Microporous Crystalline Membranes for Kr/Xe Separation: Comparison Between AlPO-18, SAPO-34, and ZIF-8

Herein we demonstrate the separation ability of AlPO-18 membranes for Kr/Xe gas mixtures. AlPO-18 membranes separated Kr/Xe mixtures with average Kr/Xe separation selectivity of 6.4 and unprecedented high average Kr permeance of 1.6 × 10–7 mol/m2·s·Pa. Diffusivity difference between Kr and Xe was identified as the dominant separation mechanism. We compared the separation performance of AlPO-18 membranes versus two other types of microporous crystalline membranes, namely, SAPO-34 and ZIF-8. SAPO-34 membranes displayed the best overall separation performance, while AlPO-18 membranes displayed the highest Kr permeances. We identified the key factors affecting both the separation selectivity and permeance. For these three distinctive types of microporous materials, the presence of rigid micropores with size lying between Kr and Xe atomic sizes, lower Xe/Kr uptakes (adsorption selectivity), and lower concentration of nonselective pores led to the highest observed s Kr/Xe separation selectivities among these three microporous crystals. The Kr permeances for these three microporous crystalline membranes decreased exponentially as membrane thickness increased.