Separation of CO2 from CH4 is an important step in the purification of raw natural gas; a separation that is in high demand as natural gas production and utilization continues to grow. While this separation has been historically achieved through amine scrubbing technology, membrane driven separations have become increasingly attractive as our fundamental understanding of membrane science continues to advance. In the field of membrane-based gas separations, a detailed understanding of molecular level structure-property relationships is critical in overcoming permeability-selectivity tradeoff behavior. This talk will highlight our recent work in understanding the role that fluorine content plays in modulating transport of CO2 and CH4 through dense polymeric membranes. This discussion will focus on the design and synthesis of a series of fluoroalkoxysilyl-substituted vinyl-addition polynorbornenes (VAPNBs), characterization of isolated polymers, and the discriminatory transport observed following the incremental incorporation of fluorinated units. The incorporation of fluorinated motifs in these VAPNBs yields a series of polymers whose CO2/CH4 permselectivity increases as a function of fluorine content. Notably, this enhanced selectivity performance is achieved with little to no decrease in CO2 permeability (performance that directly violates tradeoff behavior). While our initial hypothesis was largely centered around a proposed reduction in CH4 solubility —and subsequent reduction in CH4 permeability— due to mutual immiscibility of hydrocarbon and fluorocarbon materials, detailed sorption analysis revealed that CH4 solubility remained virtually unchanged while CO2 solubility increased by as much as 80% across one of the studied copolymer series. Electronic structure theory computations provided some insight when comparing performance between non-fluorinated materials, as well as in comparing performance between fluorinated materials. However, the correlation between experimental findings and computational findings falls short in comparing performance between non-fluorinated and fluorinated polymer materials highlighting a need for more sophisticated computational approach in investigating polymer-gas interactions in this emerging class of material.

Vinyl-addition fluoroalkoxysilyl polynorbornenes: Probing the influence of fluorine content on CO2/CH4 solubility-selectivity

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