Mixed-matrix membranes containing ZIF-8@s5A composite adsorbents with core-shell structure for $\mathrm{CO_2}$/$\mathrm{CH_4}$ separation

Abstract

Recently, due to excessive use of fossil fuels, $\mathrm{CO_2}$ emissions have increased, resulting in various environmental problems. To solve this problem, renewable natural gas (RNG) has emerged, and mixed-matrix membranes as an economically efficient way to upgrade biogas, is drawing attention. However, when fabricating mixed-matrix membranes, the poor adhesion between the inorganic filler and the organic polymer easily causes defects. In particular, a 'sieve-in-a-cage' void acts as a nonselective bypass, which is a problem by greatly reducing the selectivity. In this study, the core-shell structured filler capable of improving organic/inorganic compatibility by synthesizing nanostructured MOFs on the inorganic surface was developed. Nanowhisker-shaped ZIF-8 was synthesized specifically on the surface through a simple ion-exchange method using LTA zeolite as a template, and thus core-shell composites with a memory effect were successfully synthesized. It was confirmed that when the membrane fabricated using this as the filler was applied to $\mathrm{CO_2}$/$\mathrm{CH_4}$ separation, the interfacial interaction between the polymer-filler was improved due to the increased surface roughness and the organic linker constituting the ZIF-8 framework. Moreover, due to the synergistic effects of s5A zeolite and nanowhisker-shaped ZIF-8, it showed excellent $\mathrm{CO_2}$/$\mathrm{CH_4}$ selectivity up to 51.2 at 20 wt% ZIF-8@s5A-50.