Pre-assembly and post-annealing of ZIF-7-NH2-based mixed matrix membranes for improved interfacial compatibility

Abstract

Zeolitic imidazolate frameworks (ZIFs) have been considered the ideal fillers in the mixed matrix membranes (MMMs) for sieving hydrogen (H₂), owing to their ordered microporous structures characterized by pore dimensions approximating H₂ dynamic diameter. Nevertheless, enhancing the interfacial compatibility between ZIFs and polymers within MMMs has been an ongoing area of investigation. This study introduces a dual optimization strategy to improve interfacial compatibility via the pre-assembled fillers and post-annealing treatment. On the one hand, the electrostatic pre-assembly promotes the formation of hydrogen bonds between the –COOH groups of carboxylated polymers of intrinsic microporosity (PIM-COOH) and the –NH₂ groups of ZIF-7-NH₂ fillers, while simultaneously establishing π-π interactions with PIM-1, thereby serving as a molecular bridge. On the other hand, the post-annealing process emerges as a crucial step in the reorganization of polymer chains to reduce the non-selective voids by π-π interaction. The optimized MMMs, with a 30 wt% ratio of ZIF-7-NH₂@PIM-COOH (ZIFPC) fillers and subjected to a 2-h annealing treatment at 300 °C, demonstrate H₂ permeability of 1840 ± 123 barrer and H₂/CH₄ selectivity of 103.9 ± 5.43. The H₂ permeability and H₂/CH₄ selectivity performances are 174 % and 981 % higher than those of pristine PIM-1 membranes, surpassing the 2015 upper bound.