Enhanced High-Pressure Mixed-Gas Sieving Properties of Thermally Cross-Linked Polyimide Membranes

Polymeric membranes are susceptible to aging and plasticization, which alters their separation properties since polymeric chain mobility within the membrane matrix is greatly affected. Hence, the implementation of these materials in industrial applications has been limited. However, cross-linking of polymeric membranes is a robust methodology to enhance their physical and mechanical properties. In this regard, thermal cross-linking is advantageous over chemical cross-linking since it could be applied directly to membranes without the addition of exogenous reagents. In this paper, we report the synthesis of a CARDO(OH)-containing copolyimide, amenable to thermal cross-linking due to the hydroxyl groups within the CARDO(OH) monomer. Membranes prepared from the 6FDA-Durene/CARDO(OH) (3:1) copolyimide underwent thermal cross-linking at 200 °C for varying durations (up to 96 h), and their pure- and mixed-gas separation performances were assessed. The mixed-gas studies were conducted under aggressive conditions of high feed pressures (20.7–62.0 bar) and operational temperatures (25–55 °C), aimed to closely simulate real-world scenarios for natural gas purification. Notably, the thermally cross-linked 6FDA-Durene/CARDO(OH) (3:1) for 96 h exhibited a mixed-gas CO2 permeability coefficient of 219 Barrer and a CO2/CH4 selectivity of 26.1 at 34.5 bar and 45 °C. Moreover, the membrane did not show any sign of plasticization under elevated feed pressures of up to 62.0 bar. This work demonstrates the effectiveness of thermal cross-linking in developing high-performance polymeric membranes under realistic conditions of pressure and temperature. Upcoming research endeavors will concentrate on creating and evaluating hollow fiber modules, aiming to replicate similar morphologies employed in industrial applications.