A Combined Experimental-Computational Investigation of Methane Adsorption and Selectivity in a Series of Isoreticular Zeolitic Imidazolate Frameworks

Zeolitic imidazolate framework (ZIF) materials have received considerable attention recently due to their potential as materials for gas separation applications. In this work, we study, both experimentally and with molecular modeling, methane adsorption in a series of five ZIFs (ZIF-25, -71, -93, -96, and -97) that share a common structural topology (RHO), but differ in imidazolate functionalization. Such a series allows for the direct assessment of the role that functionalization plays in determining methane adsorption. Experimental measurements of methane adsorption up to 1 bar at various temperatures are well reproduced by molecular simulations, which are further used to examine adsorption up to higher pressures of 80 bar, and to analyze the preferred binding sites within the structure. We find that CH4 uptake in these ZIFs is roughly proportional to the Brunauer-Emmett-Teller (BET) surface area, in contrast to our earlier work on the adsorption of CO2 for this series [J. Am. Chem. Soc. 2010, 132, 11006], which showed a significant enhancement of CO2 adsorption, due to electrostatic effects, in asymmetrically functionalized ZIFs (ZIF-93, -96, -97) over those with symmetric functionalization (ZIF-25, -71). Furthermore, the ideal adsorbed solution theory (IAST) is used to predict selectivity of CO2 over CH4 in these RHO ZIFs by fitting CH4 adsorption measurements in this work and the CO2 experimental isotherms from our earlier work [J. Am. Chem. Soc. 2010, 132, 11006].