Zeolitic imidazolate frameworks for separation of binary mixtures of CO2, CH4, N2 and H2: A computer simulation investigation

We studied adsorption and dynamics of CO2, CH4, N-2, H-2 and binary mixtures thereof in the zeolitic imidazolate frameworks ZIF-2 to ZIF-10, using computer simulations. The results of grand canonical Monte Carlo calculations showed that the amount of gas adsorbed in these materials is smaller than what can be stored in other organic frameworks, due to the small-pore structure of these crystals. The results of molecular dynamics simulations showed that three of these materials (ZIF-5, ZIF-7 and ZIF-9) fulfill the condition for molecular sieving separation of H-2, whose diffusion is orders of magnitude faster than that of other species. Analysis of the adsorption and permeance selectivity for the other materials, in which all the species considered have comparable self-diffusion coefficients, points out ZIF-4 as the best performing material for a large variety of gas separations. Our results indicate the importance of complementing Monte Carlo data with molecular dynamics simulations in assessing the performance of small-pore organic frameworks for gas adsorption and separation. In particular, the explicit modeling of framework flexibility is crucial to obtain reliable gas-diffusion properties of small-pore frameworks. (C) 2011 Elsevier Inc. All rights reserved.