Polyimide/ZIF-7 mixed-matrix membranes: understanding the in situ confined formation of the ZIF-7 phases inside a polymer and their effects on gas separations

Abstract

Polymer-modification-enabled in situ metal–organic framework formation (PMMOF) is potentially a paradigm-shifting preparation method for polymer/MOF mixed-matrix membranes (MMMs). However, the actual reaction conditions of the in situ formation of MOFs in a confined polymer free volume are expected to be quite different from that in a bulk solution. ZIF-7 is an interesting filler material not only due to its use in selective light gas separations but also for its three different crystal phases. Herein, we carried out systematic investigations on the in situ confined formation of ZIF-7 phases inside a polymer (6FDA-DAM) by PMMOF. The reaction conditions of ZIF-7 in the polymer free volume were deduced based on a bulk-phase ZIF-7 phase diagram constructed by varying the ZIF-7 precursor concentrations and ratios. Based on the understanding of the reaction conditions, the ZIF-7 crystal phases formed inside the polymer during the PMMOF process were controlled, yielding 6FDA-DAM/ZIF-7 MMMs with three different crystal phases. The ZIF-7 phases had significant effects on the gas transport of MMMs with layered ZIF-7-III fillers exhibiting the highest performance enhancement for H₂/CO₂ separation (i.e., H₂ permeability of ∼1630 Barrer and H₂/CO₂ selectivity of ∼3.8) among other phases. Furthermore, the MMMs obtained by the PMMOF process showed enhanced H₂/CO₂ separation performance, surpassing the upper bound.