Selective Sorption of Light Hydrocarbons on a Family of Metal

Mar 30, 2016 - Four isostructural pillared-layer metal–organic frameworks (as 1-ims series) are successfully synthesized via the charge-balance appr...
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Selective Sorption of Light Hydrocarbons on a Family of Metal− Organic Frameworks with Different Imidazolate Pillars Hong-Ru Fu and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35002, China S Supporting Information *

ABSTRACT: Four isostructural pillared-layer metal−organic frameworks (as 1-ims series) are successfully synthesized via the charge-balance approach, where trigonal [Zn2(CO2)3] cluster-based cationic layers [Zn2(NH2−BTB)]+ (NH2−H3BTB = 1,3,5-(3-benzoic acid) aniline) are connected by a series of 2-substituted imidazolates. The 1-ims supply a systematic platform to explore the adsorption of light hydrocarbons. Especially, 1-ims shows good adsorption selectivity of C3/C1 and C2/C1, and the selectivities of C3H8/CH4 are all over 100 at room temperature. These results demonstrate that introduction of functional group plays a key role on tuning the channel and pore size as well as improving the adsorption selectivity.

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INTRODUCTION In recent years, porous metal−organic frameworks (MOFs) have been emerging as a class of crystalline materials that is extensively attractive for potential applications in gas storage/ separation.1 Compared to the conventional porous materials, such as zeolites, the great advantages of MOFs lie in the pore dimension and tunable features (such as tunable pore geometries and adjustable surface functionality).2 An increasingly inviting area in which MOFs have shown excellent potential is adsorption and selective capture of light hydrocarbons.3 Nevertheless, to date, a systematic investigation into the effect of functional groups of isoreticular MOFs on the adsorption and separation of light hydrocarbons is still seldom. For the synthesis of functional materials with tailored properties, structure predictability has been proved to be an efficient strategy for the design of MOFs.4 Among the methods to construct porous MOFs, the concordance of functionalization of the organic linker and the pillar−layer architecture is an attractive approach to tune the properties of these materials, because the functionalization of the pillared linkers not only enhances the modification of the physicochemical properties of the framework without a network rearrangement but also generates the important effect on shape/size/enantioselective catalysis and the selective adsorption of guests.5 Here we report a systematic approach toward fine-tuning the pore and small hydrocarbons sorption behavior of the pillartype MOFs. The isostructural porous materials of [Zn2(NH2− BTB)(2R-im)] (denoted as 1-ims, NH2−H3BTB = 1,3,5-(3benzoic acid) aniline, 2R-im = 2-substituted imidazolate) exhibit high porosity. Moreover, the series of 1-ims show light hydrocarbon storage capacity as well as good C3H8/CH4, C2H6/CH4, C2H4/CH4, and C2H2/CH4 selective separation, © XXXX American Chemical Society

also illustrate structure−property relationship via a combination of functionalized linkers.

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EXPERIMENTAL SECTION

Materials and Instruments. The 1,3,5-(tribenzoic acid)aniline ligand was synthesized via Suzuki reaction.6 Powder X-ray diffraction (PXRD) patterns were measured on a MiniFlex-II diffractometer using Cu (λ = 1.541 78 Å) radiation. Thermogravimetric (TG) analysis was performed on Netzsch STA449C. Gas sorption experiments were performed with ASAP 2020 system. The materials maintain the crystallinity after the removal of solvent molecules. Elemental analyses were characterized by a Vario EL III elemental analyzer. Synthesis of 1-mim. A mixture of 2-methylimidazole (2-mim, 0.0155 g, 0.19 mmol), NH2−H3BTB (0.0245 g, 0.057 mmol), Zn(NO3)2·6H2O (0.0560 g, 0.2 mmol), tetramethylammonium bromide (0.0153 g, 0.10 mmol), and dimethylformamide (DMF)/ H2O (v/v = 3:0.5, 3.5 mL) was heated in a 20 mL scintillation vial at 100 °C for 24 h, followed by cooling to room temperature. The resulting colorless crystals were washed with DMF and water and dried in air. Anal. Calcd for [Zn2(NH2−BTB)][2-mim]·3DMF·7H2O: Calcd C, 47.43; H, 4.74; N, 8.30; found C, 47.98; H, 4.78; N, 8.81%. Synthesis of 1-eim. A mixture of 2-ethylimidazole (2-eim, 0.0120 g, 0.13 mmol), NH2−H3BTB (0.0195 g, 0.044 mmol), Zn(NO3)2· 6H2O (0.053g, 0.19 mmol), tetramethylammonium bromide (0.020g, 0.13 mmol), and DMF/EtOH/H2O (v/v/v = 4:4:1, 9 mL) was heated in a 20 mL scintillation vial at 100 °C for 24 h, followed by cooling to room temperature. The resulting colorless crystals were washed with DMF and water and dried in air. Anal. Calcd for [Zn2(NH2−BTB)][2eim]·3DMF·5H2O: Calcd C, 49.70; H, 4.65; N, 8.48; found C, 49.54; H, 4.60; N, 8.17%. Received: January 22, 2016

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DOI: 10.1021/acs.inorgchem.6b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Synthesis of 1-pim. A mixture of 2-propylimidazole (2-pim, 0.0150 g, 0.14 mmol), NH2−H3BTB (0.0150 g, 0.035 mmol), Zn(NO3)2·6H2O (0.0400 g, 0.14 mmol), tetramethylammonium bromide (0.0200 g, 0.13 mmol), and DMF/H2O (v/v = 3:0.5, 3.5 mL) was heated in a 20 mL scintillation vial at 100 °C for 24 h, followed by cooling to room temperature. The resulting colorless block crystals were washed with DMF and water and dried in air. Anal. Calcd for [Zn2(NH2−BTB)][2-pim]·3DMF·6H2O: Calcd C, 49.31; H, 4.89; N, 8.22; found C, 48.9; H, 4.73; N, 8.11%. Synthesis of 1-buim. A mixture of 2-buthylimidazole (2-buim, 0.0150 g, 0.12 mmol), NH2−H3BTB (0.0180 g, 0.042 mmol), Zn(NO3)2·6H2O (0.0560 g, 0.2 mmol), tetramethylammonium bromide (0.0085 g, 0.055 mmol), and DMF/EtOH/H2O (v/v/v = 4:2:1, 7 mL) was heated in a 20 mL scintillation vial at 100 °C for 24 h, followed by cooling to room temperature. The resulting crystals were washed with DMF and water and dried in air. Anal. Calcd for [Zn2(NH2−BTB)][2-buim]·3DMF·5H2O: Calcd C, 50.69; H, 4.91; N, 8.25; found C, 48.79; H, 4.68; N, 7.73%. X-ray Diffraction Analysis. Single-crystal X-ray structural measurement analyses were performed on a Agilent SuperNova Xray diffractometer with Cu Kα radiation (λCu−Kα = 1.541 78 Å) at 100 K. The structures were solved and refined by using the SHELX-97 program package.7 Part solvent molecules in the structure were disordered. The SQUEEZE program of PLATON was used to eliminate the contribution of these disordered guest molecules. Crystallographic data for four compounds were listed in Tables S1 and S2.

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RESULTS AND DISCUSSION Four pillared−layer MOFs were of the isostructural family [Zn2(NH2−BTB)(2R-im)], where R signifies the substituted group on the imidazole ligand. Figure 1 shows the related

Figure 2. (a) The pillared-layer framework of 1-mim along the b-axis (green polyhedra: [Zn2(CO2)3] units); (b) The framework of 1-buim along the c-axis.

Figure 1. Basic structure in 1-ims and four kinds of imidazolate linkers.

building units and the structures of all ligands. Here, we describe two representative structures, namely, 1-mim and 1buim, in detail. As shown in Figure 1, the asymmetric unit of 1mim contains a dinuclear zinc unit, one NH2−BTB, and one 2mim ligand. The dinuclear Zn unit is surrounded by six carboxylate groups from three NH2−BTB3− ligands, forming a tritopic paddle-wheel building unit. A triangle-planar Zn2(NH2−BTB) layer is formed by [Zn2(COO)3]+ paddlewheel units8 and NH2−BTB3− ligands. The μ2-mim acts as a pillar interconnecting the Zn2((NH2−BTB) layers in [100] direction, forming a three-dimensional neutral framework [Zn2(NH2−BTB)(2-mim)] (Figure 2). Because of the large size of NH2−BTB ligand, the pores are large enough for a second net, which interpenetrates the first one (Figure 3). Although the blockage of long side chain of the 2-butyl imidazole linker, the interpenetration also occurred in 1-buim (Figure S2). Therefore, the free solvent-accessible volume per

Figure 3. (a, b) View of the interpenetrated framework along a- or baxis; (c) view of the interpenetrated framework along c-axis, showing two kinds of windows.

unit cell volume of the isolated structure becomes gradually smaller (36.3% in 1-mim and 20.2% in 1-buim), estimated by using the PLATON program.9 The alkane groups of the imidazolate ligands are limited on the wall of the channels, B

DOI: 10.1021/acs.inorgchem.6b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry having no effect on the distribution of channels (Figure 3a,b). Thus, the size and shape of pores in these MOFs are very close to each other. The sizes of two kinds of windows along the caxis are 6.7 × 9.2 and 12.3 × 10.7 Å2, respectively (Figure 3c). The TG analyses of these compounds were investigated (Figure S12), and they exhibit similar weight loss steps. The first weight loss between 40 to ∼190 °C can be assigned to the loss of the solvent water and DMF molecules. The second weight loss is corresponding to the decomposition of the frameworks. There are obvious plateaus between two weight loss steps, and the frameworks do not collapse until near 500 °C, indicating these materials own excellent thermal stability. The TG analyses of activated frameworks were performed, the plateaus range from 40 to 500 °C show well that the initial guest molecules can be fully exchanged by methanol, and methanol molecules can be completely removed by the thermal/vacuum activation at 333 K. The results of the effective activation laid the foundation for gas sorption. To investigate the pore characterization and adsorption behavior of these materials, N2 physisorption measurements were performed at 77 K. Because of the microporosity feature of these MOFs, all of the compounds displayed typical type-I gas sorption isotherms (Figure 4). Their surface areas

The slight differences of the microporous environment may drastically affect adsorption behaviors. That prompted us to further explore their adsorptive properties of light hydrocarbons. 1-ims exhibit different adsorption capacities upon response to C3H8, C2H6, C2H4, C2H2, and CH4 at 273 and 297 K (Table 1 and Figure 5). In general, 1-ims have an adsorption

Figure 4. N2 sorption isotherms of 1-ims at 77 K.

Figure 5. (a) C2H2 sorption isotherms of 1-ims at 273 K; (b) C3H8 sorption isotherms of 1-ims at 273 K.

(Brunauer−Emmett−Teller model) are listed as follows: 1mim, 940.26 m2 g−1; 1-eim, 877.41 m2 g−1; 1-pim, 839.04 m2 g−1; 1-buim, 769.67 m2 g−1. Obviously, the steric bulk of the -R group reduced the pore space, decreasing the surface areas. The pore size of each compound is ∼10.2 Å calculated from Horvath−Kawazoe equation (Figure S4). Moreover, the adsorption of CO2 was also performed at 273 K and room temperature (Table 1). The results exhibit that the series of functionalized ligands do not affect CO2 adsorption significantly.

capacity with the following trend: C3 > C2 > C1. Moreover, the gas uptake capacities basically obey the order: 1-mim > 1-eim > 1-pim > 1-buim, in line with the order of surface areas and porous volumes of these frameworks (Table 1 and Figures S5− S9). The combination of pillared−layer frameworks and the substituents offers the advantage for a fine-tuning of such properties. To further understand the gas adsorption behaviors, isosteric heats of adsorption were calculated from the 273 and 297 K data, using the virial equation. A summary of Qst for different

Table 1. Uptake Values of Small Hydrocarbons for Four Compounds at 273 and 297 K and 1 bar Va (cm3 g CH4 1-mim 1-eim 1-pim 1-buim

14.65 19.32 16.24 14.08

C2H2 10.64 11.48 9.70 8.86

119.42 117.84 102.42 93.54

−1

) (273 K/297 K)

C2H4 76.26 73.70 65.00 56.14

92.37 87.30 84.54 73.16

C2H6 64.95 61.29 53.72 48.70 C

101.03 99.348 93.78 81.77

C3H8 79.91 75.38 71.65 63.00

102.92 97.36 97.31 83.46

CO2 96.87 86.60 83.29 75.22

66.84 70.25 62.24 54.38

37.82 37.50 42.30 29.40

DOI: 10.1021/acs.inorgchem.6b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

adsorption capacity is commonly CH4 < C2H4 < C2H6 < C2H2 < C3H8, making the feasibility of these MOFs for the separation of light hydrocarbons. Four compounds all display great separation ratio of C2H4/CH4 and C2H6/CH4 at 297 K. The separation ratio value of 1-mim or 1-pim is ∼12, higher than that of ZJU-48a (6),13 ZIF-8 (10),14 and Mg-MOF-74 (7)15 but lower than that of Fe-MOF-74 (18).15 Noteworthily, the selectivity of C3H8/CH4 for each compound reported here is over 100 at room temperature, much higher than those of UTSA-35a (80)16 and JLU-Liu18 (108.2).17 Such high selectivities reveal that these compounds have the great potential on separation of light hydrocarbons.

gases were listed in Table 2. Among these frameworks, the isosteric heat values of isolated MOF basically follow the trend: Table 2. Summary of the Enthalpies (Qst,n=0, kJ mol−1) of Gas Adsorption on Four Compounds CH4 C2H2 C2H4 C2H6 C3H8 CO2

1-mim

1-eim

1-pim

1-buim

16.15 16.13 15.5 19.51 21.39 18.08

16.58 18.13 19.78 21.12 27.89 19.51

14.62 15.15 19.00 22.76 28.41 22.04

14.99 17.55 20.94 22.95 27.97 21.17

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C3H8 > C2H6 > C2H4 > C2H2 > CH4. The increasing isosteric heats are proportional to the polarity of gas molecules.10 Moreover, it also can be attributed to the confined pore space generated by the substituded alkyl groups of the pore, which enhance the gas affinity of porous materials.11 However, no linear relationship of isosteric heats can be found among these compounds. The adsorption selectivities of C3/C1 and C2/C1 (equimolar binary mixtures) were calculated by the ideal adsorption solution theory (IAST)12 at 273 and 297 K (Figure 6 and Figure S12). The IAST calculations show that the hierarchy of

CONCLUSION

In summary, we designed a family of pillared−layer frameworks with Zn2(NH2−BTB) layers and different imidazole pillars. 1ims exhibit exceptionally high hydrocarbon uptake and show good adsorption selectivity of C3/C2 and C2/C1. Especially, the selectivities of C3H8/CH4 for 1-ims are over 100 at room temperature. These results reveal a fine-tuning approach for the precise design and application of porous MOFs.

Figure 6. IAST-predicted adsorption selectivity of C3/C1, C2/C1, and CO2/CH4 on 1-ims at 297 K (C3/C1 and C2/C1 equimolar binary mixtures). (a) 1-mim; (b) 1-eim; (c) 1-pim; (d) 1-buim. D

DOI: 10.1021/acs.inorgchem.6b00136 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00136. Additional figures, TGA, powder X-ray diffraction patterns, adsorption isotherms. (PDF) X-ray crystallographic information (CCDC Nos. 1440818−1440821). (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by 973 program (2012CB821705), and NSFC (21425102, 21521061). REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00136 Inorg. Chem. XXXX, XXX, XXX−XXX