Strong Interplay between Polymer Surface Charge and MOF Cage Chemistry in Mixed-Matrix Membrane for Water Treatment Applications

Despite the large number of reports on the utilization of highly microporous solids, most relevant are metal-organic frameworks (MOFs), in different demanding applications, the successful hybridization of MOFs and moldable polymer matrices into flexible, water-permeable membranes exhibiting strong entanglement of the MOF and the polymer matrix properties is still lacking. We describe herein an efficient pathway to construct a mixed-matrix membrane (MMM) comprising a water-stable metal-organic framework (UiO-66-NH2), as the active sorbent, and cellulose acetate (CA), as the polymer matrix, to construct a flexible membrane for water treatment applications. The MOF@CA MMM demonstrated superior performance in terms of exceptional removal of organic dyes (both cationic and anionic species) as well as hexavalent Cr ions, compared to the control CA membrane. The recorded high uptake of the MOF@CA MMM for this wide array of contaminants demonstrated the accessibility of the MOF nanocages immobilized within the MMM, in contrast to the common perception that the polymer matrix might act as a physical barrier to block the accessibility of the MOF cages. The negative surface charge of the matrix exerted a notable action to affect the diffusion of the negatively charged contaminants to reach the active sorbent filler. Moreover, the formed membrane demonstrated high durability and recyclability with no detected loss of performance over numerous cycles. This approach outlines the ability to formulate one of the most water-stable MOFs, as exceptional microporous sorbent, into a usable membrane form compatible with real-life applications.