Ideal and mixture permeation selectivity of flexible prototypical zeolitic imidazolate framework – 8 Membranes

Abstract

Permeance, ideal and mixture permeation selectivity have been evaluated based on molecular simulations of gas adsorptions by the grand canonical Monte Carlo simulation and diffusion by molecular dynamics simulation under a flexible framework. Additional transport phenomena are also obtained using transition state theory under rigid framework assumptions. A number of gas systems in the simulation involving H2, CO2, CH4, N2, O2 and Ar penetrating through a thin layer of ZIF-8 membrane are analyzed and compared with permeance, ideal and mixture permeation selectivity experiments. Three contributions have been achieved in this work. Firstly, it is demonstrated that the assumption of a rigid framework may be sufficient for simulating adsorption isotherms while diffusivity requires a flexible framework, especially when a diffusing molecule is bigger than the pore window of the MOFs, with an appropriate force-field and charges. Secondly, simulations of the ideal permeation selectivity are higher than those of mixture permeation selectivity. However, both calculations with flexible framework accuracy may be acceptable for the purpose of screening membrane materials when compared with experiments, especially the permeation selectivity of an equimolar mixture. Finally, even though simulated isotherms, diffusivity and mixture selectivity are consistent with available experiments and the same data have been used for estimating permeance of the membrane, the difference between the estimation and measurement of the permeation at low pressure is considerable. Therefore, it is confirmed that the effects of defections and multi-scale (macroscopic) diffusion cannot be ignored.