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A new tetrazolate zeolite-like framework for highly selective CO2/CH4 and CO2/N2 separation† Shunshun Xiong,abc Youjin Gong,a Hongxia Wang,a Hailong Wang,b Qiang Liu,a Mei Gu,a Xiaolin Wang,*a Banglin Chen*bd and Zhiyong Wangc
DOI: 10.1039/c4cc05729e www.rsc.org/chemcomm
A new tetrazolate zeolite-like framework with a diamond topology, UTSA-49, was synthesized. UTSA-49 shows high selectivity for CO2/ CH4 and CO2/N2 indicating a synergistic effect of the suitable pore size/shape and functional groups.
Selective carbon dioxide capture from natural gas and fuel gas has attracted great interest and has become a critical issue because of environmental concerns over the climate change and demands in natural gas purification.1 The emerging porous metal–organic frameworks (MOFs) have shown promise as cost-effective and efficient materials for selective CO2 capture and separation due to their high thermal stability, high surface areas, and controllable pore structures.2 In order to enhance CO2 adsorption capacity and selectivity, various efforts have been made including controlling the pore size and shape,1,3 generating high density open metal sites or donor sites,4 immobilizing functional groups (–NH2, –NO2, –OH, –Me, etc.) in their pore surfaces,5,6 and increasing the field potential of the pores.7 Moreover, combining multiple functional groups into a single MOF may improve CO2 uptake and selectivity more effectively.8 While the investigation on the impact of the above-mentioned individual factors on CO2 uptake and adsorption selectivity of MOFs may be intriguing, these factors could function cooperatively and be beneficial to enhance the CO2 uptake and selectivity. And it is important to study the synergistic effect of
a
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, P. R. China. E-mail: [email protected] b Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-069, USA. E-mail: [email protected]; Fax: +1-210-458-7428 c Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key laboratory of soft Matter Chemistry and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China d Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia † Electronic supplementary information (ESI) available: Experimental details, crystallographic data, structure representation, PXRD spectra, TGA diagrams, and calculation details of IAST selectivity. CCDC 988248. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc05729e
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different factors such as the pore size/shape and functional groups. Zeolitic and zeolite-like metal imidazolate frameworks (ZIFs), as special types of MOFs, have received considerable attention because of their potential applications in CO2 capture and separation.9 At the same time, due to their strong M–N bonds, ZIFs show better chemical and thermal stabilities in contrast with most carboxylatebased MOFs, providing the potential for industrial application. Nevertheless, most ZIFs exhibit weak adsorption and separation ability for CO2 compared with other types of MOFs, because of the lack of open metal sites and other active sites in their pore surface.10 By employing some triazolate, tetrazolate ligands instead of the imidazole ligand, the functionalized ZIFs containing uncoordinated N donors in their pore surface exhibit much higher CO2 capture and selectivity than un-functionalized ones.10–12 Banerjee et al. synthesized a zeolitic tetralate framework (ZTF-1) with a dia topology and high CO2 uptake capacity using 5-aminotetrazole.13 Chen et al. used 3-amino-1,2,4-triazole to construct a zeolitic triazolate framework (MAF-66) with a dia topology and high CO2 adsorption capacity.12 However, reports on experimentally systematic investigation of the synergistic effect of the pore size and multiple functional groups on CO2 uptake and selectivity in porous MOFs with similar structures have been rare.3–8 Recently, Zaworotko et al. reported a crystal engineering strategy that can enhance CO2 uptake and adsorption selectivity by controlling the pore functionality and the pore size.14 Herein, we report a new tetrazolate zeolite-like framework [Zn(mtz)2] (UTSA-49, UTSA = University of Texas at San Antonio; Hmtz = 5-methyl-1H-tetrazole) incorporated with uncoordinated N atoms and methyl functional groups for high CO2 uptake capacity and highly selective CO2/CH4 and CO2/N2 separation at room temperature. UTSA-49 has a dia topology network and possesses similar pore structures with two other triazolate/tetrazolate zeolite-like frameworks, MAF-66 and ZTF-1, but with different multiple functional groups and subtle differences in pore sizes/ shapes. The investigation and systematic comparison of the synergistic effect of the suitable pore size/shape and functional groups on CO2 uptake and separation have also been carried out.
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A single colorless block crystal of UTSA-49 was obtained via a solvothermal reaction of Hmtz and Zn(NO3)26H2O in DMF– EtOH (5 : 1) solution at 100 1C for 24 hours. Single crystal X-ray analysis reveals that UTSA-49 crystallizes in the monoclinic space group Pc. The asymmetric unit of UTSA-49 contains four Zn centers and eight mtz ligands. The Zn center is coordinated to four nitrogen atoms from four mtz ligands to accomplish a distorted tetrahedral geometry [Zn–N 1.947–2.048 Å; N–Zn–N 100.77–119.881]. Only the neighbouring two nitrogen atoms of methyl on the mtz ring bind with the zinc center, which leaves the other nitrogen atoms uncoordinated and exposed on the pore surface. The adjacent Zn Zn distance in UTSA-49 ranges from 6.068 to 6.117 Å, which is slightly longer than those in reported zinc imidazolates (ca. 6.0 Å). While the Zn-mtz-Zn coordination angle ranges from 146.31 to 155.91, which is larger than those of other reported ZIFs or ZTFs (zeolitic tetrazolate frameworks) (from 1351 to 1491).15 This could be attributed to the steric effect of methyl on mtz ligands. Regarding the Zn atoms as 4-connected nodes and the mtz ligands as linkers, the topology of UTSA-49 can be simplified as dia. The dia-type framework of UTSA-49 is same as that of MAF-66 and ZTF-1. The large and wide variation of the Zn-mtz-Zn angle for UTSA-49 may be the main reason for the formation of the dia topology with Hmtz rather than the sod topology.13 In the network, 10 Zn2+ ions and 12 mtz ligands assemble to a adamantine cage which is constructed by four chair-type 6-membered Zn-mtz-Zn rings. Note that, methyl groups in UTSA-49 exhibit more stronger steric effects and could not form the weak hydrogen bond with uncoordinated tetrazolate nitrogen atoms compared with amino-groups in MAF-66 or ZTF-1 (Fig. 1).12,13 As shown in Fig. 2, the adamantine of UTSA-49 has two kinds of 6-membered ring windows with the effective aperture sizes of 2.9 Å 3.6 Å for windows-1 and 3.6 Å 4.0 Å for windows-2, respectively (considering the van der Waals radii of constituting atoms). These values are smaller than those of MAF-66 (4.3 Å 5.2 Å) and ZTF-1 (5.0 Å 5.1 Å) (as shown in the ESI,† Fig. S3). UTSA-49 possesses a B4.3 Å diameter cavity which is also smaller than those of MAF-66 or ZTF-1. The void volume in UTSA-49 is 50.6% calculated by the PLATON software.
Fig. 1 (a) The Zn-mtz-Zn bridging angle in the UTSA-49 ligand. (153.41 r y r 154.91) (b) the adamantine cage (constructed by 10 Zn2+ ions and 12 mtz ligands) in UTSA-49 and the dia topology network of UTSA-49 (c) the perspective view of the 3D network of UTSA-49 (Zn, purple; C, gray; N, blue).
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Fig. 2 The ball-and-stick model (left) and the space-filling model (right) showing the two different 6-membered ring windows: windows-1 (a) and windows-2 (b) in the adamantine cage of UTSA-49. The green lines represent the effective aperture size.
Thermal gravimetric analysis (TGA) of UTSA-49 showed that approximately 28.5% weight loss occurred from 31 to 210 1C, which is attributed to the release of solvent molecules. There was no weight loss and a platform from 210 to 290 1C could be seen before decomposition, which indicates that UTSA-49 has good thermal stability. Methanol-exchanged UTSA-49 was activated at room temperature under high vacuum to obtain the activated sample UTSA-49a. The N2 sorption isotherm of UTSA-49a at 77 K indicates a typical type I sorption behavior, characteristic of microporous materials with a saturated sorption amount of N2 of 263.4 cm3 g 1 (STP), corresponding to a pore volume of 0.407 cm3 g 1, which is similar to that of MAF-66. The BET and Langmuir surface areas were calculated to be 710.5 m2 g 1 and 1046.6 m2 g 1, respectively. These values are slightly lower than those calculated for MAF-66 (SBET = 1014 m2 g 1, SLangmuir = 1196 m2 g 1) and much higher than those for ZTF-1 (SBET = 355.3 m2 g 1, SLangmuir = 443.8 m2 g 1). The establishment of the permanent porosity encouraged us to examine its potential application in separating CO2 over CH4 and N2. The CO2 low-pressure adsorption–desorption isotherms were measured at 298 K and 273 K. As shown in Fig. 3a, the CO2 uptake is 69.0 cm3 g 1 (STP, standard temperature and pressure; 13.6 wt%, 78 v/v) at 298 K. At 273 K, the CO2 uptake amount is 108.5 cm3 g 1 (STP, standard temperature and pressure; 21.3 wt%, 123 v/v) (as shown in the ESI,† Fig. S4). Although UTSA-49 has lower CO2 uptake (21.3 wt% at 273 K) than ZTF-1 (23.5 wt% at 273 K) and MAF-66 (27.6 wt% at 273 K), the CO2 uptake of UTSA-49 outperforms other ZIFs.9,15 This can be attributed to the lower pore volume and narrower pore channels compared with ZTF-1 and MAF-66. The N2 uptake capacities are 2.5 cm3 g 1 (STP) at 298 K and 3.8 cm3 g 1 (STP) at 273 K, which compare well with those of
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Fig. 3 (a) Adsorption (solid) and desorption (open) isotherms of carbon dioxide (red circles), methane (blue squares), and nitrogen (green triangles) on UTSA-49a at 298 K. (b) Mixture adsorption isotherms and adsorption selectivity predicted by IAST of UTSA-49a for CO2 (50%) and CH4 (50%) at 298 K. (c) Mixture adsorption isotherms predicted by IAST of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85, and 20 : 80) at 298 K. (d) Mixture adsorption selectivity predicted by IAST of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85, and 20 : 80) at 298 K.
ZTF-1 and MAF-66. The CH4 uptake capacities of UTSA-49a are 8.0 cm3 g 1 (STP) at 298 K and 12.8 cm3 g 1 (STP) at 273 K, which are significantly lower than those of MAF-66 (29.4 cm3 g 1 at 298 K and 50.1 cm3 g 1 at 273 K). This can be due to the weak interaction between CH4 molecules and uncoordinated N atoms or methyl groups and enhanced size-exclusive effects caused by the narrow pore size/shape. The coverage-dependent adsorption enthalpies for CO2 and CH4 were calculated by using the Clausius–Clapeyron equation. The isosteric enthalpies of adsorption (Qst,n=0) for CO2 at the zero surface coverage is 27.0 kJ mol 1. The fact that CO2 enthalpy of UTSA-49a is comparable to the values of MAF-66 (26.0 kJ mol 1) and ZTF-1 (25.4 kJ mol 1) indicates that the uncoordinated N atoms and methyl groups exposed in the pores and the narrow pore size can also enhance the interactions with CO2 gas molecules effectively. It should be noted that CH4 enthalpy of UTSA-49a at the zero coverage is 15.5 kJ mol 1 which is significantly lower than the value of MAF-66 (19.5 kJ mol 1) and decreases obviously when the uptake amount increases, which indicates the weak interaction between CH4 molecules and the narrow pore surfaces. The above-mentioned results indicate that UTSA-49a can selectively adsorb CO2 over CH4 and N2. In order to predict CO2–CH4 and CO2–N2 binary mixture selectivity, an ideal adsorbed solution theory (IAST) calculation based on a dual-site Langmuir– Freundlich (DSLF) simulation was employed on the basis of the single-component CO2, CH4, and N2 adsorption isotherms (see details in the ESI,† Table S5). The accuracy of IAST for prediction of gas mixture adsorption in a large number of zeolites and MOFs has been well established.16 Fig. 3b shows the predicted mixture adsorption isotherms and adsorption selectivity of UTSA-49a for CO2 (50%) and CH4 (50%) at 298 K. The CO2/CH4 selectivity values of UTSA-49a are estimated to be 33.7 at 298 K and 34.8 at 273 K,
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respectively, which are significantly higher than the values of MAF-66 (the Henry’s law CO2/CH4 selectivity of MAF-66 is 5.8 at 298 K and 7.5 at 273 K). These values are also higher than those reported for other ZIFs and are comparable to those of MOFs with highest selectivity reported in the literature. Fig. 3d shows the predicted mixture adsorption selectivity of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85 and 20 : 80) at 298 K. The CO2/N2 selectivity values are estimated to be 90.5, 93.5 and 95.8 at 298 K and 188.8, 193.7 and 197.7 at 273 K for three different compositions (CO2/N2 = 10 : 90, 15 : 85, 20 : 80), respectively. Although these values are lower than those of MAF-66 (225 at 298 K and 403 at 273 K), they are higher than those reported for other ZIFs and are still considered very high selectivity values in MOFs. The high values of CO2 selectivity over CH4 and N2 could be due to the synergistic effect of multiple functional groups and the pore size/shape. The uncoordinated tetrazolate nitrogen atoms and methyl groups can be introduced by the Hmtz ligand into the framework of UTSA-49 simultaneously. The uncoordinated N atoms from N-rich aromatic rings have a positive effect on the adsorption of CO2 by facilitating dipole–quadrupole interactions between uncoordinated tetrazolate N atoms and CO2 molecules, which has been demonstrated by IFMC-1.11a The methyl groups may not be beneficial to further enhance the interaction with CO2 molecules as amino groups in MAF-66 or ZTF-1 do, but they adjust the pore structure delicately and make the pore size or shape narrower. Within such small dimensions, the slight variation introduced by methyl groups in the pore size or shape can cause a dramatic change in the adsorption affinity for CH4. Furthermore, its more hydrophobic pore nature enforces its framework stability under humid environments, which is favorable for its potential application in CO2 capture and separation. In summary, we have synthesized a new microporous tetrazolate zeolite-like framework with uncoordinated tetrazolate N atoms and functionalized methyl groups on the pore surfaces. The uncoordinated tetrazolate N atoms induce strong interaction with CO2 and the functionalized methyl groups modify the pore size/shape reducing the affinity with CH4, which demonstrate the potential of UTSA-49 to be a promising candidate material for CO2 capture and separation from natural gas and fuel gas. The introduction of multiple functional groups into a single MOF to produce a synergistic effect to improve interaction with gas molecules by immobilizing functional groups and reinforcing the size-exclusive effect by pore size/shape control simultaneously may be a promising strategy to enhance gas adsorption capacity and separation performance of MOFs. This work was financially supported by the Science and Technology Development of China Academy of Engineering Physics (No. 2014B0301034) and the Welch Foundation (AX-1730).
Notes and references 1 (a) S. Xiang, Y. He, Z. Zhang, H. Wu, W. Zhou, R. Krishna and B. Chen, Nat. Commun., 2012, 3, 954; (b) J. R. Li, J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869–932; (c) Z. Zhang, Y. Zhao, Q. Gong and J. Li, Chem. Commun., 2013, 49, 653–661. 2 (a) D. M. D’Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 6058–6082; (b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long,
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A new tetrazolate zeolite-like framework for highly selective CO2/CH4 and CO2/N2 separation† Shunshun Xiong,abc Youjin Gong,a Hongxia Wang,a Hailong Wang,b Qiang Liu,a Mei Gu,a Xiaolin Wang,*a Banglin Chen*bd and Zhiyong Wangc
DOI: 10.1039/c4cc05729e www.rsc.org/chemcomm
A new tetrazolate zeolite-like framework with a diamond topology, UTSA-49, was synthesized. UTSA-49 shows high selectivity for CO2/ CH4 and CO2/N2 indicating a synergistic effect of the suitable pore size/shape and functional groups.
Selective carbon dioxide capture from natural gas and fuel gas has attracted great interest and has become a critical issue because of environmental concerns over the climate change and demands in natural gas purification.1 The emerging porous metal–organic frameworks (MOFs) have shown promise as cost-effective and efficient materials for selective CO2 capture and separation due to their high thermal stability, high surface areas, and controllable pore structures.2 In order to enhance CO2 adsorption capacity and selectivity, various efforts have been made including controlling the pore size and shape,1,3 generating high density open metal sites or donor sites,4 immobilizing functional groups (–NH2, –NO2, –OH, –Me, etc.) in their pore surfaces,5,6 and increasing the field potential of the pores.7 Moreover, combining multiple functional groups into a single MOF may improve CO2 uptake and selectivity more effectively.8 While the investigation on the impact of the above-mentioned individual factors on CO2 uptake and adsorption selectivity of MOFs may be intriguing, these factors could function cooperatively and be beneficial to enhance the CO2 uptake and selectivity. And it is important to study the synergistic effect of
a
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, P. R. China. E-mail: [email protected] b Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-069, USA. E-mail: [email protected]; Fax: +1-210-458-7428 c Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key laboratory of soft Matter Chemistry and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China d Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia † Electronic supplementary information (ESI) available: Experimental details, crystallographic data, structure representation, PXRD spectra, TGA diagrams, and calculation details of IAST selectivity. CCDC 988248. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc05729e
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different factors such as the pore size/shape and functional groups. Zeolitic and zeolite-like metal imidazolate frameworks (ZIFs), as special types of MOFs, have received considerable attention because of their potential applications in CO2 capture and separation.9 At the same time, due to their strong M–N bonds, ZIFs show better chemical and thermal stabilities in contrast with most carboxylatebased MOFs, providing the potential for industrial application. Nevertheless, most ZIFs exhibit weak adsorption and separation ability for CO2 compared with other types of MOFs, because of the lack of open metal sites and other active sites in their pore surface.10 By employing some triazolate, tetrazolate ligands instead of the imidazole ligand, the functionalized ZIFs containing uncoordinated N donors in their pore surface exhibit much higher CO2 capture and selectivity than un-functionalized ones.10–12 Banerjee et al. synthesized a zeolitic tetralate framework (ZTF-1) with a dia topology and high CO2 uptake capacity using 5-aminotetrazole.13 Chen et al. used 3-amino-1,2,4-triazole to construct a zeolitic triazolate framework (MAF-66) with a dia topology and high CO2 adsorption capacity.12 However, reports on experimentally systematic investigation of the synergistic effect of the pore size and multiple functional groups on CO2 uptake and selectivity in porous MOFs with similar structures have been rare.3–8 Recently, Zaworotko et al. reported a crystal engineering strategy that can enhance CO2 uptake and adsorption selectivity by controlling the pore functionality and the pore size.14 Herein, we report a new tetrazolate zeolite-like framework [Zn(mtz)2] (UTSA-49, UTSA = University of Texas at San Antonio; Hmtz = 5-methyl-1H-tetrazole) incorporated with uncoordinated N atoms and methyl functional groups for high CO2 uptake capacity and highly selective CO2/CH4 and CO2/N2 separation at room temperature. UTSA-49 has a dia topology network and possesses similar pore structures with two other triazolate/tetrazolate zeolite-like frameworks, MAF-66 and ZTF-1, but with different multiple functional groups and subtle differences in pore sizes/ shapes. The investigation and systematic comparison of the synergistic effect of the suitable pore size/shape and functional groups on CO2 uptake and separation have also been carried out.
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A single colorless block crystal of UTSA-49 was obtained via a solvothermal reaction of Hmtz and Zn(NO3)26H2O in DMF– EtOH (5 : 1) solution at 100 1C for 24 hours. Single crystal X-ray analysis reveals that UTSA-49 crystallizes in the monoclinic space group Pc. The asymmetric unit of UTSA-49 contains four Zn centers and eight mtz ligands. The Zn center is coordinated to four nitrogen atoms from four mtz ligands to accomplish a distorted tetrahedral geometry [Zn–N 1.947–2.048 Å; N–Zn–N 100.77–119.881]. Only the neighbouring two nitrogen atoms of methyl on the mtz ring bind with the zinc center, which leaves the other nitrogen atoms uncoordinated and exposed on the pore surface. The adjacent Zn Zn distance in UTSA-49 ranges from 6.068 to 6.117 Å, which is slightly longer than those in reported zinc imidazolates (ca. 6.0 Å). While the Zn-mtz-Zn coordination angle ranges from 146.31 to 155.91, which is larger than those of other reported ZIFs or ZTFs (zeolitic tetrazolate frameworks) (from 1351 to 1491).15 This could be attributed to the steric effect of methyl on mtz ligands. Regarding the Zn atoms as 4-connected nodes and the mtz ligands as linkers, the topology of UTSA-49 can be simplified as dia. The dia-type framework of UTSA-49 is same as that of MAF-66 and ZTF-1. The large and wide variation of the Zn-mtz-Zn angle for UTSA-49 may be the main reason for the formation of the dia topology with Hmtz rather than the sod topology.13 In the network, 10 Zn2+ ions and 12 mtz ligands assemble to a adamantine cage which is constructed by four chair-type 6-membered Zn-mtz-Zn rings. Note that, methyl groups in UTSA-49 exhibit more stronger steric effects and could not form the weak hydrogen bond with uncoordinated tetrazolate nitrogen atoms compared with amino-groups in MAF-66 or ZTF-1 (Fig. 1).12,13 As shown in Fig. 2, the adamantine of UTSA-49 has two kinds of 6-membered ring windows with the effective aperture sizes of 2.9 Å 3.6 Å for windows-1 and 3.6 Å 4.0 Å for windows-2, respectively (considering the van der Waals radii of constituting atoms). These values are smaller than those of MAF-66 (4.3 Å 5.2 Å) and ZTF-1 (5.0 Å 5.1 Å) (as shown in the ESI,† Fig. S3). UTSA-49 possesses a B4.3 Å diameter cavity which is also smaller than those of MAF-66 or ZTF-1. The void volume in UTSA-49 is 50.6% calculated by the PLATON software.
Fig. 1 (a) The Zn-mtz-Zn bridging angle in the UTSA-49 ligand. (153.41 r y r 154.91) (b) the adamantine cage (constructed by 10 Zn2+ ions and 12 mtz ligands) in UTSA-49 and the dia topology network of UTSA-49 (c) the perspective view of the 3D network of UTSA-49 (Zn, purple; C, gray; N, blue).
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Fig. 2 The ball-and-stick model (left) and the space-filling model (right) showing the two different 6-membered ring windows: windows-1 (a) and windows-2 (b) in the adamantine cage of UTSA-49. The green lines represent the effective aperture size.
Thermal gravimetric analysis (TGA) of UTSA-49 showed that approximately 28.5% weight loss occurred from 31 to 210 1C, which is attributed to the release of solvent molecules. There was no weight loss and a platform from 210 to 290 1C could be seen before decomposition, which indicates that UTSA-49 has good thermal stability. Methanol-exchanged UTSA-49 was activated at room temperature under high vacuum to obtain the activated sample UTSA-49a. The N2 sorption isotherm of UTSA-49a at 77 K indicates a typical type I sorption behavior, characteristic of microporous materials with a saturated sorption amount of N2 of 263.4 cm3 g 1 (STP), corresponding to a pore volume of 0.407 cm3 g 1, which is similar to that of MAF-66. The BET and Langmuir surface areas were calculated to be 710.5 m2 g 1 and 1046.6 m2 g 1, respectively. These values are slightly lower than those calculated for MAF-66 (SBET = 1014 m2 g 1, SLangmuir = 1196 m2 g 1) and much higher than those for ZTF-1 (SBET = 355.3 m2 g 1, SLangmuir = 443.8 m2 g 1). The establishment of the permanent porosity encouraged us to examine its potential application in separating CO2 over CH4 and N2. The CO2 low-pressure adsorption–desorption isotherms were measured at 298 K and 273 K. As shown in Fig. 3a, the CO2 uptake is 69.0 cm3 g 1 (STP, standard temperature and pressure; 13.6 wt%, 78 v/v) at 298 K. At 273 K, the CO2 uptake amount is 108.5 cm3 g 1 (STP, standard temperature and pressure; 21.3 wt%, 123 v/v) (as shown in the ESI,† Fig. S4). Although UTSA-49 has lower CO2 uptake (21.3 wt% at 273 K) than ZTF-1 (23.5 wt% at 273 K) and MAF-66 (27.6 wt% at 273 K), the CO2 uptake of UTSA-49 outperforms other ZIFs.9,15 This can be attributed to the lower pore volume and narrower pore channels compared with ZTF-1 and MAF-66. The N2 uptake capacities are 2.5 cm3 g 1 (STP) at 298 K and 3.8 cm3 g 1 (STP) at 273 K, which compare well with those of
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Fig. 3 (a) Adsorption (solid) and desorption (open) isotherms of carbon dioxide (red circles), methane (blue squares), and nitrogen (green triangles) on UTSA-49a at 298 K. (b) Mixture adsorption isotherms and adsorption selectivity predicted by IAST of UTSA-49a for CO2 (50%) and CH4 (50%) at 298 K. (c) Mixture adsorption isotherms predicted by IAST of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85, and 20 : 80) at 298 K. (d) Mixture adsorption selectivity predicted by IAST of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85, and 20 : 80) at 298 K.
ZTF-1 and MAF-66. The CH4 uptake capacities of UTSA-49a are 8.0 cm3 g 1 (STP) at 298 K and 12.8 cm3 g 1 (STP) at 273 K, which are significantly lower than those of MAF-66 (29.4 cm3 g 1 at 298 K and 50.1 cm3 g 1 at 273 K). This can be due to the weak interaction between CH4 molecules and uncoordinated N atoms or methyl groups and enhanced size-exclusive effects caused by the narrow pore size/shape. The coverage-dependent adsorption enthalpies for CO2 and CH4 were calculated by using the Clausius–Clapeyron equation. The isosteric enthalpies of adsorption (Qst,n=0) for CO2 at the zero surface coverage is 27.0 kJ mol 1. The fact that CO2 enthalpy of UTSA-49a is comparable to the values of MAF-66 (26.0 kJ mol 1) and ZTF-1 (25.4 kJ mol 1) indicates that the uncoordinated N atoms and methyl groups exposed in the pores and the narrow pore size can also enhance the interactions with CO2 gas molecules effectively. It should be noted that CH4 enthalpy of UTSA-49a at the zero coverage is 15.5 kJ mol 1 which is significantly lower than the value of MAF-66 (19.5 kJ mol 1) and decreases obviously when the uptake amount increases, which indicates the weak interaction between CH4 molecules and the narrow pore surfaces. The above-mentioned results indicate that UTSA-49a can selectively adsorb CO2 over CH4 and N2. In order to predict CO2–CH4 and CO2–N2 binary mixture selectivity, an ideal adsorbed solution theory (IAST) calculation based on a dual-site Langmuir– Freundlich (DSLF) simulation was employed on the basis of the single-component CO2, CH4, and N2 adsorption isotherms (see details in the ESI,† Table S5). The accuracy of IAST for prediction of gas mixture adsorption in a large number of zeolites and MOFs has been well established.16 Fig. 3b shows the predicted mixture adsorption isotherms and adsorption selectivity of UTSA-49a for CO2 (50%) and CH4 (50%) at 298 K. The CO2/CH4 selectivity values of UTSA-49a are estimated to be 33.7 at 298 K and 34.8 at 273 K,
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respectively, which are significantly higher than the values of MAF-66 (the Henry’s law CO2/CH4 selectivity of MAF-66 is 5.8 at 298 K and 7.5 at 273 K). These values are also higher than those reported for other ZIFs and are comparable to those of MOFs with highest selectivity reported in the literature. Fig. 3d shows the predicted mixture adsorption selectivity of UTSA-49a for CO2 and N2 (10 : 90, 15 : 85 and 20 : 80) at 298 K. The CO2/N2 selectivity values are estimated to be 90.5, 93.5 and 95.8 at 298 K and 188.8, 193.7 and 197.7 at 273 K for three different compositions (CO2/N2 = 10 : 90, 15 : 85, 20 : 80), respectively. Although these values are lower than those of MAF-66 (225 at 298 K and 403 at 273 K), they are higher than those reported for other ZIFs and are still considered very high selectivity values in MOFs. The high values of CO2 selectivity over CH4 and N2 could be due to the synergistic effect of multiple functional groups and the pore size/shape. The uncoordinated tetrazolate nitrogen atoms and methyl groups can be introduced by the Hmtz ligand into the framework of UTSA-49 simultaneously. The uncoordinated N atoms from N-rich aromatic rings have a positive effect on the adsorption of CO2 by facilitating dipole–quadrupole interactions between uncoordinated tetrazolate N atoms and CO2 molecules, which has been demonstrated by IFMC-1.11a The methyl groups may not be beneficial to further enhance the interaction with CO2 molecules as amino groups in MAF-66 or ZTF-1 do, but they adjust the pore structure delicately and make the pore size or shape narrower. Within such small dimensions, the slight variation introduced by methyl groups in the pore size or shape can cause a dramatic change in the adsorption affinity for CH4. Furthermore, its more hydrophobic pore nature enforces its framework stability under humid environments, which is favorable for its potential application in CO2 capture and separation. In summary, we have synthesized a new microporous tetrazolate zeolite-like framework with uncoordinated tetrazolate N atoms and functionalized methyl groups on the pore surfaces. The uncoordinated tetrazolate N atoms induce strong interaction with CO2 and the functionalized methyl groups modify the pore size/shape reducing the affinity with CH4, which demonstrate the potential of UTSA-49 to be a promising candidate material for CO2 capture and separation from natural gas and fuel gas. The introduction of multiple functional groups into a single MOF to produce a synergistic effect to improve interaction with gas molecules by immobilizing functional groups and reinforcing the size-exclusive effect by pore size/shape control simultaneously may be a promising strategy to enhance gas adsorption capacity and separation performance of MOFs. This work was financially supported by the Science and Technology Development of China Academy of Engineering Physics (No. 2014B0301034) and the Welch Foundation (AX-1730).
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