Size-reduced low-crystallinity ZIF-62 for the preparation of mixed-matrix membranes for CH4/N2 separation

Abstract

CH4/N2 separation is of great significance for the utilization of unconventional natural gas, however facing great difficulty due to the similarity of physical and chemical properties between components. In this work, a liquid-assisted mechanochemical route was developed for the synthesis of low-crystallinity ZIF-62 with reduced particle size and abundant open metal sites (denoted as ZIF-62-L), and dual-zinc sources (Zn(OH)2 and Zn(OAc)2·2H2O) was found to be crucial for the acquisition of pure phase ZIF-62. The small particle size ensured good compatibility between ZIF-62-L and polymer matrix (polyethyleneimine, PEI), while the abundant open metal sites enhanced the interaction between ZIF-62-L and CH4 molecules. As a result, defect-free ZIF-62-L/PEI mixed-matrix membranes (MMMs) with high filler loadings and largely improved CH4/N2 separation performance were successfully prepared on modified polysulfone (MPSf) ultrafiltration substrate. Specifically, at the ZIF-62-L loading of 50 wt%, the composite membrane exhibit CH4 permeance of 2726.6 gas permeation unit (GPU) and a CH4/N2 selectivity of 3.96, which are 505% and 230% higher than that of the pure PEI/MPSf composite membrane, respectively. These results indicate the great potential of constructing low-crystallinity metal-organic frameworks with increased open metal sites to develop high-performance membranes for CH4/N2 separation.