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Cite this: Chem. Commun., 2013, 49, 11329 Received 4th June 2013, Accepted 8th October 2013
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Modular structure of a robust microporous MOF based on Cu2 paddle-wheels with high CO2 selectivity† ´ M. Seco,a David Fairen-Jimenez,*b Antonio J. Calahorro,c Laura Me ´ndez-Lin ´n,c Jose ˜a c d c ´rez-Mendoza, Nicola Casati, Enrique Colacio and Manuel Pe ´guez*c Antonio Rodrı´guez-Die
DOI: 10.1039/c3cc44193h www.rsc.org/chemcomm
The synthesis of a new MOF with Cu2 paddle-wheels connected to glutarate and 1,3-bis(4-pyridyl)propane linkers has been explored. Experimental gas adsorption measurements reveal that the MOF is essentially non-porous to methane whereas it presents a type III isotherm upon CO2 adsorption, leading to high capacity and outstanding CO2 selectivity.
The use of porous adsorbents (e.g. zeolites and activated carbons) in industrial separation and purification processes plays an important role in the global economy.1 Extensive research has been performed to develop selective adsorbents for applications such as the purification of hydrogen and natural gas, mainly composed of methane, as well as carbon capture.2 These efforts towards the development of these selective materials have turned in the last few years to metal–organic frameworks (MOFs) due to their structural diversity and functional properties.3 MOFs are obtained by the self-assembly of metal clusters and organic linkers, resulting in tailored nanoporous host materials. The high internal surface areas and large pore volumes make MOFs promising candidates for gas adsorption and separation applications. Many adsorption isotherms of small gas molecules on MOFs at room temperature show the most common type I (i.e. Langmuir) shape. However, some MOFs exhibit unusual adsorption behaviours, such as the existence of steps during the adsorption process. The existence of steps could be explained either by the a
´n, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas de San Sebastia Universidad del Paı´s Vasco, Spain b Dept. of Chemical Engineering & Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK. E-mail: [email protected]; Web: http://people.ds.cam.ac.uk/df334 c ´nica, Universidad de Granada, Departamento de Quı´mica Inorga Avda Fuentenueva s/n, 18071, Granada, Spain. E-mail: [email protected] d Paul Scherrer Institute, WLGA/229, Villigen PSI, 5232, Switzerland † Electronic supplementary information (ESI) available: Synthesis of compound 1, XRD and IR, bond distance and angle tables, PSD, experimental gas adsorption, details of molecular simulations and high-pressure X-ray diffraction. CCDC 941413–941417. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc44193h
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sequential filling of the different cavities or adsorption sites of the structure,4 especially at low temperature, or by the existence of flexible structures in the phenomenon described as a gate opening, breathing or swing effect (e.g. MIL-53 and ZIF-8).5 In the latter flexible cases, the increase in pressure provokes structural changes that induce a modification in the pore shape and volume. On the other hand, unusual type V (i.e. sigmoid) isotherms have been observed in the absence of structural changes during the adsorption of CO2 and CH4, as a result of relatively weak gas–MOF versus gas–gas interactions.6 The existence of isotherms with unusual shapes on MOFs, such as type V, offers outstanding possibilities for the design of materials for gas storage and gas separation applications. When a gas is adsorbed, the adsorbent is regenerated by reducing the pressure. A good adsorbent for a ‘‘pressure-swing’’ process needs to have a high selectivity but also a high deliverable capacity, i.e. the difference between the amount adsorbed at the maximum adsorption pressure and the amount adsorbed at the regeneration pressure. As shown in Fig. S2 in the ESI,† an isotherm that is convex to the pressure axis but has a high capacity, such as type III, has this feature. In order to obtain a material with high selectivity, we have recently explored the gas purification capabilities of a MOF constituted of 5-bromonicotinic acid linked to cobalt metal corners, demonstrating that it is possible to synthesize highly selective materials by using classical ligands.7 However, the narrow porosity of this MOF, which allows for molecular sieving of H2–CH4 mixtures, also implies low pore volumes and therefore low adsorption capacities. The design of a porous material with high pore volumes and molecular sieving behaviour with high selectivities remains an open issue. In this work, we have designed the synthesis of a new MOF (1) with Cu2 paddle-wheels connected to glutarate (glu) and 1,3-bis(4-pyridyl)propane (bpp) ligands. We believe that the conformational freedom of the glutarate ligand could promote the self-assembly of MOFs with novel topologies. Herein, we describe the synthesis,8 crystal structure,9 TG, high pressure X-ray experiments up to 4.5 GPa, gas adsorption properties and Chem. Commun., 2013, 49, 11329--11331
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Fig. 2 Differences between 1 (top) and 2 (bottom). Note the distinctive tilt in the pyridyl ligands with respect to the propane chains that connect them. The different molecular fragments have been indicated in red and green for clarity.
Fig. 1 (top) Perspective view through the c axis of the channels in the threedimensional network of 1 and, (bottom) through the b axis of the bpp pillars linking different sheets. Color code N = blue, O = red, C = black and Cu = orange. Hydrogen atoms have been omitted for clarity.
grand canonical Monte Carlo simulations of the MOF [Cu2(glu)2(m-bpp)]2H2O (1). This compound presents unexpected adsorption behaviour with type I and type III isotherms, for H2 and CO2, respectively, high CO2/CH4 selectivity and a high adsorption capacity. Fig. 1 shows the single-crystal XRD structure of 1, solved in the space group C2/c. Table S1 (ESI†) lists selected bond distances and angles. The copper metal centre of the paddlewheel adopts a slightly distorted square pyramidal coordination geometry with the bond angles only deviating slightly from 901 and 1801. The Cu(II) atom is bonded to four oxygen atoms of the bridging carboxylate groups pertaining to four different glutarate ligands (Cu–O = 1.907(6)–2.004(6) Å) in the basal plane. The Cu(II) atom is also bonded to one bpp ligand (Cu–N = 2.162(6) Å) in the axial position to complete the distorted square-pyramidal coordination geometry. In the structure, Cu2 units are bridged by glutarate dianions to form a distorted 2D square grid, with Cu2 units linked along the [011] and [011% ] directions. These 2D square grids are further pillared by bpp ligands extending from the axial sites of the Cu2 paddle wheels along the [102] direction to form the MOF. The activation process to remove the solvent does not affect the integrity of structure 1 (Fig. S4, ESI,† left). With a similar approach, Hwang et al. have recently published a MOF with the same metal ions and ligands but different solvent [{Cu2(glu)2(m-bpp)}(C3H6O)]n (2).10 In contrast to 1, this structure 11330
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presents a very different arrangement of the bpp ligands based on the tilt of the pyridyl fragments with respect to the propane chain that interconnects them (Fig. 2 and Fig. S3, ESI†). The dihedral angles between the pyridyl rings and the propane chain are 5.991 and 82.831 for 1 and 2, respectively. The co-planarity shown by 1 causes the stacking of the bpp ligands in an ‘‘accordion’’ fashion along the c axis (Fig. 1, bottom), an aspect that does not take place in 2. We believe that this characteristic is fundamental for the behaviour of 1 in terms of gas adsorption. We first analysed the porosity of 1 geometrically. Fig. S4 (ESI†), right, shows the calculated pore size distribution (PSD). Structure 1 presents an open-porosity centred at ca. 6 Å, broad enough to be, in principle, accessible for gas adsorption. However, when running the experimental gas adsorption isotherms on 1, we found that it was essentially non-porous to N2 at 1 bar and 77 K. On the other hand, Fig. 3 shows how CO2 and H2 can access into the porosity when increasing the pressure up to 30 bar at 273 K and 77 K, respectively, whereas CH4 (273 K) access remains impeded. More interesting is the shape of the isotherms: while the H2 isotherm can be classified as a type I, the CO2 isotherm presents a type III shape, and almost no adsorption up to 10 bar. Under the studied conditions, 1 showed a maximum uptake of 2.24, 15.75 and 0.25 wt% for H2, CO2 and CH4, respectively. High H2 and CH4 selectivity in MOFs has been reported previously.11 However, and to the best of our knowledge, this is the first time that a microporous material, including not only MOFs but also zeolites and activated carbons, shows this behaviour. The existence of type III isotherms generally implies a low interaction between the adsorbate (i.e. the gas molecules) and the adsorbent (i.e. the MOF).6a,12 This could suggest that the interaction between 1 and CO2 (type III) is much lower This journal is
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Fig. 3 Adsorption isotherms on 1 obtained at 77 K (H2, green circles) and 273 K (CO2, blue diamonds; CH4, red triangles).
completely mirrored by a change in the beta angle. While this complex mechanism seems to be largely due to the flexibility of the Cu-glutarate paddle-wheel, in 2 the difference in the bpp conformation inhibits the distorted 2D layers from shifting with respect to each other and no flexibility can therefore be observed. Further experimental work using in situ XRD to confirm the existence or not of structural changes during gas adsorption or the existence of densified surface layers or structural defects preventing the access to the porosity is in progress, with the aim of studying the mechanism of action of this metal–organic framework. This work was supported by the Junta de Andalucı´a (FQM4228) (A. J. Calahorro for a predoctoral grant) and the MEC of Spain (Project CTQ2011-24478). D. F.-J. thanks the Royal Society for a University Research Fellowship.
Notes and references compared to that of the H2 (type I). This idea is counterintuitive since the CO2 molecules generally present higher interaction with an adsorbent due to the higher molecular weight and the presence of a strong quadrupole moment. A second possibility to this behaviour would be the existence of a molecular sieve effect, where the differences between the size of the H2, CO2 and CH4 molecules play an important role in the diffusion of the gases through the porosity. In this case, a small molecule such as H2 may diffuse through a narrow porosity, whereas CH4 and CO2 are hindered. Only high pressures allow enough potential energy to either overdue the kinetic barrier in the diffusion, or to provoke a gate opening or breathing effect and therefore a structural change. This explanation is, however, not compatible with the relatively broad 6 Å porosity found in this material. Moreover, the use of grand canonical Monte Carlo (GCMC) simulations for CO2 adsorption on a crystallographic rigid model, shown in Fig. S5 (ESI†), confirms the goodness of the experimental maximum capacity value (estimated to be ca. 23 wt%), but not the shape of the isotherm. Since GCMC simulations are in thermodynamic equilibrium, the existence of experimental kinetic barriers due to the presence of narrow porosity will not be detected using this technique. In a similar way, Feldblyum et al. were able to explain the disconnection between porous performance, as predicted by crystallography, and porous texture measured by gas adsorption.13 Using positron annihilation lifetime spectroscopy, they explained these differences due to the presence of densified layers at the surface of the material, preventing the entry of small molecular species into the bulk porosity. To further analyse structure 1, we studied the most flexible modes of the framework using a high pressure X-ray diffraction study up to 4.5 GPa. Despite the possible conformational rearrangements, the bpp ligand in 1 remains stiff and largely unchanged throughout compression, while the glutarate-Cu moiety experiences a small but progressive distortion. The rearrangement of the distorted glutarate-Cu plane allows the bpp pillars to bend rigidly with respect to it, closing the ‘accordion’ and bringing different planes significantly closer, by shifting them laterally with respect to each other. As symmetry implies this is
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1 (a) B. Chen, S. Ma, F. Zapata, F. R. Fronczek, E. B. Lobkovsky and H.-C. Zhou, Inorg. Chem., 2007, 46, 1233; (b) H. B. T. Jeazet, C. Staudt and C. Janiak, Chem. Commun., 2012, 48, 2140; (c) H. Z. Chen and T.-S. Chung, Int. J. Hydrogen Energy, 2012, 37, 6001. 2 J. Shang, G. Li, R. Singh, Q. Gu, K. M. Nairn, T. J. Bastow, N. Medhekar, C. M. Doherty, A. J. Hill, J. Z. Liu and P. A. Webley, J. Am. Chem. Soc., 2012, 134, 19246. 3 H.-C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673. 4 J. Getzschmann, I. Senkovska, D. Wallacher, M. Tovar, D. Fairen¨ren, J. M. van Baten, R. Krishna and S. Kaskel, Jimenez, T. Du Microporous Mesoporous Mater., 2010, 136, 50. 5 (a) K. Uemura, R. Matsuda and S. Kitagawa, J. Solid State Chem., 2005, 178, 2420; (b) N. A. Ramsahye, G. Maurin, S. Bourrelly, P. L. Llewellyn, T. Loiseau, C. Serre and G. Ferey, Chem. Commun., 2007, 3261; (c) D. Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, ¨ren, J. Am. Chem. Soc., 2011, P. A. Wright, S. Parsons and T. J. Du 133, 8900. ¨ren, Langmuir, 2010, 6 (a) D. Fairen-Jimenez, N. A. Seaton and T. Du 26, 14694; (b) A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998. ´pez-Viseras, A. Salinas-Castillo, D. Fairen-Jimenez, 7 A. J. Calahorro, M. E. Lo ´guez, CrystEngComm, 2012, E. Colacio, J. Cano and A. Rodrı´guez-Die 14, 6390. 8 Synthesis of [Cu2(glu)2(m-bpp)]2H2O (1). The reaction in water solvent of the copper sulfate (0.25 mmol, 0.0628 g), glutaric acid (0.25 mmol, 0.033 g), urea (0.872 mmol, 0.054 g) and 1,3-bis(4-pyridyl)propane (0.25 mmol, 0.046 g) led to a blue solution, which kept at room temperature for one week gave rise to green crystals of complex 1, which were filtered off and air-dried. Yield: 60%. Anal. calc. for C23H26Cu2N2O8: C, 47.18; H, 4.48; N, 4.78. Experimental: C, 47.05; H, 4.61; N, 4.83. 9 Crystal data: C23H26Cu2N2O8, fw = 585.54 g mol 1; monoclinic, C2/c, a = 28.265(5), b = 13.127(5), c = 8.729(5), b = 106.779(5), V = 3101(2) Å3; Z = 4; T = 293 K; rcalc = 1.254 g cm 3; F(000) = 1200; m(Mo-Ka) = 1.411 cm 1. Data collected on a Bruker Axs APEX automated diffractometer at RT, R1(Fo) = 0.0326 (wR2(Fo2) = 0.0752) for 17 018 unique reflections (Rint = 0.0865) with a goodness-of-fit on F2 1.048. For both structures, data were collected by oyv/2h scans (2ymax = 561) on a Bruker SMART CCD diffractometer with graphite-monochromated MoKa radiation (l = 0.71073). The structures were solved by direct methods and refined on F2 by the SHELX-97 program.11. 10 I. H. Hwang, J. M. Bae, W.-S. Kim, Y. D. Jo, C. Kim, Y. Kim, S.-J. Kim and S. Huh, Dalton Trans., 2012, 41, 12759. 11 (a) B. Chen, S. Ma, F. Zapata, F. R. Fronczek, E. B. Lobkovsky and H.-C. Zhou, Inorg. Chem., 2007, 46, 1233; (b) M. Xue, S. Ma, Z. Jin, R. M. Schaffino, G. S. Zhu, E. B. Lobkovsky, S. L. Qiu and B. L. Chen, Inorg. Chem., 2008, 47, 6825. 12 J. Rouquerol, F. Rouquerol and K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, San Diego, CA, 1999. 13 J. I. Feldblyum, M. Liu, D. W. Gidley and A. J. Matzger, J. Am. Chem. Soc., 2011, 133, 18257.
Chem. Commun., 2013, 49, 11329--11331
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COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 11329 Received 4th June 2013, Accepted 8th October 2013
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Modular structure of a robust microporous MOF based on Cu2 paddle-wheels with high CO2 selectivity† ´ M. Seco,a David Fairen-Jimenez,*b Antonio J. Calahorro,c Laura Me ´ndez-Lin ´n,c Jose ˜a c d c ´rez-Mendoza, Nicola Casati, Enrique Colacio and Manuel Pe ´guez*c Antonio Rodrı´guez-Die
DOI: 10.1039/c3cc44193h www.rsc.org/chemcomm
The synthesis of a new MOF with Cu2 paddle-wheels connected to glutarate and 1,3-bis(4-pyridyl)propane linkers has been explored. Experimental gas adsorption measurements reveal that the MOF is essentially non-porous to methane whereas it presents a type III isotherm upon CO2 adsorption, leading to high capacity and outstanding CO2 selectivity.
The use of porous adsorbents (e.g. zeolites and activated carbons) in industrial separation and purification processes plays an important role in the global economy.1 Extensive research has been performed to develop selective adsorbents for applications such as the purification of hydrogen and natural gas, mainly composed of methane, as well as carbon capture.2 These efforts towards the development of these selective materials have turned in the last few years to metal–organic frameworks (MOFs) due to their structural diversity and functional properties.3 MOFs are obtained by the self-assembly of metal clusters and organic linkers, resulting in tailored nanoporous host materials. The high internal surface areas and large pore volumes make MOFs promising candidates for gas adsorption and separation applications. Many adsorption isotherms of small gas molecules on MOFs at room temperature show the most common type I (i.e. Langmuir) shape. However, some MOFs exhibit unusual adsorption behaviours, such as the existence of steps during the adsorption process. The existence of steps could be explained either by the a
´n, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas de San Sebastia Universidad del Paı´s Vasco, Spain b Dept. of Chemical Engineering & Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK. E-mail: [email protected]; Web: http://people.ds.cam.ac.uk/df334 c ´nica, Universidad de Granada, Departamento de Quı´mica Inorga Avda Fuentenueva s/n, 18071, Granada, Spain. E-mail: [email protected] d Paul Scherrer Institute, WLGA/229, Villigen PSI, 5232, Switzerland † Electronic supplementary information (ESI) available: Synthesis of compound 1, XRD and IR, bond distance and angle tables, PSD, experimental gas adsorption, details of molecular simulations and high-pressure X-ray diffraction. CCDC 941413–941417. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc44193h
This journal is
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The Royal Society of Chemistry 2013
sequential filling of the different cavities or adsorption sites of the structure,4 especially at low temperature, or by the existence of flexible structures in the phenomenon described as a gate opening, breathing or swing effect (e.g. MIL-53 and ZIF-8).5 In the latter flexible cases, the increase in pressure provokes structural changes that induce a modification in the pore shape and volume. On the other hand, unusual type V (i.e. sigmoid) isotherms have been observed in the absence of structural changes during the adsorption of CO2 and CH4, as a result of relatively weak gas–MOF versus gas–gas interactions.6 The existence of isotherms with unusual shapes on MOFs, such as type V, offers outstanding possibilities for the design of materials for gas storage and gas separation applications. When a gas is adsorbed, the adsorbent is regenerated by reducing the pressure. A good adsorbent for a ‘‘pressure-swing’’ process needs to have a high selectivity but also a high deliverable capacity, i.e. the difference between the amount adsorbed at the maximum adsorption pressure and the amount adsorbed at the regeneration pressure. As shown in Fig. S2 in the ESI,† an isotherm that is convex to the pressure axis but has a high capacity, such as type III, has this feature. In order to obtain a material with high selectivity, we have recently explored the gas purification capabilities of a MOF constituted of 5-bromonicotinic acid linked to cobalt metal corners, demonstrating that it is possible to synthesize highly selective materials by using classical ligands.7 However, the narrow porosity of this MOF, which allows for molecular sieving of H2–CH4 mixtures, also implies low pore volumes and therefore low adsorption capacities. The design of a porous material with high pore volumes and molecular sieving behaviour with high selectivities remains an open issue. In this work, we have designed the synthesis of a new MOF (1) with Cu2 paddle-wheels connected to glutarate (glu) and 1,3-bis(4-pyridyl)propane (bpp) ligands. We believe that the conformational freedom of the glutarate ligand could promote the self-assembly of MOFs with novel topologies. Herein, we describe the synthesis,8 crystal structure,9 TG, high pressure X-ray experiments up to 4.5 GPa, gas adsorption properties and Chem. Commun., 2013, 49, 11329--11331
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Fig. 2 Differences between 1 (top) and 2 (bottom). Note the distinctive tilt in the pyridyl ligands with respect to the propane chains that connect them. The different molecular fragments have been indicated in red and green for clarity.
Fig. 1 (top) Perspective view through the c axis of the channels in the threedimensional network of 1 and, (bottom) through the b axis of the bpp pillars linking different sheets. Color code N = blue, O = red, C = black and Cu = orange. Hydrogen atoms have been omitted for clarity.
grand canonical Monte Carlo simulations of the MOF [Cu2(glu)2(m-bpp)]2H2O (1). This compound presents unexpected adsorption behaviour with type I and type III isotherms, for H2 and CO2, respectively, high CO2/CH4 selectivity and a high adsorption capacity. Fig. 1 shows the single-crystal XRD structure of 1, solved in the space group C2/c. Table S1 (ESI†) lists selected bond distances and angles. The copper metal centre of the paddlewheel adopts a slightly distorted square pyramidal coordination geometry with the bond angles only deviating slightly from 901 and 1801. The Cu(II) atom is bonded to four oxygen atoms of the bridging carboxylate groups pertaining to four different glutarate ligands (Cu–O = 1.907(6)–2.004(6) Å) in the basal plane. The Cu(II) atom is also bonded to one bpp ligand (Cu–N = 2.162(6) Å) in the axial position to complete the distorted square-pyramidal coordination geometry. In the structure, Cu2 units are bridged by glutarate dianions to form a distorted 2D square grid, with Cu2 units linked along the [011] and [011% ] directions. These 2D square grids are further pillared by bpp ligands extending from the axial sites of the Cu2 paddle wheels along the [102] direction to form the MOF. The activation process to remove the solvent does not affect the integrity of structure 1 (Fig. S4, ESI,† left). With a similar approach, Hwang et al. have recently published a MOF with the same metal ions and ligands but different solvent [{Cu2(glu)2(m-bpp)}(C3H6O)]n (2).10 In contrast to 1, this structure 11330
Chem. Commun., 2013, 49, 11329--11331
presents a very different arrangement of the bpp ligands based on the tilt of the pyridyl fragments with respect to the propane chain that interconnects them (Fig. 2 and Fig. S3, ESI†). The dihedral angles between the pyridyl rings and the propane chain are 5.991 and 82.831 for 1 and 2, respectively. The co-planarity shown by 1 causes the stacking of the bpp ligands in an ‘‘accordion’’ fashion along the c axis (Fig. 1, bottom), an aspect that does not take place in 2. We believe that this characteristic is fundamental for the behaviour of 1 in terms of gas adsorption. We first analysed the porosity of 1 geometrically. Fig. S4 (ESI†), right, shows the calculated pore size distribution (PSD). Structure 1 presents an open-porosity centred at ca. 6 Å, broad enough to be, in principle, accessible for gas adsorption. However, when running the experimental gas adsorption isotherms on 1, we found that it was essentially non-porous to N2 at 1 bar and 77 K. On the other hand, Fig. 3 shows how CO2 and H2 can access into the porosity when increasing the pressure up to 30 bar at 273 K and 77 K, respectively, whereas CH4 (273 K) access remains impeded. More interesting is the shape of the isotherms: while the H2 isotherm can be classified as a type I, the CO2 isotherm presents a type III shape, and almost no adsorption up to 10 bar. Under the studied conditions, 1 showed a maximum uptake of 2.24, 15.75 and 0.25 wt% for H2, CO2 and CH4, respectively. High H2 and CH4 selectivity in MOFs has been reported previously.11 However, and to the best of our knowledge, this is the first time that a microporous material, including not only MOFs but also zeolites and activated carbons, shows this behaviour. The existence of type III isotherms generally implies a low interaction between the adsorbate (i.e. the gas molecules) and the adsorbent (i.e. the MOF).6a,12 This could suggest that the interaction between 1 and CO2 (type III) is much lower This journal is
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Fig. 3 Adsorption isotherms on 1 obtained at 77 K (H2, green circles) and 273 K (CO2, blue diamonds; CH4, red triangles).
completely mirrored by a change in the beta angle. While this complex mechanism seems to be largely due to the flexibility of the Cu-glutarate paddle-wheel, in 2 the difference in the bpp conformation inhibits the distorted 2D layers from shifting with respect to each other and no flexibility can therefore be observed. Further experimental work using in situ XRD to confirm the existence or not of structural changes during gas adsorption or the existence of densified surface layers or structural defects preventing the access to the porosity is in progress, with the aim of studying the mechanism of action of this metal–organic framework. This work was supported by the Junta de Andalucı´a (FQM4228) (A. J. Calahorro for a predoctoral grant) and the MEC of Spain (Project CTQ2011-24478). D. F.-J. thanks the Royal Society for a University Research Fellowship.
Notes and references compared to that of the H2 (type I). This idea is counterintuitive since the CO2 molecules generally present higher interaction with an adsorbent due to the higher molecular weight and the presence of a strong quadrupole moment. A second possibility to this behaviour would be the existence of a molecular sieve effect, where the differences between the size of the H2, CO2 and CH4 molecules play an important role in the diffusion of the gases through the porosity. In this case, a small molecule such as H2 may diffuse through a narrow porosity, whereas CH4 and CO2 are hindered. Only high pressures allow enough potential energy to either overdue the kinetic barrier in the diffusion, or to provoke a gate opening or breathing effect and therefore a structural change. This explanation is, however, not compatible with the relatively broad 6 Å porosity found in this material. Moreover, the use of grand canonical Monte Carlo (GCMC) simulations for CO2 adsorption on a crystallographic rigid model, shown in Fig. S5 (ESI†), confirms the goodness of the experimental maximum capacity value (estimated to be ca. 23 wt%), but not the shape of the isotherm. Since GCMC simulations are in thermodynamic equilibrium, the existence of experimental kinetic barriers due to the presence of narrow porosity will not be detected using this technique. In a similar way, Feldblyum et al. were able to explain the disconnection between porous performance, as predicted by crystallography, and porous texture measured by gas adsorption.13 Using positron annihilation lifetime spectroscopy, they explained these differences due to the presence of densified layers at the surface of the material, preventing the entry of small molecular species into the bulk porosity. To further analyse structure 1, we studied the most flexible modes of the framework using a high pressure X-ray diffraction study up to 4.5 GPa. Despite the possible conformational rearrangements, the bpp ligand in 1 remains stiff and largely unchanged throughout compression, while the glutarate-Cu moiety experiences a small but progressive distortion. The rearrangement of the distorted glutarate-Cu plane allows the bpp pillars to bend rigidly with respect to it, closing the ‘accordion’ and bringing different planes significantly closer, by shifting them laterally with respect to each other. As symmetry implies this is
This journal is
c
The Royal Society of Chemistry 2013
1 (a) B. Chen, S. Ma, F. Zapata, F. R. Fronczek, E. B. Lobkovsky and H.-C. Zhou, Inorg. Chem., 2007, 46, 1233; (b) H. B. T. Jeazet, C. Staudt and C. Janiak, Chem. Commun., 2012, 48, 2140; (c) H. Z. Chen and T.-S. Chung, Int. J. Hydrogen Energy, 2012, 37, 6001. 2 J. Shang, G. Li, R. Singh, Q. Gu, K. M. Nairn, T. J. Bastow, N. Medhekar, C. M. Doherty, A. J. Hill, J. Z. Liu and P. A. Webley, J. Am. Chem. Soc., 2012, 134, 19246. 3 H.-C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673. 4 J. Getzschmann, I. Senkovska, D. Wallacher, M. Tovar, D. Fairen¨ren, J. M. van Baten, R. Krishna and S. Kaskel, Jimenez, T. Du Microporous Mesoporous Mater., 2010, 136, 50. 5 (a) K. Uemura, R. Matsuda and S. Kitagawa, J. Solid State Chem., 2005, 178, 2420; (b) N. A. Ramsahye, G. Maurin, S. Bourrelly, P. L. Llewellyn, T. Loiseau, C. Serre and G. Ferey, Chem. Commun., 2007, 3261; (c) D. Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, ¨ren, J. Am. Chem. Soc., 2011, P. A. Wright, S. Parsons and T. J. Du 133, 8900. ¨ren, Langmuir, 2010, 6 (a) D. Fairen-Jimenez, N. A. Seaton and T. Du 26, 14694; (b) A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998. ´pez-Viseras, A. Salinas-Castillo, D. Fairen-Jimenez, 7 A. J. Calahorro, M. E. Lo ´guez, CrystEngComm, 2012, E. Colacio, J. Cano and A. Rodrı´guez-Die 14, 6390. 8 Synthesis of [Cu2(glu)2(m-bpp)]2H2O (1). The reaction in water solvent of the copper sulfate (0.25 mmol, 0.0628 g), glutaric acid (0.25 mmol, 0.033 g), urea (0.872 mmol, 0.054 g) and 1,3-bis(4-pyridyl)propane (0.25 mmol, 0.046 g) led to a blue solution, which kept at room temperature for one week gave rise to green crystals of complex 1, which were filtered off and air-dried. Yield: 60%. Anal. calc. for C23H26Cu2N2O8: C, 47.18; H, 4.48; N, 4.78. Experimental: C, 47.05; H, 4.61; N, 4.83. 9 Crystal data: C23H26Cu2N2O8, fw = 585.54 g mol 1; monoclinic, C2/c, a = 28.265(5), b = 13.127(5), c = 8.729(5), b = 106.779(5), V = 3101(2) Å3; Z = 4; T = 293 K; rcalc = 1.254 g cm 3; F(000) = 1200; m(Mo-Ka) = 1.411 cm 1. Data collected on a Bruker Axs APEX automated diffractometer at RT, R1(Fo) = 0.0326 (wR2(Fo2) = 0.0752) for 17 018 unique reflections (Rint = 0.0865) with a goodness-of-fit on F2 1.048. For both structures, data were collected by oyv/2h scans (2ymax = 561) on a Bruker SMART CCD diffractometer with graphite-monochromated MoKa radiation (l = 0.71073). The structures were solved by direct methods and refined on F2 by the SHELX-97 program.11. 10 I. H. Hwang, J. M. Bae, W.-S. Kim, Y. D. Jo, C. Kim, Y. Kim, S.-J. Kim and S. Huh, Dalton Trans., 2012, 41, 12759. 11 (a) B. Chen, S. Ma, F. Zapata, F. R. Fronczek, E. B. Lobkovsky and H.-C. Zhou, Inorg. Chem., 2007, 46, 1233; (b) M. Xue, S. Ma, Z. Jin, R. M. Schaffino, G. S. Zhu, E. B. Lobkovsky, S. L. Qiu and B. L. Chen, Inorg. Chem., 2008, 47, 6825. 12 J. Rouquerol, F. Rouquerol and K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, San Diego, CA, 1999. 13 J. I. Feldblyum, M. Liu, D. W. Gidley and A. J. Matzger, J. Am. Chem. Soc., 2011, 133, 18257.
Chem. Commun., 2013, 49, 11329--11331
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