Vinyl-Addition Fluoroalkoxysilyl-Substituted Polynorbornene Membranes for CO₂/CH₄ Separation

Natural gas production and utilization have grown tremendously in recent decades, which highlights the need for improved, high-throughput purification methods. In the field of polymeric membrane-based gas separations, a detailed understanding of molecular structure–property relationships are critical to overcoming the permeability–selectivity trade-off. Prior work has highlighted that alkoxysilyl-substituted vinyl-addition polynorbornenes (VAPNBs) are promising candidates for natural gas purification, having exceptional H₂S/CH₄ and modest CO₂/CH₄ permselectivity. To improve their CO₂/CH₄ separation performance, we herein describe a series of fluoroalkoxysilyl-substituted VAPNBs. We demonstrate that the incorporation of fluoroalkoxysilyl substituents yields a series of polymeric materials whose CO₂/CH₄ permselectivity increases as a function of fluorine incorporation. Interestingly, these enhanced selectivity characteristics are realized with minimal decreases to overall CO₂ permeability. While it was initially hypothesized that introduction of fluorinated units would decrease CH₄ solubility, detailed sorption analysis revealed that the observed increases in CO₂/CH₄ permselectivity were due almost exclusively to enhanced CO₂ sorption in fluorine-containing VAPNBs. Computational studies provided insights into the electronic interaction energies between gas molecules and polymer repeat units across the nonfluorinated and fluorinated series, respectively.