Microporosity effect of intrinsic microporous polyimide membranes on their helium enrichment performance after direct fluorination

The efficient enrichment of Helium (He) from low concentrated natural gas seems the only way to balance the He shortage. However, to reinforce this process, one of the biggest challenges is the membrane with both high He permeability and He/CH4 selectivity. Here, we applied a direct fluorination technique to in-situ modify two intrinsic microporous polyimides (SPI and TPI) with different surface areas. The fluorine reacted with the polyimide backbone at the membrane skin and inner surface that were proved by FT-IR, XPS and EDS analysis. The resulting fluorinated SPI (FSPI) and TPI (FTPI) manifested significantly enhanced He/CH4 separation properties by huge improved He/CH4 diffusion selectivity. The overall performance of He/CH4, He/CO2, H2/CO2 and H2/CH4 were much better than their precursor membranes. In which, the FSPI-60 demonstrated a state-of-the-art He permeability of 590 Barrer combined with He/CH4 selectivity of 634. This figure-of-merit outperformed most of polymer membranes and beat the latest perfluorinated trade-off line. Notably, the FSPI-60 with higher BET surface area precursor demonstrated simultaneously larger He permeability and He/CH4 selectivity than the corresponding FTPI-60 from low surface area precursor. The higher permeability of FSPI originated from its larger concentration of micropores that provides more pathways for He transport. The higher selectivity of FSPI-60 derived from its much larger He/CH4 diffusion selectivity, that is, 6375 vs 2334. Additionally, this FSPI-60 membrane also showed plasticization resistant and excellent He/CH4 (1/99) mixed-gas separation performance, which provides great perspective in He recovery from natural gas applications.