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 poly-norbornenes (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.