Highly Propylene-Selective Mixed-Matrix Membranes by in Situ Metal–Organic Framework Formation Using a Polymer-Modification Strategy

Despite the potential of C₃H₆/C₃H₈ separation, there have been no industrial applications of zeolitic-imidazole framework-8 (ZIF-8) mixed-matrix membranes (MMMs) because of the moderate separation performances and several challenging processing issues. Herein, we present a new paradigm of MMM fabrication, named polymer-modification-enabled in situ metal-organic framework formation (PMMOF), enabling in situ formation of ZIF-8 fillers inside the 4,4-(hexafluoroisopropylidene)diphthalic anhydride 2,4,6-trimethyl-1,3-phenylenediamine polymer. PMMOF consists of four steps including hydrolysis of a polymer, ion-exchange, ligand treatment, and imidization. Each step was thoroughly analyzed and important processing parameters were identified, enabling the structural control of MMMs by PMMOF. The binary C₃H₆/C₃H₈ separation performance of the MMMs showed much higher separation factors than conventionally prepared MMMs at similar filler loadings, satisfying the commercial C₃H₆/C₃H₈ separation performance criteria. PMMOF was successfully applied for other MOFs, demonstrating that the process could be general. Finally, as a proof of concept, asymmetric mixed-matrix hollow fiber membranes (i.d. of 0.45 mm and o.d. of 0.63 mm) with ultrathin selective skin layers were prepared by PMMOF, showing C₃H₆ permeance of 2.17 GPU and C₃H₆/C₃H₈ separation factor of ∼20.