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Cite this: Chem. Commun., 2014, 50, 4683 Received 13th December 2013, Accepted 26th February 2014
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‘‘Click’’-extended nitrogen-rich metal–organic frameworks and their high performance in CO2-selective capture† Pei-Zhou Li,‡ab Xiao-Jun Wang,‡a Kang Zhang,c Anjaiah Nalaparaju,c Ruyi Zou,a Ruqiang Zou,*b Jianwen Jiang*c and Yanli Zhao*ad
DOI: 10.1039/c3cc49457h www.rsc.org/chemcomm
Metal–organic frameworks (MOFs), NTU-111, 112, 113, were assembled from diversely ‘‘click’’-extended tetracarboxylates. Since the MOFs were incorporated with exposed metal sites and accessible nitrogen-rich triazole units, they exhibited high CO2-selective capture capabilities supported by experimental investigations and molecular simulation studies.
On account of their large surface area, tunable pore size, adjustable composition and functionalizable pore surface, metal–organic frameworks (MOFs) have emerged as a new class of promising adsorbent materials for potential applications in adsorption-based storage and separation technologies towards small molecules.1 Among the small molecules, CO2 has been cited as the primary anthropogenic greenhouse gas as well as the leading culprit in climate change.1,2 Theoretical and experimental investigations have revealed that unsaturated or exposed metal sites in MOFs have a very high affinity towards CO2 and can increase the CO2 adsorption performance of the host frameworks.1,3 CO2 adsorption studies have also demonstrated that the incorporation of accessible nitrogen-donor groups, such as amine, pyridine, imidazole, triazole and tetrazole, into MOFs can dramatically influence the CO2 uptake capacity and selectivity owing to the dipole–quadrupole interactions between the polarizable CO2 molecule and the accessible nitrogen site.2,4 However, competitive coordination of these Lewis basic nitrogen units with exposed metal sites is a great challenge in direct synthesis of MOFs incorporating both CO2-philic exposed metal sites and accessible nitrogen-donor
groups.5 In our recent work, we have overcome such an issue by constructing a group of MOF-based adsorbents incorporated with exposed metal sites and accessible nitrogen-donor groups for CO2 capture. Herein, we present the successful fabrication of three highly porous MOFs, referred as NTU-111, 112, 113, containing both unsaturated Cu sites and accessible nitrogen-rich triazole units, which exhibit extremely high performance in CO2-selective capture supported by experimental investigations and further confirmed by molecular simulation studies. By utilizing versatile ‘‘click chemistry’’,6 nitrogen-rich tetracarboxylate ligands (H4L), i.e., 5,50 -(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H4L1), 5,50 -(benzene-1,4-diyl)di(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H4L2), and 5,50 -(benzene-1,3-diyl)di-(1H-1,2,3triazole-1,4-diyl)diisophthalic acid (H4L3), were designed and synthesized via diverse extension of diisophthalates (see ESI† for details). Subsequently, the crystals of NTU-111, 112, 113 were successfully obtained by reacting the ligands with Cu(II) ions (Fig. S1, ESI†). Structural investigation reveals that NTU-111 is a three-dimensional (3D) porous MOF possessing a framework formula of [Cu2(L1)]n. As shown in Fig. 1, two neighboring Cu(II) ions are bridged by four
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. E-mail: [email protected] b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China. E-mail: [email protected] c Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore. E-mail: [email protected] d School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore † Electronic supplementary information (ESI) available: Experimental details. CCDC 957323, 957324 and 957458. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc49457h ‡ These authors contributed equally to this work.
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Fig. 1 Crystal structures for (a) ‘‘click’’-extended diisophthalates and unsaturated paddle-wheel Cu2 clusters, (b) arrangements of regular laminar frameworks, and (c) ‘‘click’’-extended nitrogen-rich 3D frameworks: (i) NTU-111, (ii) NTU-112, and (iii) NTU-113.
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distributed carboxylates from four different L1 ligands around the Cu–Cu axis, resulting in the formation of an unsaturated paddlewheel Cu2 cluster in square-planar 4-connected geometry.7,8 From the perspective of the organic ligand, each L1 connects four different paddle-wheel Cu2 clusters through the four carboxylates, also forming a 4-connected node. The assembly of the two types of 4-connected nodes makes NTU-111 into an NbO-type network with 6482 topology (Fig. S2, ESI†).7,8 In NTU-111, the end isophthalate moieties of L1 acting as short linkers connect the paddle-wheel Cu2 clusters to form 2D parallel layers with the unsaturated Cu2 clusters arranged alternately in equilateral triangle and regular hexagon geometries, between which the ‘‘clicked’’ triazole rings from L1 serving the extension groups are uniformly located. Such arrangement in NTU-111 successfully leads to the formation of 3D frameworks incorporated with both exposed metal sites and accessible nitrogenrich triazole units. Structural analyses of NTU-112 and 113 indicate that they are also 3D porous MOFs consisting of 4-connected unsaturated paddle-wheel Cu2 clusters and corresponding 4-connected nitrogen-rich L ligands with a framework formula of [Cu2(L)]n. In NTU-112, the end isophthalate moieties of L2 connect with the unsaturated Cu2 clusters to form regular laminar frameworks as that in NTU-111, which are extended by the nitrogen-rich 1,4-ditriazolebenzene groups along the vertical direction to result in porous frameworks with uniformly distributed exposed metal sites and nitrogen-rich triazole units. However, because of the torsion of C–C single bonds between the benzene ring and triazole rings in L2, NTU-112 exhibits a topologically rare acs net (Fig. 1 and Fig. S3, ESI†).9 In NTU-113, the end isophthalate moieties of L3 connect with the unsaturated Cu2 clusters to form regular laminar frameworks, where the Cu2 clusters show an arrangement of a quadrilateral lattice. Then, the extended nitrogenrich 1,3-ditriazolebenzene groups of L3 are uniformly located between the parallel 2D quadrilateral lattices, leading to the PtS type network of 4284 topology (Fig. 1 and Fig. S4, ESI†).10 Structural analyses reveal that all the three MOFs incorporating both exposed metal sites and accessible nitrogen-rich triazole units exhibit unique structural features, indicating that subtle variation of ‘‘clicked’’ extension of the diisophthalate group could lead to a high diversity of the target frameworks. The total solvent-accessible volumes of NTU-111, 112, and 113 were calculated to be 68.6%, 71.5% and 71.3%, respectively, using PLATON/VOID routine, revealing that all the three MOFs possess high porosity.11 After the activation, the MOFs were subjected to nitrogen sorption at 77 K. As illustrated in Fig. 2a, all MOFs exhibit reversible type I sorption isotherms,12 demonstrating that all of them have the features of micropores.6–10 The overall nitrogen uptake of NTU-111 is 660 cm3 g1 at 1 atm with a Brunauer–Emmett–Teller (BET) surface area of 2450 m2 g1, which is comparable with reported MOFs having similar structures, such as NTU-101-Cu,6a ZJU-5,7 HNUST-1,8 and NOTT-101.10 NTU-112 and 113 exhibit a higher nitrogen uptake capability than NTU-111, which should be contributed to larger ‘‘clicked’’ extensions. NTU-112 shows a N2 uptake value of 829 cm3 g1 at 1 atm with a BET surface area of 2992 m2 g1, while NTU-113 gives a N2-uptake value of 873 cm3 g1 under the same conditions with a BET surface area of 3095 m2 g1. The pore size distributions calculated from the nitrogen sorption isotherms at 77 K
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Fig. 2 (a) N2 sorption isotherms at 77 K and (b) CO2 and N2 sorption isotherms at 273 K for activated MOFs: (i) NTU-111, (ii) NTU-112, and (iii) NTU-113.
reveal that the pore size of NTU-111 is 8.9 Å, while those of NTU-112 and 113 are 8.6 and 9.6 Å, respectively (Fig. S11–S13, ESI†), which are consistent well with the observations from the crystal structures. High porosity together with the exposed metal sites and nitrogenrich triazole units in these ‘‘click’’-extended MOFs inspired us to investigate their uptake capacity and selectivity towards CO2. As shown in Fig. 2b, an increase of the CO2-uptake capability from NTU-111 to 113 was observed. NTU-111 shows a CO2-uptake value of 124.6 cm3 g1 at 273 K and 1 atm. Sharp increases of CO2 uptake capabilities for NTU-112 and 113 were achieved under the same conditions, with the uptake values of 158.5 and 166.8 cm3 g1, respectively. The CO2-uptake values are higher than that of reported MOFs possessing similar structures, such as NTU-101-Cu and HNUST-1, and are even comparable with that of some reported MOFs having higher porosity,6a,8 which should be contributed to the successful introduction of both exposed metal sites and accessible nitrogen-rich triazole units in the fabricated MOFs. The N2 uptake capabilities were also measured at 273 K and 1 atm, showing values of only 10.5, 11.0 and 11.9 cm3 g1 for NTU-111, 112, and 113, respectively. The remarkable CO2-selective uptake capability makes these ‘‘click’’-extended nitrogen-rich MOFs excellent candidates for selective CO2 capture. Then, the isosteric heats of adsorption (Qst) for CO2 capture were calculated based on the adsorption isotherms at 273 and 298 K (Fig. S14–S16, ESI†) through the Clausius–Clapeyron equation.1b It was found that Qst values at a low loading range are B30.7, B32.0 and B33.2 kJ mol1 for NTU-111, 112, and 113, respectively, followed by the convergence into a pseudoplateau with a relatively high uptake, showing high CO2–framework interactions within these MOFs. Furthermore, all the three MOFs also exhibit good H2 adsorption capabilities, exhibiting adsorption values of 163.7, 214.9 and 239.8 cm3 g1 at 1 atm and 77 K for NTU-111, 112, and 113, respectively (Fig. S17, ESI†). To investigate the interactions between the adsorbate CO2 and the constructed frameworks, NTU-113 was taken as the representative for molecular simulation studies,13 and radial distribution functions g(r) were calculated by using the following equation: gij ðrÞ ¼
Nij ðr; r þ DrÞ V 4pr2 Dr Ni Nj
where r is the distance between atoms i and j, Nij(r, r + Dr) is the number of atom j around i within a shell from r to r + Dr, V is the system volume, and Ni and Nj are the numbers of atoms i and j, respectively. The g(r) of CO2 around the Cu, N1, N2, and N3 atoms in NTU-113 at 1, 10, and 100 kPa is shown in Fig. 3b. Pronounced peaks were observed at a low pressure (e.g. 1 kPa), indicating that the Cu, N1,
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Fig. 4 Simulated breakthrough curves for CO2 and N2 through a fixed bed packed with NTU-113. The inlet CO2–N2 mixture (0.15/0.85) is at 273 K and 100 kPa.
Fig. 3 (a) Fragment of NTU-113, (b) radial distribution functions of CO2 around the Cu, N1, N2 and N3 atoms at 273 K and 1, 10, 100 kPa, respectively, (c) simulation snapshots of CO2 adsorption at 273 K and 1, 10, 100 kPa (from left to right). Cu, orange; N, blue; C, gray; O, red; H, white; CO2 molecules, green.
N2, and N3 atoms have strong affinities for CO2. Among these four atoms, the Cu and N1 atoms show higher peaks than N2 and N3, implying that the Cu and N1 atoms are more preferential sites than N2 and N3 atoms for CO2 adsorption. Upon increasing the pressure, all the peaks around the Cu and N atoms decrease, indicating that CO2 molecules tend to be adsorbed onto a weak site when the preferential sites are fully occupied at high loadings. The simulation snapshots (Fig. 3c) of CO2 adsorption in NTU-113 at 273 K and 1, 10 and 100 kPa apparently show that the adsorbed CO2 molecules are located near the unsaturated Cu sites at a low pressure (1 kPa). At a medium pressure (10 kPa), CO2 molecules start to accumulate around the N atoms. When the pressure is further increased (100 kPa), newly adsorbed CO2 molecules are populated in the open channels. The molecular simulation studies provide clear evidence of strong interactions between CO2 and both exposed metal sites and nitrogen-rich moieties in the framework, which are consistent well with the experimental observations. To further investigate the performance of these effective MOF-based adsorbents in the application of post-combustion CO2 capture from a flue gas, the breakthrough curves for a CO2–N2 mixture in NTU-113 were predicted by using the isotherms of pure CO2 and N2.14 A fixed bed was assumed to be free of adsorbate initially, and the inlet CO2–N2 mixture was set at 273 K and 100 kPa with a CO2/N2 molar ratio of 0.15 : 0.85 for mimicking a flue gas. In the fixed bed packed with NTU-113 as the adsorbent, the breakthrough times, defined as the time when the composition at the outlet is 0.01%, were calculated to be 13.8 for N2 and 181.2 for CO2 (Fig. 4). Such an extremely large difference in breakthrough time makes NTU-113 a promising candidate for CO2/N2 separation, demonstrating that the effective adsorbents for post-combustion CO2 capture could be achieved by incorporating both exposed metal sites and nitrogenrich moieties into porous frameworks to serve as the interaction sites. In summary, diverse nitrogen-rich MOFs have been successfully assembled from ‘‘click’’-extended tetracarboxylate ligands and Cu(II)
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ions, which exhibit extremely high CO2-selective uptake capability. The experimental investigations and molecular simulation studies have clearly demonstrated that the incorporation of both exposed metal sites and nitrogen-rich moieties into the frameworks serving as the interaction sites with CO2 could lead to effective adsorbents for CO2 capture and separation. We thank the financial support from the Singapore National Research Foundation Fellowship (NRF2009NRF-RF001-015), the Singapore National Research Foundation CREATE program— Singapore Peking University Research Centre for a Sustainable LowCarbon Future, and the NTU-A*Star Centre of Excellence for Silicon Technologies (A*Star SERC No.: 112 351 0003).
Notes and references 1 (a) J.-R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P. B. Balbuen and H.-C. Zhou, Coord. Chem. Rev., 2011, 255, 1791–1823; (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, Chem. Rev., 2012, 112, 724–781. 2 (a) S. Choi, J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796–854; (b) P.-Z. Li and Y. Zhao, Chem.–Asian J., 2013, 8, 1680–1691. 3 A. L. Dzubak, L.-C. Lin, J. Kim, J. A. Swisher, R. Poloni, S. N. Maximoff, B. Smit and L. Gagliardi, Nat. Chem., 2012, 4, 810–816. 4 K. D. Vogiatzis, A. Mavrandonakis, W. Klopper and G. E. Froudakis, ChemPhysChem, 2009, 10, 374–383. 5 (a) G. Aromı´, L. A. Barrios, O. Roubeau and P. Gamez, Coord. Chem. Rev., 2011, 255, 485–546; (b) J.-P. Zhang, Y.-B. Zhang, J.-B. Lin and X.-M. Chen, Chem. Rev., 2012, 112, 1001–1033. 6 (a) X.-J. Wang, P.-Z. Li, L. Liu, Q. Zhang, P. Borah, J. D. Wong, X. X. Chan, G. Rakesh, Y. Li and Y. Zhao, Chem. Commun., 2012, 48, 10286–10288; (b) X.-J. Wang, P.-Z. Li, Y. Chen, Q. Zhang, H. Zhang, X. X. Chan, R. Ganguly, Y. Li, J. Jiang and Y. Zhao, Sci. Rep., 2013, 3, 1149; (c) P.-Z. Li, X.-J. Wang, R. H. D. Tan, Q. Zhang, R. Zou and Y. Zhao, RSC Adv., 2013, 3, 15566–15570. 7 X. Rao, J. Cai, J. Yu, Y. He, C. Wu, W. Zhou, T. Yildirim, B. Chen and G. Qian, Chem. Commun., 2013, 49, 6719–6721. 8 B. Zheng, H. Liu, Z. Wang, X. Yu, P. Yi and J. Bai, CrystEngComm, 2013, 15, 3517–3520. 9 W. Qiu, J. A. Perman, Ł. Wojtas, M. Eddaoudi and M. J. Zaworotko, Chem. Commun., 2010, 46, 8734–8736. 10 X. Lin, I. Telepeni, A. J. Blake, A. Dailly, C. M. Brown, J. M. Simmons, M. Zoppi, G. S. Walker, K. M. Thomas, T. J. Mays, P. Hubberstey, N. R. ¨der, J. Am. Chem. Soc., 2009, 131, 2159–2171. Champness and M. Schro 11 A. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148. 12 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, P. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603–619. 13 (a) G. Garberoglio, A. I. Skoulidas and J. K. Johnson, J. Phys. Chem. B, 2005, 109, 13094–13103; (b) A. I. Skoulidas and D. S. Sholl, J. Phys. Chem. B, 2005, 109, 15760–15768. 14 K. Zhang, A. Nalaparaju, Y. F. Chen and J. W. Jiang, RSC Adv., 2013, 3, 16152–16158.
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Cite this: Chem. Commun., 2014, 50, 4683 Received 13th December 2013, Accepted 26th February 2014
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‘‘Click’’-extended nitrogen-rich metal–organic frameworks and their high performance in CO2-selective capture† Pei-Zhou Li,‡ab Xiao-Jun Wang,‡a Kang Zhang,c Anjaiah Nalaparaju,c Ruyi Zou,a Ruqiang Zou,*b Jianwen Jiang*c and Yanli Zhao*ad
DOI: 10.1039/c3cc49457h www.rsc.org/chemcomm
Metal–organic frameworks (MOFs), NTU-111, 112, 113, were assembled from diversely ‘‘click’’-extended tetracarboxylates. Since the MOFs were incorporated with exposed metal sites and accessible nitrogen-rich triazole units, they exhibited high CO2-selective capture capabilities supported by experimental investigations and molecular simulation studies.
On account of their large surface area, tunable pore size, adjustable composition and functionalizable pore surface, metal–organic frameworks (MOFs) have emerged as a new class of promising adsorbent materials for potential applications in adsorption-based storage and separation technologies towards small molecules.1 Among the small molecules, CO2 has been cited as the primary anthropogenic greenhouse gas as well as the leading culprit in climate change.1,2 Theoretical and experimental investigations have revealed that unsaturated or exposed metal sites in MOFs have a very high affinity towards CO2 and can increase the CO2 adsorption performance of the host frameworks.1,3 CO2 adsorption studies have also demonstrated that the incorporation of accessible nitrogen-donor groups, such as amine, pyridine, imidazole, triazole and tetrazole, into MOFs can dramatically influence the CO2 uptake capacity and selectivity owing to the dipole–quadrupole interactions between the polarizable CO2 molecule and the accessible nitrogen site.2,4 However, competitive coordination of these Lewis basic nitrogen units with exposed metal sites is a great challenge in direct synthesis of MOFs incorporating both CO2-philic exposed metal sites and accessible nitrogen-donor
groups.5 In our recent work, we have overcome such an issue by constructing a group of MOF-based adsorbents incorporated with exposed metal sites and accessible nitrogen-donor groups for CO2 capture. Herein, we present the successful fabrication of three highly porous MOFs, referred as NTU-111, 112, 113, containing both unsaturated Cu sites and accessible nitrogen-rich triazole units, which exhibit extremely high performance in CO2-selective capture supported by experimental investigations and further confirmed by molecular simulation studies. By utilizing versatile ‘‘click chemistry’’,6 nitrogen-rich tetracarboxylate ligands (H4L), i.e., 5,50 -(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H4L1), 5,50 -(benzene-1,4-diyl)di(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H4L2), and 5,50 -(benzene-1,3-diyl)di-(1H-1,2,3triazole-1,4-diyl)diisophthalic acid (H4L3), were designed and synthesized via diverse extension of diisophthalates (see ESI† for details). Subsequently, the crystals of NTU-111, 112, 113 were successfully obtained by reacting the ligands with Cu(II) ions (Fig. S1, ESI†). Structural investigation reveals that NTU-111 is a three-dimensional (3D) porous MOF possessing a framework formula of [Cu2(L1)]n. As shown in Fig. 1, two neighboring Cu(II) ions are bridged by four
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. E-mail: [email protected] b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China. E-mail: [email protected] c Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore. E-mail: [email protected] d School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore † Electronic supplementary information (ESI) available: Experimental details. CCDC 957323, 957324 and 957458. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc49457h ‡ These authors contributed equally to this work.
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Fig. 1 Crystal structures for (a) ‘‘click’’-extended diisophthalates and unsaturated paddle-wheel Cu2 clusters, (b) arrangements of regular laminar frameworks, and (c) ‘‘click’’-extended nitrogen-rich 3D frameworks: (i) NTU-111, (ii) NTU-112, and (iii) NTU-113.
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distributed carboxylates from four different L1 ligands around the Cu–Cu axis, resulting in the formation of an unsaturated paddlewheel Cu2 cluster in square-planar 4-connected geometry.7,8 From the perspective of the organic ligand, each L1 connects four different paddle-wheel Cu2 clusters through the four carboxylates, also forming a 4-connected node. The assembly of the two types of 4-connected nodes makes NTU-111 into an NbO-type network with 6482 topology (Fig. S2, ESI†).7,8 In NTU-111, the end isophthalate moieties of L1 acting as short linkers connect the paddle-wheel Cu2 clusters to form 2D parallel layers with the unsaturated Cu2 clusters arranged alternately in equilateral triangle and regular hexagon geometries, between which the ‘‘clicked’’ triazole rings from L1 serving the extension groups are uniformly located. Such arrangement in NTU-111 successfully leads to the formation of 3D frameworks incorporated with both exposed metal sites and accessible nitrogenrich triazole units. Structural analyses of NTU-112 and 113 indicate that they are also 3D porous MOFs consisting of 4-connected unsaturated paddle-wheel Cu2 clusters and corresponding 4-connected nitrogen-rich L ligands with a framework formula of [Cu2(L)]n. In NTU-112, the end isophthalate moieties of L2 connect with the unsaturated Cu2 clusters to form regular laminar frameworks as that in NTU-111, which are extended by the nitrogen-rich 1,4-ditriazolebenzene groups along the vertical direction to result in porous frameworks with uniformly distributed exposed metal sites and nitrogen-rich triazole units. However, because of the torsion of C–C single bonds between the benzene ring and triazole rings in L2, NTU-112 exhibits a topologically rare acs net (Fig. 1 and Fig. S3, ESI†).9 In NTU-113, the end isophthalate moieties of L3 connect with the unsaturated Cu2 clusters to form regular laminar frameworks, where the Cu2 clusters show an arrangement of a quadrilateral lattice. Then, the extended nitrogenrich 1,3-ditriazolebenzene groups of L3 are uniformly located between the parallel 2D quadrilateral lattices, leading to the PtS type network of 4284 topology (Fig. 1 and Fig. S4, ESI†).10 Structural analyses reveal that all the three MOFs incorporating both exposed metal sites and accessible nitrogen-rich triazole units exhibit unique structural features, indicating that subtle variation of ‘‘clicked’’ extension of the diisophthalate group could lead to a high diversity of the target frameworks. The total solvent-accessible volumes of NTU-111, 112, and 113 were calculated to be 68.6%, 71.5% and 71.3%, respectively, using PLATON/VOID routine, revealing that all the three MOFs possess high porosity.11 After the activation, the MOFs were subjected to nitrogen sorption at 77 K. As illustrated in Fig. 2a, all MOFs exhibit reversible type I sorption isotherms,12 demonstrating that all of them have the features of micropores.6–10 The overall nitrogen uptake of NTU-111 is 660 cm3 g1 at 1 atm with a Brunauer–Emmett–Teller (BET) surface area of 2450 m2 g1, which is comparable with reported MOFs having similar structures, such as NTU-101-Cu,6a ZJU-5,7 HNUST-1,8 and NOTT-101.10 NTU-112 and 113 exhibit a higher nitrogen uptake capability than NTU-111, which should be contributed to larger ‘‘clicked’’ extensions. NTU-112 shows a N2 uptake value of 829 cm3 g1 at 1 atm with a BET surface area of 2992 m2 g1, while NTU-113 gives a N2-uptake value of 873 cm3 g1 under the same conditions with a BET surface area of 3095 m2 g1. The pore size distributions calculated from the nitrogen sorption isotherms at 77 K
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Fig. 2 (a) N2 sorption isotherms at 77 K and (b) CO2 and N2 sorption isotherms at 273 K for activated MOFs: (i) NTU-111, (ii) NTU-112, and (iii) NTU-113.
reveal that the pore size of NTU-111 is 8.9 Å, while those of NTU-112 and 113 are 8.6 and 9.6 Å, respectively (Fig. S11–S13, ESI†), which are consistent well with the observations from the crystal structures. High porosity together with the exposed metal sites and nitrogenrich triazole units in these ‘‘click’’-extended MOFs inspired us to investigate their uptake capacity and selectivity towards CO2. As shown in Fig. 2b, an increase of the CO2-uptake capability from NTU-111 to 113 was observed. NTU-111 shows a CO2-uptake value of 124.6 cm3 g1 at 273 K and 1 atm. Sharp increases of CO2 uptake capabilities for NTU-112 and 113 were achieved under the same conditions, with the uptake values of 158.5 and 166.8 cm3 g1, respectively. The CO2-uptake values are higher than that of reported MOFs possessing similar structures, such as NTU-101-Cu and HNUST-1, and are even comparable with that of some reported MOFs having higher porosity,6a,8 which should be contributed to the successful introduction of both exposed metal sites and accessible nitrogen-rich triazole units in the fabricated MOFs. The N2 uptake capabilities were also measured at 273 K and 1 atm, showing values of only 10.5, 11.0 and 11.9 cm3 g1 for NTU-111, 112, and 113, respectively. The remarkable CO2-selective uptake capability makes these ‘‘click’’-extended nitrogen-rich MOFs excellent candidates for selective CO2 capture. Then, the isosteric heats of adsorption (Qst) for CO2 capture were calculated based on the adsorption isotherms at 273 and 298 K (Fig. S14–S16, ESI†) through the Clausius–Clapeyron equation.1b It was found that Qst values at a low loading range are B30.7, B32.0 and B33.2 kJ mol1 for NTU-111, 112, and 113, respectively, followed by the convergence into a pseudoplateau with a relatively high uptake, showing high CO2–framework interactions within these MOFs. Furthermore, all the three MOFs also exhibit good H2 adsorption capabilities, exhibiting adsorption values of 163.7, 214.9 and 239.8 cm3 g1 at 1 atm and 77 K for NTU-111, 112, and 113, respectively (Fig. S17, ESI†). To investigate the interactions between the adsorbate CO2 and the constructed frameworks, NTU-113 was taken as the representative for molecular simulation studies,13 and radial distribution functions g(r) were calculated by using the following equation: gij ðrÞ ¼
Nij ðr; r þ DrÞ V 4pr2 Dr Ni Nj
where r is the distance between atoms i and j, Nij(r, r + Dr) is the number of atom j around i within a shell from r to r + Dr, V is the system volume, and Ni and Nj are the numbers of atoms i and j, respectively. The g(r) of CO2 around the Cu, N1, N2, and N3 atoms in NTU-113 at 1, 10, and 100 kPa is shown in Fig. 3b. Pronounced peaks were observed at a low pressure (e.g. 1 kPa), indicating that the Cu, N1,
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Fig. 4 Simulated breakthrough curves for CO2 and N2 through a fixed bed packed with NTU-113. The inlet CO2–N2 mixture (0.15/0.85) is at 273 K and 100 kPa.
Fig. 3 (a) Fragment of NTU-113, (b) radial distribution functions of CO2 around the Cu, N1, N2 and N3 atoms at 273 K and 1, 10, 100 kPa, respectively, (c) simulation snapshots of CO2 adsorption at 273 K and 1, 10, 100 kPa (from left to right). Cu, orange; N, blue; C, gray; O, red; H, white; CO2 molecules, green.
N2, and N3 atoms have strong affinities for CO2. Among these four atoms, the Cu and N1 atoms show higher peaks than N2 and N3, implying that the Cu and N1 atoms are more preferential sites than N2 and N3 atoms for CO2 adsorption. Upon increasing the pressure, all the peaks around the Cu and N atoms decrease, indicating that CO2 molecules tend to be adsorbed onto a weak site when the preferential sites are fully occupied at high loadings. The simulation snapshots (Fig. 3c) of CO2 adsorption in NTU-113 at 273 K and 1, 10 and 100 kPa apparently show that the adsorbed CO2 molecules are located near the unsaturated Cu sites at a low pressure (1 kPa). At a medium pressure (10 kPa), CO2 molecules start to accumulate around the N atoms. When the pressure is further increased (100 kPa), newly adsorbed CO2 molecules are populated in the open channels. The molecular simulation studies provide clear evidence of strong interactions between CO2 and both exposed metal sites and nitrogen-rich moieties in the framework, which are consistent well with the experimental observations. To further investigate the performance of these effective MOF-based adsorbents in the application of post-combustion CO2 capture from a flue gas, the breakthrough curves for a CO2–N2 mixture in NTU-113 were predicted by using the isotherms of pure CO2 and N2.14 A fixed bed was assumed to be free of adsorbate initially, and the inlet CO2–N2 mixture was set at 273 K and 100 kPa with a CO2/N2 molar ratio of 0.15 : 0.85 for mimicking a flue gas. In the fixed bed packed with NTU-113 as the adsorbent, the breakthrough times, defined as the time when the composition at the outlet is 0.01%, were calculated to be 13.8 for N2 and 181.2 for CO2 (Fig. 4). Such an extremely large difference in breakthrough time makes NTU-113 a promising candidate for CO2/N2 separation, demonstrating that the effective adsorbents for post-combustion CO2 capture could be achieved by incorporating both exposed metal sites and nitrogenrich moieties into porous frameworks to serve as the interaction sites. In summary, diverse nitrogen-rich MOFs have been successfully assembled from ‘‘click’’-extended tetracarboxylate ligands and Cu(II)
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ions, which exhibit extremely high CO2-selective uptake capability. The experimental investigations and molecular simulation studies have clearly demonstrated that the incorporation of both exposed metal sites and nitrogen-rich moieties into the frameworks serving as the interaction sites with CO2 could lead to effective adsorbents for CO2 capture and separation. We thank the financial support from the Singapore National Research Foundation Fellowship (NRF2009NRF-RF001-015), the Singapore National Research Foundation CREATE program— Singapore Peking University Research Centre for a Sustainable LowCarbon Future, and the NTU-A*Star Centre of Excellence for Silicon Technologies (A*Star SERC No.: 112 351 0003).
Notes and references 1 (a) J.-R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P. B. Balbuen and H.-C. Zhou, Coord. Chem. Rev., 2011, 255, 1791–1823; (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, Chem. Rev., 2012, 112, 724–781. 2 (a) S. Choi, J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796–854; (b) P.-Z. Li and Y. Zhao, Chem.–Asian J., 2013, 8, 1680–1691. 3 A. L. Dzubak, L.-C. Lin, J. Kim, J. A. Swisher, R. Poloni, S. N. Maximoff, B. Smit and L. Gagliardi, Nat. Chem., 2012, 4, 810–816. 4 K. D. Vogiatzis, A. Mavrandonakis, W. Klopper and G. E. Froudakis, ChemPhysChem, 2009, 10, 374–383. 5 (a) G. Aromı´, L. A. Barrios, O. Roubeau and P. Gamez, Coord. Chem. Rev., 2011, 255, 485–546; (b) J.-P. Zhang, Y.-B. Zhang, J.-B. Lin and X.-M. Chen, Chem. Rev., 2012, 112, 1001–1033. 6 (a) X.-J. Wang, P.-Z. Li, L. Liu, Q. Zhang, P. Borah, J. D. Wong, X. X. Chan, G. Rakesh, Y. Li and Y. Zhao, Chem. Commun., 2012, 48, 10286–10288; (b) X.-J. Wang, P.-Z. Li, Y. Chen, Q. Zhang, H. Zhang, X. X. Chan, R. Ganguly, Y. Li, J. Jiang and Y. Zhao, Sci. Rep., 2013, 3, 1149; (c) P.-Z. Li, X.-J. Wang, R. H. D. Tan, Q. Zhang, R. Zou and Y. Zhao, RSC Adv., 2013, 3, 15566–15570. 7 X. Rao, J. Cai, J. Yu, Y. He, C. Wu, W. Zhou, T. Yildirim, B. Chen and G. Qian, Chem. Commun., 2013, 49, 6719–6721. 8 B. Zheng, H. Liu, Z. Wang, X. Yu, P. Yi and J. Bai, CrystEngComm, 2013, 15, 3517–3520. 9 W. Qiu, J. A. Perman, Ł. Wojtas, M. Eddaoudi and M. J. Zaworotko, Chem. Commun., 2010, 46, 8734–8736. 10 X. Lin, I. Telepeni, A. J. Blake, A. Dailly, C. M. Brown, J. M. Simmons, M. Zoppi, G. S. Walker, K. M. Thomas, T. J. Mays, P. Hubberstey, N. R. ¨der, J. Am. Chem. Soc., 2009, 131, 2159–2171. Champness and M. Schro 11 A. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148. 12 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, P. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603–619. 13 (a) G. Garberoglio, A. I. Skoulidas and J. K. Johnson, J. Phys. Chem. B, 2005, 109, 13094–13103; (b) A. I. Skoulidas and D. S. Sholl, J. Phys. Chem. B, 2005, 109, 15760–15768. 14 K. Zhang, A. Nalaparaju, Y. F. Chen and J. W. Jiang, RSC Adv., 2013, 3, 16152–16158.
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