Poly(ionic liquid) composite membranes bearing different anions as biocatalytic membranes for CO2 capture

The enzyme carbonic anhydrase (CA) has gainned considerable attention from the literature and the industry in the context of CO₂ capture. CA immobilization in gas-liquid membrane contactors, and more specifically, on poly(ionic liquid) (PIL) composite membranes has been demonstrated to be a potential strategy to facilitate its industrial implementation. These membranes were comprised of a PIL layer coating on a porous hydrophobic polymeric support. In this work, the composition of the PIL layer was tuned by anion exchange to yield a variety of enzyme carriers. The following anions were compared: bromide [Br], acetate [Ac], tetrafluoroborate [BF₄], and bis(trifluoromethylsulfonyl)imide [NTf₂]. The surface morphology, chemistry, and properties of these composite membranes were characterized by SEM, EDX, ATR-FTIR, and water contact angle. The activity of the different biocatalytic composite membranes was determined by the p-nitrophenyl acetate hydrolysis model reaction. It was found that the anion exchange salts had a detrimental effect on the immobilized enzyme activity. In light of these results, the enzyme immobilization step was conducted after anion exchange. The resulting biocatalytic membranes displayed slight differences in immobilized enzyme activities and thermal stabilities following the order [Br]>[BF₄]>[Ac]>[NTf₂] and [BF₄]>[Br]≈[Ac]>[NTf₂], respectively. The differences were more pronounced and detrimental for the most hydrophobic anion, [NTf₂]. Parallel trends were noted when the membranes were tested for CO₂ absorption in a gas-liquid membrane contactor set-up suggesting that the CO₂ mass transfer is strongly influenced by the activity of the immobilized enzymes. In addition, the effect of the absorption conditions, i.e., solvent flow rate, solvent saturation, and solvent concentration were evaluated. Under the best conditions, the novel biocatalytic membranes outperformed the commercial PVDF support by about a factor of 4 in terms of overall mass transfer coefficient. Such improvement would result in significant reductions in the required membrane area to capture CO₂ by a gas-liquid membrane contactor.