Models used for permeability predictions of nanoporous materials revisited for H2/CH4 and H2/CO2 mixtures

High throughput screening of new generation nanoporous materials, e.g. metal organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs), for gas mixture separations have produced large amounts of data. Reported gas permeabilities have been mainly calculated using a simplified (approximate) approach. Perme-ability predictions of an alternative method (new method), proposed previously by our group, have shown to significantly improve the predictions of the approximate method for noble gas mixtures. Permeabilities calculated using Onsager coefficients, detailed method, are accepted as correct answers, however constructing Onsager coefficient matrix is computationally cumbersome and not feasible especially for the high throughput screening purposes. In this work, we question in what accuracy the approximate and new methods can predict gas permeabilities and permeation selectivities of gases with respect to the detailed method. We perform Grand Canonical Monte Carlo and Molecular Dynamics simulations for six ZIFs, namely ZIF-6, ZIF-10, ZIF-60, ZIF-69, ZIF-79, and ZIF-81. Permeabilities of H-2/CH4 and H-2/CO(2 )mixtures are selected as cases, since the dominating interactions of the gas species with the pores of the membranes are different. For the H-2/CH4 mixture, approximate and new methods predict H-2 permeability results of the detailed method sufficiently good. CH4 permeabilities of the approximate method reveal deviations from the correct answers, yet the new method improves the predictions of the approximate approach. In the case of H-2/CO2 mixture, H-2 permeabilities calculated by the approximate approach are in agreement with the detailed method, except for ZIF-60 and ZIF-79. For the CO2 permeability, approximate and new methods give significant deviations from the results calculated using the detailed method.

Automated summary

This study investigates the accuracy of approximate and new methods in predicting gas permeabilities and selectivities of nanoporous materials for gas mixture separations. It compares the predictions of these methods with a detailed method using Monte Carlo and Molecular Dynamics simulations.