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A robust microporous ytterbium metal-organic framework with open metal sites for highly selective adsorption of CO2 over CH4
Inorganic Chemistry Communications 86 (2017) 137–139
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A robust microporous ytterbium metal-organic framework with open metal sites for highly selective adsorption of CO2 over CH4 Fengxian Gao ⁎, Yue Li, Yingjie Ye, Longtao Zhao Department of Material and Chemistry Engineering, Henan Institute of Engineering, Zhengzhou 450007, PR China
a r t i c l e
i n f o
Article history: Received 18 September 2017 Received in revised form 8 October 2017 Accepted 11 October 2017 Available online 12 October 2017 Keywords: Lanthanide metal–organic framework Helical chain SBU CO2 capture Selective gas sorption
a b s t r a c t A rare three-dimensional (3D) microporous lanthanide metal–organic framework, {[Yb(BTB)(H2O)](MeOH)2(H2O)}n (1), with 1D nano-sized channels has been constructed by bridging helical chain secondary building units with 1,3,5-Benzenetrisbenzoic acid (H3BTB) ligand. The highly rigid and stable framework contains a 1D triangular channel-type structure with highly polar pore surfaces decorated with abundant open metal sites. The desolvated framework {[Yb(BTB)]}n is found to exhibit permanent porosity with high CO2 storage capacities and significant selective sorption of CO2 over CH4 around room temperature. © 2017 Published by Elsevier B.V.
The design and construction of lanthanide metal–organic frameworks (Ln-MOFs) have undergone a booming development in the last two decades for their intrinsic porosity characteristics and physical properties coming from the lanthanide ions [1–8]. Till now, research on Ln-MOFs has mainly been focused on their fluorescent properties, while the construction of robust porous Ln-MOFs and their potential application in selective gas adsorption have been much less reported [9]. This is mainly because of the difficulty in constructing porous LnMOFs: the large coordination sphere and flexible coordination geometry of lanthanide ions tend to the formation of either condensed frameworks or only “structurally porous” frameworks which are readily collapsed once the terminal and free solvent molecules are removed during the activation. On the other hand, the organic ligand plays an important role in the construction of porous Ln-MOFs because it could not only guide the formation of the secondary building units, but can also determine the pore shapes and surroundings of the obtained products [10–12]. MOFs prepared with ligands of high symmetry have been well-studied because of synthetic and crystallographic considerations. As the elongated ligand of H3BTC, H3BTB has been widely used in the construction of Ln-MOFs. However, the combination of cluster-based secondary building units with H3BTB ligand tends to afford interpenetrating frameworks that show reduced pore volume [13–15]. It has been reported that the employment of rod-like SBUs could avoid the framework interpenetration, thus leading to MOFs with high porosity [16].
⁎ Corresponding author. E-mail address: [email protected] (F. Gao).
https://doi.org/10.1016/j.inoche.2017.10.010 1387-7003/© 2017 Published by Elsevier B.V.
In this contribution, we present the synthesis and the structural analysis of a highly porous Yb–organic network {[Yb(BTB)(H2O)](MeOH)2(H2O)}n. This MOF is composed of novel 1D helical chain building units and BTB3 − ligand, which represents the first example of Ln-MOFs based on H3BTB ligand showing 1D helical chain building units. A single component gas sorption test around room temperature reveals that the activated MOF shows excellent adsorption capabilities for CO2 and significant selective sorption of CO2 over CH4. The solvothermal reaction of Yb(NO3)3·6H2O and H3BTB in a mixed solvent of MeOH and H2O afforded complex 1 as light yellow crystals. Single-crystal X-ray diffraction reveals that 1 crystallizes in a highly symmetric and chiral hexagonal space group P6122 and the 3D coordination network is constructed through the connection of infinite 1D helical chain building units and the BTB3 − ligands. The asymmetric coordination unit consists of one Yb(III) ion situated on a symmetry site with one half occupancy, half BTB3 − ligand and one coordinated water molecule. As shown in Fig. 1a, the Yb(III) ion is sevencoordinated by six carboxylic acid O atoms from six different BTB3− ligands and one coordinated water molecule, resulting in a pentagonal bipyramid geometry. The Yb\\O bond distances are in the range of 2.212(3) to 2.609(5) Å. Each Yb(III) atom is connected with the neighboring ones though three carboxylic groups and each BTB3 − ligand binds with six Yb ions (Fig. S1). In this way, such a connection mode leads to the formation of a 1D right-handed chain along a 61 axis, which represent a rare case of Ln-based helical chain building units according to the CCDC database (Figs. 1b and S2). The pitch of the helical chain is 21.832(3) Å. Furthermore, such 1D helical chain building units are further linked by BTB3− ligand though its three carboxylate groups
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F. Gao et al. / Inorganic Chemistry Communications 86 (2017) 137–139
Fig. 1. (a) View of the asymmetric unit in 1; (b) view of 1D helical chain building units in 1; (c) view of the 1D triangular channels in 1; (d) The (6,6)-connected topology for 1.
to afford a 3D non-interpenetrating framework with 1D triangular channels with coordinated water molecule pointing to the channel center (Figs. 1c and S3). Based on the crystallographic data and also considering the van der Waals radii of atoms, the pore size for the triangular channel is 5.4 Å. From the viewpoint of net topology, this 3-D framework with 1D helical chain building units can be simplified to a 2nodal (6,6)-connected network with the point symbol of (410.65)(47.68) which has not been observed in MOF chemistry (Fig. 1d). In this work, each Yb(III) ion serves as a 6-connected node and each BTB ligand links six Yb(III) ions. The effective free volume of 1 without coordinated water molecules is 58.3% of the crystal volume (3569 Å3 of the 6126 Å3 unit cell volume), calculated with PLATON software [17].
PXRD experiment was carried out to verify the phase purity and structure stability after desolvation. As shown in Fig. S5, the diffraction peaks of compound 1 and desolvated samples are in good agreement with that of the simulated one based on the single crystal diffraction data, indicating the pure phase and structure stability after desolvation. From the thermogravimetric curve of compound 1, we found that it shows a weight loss of 14% from 25 to 140 °C, which corresponds to the release of one coordinated water molecule, two lattice MeOH and one lattice water molecule. Then, the desolvated sample can be stable up to 346 °C without any weight loss. After 346 °C, a dramatic weight loss occurs, indicating the collapse of the framework (Fig. S6). The high solvent-accessible volume together with the open metal sites (OMSs) functionalized channels promotes us to study its gas
Fig. 2. a) The CO2 and CH4 sorption isotherms around room temperature; b) CO2/CH4 adsorption selectivities around room temperature.
F. Gao et al. / Inorganic Chemistry Communications 86 (2017) 137–139
sorption properties. The permanent porosity in 1a was unambiguously established by nitrogen sorption at 77 K. Prior to gas adsorption measurement, the as-synthesized 1 was solvent-exchanged with dry MeOH and then vacuum activated at 353 K to generate the guest-free 1a. As shown in Fig. S7, the N2 sorption isotherm at 77 K indicates that 1a displays a typical type-I sorption behaviour with an N2 uptake of 418 cm3/g. Accordingly, the BET surface area derived from N2 sorption isotherm is 1932 m2/g and the pore volume is 0.58. These values are systematically higher than most of the Ln-MOFs based on the H3BTB ligand such as the MIL-103, and are also much higher than those of some benchmark MOFs functionalized with OMSs such as the Zn-MOF-74 and Cd-MOF-74 [18–20]. The sorption behaviours of 1a toward CH4 and CO2 were studied at 273 and 298 K/1 bar, all of which are fully reversible and do not reach the saturated adsorption (Fig. 2a). 1a shows a maximum CH4 uptake of 14 cm3/g at 273 K/1 bar and 9.3 cm3/g at 298 K/1 bar. In comparison, 1a could uptake much more amount of CO2 under the same conditions, with an uptake capacity of 92 cm3/g at 273 K and 60 cm3/g at 298 K. It is clearly shown that the pore structure of 1a is readily accessible to CO2. The amount of CO2 adsorbed on 1a at room temperature is comparable to some MOFs with high density of OMSs such as JLU-Jiu2 and [(CH3)NH2]3[(Cu4Cl)3(btc)8] [21,22]. The relatively high CO2 and little CH4 uptake at ambient temperature prompted us to investigate the capacity of 1a to selectively adsorb CO2 over CH4. The dual-site Langmuir–Freundlich equation fits extremely well with the single-component isotherms at 273 and 298 K, and the fitting parameters are listed in Fig. S8. Remarkably, 1a has a high selectivity toward CO2 for the both 50/50 and 5/95 CO2/CH4 mixture compositions. Such selectivities are comparable to some benchmark Ln-MOFs with high density of unsaturated metal centers or amine groups [23–26]. The high selectivity for CO2 adsorption may be due to the effect of the OMSs sites located within the wall of the porous surface, and the OMSs may strengthen the electrostatic interactions between the porous surface and CO2 molecules To understand the framework-CO2 interaction strength, isosteric heat of adsorption (Qst) was calculated by analyzing the isotherms at 273 and 298 K using the viral equation. As shown in Fig. S9, an initial Qst value of 34.8 kJ mol− 1 was obtained, and then it decreased slowly to 27.5 kJ mol− 1. The Qst of the 1a at zero coverage is relatively high among the Ln-MOFs such as the 27 kJ mol− 1 in Y-ftw-MOF-2, 33.1 kJ mol−1 in Tb-1,4-NDC and 28.23 kJ mol−1 in TbL, reflecting strong interactions between the porous material and CO2 [27–29]. Moreover, the smaller kinetic diameter of CO2 (3.30 Å) compared to CH4 (3.80 Å) might promote CO2 molecules being more easily to access the OMSs sites in 1a. In conclusion, a robust porous Yb-MOF with high pore volume and high density of OMSs has been successfully constructed using the H3BTB ligand, which resents the example of Ln-MOFs based on H3BTB ligand showing 1D helical chain building units. As expected, the resulting MOF shows a non-interpenetrated framework with 1D triangular channels running along the c axis, which shows highly selective adsorption of CO2 over CH4. It is expected that this work will initiate more investigations on the construction of non-interpenetrated LnMOFs for selective gas sorption studies. Acknowledgements This research was supported by Foundation of He'nan Educational Committee (No. 15A150043), the Doctoral Foundation of Henan
139
Institute of Engineering (No. D2014021), and the National Natural Science Foundation of China (No. 51608175). Appendix A. Supplementary data CCDC 1534274 (1) contains the supplementary crystallographic data for the title compound. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or e-mail: [email protected]. uk. Supplementary data associated with this article can be found, in the online version, at doi:https://doi.org/10.1016/j.inoche.2017.10.010.
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A robust microporous ytterbium metal-organic framework with open metal sites for highly selective adsorption of CO2 over CH4 Fengxian Gao ⁎, Yue Li, Yingjie Ye, Longtao Zhao Department of Material and Chemistry Engineering, Henan Institute of Engineering, Zhengzhou 450007, PR China
a r t i c l e
i n f o
Article history: Received 18 September 2017 Received in revised form 8 October 2017 Accepted 11 October 2017 Available online 12 October 2017 Keywords: Lanthanide metal–organic framework Helical chain SBU CO2 capture Selective gas sorption
a b s t r a c t A rare three-dimensional (3D) microporous lanthanide metal–organic framework, {[Yb(BTB)(H2O)](MeOH)2(H2O)}n (1), with 1D nano-sized channels has been constructed by bridging helical chain secondary building units with 1,3,5-Benzenetrisbenzoic acid (H3BTB) ligand. The highly rigid and stable framework contains a 1D triangular channel-type structure with highly polar pore surfaces decorated with abundant open metal sites. The desolvated framework {[Yb(BTB)]}n is found to exhibit permanent porosity with high CO2 storage capacities and significant selective sorption of CO2 over CH4 around room temperature. © 2017 Published by Elsevier B.V.
The design and construction of lanthanide metal–organic frameworks (Ln-MOFs) have undergone a booming development in the last two decades for their intrinsic porosity characteristics and physical properties coming from the lanthanide ions [1–8]. Till now, research on Ln-MOFs has mainly been focused on their fluorescent properties, while the construction of robust porous Ln-MOFs and their potential application in selective gas adsorption have been much less reported [9]. This is mainly because of the difficulty in constructing porous LnMOFs: the large coordination sphere and flexible coordination geometry of lanthanide ions tend to the formation of either condensed frameworks or only “structurally porous” frameworks which are readily collapsed once the terminal and free solvent molecules are removed during the activation. On the other hand, the organic ligand plays an important role in the construction of porous Ln-MOFs because it could not only guide the formation of the secondary building units, but can also determine the pore shapes and surroundings of the obtained products [10–12]. MOFs prepared with ligands of high symmetry have been well-studied because of synthetic and crystallographic considerations. As the elongated ligand of H3BTC, H3BTB has been widely used in the construction of Ln-MOFs. However, the combination of cluster-based secondary building units with H3BTB ligand tends to afford interpenetrating frameworks that show reduced pore volume [13–15]. It has been reported that the employment of rod-like SBUs could avoid the framework interpenetration, thus leading to MOFs with high porosity [16].
⁎ Corresponding author. E-mail address: [email protected] (F. Gao).
https://doi.org/10.1016/j.inoche.2017.10.010 1387-7003/© 2017 Published by Elsevier B.V.
In this contribution, we present the synthesis and the structural analysis of a highly porous Yb–organic network {[Yb(BTB)(H2O)](MeOH)2(H2O)}n. This MOF is composed of novel 1D helical chain building units and BTB3 − ligand, which represents the first example of Ln-MOFs based on H3BTB ligand showing 1D helical chain building units. A single component gas sorption test around room temperature reveals that the activated MOF shows excellent adsorption capabilities for CO2 and significant selective sorption of CO2 over CH4. The solvothermal reaction of Yb(NO3)3·6H2O and H3BTB in a mixed solvent of MeOH and H2O afforded complex 1 as light yellow crystals. Single-crystal X-ray diffraction reveals that 1 crystallizes in a highly symmetric and chiral hexagonal space group P6122 and the 3D coordination network is constructed through the connection of infinite 1D helical chain building units and the BTB3 − ligands. The asymmetric coordination unit consists of one Yb(III) ion situated on a symmetry site with one half occupancy, half BTB3 − ligand and one coordinated water molecule. As shown in Fig. 1a, the Yb(III) ion is sevencoordinated by six carboxylic acid O atoms from six different BTB3− ligands and one coordinated water molecule, resulting in a pentagonal bipyramid geometry. The Yb\\O bond distances are in the range of 2.212(3) to 2.609(5) Å. Each Yb(III) atom is connected with the neighboring ones though three carboxylic groups and each BTB3 − ligand binds with six Yb ions (Fig. S1). In this way, such a connection mode leads to the formation of a 1D right-handed chain along a 61 axis, which represent a rare case of Ln-based helical chain building units according to the CCDC database (Figs. 1b and S2). The pitch of the helical chain is 21.832(3) Å. Furthermore, such 1D helical chain building units are further linked by BTB3− ligand though its three carboxylate groups
138
F. Gao et al. / Inorganic Chemistry Communications 86 (2017) 137–139
Fig. 1. (a) View of the asymmetric unit in 1; (b) view of 1D helical chain building units in 1; (c) view of the 1D triangular channels in 1; (d) The (6,6)-connected topology for 1.
to afford a 3D non-interpenetrating framework with 1D triangular channels with coordinated water molecule pointing to the channel center (Figs. 1c and S3). Based on the crystallographic data and also considering the van der Waals radii of atoms, the pore size for the triangular channel is 5.4 Å. From the viewpoint of net topology, this 3-D framework with 1D helical chain building units can be simplified to a 2nodal (6,6)-connected network with the point symbol of (410.65)(47.68) which has not been observed in MOF chemistry (Fig. 1d). In this work, each Yb(III) ion serves as a 6-connected node and each BTB ligand links six Yb(III) ions. The effective free volume of 1 without coordinated water molecules is 58.3% of the crystal volume (3569 Å3 of the 6126 Å3 unit cell volume), calculated with PLATON software [17].
PXRD experiment was carried out to verify the phase purity and structure stability after desolvation. As shown in Fig. S5, the diffraction peaks of compound 1 and desolvated samples are in good agreement with that of the simulated one based on the single crystal diffraction data, indicating the pure phase and structure stability after desolvation. From the thermogravimetric curve of compound 1, we found that it shows a weight loss of 14% from 25 to 140 °C, which corresponds to the release of one coordinated water molecule, two lattice MeOH and one lattice water molecule. Then, the desolvated sample can be stable up to 346 °C without any weight loss. After 346 °C, a dramatic weight loss occurs, indicating the collapse of the framework (Fig. S6). The high solvent-accessible volume together with the open metal sites (OMSs) functionalized channels promotes us to study its gas
Fig. 2. a) The CO2 and CH4 sorption isotherms around room temperature; b) CO2/CH4 adsorption selectivities around room temperature.
F. Gao et al. / Inorganic Chemistry Communications 86 (2017) 137–139
sorption properties. The permanent porosity in 1a was unambiguously established by nitrogen sorption at 77 K. Prior to gas adsorption measurement, the as-synthesized 1 was solvent-exchanged with dry MeOH and then vacuum activated at 353 K to generate the guest-free 1a. As shown in Fig. S7, the N2 sorption isotherm at 77 K indicates that 1a displays a typical type-I sorption behaviour with an N2 uptake of 418 cm3/g. Accordingly, the BET surface area derived from N2 sorption isotherm is 1932 m2/g and the pore volume is 0.58. These values are systematically higher than most of the Ln-MOFs based on the H3BTB ligand such as the MIL-103, and are also much higher than those of some benchmark MOFs functionalized with OMSs such as the Zn-MOF-74 and Cd-MOF-74 [18–20]. The sorption behaviours of 1a toward CH4 and CO2 were studied at 273 and 298 K/1 bar, all of which are fully reversible and do not reach the saturated adsorption (Fig. 2a). 1a shows a maximum CH4 uptake of 14 cm3/g at 273 K/1 bar and 9.3 cm3/g at 298 K/1 bar. In comparison, 1a could uptake much more amount of CO2 under the same conditions, with an uptake capacity of 92 cm3/g at 273 K and 60 cm3/g at 298 K. It is clearly shown that the pore structure of 1a is readily accessible to CO2. The amount of CO2 adsorbed on 1a at room temperature is comparable to some MOFs with high density of OMSs such as JLU-Jiu2 and [(CH3)NH2]3[(Cu4Cl)3(btc)8] [21,22]. The relatively high CO2 and little CH4 uptake at ambient temperature prompted us to investigate the capacity of 1a to selectively adsorb CO2 over CH4. The dual-site Langmuir–Freundlich equation fits extremely well with the single-component isotherms at 273 and 298 K, and the fitting parameters are listed in Fig. S8. Remarkably, 1a has a high selectivity toward CO2 for the both 50/50 and 5/95 CO2/CH4 mixture compositions. Such selectivities are comparable to some benchmark Ln-MOFs with high density of unsaturated metal centers or amine groups [23–26]. The high selectivity for CO2 adsorption may be due to the effect of the OMSs sites located within the wall of the porous surface, and the OMSs may strengthen the electrostatic interactions between the porous surface and CO2 molecules To understand the framework-CO2 interaction strength, isosteric heat of adsorption (Qst) was calculated by analyzing the isotherms at 273 and 298 K using the viral equation. As shown in Fig. S9, an initial Qst value of 34.8 kJ mol− 1 was obtained, and then it decreased slowly to 27.5 kJ mol− 1. The Qst of the 1a at zero coverage is relatively high among the Ln-MOFs such as the 27 kJ mol− 1 in Y-ftw-MOF-2, 33.1 kJ mol−1 in Tb-1,4-NDC and 28.23 kJ mol−1 in TbL, reflecting strong interactions between the porous material and CO2 [27–29]. Moreover, the smaller kinetic diameter of CO2 (3.30 Å) compared to CH4 (3.80 Å) might promote CO2 molecules being more easily to access the OMSs sites in 1a. In conclusion, a robust porous Yb-MOF with high pore volume and high density of OMSs has been successfully constructed using the H3BTB ligand, which resents the example of Ln-MOFs based on H3BTB ligand showing 1D helical chain building units. As expected, the resulting MOF shows a non-interpenetrated framework with 1D triangular channels running along the c axis, which shows highly selective adsorption of CO2 over CH4. It is expected that this work will initiate more investigations on the construction of non-interpenetrated LnMOFs for selective gas sorption studies. Acknowledgements This research was supported by Foundation of He'nan Educational Committee (No. 15A150043), the Doctoral Foundation of Henan
139
Institute of Engineering (No. D2014021), and the National Natural Science Foundation of China (No. 51608175). Appendix A. Supplementary data CCDC 1534274 (1) contains the supplementary crystallographic data for the title compound. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or e-mail: [email protected]. uk. Supplementary data associated with this article can be found, in the online version, at doi:https://doi.org/10.1016/j.inoche.2017.10.010.
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