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A comparative study on conversion of porous and non-porous metal–organic frameworks (MOFs) into carbon-based composites for carbon dioxide capture
Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
A comparative study on conversion of porous and non-porous metal–organic frameworks (MOFs) into carbon-based composites for carbon dioxide capture Yingdian He a, Jin Shang a,b,⇑, Qinghu Zhao a, Qinfen Gu c,⇑, Ke Xie a, Gang Li d, Ranjeet Singh a, Penny Xiao a,b, Paul A. Webley a,b,⇑ a
Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Melbourne, Australia c Australian Synchrotron, 800 Blackburn Rd, Clayton, Victoria 3168, Australia d Australia Centre for LNG Futures, School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6009, Australia b
a r t i c l e
i n f o
Article history: Received 31 March 2016 Accepted 12 May 2016 Available online xxxx Keywords: Metal–organic framework Nanoporous carbon Carbon dioxide capture Thermal stability Porosity generation
a b s t r a c t Nanoporous carbon-based composites derived from metal–organic frameworks (MOFs) have drawn increasing attention and hold promising potential in the application of gas adsorption and separation. Herein, we report the preparation and characterization of four novel carbon-based materials, converted from a non-porous Mg-MOF and a porous Zn-MOF which were both constructed by biphenyl-4,40 -dicarboxylic acid (BPDC) as bridging linkers in the structures. The phase transformation and structural evolution of the material were studied by in situ synchrotron powder X-ray diffraction with variable temperature. Interestingly, the results indicate the porosity generated by carbonization would be more dependent on the thermal stability rather than crystallographic intactness of the template MOFs. Moreover, the derived carbon materials selectively adsorb CO2 over N2 at moderate conditions, which would be promising for post-combustion carbon dioxide capture. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide is the largest anthropogenic greenhouse gas contributing to global warming with combustion of fossil fuels being the major emission source. A range of technologies such as solvent scrubbing, membrane, and adsorption [1], have been developed to capture CO2 from flue gas mixtures. Therein, post-combustion carbon capture predominantly involves CO2/N2 gas separation at low pressures [2]. In particular, adsorption based technology is very promising due to its advantages including low cost, simple operation, versatile processes, etc. Adsorbent materials with high CO2 selectivity and capacity are critical to the improvement of separation efficiency and reduction of energy consumption of the separation processes. Porous carbon and carbon–metal hybrid materials, featuring high thermal and chemical stability, have been intensively studied
⇑ Corresponding authors at: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. Tel.: +61 3 90357873; fax: +61 3 8344 4153 (P.A. Webley). E-mail addresses: [email protected] (J. Shang), Qinfen.Gu@synchrotron. org.au (Q. Gu), [email protected] (P.A. Webley).
in gas adsorption and separation [3–5]. To achieve better control of the amorphous structure of the carbon materials, a synthetic strategy known as templating has been developed and is attracting increasing attention [6]. A variety of adsorbents (such as zeolites, alumina, silica, etc.) have been employed as templates for the synthesis of porous carbon materials [7–12]. Recently, growing interest has been drawn to the synthesis of porous carbon materials from metal-organic frameworks (MOFs) or PCP (porous coordination polymers) which are constructed from metal nodes and organic linkers [13,14]. On one hand, MOFs feature ultra-high porosity, structural diversity and tunability, providing great advantages as a starting material for carbon-based compounds. Templating with MOFs offers an exotic carbon source, low solubility, and non-volatility, along with pre-arranged organic ligand to enhance the controllability of chemical/physical properties of the carbonbased materials [15]. On the other hand, since a great number of MOF materials are not structurally stable and exhibit non-porous properties, it is a logical attempt to convert those MOFs into compounds with permanent porosity for gas adsorption and separation [16–18]. For example, nanoporous carbons with accessible N dopants were synthesized via the pyrolysis of pyridinedicarboxylate containing Zn-MOFs, showing high CO2 capacities and
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excellent cyclic performance over repeated CO2 adsorption/desorption [19]. Yamauchi et. al. reported a nano-porous carbon (NPC), converted from Al(OH)(1,4-NDC)2H2O (1,4-NDC = 1,4-naphthanedicarboxylate), exhibiting extremely high surface area (5500 m2 g1) [20]. However, the limited understanding of the role played by templates in MOF-derived carbon materials in adsorptive gas separation restricts its further application. One important factor that requires detailed investigations is the impact of thermal stability of MOF templates on the porosity and thus the gas sorption properties of the derived pyrolysis products as adsorbents. Herein, we report the preparation and characterization of four novel carbon-based materials converted from a non-porous Mg3(BPDC)34DMF and a porous Zn3(BPDC)3(BiPY) based on the same bridging linkers (where BPDC = 4,40 -biphenyldicarboxylate, BiPY = 4,40 -dipyridyl, DMF = N,N-dimethylformamide). The asobtained carbon materials selectively adsorb CO2 over N2 at low pressure and moderate temperature, which could be promising adsorbents for post-combustion carbon dioxide capture. 2. Experimental 2.1. Chemicals All reagents were purchased from commercial sources and used without further purification. Specifically, Zinc acetate dehydrate, Mg(CH3COO)24H2O, Zn(CH3COO)22H2O, 4,40 -biphenyldicarboxylic acid (BPDC), and 4,40 -dipyridyl (BiPY) (98%) were purchased from Sigma–Aldrich. N,N-dimethylformamide (DMF) (99.9%) was purchased from PROLABO CHEMICALS. 2.2. Preparation of MOF precursors Zn3(BPDC)3(BiPY) and Mg3(BPDC)34DMF Zn3(BPDC)3(BiPY), denoted as Zn-MOF, was prepared following a similar procedure in the literature except for a few minor changes [21]. Specifically, a mixture of Zinc (II) acetate dehydrate (1 mmol, 0.2195 g), 40 -biphenyldicarboxylic acid (1 mmol, 0.242 g), and 4,40 -dipyridyl (0.5 mmol, 0.0788 g) in 20 ml DMF was stirred for 3 h and then transferred to a 40 ml Teflon-lined
stainless steel autoclave. The autoclave was placed in a pre-heated oven at 120 °C for 72 h before cooled down slowly. The colourless crystals were filtered and washed with DMF (15 ml) three times and dried in a vacuum oven for 2 h. Mg3(BPDC)34DMF, denoted as Mg-MOF, was prepared according to the following procedure. A mixture of Magnesium (II) acetate tetrahydrate (1 mmol, 0.2145 g), 40 -biphenyldicarboxylic acid (1 mmol, 0.242 g), and 4,40 -dipyridyl (0.5 mmol, 0.0781 g) in 20 ml DMF was stirred for 3 h and then transferred to a 40 ml Teflon-lined stainless steel autoclave. The autoclave was placed in a pre-heated oven at 120 °C for 60 h before being cooled down slowly. The colourless crystals were filtered and washed with DMF (15 ml) three times and dried in a vacuum oven for 2 h. 2.3. Preparation of metal–carbon hybrids and porous carbons 2.3.1. Zinc nano-particle imbedded carbon matrix 1 Approximately 0.35 g of dried Zn3(BPDC)3(BiPY) was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 650 °C with ramping rate of 2 °C/min. After being held at 650 °C for 5 h, the sample was cooled down slowly to obtain material 1. 2.3.2. Porous carbon derived from Zn-MOF 2 the sample of 1 was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 1000 °C with ramping rate of 2 °C/min. After being held at 1000 °C for 5 h, the sample was cooled down slowly to obtain material 2. 2.3.3. Magnesium nano-particle imbedded carbon matrix 3 Approximately 0.32 g of dried Mg3(BPDC)34DMF was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 650 °C with ramping rate of 2 °C/min. After being held at 650 °C for 5 h, the furnace was powder off to allow slow cooling down.
Fig. 1. Synchrotron powder XRD patterns of as-synthesized and activated samples Zn3(BPDC)3(BiPY) and structureviewing down crystallographic axis of Zn3(BPDC)3(BiPY) (a and b); Synchrotron powder XRD patterns of as-synthesized and activated samples and structure viewing down crystallographic c axis of Mg3(BPDC)34DMF (c and d). Colour scheme: Zn (cyan), Mg (yellow), C (grey), O (red), N (blue). (Color online.)
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2.3.4. Porous carbon/Mg hybrid 4 the sample of 3 was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 1000 °C with ramping rate of 2 °C/min. After being held at 1000 °C for 5 h, the sample was cooled down slowly to obtain material 4. 2.4. Thermogravimetric analysis To investigate thermal stability and composition of the samples, thermogravimetric analyses (TGA) were conducted using a Mettler Toledo TGA-SDTA851 analyser (Switzerland) from 30 °C up to 1000 °C with ramping rate of 2 °C/min, under nitrogen (45 ml/min).
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diffraction beamline at Australian synchrotron. The wavelength was calibrated by NIST LaB6 660b standard of 0.7286 Å. Prior to measurements, the samples were dried at 60 °C in a vacuum oven and directly loaded into 0.7 mm (outside diameter) glass capillaries and sealed with wax. 2.6. Scanning electron microscopy The morphology of the samples was investigated on a JEOL 7001F FEG scanning electron microscope (SEM), operated at 5.0– 15.0 kV. The sample was crushed and sprinkled on carbon tape mounted on a metal stub and coated with a 1 nm layer of platinum metal.
2.5. X-ray crystallography
2.7. Gas adsorption
For phase identification and structure determination, highresolution synchrotron powder X-ray diffraction (PXRD) data were collected on all samples by a Mythen-II detector at powder
To examine the gas adsorption property of the novel adsorbents, both volumetric and gravimetric measurements were performed.
Fig. 2. In situ variable temperature synchrotron powder XRD and TGA patterns (insets) of the as-synthesized Zn3(BPDC)3(BiPY) (a) and Mg3(BPDC)34DMF (b).
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Volumetric measurement: Adsorption properties were studied by volumetric apparatus ASAP 2010 (Micromeritics, USA). High purity N2 (99.999%) and CO2 (99.995%) were used. Approximately 120 mg of the as-synthesized samples of 1 and 3 were heated up to 190 °C (2 °C/min) under high vacuum (below 10–2 Pa) and being held at 120 °C for 10 h to obtain desolvated samples. Gravimetric measurement: around 5 mg of the as-synthesized 2 and 4 were put in pans in the Mettler Toledo TGA-SDTA851 analyser. Under Helium flow (50 ml/min), the samples were outgassed at elevated temperature up to 190 °C (ramping rate at 5 °C/min), and being held at 190 °C for 30 min. Then the gas was switched to CO2 at a flow rate of 50 ml/min for 10 min for equilibrium adsorption. CO2 desorption was achieved by 50 ml/min helium follow at elevated temperature up to 190 °C (10 °C/min) and being held at 190 °C for 10 min. The CO2 adsorption–desorption cycle was repeated for five times.
3. Results and discussion The high purity and crystallinity of the two as-synthesized MOF structures were confirmed by the synchrotron powder XRD (Fig. 1a and c). Their structures are similar to the ones reported in the literature, in spite of some variance in the reaction conditions in this work compared with the reported synthetic procedure [21,22] (ESI, Table S1). The crystal structure of the Zn-MOF consists of trinuclear Zn clusters which are connected to the carboxylate groups of six BPDC bridging linkers to form 2D double layers which pillared by the secondary ligands BiPY to generate a 3D framework (Fig. 1b). In contrast, the BiPY would act as template in the process of selfassembly formation of Mg3(BPDC)34DMF under the synthetic conditions. The second building unit of the Mg-MOF features the Mg trimers, which are octahedrally coordinated by six oxygen centers from six different carboxylate ligands. The two outer metals are each bound to two solvating DMF molecules (Fig. 1d). Along with
the square channels running down the crystallographic c-axis from the structure, open metal sites would be accessible since the structure contains coordinated solvent molecules if the crystallographic structure could sustain after the desolvation. Importantly, the crystallographic structure of the Zn-MOF maintains as evidenced by the backbone frameworks being intact upon the removal of the guest solvent molecules (Fig. 1a). In contrast, the structure of the Mg-MOF collapsed once the accommodated solvent molecules were evacuated as evident by peak intensities change in PXRD pattern (Fig. 1c). Intrinsically, the structural characteristics would attribute to the structural stability upon the porosity activation. To elucidate the structural thermal stability of the two MOF precursors which would highly impact on the atomic arrangements of the pyrolysis products, we conducted thermal gravimetric analysis (TGA) and in situ variable temperature synchrotron powder XRD measurements. The TGA results indicate the two MOFs are thermally stable up to approximately 470 °C for Mg3(BPDC)34DMF and 380 °C for Zn3(BPDC)3(BiPY) under N2 atmosphere, as evidenced by the plateau in the TGA profiles prior to the weight loss steps attributed to the decomposition of the organic ligands (Fig. 2 insets). Temperature-dependent in situ synchrotron powder XRD results provided more insights to the crystallographic stability upon desolvation. From the literature, some MOF materials experience crystallographic degradation and even their structures collapse before the observation of the weight loss steps by thermogravimetric analysis [23]. As can be seen in Fig. 2b, the crystal structure of the Mg-MOF degraded significantly when the guest molecules were removed, indicating considerable crystallinity degradation from 200 °C. The Mg-MOF, featuring structure collapse on guest removal, can be classified as the first generation of MOFs [24]. Because its potential porosity can not be activated by alternative activation procedures, such as exchange high boiling-point DMF with low boiling solvents, supercritical CO2 drying [25], or freeze drying [26], which have been demonstrated to reserve the structural intactness for other MOFs. In contrast,
Fig. 3. SEM images: (a) the as-synthesized sample of Zn3(BPDC)3(BiPY), Zn-MOF derived carbon products at 650 °C (material 1) and 1000 °C (material 2); (b) the assynthesized sample of Mg3(BPDC)34DMF Mg-MOF derived carbon products at 650 °C (material 3) and 1000 °C (material 4).
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the Zn-MOF remained crystallographic structure intact at elevated temperature up to 350 °C although there is slight structure distortion when DMF molecules were evacuated from the pore channels, as indicated in Fig. 2a. From SEM images in Fig. 3, it can be observed that the monolith single crystals (approximately 200 50 50 lm) of the Zn-MOF
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and the column-like polycrystal cluster features of the Mg-MOF. Interestingly, the MOF-derived products, synthesized at various carbonization temperatures (650 °C and 1000 °C), exhibit considerably similar morphologies to their MOF precursor (Fig. 3, ESI, Figs. S5–S8). The synthetic strategy for converting MOFs into carbon composites would provide a new route for the morphology
Fig. 4. (a) N2 adsorption isotherms of Zn3(BPDC)3(BiPY) (in black) and its derived material 1 (in red); (b) N2 adsorption isotherms of Mg3(BPDC)3 (in black) and its derived material 3 (in red); adsorption and desorption isotherms of CO2 (square, in black) and N2 (up-triangle in blue) for the materials 1 (c) and 3 (d) at 298 K; (e) CO2 adsorption/ desorption cycles of the material 2 and 4. (Color online.)
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control which could have significant impact on gas adsorption and separation process. The elemental analysis results indicated 1 and 3 are metal/carbon hybrid materials with Zn/Mg nano-particle deposited in the carbon matrix (ESI, Figs. S1, S3). The material 2 features metal-free carbon (ESI, Fig. S2), suggesting all Zn has evaporated at 1000 °C. However, magnesium nano-particles cannot be completely removed from the material 4 at 1000 °C due to the high boiling point of magnesium (1091 °C) compared with that of zinc (907 °C) (ESI, Fig. S4). The four MOF-derived materials exhibit permanent porosity evidenced by N2 adsorption isotherms of 1 and 3 at 77 K (Fig. 4a and b) and CO2 adsorption–desorption cycles of 2 and 4 (Fig. 4e). Interestingly, the BET surface area of 1 and 3 were approximately 365 m2/g and 514 m2/g, respectively, calculated by the N2 adsorption isotherms at 77 K. Although the Zn-MOF exhibits permanent porosity with BET surface area of 872 m2/g (Fig. 4a), it is noteworthy that material 3, which was derived from the non-porous MgMOF, features higher porosity than the Zn-MOF derived 1 (Fig. 4b). This may suggest the porosity of the MOF-derived materials would more depend on the thermal stability of the framework during decomposition rather than the pristine structural topology of the template MOFs. In other words, the carbonization process would degrade porosity of the porous MOFs but promote porosity in the non-porous MOF precursors. In this case, the decomposition temperature of the organic ligands in the Mg-MOF is higher than that for the Zn-MOF (Fig. 2), which would contribute to the newly generated porosity via the carbonization process (Fig. 4a and b). Moreover, material 1 and 3 selectively adsorb CO2 over N2 at moderate conditions. At 298 K and 1 bar, the CO2/N2 separation factor of 1 and 3 were approximately 7.6 and 6.2, respectively (Fig. 4c and d), which would have potential applications in the postcombustion CO2 capture. To assess recyclability of 2 and 4 for CO2 capture, repeated cycles of CO2 adsorption and desorption were conducted by thermal gravimetric analysis technique. The CO2 adsorption capacities of 2 and 4 were almost identical over five cycles, with the values of approximately 8.7 wt% and 4.8 wt%, respectively (Fig. 4e), suggesting very good reproducibility. 4. Conclusion In summary, we prepared four novel carbon-based products including Zn/Mg nano-particle imbedded carbon matrix and nano-porous carbons from MOFs via carbonization process. Carbons derived from the non-porous Mg-MOF template showed substantially greater surface area than that of the carbons converted by the porous Zn-MOF. This study provides insightful understanding of the relationship between the porosity generated by carbonization and the thermal/crystallographic stability of the template MOFs. Furthermore, the MOFs-derived carbon materials selectively adsorb CO2 over N2 at mild conditions, suggesting a great potential in carbon capture from low pressure flue gases. Acknowledgments Y.H. acknowledges the Chinese Scholarship Council (CSC). P.A. W and J.S. acknowledge Australian Research Council (ARC) for providing the funding (DP2013000024). G.L. is the recipient of an Australian Research Council Discovery Early Career Researcher Award (DE140101824). Part of work was undertaken on the PD beamline, Australian Synchrotron.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2016.05.027. References [1] B. Metz, O. Davidson, H. De Coninck, M. Loos, L. Meyer, Carbon dioxide capture and storage, (2005). [2] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.H. Bae, J.R. Long, Carbon dioxide capture in metal–organic frameworks, Chem. Rev. 112 (2012) 724. [3] J.-K. Sun, Q. Xu, Functional materials derived from open framework templates/ precursors: synthesis and applications, Energy Environ. Sci. 7 (2014) 2071. [4] X. Pei, Y. Chen, S. Li, S. Zhang, X. Feng, J. Zhou, B. Wang, Metal‐organic frameworks derived porous carbons: syntheses, porosity and gas sorption properties, Chin. J. Chem. (2016). [5] W. Chaikittisilp, K. Ariga, Y. Yamauchi, A new family of carbon materials: synthesis of MOF-derived nanoporous carbons and their promising applications, J. Mater. Chem. A 1 (2013) 14. [6] Y. Xia, Z. Yang, R. Mokaya, Templated nanoscale porous carbons, Nanoscale 2 (2010) 639. [7] T. Kyotani, T. Nagai, S. Inoue, A. Tomita, Formation of new type of porous carbon by carbonization in zeolite nanochannels, Chem. Mater. 9 (1997) 609. [8] Z. Yang, Y. Xia, R. Mokaya, Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials, J. Am. Chem. Soc. 129 (2007) 1673. [9] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Ordered mesoporous carbons, Adv. Mater. 13 (2001) 677. [10] J. Ren, J. Ding, K.-Y. Chan, H. Wang, Dual-porosity carbon templated from monosize mesoporous silica nanoparticles, Chem. Mater. 19 (2007) 2786. [11] T. Kyotani, L.-F. Tsai, A. Tomita, Formation of ultrafine carbon tubes by using an anodic aluminum oxide film as a template, Chem. Mater. 7 (1995) 1427. [12] A. Popp, J. Engstler, J.J. Schneider, Porous carbon nanotube-reinforced metals and ceramics via a double templating approach, Carbon 47 (2009) 3208. [13] J.-K. Sun, Q. Xu, Functional materials derived from open framework templates/ precursors: synthesis and applications, Energy Environ. Sci. 7 (2014) 2071. [14] B. Liu, H. Shioyama, T. Akita, Q. Xu, Metal-organic framework as a template for porous carbon synthesis, J. Am. Chem. Soc. 130 (2008) 5390. [15] S. Lim, K. Suh, Y. Kim, M. Yoon, H. Park, D.N. Dybtsev, K. Kim, Porous carbon materials with a controllable surface area synthesized from metal–organic frameworks, Chem. Commun. 48 (2012) 7447. [16] H.B. Aiyappa, P. Pachfule, R. Banerjee, S. Kurungot, Porous carbons from nonporous MOFs: influence of ligand characteristics on intrinsic properties of end carbon, Cryst. Growth Des. 13 (2013) 4195. [17] W. Wang, D. Yuan, Mesoporous carbon originated from non-permanent porous MOFs for gas storage and CO2/CH4 separation, Sci. Rep. 4 (2014). [18] Y. Yang, S. Hao, H. Zhao, Y. Wang, X. Zhang, Hierarchically porous carbons derived from nonporous metal-organic frameworks: synthesis and influence of struts, Electrochim. Acta 180 (2015) 651. [19] J. Kim, A.G. Oliver, J.C. Hicks, Enhanced CO2 capture capacities and efficiencies with N-doped nanoporous carbons synthesized from solvent-modulated, pyridinedicarboxylate-containing Zn-MOFs, CrystEngComm 17 (2015) 8015. [20] M. Hu, J. Reboul, S. Furukawa, N.L. Torad, Q. Ji, P. Srinivasu, K. Ariga, S. Kitagawa, Y. Yamauchi, Direct carbonization of Al-based porous coordination polymer for synthesis of nanoporous carbon, J. Am. Chem. Soc. 134 (2012) 2864. [21] J.Y. Lee, L. Pan, S.P. Kelly, J. Jagiello, T.J. Emge, J. Li, Achieving high density of adsorbed hydrogen in microporous metal organic frameworks, Adv. Mater. 17 (2005) 2703. [22] J.A. Rood, B.C. Noll, K.W. Henderson, Cubic networks and 36 tilings assembled from isostructural trimeric magnesium aryldicarboxylates, Main Group Chem. 5 (2006) 21. [23] P.D. Dietzel, R. Blom, H. Fjellvåg, Base-induced formation of two magnesium metal–organic framework compounds with a bifunctional tetratopic ligand, Eur. J. Inorg. Chem. 2008 (2008) 3624. [24] S. Horike, S. Shimomura, S. Kitagawa, Soft porous crystals, Nat. Chem. 1 (2009) 695. [25] A.P. Nelson, O.K. Farha, K.L. Mulfort, J.T. Hupp, Supercritical processing as a route to high internal surface areas and permanent microporosity in metal– organic framework materials, J. Am. Chem. Soc. 131 (2008) 458. [26] L. Ma, A. Jin, Z. Xie, W. Lin, Freeze drying significantly increases permanent porosity and hydrogen uptake in 4,4-connected metal-organic frameworks, Angew. Chem., Int. Ed. Engl. 48 (2009) 9905.
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Contents lists available at ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
A comparative study on conversion of porous and non-porous metal–organic frameworks (MOFs) into carbon-based composites for carbon dioxide capture Yingdian He a, Jin Shang a,b,⇑, Qinghu Zhao a, Qinfen Gu c,⇑, Ke Xie a, Gang Li d, Ranjeet Singh a, Penny Xiao a,b, Paul A. Webley a,b,⇑ a
Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Melbourne, Australia c Australian Synchrotron, 800 Blackburn Rd, Clayton, Victoria 3168, Australia d Australia Centre for LNG Futures, School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6009, Australia b
a r t i c l e
i n f o
Article history: Received 31 March 2016 Accepted 12 May 2016 Available online xxxx Keywords: Metal–organic framework Nanoporous carbon Carbon dioxide capture Thermal stability Porosity generation
a b s t r a c t Nanoporous carbon-based composites derived from metal–organic frameworks (MOFs) have drawn increasing attention and hold promising potential in the application of gas adsorption and separation. Herein, we report the preparation and characterization of four novel carbon-based materials, converted from a non-porous Mg-MOF and a porous Zn-MOF which were both constructed by biphenyl-4,40 -dicarboxylic acid (BPDC) as bridging linkers in the structures. The phase transformation and structural evolution of the material were studied by in situ synchrotron powder X-ray diffraction with variable temperature. Interestingly, the results indicate the porosity generated by carbonization would be more dependent on the thermal stability rather than crystallographic intactness of the template MOFs. Moreover, the derived carbon materials selectively adsorb CO2 over N2 at moderate conditions, which would be promising for post-combustion carbon dioxide capture. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide is the largest anthropogenic greenhouse gas contributing to global warming with combustion of fossil fuels being the major emission source. A range of technologies such as solvent scrubbing, membrane, and adsorption [1], have been developed to capture CO2 from flue gas mixtures. Therein, post-combustion carbon capture predominantly involves CO2/N2 gas separation at low pressures [2]. In particular, adsorption based technology is very promising due to its advantages including low cost, simple operation, versatile processes, etc. Adsorbent materials with high CO2 selectivity and capacity are critical to the improvement of separation efficiency and reduction of energy consumption of the separation processes. Porous carbon and carbon–metal hybrid materials, featuring high thermal and chemical stability, have been intensively studied
⇑ Corresponding authors at: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. Tel.: +61 3 90357873; fax: +61 3 8344 4153 (P.A. Webley). E-mail addresses: [email protected] (J. Shang), Qinfen.Gu@synchrotron. org.au (Q. Gu), [email protected] (P.A. Webley).
in gas adsorption and separation [3–5]. To achieve better control of the amorphous structure of the carbon materials, a synthetic strategy known as templating has been developed and is attracting increasing attention [6]. A variety of adsorbents (such as zeolites, alumina, silica, etc.) have been employed as templates for the synthesis of porous carbon materials [7–12]. Recently, growing interest has been drawn to the synthesis of porous carbon materials from metal-organic frameworks (MOFs) or PCP (porous coordination polymers) which are constructed from metal nodes and organic linkers [13,14]. On one hand, MOFs feature ultra-high porosity, structural diversity and tunability, providing great advantages as a starting material for carbon-based compounds. Templating with MOFs offers an exotic carbon source, low solubility, and non-volatility, along with pre-arranged organic ligand to enhance the controllability of chemical/physical properties of the carbonbased materials [15]. On the other hand, since a great number of MOF materials are not structurally stable and exhibit non-porous properties, it is a logical attempt to convert those MOFs into compounds with permanent porosity for gas adsorption and separation [16–18]. For example, nanoporous carbons with accessible N dopants were synthesized via the pyrolysis of pyridinedicarboxylate containing Zn-MOFs, showing high CO2 capacities and
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Please cite this article in press as: Y. He et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.05.027
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excellent cyclic performance over repeated CO2 adsorption/desorption [19]. Yamauchi et. al. reported a nano-porous carbon (NPC), converted from Al(OH)(1,4-NDC)2H2O (1,4-NDC = 1,4-naphthanedicarboxylate), exhibiting extremely high surface area (5500 m2 g1) [20]. However, the limited understanding of the role played by templates in MOF-derived carbon materials in adsorptive gas separation restricts its further application. One important factor that requires detailed investigations is the impact of thermal stability of MOF templates on the porosity and thus the gas sorption properties of the derived pyrolysis products as adsorbents. Herein, we report the preparation and characterization of four novel carbon-based materials converted from a non-porous Mg3(BPDC)34DMF and a porous Zn3(BPDC)3(BiPY) based on the same bridging linkers (where BPDC = 4,40 -biphenyldicarboxylate, BiPY = 4,40 -dipyridyl, DMF = N,N-dimethylformamide). The asobtained carbon materials selectively adsorb CO2 over N2 at low pressure and moderate temperature, which could be promising adsorbents for post-combustion carbon dioxide capture. 2. Experimental 2.1. Chemicals All reagents were purchased from commercial sources and used without further purification. Specifically, Zinc acetate dehydrate, Mg(CH3COO)24H2O, Zn(CH3COO)22H2O, 4,40 -biphenyldicarboxylic acid (BPDC), and 4,40 -dipyridyl (BiPY) (98%) were purchased from Sigma–Aldrich. N,N-dimethylformamide (DMF) (99.9%) was purchased from PROLABO CHEMICALS. 2.2. Preparation of MOF precursors Zn3(BPDC)3(BiPY) and Mg3(BPDC)34DMF Zn3(BPDC)3(BiPY), denoted as Zn-MOF, was prepared following a similar procedure in the literature except for a few minor changes [21]. Specifically, a mixture of Zinc (II) acetate dehydrate (1 mmol, 0.2195 g), 40 -biphenyldicarboxylic acid (1 mmol, 0.242 g), and 4,40 -dipyridyl (0.5 mmol, 0.0788 g) in 20 ml DMF was stirred for 3 h and then transferred to a 40 ml Teflon-lined
stainless steel autoclave. The autoclave was placed in a pre-heated oven at 120 °C for 72 h before cooled down slowly. The colourless crystals were filtered and washed with DMF (15 ml) three times and dried in a vacuum oven for 2 h. Mg3(BPDC)34DMF, denoted as Mg-MOF, was prepared according to the following procedure. A mixture of Magnesium (II) acetate tetrahydrate (1 mmol, 0.2145 g), 40 -biphenyldicarboxylic acid (1 mmol, 0.242 g), and 4,40 -dipyridyl (0.5 mmol, 0.0781 g) in 20 ml DMF was stirred for 3 h and then transferred to a 40 ml Teflon-lined stainless steel autoclave. The autoclave was placed in a pre-heated oven at 120 °C for 60 h before being cooled down slowly. The colourless crystals were filtered and washed with DMF (15 ml) three times and dried in a vacuum oven for 2 h. 2.3. Preparation of metal–carbon hybrids and porous carbons 2.3.1. Zinc nano-particle imbedded carbon matrix 1 Approximately 0.35 g of dried Zn3(BPDC)3(BiPY) was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 650 °C with ramping rate of 2 °C/min. After being held at 650 °C for 5 h, the sample was cooled down slowly to obtain material 1. 2.3.2. Porous carbon derived from Zn-MOF 2 the sample of 1 was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 1000 °C with ramping rate of 2 °C/min. After being held at 1000 °C for 5 h, the sample was cooled down slowly to obtain material 2. 2.3.3. Magnesium nano-particle imbedded carbon matrix 3 Approximately 0.32 g of dried Mg3(BPDC)34DMF was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 650 °C with ramping rate of 2 °C/min. After being held at 650 °C for 5 h, the furnace was powder off to allow slow cooling down.
Fig. 1. Synchrotron powder XRD patterns of as-synthesized and activated samples Zn3(BPDC)3(BiPY) and structureviewing down crystallographic axis of Zn3(BPDC)3(BiPY) (a and b); Synchrotron powder XRD patterns of as-synthesized and activated samples and structure viewing down crystallographic c axis of Mg3(BPDC)34DMF (c and d). Colour scheme: Zn (cyan), Mg (yellow), C (grey), O (red), N (blue). (Color online.)
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2.3.4. Porous carbon/Mg hybrid 4 the sample of 3 was placed in a crucible in a furnace tube with N2 flow (approximately 50 ml/min). The temperature was programed to elevate from room temperature to 1000 °C with ramping rate of 2 °C/min. After being held at 1000 °C for 5 h, the sample was cooled down slowly to obtain material 4. 2.4. Thermogravimetric analysis To investigate thermal stability and composition of the samples, thermogravimetric analyses (TGA) were conducted using a Mettler Toledo TGA-SDTA851 analyser (Switzerland) from 30 °C up to 1000 °C with ramping rate of 2 °C/min, under nitrogen (45 ml/min).
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diffraction beamline at Australian synchrotron. The wavelength was calibrated by NIST LaB6 660b standard of 0.7286 Å. Prior to measurements, the samples were dried at 60 °C in a vacuum oven and directly loaded into 0.7 mm (outside diameter) glass capillaries and sealed with wax. 2.6. Scanning electron microscopy The morphology of the samples was investigated on a JEOL 7001F FEG scanning electron microscope (SEM), operated at 5.0– 15.0 kV. The sample was crushed and sprinkled on carbon tape mounted on a metal stub and coated with a 1 nm layer of platinum metal.
2.5. X-ray crystallography
2.7. Gas adsorption
For phase identification and structure determination, highresolution synchrotron powder X-ray diffraction (PXRD) data were collected on all samples by a Mythen-II detector at powder
To examine the gas adsorption property of the novel adsorbents, both volumetric and gravimetric measurements were performed.
Fig. 2. In situ variable temperature synchrotron powder XRD and TGA patterns (insets) of the as-synthesized Zn3(BPDC)3(BiPY) (a) and Mg3(BPDC)34DMF (b).
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Volumetric measurement: Adsorption properties were studied by volumetric apparatus ASAP 2010 (Micromeritics, USA). High purity N2 (99.999%) and CO2 (99.995%) were used. Approximately 120 mg of the as-synthesized samples of 1 and 3 were heated up to 190 °C (2 °C/min) under high vacuum (below 10–2 Pa) and being held at 120 °C for 10 h to obtain desolvated samples. Gravimetric measurement: around 5 mg of the as-synthesized 2 and 4 were put in pans in the Mettler Toledo TGA-SDTA851 analyser. Under Helium flow (50 ml/min), the samples were outgassed at elevated temperature up to 190 °C (ramping rate at 5 °C/min), and being held at 190 °C for 30 min. Then the gas was switched to CO2 at a flow rate of 50 ml/min for 10 min for equilibrium adsorption. CO2 desorption was achieved by 50 ml/min helium follow at elevated temperature up to 190 °C (10 °C/min) and being held at 190 °C for 10 min. The CO2 adsorption–desorption cycle was repeated for five times.
3. Results and discussion The high purity and crystallinity of the two as-synthesized MOF structures were confirmed by the synchrotron powder XRD (Fig. 1a and c). Their structures are similar to the ones reported in the literature, in spite of some variance in the reaction conditions in this work compared with the reported synthetic procedure [21,22] (ESI, Table S1). The crystal structure of the Zn-MOF consists of trinuclear Zn clusters which are connected to the carboxylate groups of six BPDC bridging linkers to form 2D double layers which pillared by the secondary ligands BiPY to generate a 3D framework (Fig. 1b). In contrast, the BiPY would act as template in the process of selfassembly formation of Mg3(BPDC)34DMF under the synthetic conditions. The second building unit of the Mg-MOF features the Mg trimers, which are octahedrally coordinated by six oxygen centers from six different carboxylate ligands. The two outer metals are each bound to two solvating DMF molecules (Fig. 1d). Along with
the square channels running down the crystallographic c-axis from the structure, open metal sites would be accessible since the structure contains coordinated solvent molecules if the crystallographic structure could sustain after the desolvation. Importantly, the crystallographic structure of the Zn-MOF maintains as evidenced by the backbone frameworks being intact upon the removal of the guest solvent molecules (Fig. 1a). In contrast, the structure of the Mg-MOF collapsed once the accommodated solvent molecules were evacuated as evident by peak intensities change in PXRD pattern (Fig. 1c). Intrinsically, the structural characteristics would attribute to the structural stability upon the porosity activation. To elucidate the structural thermal stability of the two MOF precursors which would highly impact on the atomic arrangements of the pyrolysis products, we conducted thermal gravimetric analysis (TGA) and in situ variable temperature synchrotron powder XRD measurements. The TGA results indicate the two MOFs are thermally stable up to approximately 470 °C for Mg3(BPDC)34DMF and 380 °C for Zn3(BPDC)3(BiPY) under N2 atmosphere, as evidenced by the plateau in the TGA profiles prior to the weight loss steps attributed to the decomposition of the organic ligands (Fig. 2 insets). Temperature-dependent in situ synchrotron powder XRD results provided more insights to the crystallographic stability upon desolvation. From the literature, some MOF materials experience crystallographic degradation and even their structures collapse before the observation of the weight loss steps by thermogravimetric analysis [23]. As can be seen in Fig. 2b, the crystal structure of the Mg-MOF degraded significantly when the guest molecules were removed, indicating considerable crystallinity degradation from 200 °C. The Mg-MOF, featuring structure collapse on guest removal, can be classified as the first generation of MOFs [24]. Because its potential porosity can not be activated by alternative activation procedures, such as exchange high boiling-point DMF with low boiling solvents, supercritical CO2 drying [25], or freeze drying [26], which have been demonstrated to reserve the structural intactness for other MOFs. In contrast,
Fig. 3. SEM images: (a) the as-synthesized sample of Zn3(BPDC)3(BiPY), Zn-MOF derived carbon products at 650 °C (material 1) and 1000 °C (material 2); (b) the assynthesized sample of Mg3(BPDC)34DMF Mg-MOF derived carbon products at 650 °C (material 3) and 1000 °C (material 4).
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the Zn-MOF remained crystallographic structure intact at elevated temperature up to 350 °C although there is slight structure distortion when DMF molecules were evacuated from the pore channels, as indicated in Fig. 2a. From SEM images in Fig. 3, it can be observed that the monolith single crystals (approximately 200 50 50 lm) of the Zn-MOF
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and the column-like polycrystal cluster features of the Mg-MOF. Interestingly, the MOF-derived products, synthesized at various carbonization temperatures (650 °C and 1000 °C), exhibit considerably similar morphologies to their MOF precursor (Fig. 3, ESI, Figs. S5–S8). The synthetic strategy for converting MOFs into carbon composites would provide a new route for the morphology
Fig. 4. (a) N2 adsorption isotherms of Zn3(BPDC)3(BiPY) (in black) and its derived material 1 (in red); (b) N2 adsorption isotherms of Mg3(BPDC)3 (in black) and its derived material 3 (in red); adsorption and desorption isotherms of CO2 (square, in black) and N2 (up-triangle in blue) for the materials 1 (c) and 3 (d) at 298 K; (e) CO2 adsorption/ desorption cycles of the material 2 and 4. (Color online.)
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control which could have significant impact on gas adsorption and separation process. The elemental analysis results indicated 1 and 3 are metal/carbon hybrid materials with Zn/Mg nano-particle deposited in the carbon matrix (ESI, Figs. S1, S3). The material 2 features metal-free carbon (ESI, Fig. S2), suggesting all Zn has evaporated at 1000 °C. However, magnesium nano-particles cannot be completely removed from the material 4 at 1000 °C due to the high boiling point of magnesium (1091 °C) compared with that of zinc (907 °C) (ESI, Fig. S4). The four MOF-derived materials exhibit permanent porosity evidenced by N2 adsorption isotherms of 1 and 3 at 77 K (Fig. 4a and b) and CO2 adsorption–desorption cycles of 2 and 4 (Fig. 4e). Interestingly, the BET surface area of 1 and 3 were approximately 365 m2/g and 514 m2/g, respectively, calculated by the N2 adsorption isotherms at 77 K. Although the Zn-MOF exhibits permanent porosity with BET surface area of 872 m2/g (Fig. 4a), it is noteworthy that material 3, which was derived from the non-porous MgMOF, features higher porosity than the Zn-MOF derived 1 (Fig. 4b). This may suggest the porosity of the MOF-derived materials would more depend on the thermal stability of the framework during decomposition rather than the pristine structural topology of the template MOFs. In other words, the carbonization process would degrade porosity of the porous MOFs but promote porosity in the non-porous MOF precursors. In this case, the decomposition temperature of the organic ligands in the Mg-MOF is higher than that for the Zn-MOF (Fig. 2), which would contribute to the newly generated porosity via the carbonization process (Fig. 4a and b). Moreover, material 1 and 3 selectively adsorb CO2 over N2 at moderate conditions. At 298 K and 1 bar, the CO2/N2 separation factor of 1 and 3 were approximately 7.6 and 6.2, respectively (Fig. 4c and d), which would have potential applications in the postcombustion CO2 capture. To assess recyclability of 2 and 4 for CO2 capture, repeated cycles of CO2 adsorption and desorption were conducted by thermal gravimetric analysis technique. The CO2 adsorption capacities of 2 and 4 were almost identical over five cycles, with the values of approximately 8.7 wt% and 4.8 wt%, respectively (Fig. 4e), suggesting very good reproducibility. 4. Conclusion In summary, we prepared four novel carbon-based products including Zn/Mg nano-particle imbedded carbon matrix and nano-porous carbons from MOFs via carbonization process. Carbons derived from the non-porous Mg-MOF template showed substantially greater surface area than that of the carbons converted by the porous Zn-MOF. This study provides insightful understanding of the relationship between the porosity generated by carbonization and the thermal/crystallographic stability of the template MOFs. Furthermore, the MOFs-derived carbon materials selectively adsorb CO2 over N2 at mild conditions, suggesting a great potential in carbon capture from low pressure flue gases. Acknowledgments Y.H. acknowledges the Chinese Scholarship Council (CSC). P.A. W and J.S. acknowledge Australian Research Council (ARC) for providing the funding (DP2013000024). G.L. is the recipient of an Australian Research Council Discovery Early Career Researcher Award (DE140101824). Part of work was undertaken on the PD beamline, Australian Synchrotron.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2016.05.027. References [1] B. Metz, O. Davidson, H. De Coninck, M. Loos, L. Meyer, Carbon dioxide capture and storage, (2005). [2] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.H. Bae, J.R. Long, Carbon dioxide capture in metal–organic frameworks, Chem. Rev. 112 (2012) 724. [3] J.-K. Sun, Q. Xu, Functional materials derived from open framework templates/ precursors: synthesis and applications, Energy Environ. Sci. 7 (2014) 2071. [4] X. Pei, Y. Chen, S. Li, S. Zhang, X. Feng, J. Zhou, B. Wang, Metal‐organic frameworks derived porous carbons: syntheses, porosity and gas sorption properties, Chin. J. Chem. (2016). [5] W. Chaikittisilp, K. Ariga, Y. Yamauchi, A new family of carbon materials: synthesis of MOF-derived nanoporous carbons and their promising applications, J. Mater. Chem. A 1 (2013) 14. [6] Y. Xia, Z. Yang, R. Mokaya, Templated nanoscale porous carbons, Nanoscale 2 (2010) 639. [7] T. Kyotani, T. Nagai, S. Inoue, A. Tomita, Formation of new type of porous carbon by carbonization in zeolite nanochannels, Chem. Mater. 9 (1997) 609. [8] Z. Yang, Y. Xia, R. Mokaya, Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials, J. Am. Chem. Soc. 129 (2007) 1673. [9] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Ordered mesoporous carbons, Adv. Mater. 13 (2001) 677. [10] J. Ren, J. Ding, K.-Y. Chan, H. Wang, Dual-porosity carbon templated from monosize mesoporous silica nanoparticles, Chem. Mater. 19 (2007) 2786. [11] T. Kyotani, L.-F. Tsai, A. Tomita, Formation of ultrafine carbon tubes by using an anodic aluminum oxide film as a template, Chem. Mater. 7 (1995) 1427. [12] A. Popp, J. Engstler, J.J. Schneider, Porous carbon nanotube-reinforced metals and ceramics via a double templating approach, Carbon 47 (2009) 3208. [13] J.-K. Sun, Q. Xu, Functional materials derived from open framework templates/ precursors: synthesis and applications, Energy Environ. Sci. 7 (2014) 2071. [14] B. Liu, H. Shioyama, T. Akita, Q. Xu, Metal-organic framework as a template for porous carbon synthesis, J. Am. Chem. Soc. 130 (2008) 5390. [15] S. Lim, K. Suh, Y. Kim, M. Yoon, H. Park, D.N. Dybtsev, K. Kim, Porous carbon materials with a controllable surface area synthesized from metal–organic frameworks, Chem. Commun. 48 (2012) 7447. [16] H.B. Aiyappa, P. Pachfule, R. Banerjee, S. Kurungot, Porous carbons from nonporous MOFs: influence of ligand characteristics on intrinsic properties of end carbon, Cryst. Growth Des. 13 (2013) 4195. [17] W. Wang, D. Yuan, Mesoporous carbon originated from non-permanent porous MOFs for gas storage and CO2/CH4 separation, Sci. Rep. 4 (2014). [18] Y. Yang, S. Hao, H. Zhao, Y. Wang, X. Zhang, Hierarchically porous carbons derived from nonporous metal-organic frameworks: synthesis and influence of struts, Electrochim. Acta 180 (2015) 651. [19] J. Kim, A.G. Oliver, J.C. Hicks, Enhanced CO2 capture capacities and efficiencies with N-doped nanoporous carbons synthesized from solvent-modulated, pyridinedicarboxylate-containing Zn-MOFs, CrystEngComm 17 (2015) 8015. [20] M. Hu, J. Reboul, S. Furukawa, N.L. Torad, Q. Ji, P. Srinivasu, K. Ariga, S. Kitagawa, Y. Yamauchi, Direct carbonization of Al-based porous coordination polymer for synthesis of nanoporous carbon, J. Am. Chem. Soc. 134 (2012) 2864. [21] J.Y. Lee, L. Pan, S.P. Kelly, J. Jagiello, T.J. Emge, J. Li, Achieving high density of adsorbed hydrogen in microporous metal organic frameworks, Adv. Mater. 17 (2005) 2703. [22] J.A. Rood, B.C. Noll, K.W. Henderson, Cubic networks and 36 tilings assembled from isostructural trimeric magnesium aryldicarboxylates, Main Group Chem. 5 (2006) 21. [23] P.D. Dietzel, R. Blom, H. Fjellvåg, Base-induced formation of two magnesium metal–organic framework compounds with a bifunctional tetratopic ligand, Eur. J. Inorg. Chem. 2008 (2008) 3624. [24] S. Horike, S. Shimomura, S. Kitagawa, Soft porous crystals, Nat. Chem. 1 (2009) 695. [25] A.P. Nelson, O.K. Farha, K.L. Mulfort, J.T. Hupp, Supercritical processing as a route to high internal surface areas and permanent microporosity in metal– organic framework materials, J. Am. Chem. Soc. 131 (2008) 458. [26] L. Ma, A. Jin, Z. Xie, W. Lin, Freeze drying significantly increases permanent porosity and hydrogen uptake in 4,4-connected metal-organic frameworks, Angew. Chem., Int. Ed. Engl. 48 (2009) 9905.
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