Bimetallic ZIF membranes growing on the inner-surface of ZnO ceramic hollow fibers for gas separation

Abstract

Bimetallic ZIF membranes offer additional functionalities compared to single-metal ZIFs membranes. However, the preparation of ZIF membranes through a facile and reproducible process remains a significant challenge. In this work, a high-quality bimetallic Co_xZn_1−x-ZIF membrane was successfully synthesized using a ZnO support-activation and hydrothermal approach. The effects of the length of ZnO hollow fiber support, surface activation, different Zn/Co molar ratios in the MOF framework and operating conditions on the membrane performance were carefully investigated. Experimental results indicate that the incorporation of two metals (Co and Zn) in the ZIF membrane can tune the ZIF pore framework to improve the gas selectivity while maintaining the general crystal structure of the single metal ZIF-67. The optimized bimetallic Co_0.3Zn_0.7-ZIF membrane exhibited a high H₂/CH₄ separation factor of 18.3 with a H₂ permeance of approximately 1.05 × 10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹, surpassing the single-metal ZIF-67 membrane. Noteworthy that the Co_0.3Zn_0.7-ZIF membrane demonstrated excellent long-term operation stability for 200 h and good thermal cycling stability for more than 10 days. Furthermore, the mechanical and hydrothermal stability test results imply that bimetallic ZIF membrane exhibited better stability than single metal ZIF-67 membrane. Our results also indicate that three parallel bimetallic ZIF hollow fiber membranes can be prepared in one autoclave using the similar amount of nutrient solution for one membrane growth and these membranes exhibited much similar separation properties. The employed method for bimetallic ZIF hollow fiber membrane has a very good reproducibility and scaling up capacity, paving the way for practical application.