Green synthesis of ZIF-8 tubular membranes from a recyclable 2-methylimidazole water-solvent solution by ZnO nanorods self-converted strategy for gas separation

Abstract

A defect-free ZIF-8 membrane on a porous ceramic tube was well prepared by the self-conversion of ZnO nanorods in a zinc-free 2-methylimidazole (Hmim) water-solvent solution. In the synthesis, the ZnO nanorods grown on the substrate provide the zinc source and act as the nucleation and growth sites for the ZIF-8 membrane, thus contributing to the formation of a high-quality ZIF-8 membrane with the ideal selectivity of over 40 for H2/CH4. Most importantly, it is found that after the one-time membrane synthesis, the used solution could be still recycled for the self-conversion of ZnO nanorods to ZIF-8 membranes. The results show that each self-converted synthesis merely consumed around 0.005 M ligand for achieving a continuous ZIF-8 membrane, and thus the solution with the ligand concentration of 0.5 M could be repeatedly used for at least 50 times for preparing compact ZIF-8 membranes without any treatment. Moreover, when the synthesis solution of around 0.2 M produced after the recycling for 60 times is recovered to the one of 0.5 M by adding fresh Hmim ligand, the obtained solution of 0.5 M could again act as the membrane synthesis one for the self-conversion of ZnO nanorods into the compact ZIF-8 membrane. Moreover, it could be also recycled for another 50 times for the synthesis of ZIF-8 membrane as the first-stage recycling, thus realizing a sustainable synthesis without any wastewater discharge. The present strategy is an environmental friendly and low-cost method for the scalable preparation of a ZIF-8 tubular membrane and of potential in practical applications.