Enhancing Mixed-Matrix Membrane Performance with Metal–Organic Framework Additives

Metal–organic frameworks (MOFs), as porous fillers possessing molecular sieving properties, have been combined with polymers to give mixed-matrix membranes (MMMs) with enhanced separation performance. This field of research has produced a large number of different membranes, and many MOF/polymer combinations have been tested and reported to show potential application to industrial gas separation. Although MOFs have been proposed as novel additives with high porosity and tunable pore size, which were supposed to outperform other porous fillers, due to restrictions in separation performance of the filler and challenges concerning the compatibility of polymer and MOF, only a small fraction of these works report both improved permeability and selectivity. In this review these challenges are set into the context of MOF synthesis and membrane fabrication by the choice of appropriate polymer/MOF combinations, utilization of the MOF functional sites, modification of the MOF surface chemistry or pore texture and size, and also targeted influence of the size and shape of the filler particles. The effect of the highlighted MOF additives on the gas separation performance is analyzed and discussed by comparison of the gas permeability and selectivity. This emphasizes strategies by which high performing MMMs can be achieved through accessing the full potential of the porous MOF fillers.