Polyimide Membrane Materials with Multiple Trifluoromethyl Groups for Helium Enrichment from Natural Gases

Membrane technology, especially the options using highly selective and permeable materials, is the preferred alternative to conduct direct helium enrichment from natural gas. In this work, the monomers containing multiple trifluoromethyl groups, including 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl) benzidine (TFMB), are used to synthesize specific polyimide materials. On the one hand, fluorine element can impair affinity to methane and then enhance the selective sorption for helium. On the other hand, trifluoromethyl groups have appropriate steric hindrance to increase accessible volume for helium permeation; moreover, this steric hindrance can make the chain segments more rigid to decrease temporary large-size cavities and then hinder methane permeation. The separation tests revealed that helium permeability is up to 73 Barrer, and its selectivity against methane is up to 247 in this synthesized material after aging and compacting sufficiently. On the whole, the 6FDA-TFMB polyimide materials should be highly promising for membrane-based helium enrichment.