Carbon Molecular Sieve Membranes from Poly(oxo-biphenylene-isatin) with Increasingly Bulky Fluorine Substitution: Characterization and Gas Transport Properties

Abstract

Four high-rigidity poly(oxo-biphenylene-isatin) polymers (POBIs)─poly(2-oxo-3-biphenyleneindole) (POBI-H), poly(2-oxo-3-biphenylene-1-(4-fluorophenyl)indole) (POBI-FPh), poly(2-oxo-3-biphenylene-1-(3-trifluoromethylphenyl)indole) (POBI-TFMPh), and poly(2-oxo-3-biphenylene-1-(3,5-bis(trifluoromethyl)phenyl)indole) (POBI-bTFMPh)─bearing a systematic increase in bulky lateral phenyl-fluorine groups were synthesized by a superacid catalyzed reaction. Thin-film membranes from POBIs were prepared by solution casting and pyrolyzed under an inert atmosphere (Ar) to obtain carbon molecular sieve membranes (CMSMs) at 600 °C. During the pyrolysis processes, −CF₃ decomposition produces an amorphous carbon structure with more open strands, as found from an increase between carbon strand distances, L_a and L_c, and pore volume. A comparison of POBI precursor films and CMSM gas transport properties shows that CMSM presents a 10-fold increase in permeability over POBI precursors. A high gas permeability was observed, ascribed to an improved gas sorption capability in the CMSM due to an increase in condensability and surface area between POBIs as the bulkiness of phenyl-fluorine substitution increases. Aging of CMSM POBI membranes shows a decrease in permeability compared to the initial CMSM, which is larger for pyrolyzed POBI-bTFMPh, due to the rearrangement of the amorphous turbostratic structure and a considerable increase in ideal selectivity associated with the formation of compact micropores by the rearrangements of the carbon strand with an increase in ultramicropores. The overall effect of the systematic increase in bulkiness of phenyl-fluorene substitution in the POBI precursor to obtain CMSM is an increase in P due to a higher gas sorption S resulting from an increase in the pore volume and surface area and a larger L_c and L_a among the carbonaceous strands.