Ultra-selective ferric ion-complexed membranes composed of water-based zwitterionic comb copolymers

Water-based, non-toxic, eco-friendly polymerization is carried out to synthesize a zwitterionic comb copolymer, i.e., poly(oxyethylene methacrylate)-co-sulfobetaine methacrylate (POEM-PSBMA). POEM-PSBMA undergoes a specific interaction with ferric (Fe³⁺) ions to form a three-dimensional, interconnected transporting channel for CO₂. Without any post-treatment, the as-synthesized POEM-PSBMA solution complexed with ferric ions is directly coated onto a microporous polysulfone support to form thin-film composite membranes. The chain morphology and structure are determined by employing Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), small angle X-ray scattering (SAXS), and energy-filtered transmission electron microscopy (EF-TEM). Microphase-separated structures are observed for all membranes, and the structures become smaller with increasing ferric ion loadings. This results from the selective interaction between ferric ions and POEM chains in the copolymer, leading to an increased number of CO₂-philic sites. The coordination of the ferric ions reduces the free volume in the polymer matrix and hinders the diffusion of non-polar gases such as CH₄ and N₂. As a result, the CO₂/N₂ ideal selectivity and CO₂/CH₄ mixture selectivity are dramatically enhanced from 4.01 and 8.26 to 423 and 695, respectively, which represent the highest performances reported so far, to our knowledge.