Preparation of Polysilsesquioxane-Based CO₂ Separation Membranes with Thermally Degradable Succinic Anhydride and Urea Units

New polysilsesquioxane (PSQ)-based CO2 separation membranes with succinic anhydride and monoalkylurea units as thermally degradable CO2-philic units were prepared by the copolymerization of a 1:1 mixture of [3-(triethoxysilyl)propyl]succinic anhydride (TESPS) or [3-(triethoxysilyl)propyl]urea (TESPU) and bis(triethoxysilyl)ethane (BTESE). The succinic anhydride and monoalkylurea units underwent thermal degradation to form ester and dialkylurea units, respectively, with the liberation of small molecules (e.g., CO2 and NH3) under N2 atmosphere. The effects of thermal degradation on the performance of the obtained membranes were investigated. The TESPS-BTESE- and TESPU-BTESE-based membranes calcined at 250 °C and 200 °C exhibited good CO2/N2 permselectivities of 20.2 and 14.4, respectively, with CO2 permeances of 7.7 × 10−8 and 7.9 × 10−8 mol m−2·s−1·Pa−1, respectively. When the membranes were further calcined at elevated temperatures of 350 °C and 300 °C, respectively, to promote the thermal degradation of the organic units, the CO2 permeances increased to 1.3 × 10−7 and 1.2 × 10−6 mol m−2·s−1·Pa−1 (3.9 × 102 and 3.6 × 103 GPU), although the CO2/N2 permselectivities decreased to 19.5 and 8.4, respectively. These data indicate that the controlled thermal degradation of the organic units provides a new methodology for possible tuning of the CO2 separation performance of PSQ membranes.