Separation and Purification Technology · Synthesis of MOF-801 particles was followed by previous report [22,23]. Fumaric acid (Alfa Aesar, 50mmol) and ZrOCl 2·8H 2O (Sigma-Aldrich,

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Separation and Purification Technology

journal homepage: www.elsevier.com/locate/seppur

MOF-801 incorporated PEBA mixed-matrix composite membranes for CO2

capture

Jiajia Sun, Qianqian Li, Guining Chen, Jingui Duan, Gongping Liu⁎, Wanqin JinState Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China

A R T I C L E I N F O

Keywords:MOF-801PEBAMixed-matrix membraneComposite membraneCO2/N2 separation

A B S T R A C T

The combination of CO2-selective porous framework with polymeric material is promising for developing mixed-matrix membranes. In this work, a novel metal organic framework MOF-801 nanocrystal was introduced intopolyether-block-amide (PEBA) polymer to fabricate a new mixed-matrix material for CO2 separation. A thinmembrane layer consisting of MOF-801 filler and PEBA matrix was successfully formed on a porous substrate viaspinning-coating approach. SEM, XRD, TGA, FTIR and adsorption test were applied to systematically char-acterize the as-prepared MOF-801 crystals and MOF-801/PEBA mixed-matrix composite membranes. It wasfound that uniform incorporation of microporous MOF-801 with preferential CO2 adsorption provided fast andselective transport channels for CO2 over N2, thereby achieving an increase both in CO2 permeance and CO2/N2

mixed-gas selectivity compared with pure PEBA membrane. The optimized MOF-801/PEBA mixed-matrixcomposite membrane (MOF loading of 7.5 wt%) exhibited high and stable separation performance with CO2

permeance of 22.4 GPU and CO2/N2 selectivity of 66 under mixed-gas permeation test, showing great potentialfor practical CO2 separation.

1. Introduction

CO2 capture is an important process to reduce or eliminate CO2

emission to minimize its negative impact on climate change. Traditionalseparation processes including adsorption and cryogenic purification,are energy intensive. Membrane-based technology owns advantages inenergy efficiency and carbon footprint [1]. Currently, polymericmembranes are dominant membrane products CO2 separation owing totheir scalable fabrication and low cost [2], whose wider application ishowever limited by the bottleneck of trade-off between membranepermeability and selectivity [3,4]. Although with great potential inexhibiting high separation performance, inorganic membranes gen-erally remain challenge in scalable fabrication of defect-free membrane[5].

Mixed-matrix membrane with inorganic filler dispersing in polymermatrix could combine the advantages of high-performance from in-organic membranes and easy fabrication from polymeric membranes,which is thus a kind of emerging membrane for CO2 separation in thepast two decades [6]. Metal-organic framework (MOF) is a class ofnanoporous materials consisting of metal ions or clusters and organiclinkers, exhibiting the respective properties of organic and inorganicparts [7]. With large surface areas and tunable pore size and chemistry,MOF has been widely studied for gas storage and adsorption separation

[8,9], sensing [10], catalysis [11] and other related fields. MOF crystalscan also be translated into membranes in form of either pure crystallinemembrane or mixed-matrix membrane [12]. Pure MOF membrane re-quires fabrication of a thin and continuous crystalline membrane layeron a porous substrate, which is challenging for some MOF materials.Alternatively, incorporating MOF crystals into polymer matrix to form amixed-matrix membrane offers a more convenient approach to realizeMOF application in membrane separation.

It has been well demonstrated that introducing CO2-selective MOFcrystals into polymer could successfully enhance the separation per-formance in CO2 capture, natural gas purification and biogas separation[13–15]. For instance, Mg-MOF-74 was incorporated into PIM-1 to forma crosslinked mixed-matrix membrane that shows up to 3 times higherCO2 permeability compared with pristine polymeric membrane [16].Vankelecom et al. [17] found that incoporation of Cu3(BTC)2 MOF withwindow aperture size of 0.35 nm can improve the CO2 permeability andits selectivity towards CH4 and N2 in polyimide membrane. Pan and co-workers incorporated CO2-philic KAUST-7 nanocrystals into 6FDA-Durene polyimide to increase the CO2/CH4 selectivity owing to theenhanced sorption selectivity [18]. Recently, our group developedPEBA mixed-matrix membranes compromising UiO-66 [19] or ZIF-300[20] nanocrystals showing highly enhanced CO2/N2 separation per-formance transcending 2008 Robeson upper-bound for polymeric

https://doi.org/10.1016/j.seppur.2019.02.036Received 7 January 2019; Received in revised form 18 February 2019; Accepted 18 February 2019

⁎ Corresponding author.E-mail address: [email protected] (G. Liu).

Separation and Purification Technology 217 (2019) 229–239

Available online 19 February 20191383-5866/ © 2019 Elsevier B.V. All rights reserved.

T

membranes.A new class of CO2-philic Zirconium-based MOF, MOF 801

[Zr6O4(OH)4(fumarate)6] reported by Wissmann et al. [21] has a cen-tral octahedral cage that is connected to 8 corner tetrahedron cagesthrough a triangular window with ∼6 Å in size. Several studies dis-covered that MOF-801 exhibited strong affinity to H2O molecules andtherefore was used for pervaporation [22–24]. However, there is noreport on the application of MOF-801 materials for gas separation. Inview of that the MOF-801 has a similar framework topology to UiO-66which is a kind of MOF with strong CO2 affinity owing to the hydroxylgroups coordinated to Zr cluster [25–27]. Thus, we anticipate thatMOF-801 may be a promising CO2 capture membrane material. As faras we know, gas separation has not been achieved in MOF-801 untilnow.

In this work, we explored a new mixed-matrix material by in-troducing MOF-801. Nano-sized crystals of MOF-801 were synthesizedto further promote the interaction between filler and polymer chains toform an ideal interface in the mixed-matrix material. The schematic ofMOF-801/PEBA mixed-matrix composite membrane is shown in Fig. 1.We chose a commercial copolymer polyether block amide (PEBA) as thepolymer matrix [28,29]. On this basis, mixed-matrix composite mem-brane was fabricated by spin-coating approach on a porous support.The effects of MOF loading and operating conditions on CO2/N2 se-paration performance were investigated. The MOF-801/PEBA mixed-matrix composite membranes showed remarkable improvement both inCO2 permeance and CO2/N2 selectivity compared with pristine PEBAmembrane. Sorption and diffusion coefficients were employed to studythe gas transport mechanism in the membrane. In addition, the MOF-801 mixed-matrix composite membrane showed a stable performanceduring continuous mixed-gas permeation, which could be a potentialcandidate for future CO2 separation.

2. Material and methods

2.1. Synthesis of MOF-801 crystals

Synthesis of MOF-801 particles was followed by previous report[22,23]. Fumaric acid (Alfa Aesar, 50mmol) and ZrOCl2·8H2O (Sigma-Aldrich, 50mmol) were dissolved in a N,N-Dimethylformamide (DMF)/formic acid mixed-solvent (20:7 in volume) in an autoclave. A hydro-thermal synthesis is carried out for the autoclave at 130 °C for 6 h. Thenwhite particles are precipitated and washed by using DMF and me-thanol for at least 3 times. The washed particles were solvent ex-changed with methanol for 3 days by rinsing with DMF by 3 times/day.

Activated MOF-801 were obtained by drying the solvent-exchangedparticles at 150 °C under vacuum for 12 h.

2.2. Preparation of MOF-801/PEBA mixed-matrix composite membrane

MOF-801 nanoparticles were dispersed in ethanol/water mixed-solvent (ωethanol:ωwater = 7:3) under stirring and sonication for 2 h.Subsequently, the MOF-801 dispersion was mixed with PEBA (PEBAXMH 1657, Arkema, France) under stirring and refluxing at 80 °C over-night. The resulting solution was spin coated on a porous poly-acrylonitrile (PAN) support (average pore size: 100 nm) to form theMOF-801/PEBA mixed-matrix composite membrane. As a control,pristine PEBA composite membrane was prepared by spinning coatingPEBA solution on the PAN support under the identical conditions givenabove. Dense film was also cast on a Teflon dish. The membrane wasdried at room temperature for 2–3 days to evaporate the solvent andfurther dried at 70 °C under vacuum for 12 h. The MOF-801 loading inthe membrane was defined as the weight of MOF in the total weight ofMOF and PEBA. MOF-801/PEBA(X) represents the MOF-801 loadingwas X% (X=2.5, 5, 7.5, 10, 12.5).

2.3. Characterizations

Crystal structure of the MOF nanocrystals and membranes werecharacterized by X-ray diffraction (XRD, Rigaku, Miniflex 600, Japan).The chemical groups of MOF-801 nanocrystals and membranes wereanalyzed by Fourier transform infrared (FTIR, AVATAR-FT-IR-360,Thermo Ncolet, USA). Thermal properties of the MOF-801 nanocrystalsand membranes were analyzed by thermogravimetric analysis (TGA,STA 209 F1, NETZSCH, Germany) under N2 atmosphere with a heatingrate of 10 K/min. Adsorption-desorption of CO2 and N2 in the MOF-801nanocrystals were measured by ASAP 2460. BET surface area of MOF-801 was obtained from the N2 adsorption-desorption isotherms at 77 K.Morphologies of the MOF-801 crystals and membranes were observedby field emission scanning electron microscope (FESEM, S4800,Hitachi, Japan) with energy-dispersive X-ray detector (EDX). CO2 andN2 sorption properties of the membrane at high pressure range weremeasured by Belsorp-HP at 293 K.

2.4. Gas permeation measurements

The gas permeation properties are measured by following themethods described in our previous work [30], which is also given asbelow. Pure gas permeation was measured using constant-volume

Fig. 1. Schematic of MOF-801/PEBA mixed matrix composite membrane.

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230

system at 20 °C. The membrane and the permeation system were va-cuumed overnight before measuring each gas. The gas reservoir wasfeed with gas at a given pressure and the pressure variation in thepermeate was record with a transducer. The gas permeability can becalculated as:

= × ×+

× ×P VA T

lp

dd

1760

273273 760

p

t (1)

where P is the gas permeability (cm3 (STP) cm/cm3 s cmHg), A is theeffective membrane area (cm2), V is downstream volume (cm3), T is thepermeation temperature (°C), l is the membrane thickness (cm), p is thepressure (MPa) and t is the permeation time (s). 1 Barrer= 10-10 cm3 (STP) cm/cm3 s cmHg; 1 GPU=10-6 cm3 (STP)/cm3 s cmHg(3.35×10−10 mol s−1 m−2 Pa−1).

Mixed-gas (50/50 CO2/N2) permeation was measured using con-stant-pressure system at 20 °C. Ar was used as sweep gas and the gascomposition was determined by gas chromatography. The gas per-meance (P/L) can be calculated as below:

=∅ − ∅

−P lV y

AT p x p y/ 10

273.15( )

p A

x x A y y A

6

A A (2)

where Vp is volumetric flow rate of permeate (ml/s), x is the molfraction of the component A in the feed and y is the one in the permeate,Px and Py are the feed pressure and permeate pressure (cmHg), re-spectively; ϕxA and ϕyA are fugacity coefficients in the feed andpermeate stream evaluated by thermodynamic equation.

Diffusion coefficient (D) could be calculated using the permeability(P) and solubility coefficient (S) according to the solution-diffusionmodel:

=D PS (3)

where D and S characterize the kinetic and thermodynamic contribu-tion to the gas transport, respectively. S can be obtained from thesorption measurement using the following equation:

=S cfii

i (4)

where ci is gas concentration adsorbed in the membrane and fi is thefeed pressure.

The selectivity of gas A over B can be calculated as:

= = ×α PP

SS

DDA B

A

B

A

B

A

B/ (5)

where SA/SB is the sorption selectivity and DA/DB is the diffusion se-lectivity.

3. Results and discussion

3.1. Physicochemical properties of MOF-801

The SEM image shows that the as-synthesized MOF-801 particleswith crystallized octahedral structure have a uniform size distributionwith an average particle size of 400–500 nm (Fig. 2a). These nano-sizednanoparticles would facilitate the preparation of a thin and uniformmixed-matrix composite membrane. The crystal structure of the syn-thesized MOF-801 particles were analyzed by XRD. As shown in Fig. 2b,the XRD pattern is consistent with the simulation result [38], indicatinga high phase purity of the synthesized MOF-801 nanocrystals. From theIR spectrum of MOF-801 shown in Fig. 2c, the peaks at 1402 cm−1,1584 cm−1 and 1658 cm−1 are recognized as eC]OeO bonds, and thepeaks at 1202 cm−1 and 1102 cm−1 are assigned to the eC]OeOHand eCeO arising from the functional groups of the fumaric acid inMOF-801, which are consistent with previous report [22]. The thermalstability of MOF-801 crystals was studied by TGA (Fig. 2d). It is re-vealed that the as-synthesized MOF-801 underwent mass loss up to

500 °C with three steps. The first slight weight loss of the activatedsamples before 100 °C is due to the moisture removal. The secondweight loss occurred at approximately 250 °C was corresponding to theevaporation of guest molecules (e.g., the solvents DMF and methanol)from the pores of MOF-801. With further heating, the final decom-position of MOF-801 is occurred at ∼500 °C resulting from the releaseof carboxylate groups, indicating an excellent thermal stability of MOF-801.

N2 adsorption measurements at 77 K (Fig. 3) shows that MOF-801exhibits type-I adsorption behavior, and the calculated BET surface areafor MOF-801 is 838.94m2/g. The insert shows the micropores of MOF-801 framework with diameter of 5–7 Å and average of ∼6 Å. The re-sults indicate a UiO-66-type topology of Zr-fum MOF [21]. Fig. 3bpresents the CO2 and N2 sorption isotherms at 298 K for MOF-801crystals. Notably, it showed a very low amount of N2 was absorbed onMOF-801 while shows a steep rise sorption capacity and affinity towardCO2. At the pressure of ∼1.0 bar, the sorption amounts for CO2 and N2

are 30.3 and 1.8 cm3(STP)/g, respectively, leading to a high CO2/N2

sorption selectivity of∼17. As the adsorption operating pressure raises,the amount of CO2 adsorption increases significantly, while N2 onlyslightly changes. This behavior is an indication of existence of specificadsorption sites for CO2 gases, meaning relatively strong affinities forCO2 over N2. These results imply that MOF-801 could be potentialmaterial for selective sorption of CO2 over N2.

3.2. Membrane characterization

3.2.1. MorphologyIn order to study the distribution of MOF filler in the PEBA matrix,

mixed-matrix dense films were prepared. The morphology of the as-prepared MOF-801/PEBA mixed-matrix dense films was investigated byFESEM. As seen from Fig. 4, dense and defect-free morphology can beobserved in the cross-sectional micrograph (Fig. 4a). The magnifiedimage of the sample clearly shows the crystal shape of the MOF andpolymer veins (Fig. 4b). The complete filler-polymer interface revealsgood compatibility of the MOF-polymer. EDX mapping was furthercarried out to analyze the elemental composition of the as-preparedmixed-matrix films. The grey-colored, green-colored, yellow-coloredand red-colored dots represent carbon, oxygen, zirconium and nitrogen,respectively. The MOF-801/PEBA mixed-matrix dense film is composedof Zr, C, O and N elements (Fig. 4c), which are consistent with theelements contained in the crystal and polymer. In particular, Zr element(red) from MOF-801 uniformly disperses across the film, which reflectsthe homogeneous distribution of the MOF-801 nanoparticles in thepolymer matrix. It further demonstrates the stable combination be-tween MOF-801 nanocrystals and PEBA because of the favorable in-teractions of the organic linkers on the MOF with the polymer chains[31].

Based on the good dispersion of MOF-801 in PEBA, MOF-801/PEBAmixed-matrix composite membrane was prepared by a facile spin-coating method. Fig. 5a showed a cross-sectional SEM image of thepristine PEBA composite membrane. The surface of the pristine PEBAmembrane layer was smooth without any structural protrusions whilethat of the MOF-801/PEBA mixed-matrix membrane layer was rough(Fig. 5b–f). As the MOF-801 filler was introduced into the PEBA matrixwith low loading, granular protrusions were observed on the surface ofthe membrane, and there was no obvious interfacial void and peelingphenomenon, which indicates a good compatibility between MOF-801and polymer. However, due to the increase in the content of MOF-801to 12.5 wt%, particle agglomeration occurred in the SEM image of themembrane (Fig. 5f). From the cross-section images, it is found that themixed-matrix layer and the support are firmly bonded, there is nomacroporous interfacial void and exfoliation, and the thickness of themixed-matrix layer is controlled at about 1.3 μm.

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3.2.2. Chemical and physical structureThe pristine PEBA and mixed-matrix dense films with different

loadings were studied by XRD to investigate the effect of MOF-801loading on membrane crystal structure. The peak from 15° to 25° isattributed to the PEBA, which is a semicrystalline copolymer consistingof crystalline PA6 and amorphous PEO parts [32]. Fig. 6a shows thatthe PEBA membrane embedded with MOF-801 exhibits a distinct newcharacteristic peak at 2θ=10° which clearly confirms the presence of

MOF-801 in polymer matrix in comparison to that of pristine PEBA.Moreover, it shows a growth of the characteristic peaks with increasingthe loading, indicating that the MOF-801 crystalline structure remainedunchanged after being incorporated into the PEBA matrix. TGA mea-surements were performed to analyze the thermal stability of thepristine PEBA membrane and mixed-matrix membranes, as shown inFig. 6b. The entire thermal decomposition process consists of threemajor stages of mass loss. Weight loss at 0–100 °C is caused by

Fig. 2. (a) SEM image of MOF-801 crystals; (b) XRD patterns of simulated MOF-801 crystals and as-prepared MOF-801 crystals; (c) FT-IR spectra of MOF-801crystals; (d) TGA curves of MOF-801 crystals.

Fig. 3. (a) N2 sorption-desorption isotherms at 77 K and pore size distribution curve (insert) of MOF-801 nanocrystals; (b) adsorption isotherms for CO2 and N2 withMOF-801 nanocrystals at 298 K.

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evaporation of residual solvent in the membrane. The second stage(100–400 °C) is due to the deacetylation and depolymerization of PEBA.The final decomposition stage (400–800 °C) is related to the residualdecomposition of PEBA, which well agrees with previous study [33].The final decomposition temperature of the MOF-801/PEBA mixed-matrix membrane is slightly higher than that of the pristine PEBAmembrane as the MOF loading increases, which can be attributed to thehigher thermal stability of the MOF-801 and the good compatibility ofthe MOF with the PEBA matrix. Nevertheless, the thermal stability ofMOF-801/PEBA mixed-matrix membranes is sufficient for conventionalgas separation applications.

To further confirm the successful MOF incorporated in the polymerand analyze the connection between incorporated MOF-801 and

polymer chains, FTIR analysis was conducted and the results are shownin Fig. 7. The peaks observed at 1094 cm−1, 1636 and 1732 cm−1, and3298 cm−1 are corresponding to the stretching vibration of the CeOeCfrom PEO [34], eNeHe, HeNeC]O and OeC]O from PA, respec-tively [35]. Limited chemical interactions between the PEBA chain andMOF-801 framework are expected as no new peaks are generated afterintroducing MOF-801 into the PEBA membrane.

3.2.3. Gas sorption propertiesHigh pressure adsorption (up to 10 bar) was measured on pure

PEBA and MOF-801/PEBA mixed-matrix dense films with variousloadings to obtain adsorption isotherms of CO2 and N2. The dense filmswere treated under vacuum at 150° C for 24 h for degassing. As shown

Fig. 4. (a) Cross section SEM images, (b)Enlarged cross section SEM images and (c)EDX mapping of MOF-801/PEBA (7.5)mixed-matrix dense membrane (N signal:yellow, Zn signal: red, C signal: grey, Osignal: green). (For interpretation of the re-ferences to color in this figure legend, thereader is referred to the web version of thisarticle.)

Fig. 5. Cross-section SEM images of (a) pristine PEBA, (b) MOF-801/PEBA (2.5), (c) MOF-801/PEBA (5), (d) MOF-801/PEBA (7.5) membranes, (e) MOF-801/PEBA(10) membranes, and (f) MOF-801/PEBA (12.5) mixed-matrix membrane respectively.

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in Fig. 8, the adsorbed gas concentration increases as the MOF-801loading in the PEBA membrane. The high-pressure adsorption isothermof the pristine PEBA membranes has almost linear shape. The linearrelationship between adsorption capacity and pressure indicates thatCO2 or N2 adsorption in rubbery PEBA generally follows Henry's law[59]. After the MOF-801 nanocrystals are incorporated, the adsorptionamount of gas in the mixed matrix membrane raises steeply with theincrease of the MOF amount. Meanwhile, CO2 sorption was sig-nificantly enhanced along with only a slight increase in N2 sorptioncapacity via incorporating the nanoporous CO2-philic MOF-801 crystalswhich attributed to the higher gas adsorption capacities in MOF-801than those in PEBA. This phenomenon may leads to an increase in theadsorption selectivity of the mixed-matrix membrane for CO2/N2 gaspair. Taking the 7.5 wt% MOF-801-filled PEBA mixed-matrix mem-brane as an example, the CO2/N2 sorption selectivity of pristine PEBAmembrane under the permeation conditions (3 bar) was increased from5.90 to 6.91. As expected from the isotherms of 10 wt% MOF-801/PEBAmixed-matrix membrane, the sorption amount can be further increasedby incorporating more MOF-801 crystals in PEBA membrane. The ob-tained experimentally adsorption coefficient will then be used to cal-culate the diffusion coefficient from the measured pure gas perme-ability of the dense film. In addition, it can be speculated that theadsorption capacity of MOF-801/PEBA mixed-matrix membrane maybe combined with the adsorption properties of PEBA matrix and MOF-801 filler, indicating that the surrounding PEBA chain does not affectthe porous structure of MOF-801.

3.3. Gas separation performance

3.3.1. Effect of spin-coating cycleTo form a thin and defect-free mixed-matrix composite membrane,

the effect of spin-coating cycle of the casting solution on the mor-phology and CO2/N2 gas separation performance of 7.5 wt% MOF-801/PEBA mixed-matrix composite membrane was firstly investigated. Itcan be clearly seen from the membrane cross-sectional images(Fig. 9a–e) that as the spin-coating cycle increased, the membranethickness gradually increased, resulting in a decrease in the permeanceof CO2. At the same time, the CO2/N2 selectivity increased first andthen decreased. Due to the porous framework and the preferential ad-sorption to CO2, the CO2/N2 selectivity could be increased with thespin-coating cycle (Fig. 10). However, the selectivity reduction withexcessive spin-coating cycle is due to the agglomeration of MOF par-ticles. From the surface images of the mixed-matrix composite mem-brane (Fig. 9f–j), it was found when the spin-coating exceeds 3 cycles,

Fig. 6. (a) XRD spectra and (b) TG curves of MOF-801/PEBA mixed-matrix membranes with various loading.

Fig. 7. FT-IR spectra of pristine PEBA membrane and MOF-801/PEBA mixed-matrix membranes.

0 2 4 6 8 10

0

5

10

15

20

25

Gas

ads

orpt

ion/

cm3 (S

TP)g

-1

Pressure (bar)

Pure PEBA, CO27.5% MOF-801-PEBA, CO2

10% MOF-801-PEBA, CO2

Pure PEBA, N2

10% MOF-801-PEBA, N2

7.5% MOF-801-PEBA, N2

Fig. 8. CO2 and N2 sorption isotherms of pristine PEBA membrane and MOF-801/PEBA mixed-matrix membrane with 7.5 or 10 wt% MOF-801 at 0–10 barand 25 °C.

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visible agglomeration of MOF-801 particles appears in the membrane. Abest CO2/N2 separation performance was obtained with 3 spin-coatingcycles that is thus followed by the preparation of MOF-801/PEBAmixed-matrix composite membranes with various MOF loading.

3.3.2. Effect of MOF-801 loadingA binary gas mixture measurement (CO2/N2= 50/50 vol%) was

carried out to study the actual gas separation performance of the MOF-801/PEBA mixed-matrix composite membranes varied by MOF loading.As shown in Fig. 11, by incorporating MOF-801 into PEBA, the CO2

permeance and CO2/N2 selectivity of PEBA membrane were increasedsimultaneously with gradually increasing the filler content to 7.5 wt%.The highly porous framework of MOF-801 and the preferential ad-sorption of CO2 promote the diffusion of CO2 thus increasing the per-meation rate of CO2. Although the above conditions also improved theN2 permeance, this increase was smaller than that of CO2 permeanceowing to larger kinetic diameter of N2 (3.6 Å) than that of CO2 (3.3 Å).As a result, CO2 permeance increases from 8.1 GPU to 22.4 GPU, whileN2 permeance only slight changes. While as the MOF is overloaded(> 7.5 wt%), agglomeration of the MOF particles, rigidification andblockage of filler pores might be occurred, resulting in the decrease ofgas permeance both for CO2 and N2. Meanwhile, some non-selectivedefects would be formed due to the particle aggregation, leading to thelowered selectivity with the MOF loading of 10.0 wt% and 12.5 wt%compared with the 7.5 wt%. On the other hand, the preferential sorp-tion of MOF towards CO2 over N2 can compensate some CO2/N2 per-meation selectivity, which is referred from the slightly higher se-lectivity at the loading of 12.5 wt% compared with the 10.0 wt%.Overall, in our case, the PEBA mixed-matrix composite membranesfilled with 7.5 wt% MOF-801 achieved an optimal performance withthe CO2 permeance of 22.4 GPU and CO2/N2 selectivity of 66. Com-pared to pristine PEBA membrane, the permeance and selectivity weresimultaneously enhanced by 75% and 38%. The result confirms that theaddition of nanoporous and CO2-philic MOF-801 framework is efficientto improve the CO2/N2 separation performance of PEBA membrane.

In order to further understand the mechanism of enhanced gastransport in the MOF-801/PEBA mixed-matrix membrane, the solubilitycoefficient and diffusion coefficient of CO2 and N2 in the membranewere studied in detail. The diffusion coefficient D is calculated from thepure-gas permeability (P) and sorption coefficient (S) measured in thepermeation and adsorption experiments on dense films. On the basis ofthis, the sorption selectivity (αS) and diffusion selectivity (αD) of CO2/N2 are calculated separately. As shown in Fig. 12, as increasing theMOF-801 content, the gas sorption coefficient increases while the dif-fusion coefficient decreases. Since MOF-801 has a higher gas adsorptioncapacity than PEBA, it produces a significant increase in gas solubility.And because of the CO2-philic characteristics of MOF-801 frameworks,the sorption coefficient of CO2 is much higher than that of N2, whichimproves the CO2/N2 sorption selectivity in PEBA membrane. There aresome reduced-diffusion regions that might be created by blockage ofMOF pores and/or rigidity of polymer chains. This fact notwith-standing, a higher CO2/N2 diffusion selectivity in the MOF-801/PEBAmixed-matrix membrane. The analysis of transport coefficients con-firms our hypothesis that the preferential adsorption of CO2 over N2

allows MOF-801 improving the permeability and selectivity of PEBAmembranes for CO2/N2 separation.

3.3.3. Effect of operation temperatureFig. 13 investigates the effect of permeation temperature on the

separation performance of an equal volume CO2/N2 mixture on the

Fig. 9. Surface and cross-section SEM images of 7.5 wt% MOF-801/PEBA mixed-matrix composite membranes with different spin-coating cycles, (a, f) MOF-801/PEBA (2), (b, g) MOF-801/PEBA (3), (c, h) MOF-801/PEBA (4), (d, i) MOF-801/PEBA (5) and (e, j) MOF-801/PEBA (6).

Fig. 10. Effect of spin-coating cycle on the CO2/N2 separation performances of7.5 wt% MOF-801/PEBA mixed-matrix composite membrane. Single gas per-meation tests were measured at 3 bar and 20 °C.

Fig. 11. Effect of MOF-801 loading on the CO2/N2 separation performances ofMOF-801/PEBA mixed-matrix composite membrane. Mixed gas permeationtests was measured at 1 bar and 20 °C.

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PEBA mixed-matrix composite membrane with MOF loading of 7.5 wt%. It was found that the CO2 permeance gradually increased with ele-vating temperature until reaching a maximum of ∼43 GPU at 60 °C,while the CO2/N2 selectivity is gradually reduced from 66 to 30. ThePEBA mixed-matrix membrane maintains higher CO2 permeance thanthe pristine PEBA membrane during the temperature range. It indicatesthat the diffusion dominates the gas transport through the membrane,rather than the sorption, as higher temperature favors gas diffusionwhile inhibits gas sorption. The increase in gas permeance with tem-perature can be ascribed to the highly porous framework of MOF-801providing fast diffusion channels for gases. It’s interesting to notice aslightly lower CO2/N2 selectivity for the PEBA membrane filled withMOF-801 than the pristine PEBA membrane at high temperature(50–60 °C). This might be due to the high-temperature induced moresignificant inhibition of preferential sorption towards CO2 over N2 inthe MOF-801 fillers than that in the PEBA matrix.

The relationship between gas permeability and temperature can beexpressed by the Arrhenius equation:

= ⎛⎝

− ⎞⎠

P P ERT

expi iP

,0 (6)

where Pi is the permeability of component i, Ep is the activation energy,

Pi,0 is pre-exponential factor, R is the gas constant, and T is the op-eration temperature in Kelvin. Fig. 14 calculates the Ep of gas passingthrough the membrane. The Ep values of CO2 and N2 on the pristinePEBA membrane were 9.889 and 19.27 kJ·mol−1 respectively, con-sistent with the reported values of PEBA membrane [36]. In addition,the Ep values of gases in the PEBA mixed-matrix membrane with MOF-801 were higher than those of PEBA membrane, indicating that theaddition of MOF-801 filler may increase the energy barrier of gas per-meation through the membrane [37]. Nevertheless, the Ep of CO2 inboth membranes is higher than that of N2, suggesting that CO2 exhibitslower activation energy permeating through PEBA-based membranes.

3.3.4. Effect of continuous mixed-gas permeationAs shown in Fig. 15, the long-term operation test of the mixed gas

permeation was carried out under the conditions of an atmosphericpressure and 20 °C. During the entire 120 h test period, the MOF-801/PEBA (7.5 wt%) mixed-matrix composite membrane kept stable CO2

permeance and CO2/N2 selectivity, indicating good operation stability.The average CO2 permeability is 22.4 GPU, and the CO2/N2 selectivityis 66. This result also proves the membrane can exhibit good structuralstability after the addition of MOF-801 filler, and is expected to beapplied for CO2 capture.

Fig. 12. (a) Sorption coefficient (S) and diffusion coefficient (D) and (b) sorption selectivity (αS) and diffusion selectivity (αD) of MOF-801/PEBA mixed-matrix densefilms with different MOF-801 loading for CO2 and N2 at 3 bar and 20 °C. The permeability and perm-selectivity were measured at 3 bar and 20 °C by using theidentical MOF-801/PEBA mixed-matrix dense films used for the sorption measurement.

Fig. 13. Effect of operation temperature on (a) CO2 permeance and (b) CO2/N2 selectivity of pristine PEBA membrane and MOF-801/PEBA (7.5) mixed-matrixcomposite membrane. Mixed gas permeation tests were measured at 1 bar.

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3.3.5. Comparison with literatureThe CO2/N2 separation performance of recently developed PEBA

mixed-matrix composite membranes are compared in Fig. 16 andTable 1. Different kinds of MOFs such as ZIF-7, ZIF-8, MIL-53, UiO-66,and carbon-based nanomaterials including carbon nanotube and gra-phene oxide nanosheets have been incorporated into PEBA to fabricatemixed-matrix composite membranes. It can be found that our MOF-801/PEBA mixed-matrix composite membrane with 7.5 wt% loadingshows a high CO2/N2 selectivity and good CO2 permeance, surpassingthe performance limit for most reported membranes. The obtained ex-cellent separation performance is owing to the uniform dispersion ofMOF-801 nanocrystals in the PEBA matrix and the spinning coated thinand defect-free MOF-801/PEBA mixed-matrix membrane layer on aporous support.

It’s also interesting to compare PEBA mixed-matrix membranescomprising three types of MOF filler fabricated in our lab: ZIF-300, UiO-66 and MOF-801. As shown in Fig. 17, incorporation of MOF-801 leadshigher CO2 permeability and CO2/N2 selectivity than ZIF-300 in PEBAmembrane with the same MOF loading of 7.5 wt%. Although the BETpore size of MOF-801 (0.61 nm) is smaller than that of ZIF-300(0.79 nm), the linear fumaric linker of MOF-801 may offer lowertransport resistance for gas diffusion than the bulky 2-mImH andbbImH linkers of ZIF-300, thereby resulting in higher CO2 permeability.Owing to the smaller aperture size (0.61 nm vs 0.79 nm) and highersorption selectivity (17 vs 8.8), MOF-801 shows higher enchantment ofCO2/N2 selectivity than that of ZIF-300 in the PEBA mixed-matrix

Fig. 14. (a) Arrhenius plots for CO2 and N2 permeance of pristine PEBA membrane at 1 bar. (b) Arrhenius plots for CO2 and N2 permeance of 7.5 wt% MOF-801/PEBA mixed-matrix composite membrane at 1 bar.

Fig. 15. Long-term operation test on MOF-801/PEBA mixed-matrix compositemembrane. Mixed gas permeation tests were measured at 1 bar and 20 °C.

Fig. 16. Comparison of CO2/N2 mixed gas separation performances of MOF-801/PEBA membranes with Robeson upper bound (all the data points are listedin Table 1).

Table 1Comparison of the performance of PEBA mixed-matrix composite membranesfor CO2/N2 separation.

Filler PCO2(GPU)

αCO2/N2 Test condition P (bar)/T(°C)

Ref.

ZIF-7 39 105 3.8/25 [36]ZIF-7 300 48 2.0/25 [28]ZIF-8 14 52 5.0/25 [38]ZIF-8 345 32 2.0/25 [37]ZIF-8 1.5 84 1.0/25 [39]Cu-MOF 13 53 3.0/20 [40]MIL-53 2.0 59 10/35 [5]UiO-66 7.2 60 3.0/20 [19]UiO-66 225 45 2.0/25 [28]Attapulgite 2.6 52 4.0/35 [41]Carbon nanotubes 13 59 1.0/25 [42]Carbon nanotubes 4.1 50 7.0/35 [43]Graphene oxide 1.4 49 7.0/35 [44]Zeolite 4A 6 54 5.0/25 [45]MOF-801 22 66 1.0/20 This

work

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membrane. Compared with its analogue UiO-66, MOF-801 mixed-ma-trix membrane exhibits higher CO2/N2 selectivity whereas lower CO2

permeability. It can be attributed to the more open window aperture ofUiO-66 using the same Zr cluster while much longer benzodicarboxyliclinker providing higher gas permeability and lower selectivity for CO2/N2 separation. Overall, it suggests that MOF-801 is a promising materialfor developing advanced mixed-matrix or even pure MOF membranesfor gas separation.

4. Conclusions

In summary, we report a novel mixed-matrix composite membranewith enhanced CO2/N2 separation performance by combining MOF-801filler with PEBA polymer. The synthesized MOF-801 nanocrystals ex-hibit excellent CO2 preferential adsorption toward N2 and good inter-facial adhesion to the PEBA matrix, thereby significantly enhancing theCO2 permeance and CO2/N2 selectivity of pure PEBA membrane. TheCO2 permeance of the optimized 7.5 wt% MOF-801/PEBA membranereached 22.4 GPU, which is 75% higher than the pristine PEBA mem-brane, meanwhile with 43% higher CO2/N2 selectivity (66). Sorption-diffusion analysis confirms that the incorporation of porous CO2-philicMOF-801 nanocrystals contributes to the enhanced gas separationperformance in the MOF-801/PEBA mixed-matrix membrane.Compared with other fillers reported in literature, MOF-801 exhibits ahigh efficiency for simultaneous enhancing permeance and selectivityfor CO2/N2 separation, which could be a potential candidate formembrane CO2 capture application.

Acknowledgements

This work was financially supported by the National Natural ScienceFoundation of China (Grant Nos. 21776125, 21490585, 5171101539),the Innovative Research Team Program by the Ministry of Education ofChina (Grant No. IRT17R54) and the Topnotch Academic ProgramsProject of Jiangsu Higher Education Institutions (TAPP).

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