Synthesis and characterization of zinc-organic frameworks with 1,4-benzenedicarboxylic acid and azobenzene-4,4′-dicarboxylic acid

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Synthesis and characterization of zinc-organic frameworks with 1,4-benzenedicarboxylic acid

and azobenzene-4,4'-dicarboxylic acid

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2011 Adv. Nat. Sci: Nanosci. Nanotechnol. 2 025008

(http://iopscience.iop.org/2043-6262/2/2/025008)

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025008 (5pp) doi:10.1088/2043-6262/2/2/025008

Synthesis and characterization ofzinc-organic frameworks with1,4-benzenedicarboxylic acid andazobenzene-4,4′-dicarboxylic acidVan Hung Nguyen, Ngoc Phuong Thuy Nguyen, Thi Tuyet NhungNguyen, Thi Thanh Thuy Le, Van Nghiem Le, Quoc Chinh Nguyen,That Quang Ton, Thai Hoang Nguyen and Thi Phuong Thoa Nguyen

University of Science, Vietnam National University—Ho Chi Minh City, 227 Nguyen Van Cu,District 5, Ho Chi Minh City, Vietnam

E-mail: [email protected] and [email protected]

Received 20 March 2011Accepted for publication 5 April 2011Published 3 May 2011Online at stacks.iop.org/ANSN/2/025008

AbstractThe solvothermal reactions of 1,4-bezenedicarboxylic acid (H2BDC) orazobenzene-4,4′-dicarboxylic acid (H2ABD) with zinc ions/clusters lead to the formation offour crystalline materials. All of these compounds were characterized by x-ray diffraction,optical microscopy, thermo-gravimetric analysis and nitrogen adsorption. Block-shapedcrystals (BZ1) with various shapes and sizes were obtained at H2BDC : Zn mole ratio of 1:1and H2BDC concentration of 0.1 M. At more dilute H2BDC concentration of 0.01 M andH2BDC : Zn mole ratio of 1 : 4, the reaction product was cubic crystals (BZ2) with a size of250 µm. In the H2ABD system, flat-plate-like crystals (AZ1) were obtained at H2ABD : Znmole ratio of 1 : 1 and H2ABD concentration of 0.01 M. Meanwhile, thick-block-like crystals(AZ2) were formed at the same H2ABD : Zn mole ratio but at 0.004 M H2ABD. TheLangmuir surface area (SLang) of the materials was remarkable, enhanced by diluting thereaction solution. For the compounds synthesized in N ,N ′-dimethylformamide (DMF), SLang

increased from 304.6 m2 g−1 for BZ1 to 2631 m2 g−1 for BZ2 and from 475.8 m2 g−1 for AZ1to 3428 m2 g−1 for AZ2. Meanwhile, BZ2 synthesized in N ,N ′-diethylformamide (BZ2/DEF)got the highest SLang of 4330 m2 g−1. Both AZ2 and BZ2 materials were stable up to 400 ◦C.

Keywords: benzene carboxylic acid, metal-organic frameworks, MOF-5, solvothermalreaction, zinc

Classification number: 5.19

1. Introduction

Metal-organic frameworks (MOFs) form a new class ofporous materials, which are receiving growing attentionbecause of their exceptionally high surface area, leadingto their potential applications in many areas, includingcatalysis, and sensors, especially in gas separation andstorage [1–4]. MOFs are constituted of two main components:multitopic organic linkers and metal ions/clusters. The latterare coordinated with the former to form polyhedral structures

called ‘second building blocks’ in which metal ions/clustersare vertices of the constructed frameworks [5, 6]. The organiclinkers continually act as bridges to expand these blocks,resulting in highly porous MOF structures. A wide variety ofmetal ions and organic bridges have been used to fabricatethousands of MOFs [7, 8]. Yaghi and co-workers havesuccessfully synthesized hundreds of MOFs based on thedi-, tri-, or multitopic carboxylic acid linkers and transitionmetals, such as Cu, Fe and Zn [9]. Of these, MOF-200 andMOF-205 have attracted much attention due to their surface

2043-6262/11/025008+05$33.00 1 © 2011 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025008 V H Nguyen et al

area exceeding 10 000 m2 g−1 [10]. Generally, the structure ofMOFs are constructed basing on the coordination of metalions/clusters and organic linkers [11–13]. However, severalfactors that should be borne in mind when approaching thesynthesis of new metal-organic frameworks, aside from thegeometric principle, are concentration, solvent polarity, pHand temperature [14–16]. In this work, we have focused onthe effect of the synthesis conditions on the morphologiesand properties of the conventional and novel metal-organicframework material using H2BDC and H2ABD linkers,respectively.

2. Experimental

2.1. Materials and instrumentation

N ,N ′-dimethylformamide (DMF) and N ,N ′-diethyl-formamide (DEF) were used as solvents. 1,4-benzenedicarboxylic acid (H2BDC) and azobenzene-4,4′-dicarboxylicacid (H2ABD) were used as ligands bound to zinc ions (zincnitrate tetrahydrate, Zn(NO3)2 · 4H2O). All of these reagentsand other chemicals, such as triethyleneamin (TEA) anddichloromethane (CH2Cl2), were purchased from Merck andused as received without further purification.

X-ray diffraction data were recorded on a D8 AdvanceBruker powder diffractometer with Cu-Kα radiation(λ = 1.542 Å). The thermal behavior of synthesized materialswas recorded on a TGA Q500 analyzer with a heatingrate of 10 ◦C min−1 in nitrogen. Nitrogen physisorptionmeasurements were carried out on a QuantachromeAutosorb-1C system. Prior to the measurement, the samplewas outgassed at 150–200 ◦C for 10 h.

2.2. Synthesis MOFs

2.2.1. Synthesis of MOFs from 1,4-benzenedicarboxylic acidand zinc nitrate. The reaction conditions that produce ourMOFs are based on the procedure used in [9, 17]. Zinc nitrateand H2BDC were dissolved in 10 ml DMF or DEF. The pH ofthe solutions was adjusted by TEA and varied from 3.0 to 5.0.The mole ratio of H2BDC and zinc nitrate (H2BDC : Zn) wasvaried from 1 : 1 to 1 : 5 with various H2BDC concentrationsin the range from 0.1 M to 0.0075 M. The mixtures were putin 20 ml reaction vials that were capped afterwards and heatedin an isothermal oven from 80 ◦C to 100 ◦C for 12–72 h, andthen cooled to room temperature. The cubic-shaped crystalswere obtained by decanting them from the mother liquor andthen washing them three times with 2 ml of DMF, and thenthree times with 10 ml of CH2Cl2 continuously for severaldays. Finally, the product was dried under vacuum at 200 ◦Cfor 10 h [17].

2.2.2. Synthesis of MOFs from azobenzene-1,4-dicarboxylicacid and zinc nitrate. The mixtures with mole ratios ofH2ABD to zinc nitrate (H2ABD : Zn) varying from 1 : 1 to1 : 5 were dissolved in 10 ml DMF. In each experiment with acertain H2ABD : Zn mole ratio, the concentration of H2ABDwas varied from 0.01 M to 0.001 M. The reaction mixture wasput in a 20 ml reaction vial, which was capped and heated inan isothermal oven from 80 ◦C to 100 ◦C for 12–72 h, and thencooled to room temperature. The red sheet-shaped crystalswere collected by decanting from mother liquor. The washingand solvent exchange procedures were the same as describedabove in section 2.2.1. Finally, the crystals were dried undervacuum at 150 ◦C for 10 h.

(a) (b)

200 µm 200 µm

200 µm

(c)

Figure 1. Microscope images of crystalline materials synthesized in DMF at 80 ◦C, 12 h, pH 4.18, [H2BDC] : Zn2+= 1 : 1,

[H2BDC] = 0.1 M (a, BZ1), in DMF at 100 ◦C, 24 h, pH 4.82, [H2BDC] : Zn2+= 1 : 4, [H2BDC] = 0.01 M (b, BZ2/DMF), in DEF at

100 ◦C, 24 h, pH 4.64, [H2BDC] : Zn2+= 1 : 4, [H2BDC] = 0.01 M (c, BZ2/DEF).

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025008 V H Nguyen et al

3. Results and discussion

3.1. MOF synthesis

3.1.1. MOFs from 1,4-benzenedicarboxylic acid and zincnitrate tetra hydrate. Compound BZ1 was obtained aspolyhedral crystals with ambiguous shapes from a simplereaction of zinc nitrate and H2BDC in DMF at 0.1 Mconcentration of H2BDC and equivalent H2BDC : Zn moleratio. Its microscope image is shown in figure 1(a). Anothertype of crystal, BZ2/DMF, was obtained in the same solvent,but with a H2BDC : Zn mole ratio of 1 : 4 and a remarkablydecreased concentration of H2BDC, to 0.01 M, was shownin figure 1(b). In figure 1(c), we can see the image of BZ2,synthesized under the same conditions with BZ2/DMF, butin DEF as a solvent. We name this product as BZ2/DEF.BZ2 synthesized in both studied solvents has the same cubicshape. However, comparing figures 1(b) and 1(c), it seems thatBZ2/DEF crystalline is thicker than BZ2/DMF, and we maysuppose that the porosity of the former will be higher.

3.1.2. MOFs from azobenzene-1,4-dicarboxylic acid and zincnitrate. The products of the reaction of zinc nitrate andH2ABD in DMF were coded as AZ1 and AZ2. CompoundAZ1 was formed at equivalent H2ABD : Zn mole ratio and0.010 M concentration of H2ABD (figure 2(a)). When thisconcentration was reduced to a very low value of 0.004 Mand H2ABD : Zn mole ratio was decreased to 2 : 5, the productAZ2 was obtained.

(b)

200 µm

(a)

200 µµm

Figure 2. Microscope images of crystalline materials synthesizedin DMF at 90 ◦C, [H2ABD] : Zn2+

= 1 : 1, [H2ABD] = 0.01 M(a, AZ1), at 85 ◦C, [H2ABD] : Zn2+

= 2 : 5, [H2ABD] = 0.004 M(b, AZ2).

3.2. Characterization

3.2.1. X-ray diffraction (XRD). BZ1 and BZ2 belonged totwo different crystalline structures. BZ2 had shape peaks(figure 3(a)) on the XRD diffractogram indicating that thehighly crystalline material was obtained. Furthermore, XRDpattern of BZ2 showed four major peaks at 2θ of 6.8,9.7, 13.7 and 15.4◦, which was in strong agreement withthose of MOF-5, as reported in the literature [18, 19].Hence, BZ2 was identified as MOF-5, while BZ1 was anunidentified compound. In figures 3(b) and 3(c), two verysharp peaks below 10◦ (at 2θ of 5.5) were observed onthe XRD diffractograms of the AZ1 and AZ2, exhibitingthat highly crystalline materials were achieved. The morepeaks on the XRD pattern of AZ1 showed that it was nota single-phase crystalline material. Neither AZ1 and AZ2have been identified yet since we are waiting for the structureparameters from single crystal x-ray diffraction analysis.

3.2.2. Thermo-gravimetric analysis (TGA). TGA curve ofBZ2, showing its thermal stability up to 500 ◦C. The materialexperienced two main weight changes. The first step slopeof mass loss (15%) around 200 ◦C was attributed to thevaporization of dichloromethane and DMF residing in thepore. The other weight loss accounted for the frameworkdecomposition of the organic linker (50%) between 460 ◦Cand 600 ◦C. Clearly, after being activated with a trap filledin liquid nitrogen under vacuum, the TGA curve exhibitedonly one main weight loss around 460 ◦C, meaning that guestmolecules were evacuated completely. However, due to thehumidity, the small mass loss was still observed on the BZ2activated curve (figure 4(a)). The TGA curve of AZ2 showed

(a)

(b)

((

(c)

Inte

nsity

10 15 20 25 30 5 2theta (o)

5 10 15 20 25 30 2theta (o)

Inte

nsity

Figure 3. XRD patterns of BZ2 (a), AZ2 (b) and AZ1 (c).

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025008 V H Nguyen et al

Figure 4. The TGA profile in a nitrogen atmosphere for BZ2 (a)and AZ2 (b).

a very similar pattern to that of BZ2 (figure 4(b)). The firstdeep slope of 40% weight loss around 100 ◦C was attributedto the removal of dichloromethane and DMF occluded inthe framework. The second step, ranging from 100 ◦C to470 ◦C, was assigned to the decomposition of the AZ2framework. The deeper mass loss slope of AZ2 as synthesizedas compared to BZ2 allowed us to deduce that AZ2 was moreporous than BZ2. This deduction was confirmed by nitrogenadsorption analysis in section 3.2.3.

3.2.3. Nitrogen adsorption. To confirm the permanentporosity, we obtained the N2 adsorption isotherm ofsynthesized materials using a gas sorption analyzer(Quantachrome Autosorb-1C system) after careful evacuationat 150–120 ◦C for 10 h. The N2 adsorption isotherms revealtypical type-I behavior of BZ2/DEF, AZ2 and BZ2/DMF, asplotted in figure 5.

Fitting the BET and Langmuir equations to the resultingisotherms, N2 gave the estimated highest BET and Langmuirspecific surface areas of 2414 and 4330 m2 g−1, respectively,for the BZ2/DEF compound. Then it was followed by AZ2and BZ2/DMF, with 3428 and 2631 m2 g−1 Langmuir surfacearea, respectively. AZ1 and BZ1 have the lowest specificsurface area, with SLang equal to 475.8 for the former and304.6 m2 g−1 for the latter (table 1). The results can also beseen clearly from figure 5, the isotherm slope of BZ2/DMFraised sharply as compared to BZ1, associated with dilutingthe H2BDC concentration. The slope of the BZ2 curve wasmuch steeper as replacing DMF with DEF. A similar patternwas found in the case of AZ1 and AZ2. Table 1 also indicates

Vol

ume

(ml/

g)

Relative pressure, P/Po

BZ2/DEF (MOF-5)

AZ2

BZ2/DMF (MOF-5)

BZ1

AZ1

200

400

600

800

1000

0.2 0.4 0.6 1.0 0.8

Figure 5. Adsorption-desorption isotherms of nitrogen oversynthesized materials after evacuation at 150 ◦C–200 ◦C for 10 h.

BZ2/DEF

BZ2/DMF

BZ1

AZ1

AZ2

6.0 18.0 30.0 42.0 64.0

0.05

0.15

0.25

0.35

0.45

0.55 DA method Dv (r)

Pore

vol

ume

(ml/Å

/g)

Pore Diameter (Å)

Figure 6. Pore size distribution.

that all of the synthesized compounds except for BZ1 showedtheir pore diameters in the microporous regime.

Figure 6 shows the pore volume and pore sizedistribution, the other important parameters that should betaken into consideration. It is also clear from figure 6 thatthe pore size distribution of BZ2/DEF exhibited a sharpestand highest peak, as a consequence of its highest surfacearea. Although AZ2 had a higher surface area than BZ2/DMF(table 1), its pore volume peak was lower than that ofBZ2/DMF (figure 6). This was due to the more dispersed poresize of AZ2, ranging from 12 to 37 Å, while the pore sizedistribution of BZ2/DMF is just from 12 to 25 Å. However, thetotal pore volume of AZ2 was higher than that of BZ2/DMF,with 1.16 and 0.89 cm3 g−1, respectively (table 1). The verysmall areas of peaks on pore size distribution patterns of BZ1and AZ1 indicated their lowest pore volume, and their surfaceareas as well.

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 025008 V H Nguyen et al

Table 1. Pore parameters calculated from the nitrogen adsorption data.

Sample BET SA Langmuir SA Pore volume Pore diameter (Å),(m2 g−1) (m2 g−1) (ml g−1) (DA method)

BZ1 (DMF) 201 304.6 – –BZ2/DMF (MOF-5) 1774 2631 0.89 17BZ2/DEF (MOF-5) 2414 4330 1.44 18AZ1 (DMF) 317.7 475.8 0.16 16.6AZ2 (DMF) 2320 3428 1.16 18.8

4. Conclusions

The solvothermal reactions of 1,4-benzenedicarboxylic acidor azobenzene-1,4-dicarboxylic acid with zinc nitrate tetrahydrate gave four crystalline materials coded BZ1, BZ2,AZ1 and AZ2. The structure and properties of the obtainedcrystals are dependent strongly on the synthesis conditions,especially on the reactant concentration. Polyhedral crystalsBZ1 were obtained in thick reactant concentration resulting inambiguous shape and low surface area. As this concentrationwas reduced, cubic-shaped crystals of BZ2 were achievedwith a Langmuir surface area increased by eight times.In the case of BZ2, the solvent polarity has importanteffects that should be considered carefully. When DEFinstead of DMF was used as the solvent, BZ2 porosity wasenhanced significantly (table 1). The structure determinationindicated that BZ2 was MOF-5, while the BZ1 structurewas unidentified. Encouraged by this result, herein weexpanded our work on the synthesis of new MOFsbased on azobenzene-4,4

′

-dicarboxylic acid, called AZ1 andAZ2. Similarly, AZ1 was synthesized at dense H2ABDconcentration while AZ2 resulted from diluting the reactionsolution. As a consequence, the surface area of AZ2 was eighttimes that of AZ1. The fully structural parameters of thesecompounds have not been determined yet. Of these, BZ2 andAZ2 were highly crystalline porous materials with thermalstability. BZ2/DEF could afford Langmuir surface areas ofup to 4330 m2 g−1, that is consistent with the literature [19].Remarkably, AZ2, a new compound interesting for furtherstudy, could reach a surface area of 3428 m2 g−1, much higherthan that of BZ2/DMF.

Acknowledgments

This work was supported by Grant B2009-18-03TD fromVietnam National University in Ho Chi Minh City. The

authors are grateful to Anh Phan, Giang Dao and the staffof MANAR-USA for their advice and contribution to thiswork.

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