A confined flexibility release enabled non-equilibrium stage in mixed-matrix membranes: unprecedented selectivity for olefin/paraffin deep separation

Abstract

Efficient separation of olefins/paraffins requires accurate regulation of the size of membrane channels, which is a great technical challenge. For flexible metal–organic frameworks (MOFs), there is a good opportunity for constructing desirable membrane channels, as long as the lattice flexibility can be well controlled. Here, we demonstrate a facile polymer-mediated confined flexibility release strategy for realizing sharp molecular sieving separation of propylene/propane with highly scalable mixed-matrix membranes (MMMs). In this approach, the gradual release of framework flexibility of ZIF-8 is achieved through the progressive plasticization of rigidified polymer chains chelated to zinc nodes. This results in the continuous expansion of the aperture of ZIF-8, thus the appearance of an unprecedented non-equilibrium stage with high propylene/propane and ethylene/ethane separation factors (over 200 and 20) that far exceed those of the existing MMMs and long duration of practical significance. Additionally, the non-equilibrium stage can be regenerated simply, rapidly and repeatedly, endowing the membrane with the potential of being applied in a parallel single-stage membrane system for continuous production of polymerization grade propylene. This strategy not only provides high performance membrane candidates for olefin/paraffin separation, but also inspires the exploitation of framework flexibility in molecular separation.