The performance of affordable and stable cellulose-based poly-ionic membranes in CO2/N2 and CO2/CH4 gas separation

Abstract

The majority of commercial membrane units for large-scale natural gas sweetening are based on cellulose acetate (CA). However, the low selectivity and risk for and plasticisation affect adversely the performance of CA-based systems. Herein, we present a new class of CA-derived poly(ionic liquid) (PIL) as a thin film composite (TFC) membrane for CO2 separations. CA is modified with pyrrolidinium cations through alkylation of butyl chloride, substituting the hydroxyl group in the polymer backbone, and further anion exchange to bis(trifluoromethylsulfonyl)imide, P[CA][Tf2N]. The synthesised PIL material properties are extensively studied. The CO2 separation performance of the newly synthesised materials is evaluated by gravimetric gas sorption experiments, single gas time-lag experiments on thick membranes, and mixed-gas separation experiments on TFC membranes. The results are compared to the parent material (CA) as well as a reference PIL (poly(diallyldimethyl ammonium) bis(trifluoromethylsulfonyl)imide (P[DADMA][Tf2N])). The ideal CO2/N2 sorption selectivity of P[CA][Tf2N] is constant up to 10 bar. The single gas transport measurements in P[CA][Tf2N] reveal improved ideal CO2 selectivity for the CO2/N2 gas pair and increased CO2 permeability for the CO2/CH4 gas pair compared to the reference PIL. Mixed-gas permeation tests demonstrated that P[CA][Tf2N]-based membranes with a 5 µm thick selective layer has a two-fold higher CO2 flux compared to conventional CA. These results present CA modification into PILs as a successful approach promoting the higher permeate flows and improved process stability in a wide range of concentrations and pressures of CO2/N2 and CO2/CH4 gas mixtures.