Theoretical investigation of ZIFs as adsorbents or membranes for separating noble gas mixtures: applying a newer method for predicting performances

Zeolitic imidazolate frameworks (ZIFs) possess similar topological structures with zeolites, however a number of characteristic properties of the ZIFs, such as easy incorporation of desired linkers to their structures, tunable pore sizes, high porosities, and large surface areas make them better option for targeted engineering applications. In this respect, cheaper, highly efficient, and selective separation of syngas mixtures using ZIFs has been already shown in the literature. Separation and purification of noble gases, on the other hand, have been achieved using energy demanding cryogenic distillation of air and currently we know little about noble gas separation performances of the ZIFs. Replacing energy intensive gas separation techniques with adsorbent and/or membrane based separation technologies, indeed, have been motivated for decreasing the operational costs, price of the pure noble gases, as well as opening new application areas for the noble gases. In this respect, we theoretically investigate Xe/Kr and Xe/Ar separation performances of eight ZIFs, namely ZIF-6, ZIF-11, ZIF-60, ZIF-67, ZIF-69, ZIF-78, ZIF-79, and ZIF-81 using Grand Canonical Monte Carlo and Molecular Dynamics simulations and calculate Xe permeability and permeation selectivity using a newer method suggested previously by our group. While ZIF-11 shows significant Xe uptake from the noble gas mixtures, ZIF-81 and ZIF-79 show exceptional Xe adsorption selectivities for two gas mixtures at low pressures, 22.5 and 21 for the Xe/Kr and 128 and 120 for the Xe/Ar mixture, respectively. ZIF-69 and ZIF-79 show high Xe permeability (4.2x10(5) and 3.2x10(5) Barrers) as well as high Xe permeation selectivity (17.35 and 15.85) for the Xe/Ar mixture at 2 bar feed gas pressure. For the Xe/Kr mixture, on the other hand, we observe high Xe permeability and moderate Xe permeation selectivity, especially at low pressures. Comparing with the limited literature available, these ZIFs show promise for selective separation of Xe from its binary mixtures of Kr and Ar at room temperature. We also compare the permeability and permeation selectivity predictions using different approaches, namely the new method and the approximate approach which is commonly used in the literature. Results reveal significant deviations of the approximate approach with respect to the new method, especially for the permeability calculations. Thus, using the new method for determining membrane performances is highly suggested.

Automated summary

ZIFs exhibit promising potential for syngas mixture and noble gas separation applications. While literature exists on ZIFs for syngas separation, the performance of ZIFs for noble gas separation remains largely unknown.