Highly Selective Transport of Alkali Metal Ions by Nano-channels of Polyelectrolyte Threaded MIL-53 Metal Organic Framework.

Abstract

Conventional ion-exchange polymeric membranes have limited selectivity due to their non-uniform and unstable structures. The rigid, regular, high porosity of metal organic framework (MOF) generally provides MOF membrane with ion exclusion/sieving effect but lack of electrostatic screening. Here we report for the first time a non-biological highly selective MOF membrane with particles of polyelectrolyte threaded in the nanochannel of metal organic framework (polyelectrolyte~MOF) and its selective transport of alkali metal cations. Poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al) is prepared on anodic aluminium oxide substrate via steps of MOF MIL-53(Al) growth followed by in-situ polymerization. The poly(VS-co-AA)~MIL-53(Al) membrane demonstrates highly specific selectivity in transport of alkali metal cations. Rate of ion transport correlates inversely with the hydrated diameter of the ion reaching a low limiting rate near 0.7 nm hydrated diameter. Charge exclusion is demonstrated with blockage of anion transport under a concentration gradient. The highly uniform porous nanostructure of MOF and ionic function of polyelectrolyte offers the MOF membrane with synergistic selectivity based on exclusion forces of the framework and Coulomb forces from fixed charges of polyelectrolytes in nanochannels.