Development of ZIF-8 membranes for propylene/propane separation by direct growth on a ZnO-modified support without activation

Abstract

The present study examines zeolitic imidazolate framework-8 (ZIF-8) membranes for propylene/propane separation prepared via direct growth on a zinc oxide (ZnO)-modified support, and their permeation properties. ZnO acts as a secondary zinc source and induces crystallization of the ZIF-8 selective layer, thereby actualizing a simple and more versatile preparation method. The ZnO layer was prepared by dip-coating the porous support in an aqueous solution of zinc nitrate and subsequent calcination in air at 200°C. ZIF-8 membranes with well-intergrown crystalline layers were obtained via direct growth on the modified support without activation with 2-methyimidazole. The ZIF-8 membranes prepared from the support modified by a 0.1-M aqueous solution of zinc nitrate demonstrated the highest performance with an average propylene permeance of (1.8±0.29)×10−8mol·m−2·s−1·Pa−1 and a mean propylene/propane selectivity of 25±1.8, using an equimolar feed of propylene and propane at 25°C. The intermittent, long-term propylene/propane separation measurements across the ZIF-8 membranes showed an approximately constant performance up to 20d, thus demonstrating the effectiveness of our proposed method. A molecular sieve was identified as the dominant permeation mechanism; however, selectivity was lower than that presented in previous reports. To estimate the influence of defects in the selective layer, the sum of parallel micropores and Knudsen and viscous flows were fitted to experimental data. Viscous flow was quantitatively estimated to be the main cause for the decrease in selectivity.