Origins of CH4/CO2 Adsorption Selectivity in Zeolitic Imidazolate Frameworks: A van der Waals Density Functional Study

The energetics of methane binding are calculated in a set of five zeolitic imidazolate framework (ZIF) materials using the van der Waals Density Functional 2 (vdW-DF2) [Lee, K.; et al. Phys. Rev. B 2010, 82, 081101]. These results are compared to those from previous calculations for carbon dioxide in the same ZIFs [Phys. Rev. B 2012, 85, 085410] to examine the roles of electrostatic interactions, polarization, steric constraints, and hydrogen bonding in determining the CO2/CH4 adsorption selectivity. To isolate the effect of the chemical functionalization of the imidazolate linkers, the ZIFs considered share the same zeolite RHO topology and metal atom (Zn). Methane is found to be primarily bound by dispersion forces, and thus, site geometry and steric constraints have the greatest influence on its binding. These results are in contrast to those of carbon dioxide where electrostatic interactions play a sizable role. To quantify the relative importance of dispersion forces versus electrostatic contributions to the binding energies, we isolate these different contributions through an approach combining an analysis of the nonlocal contributions to the exchange-correlation energy and a decomposition of the charge density based on Bader analysis.