Polyoctahedral Silsesquioxane Hexachlorocyclotriphosphazene Membranes for Hot Gas Separation

There is a need for gas separation membranes that can perform at high temperatures, for example, for CO₂ capture in industrial processes. Polyphosphazenes classify as interesting materials for use under these conditions because of their high thermal stability, hybrid nature, and postfunctionalization options. In this work, thin-film composite cyclomatrix polyphosphazene membranes are prepared via the interfacial polymerization reaction between polyhedral oligomeric silsesquioxane and hexachlorocyclotriphosphazene on top of a ceramic support. The prepared polyphosphazene networks are highly crosslinked and show excellent thermal stability until 340 °C. Single gas permeation experiments at temperatures ranging from 50 to 250 °C reveal a molecular sieving behavior, with permselectivities as high as 130 for H₂/CH₄ at the low temperatures. The permselectivities of the membranes persist at the higher temperatures; at 250 °C H₂/N₂ (40), H₂/CH₄ (31) H₂/CO₂ (7), and CO₂/CH₄ (4), respectively, while maintaining permeances in the order of 10^-7 to 10^-8 mol m^-2 s^-1 Pa^-1. Compared to other types of polymer-based membranes, especially the H₂/N₂ and H₂/CH₄ selectivities are high, with similar permeances. Consequently, the hybrid polyphosphazene membranes have great potential for use in high-temperature gas separation applications.