Article pubs.acs.org/crystal
Doubly Interpenetrated Metal−Organic Framework for Highly Selective C2H2/CH4 and C2H2/CO2 Separation at Room Temperature Ling Zhang,† Chao Zou,§ Min Zhao,§ Ke Jiang,† Ruibiao Lin,‡ Yabing He,∥ Chuan-De Wu,*,§ Yuanjing Cui,† Banglin Chen,*,†,‡ and Guodong Qian*,† †
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science & Engineering and §Department of Chemistry, Zhejiang University, Hangzhou 310027, China ‡ Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States ∥ College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China S Supporting Information *
ABSTRACT: A doubly interpenetrated metal−organic framework [Cu 2L(DMF)(H2 O)]·DMF·H2 O (ZJU-199, ZJU = Zhejiang University; H3BTTA = benzene-1,3,5-triacrytic acid; DMF = N,N-dimethylformamide) has been successfully synthesized. The resulting ZJU-199 exhibits moderate C2H2 uptake of 128.0 cm3·g−1 and high selectivity for separation of C2H2/CH4 (27.3 to 33.5) and C2H2/CO2 (4.0 to 5.8) at room temperature.
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INTRODUCTION Acetylene (C2H2) is one of the most important raw chemicals in the petrochemical and electronic industries. However, some impurities like CO2 and CH4 degrade tremendously the significance of C2H2. So, it is essential to separate C2H2 from the mixture gas containing CO2 and CH4. Over the past few decades, more and more researchers have paid attention to metal−organic frameworks (MOFs),1−3 the hybrid porous materials of inorganic metal and organic ligands widely applied to catalysis,4,5 sensing,6,7 laser,8,9 drug delivery,10−12 and gas storage and separation.13−15 In fact, a large number of MOFs have been reported for methane and acetylene storage,16,17 carbon dioxide capture,18,19 as well as CO2/N220 and hydrocarbon separation.21−23 Among these examples of gas storage and separation, C2H2/ CO2 separation is a quite challenging task due to the molecule shapes and sizes. In fact, although a number of MOFs have been reported for C2H2/CH4 separation,24 far fewer examples of MOFs have been discovered for C2H2/CO2 separation.25,26 During our research on porous MOFs for gas storage and separation, we have developed a new organic linker of H3BTTA (H3BTTA = benzene-1,3,5-triacrylic acid, Scheme 1), an expanded H3BTC, and thus synthesized its first MOF ZJU199 [Cu2L(DMF)(H2O)]·DMF·H2O. Single X-ray crystallographic study shows that ZJU-199 has the expected tbo topology in which the common paddle-wheel Cu2(COO)4 secondary building units (SBUs) were linked with organic linker L3− to form a three-dimensional porous structure. Although this new organic linker L3− is larger than BTC3− and the other two similar linkers we had developed before,27 ZJU199 is a doubly interpenetrated MOF, so the pores within ZJU199 are significantly reduced compared with HKUST-1,28 and © XXXX American Chemical Society
Scheme 1. Organic Linker H3BTTA Used to Construct ZJU199
ZJU-35 and -36. Furthermore, the open metal site density of ZJU-199 is much higher than those in HKUST-1 and ZJU-35 and -36. These two unique features enable ZJU-199 to be a suitably porous material for gas separation. Herein we report the structure of ZJU-199, its permanent porosity, and its potential for the separation of C2H2/CH4 and C2H2/CO2 at room temperature.
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RESULTS AND DISCUSSION The organic linker, H3BTTA, was successfully synthesized by Heck cross-coupling reactions according to the reported work.27 Reactions of Cu(NO3)2·2.5H2O with H3BTTA in the solvothermal condition formed the transparent cyan block crystals, ZJU-199. The structure of the as-synthesized compound has already been determined by single-crystal Xray diffraction analysis, and the phase purity has been further Received: September 19, 2016 Revised: October 14, 2016
A
DOI: 10.1021/acs.cgd.6b01382 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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molecules can be readily removed to generate the open Cu2+ sites, established before, for gas sorption. The overall framework is a doubly interpenetrated one, existing as threedimensional intersecting channels of about 4.7 × 7.8 Å (Figure 1e), 8.0 × 8.4 Å (Figure 1f), and 4.0 × 8.4 Å (Figure 1g), respectively. In view of its crystal nature, we explored the permanent porosity of ZJU-199. The acetone-exchanged ZJU-199 was evacuated at 273 K for 2 days and at room temperature for 5 h under high vacuum to yield the activated ZJU-199a. The PXRD analysis reveals that the activated crystal material still retains its crystalline structure, and the peaks of ZJU-199 and ZJU-199a are mainly consistent with the simulated one (Figure S3). The permanent porosity of ZJU-199a was evaluated by N2-gas sorption at 77 K. Nitrogen sorption isotherm of ZJU-199a indicates typical Type-I sorption behavior, confirming its microporous nature, and the pore size distribution is quite consistent with the aforementioned crystal structure (Figure 2). The saturation N2 physisorption reaches 353.2 cm3·g−1, and the corresponding pore volume is 0.55 cm3·g−1. Thus, the Brunauer−Emmett−Teller (BET) and Langmuir surface areas were estimated to be 987 m2·g−1 and 1532 m2·g−1, respectively. We examined the ZJU-199a for the selective separation of C2H2/CO2 and C2H2/CH4. The low-pressure sorption measurements of C2H2, CO2, and CH4 were performed at 273 and 296 K, respectively (Figure 3). ZJU-199a systematically adsorbs much more C2H2 than CO2 and CH4. At 296 K and 1 atm, the C2H2 uptake amount of ZJU-199a is 128.0 cm3· g−1, which is much higher than those of CO2 (62.4 cm3·g−1) and CH4 (14.4 cm3·g−1) under the same conditions (Figure 3b). Similarly, at 273 K and 1 atm, the C2H2 uptake amount of ZJU-199a is 169.5 cm3·g−1, much higher than the amounts of CO2 (119.0 cm3·g−1) and CH4 (25.3 cm3·g−1) (Figure 3a). It is worth noting that the C2H2 uptake of ZJU-199a (128.0 cm3· g−1) is much higher than those of other MOFs for hydrocarbon or C2H2/CO2 separations (48.0 cm3·g−1 in ZJNU-61,31 72.5 cm3·g−1 in FJI-C4,32 80.0 cm3·g−1 in [Zn2(NH2−BTB)(2nim)],33 64 cm3·g−1 in {[Co6(μ3-OH)4(Ina)8](H2O)10(DMA)2}n,34 51.8 cm3·g−1 in ZJU-30a,24 and 70.1 cm3·g−1 in UTSA-68a35) (Table 1). As is well-known, Ideal Adsorbed Solution Theory (IAST) was carried out to estimate the adsorption selectivity for the binary C2H2/CO2 (50:50, v/v) and C2H2/CH4 (50:50, v/v). Figure 4a,b expresses the IAST calculations of C2H2/CO2 and C2H2/CH4 adsorption selectivity for ZJU-199a at 273 and 296 K, respectively. At 296 K, the C2H2/CH4 selectivity of ZJU-
confirmed by powder X-ray diffraction (PXRD) and elemental analysis (EA). Single-crystal X-ray diffraction analysis indicates that ZJU199 crystallizes in the R3c̅ space group, the same as the wellknown PCN-14,29 and different from the common NbO-type frameworks, which all crystallize in the space group R3̅m.30 The underlying topology of ZJU-199 is a (4,3)-coordinated network of tbo framework, composed of paddle-wheel dinuclear Cu2(COO)4 SBUs (Figure 1a) as the site of four- coordination,
Figure 1. X-ray single crystal structure of ZJU-199 indicating that (a) each tricarboxylate ligand connects with three paddle-wheel Cu2(COO)4 clusters to compose the independent framework and the resulting twofold interpenetrated framework; (b) the cages of the independent framework; (c) and (d) the resulting frameworks without the consideration of the framework interpenetration; the pore channels of the doubly interpenetrated framework (e) of about 4.7 Å × 7.8 Å; (f) of about 8.0 Å × 8.4 Å; and (g) of about 4.0 Å × 8.4 Å in diameter viewed from different directions (C, gray and violet; O, red; Cu, blue; H atoms are omitted for the clarity).
and with H3BTTA linkers as the site of three-coordination, thus forming a doubly interpenetrated three-dimensional (3D) framework. The independent framework of ZJU-199 is analogous to HKUST-1 and ZJU-35 and -36, containing three distinct cuboctahedral cages (Figure 1b). The diameters of these types of cages are around 17.6 Å, 16.0 Å, and 7.8 Å, respectively. These three types of cages are interconnected at windows with a diameter of approximately 8.4 and 7.2 Å (Figure 1c and d), respectively. There are numbers of water and DMF molecules coordinated with Cu atoms, directly located in the axial positions. It is expected that the axial solvent
Figure 2. (a) N2 sorption isotherms of ZJU-199a at 77 K. Closed symbols, adsorption; open symbols, desorption. (b) Pore size distribution of ZJU199a. B
DOI: 10.1021/acs.cgd.6b01382 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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5.8, slightly higher than UTSA-68a (3.5 to 5) and significantly higher than {[Co6(μ3-OH)4(Ina)8](H2O)10(DMA)2}n (3.4) and ZJU-30a (1.7 to 2.4), featuring ZJU-199a as the potential candidate for C2H2/CO2 and C2H2/CH4 separation (Figure 4b). In addition, the selectivity of ZJU-199a lies in the ranges of 40.0 to 62.7 for C2H2/CH4 and 4.0 to 8.8 for C2H2/CO2 at 273 K, respectively, which are quite high as well (Figure 4a). Except for the high selectivity of C2H2/CH4 and C2H2/CO2 separation, the necessary stability of ZJU-199 still needs to be verified. Thermogravimetric analysis (TGA) demonstrated the satisfactory thermostability of ZJU-199, with no decomposition of framework occurring up to at least 240 °C (Figure S1), suitable enough for application. As shown in Figure 4c, the heats of adsorption of C2H2, CO2, and CH4 in ZJU-199a are 38.5 kJ mol−1, 29.0 kJ mol−1, and 19 kJ mol−1, respectively. Apparently, the interaction between C2H2 and the frameworks is stronger than that of CO2 and CH4, indicating that both open metal sites and suitable pore sizes play important roles for the high selectivity separation of C2H2 over CO2 and CH4 at room temperature.
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CONCLUSIONS In summary, we have designed and synthesized a new organic linker and thus a novel doubly interpenetrated metal−organic framework, ZJU-199. With high density of open metal sites and suitable pore sizes, the activated ZJU-199a takes a much higher amount of C2H2 uptake than those of CO2 and CH4, exhibiting highly selective separation for C2H2/CO2 and C2H2/CH4 at room temperature. Although a large number of organic linkers have been explored, the rich organic chemistry might still provide us with other useful organic linkers to construct porous metal−organic frameworks for their diverse applications in the near future.
Figure 3. Single-component adsorption isotherms for C2H2 (violet), CO2 (orange), and CH4 (blue) of ZJU-199a at 273 K (a) and 296 K (b), respectively.
Table 1. Comparison of Adsorption Data
MOFs ZJNU-61 FJI-C4 [Zn2(NH2−BTB) (2-nim)] {[Co6(μ3OH)4(Ina)8] (H2O)10(DMA)2}n ZJU-30 UTSA-68 ZJU-199
surface area (m2/g, BET)
C2H2 uptake (at 1.0 bar, RT, mmol/g)
selectivity for C2H2/ CH4
1059 690 893
48.0 72.5 80.0
9.9 51.0 5.0−18.0
selectivity for C2H2/ CO4
ASSOCIATED CONTENT
S Supporting Information *
631
64
9.6
3.4
228 897 987
52.6 70.1 128.0
9.58
1.7−2.4 3.5−5 4.0−5.8
27.3−33.5
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01382. Details of measurements and analyses, syntheses of the organic linker H3BTTA and ZJU-199, gas sorption measurements, TGA, IR spectrum, PXRD, Langmuir− Freundlich fit, Virial fit parameters (PDF) Accession Codes
199a lies in the range of 27.3 to 33.5, lower than that of FJI-C4 (51.0) but significantly higher than that of ZJNU-61 (9.9), {[Co6(μ3-OH)4(Ina)8](H2O)10(DMA)2}n (9.6), [Zn2(NH2− BTB)(2-nim)] (5.0 to 18.0), and ZJU-30a (9.58); and the C2H2/CO2 selectivity of ZJU-199a lies in the range of 4.0 to
CCDC 1457428 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge
Figure 4. (a) IAST calculations of C2H2/CO2 and C2H2/CH4 adsorption selectivity for ZJU-199a at 273 K. (b) IAST calculations of C2H2/CO2 and C2H2/CH4 adsorption selectivity for ZJU-199a at 296 K. (c) Isosteric heats of C2H2, CO2, and CH4 adsorption, Qst, in ZJU-199a. C
DOI: 10.1021/acs.cgd.6b01382 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51472217, 51432001, 21373180, and 21525312), the Zhejiang Provincial Natural Science Foundation of China (No. LR13E020001 and LZ15E020001), and Fundamental Research Funds for the Central Universities (No. 2015QNA4009 and 2016FZA4007), and partly supported by Welch Foundation (AX-1730).
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DOI: 10.1021/acs.cgd.6b01382 Cryst. Growth Des. XXXX, XXX, XXX−XXX
