Fluorine-containing polyimide/polysilsesquioxane carbon molecular sieve membranes and techno-economic evaluation thereof for C₃H₆/C₃H₈ separation

Herein, we investigated the effect of thermo-oxidative crosslinking of siloxanes on the C₃H₆/C₃H₈ separation performance of carbon molecular sieve (CMS) membranes derived from polymer blends of a fluorine-containing polyimide and a ladder-structured polysilsesquioxane (PI/LPSQ). The PI/LPSQ precursors self-generated fluorinated gases, which possibly lead to the cleavage of double-stranded siloxanes during pyrolysis. At the same time, residual siloxanes underwent thermo-oxidative crosslinking, which resulted in densification into nonporous inorganic SiO₂ phases. Such impermeable SiO₂ phases adversely affected the gas diffusion, decreasing C₃H₆ permeability but contributed to the significant enhancement in the C₃H₆/C₃H₈ selectivity up to as much as 67, because of the substantially enhanced diffusivity selectivity. Density functional theory-based pore size distribution analysis exhibited that the narrower pores in the range of 5.0–5.5 Å emerged for the PI/LPSQ (80/20 w/w) CMS, supporting the enhancement in C₃H₆/C₃H₈ selectivity. Also, PI/LPSQ (80/20 w/w) CMS fibers aged for 30 days showed C₃H₆ permeance of 2.9 GPU and a C₃H₆/C₃H₈ selectivity of 57. Furthermore, the techno-economic analysis verified the economic feasibility of the proposed membrane for the C₃H₆/C₃H₈ separation process. It reflected that higher C₃H₆ permeability plays a significant role in reducing the total cost of C₃H₆/C₃H₈ separation process as long as the C₃H₆/C₃H₈ selectivity is above 30, implying the significance of anti-aging in CMS hollow fiber membranes.