Cross-linked amorphous mixed matrix membranes for selective separations of CO2/CH4 mixed-gas feeds.

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 NH 2 -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.

Conventional CO2 separation in the petrochemical industry via cryogenic distillation or aminebased absorber-stripper units is energy intensive and environmentally unfriendly. Membranebased gas separation technology, on the other hand, has contributed significantly to the development of energy-efficient systems for e.g. natural gas purification. The implementation of commercial polymeric membranes in gas separation processes is restricted by their permeabilityselectivity trade-off and by their insufficient thermal and chemical stability. Herein, we present the fabrication of Matrimid ® -based membrane loaded with a breathing metal-organic framework 2 (MOF) (NH2-MIL-53(Al)) which is capable of separating binary CO2/CH4 gas mixtures with high selectivities without sacrificing much of its CO2 permeabilities. NH2-MIL-53(Al) crystals were embedded in a polyimide (PI) matrix and the mixed-matrix membranes (MMMs) were treated at elevated temperatures (up to 350 °C) in air to trigger PI crosslinking and to create PI-MOF bonds at the interface to effectively seal the grain boundary. Most importantly, the MOF transitions from its narrow-pore form to the large-pore form during this treatment, allows the PI chains to partly penetrate the pores and crosslink with the amino functions at the pore mouth of the NH2-MIL-53(Al) and stabilizes the open-pore form of NH2-MIL-53(Al). This crosslinked MMM, with MOF pore entrances made more selective by the anchored PI-chains, achieves outstanding CO2/CH4 selectivities. This approach provides significant advancement towards the design of selective MMMs with enhanced thermal and chemical stabilities which could also be applicable for other potential applications, such as separation of hydrocarbons (olefin/paraffin or isomers), pervaporation and solvent resistant nanofiltration.