Bimetallic metal-organic frameworks nanocages as multi-functional fillers for water-selective membranes

Although metal-organic frameworks (MOFs) with well-defined regular and porous structure have emerged as a family of nanoporous building blocks, the great potential of MOFs as multi-functional fillers for hybrid membranes deserves in-depth exploitation. In this study, bimetallic MOF nanocages, Fe III -HMOF-5, were prepared and incorporated into sodium alginate (SA) matrix to fabricate water-selective nanohybrid membranes. Introduction of Fe III ions into MOF-5 creates more coordinatively unsaturated sites, which leads to preferential dissolution of water molecules over ethanol molecules, thus elevating the solubility selectivity. The hollow structure of Fe III -HMOF-5 ensures the free diffusion of water molecules. The synergistic regulation of chemical composition and physical structure of MOFs endows the hybrid membranes with remarkably elevated separation factor and permeation flux. Taking dehydration of 90   wt% ethanol aqueous solution as model system, the hybrid membrane containing Fe III -HMOF-5 exhibits the highest separation performance with separation factor of 3423 and permeation flux of 1540   g/m 2   h, much higher than those for pure SA membrane and the hybrid membranes incorporating hollow MOF-5 (HMOF-5) and MOF-5. Moreover, the hybrid membranes containing Fe III -HMOF-5 display remarkably superior physicochemical stabilities and long-term operation stability. This study demonstrates a promising prospect of heterometallic hollow MOFs as multi-functional fillers in high-performance hybrid membranes.