Gas Membrane Selectivity Enabled by Zeolitic Imidazolate Framework Electrostatics

CO2 and CH4 diffusivities are calculated using molecular-dynamics simulations for a set of five RHO topology zeolitic imidazolate frameworks (ZIF-25, -71, -93, -96, and -97), which differ only in linker functionalization. It is found that CO2 diffuses more readily than CH4, by a factor of 3.25, through the channels of ZIF-93, despite being much more strongly bound to the framework. Using adsorption selectivity values from previous studies, we calculate the membrane selectivities at 1 bar for all five materials and find this quantity to be highest in ZIF-93, with a value of 26.7. First-principles calculations based on van der Waals density functional methods are used to investigate the transport barriers for CO2 in ZIF-93 and ZIF-97. These two materials have similar channel diameters, but have very different CO2/CH4 diffusion selectivities. The excellent molecular-sieving performance of ZIF-93 is determined to be enhanced by electrostatic interactions, that result in smaller energy barriers for CO2 transpo...