Use of green polar aprotic solvents TamiSolve® NxG, DMSO and methyl-THF for the synthesis of asymmetric polyimide-based biogas purification membranes

Abstract

Valorization of biogas to biomethane through membrane-based purification of its multi-component stream primarily consisting of CH4 and CO2, holds significant potential as a crucial renewable energy source. Industrial membrane units classically comprise of membranes synthesized using non-solvent induced phase separation (NIPS), with polymer processing typically performed using conventional solvents, such as N-N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), etc. However, use and subsequent waste handling of these solvents present significant hazards, due to their toxicological nature, therefore calling for a shift to employ safer and ‘greener’ materials to facilitate sustainable production of these membranes. In current work, a novel and sustainable methodology for synthesis of CO2/CH4 gas separation flat-sheet asymmetric membranes was developed with commercially available Matrimid® 5218 polyimide, using TamiSolve® NxG, dimethylsulfoxide (DMSO) and 2-methyltetrahydrofuran (MeTHF) as green solvent medium. NIPS thermodynamic and kinetic aspects of the novel system were fundamentally analyzed through determination of ternary phase diagrams and viscosity profiles. Membranes were systematically tested for gas separation performance by varying Tamisolve-DMSO-MeTHF solvent ratios. Mixed gas CO2/CH4 separation factors in the range of 13 to 41, along with CO2 permeances upto 186 GPU were achieved. Furthermore, chemical crosslinking was applied in view of plasticization resistance of the membranes. Matrimid® membranes crosslinked with hyperbranched polyethylenimine (PEI) successfully displayed stability against CO2-induced plasticization upto CO2 feed partial pressures of 22 bar. The developed membrane synthesis method provides a sustainable framework for synthesizing polyimide-based membranes with tunable gas separation performances.