Design of Mixed-Matrix MOF Membranes with Asymmetric Filler Density and Intrinsic MOF/Polymer Compatibility for Enhanced Molecular Sieving.

The separation of high-value-added chemicals from organic solvents is important for many industries. Membrane-based nanofiltration offers a more energy-efficient separation than the conventional thermal processes. Conceivably, mixed-matrix membranes (MMMs), encompassing metal-organic frameworks (MOFs) as fillers, are poised to promote selective separation via molecular sieving, synergistically combining polymers flexibility and fine-tuned porosity of MOFs. Nevertheless, conventional direct mixing of MOFs with polymer solutions results in underutilization of the MOF fillers owing to their uniform cross-sectional distribution. Therefore, in this work, a multizoning technique is proposed to produce MMMs with an asymmetric-filler density, in which the MOF fillers are distributed only on the surface of the membrane, and a seamless interface at the nanoscale. The design strategy demonstrates five times higher MOF surface coverage, which results in a solvent permeance five times higher than that of conventional MMMs while maintaining high selectivity. Practically, MOFs are paired with polymers of similar chemical nature to enhance their adhesion without the need for surface modification. The approach offers permanently accessible MOF porosity, which translates to effective molecular sieving, as exemplified by the polybenzimidazole and Zr-BI-fcu-MOF system. The findings pave the way for the development of composite materials with a seamless interface.