CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402378
A Combinatorial Approach towards Water-Stable Metal– Organic Frameworks for Highly Efficient Carbon Dioxide Separation Zhigang Hu, Kang Zhang, Mei Zhang, Zhengang Guo, Jianwen Jiang, and Dan Zhao*[a] A library of 20 UiO-66-derived metal–organic frameworks (MOFs) is synthesized in a combinatorial approach involving mixed ligand copolymerization and two post-synthetic modifications (PSMs) in tandem. Mixed ligand co-polymerization of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephthalate (SS-BDC) with zirconium tetrachloride (ZrCl4) was used to prepare 5 groups of MOFs with the same UiO-66 topology but differing amounts of sulfate groups. These MOFs exhibit excellent water stabilities in a pH range of 1 to 12, together with high CO2 uptake capacities and selectivities.
Carbon dioxide separation is playing an increasingly important role in environmental sustainability and clean energy applications. For example, it is urgent to develop efficient capture and storage techniques to curb the escalating anthropogenic CO2 emissions.[1] One of the key approaches for CO2 capture is post-combustion capture from flue gas emitted by coal-fired power stations, in which CO2 (10–15 %) needs to be separated from N2 (70–75 %) at a total pressure of around 1 bar.[2] In addition to CO2 capture, upgrading of natural gas is another important application for CO2 separation. Natural gas has CH4 as its major component, and contains substantial amounts of CO2 that need to be removed to increase its heating value and reduce pipeline corrosion during transportation.[3] A widely used approach for CO2 separation in industry is amine scrubbing, which suffers from the problems of amine degradation, equipment corrosion, and most importantly a high energy penalty (20–30 % of the output).[4] Adsorption-based CO2 capture using either pressure-swing adsorption (PSA) or vacuumswing adsorption (VSA) is considered to be a promising alternative due to its lower operation cost and smaller footprint.[5] The applicability of this technique relies heavily on the development of suitable adsorbent materials, which should possess, amongst others, the following properties: high CO2 uptake capacity; excellent CO2 selectivity (CO2 over N2 in post-combustion CO2 capture, CO2 over CH4 in natural gas upgrading); good hydrothermal stability, and reasonable cost.[6]
[a] Z. Hu, K. Zhang, M. Zhang, Dr. Z. Guo, Prof. J. Jiang, Prof. D. Zhao Department of Chemical and Biomolecular Engineering National University of Singapore, 117585 (Singapore) Fax: (+ 65) 6779-1936 E-mail: [email protected] Homepage: http://www.chbe.nus.edu.sg/faculty/chezhao Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402378.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Metal–organic frameworks (MOFs, also known as porous coordination polymers: PCPs) are very promising porous materials.[7] Because of their ultrahigh porosities, uniform yet tunable pore sizes, tailored texture structures, and rich chemical functionalities, they have been widely studied in gas storage,[8] adsorption-based gas separation,[9] catalysis,[10] and sensing[11] applications, and as precursors for other materials.[12] The recent decade has witnessed a boom in studies on MOF-based CO2 storage and separation,[13] and several promising MOFs with excellent CO2 separation performance have been identified.[14] However, most of the reported MOFs have a weak hydrothermal stability that remains the Achilles’ heel for industrial applications. Although several water-stable MOFs have been reported, their CO2 uptake capacity and selectivity still can not meet industrial requirements due to poor CO2 affinity.[15] In order to increase the CO2 affinity of MOFs while keeping their high water stabilities, many attempts have been made based on the modification of water-stable MOFs such as MIL101[15a] and UiO-66.[15c] These attempts include amine decoration,[16] ligand functionalization,[17] and metal exchange.[18] The most dramatic improvements were seen in alkylamine decorated/incorporated MOFs,[16a–d] although the risk of amine degradation and leaching after multiple cycles has yet to be studied.[19] The high CO2 affinity of zeolite is attributed to the presence of cations (e.g., Na + , K + ) in the channels, which balance the anionic frameworks and create an electric field that can polarize CO2 molecules to afford strong CO2 interactions.[6, 20] This structural property has been mimicked in other cationcontaining porous materials that exhibit excellent CO2 affinity and CO2 separation performance, such as lithium-reduced polymer,[21] PPN-6-SO3Li,[22] PPN-6-SO3NH4,[23] rho-ZMOF,[24] and others. Inspired by these approaches, we report herein a library, obtained by combinatorial methods, of 20 UiO-66-derived MOFs that resemble the characteristics of zeolites. UiO-66 was chosen as the prototype MOF owing to its high hydrothermal stability, ease of synthesis, and low cost.[15c] Mixed ligand copolymerization[25] of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephthalate (SS-BDC) with zirconium tetrachloride (ZrCl4) was used to prepare 5 groups of MOFs with the same UiO-66 topology but differing amounts of sulfate groups. The obtained MOFs then underwent two post-synthetic modifications (PSMs)[26] in tandem to introduce alkali metal cations (Li + , Na + , K + , Rb + ) into the frameworks, affording the final MOF library (Scheme 1). All of the materials showed extraordinary high water stability in the pH range 1–12, and some of them exhibited highly efficient CO2 separation performanChemSusChem 0000, 00, 1 – 5
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Scheme 1. Synthesis of UiO-66-SO3M-X involving mixed ligand copolymerization and two post-synthetic modifications (PSMs) in tandem (azure, H; black, C; red, O; yellow, S; green, Zr; violet, M that represents Li + , Na + , K + , and Rb + ).
ces, very promising for industrial application in post-combustion CO2 capture and upgrading of natural gas. BDC and SS-BDC with different ratios were co-polymerized with ZrCl4 to yield UiO-66-derived MOFs by revising a reported method (see Supporting Information for details).[27] The products then underwent the first PSM to completely protonate the sulfate groups into sulfuric acid groups by acidification in 0.1 m aqueous HCl solution (pH 1.0) and then in an ethanol solution of trifluoroacetic acid (10 vol %). The obtained MOFs were named UiO-66-SO3H-X, where X was 0 (pristine UiO-66), 0.15, 0.44, 0.64, and 1, representing the molar ligand ratio of SS-BDC over BDC determined by nuclear magnetic resonance (NMR) (Supporting Information, Figure S1). The presence of sulfuric acid groups was also confirmed by the characteristic FTIR peaks at 1170 and 1260 cm 1, attributable to O=S=O symmetric stretching vibrations (Supporting Information, Figure S2).[28] Based on field-emission scanning electron microscopy (FE-SEM) measurements, there is no obvious difference in the morphology of UiO-66-SO3H-X particles with the same value of X, indicating a homogeneous copolymerization of the two ligands with Zr4 + cations (Supporting Information, Figure S3). The uniform distribution of sulfuric acid groups inside the MOFs was doubly confirmed through elemental mapping by energy dispersive X-ray spectroscopy (EDS; Supporting Information, Figure S4). The crystallinities of UiO-66-SO3H-X were checked by powder X-ray diffraction (PXRD), which matched well with UiO66 at lower molar ratios of SS-BDC (X = 0.15, 0.44, 0.64) but deteriorated at a higher ratio (X = 1) (Supporting Information, Figure S5). The same phenomenon was reported previously and was attributed to the interaction between sulfuric acid groups and Zr6O4(OH)4(CO2)12 secondary building units (SBUs) in the UiO-66 frameworks.[27] N2 sorption data collected at 77 K and a pressure up to 1 bar were used to evaluate the surface areas of these samples using either Brunauer–Emmett–Teller (BET) or Langmuir model (Supporting Information, Figure S6). The pristine UiO-66 synthesized in this study has a BET specific surface area of 1525 m2 g 1 (1757 m2 g 1 based on Langmuir model), which is comparable to the value of 1400 m2 g 1 reported in the literature.[29] The surface areas of UiO-66-SO3H-X drop along with increasing X due to the gradually increased weight of the framework, and more importantly, the worse crystallinity (Supporting Information, Table S1). UiO-66-SO3H-X underwent the second PSM by neutralization in 0.01 m of aqueous alkali hydroxide solutions (pH 12) to introduce alkali metal cations into the framework, with the prod 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org ucts named as UiO-66-SO3M-X, where M represents Li + , Na + , K + , or Rb + . The even distribution of alkali cations inside UiO66-SO3M-X was confirmed by EDS elemental mapping (Supporting Information, Figure S4). Compared to UiO-66-SO3H-X, there is no obvious change in the crystallinity of UiO-66-SO3MX containing Li + , Na + , and Rb + , but only at lower values of X (X 0.44; Supporting Information, Figure S5). However, it is interesting to note that for the samples of UiO-66-SO3K-X, neutralization helped in regaining the crystallinity. All the four samples, including UiO-66-SO3K-1, have sharp powder XRD peaks that match well with UiO-66. Control experiments were done to enunciate the conditions for regaining crystallinity and it was found that crystallinity could be restored only by soaking in 0.01 m aqueous KOH solution (Supporting Information, Figure S5 f). This indicates the possibility of special chemical conditions involving appropriate pH value and ionic strength that can be applied for the re-crystallization of Zr-containing MOFs which are previously believed to be chemically inert.[30] Theoretically speaking, given the same molar sulfate content and crystallinity, MOFs bearing heavier alkali metal cations (e.g., K + , Rb + ) should have lower surface areas then MOFs bearing lighter alkali metal cations (e.g., Li + , Na + ) because of the increased weight in the frameworks. A 3D plot of BET surface areas versus X and M of all UiO-66-SO3M-X samples is shown in Figure 1. The surface areas decrease with the increase of X given the same cations, while they do not always decrease with the increase of cation weight under the same value of X, indicating the impact of other factors such as crystallinity on surface areas. The observed higher BET surface area of UiO-66-SO3Li-0.15 (1630 m2 g 1) than that of UiO-66 (1525 m2 g 1) could be due to a better crystallinity caused by neutralization that is similar to the previous case of K + -containing samples. The major pore size distribution decreased from ca. 8.5 to 5 ~ 6 after incorporation of alkali metal cations, mainly because of the steric constraints caused by the increased cationic radius (Figure 1). Based on thermogravimetric analysis (TGA), all of the UiO-66-SO3M-X samples are thermally stable up to 350 8C, which is comparable to pristine UiO-66
Figure 1. (a) PXRD patterns. (b) 3D plot BET surface areas. (c) N2 sorption isotherms (filled, adsorption; open, desorption). (d) Pore size distributions.
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CHEMSUSCHEM COMMUNICATIONS and grants their application and regeneration under high temperature conditions (Supporting Information, Figure S7). The post-combustion CO2 capture performance was evaluated by analyzing the data of low pressure (up to 1 bar) CO2 and N2 sorption isotherms collected at 298 K (Supporting Information, Figure S8–S11). The pristine UiO-66 can reversibly adsorb 1.79 mmol of CO2 per gram of material (denoted as mmol g 1 hereafter) at 298 K and 1 bar, which is comparable to the value of 1.7 mmol g 1 reported previously.[17b] Introducing sulfuric acid groups into the framework of UiO-66 substantially increases the CO2 uptake (2.23 mmol g 1 in UiO-66-SO3H-0.15 versus 1.79 mmol g 1 in UiO-66, 298 K 1 bar) due to the strong affinity between CO2 and sulfuric acid groups caused by hydrogenbonding interactions.[22, 31] However, the increase is only seen at X = 0.15 where a balance of high CO2 affinity and good framework crystallinity can be obtained (Supporting Information, Table S2). Thanks to the strong electric fields created by alkali cations, neutralization of sulfuric acid into alkali sulfate further improves the CO2 uptake at lower X values (X = 0.15, 0.44). For example, UiO-66-SO3Li-0.15 has a CO2 uptake capacity of 3.28 mmol g 1 at 298 K and 1 bar, which is 47 % higher than UiO-66-SO3H-0.15 and 83 % higher than pristine UiO-66. The improvement is even more obvious under 0.15 bar, a typical partial pressure of CO2 in flue gas for post-combustion CO2 capture, where the increase of 82 % and 151 % were observed for UiO-66-SO3Li-0.15 (0.93 mmol g 1) over UiO-66-SO3H-0.15 (0.51 mmol g 1) and UiO-66 (0.37 mmol g 1), respectively (Table S2). Besides UiO-66-SO3Li-0.15, several other ionized MOFs also exhibit excellent CO2 uptake capacities at 298 K and 0.15 bar, such as UiO-66-SO3K-0.15 (0.75 mmol g 1) and UiO-66-SO3Na0.15 (0.75 mmol g 1). Based on Fajan’s rule, cations with higher charge and smaller size should have a stronger polarization ability.[32] This is why UiO-66-SO3Li-0.15 has the best CO2 uptake capacity: Li + is the smallest among the four cations. However, in MOFs with higher content of sulfate groups, crystallinity and surface area start to dominate the CO2 uptake capacity, which is reflected in the K-containing samples. The isosteric heats of adsorption Qst, a term that can be used to evaluate the adsorbent–adsorbate interactions, were calculated using the Clausius–Clapeyron equation,[33] and an almost opposite trend compared with the CO2 uptake capacity was revealed in the 3D plot (Figure 2; Supporting Information, Table S3). UiO-66 has a low-coverage Qst of 24.5 kJ mol 1, which is quite close to the value of 25.5 kJ mol 1 reported previously.[17b] UiO-66-SO3Li-0.15 only demonstrates a slight increase of Qst ( 25.4 kJ mol 1), while the highest Qst of 50.9 kJ mol 1 was obtained using UiO-66-SO3H-1, which is comparable to the Qst reported in other highly efficient CO2 adsorbents such as SIFSIX-3-Zn ( 45 kJ mol 1),[14e] PPN-6SO3NH4 ( 40 kJ mol 1),[23] Mg-MOF-74 ( 39 kJ mol 1),[34] and zeolite NaX ( 43 kJ mol 1).[34] This value also fits in the ideal enthalpy range for adsorption-based CO2 separation, where a balance between separation performance and ease of regeneration needs to be obtained.[13a] Besides CO2 uptake capacity, the CO2/N2 selectivity is equally if not more important in evaluating an adsorbent’s applicability for CO2 separation. In this study, two methods were used to 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. (a) 3D plot CO2 uptake at 298 K 0.15 bar. (b) 3D plot low coverage Qst of CO2. (c) 3D plot Henry CO2/N2 selectivities. (d) 3D plot IAST CO2/N2 selectivities (CO2/N2 = 15:85, 298 K 1 bar).
evaluate the CO2/N2 selectivity. The first one is based on the constant kH of Henry’s law, which is defined as the initial slope of the isotherm that reflects the interactions between adsorbent and adsorbate at low pressure range (Supporting Information, Table S4).[35] The CO2/N2 selectivities are calculated as the kH ratio of CO2 over N2 and are plotted in Figure 2, and listed in Table S5 (Supporting Information). UiO-66-SO3H-0.64 has the highest Henry CO2/N2 selectivity of 34.8, followed by UiO-66SO3Na-1 (29) and UiO-66-SO3Na-0.15 (23.7). These values are higher than that of UiO-66 (15.3) and other MOFs such as HKUST-1 (~ 20),[36] MOF-5 (17.48),[37] and MOF-177 (17.73).[37] The second method in calculating the CO2/N2 selectivity is based on ideal adsorption solution theory (IAST), where the binary gas mixture selectivity can be estimated using singlegas isotherms.[38] The isotherms of CO2 and N2 were fitted to a dual-site Langmuir model in order to get equilibrium parameters, saturation capacities of two adsorption sites, and Langmuir constants. IAST calculations were carried out assuming a CO2/N2 binary mixture at a molar ratio of 15:85 to mimic the composition of flue gas, and the results are shown in Figure 2 and Table S6 (Supporting Information). UiO-66-SO3Na-1 exhibits the highest IAST CO2/N2 selectivity (83.6) at 298 K and 1 bar, followed by UiO-66-SO3Na-0.15 (67.6), both of which are much higher than the pristine UiO-66 (19.4) and zeolite 13X (~ 20).[6] Although these selectivities are still lower than the top MOFs and polymers reported so far, such as SIFSIX-3-Zn (1818),[14e] Mg-MOF-74 (ca. 148),[14b] and PPN-6-SO3NH4 (ca. 796),[23] the excellent hydrothermal stability and low material cost make our ionized UiO-66 materials more practical towards industrial applications. Among these materials, UiO-66-SO3Na-0.15 is the best candidate for the balanced CO2 uptake capacity of 0.75 mmol g 1 (298 K, 0.15 bar) and CO2/N2 IAST selectivity of 67.6 (298 K, 1 bar). The application of ionized UiO-66 materials in natural gas upgrading involving CO2/CH4 separation was evaluated as well. Ionization has also helped increase the CH4 uptake, but to a smaller extent compared to CO2 because of the higher polarChemSusChem 0000, 00, 1 – 5
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late (SS-BDC) with different ratios (0.686 mmol in total) were firstly dissolved in 20 mL of dimethylformamide (DMF)/acetic acid (9:1) mixed solvents, followed by the addition of ZrCl4 (160 mg, 0.686 mmol). The solutions were sonicated for 15 min and then heated at 120 8C for 40 h. The solid products were recovered by centrifuge, washed with DMF three times, and then soaked in DMF at 80 8C for 12 h to completely remove any unreacted ligands trapped inside the frameworks. The samples then underwent solvent exchange with methanol for three days, and were fully activated at 120 8C under vacuum Figure 3. (a) 3D plot Henry CO2/CH4 selectivities. (b) 3D plot IAST CO2/CH4 selectivities for 24 h. Acidification was carried out by soaking the ac(CO2/CH4 = 50:50, 298 K 1 bar). tivated samples in 0.1 m of aqueous HCl solutions for 12 h and then in ethanol solutions of trifluoroacetic acid (10 vol %) for 6 h. The acidified samples were washed izability (29.11 1025 cm3) and quadrupole moment (4.3 and exchanged with methanol for three additional days, and then 1026 esu cm2) of CO2 as compared to that of CH4 (25.93 dried under vacuum at 120 8C for 24 h to afford the final products.
1025 cm3 and 0 esu cm2, respectively).[9a] In our study, UiO-66SO3Li-0.15 shows the highest CH4 uptake of 0.70 mmol g 1 at 1 bar and 298 K, which is only 63 % higher than that of pristine UiO-66 (0.43 mmol g 1) (recall that the increase for CO2 under the same condition was 83 %). As a result, higher CO2/CH4 selectivities are achieved after ionization. For example, UiO-66SO3Na-1 exhibits the highest Henry CO2/CH4 selectivity of 20.3, which is much higher than that of UiO-66 (4.9) and other reported MOFs, such as MOF-5 (15.53)[37] and MOF-177 (4.43).[37] IAST CO2/CH4 selectivities were calculated assuming a CO2/CH4 binary mixture at a molar ratio of 50:50 under 298 K 1 bar (Supporting Information, Table S6). Figure 3 shows that all of the ionized samples have higher IAST CO2/CH4 selectivities than UiO-66 (ca. 6.1). It is worth noting that UiO-66-SO3Rb1 shows an astonishing 789 % increase in selectivity (54.2), which is among the highest of all MOFs reported so far.[14e, 16c] However, its CO2 uptake capacity still needs to be improved for practical applications. In summary, we report herein a library of 20 UiO-66-derived metal–organic frameworks (MOFs) obtained through combinatorial chemistry involving mixed ligand copolymerization and post-synthetic modifications. The MOFs exhibit excellent water stability in a pH range of 1 to 12, and high CO2 uptake capacity/selectivity as revealed by the analysis of 147 isotherms. Among these materials, UiO-66-SO3Li-0.15 has the highest CO2 uptake capacity at 298 K under both 0.15 bar (0.93 mmol g 1) and 1 bar (3.28 mmol g 1); UiO-66-SO3H-1 exhibits the highest Qst of 50.9 kJ mol 1 at low coverage; the best CO2/N2 (15:85) selectivity calculated by IAST at 298 K and 1 bar belongs to UiO-66-SO3Na-1 (83.6); and the best CO2/CH4 (50:50) selectivity calculated by IAST at 298 K and 1 bar was achieved in UiO-66SO3Rb-1 (54.2), which is 789 % higher than the pristine UiO-66. Our approach paves a way towards the systematic study of water-stable and affordable MOFs as highly efficient adsorbents for CO2 separation in the applications of post-combustion CO2 capture and upgrading of natural gas.
Experimental Section Synthesis of UiO-66-SO3H-X: The UiO-66-SO3H-X samples were synthesized by revising a reported method.[27] Briefly, mixtures of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephtha 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Synthesis of UiO-66-SO3M-X: The UiO-66-SO3M-X samples were synthesized by neutralizing UiO-66-SO3H-X with aqueous alkali hydroxide solutions. Taking UiO-66-SO3Li-0.15 for example, 100 mg of UiO-66-SO3H-0.15 was immersed in 20 mL of 0.01 m LiOH aqueous solution at room temperature for 2 h. The product was recovered by centrifuge, washed with deionized water until neutral, exchanged with methanol for three days, and then activated under vacuum at 120 8C for 24 h to afford the final product. All the rest UiO-66-SO3M-X samples were synthesized similarly.
Acknowledgements This work is supported by National University of Singapore (NUS Start-up Funding R-279-000-369-133) and the Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, R-279-000361-731). Keywords: clean energy · carbon dioxide capture · combinatorial chemistry · metal-organic frameworks · natural gas [1] R. S. Haszeldine, Science 2009, 325, 1647 – 1652. [2] D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed. 2010, 49, 6058 – 6082; Angew. Chem. 2010, 122, 6194 – 6219. [3] S. Chaemchuen, N. A. Kabir, K. Zhou, F. Verpoort, Chem. Soc. Rev. 2013, 42, 9304 – 9332. [4] G. T. Rochelle, Science 2009, 325, 1652 – 1654. [5] D. M. Ruthven, S. Farooq, K. S. Knaebel, Pressure Swing Adsorption, Wiley-VCH, Weinheim, 1993. [6] S. Choi, J. H. Drese, C. W. Jones, ChemSusChem 2009, 2, 796 – 854. [7] a) H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science 2013, 341, 1230444; b) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334 – 2375; Angew. Chem. 2004, 116, 2388 – 2430; c) G. Frey, M. Haouas, T. Loiseau, F. Taulelle, Chem. Mater. 2014, 26, 299 – 309; d) R. J. Kuppler, D. J. Timmons, Q. R. Fang, J. R. Li, T. A. Makal, M. D. Young, D. Q. Yuan, D. Zhao, W. J. Zhuang, H. C. Zhou, Coord. Chem. Rev. 2009, 253, 3042 – 3066. [8] a) D. Zhao, D. Q. Yuan, H. C. Zhou, Energy Environ. Sci. 2008, 1, 222 – 235; b) R. E. Morris, P. S. Wheatley, Angew. Chem. Int. Ed. 2008, 47, 4966 – 4981; Angew. Chem. 2008, 120, 5044 – 5059; c) S. Q. Ma, H. C. Zhou, Chem. Commun. 2010, 46, 44 – 53. [9] a) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 – 1504; b) J. R. Li, J. Sculley, H. C. Zhou, Chem. Rev. 2012, 112, 869 – 932. [10] a) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459; b) A. Corma, H. Garcia, F. X. L. Xamena, Chem. Rev. 2010, 110, 4606 – 4655.
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CHEMSUSCHEM COMMUNICATIONS [11] a) M. D. Allendorf, C. A. Bauer, R. K. Bhakta, R. J. T. Houk, Chem. Soc. Rev. 2009, 38, 1330 – 1352; b) Y. J. Cui, Y. F. Yue, G. D. Qian, B. L. Chen, Chem. Rev. 2012, 112, 1126 – 1162; c) M. Zhang, G. X. Feng, Z. G. Song, Y. P. Zhou, H. Y. Chao, D. Q. Yuan, T. T. Y. Tan, Z. G. Guo, Z. G. Hu, B. Z. Tang, B. Liu, D. Zhao, J. Am. Chem. Soc. 2014, 136, 7241 – 7244. [12] a) D. Zhao, J. L. Shui, C. Chen, X. Q. Chen, B. M. Reprogle, D. P. Wang, D. J. Liu, Chem. Sci. 2012, 3, 3200 – 3205; b) D. Zhao, J. L. Shui, L. Grabstanowicz, C. Chen, S. Commet, T. Xu, J. Lu, D. J. Liu, Adv. Mater. 2014, 26, 1093 – 1097. [13] a) S. Keskin, T. M. van Heest, D. S. Sholl, ChemSusChem 2010, 3, 879 – 891; b) Y. S. Bae, R. Q. Snurr, Angew. Chem. Int. Ed. 2011, 50, 11586 – 11596; Angew. Chem. 2011, 123, 11790 – 11801; c) G. Frey, C. Serre, T. Devic, G. Maurin, H. Jobic, P. L. Llewellyn, G. De Weireld, A. Vimont, M. Daturi, J. S. Chang, Chem. Soc. Rev. 2011, 40, 550 – 562; d) J. R. Li, Y. G. Ma, M. C. McCarthy, J. Sculley, J. M. Yu, H. K. Jeong, P. B. Balbuena, H. C. Zhou, Coord. Chem. Rev. 2011, 255, 1791 – 1823; e) J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown, J. Liu, Chem. Soc. Rev. 2012, 41, 2308 – 2322; f) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T. H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724 – 781. [14] a) S. R. Caskey, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc. 2008, 130, 10870 – 10871; b) J. A. Mason, K. Sumida, Z. R. Herm, R. Krishna, J. R. Long, Energy Environ. Sci. 2011, 4, 3030 – 3040; c) P. Aprea, D. Caputo, N. Gargiulo, F. Iucolano, F. Pepe, J. Chem. Eng. Data 2010, 55, 3655 – 3661; d) T. M. McDonald, D. M. D’Alessandro, R. Krishna, J. R. Long, Chem. Sci. 2011, 2, 2022 – 2028; e) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Q. Ma, B. Space, L. Wojtas, M. Eddaoudi, M. J. Zaworotko, Nature 2013, 495, 80 – 84. [15] a) G. Frey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surbl, I. Margiolaki, Science 2005, 309, 2040 – 2042; b) P. L. Llewellyn, S. Bourrelly, C. Serre, A. Vimont, M. Daturi, L. Hamon, G. De Weireld, J. S. Chang, D. Y. Hong, Y. K. Hwang, S. H. Jhung, G. Frey, Langmuir 2008, 24, 7245 – 7250; c) J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K. P. Lillerud, J. Am. Chem. Soc. 2008, 130, 13850 – 13851; d) A. D. Wiersum, E. Soubeyrand-Lenoir, Q. Y. Yang, B. Moulin, V. Guillerm, M. Ben Yahia, S. Bourrelly, A. Vimont, S. Miller, C. Vagner, M. Daturi, G. Clet, C. Serre, G. Maurin, P. L. Llewellyn, Chem. Asian J. 2011, 6, 3270 – 3280; e) A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe, O. M. Yaghi, Acc. Chem. Res. 2010, 43, 58 – 67. [16] a) X. R. Wang, H. Q. Li, X. J. Hou, J. Phys. Chem. C 2012, 116, 19814 – 19821; b) Y. C. Lin, Q. J. Yan, C. L. Kong, L. Chen, Sci. Rep. 2013, 3, 1859; c) Q. J. Yan, Y. C. Lin, C. L. Kong, L. Chen, Chem. Commun. 2013, 49, 6873 – 6875; d) Y. L. Hu, W. M. Verdegaal, S. H. Yu, H. L. Jiang, ChemSusChem 2014, 7, 734 – 737; e) A. Khutia, C. Janiak, Dalton Trans. 2014, 43, 1338 – 1347. [17] a) H. Jasuja, J. Zang, D. S. Sholl, K. S. Walton, J. Phys. Chem. C 2012, 116, 23526 – 23532; b) G. E. Cmarik, M. Kim, S. M. Cohen, K. S. Walton, Langmuir 2012, 28, 15606 – 15613; c) Y. T. Huang, W. P. Qin, Z. Li, Y. W. Li, Dalton Trans. 2012, 41, 9283 – 9285; d) Q. Y. Yang, S. Vaesen, F. Ragon, A. D. Wiersum, D. Wu, A. Lago, T. Devic, C. Martineau, F. Taulelle, P. L. Llewellyn, H. Jobic, C. L. Zhong, C. Serre, G. De Weireld, G. Maurin, Angew. Chem. Int. Ed. 2013, 52, 10316 – 10320; Angew. Chem. 2013, 125, 10506 – 10510; e) S. Biswas, J. Zhang, Z. B. Li, Y. Y. Liu, M. Grzywa, L. X. Sun, D. Volkmer, P. Van der Voort, Dalton Trans. 2013, 42, 4730 – 4737; f) H. Wu,
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Y. S. Chua, V. Krungleviciute, M. Tyagi, P. Chen, T. Yildirim, W. Zhou, J. Am. Chem. Soc. 2013, 135, 10525 – 10532; g) D. H. Hong, M. P. Suh, Chem. Eur. J. 2014, 20, 426 – 434; h) Q. Y. Yang, V. Guillerm, F. Ragon, A. D. Wiersum, P. L. Llewellyn, C. L. Zhong, T. Devic, C. Serre, G. Maurin, Chem. Commun. 2012, 48, 9831 – 9833. C. H. Lau, R. Babarao, M. R. Hill, Chem. Commun. 2013, 49, 3634 – 3636. a) C. S. Srikanth, S. S. C. Chuang, ChemSusChem 2012, 5, 1435 – 1442; b) W. Li, S. Choi, J. H. Drese, M. Hornbostel, G. Krishnan, P. M. Eisenberger, C. W. Jones, ChemSusChem 2010, 3, 899 – 903. a) E. Quartapelle Procopio, F. Linares, C. Montoro, V. Colombo, A. Maspero, E. Barea, J. A. R. Navarro, Angew. Chem. Int. Ed. 2010, 49, 7308 – 7311; Angew. Chem. 2010, 122, 7466 – 7469; b) T. H. Bae, M. R. Hudson, J. A. Mason, W. L. Queen, J. J. Dutton, K. Sumida, K. J. Micklash, S. S. Kaye, C. M. Brown, J. R. Long, Energy Environ. Sci. 2013, 6, 128 – 138; c) J. Juan-AlcaÇiz, R. Gielisse, A. B. Lago, E. V. Ramos-Fernandez, P. SerraCrespo, T. Devic, N. Guillou, C. Serre, F. Kapteijn, J. Gascon, Catal. Sci. Technol. 2013, 3, 2311 – 2318. O. K. Farha, Y. S. Bae, B. G. Hauser, A. M. Spokoyny, R. Q. Snurr, C. A. Mirkin, J. T. Hupp, Chem. Commun. 2010, 46, 1056 – 1058. W. G. Lu, D. Q. Yuan, J. L. Sculley, D. Zhao, R. Krishna, H. C. Zhou, J. Am. Chem. Soc. 2011, 133, 18126 – 18129. W. G. Lu, W. M. Verdegaal, J. M. Yu, P. B. Balbuena, H. K. Jeong, H. C. Zhou, Energy Environ. Sci. 2013, 6, 3559 – 3564. R. Babarao, J. W. Jiang, Energy Environ. Sci. 2009, 2, 1088 – 1093. K. Koh, A. G. Wong-Foy, A. J. Matzger, Angew. Chem. Int. Ed. 2008, 47, 677 – 680; Angew. Chem. 2008, 120, 689 – 692. S. M. Cohen, Chem. Rev. 2012, 112, 970 – 1000. M. L. Foo, S. Horike, T. Fukushima, Y. Hijikata, Y. Kubota, M. Takata, S. Kitagawa, Dalton Trans. 2012, 41, 13791 – 13794. M. G. Goesten, J. Juan-AlcaÇiz, E. V. Ramos-Fernandez, K. B. S. S. Gupta, E. Stavitski, H. van Bekkum, J. Gascon, F. Kapteijn, J. Catal. 2011, 281, 177 – 187. A. Schaate, P. Roy, A. Godt, J. Lippke, F. Waltz, M. Wiebcke, P. Behrens, Chem. Eur. J. 2011, 17, 6643 – 6651. M. Kim, J. F. Cahill, Y. X. Su, K. A. Prather, S. M. Cohen, Chem. Sci. 2012, 3, 126 – 130. Q. Y. Yang, A. D. Wiersum, P. L. Llewellyn, V. Guillerm, C. Serred, G. Maurin, Chem. Commun. 2011, 47, 9603 – 9605. K. Fajans, G. Joos, Z. Phys. 1924, 23, 1 – 46. J. L. C. Rowsell, O. M. Yaghi, J. Am. Chem. Soc. 2006, 128, 1304 – 1315. D. Britt, H. Furukawa, B. Wang, T. G. Glover, O. M. Yaghi, Proc. Natl. Acad. Sci. USA 2009, 106, 20637 – 20640. D. Mackay, W. Y. Shiu, J. Phys. Chem. Ref. Data 1981, 10, 1175 – 1199. Q. Y. Yang, C. Y. Xue, C. L. Zhong, J. F. Chen, AICHE J. 2007, 53, 2832 – 2840. D. Saha, Z. B. Bao, F. Jia, S. G. Deng, Environ. Sci. Technol. 2010, 44, 1820 – 1826. A. L. Myers, J. M. Prausnitz, AICHE J. 1965, 11, 121 – 127.
Received: May 3, 2014 Revised: June 18, 2014 Published online on && &&, 0000
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COMMUNICATIONS Z. Hu, K. Zhang, M. Zhang, Z. Guo, J. Jiang, D. Zhao* && – && A Combinatorial Approach towards Water-Stable Metal–Organic Frameworks for Highly Efficient Carbon Dioxide Separation
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
With our powers combined: A library of 20 UiO-66-derived metal–organic frameworks (MOFs) is synthesized following a combinatorial approach involving mixed ligand copolymerization and two post-synthetic modifications in tandem. The MOFs have excellent water stabilities in a pH range of 1 to 12 together with high carbon dioxide (CO2) uptake capacities and selectivities. These features make them promising adsorbents in adsorption-based CO2 separations such as post-combustion CO2 capture and upgrading natural gas.
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A Combinatorial Approach towards Water-Stable Metal– Organic Frameworks for Highly Efficient Carbon Dioxide Separation Zhigang Hu, Kang Zhang, Mei Zhang, Zhengang Guo, Jianwen Jiang, and Dan Zhao*[a] A library of 20 UiO-66-derived metal–organic frameworks (MOFs) is synthesized in a combinatorial approach involving mixed ligand copolymerization and two post-synthetic modifications (PSMs) in tandem. Mixed ligand co-polymerization of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephthalate (SS-BDC) with zirconium tetrachloride (ZrCl4) was used to prepare 5 groups of MOFs with the same UiO-66 topology but differing amounts of sulfate groups. These MOFs exhibit excellent water stabilities in a pH range of 1 to 12, together with high CO2 uptake capacities and selectivities.
Carbon dioxide separation is playing an increasingly important role in environmental sustainability and clean energy applications. For example, it is urgent to develop efficient capture and storage techniques to curb the escalating anthropogenic CO2 emissions.[1] One of the key approaches for CO2 capture is post-combustion capture from flue gas emitted by coal-fired power stations, in which CO2 (10–15 %) needs to be separated from N2 (70–75 %) at a total pressure of around 1 bar.[2] In addition to CO2 capture, upgrading of natural gas is another important application for CO2 separation. Natural gas has CH4 as its major component, and contains substantial amounts of CO2 that need to be removed to increase its heating value and reduce pipeline corrosion during transportation.[3] A widely used approach for CO2 separation in industry is amine scrubbing, which suffers from the problems of amine degradation, equipment corrosion, and most importantly a high energy penalty (20–30 % of the output).[4] Adsorption-based CO2 capture using either pressure-swing adsorption (PSA) or vacuumswing adsorption (VSA) is considered to be a promising alternative due to its lower operation cost and smaller footprint.[5] The applicability of this technique relies heavily on the development of suitable adsorbent materials, which should possess, amongst others, the following properties: high CO2 uptake capacity; excellent CO2 selectivity (CO2 over N2 in post-combustion CO2 capture, CO2 over CH4 in natural gas upgrading); good hydrothermal stability, and reasonable cost.[6]
[a] Z. Hu, K. Zhang, M. Zhang, Dr. Z. Guo, Prof. J. Jiang, Prof. D. Zhao Department of Chemical and Biomolecular Engineering National University of Singapore, 117585 (Singapore) Fax: (+ 65) 6779-1936 E-mail: [email protected] Homepage: http://www.chbe.nus.edu.sg/faculty/chezhao Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402378.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Metal–organic frameworks (MOFs, also known as porous coordination polymers: PCPs) are very promising porous materials.[7] Because of their ultrahigh porosities, uniform yet tunable pore sizes, tailored texture structures, and rich chemical functionalities, they have been widely studied in gas storage,[8] adsorption-based gas separation,[9] catalysis,[10] and sensing[11] applications, and as precursors for other materials.[12] The recent decade has witnessed a boom in studies on MOF-based CO2 storage and separation,[13] and several promising MOFs with excellent CO2 separation performance have been identified.[14] However, most of the reported MOFs have a weak hydrothermal stability that remains the Achilles’ heel for industrial applications. Although several water-stable MOFs have been reported, their CO2 uptake capacity and selectivity still can not meet industrial requirements due to poor CO2 affinity.[15] In order to increase the CO2 affinity of MOFs while keeping their high water stabilities, many attempts have been made based on the modification of water-stable MOFs such as MIL101[15a] and UiO-66.[15c] These attempts include amine decoration,[16] ligand functionalization,[17] and metal exchange.[18] The most dramatic improvements were seen in alkylamine decorated/incorporated MOFs,[16a–d] although the risk of amine degradation and leaching after multiple cycles has yet to be studied.[19] The high CO2 affinity of zeolite is attributed to the presence of cations (e.g., Na + , K + ) in the channels, which balance the anionic frameworks and create an electric field that can polarize CO2 molecules to afford strong CO2 interactions.[6, 20] This structural property has been mimicked in other cationcontaining porous materials that exhibit excellent CO2 affinity and CO2 separation performance, such as lithium-reduced polymer,[21] PPN-6-SO3Li,[22] PPN-6-SO3NH4,[23] rho-ZMOF,[24] and others. Inspired by these approaches, we report herein a library, obtained by combinatorial methods, of 20 UiO-66-derived MOFs that resemble the characteristics of zeolites. UiO-66 was chosen as the prototype MOF owing to its high hydrothermal stability, ease of synthesis, and low cost.[15c] Mixed ligand copolymerization[25] of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephthalate (SS-BDC) with zirconium tetrachloride (ZrCl4) was used to prepare 5 groups of MOFs with the same UiO-66 topology but differing amounts of sulfate groups. The obtained MOFs then underwent two post-synthetic modifications (PSMs)[26] in tandem to introduce alkali metal cations (Li + , Na + , K + , Rb + ) into the frameworks, affording the final MOF library (Scheme 1). All of the materials showed extraordinary high water stability in the pH range 1–12, and some of them exhibited highly efficient CO2 separation performanChemSusChem 0000, 00, 1 – 5
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Scheme 1. Synthesis of UiO-66-SO3M-X involving mixed ligand copolymerization and two post-synthetic modifications (PSMs) in tandem (azure, H; black, C; red, O; yellow, S; green, Zr; violet, M that represents Li + , Na + , K + , and Rb + ).
ces, very promising for industrial application in post-combustion CO2 capture and upgrading of natural gas. BDC and SS-BDC with different ratios were co-polymerized with ZrCl4 to yield UiO-66-derived MOFs by revising a reported method (see Supporting Information for details).[27] The products then underwent the first PSM to completely protonate the sulfate groups into sulfuric acid groups by acidification in 0.1 m aqueous HCl solution (pH 1.0) and then in an ethanol solution of trifluoroacetic acid (10 vol %). The obtained MOFs were named UiO-66-SO3H-X, where X was 0 (pristine UiO-66), 0.15, 0.44, 0.64, and 1, representing the molar ligand ratio of SS-BDC over BDC determined by nuclear magnetic resonance (NMR) (Supporting Information, Figure S1). The presence of sulfuric acid groups was also confirmed by the characteristic FTIR peaks at 1170 and 1260 cm 1, attributable to O=S=O symmetric stretching vibrations (Supporting Information, Figure S2).[28] Based on field-emission scanning electron microscopy (FE-SEM) measurements, there is no obvious difference in the morphology of UiO-66-SO3H-X particles with the same value of X, indicating a homogeneous copolymerization of the two ligands with Zr4 + cations (Supporting Information, Figure S3). The uniform distribution of sulfuric acid groups inside the MOFs was doubly confirmed through elemental mapping by energy dispersive X-ray spectroscopy (EDS; Supporting Information, Figure S4). The crystallinities of UiO-66-SO3H-X were checked by powder X-ray diffraction (PXRD), which matched well with UiO66 at lower molar ratios of SS-BDC (X = 0.15, 0.44, 0.64) but deteriorated at a higher ratio (X = 1) (Supporting Information, Figure S5). The same phenomenon was reported previously and was attributed to the interaction between sulfuric acid groups and Zr6O4(OH)4(CO2)12 secondary building units (SBUs) in the UiO-66 frameworks.[27] N2 sorption data collected at 77 K and a pressure up to 1 bar were used to evaluate the surface areas of these samples using either Brunauer–Emmett–Teller (BET) or Langmuir model (Supporting Information, Figure S6). The pristine UiO-66 synthesized in this study has a BET specific surface area of 1525 m2 g 1 (1757 m2 g 1 based on Langmuir model), which is comparable to the value of 1400 m2 g 1 reported in the literature.[29] The surface areas of UiO-66-SO3H-X drop along with increasing X due to the gradually increased weight of the framework, and more importantly, the worse crystallinity (Supporting Information, Table S1). UiO-66-SO3H-X underwent the second PSM by neutralization in 0.01 m of aqueous alkali hydroxide solutions (pH 12) to introduce alkali metal cations into the framework, with the prod 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org ucts named as UiO-66-SO3M-X, where M represents Li + , Na + , K + , or Rb + . The even distribution of alkali cations inside UiO66-SO3M-X was confirmed by EDS elemental mapping (Supporting Information, Figure S4). Compared to UiO-66-SO3H-X, there is no obvious change in the crystallinity of UiO-66-SO3MX containing Li + , Na + , and Rb + , but only at lower values of X (X 0.44; Supporting Information, Figure S5). However, it is interesting to note that for the samples of UiO-66-SO3K-X, neutralization helped in regaining the crystallinity. All the four samples, including UiO-66-SO3K-1, have sharp powder XRD peaks that match well with UiO-66. Control experiments were done to enunciate the conditions for regaining crystallinity and it was found that crystallinity could be restored only by soaking in 0.01 m aqueous KOH solution (Supporting Information, Figure S5 f). This indicates the possibility of special chemical conditions involving appropriate pH value and ionic strength that can be applied for the re-crystallization of Zr-containing MOFs which are previously believed to be chemically inert.[30] Theoretically speaking, given the same molar sulfate content and crystallinity, MOFs bearing heavier alkali metal cations (e.g., K + , Rb + ) should have lower surface areas then MOFs bearing lighter alkali metal cations (e.g., Li + , Na + ) because of the increased weight in the frameworks. A 3D plot of BET surface areas versus X and M of all UiO-66-SO3M-X samples is shown in Figure 1. The surface areas decrease with the increase of X given the same cations, while they do not always decrease with the increase of cation weight under the same value of X, indicating the impact of other factors such as crystallinity on surface areas. The observed higher BET surface area of UiO-66-SO3Li-0.15 (1630 m2 g 1) than that of UiO-66 (1525 m2 g 1) could be due to a better crystallinity caused by neutralization that is similar to the previous case of K + -containing samples. The major pore size distribution decreased from ca. 8.5 to 5 ~ 6 after incorporation of alkali metal cations, mainly because of the steric constraints caused by the increased cationic radius (Figure 1). Based on thermogravimetric analysis (TGA), all of the UiO-66-SO3M-X samples are thermally stable up to 350 8C, which is comparable to pristine UiO-66
Figure 1. (a) PXRD patterns. (b) 3D plot BET surface areas. (c) N2 sorption isotherms (filled, adsorption; open, desorption). (d) Pore size distributions.
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CHEMSUSCHEM COMMUNICATIONS and grants their application and regeneration under high temperature conditions (Supporting Information, Figure S7). The post-combustion CO2 capture performance was evaluated by analyzing the data of low pressure (up to 1 bar) CO2 and N2 sorption isotherms collected at 298 K (Supporting Information, Figure S8–S11). The pristine UiO-66 can reversibly adsorb 1.79 mmol of CO2 per gram of material (denoted as mmol g 1 hereafter) at 298 K and 1 bar, which is comparable to the value of 1.7 mmol g 1 reported previously.[17b] Introducing sulfuric acid groups into the framework of UiO-66 substantially increases the CO2 uptake (2.23 mmol g 1 in UiO-66-SO3H-0.15 versus 1.79 mmol g 1 in UiO-66, 298 K 1 bar) due to the strong affinity between CO2 and sulfuric acid groups caused by hydrogenbonding interactions.[22, 31] However, the increase is only seen at X = 0.15 where a balance of high CO2 affinity and good framework crystallinity can be obtained (Supporting Information, Table S2). Thanks to the strong electric fields created by alkali cations, neutralization of sulfuric acid into alkali sulfate further improves the CO2 uptake at lower X values (X = 0.15, 0.44). For example, UiO-66-SO3Li-0.15 has a CO2 uptake capacity of 3.28 mmol g 1 at 298 K and 1 bar, which is 47 % higher than UiO-66-SO3H-0.15 and 83 % higher than pristine UiO-66. The improvement is even more obvious under 0.15 bar, a typical partial pressure of CO2 in flue gas for post-combustion CO2 capture, where the increase of 82 % and 151 % were observed for UiO-66-SO3Li-0.15 (0.93 mmol g 1) over UiO-66-SO3H-0.15 (0.51 mmol g 1) and UiO-66 (0.37 mmol g 1), respectively (Table S2). Besides UiO-66-SO3Li-0.15, several other ionized MOFs also exhibit excellent CO2 uptake capacities at 298 K and 0.15 bar, such as UiO-66-SO3K-0.15 (0.75 mmol g 1) and UiO-66-SO3Na0.15 (0.75 mmol g 1). Based on Fajan’s rule, cations with higher charge and smaller size should have a stronger polarization ability.[32] This is why UiO-66-SO3Li-0.15 has the best CO2 uptake capacity: Li + is the smallest among the four cations. However, in MOFs with higher content of sulfate groups, crystallinity and surface area start to dominate the CO2 uptake capacity, which is reflected in the K-containing samples. The isosteric heats of adsorption Qst, a term that can be used to evaluate the adsorbent–adsorbate interactions, were calculated using the Clausius–Clapeyron equation,[33] and an almost opposite trend compared with the CO2 uptake capacity was revealed in the 3D plot (Figure 2; Supporting Information, Table S3). UiO-66 has a low-coverage Qst of 24.5 kJ mol 1, which is quite close to the value of 25.5 kJ mol 1 reported previously.[17b] UiO-66-SO3Li-0.15 only demonstrates a slight increase of Qst ( 25.4 kJ mol 1), while the highest Qst of 50.9 kJ mol 1 was obtained using UiO-66-SO3H-1, which is comparable to the Qst reported in other highly efficient CO2 adsorbents such as SIFSIX-3-Zn ( 45 kJ mol 1),[14e] PPN-6SO3NH4 ( 40 kJ mol 1),[23] Mg-MOF-74 ( 39 kJ mol 1),[34] and zeolite NaX ( 43 kJ mol 1).[34] This value also fits in the ideal enthalpy range for adsorption-based CO2 separation, where a balance between separation performance and ease of regeneration needs to be obtained.[13a] Besides CO2 uptake capacity, the CO2/N2 selectivity is equally if not more important in evaluating an adsorbent’s applicability for CO2 separation. In this study, two methods were used to 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. (a) 3D plot CO2 uptake at 298 K 0.15 bar. (b) 3D plot low coverage Qst of CO2. (c) 3D plot Henry CO2/N2 selectivities. (d) 3D plot IAST CO2/N2 selectivities (CO2/N2 = 15:85, 298 K 1 bar).
evaluate the CO2/N2 selectivity. The first one is based on the constant kH of Henry’s law, which is defined as the initial slope of the isotherm that reflects the interactions between adsorbent and adsorbate at low pressure range (Supporting Information, Table S4).[35] The CO2/N2 selectivities are calculated as the kH ratio of CO2 over N2 and are plotted in Figure 2, and listed in Table S5 (Supporting Information). UiO-66-SO3H-0.64 has the highest Henry CO2/N2 selectivity of 34.8, followed by UiO-66SO3Na-1 (29) and UiO-66-SO3Na-0.15 (23.7). These values are higher than that of UiO-66 (15.3) and other MOFs such as HKUST-1 (~ 20),[36] MOF-5 (17.48),[37] and MOF-177 (17.73).[37] The second method in calculating the CO2/N2 selectivity is based on ideal adsorption solution theory (IAST), where the binary gas mixture selectivity can be estimated using singlegas isotherms.[38] The isotherms of CO2 and N2 were fitted to a dual-site Langmuir model in order to get equilibrium parameters, saturation capacities of two adsorption sites, and Langmuir constants. IAST calculations were carried out assuming a CO2/N2 binary mixture at a molar ratio of 15:85 to mimic the composition of flue gas, and the results are shown in Figure 2 and Table S6 (Supporting Information). UiO-66-SO3Na-1 exhibits the highest IAST CO2/N2 selectivity (83.6) at 298 K and 1 bar, followed by UiO-66-SO3Na-0.15 (67.6), both of which are much higher than the pristine UiO-66 (19.4) and zeolite 13X (~ 20).[6] Although these selectivities are still lower than the top MOFs and polymers reported so far, such as SIFSIX-3-Zn (1818),[14e] Mg-MOF-74 (ca. 148),[14b] and PPN-6-SO3NH4 (ca. 796),[23] the excellent hydrothermal stability and low material cost make our ionized UiO-66 materials more practical towards industrial applications. Among these materials, UiO-66-SO3Na-0.15 is the best candidate for the balanced CO2 uptake capacity of 0.75 mmol g 1 (298 K, 0.15 bar) and CO2/N2 IAST selectivity of 67.6 (298 K, 1 bar). The application of ionized UiO-66 materials in natural gas upgrading involving CO2/CH4 separation was evaluated as well. Ionization has also helped increase the CH4 uptake, but to a smaller extent compared to CO2 because of the higher polarChemSusChem 0000, 00, 1 – 5
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late (SS-BDC) with different ratios (0.686 mmol in total) were firstly dissolved in 20 mL of dimethylformamide (DMF)/acetic acid (9:1) mixed solvents, followed by the addition of ZrCl4 (160 mg, 0.686 mmol). The solutions were sonicated for 15 min and then heated at 120 8C for 40 h. The solid products were recovered by centrifuge, washed with DMF three times, and then soaked in DMF at 80 8C for 12 h to completely remove any unreacted ligands trapped inside the frameworks. The samples then underwent solvent exchange with methanol for three days, and were fully activated at 120 8C under vacuum Figure 3. (a) 3D plot Henry CO2/CH4 selectivities. (b) 3D plot IAST CO2/CH4 selectivities for 24 h. Acidification was carried out by soaking the ac(CO2/CH4 = 50:50, 298 K 1 bar). tivated samples in 0.1 m of aqueous HCl solutions for 12 h and then in ethanol solutions of trifluoroacetic acid (10 vol %) for 6 h. The acidified samples were washed izability (29.11 1025 cm3) and quadrupole moment (4.3 and exchanged with methanol for three additional days, and then 1026 esu cm2) of CO2 as compared to that of CH4 (25.93 dried under vacuum at 120 8C for 24 h to afford the final products.
1025 cm3 and 0 esu cm2, respectively).[9a] In our study, UiO-66SO3Li-0.15 shows the highest CH4 uptake of 0.70 mmol g 1 at 1 bar and 298 K, which is only 63 % higher than that of pristine UiO-66 (0.43 mmol g 1) (recall that the increase for CO2 under the same condition was 83 %). As a result, higher CO2/CH4 selectivities are achieved after ionization. For example, UiO-66SO3Na-1 exhibits the highest Henry CO2/CH4 selectivity of 20.3, which is much higher than that of UiO-66 (4.9) and other reported MOFs, such as MOF-5 (15.53)[37] and MOF-177 (4.43).[37] IAST CO2/CH4 selectivities were calculated assuming a CO2/CH4 binary mixture at a molar ratio of 50:50 under 298 K 1 bar (Supporting Information, Table S6). Figure 3 shows that all of the ionized samples have higher IAST CO2/CH4 selectivities than UiO-66 (ca. 6.1). It is worth noting that UiO-66-SO3Rb1 shows an astonishing 789 % increase in selectivity (54.2), which is among the highest of all MOFs reported so far.[14e, 16c] However, its CO2 uptake capacity still needs to be improved for practical applications. In summary, we report herein a library of 20 UiO-66-derived metal–organic frameworks (MOFs) obtained through combinatorial chemistry involving mixed ligand copolymerization and post-synthetic modifications. The MOFs exhibit excellent water stability in a pH range of 1 to 12, and high CO2 uptake capacity/selectivity as revealed by the analysis of 147 isotherms. Among these materials, UiO-66-SO3Li-0.15 has the highest CO2 uptake capacity at 298 K under both 0.15 bar (0.93 mmol g 1) and 1 bar (3.28 mmol g 1); UiO-66-SO3H-1 exhibits the highest Qst of 50.9 kJ mol 1 at low coverage; the best CO2/N2 (15:85) selectivity calculated by IAST at 298 K and 1 bar belongs to UiO-66-SO3Na-1 (83.6); and the best CO2/CH4 (50:50) selectivity calculated by IAST at 298 K and 1 bar was achieved in UiO-66SO3Rb-1 (54.2), which is 789 % higher than the pristine UiO-66. Our approach paves a way towards the systematic study of water-stable and affordable MOFs as highly efficient adsorbents for CO2 separation in the applications of post-combustion CO2 capture and upgrading of natural gas.
Experimental Section Synthesis of UiO-66-SO3H-X: The UiO-66-SO3H-X samples were synthesized by revising a reported method.[27] Briefly, mixtures of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephtha 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Synthesis of UiO-66-SO3M-X: The UiO-66-SO3M-X samples were synthesized by neutralizing UiO-66-SO3H-X with aqueous alkali hydroxide solutions. Taking UiO-66-SO3Li-0.15 for example, 100 mg of UiO-66-SO3H-0.15 was immersed in 20 mL of 0.01 m LiOH aqueous solution at room temperature for 2 h. The product was recovered by centrifuge, washed with deionized water until neutral, exchanged with methanol for three days, and then activated under vacuum at 120 8C for 24 h to afford the final product. All the rest UiO-66-SO3M-X samples were synthesized similarly.
Acknowledgements This work is supported by National University of Singapore (NUS Start-up Funding R-279-000-369-133) and the Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, R-279-000361-731). Keywords: clean energy · carbon dioxide capture · combinatorial chemistry · metal-organic frameworks · natural gas [1] R. S. Haszeldine, Science 2009, 325, 1647 – 1652. [2] D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed. 2010, 49, 6058 – 6082; Angew. Chem. 2010, 122, 6194 – 6219. [3] S. Chaemchuen, N. A. Kabir, K. Zhou, F. Verpoort, Chem. Soc. Rev. 2013, 42, 9304 – 9332. [4] G. T. Rochelle, Science 2009, 325, 1652 – 1654. [5] D. M. Ruthven, S. Farooq, K. S. Knaebel, Pressure Swing Adsorption, Wiley-VCH, Weinheim, 1993. [6] S. Choi, J. H. Drese, C. W. Jones, ChemSusChem 2009, 2, 796 – 854. [7] a) H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science 2013, 341, 1230444; b) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334 – 2375; Angew. Chem. 2004, 116, 2388 – 2430; c) G. Frey, M. Haouas, T. Loiseau, F. Taulelle, Chem. Mater. 2014, 26, 299 – 309; d) R. J. Kuppler, D. J. Timmons, Q. R. Fang, J. R. Li, T. A. Makal, M. D. Young, D. Q. Yuan, D. Zhao, W. J. Zhuang, H. C. Zhou, Coord. Chem. Rev. 2009, 253, 3042 – 3066. [8] a) D. Zhao, D. Q. Yuan, H. C. Zhou, Energy Environ. Sci. 2008, 1, 222 – 235; b) R. E. Morris, P. S. Wheatley, Angew. Chem. Int. Ed. 2008, 47, 4966 – 4981; Angew. Chem. 2008, 120, 5044 – 5059; c) S. Q. Ma, H. C. Zhou, Chem. Commun. 2010, 46, 44 – 53. [9] a) J. R. Li, R. J. Kuppler, H. C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 – 1504; b) J. R. Li, J. Sculley, H. C. Zhou, Chem. Rev. 2012, 112, 869 – 932. [10] a) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459; b) A. Corma, H. Garcia, F. X. L. Xamena, Chem. Rev. 2010, 110, 4606 – 4655.
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Received: May 3, 2014 Revised: June 18, 2014 Published online on && &&, 0000
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COMMUNICATIONS Z. Hu, K. Zhang, M. Zhang, Z. Guo, J. Jiang, D. Zhao* && – && A Combinatorial Approach towards Water-Stable Metal–Organic Frameworks for Highly Efficient Carbon Dioxide Separation
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With our powers combined: A library of 20 UiO-66-derived metal–organic frameworks (MOFs) is synthesized following a combinatorial approach involving mixed ligand copolymerization and two post-synthetic modifications in tandem. The MOFs have excellent water stabilities in a pH range of 1 to 12 together with high carbon dioxide (CO2) uptake capacities and selectivities. These features make them promising adsorbents in adsorption-based CO2 separations such as post-combustion CO2 capture and upgrading natural gas.
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