Simultaneously enhanced gas perm-selectivity of carbon molecular sieve membranes via silicon-based molecular cross-linking for efficient gas separation

Abstract

Carbon molecular sieve membranes (CMSMs) face a fundamental permeability-selectivity trade-off, as defined by the Robeson upper bound. Here, we engineer CMSMs via copolymerization of hydroxy-functionalized polyimide (6FDA-6FAP) with polydimethylsiloxane (PDMS). Incorporating flexible siloxane chains promotes denser chain aggregation and provides additional gas transport channels. During pyrolysis, the decomposition of siloxane segments generates additional micropores, thereby enhancing gas permeability. Simultaneously, PDMS-induced silicon inter-layer crosslinking promotes tighter packing of carbon strands, which reduces inter-layer spacing and consequently improves gas selectivity. Systematic comparison of hydroxyl (-OH) and amine (-NH₂) functional groups reveals that –OH groups in PDMS form extensive hydrogen-bonding networks with 6FAP, inducing crosslinking that enhances precursor chain packing. The optimized PCMS-OH membrane achieves H₂ and CO₂ permeability of 10,568 and 1,768 Barrer, with H₂/CH₄ and CO₂/CH₄ selectivity of 320 and 53, respectively, which far exceed the latest Robeson upper bound. Moreover, silicon-induced crosslinking reduces aging kinetics, retaining 57 % higher H₂ permeability after 60 days. This work demonstrates a precursor engineering strategy to break the trade-off barrier for sustainable hydrogen purification and carbon dioxide capture.