Low temperature pyrolysis of thin film composite polyphosphazene membranes for hot gas separation

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Highly selective thin-film composite membranes for hot hydrogen sieving are prepared via the pyrolysis of thin cyclomatric polyphenoxy phosphazene films that are prepared via non-conventional interfacial polymerization of hexachlorocyclotriphosphazene with 1,3,5-trihydroxybenzene or m-dihydroxybenzene. The presence of the cyclic phosphazene ring within the weakly branched polymer films gives rise to a distinct thermal degradation evolution, with an onset temperature around 200 °C. For the trihydroxybenzene derived material, the hydrogen permselectivity of the films shows a maximum around a pyrolysis temperature of 450 °C. At this temperature a compact atomic structure is obtained that comprises mostly disordered carbon and accommodates P-O-C and P-O-P bonds. During thermal treatment, these films reveal molecular sieving with permselectivities exceeding 100 for H2/N2, H2/CH4, and H2/CO2, and a hydrogen permeance of 2×10–10 to 1.5×10–-8 mol m–2s–1Pa–1 (0.6-44.8GPU), at 200 °C. At ambient temperatures, thin films are very effective barriers for small gas molecules. Because of the inexpensive facile synthesis and low temperature pyrolysis, the polyphosphazene films have the potential for use in high-temperature industrial gas separations, as well as for use as barriers such as liners in high pressure hydrogen storage vessels at ambient temperature.