High-Temperature Gas Separation in Thermally Rearranged Polybenzoxazoles Achieved through Bulky CARDO Groups

The deployment of polymeric membranes for natural gas purification is constrained by the intrinsic trade-off between permeability and selectivity, as well as by performance degradation under harsh industrial conditions. Although thermally rearranged polybenzoxazoles (TR-PBOs) can surpass the permeability-selectivity trade-off, their performance stability in high-pressure, high-temperature gas streams remains a major challenge. In this work, we report the molecular design of TR-PBO membranes derived from ortho -functionalized (APAF) 6FDA-based polyimide precursors incorporating a bulky CARDO( t -Bu) monomer. This strategic design creates a rigid, contorted free volume architecture that is inherently stable, enhancing molecular sieving while mitigating plasticization. A series of 6FDA-APAF/CARDO( t -Bu) ( x: y ) random copolyimides were synthesized and thermally converted into TR-PBO membranes. Their gas separation performance was rigorously evaluated with mixed-gas feeds across a range of industrially relevant conditions, including elevated temperatures (up to 55 °C) and high pressures (up to 700 psi). The optimized membrane exhibited exceptional stability, maintaining a CO 2 permeability of 192 Barrer with a CO 2 /CH 4 selectivity of 19 at 55 °C and 500 psi, without the severe plasticization typical of conventional polymers. This combination of high performance, thermal robustness, and intrinsic processability establishes these molecularly engineered TR-PBOs as promising candidates for next-generation, energy-efficient gas separation membranes.