- Email: [email protected]
Enhancement of gas-framework interaction in a metal–organic framework by cavity modification
Sci. Bull. DOI 10.1007/s11434-016-1133-8
www.scibull.com www.springer.com/scp
Letter
Chemistry
Enhancement of gas-framework interaction in a metal–organic framework by cavity modification Denglin Fu • Yuanyuan Xu • Meng Zhao Ze Chang • Xianhe Bu
•
Received: 11 May 2016 / Revised: 5 June 2016 / Accepted: 8 June 2016 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2016
Abstract Originated from the pore space segmentation modification of a reported metal–organic framework (MOF) (NOTT-125), a new porous MOF ZnX was obtained and characterized by single-crystal X-ray diffraction, elemental analysis, X-ray powder diffraction and TGA. The ZnX exhibits remarkable selective CO2 adsorption property compared with that of the NOTT-125, which should be attributed to the enhanced gas-framework interactions induced by the fragmented pore space in ZnX. Keywords Metal–organic framework Pore segmentation Gas sorption Structure modification
The use of fossil fuels promotes the development of industrial civilization. However, many environmental problems come follow it, such as the global warming which is known as the greenhouse effect. It is widely believed that the greenhouse effect is a result of the emission of carbon dioxide (CO2) from the fossil fuel consumption [1], therefore much efforts has been devoted to the development of effective methods for CO2 capture and sequestration (CCS) [2–7].
Electronic supplementary material The online version of this article (doi:10.1007/s11434-016-1133-8) contains supplementary material, which is available to authorized users. D. Fu Y. Xu M. Zhao Z. Chang (&) X. Bu (&) School of Materials Science and Engineering, National Institute for Advanced Materials, TKL of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China e-mail: [email protected] X. Bu e-mail: [email protected]
Among the various strategies developed for the CCS, physical sorption has been proved to be an effective method, and metal–organic frameworks (MOFs) has emerged as promising porous adsorbents for their readily tailorable pore characteristics, which could facilitate their performance targeted construction [8, 9]. For CO2 adsorption in MOFs, there are three effective strategies: incorporation of unsaturated metal cation centers, metal doping and chemical functionalization [10, 11]. However, at the low pressure, the gas–framework interactions play an important role in CO2 uptaking of a framework [12, 13]. Hence a suitable pore size can maximize the gas-framework interactions to get an efficient capture of CO2 [14]. Some tactics have been reported to realize the goal [15–17], among which the segmentation of large pore into small space by ligand insertion has been proved to be a straightforward and effective one to enhance the interaction between the gas molecule and framework [18]. It should be noted that though this method could be utilized to enhance the CO2 separation and capture performances of MOFs, the implementation of it require MOFs with open metal sites (OMSs) decorated pore surfaces and organic ligands with matched size and configurations. Taking the advantage of the unambiguous structures of MOFs and the diversity of organic building blocks, the structure segmentation modification of MOFs could be a useful method for the investigation of targeted materials. However, in addition to the fundamental structural requirements, the coordination behaviour of the metal centers could also be a critical factor that could determine the modification process as well as the performance of the resulted material, which has not been investigated before. Herein, we reported a porous Zn MOF (ZnX) constructed based on oxalylbis(azanediyl)diisophthalic acid (H4L) and 4,40 -bipyridine, which was constructed aiming at the
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segmentation modification and performance optimization of the reported Cu based MOF NOTT-125 (NOTT is short for Nottingham) that showing remarkable CO2 sorption properties [19, 20]. After many attempts, it was found the targeted construction could only be achieved when the Cu2? ions in the parent NOTT-125 were replaced with Zn2?, which emphasized the role of metal centers in the segmentation modification. Furthermore, the ZnX show remarkable selective CO2 adsorption property as expected. Our research originate from the segmentation modification of NOTT-125, a Cu based MOF which has been reported to show remarkable gas sorption properties. As reported, NOTT-125 possesses two different kinds of
cages. Cage A is consisted of 6 L4- ligands and 12 Cu2(OOCR)4 paddlewheels units to give ellipsoidal interspace with about 1.2 nm in diameter. Cage B is constructed by 12 L4- ligands and 6 Cu2(OOCR)4 paddlewheels units to show a spherical interspace with about 1.3 nm in diameter. These two types of cages were interlinked by sharing edges to result in the porous framework of NOTT125. Structure analysis of the cage based framework show that the Cu2(OOCR)4 paddle-wheel units may provide OMSs for coordination of additional ligands, and the ˚ ) between these sites in the moderate distance (11.424 A Cage B (Fig. 1a) make them possible to be interconnected by 4,40 -bipyridine ligand. All these factors make NOTT-
Fig. 1 a The distance between the neighboring OMSs in NOTT-125. b The fragmented cage in ZnX. c The coordination environments of two kinds of Zn2(OOCR)4 paddle-wheel units in ZnX
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125 a good candidate for segmentation modification and the resulted compound is expected to exhibit further improved CO2 sorption performances. Based on the considerations mentioned above, targeted construction of the MOF has been proceeded. According to the successful examples of the segmentation modification of MOFs, one-pot synthesis method have been applied based on the original synthesis condition of NOTT-125 with the 4,40 -bipyridine ligand add directly into the reaction system. The experimental sections are shown in Electronic Supplementary Material (online). However, the desired MOF could not be obtained though much effort has been made. As an alternating approach, the Zn2? ions was applied to construction Zn2(OOCR)4 paddlewheel unit based analogue framework of NOTT-125, and the ZnX showing expected segmented porous framework structure was obtained. Similar to that of NOTT-125, the ZnX crystallizes in the monoclinic space group P21/c, and the main cage based framework of it is quite similar to that of NOTT-125. In contrast, there are two kinds of Zn2(OOCR)4 paddle-wheel units center in ZnX according to their coordination modes: for one kind of the units, both of the axial coordination sites were occupied by the N atoms from 4,40 -bipyridine, while for the other ones the two sites are occupied by one 4,40 -bipyridine and one water molecule respectively (Fig. 1c). As expected, all the 4,40 -bipyridine ligands were located in the cage B by coordinating to the axial sites of the paddle-wheel units, and every cage B is occupied by two 4,40 -bipyridine ligands showing parallel arrangement. As a result, the spherical innerspace in cage B is successfully segmented by the 4,40 -bipyridine (Fig. 1b). In addition to the success in the structure modification, a detailed analysis of the structure of ZnX also reveals the impact of coordination behavior of the metal center on the structure modification. It should be noted that the 4,40 bipyridine in the ZnX adopt a configuration with highly tension for coordination, and the Zn2(OOCR)4 units also show a distorted configuration compared with the Cu2(OOCR)4 units in the NOTT-125 (Fig. 1c). This phenomenon suggests that the backbone of the additional ligand and the pristine framework needs to distort to adapt the structure modification. It has been reported that the M2(OOCR)4 units constructed with different metal centers reveals distinct flexibility and the Zn2(OOCR)4 reveals relatively higher flexibility than Cu2(OOCR)4 [21]. Then the successful construction of ZnX as well as the failure in the modification of the original NOTT-125 might be attributed to the distinct flexibility of the units, which affect the deformation of the framework and the corresponding coordination of the additional ligand. In addition to the structure investigation, the ZnX was also characterized by X-ray powder diffraction (XRPD)
and thermal gravimetric analysis (TGA). The XRPD pattern of the as synthesized sample matches that from the single crystal data well, indicating the high phase purity of the bulk sample (Fig. S1 online). The TGA profile (Fig. S2 online) of the as synthesized sample shows a weight loss of 30.2 % in the range of 25–200 °C, in agreement with the weight of guest solvent molecules (calculated: 29.3 %). After that, no obvious platform is observed in the temperature range of 200–325 °C before the decomposition of the complex. The ZnX decomposes gradually in the temperature range of 325–545 °C. The considerable stability of ZnX after the removal of the guest solvent molecules promises the further investigation of its gas sorption performances. Based on the determination of purity and stability, gas sorption investigations were performed on ZnX. As shown in Fig. 2, the N2 sorption of ZnX at 77 K affords a type I isotherm, indicating the permanent microporosity of the framework. The surface areas of ZnX were estimated to be 755.69 m2/g obtained from BET method and 996.61 m2/g from Langmuir method, respectively. The surface areas of ZnX is lower than NOTT-125, probably due to the introduced 4,40 -bipyridine ligands that occupied the innerspace of the cages. The pore diameter distribution calculated form the N2 adsorption isotherm reveals a primary range around 6 nm in ZnX in contrast to that of 12 nm in NOTT125, which further confirms the success in segmentation modification (Fig. 2). On the other hand, the CO2 uptake of ZnX under 100 kPa reaches 100.75 cm3/g (19.77 %) at 273 K and 59.24 cm3/g (11.63 %) at 298 K (Fig. 3a), respectively, and the initial isosteric heat of sorption (Qst)
Fig. 2 Gas adsorption isotherms of N2 for ZnX at 77 K and the pore width distribution calculated from the adsorption isotherm (inset). Adsorption and desorption branches are shown with closed and open symbols, respectively
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Fig. 3 (Color online) a CO2 and CH4 sorption isotherms of ZnX at 273 and 298 K (left Y axis) and the corresponding IAST-predicted selectivity for the equimolar CO2/CH4 mixture (right Y axis). The filled and open symbols represent the adsorption and desorption data, respectively. b N2 and CO2 sorption isotherms of ZnX at 273 and 298 K (left Y axis) and the corresponding IAST-predicted selectivity for the 85:15 CO2/N2 mixture (right Y axis). The filled and open symbols represent the adsorption and desorption data, respectively. c Isosteric heats of adsorption of CO2 (solid line) and CH4 (dotted line in the inset) calculated from the adsorption isotherms measured at 273 and 298 K
estimated using the Virial method is about 26.70 kJ/mol at zero coverage (Figs. 3c and S3 online). Comparing with the gas sorption performance of NOTT-125a, the CO2 gas uptake of ZnX is slightly lower under the same conditions, while the Qst show a significant increase. The CH4 uptake of ZnX is 23.45 cm3/g at 273 K and 14.06 cm3/g at 298 K (Fig. 3a) under 1 bar. Under the same conditions, the material shows lower CH4 uptake and Qst (Figs. 3c and S4 online) than that of CO2, while the Qst of CH4 is increased compared with that of NOTT-125. The relatively high Qst observed in ZnX indicates stronger interaction between the framework and the gas molecules compared with that in NOTT-125, which might be attributed to the enhanced gasframework interaction induced by the fragmented pore space. Furthermore, the CO2 selectivity of ZnX was calculated using the ideal adsorbed solution theory (IAST) method to evaluate its selective CO2 sorption performances. As shown in Fig. 3b, the selectivity of CO2/N2 reaches about 63 at 273 K and low pressure, while a decrease trend was observed upon increasing the pressure (46 at 120 kPa). The value shows the same trends in the range of 38–32 at 298 K. This value is higher than many MOFs, such as NiMOF-74 (30 at 298 K), CuTATB-60 (24 at 298 K), ZIF100 (18 at 298 K) [22–25]. This result also indicated the strong interaction between CO2 molecules and the frameworks of ZnX. The selectivity of CO2/CH4 and CH4/N2 also calculate with IAST which is shown in Figs. 3 and S5 (online). It is worth noticing that the CH4/N2 selectivity at 298 K shows an increasing trend from 3.1 to 3.6. The temperature and pressure dependent CO2 selectivity of ZnX indicated its potential for CO2 separation applications at different pressure and temperature. In conclusion, the performances targeted construction of a cage based MOF, ZnX, was achieved originated from the
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segmentation modification of the well-studied NOTT-125. Compared with the parental NOTT-125, the distorted configuration of the paddle-wheel units and the in ZnX indicates that the nature of the metal centers could be a critical factor that determined the introduction of additional ligands for structure modulation. Furthermore, the fragmented pore space in ZnX further enhanced the interactions between the gas molecules and the host framework, which benefit its selective CO2 sorption performances over CH4 and N2. Acknowledgments This work was supported by the National Natural Science Foundation of China (21531005, 21421001, and 21290171) and Ministry of Education Innovation Team of China (IRT13022). Conflict of interest The authors declare that they have no conflict of interest.
References 1. Service RF (2004) Choosing a CO2 separation technology. Science 305:963 2. Sumida K, Rogow DL, Mason JA et al (2012) Carbon dioxide capture in metal–organic frameworks. Chem Rev 112:724–781 3. Petit JR, Jouzel J, Raynaud D et al (1999) Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399:429–436 4. Nugent P, Belmabkhout Y, Burd SD et al (2013) Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation. Nature 495:80–84 5. Haszeldine RS (2009) Carbon capture and storage: how green can black be? Science 325:1647–1652 6. Christopher WJ, Edward JM (2010) Materials and processes for carbon capture and sequestration. ChemSusChem 3:863–864 7. Rubin ES, Mantripragada H, Marks A et al (2012) The outlook for improved carbon capture technology. Prog Energy Combust Sci 38:630–671 8. Bourrelly S, Llewellyn PL, Serre C et al (2005) Different adsorption behaviors of methane and carbon dioxide in the
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isotypic nanoporous metal terephthalates MIL-53 and MIL-47. J Am Chem Soc 127:13519–13521 Ramsahye NA, Maurin G, Bourrelly S et al (2007) Probing the adsorption sites for CO2 in metal organic frameworks materials MIL-53 (Al, Cr) and MIL-47 (V) by density functional theory. J Phys Chem C 112:514–520 Li B, Wang H, Chen B (2014) Microporous metal–organic frameworks for gas separation. Chem Asian J 9:1474–1498 Lu W, Sculley JP, Yuan D et al (2012) Polyamine-tethered porous polymer networks for carbon dioxide capture from flue gas. Angew Chem Int Ed 51:7480–7484 Himeno S, Toshiya Komatsu A, Fujita S (2005) High-pressure adsorption equilibria of methane and carbon dioxide on several activated carbons. J Chem Eng Data 50:369–376 Sevilla M, Fuertes AB (2012) CO2 adsorption by activated templated carbons. J Colloid Interface Sci 366:147–154 Zhao X, Bu X, Zhai QG et al (2015) Pore space partition by symmetry-matching regulated ligand insertion and dramatic tuning on carbon dioxide uptake. J Am Chem Soc 137:1396–1399 Zheng ST, Bu JT, Li Y et al (2010) Pore space partition and charge separation in cage-within-cage indium-organic frameworks with high CO2 uptake. J Am Chem Soc 132:17062–17064 Zheng ST, Mao C, Wu T et al (2012) Generalized synthesis of zeolite-type metal–organic frameworks encapsulating immobilized transition-metal clusters. J Am Chem Soc 134:11936–11939 Bu F, Lin Q, Zhai Q et al (2012) Two zeolite-type frameworks in one metal–organic framework with Zn24@Zn104 cube-in-sodalite architecture. Angew Chem 51:8538–8541
18. Wang H, Xu J, Zhang DS et al (2015) Crystalline capsules: metal–organic frameworks locked by size-matching ligand bolts. Angew Chem 54:5966–5970 19. Alsmail NH, Suyetin M, Yan Y et al (2008) Analysis of high and selective uptake of CO2 in an oxamide-containing {Cu2(OOCR)4}based metal–organic framework. Chem Eur J 1085:7317–7324 20. Wang Z, Zheng B, Liu H et al (2013) High-capacity gas storage by a microporous oxalamide-functionalized NbO-Type metal– organic framework. Cryst Growth Des 13:5001–5006 21. Bureekaew S, Amirjalayer S, Schmid R (2012) Orbital directing effects in copper and zinc based paddle-wheel metal organic frameworks: the origin of flexibility. J Mater Chem 22:10249–10254 22. Dietzel PDC, Besikiotis V, Blom R (2009) Application of metal– organic frameworks with coordinatively unsaturated metal sites in storage and separation of methane and carbon dioxide. J Mater Chem 19:7362–7370 23. Kim J (2011) Control of catenation in CuTATB-n metal–organic frameworks by sonochemical synthesis and its effect on CO2 adsorption. J Mater Chem 21:3070–3076 24. Wang B, Coˆte´ AP, Furukawa H et al (2008) Colossal cages in zeolitic imidazolate frameworks as selective carbon dioxide reservoirs. Nature 453:207–211 25. Mason JA, Sumida K, Herm ZR et al (2011) Evaluating metal– organic frameworks for post-combustion carbon dioxide capture via temperature swing adsorption. Energy Environ Sci 4:3030–3040
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www.scibull.com www.springer.com/scp
Letter
Chemistry
Enhancement of gas-framework interaction in a metal–organic framework by cavity modification Denglin Fu • Yuanyuan Xu • Meng Zhao Ze Chang • Xianhe Bu
•
Received: 11 May 2016 / Revised: 5 June 2016 / Accepted: 8 June 2016 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2016
Abstract Originated from the pore space segmentation modification of a reported metal–organic framework (MOF) (NOTT-125), a new porous MOF ZnX was obtained and characterized by single-crystal X-ray diffraction, elemental analysis, X-ray powder diffraction and TGA. The ZnX exhibits remarkable selective CO2 adsorption property compared with that of the NOTT-125, which should be attributed to the enhanced gas-framework interactions induced by the fragmented pore space in ZnX. Keywords Metal–organic framework Pore segmentation Gas sorption Structure modification
The use of fossil fuels promotes the development of industrial civilization. However, many environmental problems come follow it, such as the global warming which is known as the greenhouse effect. It is widely believed that the greenhouse effect is a result of the emission of carbon dioxide (CO2) from the fossil fuel consumption [1], therefore much efforts has been devoted to the development of effective methods for CO2 capture and sequestration (CCS) [2–7].
Electronic supplementary material The online version of this article (doi:10.1007/s11434-016-1133-8) contains supplementary material, which is available to authorized users. D. Fu Y. Xu M. Zhao Z. Chang (&) X. Bu (&) School of Materials Science and Engineering, National Institute for Advanced Materials, TKL of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China e-mail: [email protected] X. Bu e-mail: [email protected]
Among the various strategies developed for the CCS, physical sorption has been proved to be an effective method, and metal–organic frameworks (MOFs) has emerged as promising porous adsorbents for their readily tailorable pore characteristics, which could facilitate their performance targeted construction [8, 9]. For CO2 adsorption in MOFs, there are three effective strategies: incorporation of unsaturated metal cation centers, metal doping and chemical functionalization [10, 11]. However, at the low pressure, the gas–framework interactions play an important role in CO2 uptaking of a framework [12, 13]. Hence a suitable pore size can maximize the gas-framework interactions to get an efficient capture of CO2 [14]. Some tactics have been reported to realize the goal [15–17], among which the segmentation of large pore into small space by ligand insertion has been proved to be a straightforward and effective one to enhance the interaction between the gas molecule and framework [18]. It should be noted that though this method could be utilized to enhance the CO2 separation and capture performances of MOFs, the implementation of it require MOFs with open metal sites (OMSs) decorated pore surfaces and organic ligands with matched size and configurations. Taking the advantage of the unambiguous structures of MOFs and the diversity of organic building blocks, the structure segmentation modification of MOFs could be a useful method for the investigation of targeted materials. However, in addition to the fundamental structural requirements, the coordination behaviour of the metal centers could also be a critical factor that could determine the modification process as well as the performance of the resulted material, which has not been investigated before. Herein, we reported a porous Zn MOF (ZnX) constructed based on oxalylbis(azanediyl)diisophthalic acid (H4L) and 4,40 -bipyridine, which was constructed aiming at the
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segmentation modification and performance optimization of the reported Cu based MOF NOTT-125 (NOTT is short for Nottingham) that showing remarkable CO2 sorption properties [19, 20]. After many attempts, it was found the targeted construction could only be achieved when the Cu2? ions in the parent NOTT-125 were replaced with Zn2?, which emphasized the role of metal centers in the segmentation modification. Furthermore, the ZnX show remarkable selective CO2 adsorption property as expected. Our research originate from the segmentation modification of NOTT-125, a Cu based MOF which has been reported to show remarkable gas sorption properties. As reported, NOTT-125 possesses two different kinds of
cages. Cage A is consisted of 6 L4- ligands and 12 Cu2(OOCR)4 paddlewheels units to give ellipsoidal interspace with about 1.2 nm in diameter. Cage B is constructed by 12 L4- ligands and 6 Cu2(OOCR)4 paddlewheels units to show a spherical interspace with about 1.3 nm in diameter. These two types of cages were interlinked by sharing edges to result in the porous framework of NOTT125. Structure analysis of the cage based framework show that the Cu2(OOCR)4 paddle-wheel units may provide OMSs for coordination of additional ligands, and the ˚ ) between these sites in the moderate distance (11.424 A Cage B (Fig. 1a) make them possible to be interconnected by 4,40 -bipyridine ligand. All these factors make NOTT-
Fig. 1 a The distance between the neighboring OMSs in NOTT-125. b The fragmented cage in ZnX. c The coordination environments of two kinds of Zn2(OOCR)4 paddle-wheel units in ZnX
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125 a good candidate for segmentation modification and the resulted compound is expected to exhibit further improved CO2 sorption performances. Based on the considerations mentioned above, targeted construction of the MOF has been proceeded. According to the successful examples of the segmentation modification of MOFs, one-pot synthesis method have been applied based on the original synthesis condition of NOTT-125 with the 4,40 -bipyridine ligand add directly into the reaction system. The experimental sections are shown in Electronic Supplementary Material (online). However, the desired MOF could not be obtained though much effort has been made. As an alternating approach, the Zn2? ions was applied to construction Zn2(OOCR)4 paddlewheel unit based analogue framework of NOTT-125, and the ZnX showing expected segmented porous framework structure was obtained. Similar to that of NOTT-125, the ZnX crystallizes in the monoclinic space group P21/c, and the main cage based framework of it is quite similar to that of NOTT-125. In contrast, there are two kinds of Zn2(OOCR)4 paddle-wheel units center in ZnX according to their coordination modes: for one kind of the units, both of the axial coordination sites were occupied by the N atoms from 4,40 -bipyridine, while for the other ones the two sites are occupied by one 4,40 -bipyridine and one water molecule respectively (Fig. 1c). As expected, all the 4,40 -bipyridine ligands were located in the cage B by coordinating to the axial sites of the paddle-wheel units, and every cage B is occupied by two 4,40 -bipyridine ligands showing parallel arrangement. As a result, the spherical innerspace in cage B is successfully segmented by the 4,40 -bipyridine (Fig. 1b). In addition to the success in the structure modification, a detailed analysis of the structure of ZnX also reveals the impact of coordination behavior of the metal center on the structure modification. It should be noted that the 4,40 bipyridine in the ZnX adopt a configuration with highly tension for coordination, and the Zn2(OOCR)4 units also show a distorted configuration compared with the Cu2(OOCR)4 units in the NOTT-125 (Fig. 1c). This phenomenon suggests that the backbone of the additional ligand and the pristine framework needs to distort to adapt the structure modification. It has been reported that the M2(OOCR)4 units constructed with different metal centers reveals distinct flexibility and the Zn2(OOCR)4 reveals relatively higher flexibility than Cu2(OOCR)4 [21]. Then the successful construction of ZnX as well as the failure in the modification of the original NOTT-125 might be attributed to the distinct flexibility of the units, which affect the deformation of the framework and the corresponding coordination of the additional ligand. In addition to the structure investigation, the ZnX was also characterized by X-ray powder diffraction (XRPD)
and thermal gravimetric analysis (TGA). The XRPD pattern of the as synthesized sample matches that from the single crystal data well, indicating the high phase purity of the bulk sample (Fig. S1 online). The TGA profile (Fig. S2 online) of the as synthesized sample shows a weight loss of 30.2 % in the range of 25–200 °C, in agreement with the weight of guest solvent molecules (calculated: 29.3 %). After that, no obvious platform is observed in the temperature range of 200–325 °C before the decomposition of the complex. The ZnX decomposes gradually in the temperature range of 325–545 °C. The considerable stability of ZnX after the removal of the guest solvent molecules promises the further investigation of its gas sorption performances. Based on the determination of purity and stability, gas sorption investigations were performed on ZnX. As shown in Fig. 2, the N2 sorption of ZnX at 77 K affords a type I isotherm, indicating the permanent microporosity of the framework. The surface areas of ZnX were estimated to be 755.69 m2/g obtained from BET method and 996.61 m2/g from Langmuir method, respectively. The surface areas of ZnX is lower than NOTT-125, probably due to the introduced 4,40 -bipyridine ligands that occupied the innerspace of the cages. The pore diameter distribution calculated form the N2 adsorption isotherm reveals a primary range around 6 nm in ZnX in contrast to that of 12 nm in NOTT125, which further confirms the success in segmentation modification (Fig. 2). On the other hand, the CO2 uptake of ZnX under 100 kPa reaches 100.75 cm3/g (19.77 %) at 273 K and 59.24 cm3/g (11.63 %) at 298 K (Fig. 3a), respectively, and the initial isosteric heat of sorption (Qst)
Fig. 2 Gas adsorption isotherms of N2 for ZnX at 77 K and the pore width distribution calculated from the adsorption isotherm (inset). Adsorption and desorption branches are shown with closed and open symbols, respectively
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Fig. 3 (Color online) a CO2 and CH4 sorption isotherms of ZnX at 273 and 298 K (left Y axis) and the corresponding IAST-predicted selectivity for the equimolar CO2/CH4 mixture (right Y axis). The filled and open symbols represent the adsorption and desorption data, respectively. b N2 and CO2 sorption isotherms of ZnX at 273 and 298 K (left Y axis) and the corresponding IAST-predicted selectivity for the 85:15 CO2/N2 mixture (right Y axis). The filled and open symbols represent the adsorption and desorption data, respectively. c Isosteric heats of adsorption of CO2 (solid line) and CH4 (dotted line in the inset) calculated from the adsorption isotherms measured at 273 and 298 K
estimated using the Virial method is about 26.70 kJ/mol at zero coverage (Figs. 3c and S3 online). Comparing with the gas sorption performance of NOTT-125a, the CO2 gas uptake of ZnX is slightly lower under the same conditions, while the Qst show a significant increase. The CH4 uptake of ZnX is 23.45 cm3/g at 273 K and 14.06 cm3/g at 298 K (Fig. 3a) under 1 bar. Under the same conditions, the material shows lower CH4 uptake and Qst (Figs. 3c and S4 online) than that of CO2, while the Qst of CH4 is increased compared with that of NOTT-125. The relatively high Qst observed in ZnX indicates stronger interaction between the framework and the gas molecules compared with that in NOTT-125, which might be attributed to the enhanced gasframework interaction induced by the fragmented pore space. Furthermore, the CO2 selectivity of ZnX was calculated using the ideal adsorbed solution theory (IAST) method to evaluate its selective CO2 sorption performances. As shown in Fig. 3b, the selectivity of CO2/N2 reaches about 63 at 273 K and low pressure, while a decrease trend was observed upon increasing the pressure (46 at 120 kPa). The value shows the same trends in the range of 38–32 at 298 K. This value is higher than many MOFs, such as NiMOF-74 (30 at 298 K), CuTATB-60 (24 at 298 K), ZIF100 (18 at 298 K) [22–25]. This result also indicated the strong interaction between CO2 molecules and the frameworks of ZnX. The selectivity of CO2/CH4 and CH4/N2 also calculate with IAST which is shown in Figs. 3 and S5 (online). It is worth noticing that the CH4/N2 selectivity at 298 K shows an increasing trend from 3.1 to 3.6. The temperature and pressure dependent CO2 selectivity of ZnX indicated its potential for CO2 separation applications at different pressure and temperature. In conclusion, the performances targeted construction of a cage based MOF, ZnX, was achieved originated from the
123
segmentation modification of the well-studied NOTT-125. Compared with the parental NOTT-125, the distorted configuration of the paddle-wheel units and the in ZnX indicates that the nature of the metal centers could be a critical factor that determined the introduction of additional ligands for structure modulation. Furthermore, the fragmented pore space in ZnX further enhanced the interactions between the gas molecules and the host framework, which benefit its selective CO2 sorption performances over CH4 and N2. Acknowledgments This work was supported by the National Natural Science Foundation of China (21531005, 21421001, and 21290171) and Ministry of Education Innovation Team of China (IRT13022). Conflict of interest The authors declare that they have no conflict of interest.
References 1. Service RF (2004) Choosing a CO2 separation technology. Science 305:963 2. Sumida K, Rogow DL, Mason JA et al (2012) Carbon dioxide capture in metal–organic frameworks. Chem Rev 112:724–781 3. Petit JR, Jouzel J, Raynaud D et al (1999) Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399:429–436 4. Nugent P, Belmabkhout Y, Burd SD et al (2013) Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation. Nature 495:80–84 5. Haszeldine RS (2009) Carbon capture and storage: how green can black be? Science 325:1647–1652 6. Christopher WJ, Edward JM (2010) Materials and processes for carbon capture and sequestration. ChemSusChem 3:863–864 7. Rubin ES, Mantripragada H, Marks A et al (2012) The outlook for improved carbon capture technology. Prog Energy Combust Sci 38:630–671 8. Bourrelly S, Llewellyn PL, Serre C et al (2005) Different adsorption behaviors of methane and carbon dioxide in the
Sci. Bull.
9.
10. 11.
12.
13. 14.
15.
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