Communication pubs.acs.org/crystal
A Calcium Coordination Framework Having Permanent Porosity and High CO2/N2 Selectivity Debasis Banerjee,*,† Zhijuan Zhang,‡ Anna M. Plonka,§ Jing Li,*,‡ and John B. Parise*,†,§,∥ †
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States § Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, United States ∥ Photon Source Division, Brookhaven National Laboratory, Upton, New York 11973, United States ‡
S Supporting Information *
ABSTRACT: A thermally stable, microporous calcium coordination network shows a reversible 5.75 wt % CO2 uptake at 273 K and 1 atm pressure, with an enthalpy of interaction of ∼31 kJ/mol and a CO2/N2 selectivity over 45 under ideal flue gas conditions. The absence of open metal sites in the activated material suggests a different mechanism for selectivity and high interaction energy compared to those for frameworks with open metal sites.
S
elective removal of CO2 from gas streams and the atmosphere is an important process in energy production and in mitigating the environmental damage resulting from energy consumption.1−3 The selectivity of zeolite molecular sieves, used commercially for the separation of gases, including CO2, is attributed to the presence of regularly spaced pores and channels and to the different interaction potentials of gases with extra-framework cations within the pores.4−7 The extraframework cations are often from s-block series.8,9 The structurally diverse porous metal−organic frameworks (MOFs)10 and coordination networks (CNs)11 emulate favorable properties for gas storage1,12−14 separation15,16 and catalysis17,18 attributed to zeolites. Frameworks constructed using s-block metals, such as environmentally abundant magnesium and calcium, are particularly promising because these metals are lightweight and have high enthalpies of interaction with adsorbed gas-molecules.11,19 As partial confirmation of this strategy, several magnesium coordination networks are reported and show permanent porosity and interesting gas-adsorption properties.19−22 For example, the presence of open metal-sites in the MOF-74(Mg) framework, akin to extra-framework sites in many zeolites, was implicated in the observed high CO2 selectivity in the absence of water vapor.23 On the other hand, the role of calcium in natural CO2 separation and storage24 suggests research directed toward synthesis of calcium based porous networks may also be fruitful. Despite several reports of calcium based coordination networks, they are found to be nonporous, after activation; removal of coordinated and/or uncoordinated solvents leads to structural © 2012 American Chemical Society
collapse, rendering the framework unsuitable for selective sorption.25−28 In this communication, we report the selective CO2/N2 gas-adsorption properties of a highly robust microporous calcium based coordination network [Ca(SDB)]·H2O (1) (SDB = 4,4′-sulfonyldibenzoate) with sustainable and accessible pores upon guest removal. Just prior to the submission of this work we became aware that this particular compound is reported by another group.29 Although they report the moderate capacity for CO2 sorption, they report neither the compound’s remarkable selectivity toward CO2 nor the correct structure of the activated compound responsible for this selectivity. The reaction of metal salt CaCl2·xH2O with the organic linker (SDB) under solvothermal conditions leads to the formation of block shaped crystals of compound 1 (see Supporting Information). Single crystal structure analysis shows that 1 is a three-dimensional (3D) network, composed of corner-sharing calcium polyhedral chains along the crystallographic [010] direction (Figure 1 and Supporting Information, Figure S1). Calcium is present in octahedral coordination, with bonds to five-carboxylate groups and a sulfonyl oxygen atom. One of the sulfonyl oxygen atoms (O5) of the linker is uncoordinated. Each of the calcium polyhedral chains is connected with six other such chains in the [100] and [001] directions by the linker molecules. Such a connectivity leads to the formation of a channel running in the [010] direction with a square-shaped cross section of average size 5.9 Å × 5.8 Å, Received: February 26, 2012 Published: March 15, 2012 2162
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Compound 1′ maintains its topology after gas-adsorption experiments and when exposed to air for 2 days, as observed from PXRD patterns (Figure S6 of the Supporting Information). Further, the single crystal diffraction and the TGA data of compound 1′ show no evidence of moisture adsorption upon exposure to air (Figures S7−8). However, both compounds 1 and 1′ transform to a different phase when placed in liquid water (see Supporting Information, Figures S9−10). The permanent porosity of compound 1′ was confirmed by pore characterization and gas-adsorption studies. The N2 adsorption isotherm at 77 K displays a type I sorption behavior, verifying its microporous nature. The BET surface area was calculated to be 145.15 m2/g. Experiments of H2 adsorption reveal a moderate uptake of 0.55 wt % at 77 K and 1 atm (see Figure S11 of the Supporting Information). The isosteric heats of H2 sorption, calculated by fitting the adsorption data at 77 K and 87 K to the Virial equation, were found to be between 7.41 and 8.92 kJ/mol in the loading range below 0.4 wt % (see Figure 3). The Qst (H2) is slightly
Figure 1. View of compound 1 along the [010] direction. [Ca, dark cyan sphere; S, yellow sphere; O, red sphere; C, light gray sphere.] Hydrogen and solvent (water) molecules are omitted for clarity.
based on the van der Waals radii of the aromatic carbon atoms. Disordered water molecules are present within the channel. PLATON30 indicates an accessible volume of 18.2% (275 Å3) upon solvent removal. The water molecules present within the channel are readily removed below 250 °C, as evident from the TG experiments (observed, 5.1%; calculated, 4.97%). The dehydrated network undergoes thermal decomposition at ∼575 °C (Figure S2). Single crystal analysis of the dehydrated network reveals that after activation compound 1 undergoes a structural transformation to a different, denser phase (denoted as 1′). Compounds 1 and 1′ crystallize in the same space group (P21/n), with very similar unit cell dimensions (Table S1), but with different origin of the unit cell and different packing of organic linkers. The main observable change is the position of sulfur atoms, influencing how the linkers surrounding the same channel are facing each other. In compound 1 linkers of the same channel are packed in a parallel manner; after activation they are turned by 45° from each other. As a consequence, the volume of the channel is smaller, changing the overall porosity of compound 1′ from 18.2% (calculated for 1) to 16%. The change in linker packing pattern results in the reorganization of the calcium coordination sphere (Figure 2) and, further, the transformation of O−M−O connectivity from corner sharing chains in compound 1 to edge sharing dimers in 1′ (Figures S3−4).
Figure 3. Isosteric heats of H2 adsorption calculated based on adsorption isotherms at 77 and 87 K using the Virial method.57−62
lower than that of MOF-74(Mg) but considerable higher than those of other porous materials, such as MOF-5,31 ZIF-8,32 and PCN-6.33 Further, 1′ was tested for CO2 adsorption at different temperatures and pressures. Compound 1′ shows a reversible uptake of 5.75 and 4.37 wt % at 273 and 298 K and 1 atm pressure, respectively (Figure 4a). The heats of CO2 adsorption at low loadings are ∼28−31 kJ/mol, significantly higher than those of N2 (∼19−23 kJ/mol) in the same loading range, as shown in Figure 4b. The values of Qst for CO2 are lower than those of the M-MOF-74 (M = Mg, Co, Ni) series under similar experimental conditions,18 but they are comparable to those of
Figure 2. Structural differences between compounds 1 and 1′. (A) cross section of the channel, showing parallel organization of organic linkers in compound 1. (B) Ca coordination sphere in 1. (C) Ca coordination sphere in 1′. (D) Cross section of the channel showing tilted organization of the SDB linkers in 1′.
Figure 4. (a, left) CO2 adsorption−desorption isotherms at 273 K (black), 288 K (red), and 298 K (blue). (b, right) Heats of adsorption (Qst) for CO2 and N2 calculated on the basis of the Virial method. 2163
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HKUST-134 and MOF-5.35 The higher Qst for the M-MOF-74 (M = Mg, Co, Ni) series is due to the presence of open metal sites in the activated framework,23 while 1 possesses no open metal sites (OMSs). For postcombustion CO2 capture, OMSs are subject to water vapor and will lead to significantly reduced capture performance.36 The relatively high Qst of compound 1′ may be attributed to its small pore size37 and the ionic nature of Ca−O bonding, which favors quadruple interactions with CO2 molecules.1,19 The ideal adsorbed solution theory (IAST), developed by Myers and Prausnitz,38 is a widely adopted method to predict multicomponent isotherms from pure gas isotherms. It has been reported that it can accurately predict gas mixture adsorption in many zeolites39−43 and more recently in MOF materials.39,44−51 Here, IAST is employed to estimate the CO2 and N2 adsorption selectivity in 1′. Adsorption isotherms predicted by IAST for mixtures of CO2/N2 (CO2/N2 = 15:85) in 1′ as a function of total bulk pressure are shown in Figure S14 of the Supporting Information. The predicted CO2/N2 selectivities at different pressures and mixture compositions for 1′ are shown in Figure 5. The values are above 45. At 0.15 bar
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental procedure for the synthesis of compound 1, PXRD patterns, TG profiles, and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: D.B.,
[email protected]; J.L., jingli@ rutgers.edu; J.B.P.,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Initial synthesis work at Stony Brook was supported by Grant NSF-DMR-0800415. Subsequent structural characterization is supported by Department of Energy (DOE) Grant DE-FG0209ER46650. The Rutgers team would like to thank the DOE for partial support through Grant No. DE-FG02-08ER46491. Stony Brook University’s single crystal diffractometer, used in this work, was obtained through NSF Grant CHE-0840483. D.B. acknowledges the help of Stony Brook colleagues Mr. Nicholas W. Mann (synthesis work) and Mr. Chris Koenigsmann and Prof. Stanislaus S. Wong (TGA data).
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Figure 5. Room temperature CO2/N2 selectivity calculated by the IAST method for three CO2 concentrations (CO2/N2: 10:90 (10%), 15:85 (15%), and 20:80 (20%)) in CO2−N2 binary mixtures.
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