Sub-Tg promoted solid-state interfacial linker exchange in ZIF-based hybrid membranes towards enhanced gas-sieving performance

Abstract

Minimizing non-selective defects in mixed matrix membranes (MMMs) is a highly desirable, yet challenging task. A generalizable strategy of sub-glass transition temperature (Tg) promoted solid-state linker exchange is presented here, which effectively seals non-selective voids and transforms insignificant defective MMMs into highly selective ones. The progression of solid-state linker exchange is initiated by the methyl groups in a thermal-oxidative environment at sub-Tg temperatures, creating the missing linker defects in the embedded ZIF-8 fillers. The essential ligand groups of benzimidazole of the polymer matrix gain the chain mobility at sub-Tg and concurrently bind to defective ZIF-8 crystals to accomplish the linker exchange, which is accompanied with the methyl group-involved covalent crosslinking in the polymer and ZIF-8, eventually yielding an ultra-selective MMM. The ideal selectivities of H2/CH4 and CO2/CH4 can be tuned up to 584 and 145, respectively, which are two to ten times greater than conventional MMMs. The linker exchange process is validated to be the most impactful factor for gas selectivity enhancement. A representative membrane modified by this strategy has an extremely high mixed-gas CO2/CH4 selectivity of 219 at −25 °C and CO2 permeability of 61 Barrer, exceeding the 2019 Robeson upper bound. The solvent-free sub-Tg promoted linker exchange strategy is employed on benzimidazole containing polyimides/ZIF-7 to demonstrate its versatility and could be extended to a wide range of polymer/MOF hybrids to manipulate interfacial structure.