Organic-inorganic double networks as highly permeable separation membranes with a chiral selector for organic solvents

Abstract

A novel design of separation membranes was realized by formation of an inorganic framework (IF) within an organic polymer membrane. This design greatly improves the stiffness and the strength of the membrane. Furthermore, the inorganic scaffold affords higher solute permeability. The new design is demonstrated on the example of a diffusion based separation membrane that consists of copolymerized and cross-linked acrylamide and (R/S)–N-(1-hydroxy-butan-2-yl) acrylamide (PAAm-co-(R/S)-HBA-l-MBAm). The IF was implemented by enzyme-induced mineralization (EIM) of calcium phosphate using alkaline phosphatase. Thereby, the stiffness and strength of the organic membrane could be improved from <1 to < 0.2 MPa to 150 and 1.5 MPa, respectively, upon EIM. The composite material is suitable as a highly permeable membrane in swelling solvents, such as methanol, as well as in non-swelling solvents, such as toluene. It was demonstrated that the solutes naproxen, BINOL, and pseudoephedrine show varying solute permeability in a range of 1.3–35.9 x 10−11 m2 s−1, which indicates separation potential for small molecules. It could also be shown on the example of (R)- and (S)-naproxen that the membranes have potential for chiral separation. The high solute permeabilities in combination with solute selectivity and even enantioselectivity and excellent mechanical properties make these membranes an alternative to commonly reported separation membranes.