Matching Analysis of Mixed Matrix Membranes for Organic Solvent Reverse Osmosis

Abstract

Existing polymeric membranes struggle to separate small molecule solvents in the liquid phase due to low selectivity from solvent-induced plasticization and dilation. Mixed matrix membranes (MMMs) can potentially alleviate this issue via diffusion-based separations within rigid framework materials. Previous work from our lab and others has shown that organic solvent reverse osmosis membranes have different responses to transmembrane pressure depending on whether the material is a rigid structure (e.g., a carbon, zeolite, or metal-organic framework) or a swollen polymer. This work combines two Maxwell−Stefan transport models, representing the flexible polymer phase and a rigid microporous filler, with the Maxwell model to predict mixed matrix membrane solvent separation performance as a function of pressure and membrane material properties. The model demonstrates that for every filler perm-selectivity, there is a filler permeability that provides the largest separation factor in the final MMM. This optimum permeability increases with the filler's perm-selectivity. Dual-layer UiO-66/Matrimid hollow fiber MMMs were created to evaluate the model's prediction on the influence of transmembrane pressure on the separation of toluene and mesitylene as a test case. The UiO-66/Matrimid membrane demonstrated a predicted decline in permeance as pressure was increased. The separation factors increased as higher pressures increased the driving force for separation, consistent with the model. UiO-66 was shown to have superior selectivity to Matrimid in toluene/mesitylene; however, we conclude that ultraselective materials are ultimately needed to enable the mixed matrix membrane concept for the most challenging solvent− solvent separations, and open questions remain about polymer−filler pairings for organic solvent reverse osmosis.