Mixed matrix membranes prepared from non-dried MOFs for CO2/CH4 separations

Abstract

Mixed matrix membranes (MMMs) aim at combining the processibility of polymers with the molecular sieving of fillers to improve gas separation performance. Metal–organic frameworks (MOFs) are a new family of materials with promising potential as fillers. The first part of this work reports on exploiting the versatility of MOF synthesis routes by forming ZIF-8 particles in polymer solutions to subsequently cast membranes directly from the solution. Although MOFs can be synthesised in a polymer medium, the decline in the synthesis yield does not allow for high loading in the MMMs. The second part describes a method for preparing MMMs with the commercial polyimide (PI) Matrimid® and ZIF-8, ZIF-7 and NH₂-MIL-53(Al) as non-dried filler with 30 wt% and 50 wt% loading. A comparison of this method with the conventional approach of drying MOFs prior to incorporation exhibits the flexibility MOFs provide in membrane synthesis, in contrast to e.g. zeolites which intrinsically have to be calcined to become useful. The membranes with non-dried MOFs show some improvement in performance as compared to the unfilled polymer-only membranes, while those with dried MOFs even lose the inherent selectivity of the polymer.