Gas Membrane Selectivity Enabled by Zeolitic lmidazolate Framework Electrostatics
KG Ray and DL Olmsted and JMR Burton and Y Houndonougbo and BB Laird and M Asta, CHEMISTRY OF MATERIALS, 26, 3976-3985 (2014).
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 transport than in ZIF-97. Thus, for channels small enough to act as molecular sieves, the electrostatic field in the channel is found to be an important factor in the design of ZIFs for the separation of gas molecules possessing non- negligible electrostatic interactions with the framework.
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