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J. Phys. Chem. C 2010, 114, 6464–6471
Gas Adsorption Study on Mesoporous Metal-Organic Framework UMCM-1 Bin Mu, Paul M. Schoenecker, and Krista S. Walton* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe NW, Atlanta, Georgia 30332 ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: February 16, 2010
A mesoporous metal-organic framework (MOF) material, UMCM-1, has been synthesized and characterized using N2 adsorption, powder X-ray diffraction, scanning electron microscopy, and Fourier transform infrared techniques. A detailed experimental study has been made of the adsorption of pure methane, hydrogen, carbon dioxide, oxygen, and nitrogen at various temperatures (298-338 K) and pressures (up to 25 bar). Multitemperature isotherms were modeled using the Dubinin-Astakhov equation to obtain useful thermodynamic properties including adsorption potential characteristic curves and isosteric heats of adsorption. Results are compared with mesoporous carbons and silicas. Large-pore materials are shown to exhibit relatively high heats of adsorption for CO2 when open metal sites are present. This phenomenon is not observed for methane, which indicates the importance of the CO2 quadrupole in influencing binding strength. Adsorption results for N2 and O2 show that selectivities in MOFs can be manipulated by the presence or absence of open metal sites. UMCM-1 and MOF-177 show a slight preference for O2 over N2. However, open metal site MOFs such as Cu-BTC show the opposite adsorption preference, which is similar to zeolite selectivities. This experimental study reveals interesting adsorption information about a novel mesoporous MOF within the context of other MOFs and traditional mesoporous adsorbents. These results can be used to advance the development of structure-property relationships for metal-organic frameworks. 1. Introduction Porous metal-organic frameworks (MOFs) have become the focus of intense study over the past decade due to their potential for advancing a variety of applications including gas storage, adsorption separation, catalysis, and gas sensing. These materials have some distinct advantages over traditional porous materials such as their well-defined structures, uniform pore sizes, chemically functionalized sorption sites, and potential for postsynthetic modification. Adsorption studies in MOFs have increased substantially in recent years, but full structure-property relations have yet to be developed. Information on host-guest interactions at low pressure, the effect of uncoordinated metal sites, pore structure and volume, chemical functionality, and surface area is crucial to elevate MOFs to an applied level in the adsorption field. Methane and hydrogen have been the most extensively studied gases for adsorption in MOFs, and gas storage for alternative fuel vehicles continues to be an important driver for this research area. Yaghi et al.1 demonstrated a series of MOF materials with remarkable methane storage capacity, among which IRMOF-6 is impressive with an uptake of 240 cm3/g at 298 K and 36 atm. Du¨ren et al.2 used molecular modeling to analyze the roles played by the surface area, free volume, heats of adsorption, and pore size distribution for methane storage in porous materials. Their simulation results confirmed that high surface area, high free volume, low adsorbent density, uniform pore size distribution, and strong interaction energy between adsorbate and adsorbent will facilitate high methane storage capacity. Wang3 used grand canonical Monte Carlo (GCMC) simulations to examine a series of 10 MOFs and reached similar conclusions. * To whom correspondence should be addressed. E-mail: krista.walton@ chbe.gatech.edu.
MOFs have also been studied for hydrogen storage, and several reviews on current progress are available.4-10 Frost et al.10 studied the influences of surface area, free volume, and heat of adsorption on hydrogen adsorption in a series of MOFs. Their grand canonical Monte Carlo (GCMC) simulation results indicated that the predominant factors affecting hydrogen uptake depend on heat of adsorption at low pressure, surface area at intermediate pressure, and free volume at high pressure. It has also been shown that structural properties such as unsaturated metal centers can be incorporated into materials to increase the magnitude of hydrogen interaction with the framework.11,12 In addition to gas storage, the capture of carbon dioxide has become one of the most urgent research topics in fields related to energy and the environment. The removal of CO2 from raw products to purify biogas is the most expensive step in biofuel upgrading.13,14 The removal of CO2 from natural gas is also an important process to help prevent pipeline corrosion. The concentration of CO2 in natural gas should be as low as 2%.13 Yaghi and co-workers reported the adsorption capacity of CO2 on a series of 10 MOF materials at room temperature, among which MOF-177 had the highest capacity of 34 mmol/g at 40 bar.15 Zhong et al.16 and Barbarao and Jiang17 studied CO2 storage capacity on different MOF materials using GCMC simulations and found that IRMOF-16 has the highest capacity of 64 mmol/g at 50 bar. Walton et al.18 studied the inflections in CO2 adsorption isotherms on MOFs with no open metal sites and found that electrostatic interactions between CO2 molecules are responsible for the unusual shape of the adsorption isotherms. Oxygen or nitrogen generation from air is an important industrial process. The applied technologies include three major types: cryogenic distillation, which has the longest application history, especially for large volumes of high-purity gas; air separation at ambient temperatures using polymeric membranes
10.1021/jp906417z 2010 American Chemical Society Published on Web 03/19/2010
Adsorption of Gases in UMCM-1
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Figure 1. Comparison of powder XRD patterns for UMCM-1 after activation to remove guest molecules (top) and as synthesized (middle), and the theoretical pattern from the single crystal data (bottom).
or porous zeolites; and high-temperature air separation using specialized ceramic ion transport membranes (ITM).19 For many applications that require a smaller volume of oxygen or nitrogen daily (i.e.,
