Ultrahigh He enrichment property of carbon molecular sieve membranes by direct fluorination

One major challenge of helium recovery from natural gas by membranes is the development of materials offers both high He permeability and He/CH 4 selectivity. Here, ultrahigh performance He separation membranes were achieved by directly fluorination of a carbon molecular sieve (CMS) membrane that derived from pyrolyzing a polyimide at 550 °C. The FCMS550-60, fluorinated for 60 s, showed an unprecedented combination of high He permeability of 3432 Barrer and ultrahigh He/CH 4 selectivity of 5329 that is located far beyond any He/CH 4 trade-off lines. Additionally, it also exhibited outstanding mixed-gas separation properties with a He permeability of 1765 Barrer and He/CH 4 selectivity of 4114 at 20 bar. The largely enhanced He/CH 4 selectivity resulted exclusively from a boost in diffusion selectivity, which is originated from the formation of C–F bonds inside the micropores of the CMS membrane that remarkably enhanced the sieving ability of the FCMS membrane. This work provides a broad prospect for the application of FCMS membranes for He recovery from natural gas and other resources.