Fuel Gas Storage and Separations by Metal-Organic Frameworks: Simulated Adsorption Isotherms for H2 and CH4 and Their Equimolar Mixture

Metal-organic frameworks (MOFs) are novel microporous materials with large surface areas, porosities, and thermal stabilities. Even though many thousand MOFs have been identified, few MOF materials have been evaluated for gas mixtures separations and fuel gas storage. In this work, using grand canonical Monte Carlo simulations, we calculated adsorption isotherms of pure and binary mixtures of hydrogen-methane in two large surface area MOFs (MOF-5 and MOF-177), two catenated MOFs (IRMOF-11 and MOF-14), and a high affinity open metal site MOF with strong Zn delta+-O delta- dipoles on the surface that create strong energetic interaction with the adsorbates (MOF-74). The pure and mixture adsorption isotherms were calculated at 298 K and up to pressures of approximately 80 bar. The results of this study indicate that separation of hydrogen from methane in these materials would be successful, since hydrogen in a 50% bulk mixture at low pressures has selectivities on the order of 25 for MOF-74, 20 for IRMOF-11, and 18 for MOF-14, compared to low selectivities values on the order of 5 for the two large surface area MOFs, MOF-5 and MOF-177. From this study, we also found that MOF-74 has a large methane storage capacity of 170 cm(3) (STP)/cm(3) at 298 K and 35 atm, close to the 180 cm(3) (STP)/cm(3) DOE target for practical methane storage. None of the materials studied has hydrogen gravimetric uptakes in excess of 0.4% wt.