State of the art and prospects of chemically and thermally aggressive membrane gas separations: Insights from polymer science

Penetrant-induced plasticization in microporous polymer membranes

Penetrant-induced plasticization has prevented the industrial deployment of many polymers for membrane-based gas separations. With the advent of microporous polymers, new structural design features and unprecedented property sets are now accessible under controlled laboratory conditions, but property sets can often deteriorate due to plasticization. Therefore, a critical understanding of the origins of plasticization in microporous polymers and the development of strategies to mitigate this effect are needed to advance this area of research. Herein, an integrative discussion is provided on seminal plasticization theory and gas transport models, and these theories and models are compared to an exhaustive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas permeation tests and mixed-gas permeation tests for pure polymers and composite films. Finally, an evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided along with suggestions for future directions of fundamental and applied research on the topic.

A Microporous Poly(Arylene Ether) Platform for Membrane-Based Gas Separation

Membrane-based gas separations are crucial for an energy-efficient future. However, it is difficult to develop membrane materials that are high-performing, scalable, and processable. Microporous organic polymers (MOPs) combine benefits for gas sieving and solution processability. Herein, we report membrane performance for a new family of microporous poly(arylene ether)s (PAEs) synthesized via Pd-catalyzed C-O coupling reactions. The scaffold of these microporous polymers consists of rigid three-dimensional triptycene and stereocontorted spirobifluorene, endowing these polymers with micropore dimensions attractive for gas separations. This robust PAE synthesis method allows for the facile incorporation of functionalities and branched linkers for control of permeation and mechanical properties. A solution-processable branched polymer was formed into a submicron film and characterized for permeance and selectivity, revealing lab data that rivals property sets of commercially available membranes already optimized for much thinner configurations. Moreover, the branching motif endows these materials with outstanding plasticization resistance, and their microporous structure and stability enables benefits from competitive sorption, increasing CO2 /CH4 and (H2 S+CO2 )/CH4 selectivity in mixture tests as predicted by the dual-mode sorption model. The structural tunability, stability, and ease-of-processing suggest that this new platform of microporous polymers provides generalizable design strategies to form MOPs at scale for demanding gas separations in industry.

Engineering the Polymer-MOF Interface in Microporous Composites to Address Complex Mixture Separations

Poor interfacial compatibility remains a pressing challenge in the fabrication of high-performance polymer-MOF composites. In response, introducing compatible chemistries such as a carboxylic acid moiety has emerged as a compelling strategy to increase polymer-MOF interactions. In this work, we leveraged compatible functionalities in UiO-66-NH2 and a carboxylic acid-functionalized PIM-1 to fabricate mixed-matrix membranes (MMMs) with improved separation performance compared to PIM-1-based MMMs in industrially relevant conditions. Under pure-gas conditions, PIM-COOH-based MMMs retained selectivity with increasing MOF loading and showed increased permeability due to increased diffusion. The composites were further investigated under industrially relevant conditions, including CO2/N2, CO2/CH4, and H2S/CO2/CH4 mixtures, to elucidate the effects of competitive sorption and plasticization. Incorporation of UiO-66-NH2 in PIM-COOH and PIM-1 mitigated the effects of CO2- and H2S-induced plasticization typically observed in linear polymers. In CO2-based binary mixed-gas tests, all samples showed similar performance as that in pure-gas tests, with minimal competitive sorption contributions associated with the amine functional groups of the MOF. In ternary mixed-gas tests, improved plasticization resistance and interfacial compatibility resulted in PIM-COOH-based MMMs having the highest H2S/CH4 and CO2/CH4 selectivity combinations among the films tested in this study. These findings demonstrate that selecting MOFs and polymers with compatible functional groups is a useful strategy in developing high-performing microporous MMMs that require stability under complex and industrially relevant conditions.