Enhancing acid-gas separations using free volume manipulation for microporous poly(arylene ether)s

To address global energy needs, traditional and renewable natural gas will likely be key energy sources for years to come. However, raw feeds require removal of impurities like hydrogen sulfide (H₂S) and carbon dioxide (CO₂) before use. In this study, we illustrate the key challenges of using traditional post-synthetic modification approaches to simultaneously enhance H₂S/CH₄ and CO₂/CH₄ selectivities in microporous polymer membranes, while also demonstrating how free volume manipulation (FVM) can overcome some of these challenges. By integrating tert-butoxycarbonyl-protected piperazinyl (PIP-tBOC) groups into a microporous poly(arylene ether) (PAE-1) and applying thermal treatment with oxygen to degrade the incorporated units in solid-state films, we successfully increased sorption capacity and diffusion selectivity. This modification enhanced the mixed-gas selectivity of H₂S/CH₄ and CO₂/CH₄ by 88% and 114%, respectively, compared to the original PAE-1 films. Consequently, the films achieved a combined acid gas (CAG) selectivity of 48, which approached the CAG upper bound for glassy polymers. The FVM process not only improved the selectivity of these membrane films but also markedly increased their resistance to plasticization, making them more suitable for industrial applications in acid-gas separation. This post-synthetic modification strategy, applicable to any glassy polymer containing a nucleophilic aromatic unit, provides a means to leverage the competitive sorption of H₂S molecules and the molecular sieving properties of the polymer.