms in MIL-101 were modeled using the Universal Force Field

(UFF).302 The Lorentz-Berthelot combining rules were used to calculate the cross LJ

interaction parameters. A number of simulation studies have shown that UFF can


53

accurately predict gas adsorption in various MOFs.39,41,191,320 The adsorption of CO2

and CH4 in MIL-101 was simulated by grand canonical Monte Carlo (GCMC) method

as described in Chapter 3.

4.2 Results and Discussion

4.2.1 Sensitivity of Framework Charges

CO2 adsorption in the dehydrated MIL-101 was simulated with the MK and

Mulliken charges on the framework atoms and the results are shown in Figure 4.5. For

comparison, the experimental data are included from the study by Llewellyn et al.133

They measured the adsorption of CO2 and CH4 in three MIL-101 samples, namely,

MIL-101a as-synthesized, MIL-101b activated by ethanol, and MIL-101c activated by

ethanol and KF.133 These samples differ primarily in the amount of residual

terephthalic acid and in the density of Lewis acid Cr sites (500, 700, and 1000 mol/g,

respectively). CH4 adsorption was found to be essentially the same in the three

samples. However, CO2 adsorption was affected by the activation method, and the

extent of adsorption increased in the order of MIL-101a < MIL-101b < MIL-101c.133

Among the three samples, MIL-101b and MIL-101c were activated more thoroughly

and contained less amount of residual terephthalic acid and more open Cr sites;

consequently, they had the stronger interactions with CO2 and the larger free volume

for adsorption than MIL-101a. The dehydrated MIL-101 model in our study is similar

to experimental MIL-101b and MIL-101c rather than MIL-101a. As seen in Figure 4.5,

the predicted CO2 adsorption from both the MK and Mulliken charges match well

with the experimental data of MIL-101b and MIL-101c. The simulation results from


54

the Mulliken charges are in good agreement with MIL-101b in the pressure range of 0

to 2000 kPa and the deviation is less than 6%. For the ESP charges, the deviation is

less than 19% at < 1000 kPa and is less than 7% at higher pressure. This validates the

models used in our study and suggests that CO2 adsorption is relatively insensitive to

the method, either MK or Mulliken, for the framework charges. Unless otherwise

stated, the results for CO4 presented below are based on the MK charges.

P (kPa)0 1000 2000 3000 4000 5000

N (

mm

ol/g

)

0

10

20

30

40

Figure 4.5. CO2 adsorption in dehydrated MIL-101. The squares, diamonds and circles are experimental data133 in MIL-101a, MIL-101b and MIL-101c, respectively.

4.2.2 United-Atom and Five-site Models of CH4

To examine the performance of united-atom and five-site models of CH4, Figure

4.6 shows the adsorption isotherms of CH4 in the dehydrated MIL-101 predicted from

both models.133 Because CH4 adsorption was found experimentally to be insensitive to

the activation method, only the experimental data in MIL-101c are included in Figure

4.6. The predictions of both united-atom and five-site model are in fairly good

agreement with the experimental data, especially at low pressures. Nevertheless, the

: ESP charges

: Mulliken charges

: exp. (MIL-101a)

: exp. (MIL-101b)

: exp. (MIL-101c)


55

united-atom model overestimates, whereas the five-site model underestimates. Such a

trend was also observed in CH4 adsorption on graphite and in graphitic slit pores, and

attributed to the better packing of the united-atom model in confined space.321 Unless

otherwise stated, the results for CH4 are based on the united-atom model.

P (kPa)0 1000 2000 3000 4000 5000

N (

mm

ol/g

)

0

5

10

15

20

Figure 4.6. CH4 adsorption in dehydrated MIL-101. The circles are the experimental data in MIL-101c.133

4.2.3 Adsorption of Pure CO2 and CH4

Figure 4.7 shows the adsorption isotherms of CO2 and CH4 in both dehydrated and

hydrated MIL-101 on a gravimetric basis. CH4 uptake increases almost linearly with

increasing pressure and is still not saturated at 50 bar because of the very porous

structure of MIL-101. This implies that the interaction between CH4 and the MIL-101

is weak and a higher pressure is needed to attain saturation. Despite a slight

overestimation at high pressures, the simulated isotherms of CH4 in both dehydrated

and hydrated MIL-101 are in good agreement with the experimental data. The

dehydrated MIL-101 exhibits greater adsorption than its hydrated counterpart at high

: united-atom model

: five-site model

: exp. (MIL-101c)


56

pressures. As will be seen, however, the opposite is observed at low pressures due to

the presence of terminal water molecules.

P (kPa)0 1000 2000 3000 4000 5000

N (

mm

ol/g

)

0

10

20

30

40

CO2

CH4

Figure 4.7. Adsorption isotherms of CO2 and CH4 on a gravimetric basis. The squares, diamonds, and circles are the experimental data in MIL-101a (as-synthesized), MIL-101b (activated by hot ethanol) and MIL-101c (activated by hot ethanol and KF), respectively.133

CO2 adsorption is apparently greater than CH4 because CO2 has a quadrupole

moment and thus interacts more strongly with the adsorbent. The simulated isotherms

of CO2 in dehydrated and hydrated MIL-101 compare fairly well with the

experimental data in MIL-101b and MIL-101c samples. The deviations can be

attributed primarily to difference between theoretical model and real sample. In our

study, the model does not contain any residual terephthalic acid, though some

probably remained in the sample. CO2 uptake in MIL-101 at 50 bar ranges from 30 to

40 mmol/g depending on the activation method, which is higher than that in IRMOF-1

(22 mmol/g)125 and Cu-BTC (11 mmol/g),125 and comparable to that in MOF-177

(33.5 mmol/g at 42 bar)125 and UMCM-1, IRMOF-10 and IRMOF-14 (40 ~ 45

mmol/g).39 Similar to CH4 adsorption, the dehydrated MIL-101 has greater adsorption

: dehydrated

: hydrated

: exp. (MIL-101a)

: exp. (MIL-101b)

: exp. (MIL-101c)


57

for CO2 at high pressures but the reverse is true at low pressures, which will be

discussed below.

P (kPa)0 50 100 150 200 250 300

0

200

400

600

800

1000

CO2

CH4

CO2

CH4

P (kPa)3000 3500 4000 4500 5000

2000

3000

6000

7000

CO2

CH4

N (

mol

ecul

es/U

C)

(a) (b)

Figure 4.8. Adsorption isotherms of CO2 and CH4 in MIL-101 (a) at low pressure and (b) high pressure based on the number of molecules per unit cell.

Figure 4.8 shows the adsorption isotherms of CO2 and CH4 on the basis of

molecules per unit cell. At low pressures, the adsorption particularly of CO2 in

hydrated MIL-101 is greater than in dehydrated framework. Interestingly, the opposite

is observed at high pressures. Such as phenomenon was recently found of CO2 and

CH4 in microporous Cu-BTC137 and of CO2/H2 mixture in charged soc-MOF.247 At

low pressures, the enhanced adsorption in hydrated MIL-101 is attributed to the

terminal water molecules that act as additional adsorption sites; thus, adsorbates

experience stronger interactions. As seen in Figure 4.8a, the enhancement is more

pronounced for CO2 than CH4. This is because CO2 is a quadrapolar molecule and

interacts strongly with the charged hydrogen and oxygen atoms of the terminal water

molecules. However, the united-atom model of CH4 can only interact with water

molecules through weak disperse interaction. At high pressures, the pores in MIL-101

are largely filled by adsorbate, and the free volume is a dominant factor to determine

: hydrated

: dehydrated

: hydrated

: dehydrated


58

adsorption capacity. Apparently, the terminal water molecules occupy part of the

volume and prevent the framework from accommodating more adsorbate molecules.

Consequently, hydrated MIL-101 exhibits lower adsorption at high pressures.

Figure 4.9. Snapshots of CO2 and CH4 in a pentagonal window in dehydrated MIL-101 at 10, 100, and 1000 kPa. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white; CO2, green; CH4, pink.

To identify the favorable adsorption sites in MIL-101, Figure 4.9 illustrates the

simulation snapshots of CO2 and CH4 in a pentagonal window in the dehydrated

MIL-101. At 10 kPa, the adsorbates are located exclusively in the supertetrahedra.

Attributed to the strong overlap of surface potentials, the microporous

supertetrahedral are the most favorable adsorption sites. At 100 kPa with increased

amount of adsorption, adsorbates appear in the pentagonal window in addition to

supertetrahedra. Compared to CH4, CO2 molecules also occupy the edge of the

pentagonal window close to the exposed Cr2 sites. At 1000 kPa, more adsorbates

CO2

CH4

10 kPa 100 kPa 1000 kPa


59

populate the pentagonal window. Though not shown here, at very high pressures the

mesoporous cages are gradually filled.

CO2-Cr1

r (Å)2 4 6 8 10 12 14 16

g(r)

0

1

2

3

4

10 kPa

100 kPa

1000 kPa

5000 kPa

CO2-Cr2

r (Å)2 4 6 8 10 12 14 16

0

1

2

3

4

10 kPa

100 kPa

1000 kPa

5000 kPa

CH4-Cr1

r (Å)2 4 6 8 10 12 14 16

g(r)

0

1

2

3

4

10 kPa100 kPa

1000 kPa

5000 kPa

CH4-Cr2

r (Å)2 4 6 8 10 12 14 16

0

1

2

3

4

10 kPa

100 kPa

1000 kPa

5000 kPa

Figure 4.10. Radial distribution functions of CO2 and CH4 around Cr1 and Cr2 atoms in dehydrated MIL-101 at 10, 100, 1000, and 5000 kPa.

The simulation snapshots in Figure 4.9 provide only qualitative information. To

quantitatively characterize the adsorption sites, the radial distribution functions of

adsorbates around the framework atoms were calculated. Figure 4.10 shows the g(r)

of CO2 and CH4 around Cr1 and Cr2 atoms in dehydrated MIL-101 at various

pressures. For all the four cases shown, pronounced peaks are observed at r = 5 ~ 9 Å,

revealing that the supertetrahedra are preferential adsorption sites. Furthermore, two

additional peaks are observed in the g(r) of CO2-Cr1. This is due to the attractions


60

between fluorine atoms (bonded to Cr1 sites) and CO2 molecules in two neighboring

supertetrahedra. As illustrated by the electrostatic potential maps in Figure 4.4, the

fluorine atoms possess large electronegativity and thus interact strongly with CO2

molecules.

There is a distinct difference in the short region of g(r) around Cr1 and Cr2. In the

dehydrated MIL-101, Cr1 sites are bonded with fluorine atoms, whereas Cr2 sites are

exposed. Small peaks are observed in the g(r) of CO2-Cr2 at r = 3.5 Å; though there

are no peaks in the g(r) of CH4-Cr2, shoulders exist in the region r = 3 ~ 4 Å. In

remarkable contrast, such peaks and shoulders are not seen around Cr1 atoms. That is,

adsorbate molecules are closer to Cr2 rather than Cr1 sites. Because of the quadrapole

moment, CO2 interacts more strongly with the Cr2 sites than CH4, leading to the peaks

in g(r) of CO2-Cr2. Figure 4.11 schematically demonstrates the locations of CO2 and

CH4 molecules around Cr2 sites. We should note that the region near Cr2 sites is

substantially smaller than the size of supertetrahedra; therefore, the peaks in g(r) at

3.5 Å are much lower and narrower than those at 5 ~ 9 Å.

Figure 4.11. Schematic locations of CO2 and CH4 near Cr3O trimer in dehydrated MIL-101. Color code: Cr, orange; F, cyan; C, blue; O, red; H, white.

CH4 CO2


61

With rising pressure, the number of adsorbed molecules increases. As defined by

eq. (3.1), g(r) is the local density relative to the overall density. In Figure 4.10, the

peaks of g(r) at 5 ~ 9 Å decrease when pressure rises, as also observed in our recent

studies for the adsorption of gas mixture and water in charged rho zeolite-like

MOF.56,242 Interestingly, different behavior is found for the peaks at 3.5 Å closed to

the exposed Cr2 sites. Specifically, the peak of CO2-Cr2 increases with rising pressure

from 10 to 100 and then to 1000 kPa, but decreases from 1000 to 5000 kPa. This

implies that CO2 molecules are increasingly adsorbed near the exposed Cr2 sites until

1000 kPa; however, these regions are small and get saturated beyond 1000 kPa.

Unlike CO2-Cr2, the shoulder of CH4-Cr2 still increases marginally when pressure

rises from 1000 to 5000 kPa. This is because the interaction of CH4-Cr2 is weaker

than that of CO2-Cr2, thus the saturation of Cr2 sites by CH4 appears at a higher

pressure. The radial distribution functions discussed here provide useful insights into

the favorable interaction sites of adsorbates in the dehydrated MIL-101.

We next consider the hydrated MIL-101, in which both Cr1 and Cr2 sites are

coordinated thus exhibit similar g(r). As shown in Figure 4.12, the g(r) are almost the

same for CO2-Cr1 and CO2-Cr2, also for CH4-Cr1 and CH4-Cr2. While there are peaks

at 5 ~ 9 Å, no peaks or shoulders are observed at 3.5 Å, which are significantly

different from Figure 4.10. The pressure dependence of the g(r) is similar to the

dehydrated framework, that is, the peak decreases with rising pressure.


62

CO2-Cr2

r (Å)2 4 6 8 10 12 14 16

0

1

2

3

4

CO2-Cr1

r (Å)2 4 6 8 10 12 14 16

g(r)

0

1

2

3

4

CH4-Cr1

r (Å)2 4 6 8 10 12 14 16

g(r)

0

1

2

3

4

CH4-Cr2

r (Å)2 4 6 8 10 12 14 16

0

1

2

3

4

Figure 4.12. Radial distribution functions of CO2 and CH4 around Cr1 and Cr2 atoms in hydrated MIL-101 at 10, 100, 1000, and 5000 kPa.

Figure 4.13 shows the radial distribution functions of CO2 and CH4 around the

oxygen atoms (OW) of the terminal water molecules in the hydrated MIL-101 at 10,

100, 1000 and 5000 kPa, respectively. Similar to the g(r) of CO2-Cr2 and CH4-Cr2 in

Figure 3.10, peaks and shoulders are observed in the g(r) of CO2-OW and CH4-OW at

r = 3.8 Å. The reason is that the terminal water molecules act as additional adsorption

sites, especially for CO2. Comparing Figures 4.10 and 4.13, we find that the peaks and

shoulders are broader in the latter, suggesting that the terminal water molecules result

in stronger interactions with adsorbates than the exposed Cr2 sites. As a consequence,

adsorption at low pressures is greater for both CO2 and CH4 in the hydrated MIL-101.

10 kPa100 kPa

1000 kPa5000 kPa

10 kPa100 kPa

1000 kPa5000 kPa

10 kPa100 kPa

1000 kPa5000 kPa

10 kPa100 kPa

1000 kPa5000 kPa


63

r (Å)2 4 6 8 10 12 14 16

g(r

)

0

1

2

3

4

10 kPa

100 kPa

1000 kPa

5000 kPa

CO2-OW

r (Å)2 4 6 8 10 12 14 16

0

1

2

3

4

10 kPa

100 kPa

1000 kPa

5000 kPa

CH4-OW

Figure 4.13. Radial distribution functions of CO2 and CH4 around the oxygen atoms of terminal water molecules in hydrated MIL-101 at 10, 100, 1000, and 5000 kPa.

4.2.4 Adsorption of CO2/CH4 Mixture

Adsorption of equimolar mixture of CO2 and CH4 was simulated in the dehydrated

and hydrated MIL-101. As shown in Figure 4.14a, the extent of adsorption of each

component is not substantially affected by hydration. With a closer look, however, we

find that CO2 adsorption at low pressures is marginally higher in the hydrated

MIL-101 than in the dehydrated counterpart. At high pressures, there is no discernable

different in the two structures. In contrast, CH4 adsorption in dehydrated MIL-101 is

slightly higher at moderate and high pressures. As shown in Figure 4.14b, with

increasing pressure the selectivity of CO2/CH4 first increases, then drops rapidly, and

finally increases slightly. The initial increase is due to the preferential adsorption of

CO2 in the multiple supertetrahedra. Upon adsorption occurring in the less attractive

windows and mesoporous cage, the selectivity drops. At high pressures, the selectivity

increases is because of the cooperative attractions between adsorbed CO2 molecules.

Interestingly, the selectivity of CO2/CH4 is higher in the hydrated MIL-101, implying


64

that the separation efficacy could be enhanced by introducing terminal H2O molecules.

This was also observed in the adsorption of CO2/CH4 in Cu-BTC137 and of CO2/H2 in

charged soc-MOF.247

P (kPa)0 1000 2000 3000 4000 5000

0

1000

2000

3000

4000

CH4

P (kPa)0 1000 2000 3000 4000 5000

3

4

5

6

7

(a) (b)

N (

mol

ecul

es/U

C)

Sel

ectiv

ity C

O2/

CH

4

CO2

Figure 4.14. Adsorption of equimolar CO2/CH4 mixture. (a) isotherm and (b) selectivity of CO2 over CH4.

4.3 Summary

We have investigated the adsorption of CO2 and CH4 in mesoporous MIL-101.

The dehydrated and hydrated MIL-101 structures were constructed on the basis of

experimental crystallographic data, molecular mechanics minimization, and

first-principles calculations. Adsorption occurs exclusively in the microporous

supertetrahedra at low pressures, and then in the mesoporous cages at high pressures.

The simulated isotherms of pure CO2 and CH4 match well with measurements, despite

variations in experimental data depending on the activation method used. The

terminal water molecules in hydrated MIL-101 have an interesting effect on

adsorption by acting as additional adsorption sites that enhance adsorption at low

pressures. The enhancement is greater for CO2 as it is a quadrapolar molecule and

: hydrated

: dehydrated

: hydrated

: dehydrated


65

interacts strongly with charged hydrogen and oxygen atoms of the water molecules.

At high pressures, however, the terminal water molecules reduce free volume and lead

to less adsorption compared to the dehydrated MIL101.

Through structural analysis, adsorbates in dehydrated MIL-101 were found to

locate preferentially near the exposed Cr2 sites as evidenced by the peaks and

shoulders in the radial distribution functions. However, such peaks and shoulders

were not seen around the fluorine saturated Cr1. In hydrated MIL-101, the radial

distribution functions behave similar for all Cr sites. Compared with those in

dehydrated MIL-101, the peaks and shoulders around the terminal water molecules

are higher and broader. This reveals that the terminal water molecules interact more

strongly than the exposed Cr2 sites for adsorbates. Therefore, the hydrated MIL-101

exhibits greater adsorption at low pressures. The adsorption selectivity of CO2 over

CH4 is slightly higher in the hydrated MIL-101. This study provides useful insights

into the microscopic adsorption behavior in a unique MOF and underlines the

interesting effect of the terminal water molecules on adsorption.

Chapter 5. Adsorption and Separation in Zn(BDC)(TED)0.5

66

Chapter 5. Adsorption and Separation in Hydrophobic

Zn(BDC)(TED)0.5

In this Chapter, adsorption and separation of CH3OH/H2O and CO2/CH4 in

Zn(BDC)(TED)0.5 are investigated using molecular simulations. The objective is to

provide the microscopic understanding of adsorption behavior in this novel MOF at a

molecular level. The study is technologically important toward the capability of

Zn(BDC)(TED)0.5 in the purification of (methanol-based) liquid fuel and natural gas.

The removal of H2O from liquid fuel and of CO2 from natural gas is crucial to their

efficacy in combustion. We compare the experimental and simulated adsorption

isotherms of single component CH3OH, H2O, CO2, and CH4, and then predict the

adsorption of mixtures. The effect of H2O on the separation of CO2/CH4 mixture is

also examined. As mentioned that among all MOFs reported, Zn(BDC)(TED)0.5

exhibits the highest adsorption for hydrocarbons. Therefore, adsorption of hexane

(n-C6) was further simulated in Zn(BDC)(TED)0.5. To provide the microscopic

understanding of adsorption, the density distributions and structural properties of

adsorbates in Zn(BDC)(TED)0.5 are presented from simulations.


Zn(BDC)(TED)0.5 possesses a paddle-wheel structure, in which the metal-oxide

building units Zn2(COO)4 are bridged by BDC linkers to form a 2D square-grid net

[Zn2(1,4-bdc)2]. The TED pillars occupy the axial sites to extend the 2D layers into a

3D framework and are disordered along the crystallographic 4-fold axis. The structure


67

has a space group of P4/mmm, and lattice constants of a = 10.929 and c = 9.608 Å.

Figure 5.1 shows the crystal structure (3 × 3 × 3 unit cells) constructed from the

experimental crystallographic data and first-principles optimization. The optimization

was conducted on a unit cell of Zn(BDC)(TED)0.5 by the periodical density functional

theory (DFT) using DMol3.312 The same basis set was used as described in Chapter 4.

The effective core-potentials were adopted which represent the interactions of nucleus

and core electrons on the valence electrons. In such a way only the wave functions of

the softer valence electrons are explicitly treated, which usually controls the chemistry,

and can significantly reduce computational cost.

XY plane XZ plane YZ plane

Figure 5.1. A unit cell of Zn(BDC)(TED)0.5 constructed from the experimental crystallographic data and first-principles optimization. Color code: Zn, pink polyhedra; N, green; C, blue; O, red; H, white.

There exist interlacing channels in the framework of Zn(BDC)(TED)0.5. Figure

5.2 shows the morphologies and diameters of the channels calculated using the HOLE

program.322 The wide open channels are along the Z axis with a diameter ranging

from 7.3 to 9.2 Å, which are connected by the small windows along the X and Y axes

with a diameter of 3.6 Å. Therefore, small molecules may enter into the windows. The

free volume freeV of Zn(BDC)(TED)0.5 was evaluated from Monte Carlo


68

simulation315 with helium as a probe. The freeV was estimated to be 0.79 cm3/g; the

porosity was 64.9%, in good agreement with the experimental accessible volume

(61.3%).150

Figure 5.2. Channels along the Z, X, and Y (from top to bottom) axes in Zn(BDC)(TED)0.5. The green regions denote the small windows.

The atomic charges of framework atoms in Zn(BDC)(TED)0.5 were also

calculated from DFT. Because of the large number of atoms in a unit cell, a

fragmental cluster shown in Figure 5.3 was used. The dangling bonds on the

fragmental cluster were terminated by hydrogen atoms. The DFT calculation used the

Becke exchange plus Lee-Yang-Parr functional and was carried out with Gaussian

03.323 The 6-31G(d) basis set was used for all atoms except Zn atoms, for which


69

LANL2DZ basis set was used. LANL2DZ is a double-zeta basis set and contains

effective pseudo-potentials to represent the potentials of nucleus and core electrons.

The atomic charges were estimated by fitting to the electrostatic potentials from DFT

calculation.313 The dispersion interactions of framework atoms in Zn(BDC)(TED)0.5

were modeled by the Dreiding force field303 which has been commonly adopted in the

simulation of MOFs.

Figure 5.3. Atomic charges in a fragmental cluster of Zn(BDC)(TED)0.5. The dangling bonds (indicated by circles) were terminated by hydrogen. Color code: Zn, pink polyhedra; N, green; C, blue; O, red; H, white.

H2O was mimicked by the three-point transferable interaction potentials (TIP3P)

model,319 in which the O-H bond length was 0.9572 Å and the HOH angle was

104.52. It has been shown that the TIP3P model gives a reasonably good interaction

potential compared to the experimental value.324 CO2 was represented as a three-site

rigid molecule as described in Chapter 4. The intermolecular interactions of H2O and

CO2 were modeled by the additive LJ and Coulombic potentials. CH3OH, CH4 and

n-C6 were represented by united-atom models with CHx as a single interaction site.

The potential parameters were adopted from the transferable potentials for phase

C1 (0.855) C2 (-0.105)

C3 (-0.142)

H1 (0.131)

O (0.743) C4 (-0.032)

H2 (0.051)

Zn (1.205) N (0.098)


70

equilibria (TraPPE) force field, developed to reproduce the critical parameters and

saturated liquid densities of alkanes and alcohols.318,325 The cross LJ parameters were

evaluated by the Lorentz-Berthelot combining rules. In addition to the LJ interaction

(and the Coulombic interaction for CH3OH), there were bending interaction in

CH3OH and n-C6

20( ) 0.5 ( )bendingu k (5.1)

Furthermore, the torsion interaction was also taken into account in n-C6

0 1 2 3( ) [1 cos ] [1 cos(2 )] [1 cos(3 )]torsionu c c c c (5.2)

Table 5.1. Potential parameters and atomic charges.

Species LJ and Coulombic Potential

Bond Length Bending Angle and Force

Constant site (Å) k (K) q (e)

CH4 CH4 3.73 148.0 0

CO2 C 2.789 29.66 0.576

rC-O = 1.18 Å θOCO = 180o

O 3.011 82.96 0.288

H2O O 3.151 76.47 0.834

rH-O = 0.9527 Å θHOH = 104.52o

H 0 0 +0.417

CH3OH

CH3 3.75 98.0 +0.265 rCH3-O = 1.43 Å

rO-H = 0.945 Å

θCH3OH = 108.5o

kθ/kB = 55400 K O 3.02 93.0 0.700

H 0 0 +0.435

n-C6 CH3 3.75 98.0 0 rCHx-CHy = 1.54 Å x y2CH CH CH 113.0

kθ/kB = 62500 K CH2 3.95 46.0 0

The force field parameters used are listed in Table 5.1. GCMC simulations were

carried out for the adsorption of pure components and mixtures in Zn(BDC)(TED)0.5.


71


5.2.1 CH3OH/H2O

Figure 5.4 shows the adsorption of pure CH3OH and H2O in Zn(BDC)(TED)0.5 at

303 K. The simulated isotherms are in fairly good agreement with experimental

results, particularly for CH3OH at low pressures (deviation ~ 10.8%) and for H2O

(deviation ~ 13.3%) over the entire pressure range under study. Note that the

saturation pressure is 21.7 kPa for CH3OH and 4.2 kPa for H2O.326 With increasing

pressure, the uptake of CH3OH increases sharply until P/P0 = 0.2 and finally

approaches a plateau. The simulation underestimates the saturation loading of CH3OH.

Specifically, the predicted saturation loading is approximately is 460 mg/g and lower

than the experimental value of 510 mg/g.150 This might imply that the force field used

to model the interactions of CH3OH-CH3OH and CH3OH-adsorbent might not be very

accurate, and the agreement could be improved by more sophisticated modeling.

P (kPa)0 5 10 15 20 25

N (

mg/

g)

0

100

200

300

400

500

P/Po0.0 0.1 0.2 0.3 0.4 0.5

0

100

200

300

400

500

N (

mg/

g)

P (kPa)0 4 8 12 16

N (

mg/

g)

0

100

200

300

400

500

P/Po0 1 2 3 4

N (

mg

/g)

0

100

200

300

400

500

(a) CH3OH (b) H2O

Figure 5.4. Isotherms of pure CH3OH and H2O at 303 K. The filled circles are experimental data. The upper and lower triangles are adsorption and desorption data from simulation. The insets show the isotherms a function of reduced pressure. The saturation pressure Po is 21.7 kPa for CH3OH and 4.2 kPa for H2O.


72

H2O uptake from simulation is negligible at P/Po < 3.0, which is consistent with

the experimental data up to P/Po = 0.4. Interestingly, a hysteresis is observed in the

simulated adsorption and desorption isotherms of H2O at P/Po between 2.6 and 3.1,

indicating the occurrence of capillary phase transition. Zn(BDC)(TED)0.5 possesses a

highly hydrophobic framework with the BDC and TED linkers surrounding the metal

oxides; therefore, the interaction with H2O is very weak. It is instructive to note that

the behavior of H2O in Zn(BDC)(TED)0.5 is significantly different from that in a

hydrophilic MOF. Our recent study revealed that H2O is strongly adsorbed in

cation-exchanged rho-ZMOFs because of the high affinity of ionic framework and

extraframework ions.242 Specifically, H2O in rho-ZMOFs exhibits a three-step

adsorption mechanism. At low pressures, H2O is preferentially adsorbed on the

extraframework ions; with increasing pressure, adsorption occurs near the framework

and finally in the large cage.


Figure 5.5. Density contours of CH3OH at 1 kPa (top) and 10 kPa (bottom).

I

I II

III

III

III III I, II I

II


73

Figure 5.5 shows the density contours of CH3OH in Zn(BDC)(TED)0.5 at 1 and

10 kPa, respectively. On the XY plane, CH3OH molecules exclusively occupy the

open square channels. Nevertheless, the channels are not homogeneous due to the

asymmetrical locations of dimethylenes (–CH2–CH2–). Based on the adsorbate density,

three adsorption sites (I, II, and III) are roughly identified in each channel. Site I is at

the corner and proximal to the metal oxide, phenyl ring, and dimethylene. As a polar

molecule, CH3OH experiences strong interaction at site I with the metal oxides and

the overlap of surface potentials. Compared with site I, sites II and III are less

favorable. This is because the steric hindrance of dimethylenes prevents CH3OH at

sites II and III from being near the metal oxides. It is clearly seen on the XZ and YZ

planes that the distribution of CH3OH is extended along the Z axis. CH3OH is also

located in the mouth of small windows. With increasing pressure from 1 to 10 kPa,

the density distribution in the channels is less inhomogeneous and the small windows

are partially occupied. However, the density in the small windows is very low.

The observed change in the density contours with increasing pressure can be

further elucidated from the radial distribution functions g(r) of CH3OH around typical

framework atoms. As shown in Figure 5.6, the peak heights in the g(r) around Zn and

N atom rise when pressure increases from 1 to 10 kPa. This indicates that CH3OH

molecules are located closer to the metal oxides at 10 kPa, particularly for molecules

at sites II and III. Consequently, the density difference among the three types of

adsorption sites reduces and the distribution tends to be homogeneous, which is

observed in Figure 5.5. CH3OH molecules also stay near the phenyl rings at 10 kPa,


74

evidenced from the rising peak height in the g(r) around C2 atom. Nevertheless, the

peak height in the g(r) around C4 atom remains essentially unchanged.

C2

r (Å)2 4 6 8 10 12

Zn

r (Å)2 4 6 8 10 12

g (r

)

0.0

0.5

1.0

1.5

2.0

2.5

1 kPa10 kPa

N

r (Å)2 4 6 8 10 12

C4

r (Å)2 4 6 8 10 12

1 kPa10 kPa

1 kPa10 kPa

1 kPa10 kPa

Figure 5.6. Radial distribution functions of CH3OH around Zn, N, C2, and C4 atoms of Zn(BDC)(TED)0.5 at 1 and 10 kPa.

P (kPa)1 2 3 4 5 6 7 8 9 10

N (

mg/

g)

0

100

200

300

400

P (kPa)1 2 3 4 5 6 7 8 9 10

SC

H3O

H/H

2O

0

5

10

15

20

25

CH3OH

H2O

(a) (b)

Figure 5.7. (a) Adsorption and (b) selectivity of CH3OH/H2O mixture at 303 K.

The adsorption of equimolar CH3OH/H2O mixture in Zn(BDC)(TED)0.5 at 303 K

is shown in Figure 5.7a. The loading of CH3OH is much higher than H2O, as

discussed above, which is attributed to the stronger interaction of CH3OH with the

hydrophobic framework. In contrast to the negligible adsorption of pure H2O,

appreciable amount of H2O is adsorbed from the CH3OH/H2O mixture. This is due to

the co-adsorption between H2O and the already adsorbed CH3OH. As seen in Figure

5.7b, the selectivity CH3OH over H2O is approximately 20 at 1 kPa. With increasing

pressure, the selectivity decreases gradually as a consequence of entropy (packing)


75

effect. H2O molecule is smaller compared to CH3OH and thus can fit into the

channels more effectively at high pressures, leading to the decrease in selectivity. The

high selectivity at low pressures suggests that Zn(BDC)(TED)0.5 is a promising

candidate for separation of CH3OH/H2O. Experimental study has shown that in a

pressure swing adsorption process, the purity of produced CH3OH reaches 94.7%

after the first cycle.150

5.2.2 CO2/CH4

Figure 5.8 shows the adsorption of pure CO2 and CH4 in Zn(BDC)(TED)0.5 at

298 K. CO2 adsorption is greater than CH4 due to two reasons. First, CO2 has stronger

dispersion and electrostatic interactions with the framework compared to CH4. Second,

the temperature 298 K considered is subcritical for CO2 (Tc = 304.4 K), but

supercritical for CH4 (Tc = 190.5 K); that is, CO2 is substantially more condensable

than CH4 at 298 K. Fairly good agreement is found between simulated and

experimental isotherms, particularly for CH4. The overestimation by simulation may

attribute to the fact that real samples usually contain impurities (solvents, like DMF),

which block the channels and reduce the adsorption. At 30 bar the amount of CO2

adsorption is about 600 mg/g (13.6 mmol/g) in Zn(BDC)(TED)0.5, larger than that in

silicalite, carbon nanotube, F-MOF-1, Mn-MOF, and Cu-BTC.48,147 This is because

Zn(BDC)(TED)0.5 has a greater free volume, which governs adsorption capacity at a

high pressure.


76

P (kPa)0 500 1000 1500 2000 2500 3000

Ne

x (m

g/g

)0

100

200

300

400

500

600CO2

CH4

Figure 5.8. Adsorption of pure CO2 and CH4 at 298 K. The open symbols are simulation results and the filled symbols are experimental data.327,328


Figure 5.9. Density contours of CO2 at 10, 100, and 3000 kPa (from top to bottom).


77

To identify the adsorption sites for CO2 in Zn(BDC)(TED)0.5, the density

contours of CO2 at 10, 100, and 3000 kPa are shown in Figure 4.9. The contours of

CO2 differ remarkably from those of CH3OH in Figure 4.5. As discussed above,

CH3OH is a polar molecule and has strong interaction with the metal oxides. However,

CO2 is nonpolar and located preferentially near the phenyl rings as seen on the XZ

and YZ planes at 10 kPa. With increasing pressure, further adsorbed CO2 molecules

tend to be close to the metal oxides and dimethylenes. At 3000 kPa, the regions

between dimethylenes are densely occupied because of the large available space for

adsorption; furthermore, CO2 molecules also extend into the small windows. It is

worthwhile to note that compared with CH3OH, CO2 is more likely to enter the small

windows because of its linear shape.

Zn

r (Å)2 4 6 8 10 12

g (r

)

0.0

0.5

1.0

1.5

2.0

2.5

10 kPa100 kPa3000 kPa

N

r (Å)2 4 6 8 10 12

C2

r (Å)2 4 6 8 10 12

C4

r (Å)2 4 6 8 10 12




Figure 5.10. Radial distribution functions of CO2 around Zn, N, C2, and C4 atoms of Zn(BDC)(TED)0.5 at 10, 100, and 3000 kPa.

As an additional support for the observed shift in the adsorption sites, Figure

4.10 shows the radial distribution functions of CO2 around typical framework atoms

at different pressure. With increasing pressure, the peak heights in the g(r) around Zn

and N atoms rise and demonstrate that the locations of CO2 molecules move toward

the metal oxides at high pressures. The (first) peak height reduces in the g(r) around

C2 atom, but rises in the g(r) around C4 atom. This indicates more CO2 molecules are


78

located near the dimethylenes when pressure increases, as observed above.

The adsorption of equimolar CO2/CH4 mixture is shown in Figure 5.11. CO2

isotherm increases rapidly at low pressures and then slowly with increasing pressure.

As a comparison, the extent of CH4 uptake is much smaller and approaches saturation

at a low pressure. The selectivity of CO2 over CH4 exhibits similar behavior to CO2

isotherm and increases from 4 to 7 with increasing pressure. The increase in

selectivity is attributed to the increased cooperative interactions between adsorbed

CO2 molecules. The selectivity in Zn(BDC)(TED)0.5 is similar to that in neutral MOFs

such as IRMOF-1, IRMOF-13, IRMOF-14, Cu-BTC, PCN-6, and PCN-6’. Compared

with charged soc-MOF and rho-ZMOF, however, the selectivity in Zn(BDC)(TED)0.5

is several orders of magnitude smaller.42,246

P (kPa)0 500 1000 1500 2000 2500 3000

N (

mg/

g)

0

100

200

300

400

500

600

CO2

CH4

CO2 (with 0.1% H2O)

CH4 (with 0.1% H2O)

P (kPa)0 500 1000 1500 2000 2500 3000

SC

O2/

CH

4

3

4

5

6

7

8

CO2/CH4

CO2/CH4 (with 0.1% H2O)

(a) (b)

Figure 5.11. (a) Adsorption and (b) selectivity of CO2/CH4 equimolar mixture at 298 K. The filled symbols refer to the CO2/CH4 mixture with 0.1% H2O.

In practice, gas mixture usually contains a small amount of moisture. The

presence of H2O may be adverse to the separation of gas mixture. For instance, upon

the addition of H2O into CO2/CH4 mixture, the interaction between CO2 and Na+ in

Na-rho-ZMOF is substantially reduced. Consequently, CO2 adsorption drops and the


79

selectivity decreases by one order of magnitude.329 In other cases, however, the

addition of H2O is beneficial, e.g., the presence of pre-adsorbed H2O in BaX zeolite

increases the selectivity toward p-xylene in p-xylene/m-xylene mixture.330 The

adsorption of CO2 and CH4 in Cu-BTC is enhanced by H2O molecules coordinated to

the open metal sites.137 With a trace amount of H2O, the selectivity of CO2 over H2 in

soc-MOF increases at low pressures due to the promoted adsorption of CO2 by H2O

bound onto the indium atoms, but decreases at high pressures as a result of the

competitive adsorption of H2O over CO2.247

To examine the effect of H2O here, the adsorption of CO2/CH4 mixture in the

presence of 0.1% H2O was simulated in Zn(BDC)(TED)0.5. As seen in Figure 4.11, the

addition of H2O has a marginal effect on the uptake of CO2 or CH4 and subsequently

the selectivity. The reason is that the hydrophobic Zn(BDC)(TED)0.5 has very weak

affinity for H2O and the adsorption of H2O is negligible. This implies that a pre-water

treatment is probably not required prior to separation process. Interestingly, the effect

of H2O on the separation of gas mixture in Zn(BDC)(TED)0.5 is significantly different

from that in other MOFs mentioned above, in which a pre-water treatment is crucial.

5.2.3 Hexane

Adsorption of n-C6 was simulated in Zn(BDC)(TED)0.5 at 313 K. Figure 5.12

shows that the simulated and experimental isotherms exhibit similar trend. As a

function of pressure, the adsorption increases rapidly at low pressures and then

reaches saturation. The saturation capacity is approximately 450 mg/g from

simulation, larger than the experimental value 400 mg/g. The deviation (< 15%) may


80

attribute to the difference of the crystal structures used. In the simulation, a perfect

crystal was adopted; however, experimental sample may contain impurities and

defects. As a consequence, the free volume and capacity in experiment are reduced.

Our results are in good agreement with the simulation results by Krishna and van

Baten.265 A recent experimental study reported the kinetic separation of C6 isomers

using fixed-bed adsorption.174 The measured loading of n-C6 is one order of

magnitude lower than the simulation results. This is probably because of the phase

transition or contamination of Zn(BDC)(TED)0.5 sample during adsorption process.

P (torr)0 20 40 60 80 100

N (

mg/

g)

0

100

200

300

400

500

Figure 5.12. Adsorption of hexane at 313 K. The open symbols are simulation results and the filled symbols are experimental data.150

The density contours of n-C6 at 0.001 and 10 kPa are shown in Figure 5.13.

Similar to the contours of CO2, n-C6 interacts strongly with the phenyl rings rather

than the metal oxides. Consequently, at 0.001 kPa the preferential site is near the

phenyl rings. At 10 kPa, more adsorbed n-C6 molecules are closer to the metal oxides

and dimethylenes, as observed in CO2 adsorption at a high pressure. Compared to

CO2, however, n-C6 is larger in size and cannot extend into the small windows.


81

XY plane YZ plane

Figure 5.13. Density contours of hexane at 0.001 kPa (top) and 10 kPa (bottom).

5.3 Summary

We have investigated the adsorption and separation of CH3OH/H2O and CO2/CH4

mixtures in highly hydrophobic Zn(BDC)(TED)0.5. The framework contains wide

open channels along the Z axis, which are connected by small windows along the X

and Y axes. In general, the simulated isotherms of all pure components (CH3OH, H2O,

CO2, CH4, and n-C6) under study are in fairly good agreement with experimental

results. Because of the hydrophobic BDC and TED linkers, the affinity for H2O is

very weak and thus H2O adsorption is negligible. The selectivity CH3OH over H2O is

approximately 20 at 1 kPa, and decreases gradually with increasing pressure due to

entropy effect. The high selectivity at low pressures suggests that Zn(BDC)(TED)0.5

could be used for purification of CH3OH (or other alcohols) from H2O. Three

favorable adsorption sites are identified in Zn(BDC)(TED)0.5 for CH3OH. At site I


82

near the corner, CH3OH interacts strongly with the metal oxides and experiences the

strong surface potentials of the phenyl rings and dimethylenes. Sites II and III are less

favorable because of the steric hindrance of bulky TED linkers. With increasing

pressure, the density distribution in the square channels is less inhomogeneous and the

small windows are partially occupied. From the analysis of radial distribution

functions, CH3OH molecules are located near the metal oxides at a high pressure.

In contrast to polar CH3OH that has strong interaction with the metal oxides,

CO2 is nonpolar and the region near the phenyl rings is the preferential adsorption site.

With increasing pressure, CO2 molecules are proximal to the metal oxides and

dimethylenes, and also extend into the small windows. The selectivity of CO2 over

CH4 increases with increasing pressure, attributed to the increased cooperative

interactions between CO2 molecules. In the presence of 0.1% H2O in CO2/CH4

mixture, the adsorption and selectivity remain essentially the same. This implies that

H2O has a marginal effect on the separation of CO2/CH4 in Zn(BDC)(TED)0.5 and a

pre-water treatment is perhaps not required.

Chapter 6. CO2 Capture in Bio-MOF-11

83


In this Chapter, we report the first molecular simulation study for mixture

adsorption in a bio-MOF; more specifically, the adsorption of CO2/H2 and CO2/N2

mixtures in bio-MOF-11. While it is relatively straightforward to experimentally

determine the adsorption of pure gases, quantitative measurement on the adsorption of

gas mixtures is challenging. First, the models and force fields are validated by

comparing simulated and experimental adsorption of pure CO2, H2, and N2; then the

favorable adsorption sites are identified in bio-MOF-11. Finally, the adsorption and

separation of CO2/H2 and CO2/N2 mixtures are examined and compared with those in

other porous materials. A gas mixture usually contains moisture; therefore, the effect

of H2O on the separation is also investigated.


Rosi and coworkers168 synthesized bio-MOF-11 by a solvothermal reaction. It has

a formula of Co2(ad)2(CO2CH3)2 and is thermally stable up to 200 °C. The framework

consists of paddle-wheel cobalt-adeninate-acetate cluster (Figure 6.1 a) as the

secondary building unit (SBU). There are five types of nitrogens, among which N1

and N6 are Lewis basic sites; N3, N7, and N9 are covalently bonded to cobalt atoms.

A three-dimensional structure with augmented lvt network topology is generated by

the SBUs. The structure is tetragonal with lattice length of a = b = 15.4355 Å and c =

22.775 Å. The cavities with diameter of 5.8 Å are periodically distributed throughout

the structure (Figure 6.1 b). These cavities form interlacing narrow channels along the


84

crystallographic a and b dimensions.168

Figure 6.1. (a) Cobalt-adeninate-acetate cluster. N1 and N6 are the Lewis basic pyrimidine and amino groups, while N3, N7, and N9 are bonded with cobalt. (b) A unit cell of bio-MOF-11. The cavities are indicated by the green circles. Co: pink, O: red, C: grey, H: white, N1: green, N6: blue, N3, N7, and N9: cyan.

To estimate the charges of bio-MOF-11 framework atoms, a fragmental cluster

(Figure 6.2) was cleaved and saturated by hydrogen atoms. The electrostatic

potentials around the cluster were calculated by density-functional theory (DFT). It

has been widely recognized that first-principles derived charges fluctuate appreciably

when small basis set is used; however, they tend to converge beyond 6-31G(d) basis

set.331 Consequently, 6-31G(d) basis set was used in the DFT calculation for all the

atoms except Co metals, for which LANL2DZ basis set was used with effective

pseudopotentials. The DFT calculation used the Lee-Yang-Parr correlation functional

(B3LYP) and was carried out with Gaussian 03.323 The concept of atomic charges is

solely an approximation and no unique straightforward method is currently available

to determine atomic charges rigorously. In this study, the atomic charges (Table 6.1)

were estimated by fitting to the electrostatic potential using the CHelpG scheme.332 In

addition to the Coulombic interactions, the dispersion interactions of the framework

cavity

N6

N1

N3

N7

N9

(b)

cavity (a)


85

atoms were presented by Lennard-Jones (LJ) potential with parameters adopted from

the Universal Force Field (UFF).302 A number of simulation studies have

demonstrated that UFF can accurately predict gas adsorption in various

MOFs.39,191,192,320,333

Figure 6.2. A fragmental cluster of bio-MOF-11 used to calculate atomic charges. The dangling bonds (indicated by circles) were terminated by hydrogen atoms. Color code: Co, pink; O, red; N, cyan; C, grey; H, white. Table 6.1. Atomic charges in the fragmental cluster of bio-MOF-11.

No. 1 2 3 4 5 6 7 8 9

Atom Co O N1 N3 N6 N7 N9 C1 C2

q (e) 0.728 -0.749 -0.713 -0.613 -0.952 -0.096 -0.393 0.104 0.568

No. 10 11 12 13 14 15 16 17 18

Atom C3 C4 C5 C6 C7 H1 H2 H3 H4

q (e) 0.563 0.760 -0.310 0.964 -0.462 0.068 0.008 0.442 0.130

Adsorbate CO2 was represented by the elementary physical model (EPM), which

was fitted to the experimental vapor-liquid equilibrium data of bulk CO2.334 The CO

bond was assumed to be rigid at 1.161 Å, while the OCO bond was flexible and

1 2

4

3 6

5

8 9 10

1112

13

14

15 16

17

18

7


86

governed by a harmonic potential ½k (0)2 with force constant k /kB = 153355.79

(K/rad2) and equilibrium angle 0 = 180. The intermolecular CO2CO2 interactions

were modeled by the additive pair-wise site-site LJ and Coulombic potentials as stated

in Chapter 3. Both H2 and N2 were mimicked by two-site models. The bond length

was 0.74 Å for H-H and 1.10 Å for N-N. The LJ potential parameters for H2 and N2

were fitted respectively to their experimental bulk properties.335,336 H2O was

represented by the three-point transferable interaction potentials (TIP3P) model, in

which the O-H bond length was 0.9572 Å and the HOH angle was 104.52.319 Table

6.2 lists the LJ potential parameters and charges for CO2, H2, N2, and H2O. The

adsorption of pure CO2, H2, and N2 as well as CO2/H2 and CO2/N2 mixtures were

simulated by GCMC method.

Table 6.2. LJ Potential Parameters and Charges for CO2, H2, N2, and H2O.

Adsorbate Site σ (Å) ε / kB (K) q (e)

CO2 C 2.785 28.999 +0.6645

O 3.064 82.997 -0.33225

H2 H 2.50 14.5 0

N2 N 3.32 36.4 0

H2O H 0 0 +0.417

O 3.151 76.47 -0.834


87


6.2.1 Pure Gases

Figure 6.3 shows the adsorption isotherms of pure CO2, H2, and N2 in

bio-MOF-11. All the isotherms belong to type I, which is the characteristic feature of

adsorption in a microporous adsorbent. Perfect agreement is found between the

simulated and experiment results for CO2 with deviation of less than 4.3%, indicating

the accuracy of the models and force fields used in the study. The deviations for H2

and N2 are a little bit larger, about 13% and 40%, respectively. On this basis, we

envision that the simulated selectivities of mixtures shown below are reliable, though

no experimental data are available for comparison.

P (kPa)0 20 40 60 80 100

N (

mm

ol/g

)

0

1

2

3

4

wt%

0

1

2

3

4

CO2

H2

N2

Figure 6.3. Adsorption isotherms of pure CO2 and N2 at 298 K and of H2 at 77 K, respectively. The open symbols are from simulation and the filled symbols are from experiment. The lines are fits of the dual-site Langmuir-Freundlich equation to the simulation data.

A closer look at Figure 6.3 reveals that simulation slightly underestimates

experiment particularly for H2 at low pressures, but overestimates at high pressures.


88

The deviations might attribute to the factor that the structure used in simulation is a

perfect crystal, while experimental samples usually contain impurities. The impurities

in the channels cause stronger interaction with adsorbate and consequently enhance

adsorption at low pressures. Another effect of the impurities is that they block the

channels and decrease free volume, then reduce adsorption capacity at high pressures.

Furthermore, the simulated isotherms were fitted to the dual-site

Langmuir-Freundlich equation, 337

1 1 2 2

21

1 2

21( )

11

n n

n nN k P N k P

N Pk Pk P

(6.1)

where P is the pressure, Ni is maximum loading in site i ( = 1 and 2), ki is the affinity

constant, and ni is used to characterize the deviation from the simple Langmuir

equation. The fitted parameters are listed in Table 6.3 and will be used to predict the

adsorption of binary mixtures by the ideal-adsorbed-solution theory (IAST).338

Table 6.3. Parameters in the dual-site Langmuir-Freundlich equation fitted to the adsorption of pure CO2, H2, and N2.

Adsorbate N1 k1 n1 N2 k2 n2

CO2 3.809 8.76 10-3 1.354 4.207 1.18 10-2 0.665

H2 8.002 3.67 10-5 1.008 5.97 10-4 9.59 10-7 3.581

N2 3.666 5.04 10-4 1.059 2.865 9.59 10-7 0.702

CO2 capacity in bio-MOF-11 is 4.1 mmol/g at 100 kPa, about 1-3 times higher

than that in most MOFs (e.g. MOF-2, MOF-177, MOF-505, IRMOF-1, -3, -6 and

-11).125 Bio-MOF-11 also outperforms many zeolites and activated carbons,339

amine-functionalized and imidazole-based MOFs in terms of CO2 capacity.168 H2


89

adsorption in bio-MOF-11 is notably high with a capacity of 1.5 wt% at 77 K and 1

bar, which is higher than that in numerous MOFs (e.g. MOF-177, ZIF-8, MIL-100,

IRMOF-2, -3, -9, -18, and -20).58 The high capacities observed in bio-MOF-11 are

attributed to the narrow channels and the Lewis basic sites present in the framework.

The channels have narrow diameter of 4.2 ~ 5.2 Å, leading to a substantial overlap of

the potential energy fields for adsorbate molecules. The linker adenine consists of

two Lewis basic sites, one amino group and the other pyrimidine nitrogen. A total of

four amino groups and four pyrimidine nitrogens are directly exposed to each

cavity.168 These electron-rich sites interact strongly with adsorbate, particularly CO2

that has a quadrupole moment.

To identify the favorable adsorption sites of CO2 in bio-MOF-11, Figure 6.4

shows the radial distribution functions g(r) of CO2 around N1, N6, and Co atoms at

298 K and 10 kPa. N1 and N6 are in the pyrimidine and amino groups, respectively.

Co

r2 4 6 8 10 12

g (r

)

0.0

0.5

1.0

1.5

2.0

2.5

(Å)

N1 N6

Figure 6.4. Radial distribution functions of CO2 around N1, N6, and Co atoms in bio-MOF-11 at 298 K and 10 kPa. N1 and N6 are in the pyrimidine and amino groups, respectively.


90

A pronounced peak is centered at r 4.0 Å in the g(r) around both N1 and N6. The

g(r) around Co is essentially zero at r < 4.0 Å and exhibits a lower peak at r 8.0 Å.

This structural analysis reveals that CO2 molecules are preferentially adsorbed onto

the Lewis basic sites in adenine linkers with the shortest distance approximately 2.7 Å,

rather than proximal to the Co atoms. Such a behavior is remarkably different from

common MOFs, for example, the preferentially sites in IRMOF-1 were found to be

close to metal clusters.340 Nevertheless, the behavior is similar to ZIF-8 in which the

adsorption sites are near 2-methylimidazolate linkers.341 We therefore infer that the

biomolecular linkers in bio-MOF-11 should be tuned in order to further enhance

adsorption capacity of CO2.

Figure 6.5. (a) Simulation snapshot and (b) density contour of CO2 in bio-MOF-11 at 298 K and 10 kPa. CO2 molecules are represented by sticks. The density has a unit of 1/Å3 and brighter color indicates a higher density. Co: pink, O: red, C: grey, H: white, N1: green, N6: blue, N3, N7, and N9: cyan.

Figure 6.5 shows the simulation snapshot and density contour of CO2 adsorption

in bio-MOF-11 at 298 K and 10 kPa. For clarity, only one cavity in the framework is

illustrated. Apparently, CO2 molecules are located dominantly in the cavity and

adsorbed onto the amino and pyrimidine groups. A few CO2 molecules seem to be

(a) (b)


91

located in the diagonal regions of the cavity. However, they are indeed in a cavity

along the perpendicular dimension, as can been seen by rotating Figure 5.5 by 90°.

6.2.2 CO2/H2 Mixture

The adsorption of CO2/H2 mixture with a composition of 15:85 was simulated in

bio-MOF-11 to represent pre-combustion CO2 capture. The composition is typically

encountered for CO2/H2 mixture in H2 production. As shown in Figure 5.6, both CO2

and H2 in the mixture are preferentially located in the cavities. However, there are two

major differences between CO2 and H2. First, H2 has a much lower extent of

adsorption and therefore a lower density in the cavities. Second, H2 is smaller and can

approach the adenine linkers more closely than CO2, as observed in the broader

distribution of H2.

Figure 6.6. Density contours of CO2 and H2 for CO2/H2 mixture (15:85) in bio-MOF-11 at 298 K and 100 kPa. The density has a unit of 1/Å3. The density distributions are largely similar to Figure 5.5b for pure CO2.

Figure 6.7a shows the adsorption isotherm at 298 K as a function of total pressure.

CO2 uptake increases sharply with increasing pressure and then approaches to a

plateau, while H2 uptake is vanishingly small. Over the entire range of pressure, CO2

H2 CO2


92

is predominantly adsorbed than H2 due to three reasons. First, CO2 is a three-site

molecule and has a stronger interaction with the framework than two-site H2. Second,

the temperature 298 K considered is subcritical for CO2 (Tc = 304.4 K), but

supercritical for H2 (Tc = 33.2 K); that is, CO2 is more condensable than H2 at 298 K.

Third, the presence of Lewis basic sites in adenines significantly enhance the

interactions with CO2.

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

N (

mm

ol/g

)

0

1

2

3

4

5

6

CO2

H2

CO2 (0.1% H2O) H2 (0.1% H2O)

CO2

(a)

H2

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

SC

O2/

H2

200

250

300

350

400

dry CO2/H2

wet CO2/H2

(b)

Figure 6.7. (a) Adsorption isotherm and (b) selectivity of CO2/H2 mixture (15:85) in bio-MOF-11 as a function of total pressure in the absence and presence of 0.1 % H2O.

Figure 6.7b shows the selectivity of CO2 over H2 as a function of total pressure.

With increasing pressure, the selectivity increases sharply, passes a maximum of 375

at 400 kPa, and then decreases. It is expected that the selectivity will reach a constant

upon further increasing pressure. The initial increase at low pressures is caused by the

strong interactions between CO2 molecules and the multiple adsorption sites in

bio-MOF-11, and further promoted by the cooperative intermolecular interactions of

adsorbed CO2 molecules. The decrease in selectivity is attributed primarily to the

entropy (packing) effect at high pressures. H2 molecule is smaller in size and can fit


93

into the channels more effectively. In the pressure range under study, the selectivity of

CO2/H2 mixture in bio-MOF-11 is between 230 and 375. Numerous experimental and

simulation have examined the separation of CO2/H2 mixture in other porous materials,

as listed in Table 6.4 for comparison. While bio-MOF-11 has a lower CO2 capacity

than other materials, it exhibits a higher selectivity compared with zeolites and

non-ionic MOFs.

Table 6.4. Selectivities and capacities for the adsorption of CO2/H2 mixture in porous materials. The capacities are for CO2 at a total pressure of 1 bar for mixture.

Material Temperature,

Pressure

Composition

(CO2:H2) Selectivity

Capacity

(mmol/g)

ETS-10230 298 K, 10 bar 50:50 3.5 2.45

MFI215 298 K, 10 bar 50:50 5.0 2.95

Na-4A342 298K, low pressure 1.4:98.6 70.65 1.3

IRMOF-n

(n = 9 ~ 14)343 298 K, 0 ~ 2 MPa

5:95, 30:70,

50:50, 70:30,

95:5

10 ~ 100

Cu-BTC344 298 K, 0 ~6 MPa 50:50 70 ~ 150

soc-MOF247 298 K, 0 ~ 3000 kPa 15:85 300 ~ 600 2.7

rho-ZMOF42 298 K, 0 ~ 3000 kPa 15:85 1.6 105 ~ 200

4.3

bio-MOF-11 298 K, 0 ~ 3000 kPa 15:85 230 ~ 375 1.3

A gas mixture usually contains moisture which would affect adsorption and

separation. For example, in the presence of H2O, the selectivity of CO2 over H2 in

soc-MOF increases at low pressures due to the promoted adsorption of CO2 by H2O

bound onto metal atoms, but decreases at high pressures as a result of the competitive

adsorption of H2O over CO2.247 In Na-rho-ZMOF, the interaction between CO2 and

Na+ is substantially reduced by a trace amount of H2O added into CO2/CH4 mixture;

consequently, CO2 adsorption drops and the selectivity decreases by one order of


94

magnitude.329 To examine the effect of H2O in this study, the adsorption of CO2/H2

mixture was simulated in the presence of 0.1% H2O (mole fraction). As seen in Figure

6.7, H2O has a negligible effect on the adsorption of both CO2 and H2; and the CO2/H2

selectivity is slightly enhanced due to co-adsorption between CO2 and adsorbed H2O.

The predictions from IAST are shown in Figure 5.8 for CO2/H2 mixture using

the parameters given in Table 6.3. While the adsorption isotherm is well predicted

upon comparison with simulation, the selectivity is underestimated by IAST. In IAST

prediction, even a small inaccuracy in the mole fractions would lead to a large

deviation in the selectivity. The deviation of 23% between IAST and GCMC

simulation may be attributed to the non-ideal gas mixture resulted from the different

polarities of CO2 and H2.345

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

SC

O2/H

2

200

250

300

350

400

Sim.IAST

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

N (

mm

ol/g

)

0

1

2

3

4

5

6

CO2 (Sim.)

H2 (Sim.)

CO2 (IAST)

H2 (IAST)

CO2(a) (b)

H2

Figure 6.8. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure. The open symbols are from simulation and the filled symbols are from IAST.

6.2.3 CO2/N2 Mixture

To examine post-combustion CO2 capture by bio-MOF-11, the adsorption of

CO2/N2 mixture with a composition of 15:85 was simulated. Figure 6.9a shows the


95

adsorption isotherm of the mixture at 298 K as a function of the total pressure. Similar

to Figure 6.7a, CO2 uptake increases sharply with increasing pressures and reaches

saturation at high pressures. In contrast, N2 uptake only increases slightly at low

pressures. CO2 has a substantially greater adsorption than N2, because of the three

reasons mentioned above for CO2/H2 mixture. Nevertheless, the selectivity of CO2/N2

versus pressure is qualitatively different from that of CO2/H2. As shown in Figure

6.9b, the selectivity increases monotonically with increasing pressure and approaches

a constant at high pressures. The increase of selectivity is due to the stronger

CO2-CO2 intermolecular interactions as pressure increases. The decrease of selectivity

seen in Figure 6.7b for CO2/H2 is not observed for CO2/N2. This is because the

molecular sizes of CO2 and N2 are not significantly different, unlike CO2 and H2 in

which H2 is much smaller than CO2; therefore, the entropy effect is not dominant for

CO2/N2 mixture at high pressures.

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

N (

mm

ol/g

)

0

1

2

3

4

5

CO2 N2 CO2 (0.1% H2O) N2 (0.1% H2O)

(a)CO2

N2

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

SC

O2/

N2

20

40

60

80

100

dry CO2/N2

wet CO2/N2

(b)

Figure 6.9. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure in the absence and presence of 0.1 % H2O.


96

CO2/N2 separation has been investigated in other nanoporous materials as listed

in Table 6.5. Upon comparison, bio-MOF-11 has a selectivity of 30 ~ 77, exhibiting a

better separation capability than zeolites and non-ionic MOFs. We note that the

selectivities of both CO2/H2 and CO2/N2 mixtures in bio- MOF-11 are smaller than in

rho-ZMOF.42 This is not unexpected because rho-ZMOF consists of a highly ionic

framework and extraframework cations, which induce substantially strong

interactions with CO2 molecules and exceptionally high selectivities.

Table 6.5. Selectivities and capacities for the adsorption of CO2/N2 mixture in porous materials. The capacities are for CO2 at a total pressure of 1 bar for mixture.

Material Temperature,

Pressure

Composition

(CO2:N2) Selectivity

Capacity

(mmol/g)

Na-4A342 298 K, low pressure 5.1:94.9 18.75 2.1

silicalite, ITQ-3,

ITQ-7346 308 K, P > 10 bar 50:50, 10:90 30, 100, 10 0.5, 2.4, 1.6

MFI347 300 K, 420 kPa 50:50 13.7

FAU348 298 K, 1 bar 50: 50 20

MOF-508b349 303 K, 1 ~ 4 bar 50:50 3 ~ 6 1.58

zinc-paddle

wheel MOFs170 298 K, 0 ~ 8 bar 15:85 25~45

Cu-BTC250 298 K, 1 ~ 5 MPa 15.6:84.4 20 ~ 38

rho-ZMOF42 298 K, 3000 kPa 15:85 1.9 104 ~ 100 4.4

bio-MOF-11 298 K, 3000 kPa 15:85 30 ~ 77 1.2

The effect of H2O on the adsorption of CO2/N2 mixture is also simulated. As

seen in Figure 6.9, the addition of H2O slightly increases the selectivity of CO2/N2,

more obviously at high pressures. The performance of IAST for CO2/N2 mixture

shown in Figure 6.10 is similar to that for CO2/H2 mixture. While the adsorption

isotherm is well predicted, the selectivity is only in qualitative agreement with

simulation.


97

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

SC

O2/

N2

20

30

40

50

60

70

80

Sim.IAST

Ptotal (kPa)0 500 1000 1500 2000 2500 3000

N (

mm

ol/g

)

0

1

2

3

4

5

CO2 (Sim.)

N2 (Sim.)

CO2 (IAST)

N2 (IAST)

(a) (b)CO2

N2

Figure 6.10. (a) Adsorption isotherm and (b) selectivity of CO2/N2 mixture (15:85) in bio-MOF-11 as a function of total pressure at 298 K. The open symbols are from simulation and the filled symbols are from IAST.

6.3 Summary

We have reported the first molecular simulation study for CO2 capture in a

bio-metal organic framework (bio-MOF-11). The bio-MOF-11 consists of

paddle-wheel cobalt-adeninate-acetate clusters and interlacing narrow channels. The

amino and pyrimidine groups are the Lewis basic sites, preferentially for adsorption.

This contrasts remarkably to most MOFs in which the metal clusters are the favorable

sites. To further facilitate CO2 adsorption in bio-MOF-11, we suggest that the adenine

linkers rather than the metals should be functionalized. Because of the Lewis basic

sites and the narrow channels in the framework, bio-MOF-11 exhibits large capacities

of CO2 and H2, and indeed larger than many zeolites, activated carbons, and MOFs.

The simulated adsorption isotherms of CO2, H2, and N2 agree very well with

experimental data, which demonstrates the accuracy of the models and potential used.

In the attempt to evaluate the capability of bio-MOF-11 for pre- and

post-combustion CO2 capture, it was found that CO2 is more favourably adsorbed


98

than H2 and N2. As a function of pressure, the selectivities of CO2/N2 and CO2/H2

mixtures behave quantitatively different. Due to the strong interactions between CO2

and framework, the selectivity of CO2/H2 at low pressures increases with increasing

pressure. At high pressures, however, it decreases because H2 has a smaller molecular

size and the entropy effect comes into play. The selectivity of CO2/N2 increases

monotonically at low pressures and gradually approaches a constant. The selectivities

of both mixtures in bio-MOF-11 are higher than in most zeolites and non-ionic MOFs.

In the presence of H2O (0.1% in mole fraction), the selectivities are slightly enhanced,

particularly for CO2/N2 mixture at high pressures. This study suggests that

adenine-based bio-MOFs might be useful for CO2 capture. The microscopic insight

from molecular simulation is important for the quantitative understanding of

adsorption mechanism and the rational design of new bio-MOFs for emerging

applications.

Chapter 7. CO2 Adsorption in Cation-Exchanged MOFs

99


In this Chapter, a simulation study is reported to investigate the adsorption of CO2,

as well as CO2/H2 mixture, in cation-exchanged rho-ZMOFs. The cations considered

include mono-, di- and trivalent Na+, K+, Rb+, Cs+, Mg2+, Ca2+ and Al3+. Following

this section, the models used for rho-ZMOF framework, cations and adsorbates are

briefly described in 7.1. The simulation methods are introduced in 7.2. In section 7.3,

the cations in rho-MOFs are first characterized, and then the isosteric heat and

Henry’s constant of CO2 adsorption at infinite dilution are presented. In addition,

adsorption isotherm and adsorbate structure are examined. Finally, the adsorption of

CO2/H2 mixture is discussed, along with the effect of H2O.

7.1 Models

As the first example of 4-connected MOF with rho topology, rho-ZMOF was

synthesized by the directed assembly of In atoms and H3ImDC.18 It has a space group

of Im-3m and a lattice constant of 31.062Å. In rho-ZMOF, each In atom is

coordinated to four N atoms and four O atoms of four separate doubly deprotonated

H3ImDC (HImDC) ligands to form an eight-coordinated dodecahedron. Each

independent HImDC is coordinated to two In atoms via N-, O-hetero-chelation

resulting in two rigid five-membered rings. As illustrated in Figure 7.1, rho-ZMOF

structure contains truncated cuboctahedron (-cage) linked via double

eight-membered ring (D8MR). The substitution of oxygen in rho-zeolite by HImDC

generates a very open structure with extra-large cavity of 18.2 Å in diameter. Unlike


100

rho-zeolite and other rho-aluminosilicate or aluminophosphate, rho-ZMOF possesses

twice as many positive charges (48 vs. 24) in a unit cell to neutralize the anionic

framework. As mentioned, the HPP cations in parent rho-ZMOF can be exchanged

with other cations. In addition, experimental thermogravimetric analysis showed that

all residential water molecules in rho-ZMOF can be completely evacuated.18

Figure 7.1. Crystal structure of rho-ZMOF. Color code: In, cyan; N, blue; C, grey; O, red; and H, white. The a-cage, double eight-membered ring (D8MR), 6-membered ring (6MR) and 4-membered ring (4MR) are indicated. The yellow spheres in the D8MR represent inaccessible cages.

In this study, seven cation-exchanged rho-ZMOFs are considered with

monovalent Na+, K+, Rb+, Cs+, divalent Mg2+, Ca2+ and trivalent Al3+. The cations

were modeled as charged Lennard-Jones (LJ) particles with parameters listed in Table

7.1. The LJ potential parameters were adopted from the Universal Force Field

(UFF).302

-cage

6MR

4MR


101

Table 7.1. Charges Z, well depths /kB and collision diameters σ of cations.

Cation Z(e)* /kB (K) (Å)

Na+ +1 15.083 2.658

K+ +1 17.597 3.396

Rb+ +1 20.111 3.665

Cs+ +1 22.624 4.024

Mg2+ +2 55.807 2.691

Ca2+ +2 119.658 3.028

Al3+ +3 253.897 4.008

Figure 7.2. Atomic charges in a fragmental cluster of rho-ZMOF. Color code: In, cyan; N, blue; C, grey; O, red; and H, white.

The atomic charges of rho-ZMOF framework atoms were calculated from

density-functional theory (DFT) using a fragmental cluster (in Figure 7.2) and same

method as stated in Chapter 6. The atomic charges were estimated by fitting to the

Electrostatic Potentials (ESP). In addition, the van der Waals interactions of the

framework atoms were mimicked by LJ potential with parameters from the UFF (in

Table 7.2).

In (2.840) N (-0.381)

C1 (0.950) O2 (-0.813)

O1 (-0.751) C2 (-0.257)

C3 (-0.525)

H2 (0.683)

H1 (0.426)


102

Table 7.2. Lennard-Jones parameters of framework atoms in rho-ZMOF.

Atom σ (Å) /kB (K)

In 3.976 301.157

N 3.260 34.690

O 3.118 30.166

C 3.431 52.790

H 2.571 22.122

CO2 was modeled by a three-site rigid molecule and its intrinsic quadrupole

moment was described by a partial charge model.317 The partial charges on C and O

atoms were qC = 0.576e and qO = –0.288e (e = 1.6022 ×10-19), respectively. The CO

bond length was 1.18 Å and the bond angle OCO was 180. The intermolecular

interactions of CO2 were represented by a combination of LJ and Coulombic

potentials. In addition, the adsorption of CO2/H2 mixture was also examined. H2 was

mimicked as a three-site model with explicit charge.350 The H-H bond length was 0.74

Å and the charge on H atom were +0.468e. The center-of-mass between the two H

atoms was a LJ core with a charge of 0.936e. This model gave a quadrupole moment

of 2.05 10-40 Cm2 for H2.

7.2 Methods

In each cation-exchanged rho-ZMOF with Na+, K+, Rb+, Cs+, Mg2+, Ca2+ or Al3+,

the locations of cations were determined by simulated annealing in a canonical

ensemble. The initial temperature was 600 K and decreased to 300 K with an interval

of 20 K. The simulation box contained one unit cell of rho-ZMOF with appropriate


103

number of cations (48 Na+, K+, Rb+, Cs+, 24 Mg2+, Ca2+ or 16 Al3+). The framework

was assumed to be rigid during simulation and periodic boundary conditions were

applied in three dimensions. The unit cell was divided into fine grids with energy

landscape tabulated in advance and then used by interpolation during simulation. In

such a way, the simulation was accelerated by two orders of magnitude. A spherical

cutoff of 15.0 Å was used to evaluate the LJ interactions with long-range corrections

added. For the Coulombic interactions, a simple spherical truncation could result in

significant errors; consequently, the Ewald sum with a tin-foil boundary condition was

used. The real/reciprocal space partition parameter and the cutoff for reciprocal lattice

vectors were chosen to be 0.2 Å-1 and 8, respectively, to ensure the convergence of the

Ewald sum. These methods to calculate the LJ and Coulombic interactions were also

used in the simulation described below. The cations were introduced into the box

randomly and followed by 107 trial moves. Specifically, two types of trial moves

including translation and regrowth were used with equal probability. After equilibrium,

the free volume in each cation-exchanged rho-ZMOF was estimated using helium as a

probe

He

ad Bfree exp[ ( ) / ] dr rV

V u k T= -ò (7.1)

where He

adu is the interaction between helium and framework, He 2.58s = Å and

BHe / 10.22 Kke = .351

The isosteric heat and Henry’s constant of CO2 adsorption at infinite dilution and

298 K were calculated by Monte Carlo (MC) simulation in a canonical ensemble. A

single gas molecule was added into rho-ZMOF and subject to three types of trial


104

moves, namely translation, rotation and regrowth. As shown in Figure 7.1, there exist

quasi-spherical inaccessible cages within the D8MR. They are isolated from the

α-cage and inaccessible by gas molecules, thus were blocked during simulation.

Nevertheless, it should be point out that these cages are very small and have

negligible effect on the adsorption. It was found that a simulation without blocking

these cages gave, within statistical uncertainty, nearly identical results. Specifically,

the isosteric heat was evaluated by

o ost aQ RT U

(7.2)

where o

aU is the ensemble averaged adsorption energy of a gas molecule with

framework. The Henry’s constant HK was evaluated by

H aexp[ ( , )] d duK r r (7.3)

where a( , )u r is the adsorption energy for a gas molecules at position r and

orientation .

The adsorption isotherms of CO2 or CO2/H2 mixture at 298 K were simulated

using GCMC method stated in Chapter 3. CO2/H2 mixture was assumed to have a

bulk composition of 15/85, which is practically found in H2 production. A gas mixture

usually contains moisture or the separation process might be operated under humid

condition. To examine the effect of H2O, the adsorption of CO2/H2/H2O mixture was

also simulated with 0.1% of H2O (mole fraction). The cations were allowed to move

during the GCMC simulation.


First, the cations in rho-ZMOFs are characterized, particularly in terms of their


105

equilibrium locations. Then, the isosteric heat and Henry’s constant of CO2 adsorption

are presented and related with the charge-to-diameter ratio of cation. The adsorption

isotherm, density contour and radial distribution function of CO2 molecules are shown

as a function of pressure. Finally, the adsorption selectivities of CO2/H2 and

CO2/H2/H2O mixtures are examined.

7.3.1 Characterization of cations

The equilibrium locations of cations are tabulated in appendix (Tables A1-A7),

along with the simulation snapshots. For cations with identical valence (e.g. Na+, K+,

Rb+ and Cs+; Mg2+ and Ca2+), the locations are largely similar. As identified in our

previous study,42 there are two types of favorable sites for Na+ ions in Na-rho-ZMOF.

Site I is in the single eight-membered ring (S8MR) and at the entrance to the -cage.

Site II is in the -cage and proximal to the moiety of organic link. Compared with site

II, site I has a larger coordination number with neighboring atoms in the S8MR and

thus a stronger interaction with the framework. It was also revealed that the mobility

of cations is very small and can be regarded as local vibration at the favorable sites.42

Figure 7.3. Equilibrium and initial locations of cations (a) K+ (b) Ca2+ (c) Al3+. The initial locations are indicated in pink.

(a) (b) (c)


106

Table 7.3. Porosity, isosteric heat and Henry’s constant of CO2 adsorption in rho-ZMOFs.

MOF Porosity o

st (kJ/mol)Q 3H (mmol/cm kPa)K

Na-rho-ZMOF 0.548 58.25 50.37

K-rho-ZMOF 0.512 53.99 9.52 Rb-rho-ZMOF 0.491 51.73 5.98 Cs-rho-ZMOF 0.466 40.27 0.82 Mg-rho-ZMOF 0.572 84.42 1.66 105 Ca-rho-ZMOF 0.566 73.71 2.64 104 Al-rho-ZMOF

0.559 89.91 2.78 106

It is interesting to examine how the equilibrium locations shift from the initial

locations at which cations are randomly introduced into simulation cell. Figure 7.3

illustrates typically the equilibrium and initial locations of three cations (K+, Ca2+ and

Al3+). Apparently, there are deviations between the two locations. The initial positions

are in a random pattern, whereas the equilibrium locations are more ordered at the

favorable sites.

For each cation-exchanged rho-ZMOF, the free volume was estimated and

porosity was calculated. By definition, the porosity is equal to the ratio of free

volume to specific volume, free /V V . As indicated in Table 7.3, decreases with

increasing cation diameter for cations with identical valence. To further quantify,

Figure 7.4 shows versus the packing fraction of cation . The latter is defined as

3cation cation( / )N V , where cationN is the number of cations. Clearly, decreases

monotonically with increasing . A good correlation is found between and

0.59 1.23 (7.4)


107

With this correlation, the porosity can be predicted for other cation-exchanged

rho-ZMOFs.

= 0.59 1.23

0.40

0.45

0.50

0.55

0.60

0.00 0.02 0.04 0.06 0.08 0.10 0.12

cation

Po

rosi

ty

Figure 7.4. Porosity versus the packing fraction of cation in rho-ZMOFs. The solid line is a linear correlation between and .

7.3.2 Isosteric Heat and Henry’s Constant

The isosteric heats ostQ

and Henry’s constants KH for CO2 adsorption at infinite

dilution are listed in Table 7.3. Due to the strong electrostatic interactions between

CO2 and cation/ionic framework, the ostQ and KH are substantially larger than those in

neutral MOFs.39 The ostQ

in Na, K, Rb and Cs-rho-ZMOFs are comparable to

experimentally measured ostQ in Na-ZSM5 (50 kJ/mol)352 and Na-MOR (65

kJ/mol).353 Intriguingly, the ostQ

and KH vary drastically with different cations. For

monovalent cations (Na+, K+, Rb+ and Cs+), the ostQ

and KH decrease in the order of

Na+ > K+ > Rb+ > Cs+, i.e., with increasing cation diameter. This is also observed for

divalent cations Mg2+ > Ca2+. In addition, the ostQ

and KH in Mg-rho-ZMOF and


108

Ca-rho-ZMOF are significantly larger than in monovalent cation-rho-ZMOFs. Among

the seven rho-ZMOFs, Al-rho-ZMOF exhibits the highest ostQ

and KH. Such a trend

was previously found for adsorption in cation-exchanged zeolites. The enthalpy of

CO2 adsorption in alkali cation-exchanged zeolite-Y follows Li+ > Na+ > K+ > Cs+.354

In monovalent and divalent zeolite-X, the enthalpy of N2 decreases as Li+ > Na+ > K+

and Mn2+ > Ca2+ > Sr2+ > Ba2+.355 In zeolite-X and MOR, the enthalpy and Henry’s

constant of N2 and Ar also decreases with increasing cation diameter.356-358

Figure 7.5 shows the behavior of ostQ

and KH versus the charge-to-diameter ratio

Z/cation of cation, which can be used to quantify the electric field generated by cation.

Both ostQ

and KH increase monotonically with increasing Z/cation as attributed to the

strong electrostatic interactions between CO2 and cations as well as ionic framework.

Z/cation

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Qst (

kJ/m

ol)

10

30

50

70

90

Z/cation

0.2 0.3 0.4 0.5 0.6 0.7 0.8

log

(KH)

0

2

4

6

Cs Rb KNaCaMgAl Cs Rb KNaCaMgAl

(a) (b)

Figure 7.5. (a) Isosteric heats and (b) Henry’s constants for CO2 adsorption in rho-ZMOFs versus the charge-to-diameter ratio of cation. The dotted lines are to guide the eye.

Figure 7.6 shows the adsorption isotherms of CO2 in cation-exchanged

rho-ZMOFs at 298 K. At low pressures (< 100 kPa), CO2 uptake increases in the

sequence Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ < Al3+, despite small difference


109

between Mg- and Al-rho-ZMOFs. This is analogous to the behavior of ostQ

and KH,

following the increasing order of Z/σcation. It implies that electrostatic interactions

predominantly govern CO2 adsorption at low pressures. Similar trend was reported by

a number of studies in cation-exchanged zeolites. For example, in zeolite-X

exchanged with monovalent and divalent cations, N2 and Ar exhibit increasing

capacity K+ < Na+ < Li+ and Ba2+ < Sr2+ < Ca2+ < Mn2+.355,358 The capacities of N2, O2

and CO in cation-exchanged zeolite-X and MOR increase in the sequence of Cs+ < K+

< Na+ < Li+.356,357 CO2 capacity in zeolite-X increases as Cs+ < Rb+ < K+ < Na+ < Li+,

and also in zeolite-Y.359 In alkali cation-exchanged chabazite, CO2 capacity in

K-CHA is 78.7% and 70.1% of those in Na-CHA and Li-CHA, respectively.360 At

high pressures, CO2 uptake largely follows the trend at low pressures, i.e., increases

P (kPa)0 20 40 60 80 100

CO

2 (m

mol

/g)

0

2

4

6

8

10

Na+

K+

Rb+

Cs+

Mg2+

Ca2+

Al3+

P (kPa)0 200 400 600 800 1000

CO

2 (m

mol

/g)

0

2

4

6

8

10

12

Na+

K+

Rb+

Cs+

Mg2+

Ca2+

Al3+

(a) (b)

Figure 7.6. Adsorption isotherms of CO2 in rho-ZMOFs (a) low-pressure regime and (b) high pressure regime.

with increasing Z/σcation. However, Mg-rho-ZMOF exhibits a marginally higher

saturation capacity than Al-rho-ZMOF. This is attributed to the free volume of

framework. As listed in Table 7.3, the porosity in Mg-rho-ZMOF is 0.572, slightly


110

larger than that in Al-rho-ZMOF (0.559). Consequently, Mg-rho-ZMOF has a larger

free volume available for adsorption and a higher saturation capacity. The effect of

free volume for CO2 adsorption is not obvious at low pressures because electrostatic

interactions play a major role, but becomes increasingly more important at high

pressures, particularly near saturation.

To examine how the locations of CO2 molecules adsorbed in the framework shift

with pressure, Figure 7.7 shows the density contours of CO2 in Na-rho-ZMOF at 10,

100 and 1000 kPa. At a low pressure (10 kPa), CO2 molecules are mostly located in

the 6-membered ring (6MR), but also in the α-cage and 4-membered ring (4MR). In

particularly, it is observed that CO2 molecules are adsorbed proximately to Na+ ions.

Therefore, Na+ ions can be regarded as preferential sites for CO2 adsorption due to

strong electrostatic interactions. In addition, the cations are located slightly different

at 10, 100 and 1000 kPa. This implies that the locations of cations are shifted upon

CO2 adsorption.

10 kPa 100 kPa 1000 kPa

Figure 7.7. Density contours of CO2 in Na-rho-ZMOF at 10, 100 and 1000 kPa. The locations of Na+ ions are indicted by the large spheres. The density scale is the number of CO2 molecules per Å3.


111

Figure 7.8a shows the g(r) for CO2 around Na+ ions, N and In atoms in

Na-rho-ZMOF at 10 kPa. The peak position in the g(r) of CO2-Na+ is at r = 3.6 Å,

shorter than those of CO2-N and CO2-In. In addition, the peak height of CO2-Na+ is

more pronounced than the other two. This confirms that Na+ ions are preferential sites

for CO2 adsorption. With increasing pressure to 100 and then to 1000 kPa, Figure 7.7

shows that more CO2 molecules are adsorbed in the a-cage and Na+ ions appear to be

solvated by CO2. Thus, it can be expected the electrostatic interactions between CO2

and Na+ ions are largely reduced. Figure 6.8b compares the g(r) of CO2-Na+ at 10,

100 and 1000 kPa. With increasing pressure, the peak height in g(r) drops as a

consequence of the fact that more CO2 molecules are adsorbed further away from the

Na+ ions. Although not shown here, it was observed in our previous study that the

distance between CO2 molecules becomes shorter with increasing pressure.42 This

indicates an enhancement in cooperative interactions between CO2 molecules.

r (A)2 3 4 5 6 7 8

g(r

)

0

1

2

3

4

5

6

In-CO2

N-CO2

Na+-CO2(a) 10 kPa

r (A)2 3 4 5 6 7 8

g(r)

0

1

2

3

4

5

6

1000 kPa

100 kPa

10 kPa (b) Na+-CO2

Figure 7.8. Radial distribution functions (a) CO2 around Na+ ions, N and In atoms in Na-rho-ZMOF at 10 kPa (b) CO2 around Na+ ions in Na-rho-ZMOF at 10, 100 and 1000 kPa.


112

7.3.3 CO2/H2 Mixture

Figure 7.9 shows the selectivity of CO2/H2 in cation-exchanged rho-ZMOFs as a

functional of total pressure. At a given pressure, the selectivity increases as Cs+ < Rb+

< K+ < Na+ < Ca2+ < Mg2+ Al3+, which largely follows the increasing order of the

charge-to-diameter ratio of cation. With increasing pressure, the selectivity in each

rho-ZMOF decreases sharply as a consequence of two factors. First, the adsorption

sites in rho-ZMOF are heterogeneous and adsorbate molecules start to occupy less

favorable sites at high pressures. Particularly as shown in Figure 7.7, more CO2

molecules are adsorbed in the a-cage when pressure increases and the electrostatic

interactions between CO2 and Na+ ions reduce substantially. Second, H2 is smaller

than CO2 and can pack into the partially filled cages more easily with increasing

pressure. At ambient conditions (298 K and 100 kPa), the selectivity ranges from 800

in Cs-rho-ZMOF to 3000 in Al-rho-ZMOF. The simulated selectivity of CO2/H2

mixture in rho-ZMOFs is significantly higher than that predicted in other porous

materials. For example, the selectivity ranges from 300 to 600 in soc-MOF,247

approximately 5 in MFI and 3.5 in ETS-10 for an equimolar CO2/H2 mixture.361 In an

activated carbon, the selectivity is between 60 and 90 for CO2/H2 mixtures with

different mole fractions.362 In zeolite Na-4A, the selectivity is 70 for a mixture with

98.6% H2 and 1.4% CO2.342 The selectivity is 40 in IRMOF-1 and 150 in Cu-BTC at

298 K and 1 atm.344


113

P (kPa)1 10 100 1000

SC

O2/

H2

1e+2

1e+3

1e+4

1e+5

NaKRbCsMgCaAl

Figure 7.9. Selectivities of CO2/H2 mixture in rho-ZMOFs. The composition of CO2/H2 mixture is 15/85.

A gas mixture usually contains moisture or the separation process might be

carried out under humid condition. Consequence, the separation performance could be

affected by H2O and this has indeed been observed in experimental and simulation

studies. A very small amount of H2O in Li-LSX significantly affects the adsorption

capacity of N2, O2 and Ar due to the shielding of cations by H2O.363 In cationic

zeolite-X, the Henry’s constant of CO2 declines exponentially with the loading of

H2O.364 For CO2/CH4 mixture in rho-ZMOF, the interaction between CO2 and Na+ is

substantially reduced upon adding H2O; the selectivity is thus decreased by one order

of magnitude.329 In other cases, however, the presence of H2O is beneficial for

adsorption and separation. For example, the presence of pre-adsorbed H2O in BaX

increases the selectivity toward p-xylene in an equimolar p-xylene/m-xylene

mixture.330 The adsorption of CO2 and CH4 is enhanced in Cu-BTC via the occupation

of open metal sites by coordinated water molecules.137 In the presence of H2O, the

selectivity of CO2 over H2 in soc-MOF increases at low pressures due to the promoted

adsorption of CO2 by H2O bound onto metal atoms, but decreases at high pressures as


114

a result of the competitive adsorption of H2O over CO2.247

To examine the effect of H2O on the separation of CO2/H2 mixture in rho-ZMOFs,

a CO2/H2/H2O mixture was considered with 0.1% of H2O (mole fraction). As shown

in Figure 7.10, the selectivities in all the seven rho-ZMOFs decrease substantially

compared with Figure 7.9. This is attributed to the competitive adsorption between

H2O and CO2. H2O is highly polar and interacts with cations more strongly than CO2,

thus has an adverse effect on CO2 adsorption and CO2/H2 separation. Therefore, it is

important to remove H2O before separation process. Furthermore, it is observed that

the degree of decrease in selectivity is about 1 order of magnitude in rho-ZMOFs

exchanged with monovalent cations, and 1 ~ 2 orders of magnitude in rho-ZMOFs

exchanged with di- and trivalent cations. Apparently, H2O has a greater effect on the

selectivities in rho-ZMOFs exchanged with higher valent cations. Interestingly, in the

presence of H2O, the selectivity is close in the seven cation-exchanged rho-ZMOFs.

This is because the electrostatic interactions by cations are largely shielded by H2O

and the difference is small among various cation-exchanged rho-ZMOFs.

P (kPa)1 10 100 1000

SC

O2/

H2

10

100

1000

NaKRbCsMgCaAl

Figure 7.10. Selectivity of CO2/H2/H2O mixture in rho-ZMOFs. The mole fraction of H2O in the mixture is 0.1%.


115

7.4 Conclusions

We have investigated CO2 adsorption in cation-exchanged rho-ZMOFs. The

cations include monovalent Na+, K+, Rb+, Cs+, divalent Mg2+, Ca2+ and trivalent Al3+.

In all the seven cation-exchanged rho-ZMOFs studied, the isosteric heat and Henry’s

constant at infinite dilution increase following Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ <

Al3+, in accord with the increasing order of the charge-to-diameter ratio of cation. At

low pressures, cations act as preferential sites for CO2 adsorption and the electrostatic

interactions of CO2-cations govern capacity, which increases in the order of Cs+ < Rb+

< K+ < Na+ < Ca2+ < Mg2+ < Al3+. At high pressures, the electrostatic interactions are

largely reduced because CO2 molecules are located more in the a-cage. Consequently,

the free volume plays a more important role and Mg-rho-ZMOF exhibits a higher

capacity than Al-rho-ZMOF. For the adsorption of CO2/H2 mixture, the selectivity at a

given pressure increases as Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ Al3+, and largely

follows the increasing order of the charge-to-diameter ratio. With increasing pressure,

the selectivity decreases sharply due to the occupation of adsorbate molecules in less

favorable sites and the smaller size of H2. At 298 K and 1 bar, the selectivity ranges

from 800 to 3000, significantly higher than in other nanoporous materials such as

silicalite, activated carbon and non-ionic MOFs. Upon adding 0.1% H2O, the

selectivity decreases substantially because H2O is more competitively adsorbed

against CO2. Particularly, H2O induces a strong shielding on cations and the

selectivity exhibits approximately the same value in all the seven cation-exchanged

rho-ZMOFs.

Chapter 8. Ionic Liquid/MOF Composite for CO2 Capture

116


In this Chapter, a composite of ionic liquid (IL) [BMIM][PF6] supported on

metal-organic framework IRMOF-1 is investigated for CO2 capture by molecular

computation. The microscopic properties of IL confined in the MOF are also provided;

this fundamental understating is indispensable for the synthesis of novel MOFs using

ILs as templates365 or bridging ligands.366 The structural and dynamic properties of IL

in MOF are presented and the adsorption behavior of CO2/N2 mixture in IL/MOF

composite is examined.


8.1.1 [BMIM][PF6]

The IL considered is 1-butyl-3-methylimidazolium hexafluorophosphate

[BMIM][PF6], which is one of the commonly studied imidazolium-based ILs. Figure

8.1 illustrates the molecular structure of [BMIM][PF6].

Figure 8.1. Atomic types in [BMIM]+ and [PF6].

The interactions of IL molecules include bonded stretching, bending, torsional

potentials and non-bonded Lennard-Jones (LJ), electrostatic potentials. The potential

F

H1

C1

C2 C3

C4 C5

C6 C7

C8 H3 H4

H5

H2

H6

H7

H8

H9

H10

H11 H13

H14

H12

[BMIM]


117

parameters of anion [PF6] were adopted from Lopes et al.367 For cation [BMIM]+, the

bonded interactions and non-bonded LJ interactions were represented by the AMBER

force field.306 To estimate the atomic charges of [BMIM]+, density functional theory

(DFT) calculations with the Lee-Yang-Parr correlation (B3LYP) functional were

carried out using GAUSSIAN 03 package.323 Initially, [BMIM]+ was geometrically

optimized at 6-31G(d) basis set and the electrostatic potentials were calculated at

6-311+G(d,p) basis set. Thereafter, the atomic charges were estimated by fitting the

electrostatic potentials with Merz-Lollman (MK) scheme.301 Table 8.1 lists the atomic

charges of [BMIM]+ and [PF6].

Table 8.1. Atomic charges in [BMIM]+ and [PF6]

.

Atom C1 C2 C3 C4 C5 C6 C7 C8 N1 N2

Charge -0.158 -0.151 -0.236 -0.316 -0.342 0.136 0.112 -0.315 0.218 0.276

Atom H1 H2 H3 H4 H5 H6 H7 H8 H9 H10

Charge 0.205 0.251 0.162 0.164 0.170 0.143 0.145 0.009 0.009 0.009

Atom H11 H12 H13 H14 H15

Charge 0.008 0.082 0.111 0.079 0.230

Atom P F

Charge 1.34 -0.39

To validate the force field, the density of [BMIM][PF6] was estimated from NPT

(constant temperature and pressure) MD simulation using GROMACS package

v.4.5.3.368 The initial velocities in MD simulation were assigned according to the

Maxwell-Boltzmann distribution at 300 K. The equations of motion were integrated

with a time step of 2 fs by Leapfrog algorithm.369 Temperature was controlled by

velocity-rescaled Berendsen thermostat with a relaxation time of 0.1 ps and pressure


118

was maintained at 1 atm using Berendsen algorithm.370 The LJ interactions were

evaluated with a cutoff value of 14 Å and the electrostatic interactions were calculated

using particle-mesh Ewald method with a grid spacing of 1.2 Å and a fourth-order

interpolation. The simulation trajectories were saved with a time interval of 2 ps.

Table 8.2 gives the simulated densities of [BMIM][PF6] at five different temperatures

(25, 40, 50, 60 and 70 C). Good agreement is observed between the simulated and

experimental data. Though the simulations slightly underestimate experimental data,

the deviations are less than 0.5%. To a certain extent, this suggests that the force field

used for [BMIM][PF6] is accurate.

Table 8.2. Simulated and experimental densities of [BMIM][PF6] at 1 atm.

Temperature (°C)

Simulated Density (kg/m3)

Experimental Density371 (kg/m3)

25 1354 1360 40 1344 1348 50 1337 1340 60 1328 1332 70 1320 1323

8.1.2 IRMOF-1

The MOF support in this study for IL/MOF composite is IRMOF-1, which is a

prototype MOF. As shown in Figure 8.2, IRMOF-1 has a cubic structure and a lattice

constant of 25.832 Å.5 The formula of IRMOF-1 is Zn4O(BDC)3, where BDC is

1,4-benzenedicarboxylate. Each oxide-centered Zn4O tetrahedron is edge-bridged by

six carboxylate linkers resulting in an octahedral Zn4O(O2C)6 building unit, which

reticulates into a three-dimensional structure. Straight channels exist in the framework


119

with alternating size of 15 and 12 Å. The atomic charges of IRMOF-1 framework

atoms were calculated from DFT based on a fragmental cluster shown in Figure 8.3.

Figure 8.2. IRMOF-1 structure. Color code: Zn, orange; O, red; C, grey; H, white.

Figure 8.3. Atomic charges in a fragmental cluster of IRMOF-1. The dangling bonds indicated by dashed circles are terminated by methyl groups.

The dangling bonds of the cluster were terminated by methyl groups. The 6-31G(d)

basis set was used for all atoms except metal atoms, for which LANL2DZ basis set

was used. After the DFT calculation, the atomic charges were fitted to electrostatic

potentials, as listed in Figure 8.3. The dispersion interactions of IRMOF-1 framework

atoms were represented by LJ potential with parameters from the UFF.302 The

Zn (1.51))O2 (-0.72))O1 (-1.79))

H (0.16)) C3 (-0.25))

C2 (0.20))


120

Lorentz-Berthelot combining rules were used to evaluate cross interaction parameters.

As shown in Figure 8.4 for CO2 adsorption in IRMOF-1,39 the agreement between

simulation and experiment is fairly good.

P (kPa)1 10 100 1000

N (

mm

ol/g

)

0

5

10

15

20

25

30

0 1500 3000 45000

5

10

15

20

25

30

5000

Figure 8.4. Adsorption isotherm of CO2 in IRMOF-1 at 300 K.39 The filled symbols are from simulation and the open symbols are from experiment.

8.1.3 IL/IRMOF-1 Composite and Adsorption of CO2/N2 Mixture

To examine the properties of IL/IRMOF-1 composite, we consider four different

weight ratios of IL over IRMOF-1 (WIL/IRMOF-1 = 0, 0.4, 0.86, 1.27, 1.5). At each ratio,

the desired number of [BMIM][PF6] were added into IRMOF-1 and subject to energy

minimization using the steepest descent method. Then, MD simulation was conducted

for [BMIM][PF6] in IRMOF-1 framework at 300 K. The LJ and electrostatic

interactions were evaluated using the same manner as described above for

[BMIM][PF6]. The time step was 2 fs and the trajectory was saved with a time

interval of 2 ps. The total MD simulation duration was 20 ns, with the final 10 ns used

for analysis. Figure 8.5 schematically illustrates an equilibrium snapshot of the


121

IL/IRMOF-1 composite at WIL/IRMOF-1 = 0.4.

Figure 8.5. [BMIM][PF6]/IRMOF-1 composite at a weight ratio WIL/IRMOF-1 = 0.4. N: blue, C in [BMIM]+: green, P: pink, F: cyan; Zn: orange, O: red, C in IRMOF-1: grey, H: white.

The capability of IL/IRMOF-1 for CO2 capture was evaluated by simulating

adsorptive separation of CO2/N2 mixture in the composite. GCMC method was used

in the simulation. CO2 was represented as a three-site molecule and its intrinsic

quadrupole moment was described by a partial charge model. The partial charges on C

and O atoms were qC = 0.576e and qO = –0.288e (e = 1.6022 ×10-19), respectively. The

CO bond length used was 1.18 Å and the bond angle OCO was 180. CO2CO2

interactions were modeled as a combination of LJ and electrostatic potentials.317 N2

was mimicked as a two-site model with the LJ potential parameters fitted to the

experimental bulk properties.336 The bulk composition of CO2/N2 mixture was

assumed to be 0.15:0.85, representing a typical flue gas. The IL/IRMOF-1 composite

was treated as rigid during adsorption simulation.


122


8.2.1 Structure and Dynamics of IL in IL/IRMOF-1

Figure 8.6 shows the g(r) of [BMIM]+ and [PF6] in IL/IRMOF-1 composite at

weight ratio WIL/IRMOF-1 = 0.4. For comparison, the g(r) of IL in bulk phase are also

plotted. Due to the confinement effect of IRMOF-1 framework, IL molecules are

packed more ordered in the composite. Consequently, the peaks of all ion pairs in the

composite are observed to be higher than in bulk phase. In particular, the attractive

[BMIM]+-[PF6]pair exhibits a pronounced peak at r = 4.4 Å. Nevertheless, the

[BMIM]+-[BMIM]+ or [PF6]-[PF6]

pair has a lower peak because of unfavorable

repulsive interaction. The peak position for [BMIM]+-[BMIM]+ or [PF6]-[PF6]

pair

in the composite is slightly shifted to a greater value compared to that in bulk phase,

indicating the distance of cation-cation or anion-anion increases. This is because the

confinement effect in the composite leads to a sharper g(r) for cation-anion pair, and

thus each cation (or anion) is surrounded by a larger number of anions (or cations).

(Å)r2 4 6 8 10 12

g (r

)

0

2

4

6

8

10

[BMIM]+-[BMIM]+

[BMIM]+-[PF6]-

[PF6]--[PF6]

-

[BMIM]+-[BMIM]+

[BMIM]+-[PF6]-

[PF6]--[PF6]

-

Figure 8.6. Radial distribution functions of [BMIM]+ and [PF6] in IL/IRMOF-1 at

WIL/IRMOF-1 = 0.4 and in bulk phase, respectively. The solid lines are in IL/IRMOF-1 and the dash lines are in bulk phase.


123

r 2 4 6 8 10 12

g (r

)

0

1

2

3

4[BMIM]+ - O1

[BMIM]+ - O2

[BMIM]+ - Zn[BMIM]+ - C3

(Å) (Å)

(a) (b)

r 2 4 6 8 10 12

g (r

)

0

1

2

3

4

5[PF6]

- - O1

[PF6]- - O2

[PF6]- - Zn

[PF6]- - C3

(b)

Figure 8.7. Radial distribution functions of (a) [BMIM]+ and (b) [PF6] around O1, O2,

Zn, and C3 atoms of IRMOF-1 at WIL/IRMOF-1 = 0.4.

Figure 8.7 further shows the g(r) of [BMIM]+ and [PF6]around IRMOF-1

framework atoms (O1, O2, Zn, and C3 atoms) at WIL/IRMOF-1 = 0.4. Non-zero g(r) is

observed at r < 5 Å for [BMIM]+-C3 and [BMIM]+-O2, but at r > 5 Å for [BMIM]+-Zn

and [BMIM]+-O1. This indicates [BMIM]+ is proximal to the benzene ring and

carboxylate group in IRMOF-1 rather than the metal cluster. With a chain-like

structure, [BMIM]+ prefers to reside in the open pore of IRMOF-1 due to

configuration entropy effect. The small corner near the metal cluster cannot

accommodate [BMIM]+. However, pronounced peaks are observed for [PF6] around

all the four atoms at r < 5 Å. Particularly, the peaks around O1 and Zn atoms are

higher, suggesting [PF6] is preferentially located in the corner near the metal cluster.

Compared to [BMIM]+, [PF6] exhibits higher peaks around the four atoms; therefore,

[PF6] possesses a stronger interaction with the framework. Note that the peaks at r =

10 ~ 11 Å are due to the presence of framework atoms on the other side of the

structure.


124

t (ps) 0 1000 2000 3000 4000 5000 6000

MS

D (

)

0

5

10

15

20

25

30

[BMIM]+ 0.4[BMIM]+ 0.86[BMIM]+ 1.27[PF6]

- 0.4

[PF6]- 0.86

[PF6]- 1.27

Å2

Figure 8.8. Mean-squared displacements of [BMIM]+ and [PF6] in IL/IRMOF-1at

WIL/IRMOF-1 = 0.4, 0.86 and 1.27.

The dynamics of IL is evaluated by mean-squared displacement (MSD). Figure

8.8 shows the MSDs of [BMIM]+ and [PF6] in IL/IRMOF-1 at WIL/IRMOF-1 = 0.4, 0.86

and 1.27. Though not shown, the mobility of IL in bulk phase is greater than in the

composite. The mobility of both [BMIM]+ and [PF6] reduces with increasing

WIL/IRMOF-1, as attributed to enhanced confinement effect. At a given WIL/IRMOF-1, the

mobility of bulky [BMIM]+ is greater than [PF6]. Such an interesting phenomenon

was also observed in simulations372,373 and experiments374,375 for imidazolium-based

bulk ILs, and interpreted as a result of the less hindered displacement of [BMIM]+

ring along the direction of C1 atom (see Figure 8.1).376 In the IL/IRMOF-1 composite

under study, [PF6] interacts more strongly with the framework as discussed above;

this also contributes to the observed smaller mobility of [PF6].


125

t (ps) 0 1 2 3 4

C(t

)/C

(0)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

[BMIM]+ 0.4[BMIM]+ 0.86[BMIM]+ 1.27[BMIM]+ 1.5

(a)

t (ps)0 1 2 3 4

C(t

)/C

(0)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

(b)

[PF6]- 0.4

[PF6]- 0.86

[PF6]- 1.27

[PF6]- 1.5

Figure 8.9. Reduced velocity correlation functions of [BMIM]+ and [PF6] in

IL/IRMOF-1 at WIL/IRMOF-1 = 0.4, 0.86, 1.27 and 1.5.

To provide further insight into the dynamics of IL in the IL/IRMOF-1 composite,

velocity correlation functions were calculated by

( ) (0) ( )C t t v v (8.1)

where v(t) is the velocity of center-of-mass of cation or anion of at time t. To calculate

C(t), a 100 ps trajectory was generated with a small time interval of 0.1 ps after 20 ns

MD simulation. Figure 8.9 shows the reduced velocity correlation functions C(t)/C(0)

of [BMIM]+ and [PF6] in the composite. The trend at different WIL/IRMOF-1 is largely

identical. For either cation or anion, the C(t)/C(0) approaches zero in less than 2 ps

and then exhibits marginal oscillation. Therefore, the velocity of [BMIM]+ or [PF6]

becomes uncorrelated rapidly within a picosecond scale. This is similar to the time

scale observed for IL in bulk phase,376 and reveals that confinement has an

insignificant effect on the velocity correlation of IL.

8.2.2 Separation of CO2/N2 Mixture in IL/IRMOF-1

Adsorption of CO2/N2 mixture in IL/IRMOF-1 was simulated to evaluate the


126

performance of IL/IRMOF-1 composite for CO2 capture. The CO2/N2 mixture was

assumed to possess a composition of 15:85 to mimic a typical flue gas. Figure 8.10

shows the simulation snapshot of CO2/N2 mixture with a total pressure of 1000 kPa in

the composite at WIL/IRMOF-1 = 0.4. [PF6] anions are observed to preferentially locate

in the corner of IRMOF-1, which was evidenced by the g(r) in Figure 8.7. Compared

to N2, more CO2 molecules are adsorbed in the composite and proximal to the ions.

This is attributed to the strong interaction between CO2 and ions.

Figure 8.10. Simulation snapshot of CO2/N2 mixture (Ptotal = 1000 kPa) in IL/IRMOF-1 at WIL/IRMOF-1 = 0.4.

To identify the favorable sites in the composite for CO2 adsorption, Figure 8.11

shows the g(r) of CO2 around the framework (Zn) and IL (N1, N2 and P) atoms. A

sharp peak is observed in the g(r) of CO2-P at r 4.1 Å, indicating [PF6] anion is the

most favorable site. The g(r) of CO2-N1 and CO2-N2 are less pronounced with broad

peaks at r 5.0 and 5.2 Å, respectively. As a comparison, the g(r) of CO2-Zn has the

lowest peak at r 4.5 Å. The structural analysis reveals that CO2 is preferentially

adsorbed onto IL, particularly [PF6]anion, in the IL/IRMOF-1 composite. Such

CO2

N2


127

behavior is remarkably different from the adsorption in IRMOF-1, in which the

favorable site was found to be near the metal cluster.39 In the IL/IRMOF-1 composite,

the cations and anions (especially [PF6]) occupy the corner of the metal cluster and

act as adsorption sites for CO2 adsorption.

r 2 4 6 8 10 12

g (r

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Zn in IRMOF-1N1 in IL

N2 in IL

P in IL

(Å)

Figure 8.11. Radial distribution functions of CO2 (Ptotal = 10 kPa) around Zn, N1, N2, and P atoms in IL/IRMOF-1 at WIL/IRMOF-1 = 0.4.

Figure 8.12a shows the g(r) of CO2 around P atom of [PF6] at 10, 100 and 1000

kPa in the IL/IRMOF-1 composite at WIL/IRMOF-1 = 0.4. The peak in g(r) of CO2-P

drops with increasing pressure, whereas the coordination number of CO2 molecules

around P atoms increases (data not shown). This is because the local density of CO2

increases less rapidly than overall density. Similar behavior was also found in our

previous studies for CO2 capture in rho zeolite-like MOF42. As shown in Figure 8.12b,

the peak in g(r) of CO2-P also drops with increasing WIL/IRMOF-1 in the IL/IRMOF-1

composite. At a low WIL/IRMOF-1, only a few IL molecules exist and reside closely to

the IRMOF-1 framework; consequently, CO2 molecules are largely localized because


128

the ions act as favorable adsorption sites. With increasing WIL/IRMOF-1, however, the

ions start to occupy the open pores and distribute homogenously in the IRMOF-1

framework; therefore, CO2 molecules are adsorbed uniformly and the local density

relative to average density drops.

r 2 4 6 8 10 12

g (r

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0


(Å)

(a)

r 2 4 6 8 10 12

g (r

)0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.4 0.861.271.5

(Å)

(b)

Figure 8.12. Radial distribution functions of CO2 around P atom in IL/IRMOF-1 (a) Ptotal = 10, 100, 1000 kPa and WIL/IRMOF-1 = 0.4, (b) Ptotal = 100 kPa and WIL/IRMOF-1 = 0.4, 0.86, 1.27 and 1.5.

Figure 8.13 shows the selectivity 2 2CO / NS in the IL/IRMOF-1 composite at

different WIL/IRMOF-1. In the absence of IL, 2 2CO / NS remains nearly 5 over the entire

pressure range; however, it increases with increasing WIL/IRMOF-1. This is because IL

acts as favorable sites for CO2 adsorption and more such sites are available when

WIL/IRMOF-1 increases. Nevertheless, 2 2CO / NS

decreases with increasing pressure,

particularly at high WIL/IRMOF-1 because of the number of adsorption sites decreases at

high pressures. At the highest WIL/IRMOF-1 = 1.5 in this study, 2 2CO / NS reaches

approximately 90 at infinite dilution and 70 at 1 bar. This value is higher than the

selectivities reported for CO2/N2 mixture in [BMIM][PF6] (22.6),290 in PVDF

supported [BMIM][PF6] (20),296 and in many other supported ILs (10 – 56).297


129

Ptotal (kPa)0 200 400 600 800 1000

SC

O2/

N2

0

20

40

60

80

100

1.51.270.860.40

Figure 8.13. Selectivity of CO2/N2 mixture in IL/IRMOF-1 at WIL/IRMOF-1 = 0, 0.4, 0.86, 1.27 and 1.5.

8.3 Conclusions

We have examined the microscopic properties of [BMIM][PF6] in IL/IRMOF-1

composite and the capability of the composite for CO2 capture. IL molecules in the

composite are packed more ordered than in bulk phase, particularly for the

electrostatic attractive [BMIM]+-[PF6]pair. This is attributed to the confinement

effect of nanoporous IRMOF-1 framework. The bulky chain-like [BMIM]+ cation

tends to reside in the open pore of IRMOF-1. The small [PF6] has a stronger

interaction with the framework than [BMIM]+ and stays preferentially in the corner

near the metal cluster. As also observed in bulk phase, the mobility of [BMIM]+ in the

IL/IRMOF-1 composite is greater than [PF6]. Due to the enhanced confinement

effect when the ratio of IL in the composite increase, the mobility of both [BMIM]+

and [PF6] reduces. Nevertheless, confinement has no significant effect on the

velocity correlation. It is found that the velocity of [BMIM]+ or [PF6] is essentially

uncorrelated after 2 picoseconds, which is close to the time scale for bulk ILs.


130

In the IL/IRMOF-1 composite, CO2 is more strongly adsorbed than N2 from

CO2/N2 mixture. From structural analysis, [PF6]anion is identified to be the most

favorable site for CO2 adsorption in the composite. In IRMOF-1, however, the

favorable site is in the corner of the metal cluster. Upon adding IL into IRMOF-1, ions

especially [PF6] occupy the corner and act as strong ionic adsorption sites for CO2.

With increasing ratio of IL in the composite, ions start to occupy the open pores and

distribute more homogenously, thus CO2 molecules tend to locate uniformly in the

composite. Compared with IRMOF-1, the IL/IRMOF-1 composite has a substantially

higher selectivity for CO2/N2 separation. With increasing ratio of IL, the selectivity in

the composite increases as the number of ionic adsorption sites becomes larger. At a

weight ratio IL/IRMOF-1 = 1.5, the selectivity is about 70 at ambient conditions (300

K and 1 bar) and higher than in most supported ILs. The simulation results suggest

that the IL/IRMOF-1 composite is potentially a good material for CO2 capture. In the

current study, the common [BMIM][PF6] and prototype IRMOF-1 are considered.

Nevertheless, thousands of ILs and MOFs have been synthesized with their readily

tunable structures; therefore, new candidates will be further explored.

Chapter 9. Conclusions and Recommendation

131


9.1 Conclusions

This thesis aims to investigate the adsorption and separation of gas mixtures

(particularly CO2-containing mixtures) in various MOFs with diverse structures and

functionalities. Molecular simulations as well as first-principles calculations have

been employed to evaluate adsorption mechanisms and microscopic insights. The

main contents of the thesis are summarized below.

1. Adsorption of CO2 and CH4 in dehydrated and hydrated mesoporous MIL-101

is examined. Adsorption occurs exclusively in the microporous supertetrahedra at low

pressures and also in the mesoporous cages at high pressures. The simulated

isotherms match well with experimentally measured data that depend on activation

method. The terminal water molecules act as additional adsorption sites and enhance

adsorption at low pressures. This is because the terminal water molecules interact

more strongly than the exposed Cr sites for adsorbates. The enhancement is greater

for CO2 as it is a quadrapolar molecule and interacts strongly with the charged

hydrogen and oxygen atoms of the water molecules. The opposite occurs at high

pressures because the terminal water molecules reduce free volume and lead to less

adsorption. This study provides useful insights into the microscopic adsorption

behavior in a unique MOF and underlines the interesting effect of the terminal water

molecules on adsorption.

2. Adsorption and separation of CH3OH/H2O and CO2/CH4 in Zn(BDC)(TED)0.5


132

are studied. Zn(BDC)(TED)0.5 possesses a highly hydrophobic framework because of

the existence of BDC and TED linkers. The simulated isotherms of CH3OH and H2O

are in fairly good agreement with experimental data. While H2O adsorption is

vanishingly small, CH3OH has a much strong adsorption. The selectivity is

approximately 20 at low pressures and decreases with increasing pressure. CH3OH

interacts strongly with the metal oxides, particularly at low pressures. The isotherms

of CO2 and CH4 from simulation also match well with experimental data. As a

nonpolar molecule, CO2 exhibits different favorable sites from polar CH3OH. At low

pressures, CO2 is located preferentially near the phenyl rings, then the metal oxides

and dimethylenes with increasing pressure. The selectivity of CO2 over CH4 increases

as a function of pressure, with a magnitude similar to other neutral MOFs. H2O has a

marginal effect on the selectivity of CO2/CH4. This study provides a quantitative

understanding of adsorption and separation in Zn(BDC)(TED)0.5 and suggests that

Zn(BDC)(TED)0.5 is a good candidate for the purification of liquid fuels.

3. Biological metal-organic framework (bio-MOF-11) for CO2 capture is

explored. The biomolecular linkers (adenines) in bio-MOF-11 contain Lewis basic

amino and pyrimidine groups as the preferential adsorption sites. The simulated and

experimental adsorption isotherms of CO2, H2, and N2 are in perfect agreement. As

attributed to the presence of multiple Lewis basic sites and nano-sized channels,

bio-MOF-11 exhibits larger adsorption capacities compared with zeolites, activated

carbons, and many other MOFs. For the adsorption of CO2/H2 and CO2/N2 mixtures

in bio-MOF-11, CO2 is more dominantly adsorbed than H2 and N2. With increasing


133

pressure, the selectivity of CO2/H2 initially increases due to the strong interactions

between CO2 and framework, and then decreased as a consequence of entropy effect.

However, the selectivity of CO2/N2 increases monotonically with increasing pressure

and finally reaches a constant. The selectivities in bio-MOF-11 are higher than in

many nanoporous materials. In addition, the simulation results indicate that a small

amount of H2O has an insignificant effect on the separation. The simulation study

provides microscopic insights into adsorption mechanism in bio-MOF-11 and

suggests that bio-MOFs might be interesting for CO2 capture.

4. CO2 adsorption is investigated in rho-ZMOFs exchanged with a series of

cations (Na+, K+, Rb+, Cs+, Mg2+, Ca2+ and Al3+). The isosteric heat and Henry’s

constant at infinite dilution increase monotonically with increasing charge-to-diameter

ratio of cation (Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ < Al3+). At low pressures, cations

act as preferential adsorption sites and the capacity follows the charge-to-diameter

ratio. However, the free volume of framework becomes predominant with increasing

pressure and Mg-rho-ZMOF exhibits the highest saturation capacity. For CO2/H2

mixture, the selectivity increases as Cs+ < Rb+ < K+ < Na+ < Ca2+ < Mg2+ Al3+. At

ambient conditions, the selectivity is in the range of 800 ~ 3000 and significantly

higher than in other MOFs. In the presence of 0.1% H2O, the selectivity decreases

drastically because of the competitive adsorption between H2O and CO2, and shows

similar value in all the cation-exchanged rho-ZMOFs. This simulation study reveals

the important role of cations in governing gas adsorption and separation, and suggests

that the performance of ionic rho-ZMOFs can be tailored by cations.


134

5. A composite of ionic liquid (IL) [BMIM][PF6] supported on IRMOF-1 is

proposed for CO2 capture. Due to the confinement effect, IL in the composite exhibits

ordered structure. The bulky [BMIM]+ cation resides in the open pore of IRMOF-1,

whereas the small [PF6]¯ anion prefers to locate in the metal cluster corner and

possesses a strong interaction with the framework. [BMIM]+ exhibits a greater

mobility than [PF6]¯, which is also observed in simulation and experimental studies of

imidazolium-based ILs in bulk phase. With increasing IL ratio in the composite and

thus enhancing confinement effect, the mobility of [BMIM]+ and [PF6]¯ reduces. Ions

in the composite interact strongly with CO2, particularly [PF6]¯ is the most favorable

site for CO2. The composite selectively adsorbs CO2 from CO2/N2 mixture, with

selectivity significantly higher than polymer-supported ILs. Furthermore, the

selectivity increases with increasing IL ratio in the composite. This computational

study demonstrates MOF-supported IL might be potentially useful for CO2 capture.

Table 9.1 summarizes CO2 selectivity at ambient conditions over other gases in

the five MOFs. It can be concluded that MOFs contain functional groups

(bio-MOF-11), ionic framework (rho-ZMOF) and more adsorption sites (IL/MOF

composite) possess better performance for CO2 capture. Overall, the

recommendations for MOFs well-suited for CO2 capture should have the following

characteristics: proper pore size, high density of open metal site, high polar functional

group, ionic framework, and more adsorption sites. In addition, H2O has different

effects on CO2 capture in various MOFs. In MIL-101, terminal H2O enhances

selectivity; in rho-ZMOF the presence of H2O largely decreases the selectivity; in


135

Zn(BDC)(TED)0.5 and bio-MOF-11, H2O has a negligible effect on CO2 selectivity.

Table 9.1. CO2 selectivities at ambient conditions in different MOFs.

Gas mixture MOF CO2 Selectivity at ambient conditions

CO2/CH4

MIL-101 5

Zn(BDC)(TED)0.5 4.5

CO2/H2

bio-MOF-11 339

rho-ZMOF 800-3000 (Cs+-Mg2+)

CO2/N2

bio-MOF-11 37

IL/IRMOF-1 4.8-70 (IL:0-1.5)

9.2 Recommendation

In this thesis, we have focused on the potential application of various MOFs with

unique structures and functionalities for CO2 capture. However, MOFs are versatile

materials for a wide range of applications such as storage, separation, catalysis, and

biosensing. In addition, tens of thousands of MOFs have been synthesized and a few

have been commercially available. Several interesting and practically important topics

are recommended for future simulation studies.

1. The removal of harmful NOx in exhaust gas is of central importance for air

quality. Currently, Ba-based materials are often used for NOx storage. Although the

nature of storage mechanism is not well recognized, it is generally recognized that the

basicity of Ba-based materials and alkali/alkaline earth metals (K, Mg, and Ca) play a

key role in NOx storage.377 MOFs with Lewis basic sites, alkali/alkaline earth metal


136

ions could be potentially used for NOx storage. Studies have shown that NO can be

adsorbed in MOFs due to the coordination between NO and open metal sites.68,378

Toward the screening and design of high-performance MOFs for NOx storage, it is

important to elucidate the interactions and microscopic properties of NOx in MOFs,

particularly those with open metal sites.

2. Most current experimental and simulations for MOFs have been concentrated

on gas storage and separation, and thus endeavors for liquid separation are lagged

well behind. Nevertheless, a recent trend has been to explore the use of MOFs for

liquid separation.379,380 Molecular simulation study for liquid separation in MOFs is

scarce due to the significant amount of computational time required to sample liquid

phase. Consequently, the microscopic understanding of liquid separation in MOFs is

far from complete. To the best of our knowledge, only two simulation studies have

been reported in this area, one for water desalination381 and the other for biofuel

purification.382 With increasing demands for clean water, liquid fuels and other

liquid-based applications, more efforts are expected in order to provide deep

molecular insights.

3. MOFs may find application in the removal of toxic compounds. For instance,

paracresol is a protein-bound uremic toxin and associated with cardiovascular disease.

It is difficult to remove paracresol by conventional dialysis method.383 As an

alternative, uremic toxins can be separated by adsorption in porous adsorbents for

blood purification. Wernert384,385 and Boulet et al.385-387 have studied the adsorption of

prarcresol in pure silica and cation-compensated alumino-silicalite. It was found that


137

silicalite has a high affinity for parecresol and can eliminate 60% paracresol, which is

2 times higher than the conventional dialysis method.384 MOFs with high porosities

and large surface areas could be better used for this purpose. Currently, there is no

study on this topic and useful microscopic information can be obtained from

simulations.

4. Porous materials are potential candidates for drug loading/delivery. There are

several factors determining the loading capacity and release rate of drug in a porous

carrier, such as pore size, shape, and host affinity. In traditional porous materials

including inorganic silica and polymeric matrixes, drug loading capacity is usually not

sufficiently high and encapsulated drug is difficult to be released specifically. To

achieve a high loading and a controlled release, porous carriers with large volumes

and well-defined structures are desired. In this regard, mesoporous MOFs provide a

wealth of opportunities for drug delivery. However, very few simulation studies are

available on this topic48,49 and the fundamental understanding of drug-MOF

interactions remains largely elusive. Therefore, molecular-level study is crucial for the

development of new MOFs as novel drug devices.

References

138

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Appendix

151

Appendix

Table A1. Equilibrium locations of Na+ ions.

Ion index X Y Z

1 23.193 17.440 9.239

2 18.263 27.145 31.033

3 25.030 22.456 30.241

4 0.017 27.937 13.034

5 0.170 18.057 3.263

6 15.571 11.903 28.243

7 19.426 15.517 2.802

8 15.563 11.732 3.061

9 22.459 6.035 30.143

10 28.440 15.546 12.133

11 29.614 6.138 23.137

12 8.136 8.829 17.770

13 8.828 23.616 16.960

14 3.356 12.867 30.972

15 15.623 28.516 18.896

16 6.219 22.471 30.305

17 27.532 12.943 0.062

18 6.208 29.366 23.624

19 16.453 7.012 21.668

20 30.998 18.241 27.321

21 31.052 3.541 12.800

22 29.899 24.873 8.444

23 3.201 0.094 18.232

24 27.650 31.057 18.132

25 6.800 30.515 8.832

26 23.981 9.246 16.387

27 18.623 30.982 27.159

28 28.296 15.460 19.183

29 0.920 8.644 5.920

30 12.760 0.003 27.403

31 8.033 6.441 29.606

32 18.340 30.975 3.800

33 18.761 2.478 15.560

34 11.691 15.597 2.853

35 12.160 2.529 15.530

36 21.951 23.590 17.014

37 7.226 14.493 10.093

Appendix

152

38 9.129 16.618 23.813

39 16.615 7.063 9.619

40 15.491 19.145 28.366

41 15.538 28.308 11.547

42 29.458 24.708 23.021

43 24.734 1.642 7.863

44 13.803 21.990 7.479

45 2.843 19.263 15.497

46 12.848 31.022 3.394

47 21.593 16.508 23.932

48 2.407 12.314 15.673

Appendix

153

Table A2. Equilibrium locations of K+ ions.

Ion index X Y Z

1 21.638 14.857 7.271

2 18.565 27.698 31.050

3 24.001 23.876 2.209

4 3.852 31.041 12.147

5 27.906 18.462 31.026

6 19.072 15.554 27.988

7 15.540 18.515 2.885

8 15.520 11.897 3.137

9 24.821 8.753 0.250

10 22.319 0.069 24.766

11 15.538 3.055 18.973

12 17.138 22.732 23.045

13 0.027 15.592 7.852

14 8.509 22.745 17.714

15 3.841 12.237 0.009

16 15.377 9.398 24.309

17 6.887 15.177 21.931

18 3.439 18.686 0.007

19 30.979 3.804 12.158

20 8.745 0.296 6.367

21 3.548 30.965 18.751

22 28.689 28.633 12.402

23 22.429 22.681 13.135

24 21.599 6.760 15.171

25 0.349 8.575 5.853

26 12.847 28.283 31.043

27 8.537 0.389 24.887

28 18.538 3.335 0.062

29 15.644 3.247 11.817

30 12.445 28.182 15.516

31 18.665 28.206 15.580

32 3.111 18.883 15.555

33 9.360 7.230 14.814

34 9.394 15.049 6.948

35 12.232 15.455 27.997

36 22.552 0.500 6.620

37 7.215 24.101 28.829

38 2.066 23.995 7.137

39 14.887 23.970 9.238

40 27.888 11.989 15.541

41 12.291 3.648 0.039

Appendix

154

42 24.423 15.372 21.646

43 3.210 12.008 15.581

44 29.168 23.709 23.900

45 30.968 12.368 27.476

46 28.011 18.907 15.534

47 27.610 0.010 18.605

48 30.988 6.204 22.323

Appendix

155

Table A3. Equilibrium locations of Rb+ ions.

Ion index X Y Z

1 24.239 15.534 9.508

2 18.569 0.046 27.918

3 22.364 24.721 0.094

4 3.339 0.048 18.724

5 0.234 22.397 6.320

6 15.555 12.213 27.842

7 18.948 15.628 3.252

8 19.077 3.340 15.528

9 23.833 6.999 1.774

10 23.974 1.869 23.753

11 6.915 9.356 15.756

12 17.711 22.556 22.664

13 31.027 24.624 22.588

14 0.002 12.354 3.333

15 15.449 6.854 21.735

16 3.228 18.676 31.058

17 30.985 3.198 12.433

18 15.375 21.752 6.871

19 8.624 31.047 24.584

20 8.398 30.789 6.802

21 27.704 12.050 15.616

22 0.022 6.457 22.390

23 12.462 27.868 0.001

24 6.047 8.700 30.874

25 18.780 0.041 3.460

26 15.441 11.983 3.369

27 11.959 3.272 15.520

28 22.070 24.148 15.356

29 8.999 24.045 15.373

30 7.018 23.656 29.394

31 9.448 15.751 6.814

32 15.537 18.54 28.066

33 24.763 31.008 8.623

34 0.041 27.628 12.278

35 15.560 27.723 11.964

36 3.167 17.715 17.661

37 12.262 3.401 0.052

38 22.545 13.441 22.651

39 3.373 15.483 11.965

40 31.055 15.517 23.300

41 28.821 12.346 28.702

Appendix

156

42 27.746 18.999 15.583

43 27.928 0.032 18.562

44 9.391 15.516 24.136

45 15.583 6.742 9.383

46 15.445 28.151 18.377

47 27.600 18.871 0.041

48 1.867 7.228 7.288

Appendix

157

Table A4. Equilibrium locations of Cs+ ions.

Ion index X Y Z

1 22.746 15.561 7.146

2 18.899 27.914 31.050

3 24.650 22.543 0.007

4 3.106 0.042 18.798

5 22.686 30.936 6.253

6 18.766 15.575 27.654

7 15.510 18.945 3.563

8 15.512 13.334 2.411

9 24.535 8.524 30.928

10 31.036 6.556 22.576

11 15.582 3.411 18.962

12 15.525 22.406 23.901

13 0.073 24.575 22.494

14 31.058 12.069 3.264

15 6.946 15.548 21.890

16 0.053 18.938 27.807

17 3.229 31.049 12.214

18 8.484 31.019 6.304

19 8.444 30.997 24.600

20 0.033 24.909 8.559

21 27.543 12.093 15.414

22 12.085 27.732 31.047

23 6.402 8.494 0.070

24 18.995 3.308 31.040

25 21.941 7.173 15.568

26 12.683 27.920 15.525

27 23.940 22.267 15.526

28 8.740 15.545 7.251

29 9.223 7.039 15.593

30 31.035 6.526 8.473

31 12.383 15.618 27.680

32 22.579 30.926 24.697

33 15.605 24.051 9.085

34 3.420 17.811 13.252

35 12.114 3.206 31.048

36 3.430 12.275 15.538

37 23.927 15.621 21.978

38 31.042 12.117 27.816

39 28.715 17.686 15.554

40 27.825 31.058 18.942

41 15.639 8.866 7.168

Appendix

158

42 8.365 22.736 16.973

43 31.036 18.878 3.255

44 27.725 30.963 12.041

45 15.618 8.867 24.006

46 15.478 3.294 12.387

47 6.115 22.573 0.093

48 18.909 27.589 15.603

Appendix

159

Table A5. Equilibrium locations of Mg2+ ions.

Ion index X Y Z

1 19.241 15.574 2.786

2 18.069 27.787 31.024

3 31.026 27.329 18.332

4 15.508 2.755 19.404

5 1.881 24.480 7.809

6 15.490 11.841 28.244

7 11.506 15.523 2.892

8 15.474 2.898 11.547

9 22.648 5.758 1.055

10 31.030 12.709 27.087

11 2.587 12.172 15.534

12 7.078 16.639 21.404

13 28.581 18.671 15.52

14 16.997 21.736 23.575

15 6.315 23.075 29.520

16 0.082 12.703 3.938

17 19.113 28.342 15.547

18 23.935 9.609 16.679

19 27.365 18.378 30.998

20 2.847 2.774 12.694

21 14.113 23.597 9.370

22 6.233 1.385 22.923

23 27.216 31.053 12.767

24 13.044 3.200 0.041

Appendix

160

Table A6. Equilibrium locations of Ca2+ ions.

Ion index X Y Z

1 21.297 16.223 6.999

2 18.585 28.481 2.532

3 27.800 19.464 15.480

4 27.293 31.032 12.365

5 0.613 25.269 22.498

6 15.549 11.754 27.881

7 12.014 15.485 3.101

8 15.496 3.184 11.703

9 25.297 8.673 0.640

10 0.080 3.684 18.656

11 12.850 3.165 18.290

12 9.786 24.000 14.826

13 3.206 15.514 11.604

14 3.184 15.515 19.495

15 3.169 12.682 31.039

16 12.222 2.896 2.935

17 15.575 19.389 27.945

18 27.677 18.432 31.045

19 3.717 0.023 12.411

20 19.013 27.977 15.519

21 12.405 31.019 27.330

22 5.852 22.664 0.759

23 27.866 11.705 15.531

24 22.396 0.571 25.083

Appendix

161

Table A7. Equilibrium locations of Al3+ ions.

Ion index X Y Z

1 15.490 24.069 9.470

2 12.431 28.390 31.001

3 28.544 28.503 11.995

4 28.059 18.891 0.122

5 18.804 15.524 27.553

6 17.685 8.605 8.705

7 18.967 3.122 30.985

8 31.013 5.283 22.277

9 13.169 3.513 17.795

10 17.990 27.601 17.956

11 5.781 23.049 22.649

12 2.987 12.232 0.035

13 2.619 2.632 11.893

14 13.169 15.545 2.724

15 3.478 15.528 12.302

16 27.907 15.547 18.260

Publications

162

Publications

1. Y. F. Chen, Z. Q. Hu, K. M. Gupta. J. W. Jiang. Ionic Liquid/Metal-Organic

Framework Composite for CO2 Capture: A Computational Investigation. Journal

of Physical Chemistry C. 2011, 115, 21736-21742.

2. Y. F. Chen, J. W. Jiang. A Bio-Metal-Organic Framework for Highly Selective

CO2 Capture: A Molecular Simulation Study. ChemSusChem. 2010, 3, 982-988.

3. Y. F. Chen, J. Y. Lee, R. Babarao, J. Li, and J. W. Jiang. A Highly Hydrophobic

Metal-Organic Framework Zn(BDC)(TED)0.5 for Adsorption and Separation of

CH3OH/H2O and CO2/CH4. Journal of Physical Chemistry C. 2010, 114, 6602 -

6609.

4. Y. F. Chen, R. Babarao, S. I. Sandler, and J. W. Jiang. Metal-Organic Framework

MIL-101 for Adsorption and Effect of Terminal Water Molecules: From Quantum

Mechanics to Molecular Simulation. Langmuir 2010, 26, 8743-8750.

5. Z. Q. Hu, Y. F. Chen, and J. W. Jiang. Zeolitic Imidazolate Framework-8 as a

Reverse Osmosis Membrane for Water Desalination: Insight from Molecular

Simulation. Journal of Chemical Physics. 2011, 134, 134705.

6. P. Pachfule, Y. F. Chen, J. W. Jiang, R. Banerjee. Fluorinated Metal Organic

Frameworks (F-MOFs): Advantageous for Higher H2 and CO2 Adsorption or not?

Chemistry-A European Journal. 2012, 18, 688-694.

7. P. Pachfule, Y. F. Chen, S. C. Sahoo, J. W. Jiang, R. Banerjee. Structural

Isomerism and effect of Fluorination of Gas Adsorption in Copper-Tetrazolate

Based Metal Organic Frameworks. Chemistry of Materials. 2011, 23, 2908-2916.

8. P. Pachfule, Y. F. Chen, J. W. Jiang, R. Banerjee. Experimental and Computational

Approach of Understanding the Gas Adsorption in Amino Functionalized

Interpenetrated Metal Organic Frameworks. Journal of Materials Chemistry. 2011,

21, 17737-17745.

Publications

163

9. T. Panda, P. Pachfule, Y. F. Chen, J. W. Jiang. And R. Banerjee. Amino

Functionalized Zeolitic Tetrazolate Framework (ZTF) with High Capacity for

Storage of Carbon Dioxide. Chemical Communications. 2011, 47, 2011-2013.

Presentations

1. Y. F. Chen, A. Nalaparaju, J. W. Jiang, Adsorption and Separation of CO2/H2 in

Mono-, Di- and Trivalent Cation-Exchanged Zeolite-like Metal-Organic

Frameworks: Atomistic Simulation Study, 14th Asia Pacific Confederation of

Chemical Engineering Congress, Singapore (Feb. 2012).

2. R. Babarao, Y. F. Chen, J. W. Jiang, Effect of Water on Adsorption in

Metal-Organic Frameworks: Insight from Molecular Simulations, AIChE Annual

Conference, Salt Lake City, Utah, USA (Nov. 2010).

3. Y. F. Chen, J. Lee, J. Li, J. W. Jiang, Adsorption and Separation of CO2/CH4 and

CH3OH/H2O in Highly Hydrophobic Zn(BDC)(TED)0.5. 2nd International

Conference on Metal-Organic Frameworks and Open Framework Compounds,

Marseille, France (Sep. 2010).

4. Y. F. Chen, R. Babarao, J. W. Jiang, Metal-Organic Framework MIL-101 for

Adsorption and Effect of Terminal Water Molecules: A Computational Study. 2nd

International Conference on Metal-Organic Frameworks and Open Framework

Compounds, Marseille, France (Sep. 2010).

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Licensed content publication Science

Licensed content title Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage

Licensed content author Mohamed Eddaoudi, Jaheon Kim, Nathaniel Rosi, David Vodak, Joseph Wachter, Michael O'Keeffe, Omar M. Yaghi

Licensed content date Jan 18, 2002

Volume number 295

Issue number 5554

Type of Use Thesis / Dissertation

Requestor type Scientist/individual at a research institution

Format Print and electronic

Portion Figure

Number of figures/tables 1

Order reference number

Title of your thesis / dissertation

Molecular simulations for CO2 capture in metal-organic frameworks

Expected completion date May 2012

Estimated size(pages) 176

Total 0.00 USD

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