Reverse-Selective PTMSP Supported Ionic Liquid Membranes for Light Hydrocarbon Recovery

Rich in hydrocarbons other than methane, growing unconventional gas production motivates new separations challenges for the growing supply of natural gas liquids (NGLs). Membranes are advantageous for pre-fractionating NGLs from methane in small-scale, decentralized shale operations. In this work we report a reverse-selective supported ionic liquid membrane (SILM) platform based on varying compositions of the polyacetylene PTMSP and the ionic liquid [hmim][Tf₂N]. The thermal stability, mechanical properties, and ionic liquid dispersion in these membranes is characterized in addition to their permeation properties. Gas permeability in SILMs is modest compared to those in the neat PTMSP. Ethane and propane permeability in SILMs increase nonlinearly with higher transmembrane pressure, which translates to highly pressure dependent pure-gas C₂H₆/CH₄ and C₃H₈/CH₄ selectivities. It is hypothesized that the ionic liquid first occupies the accessible free volume in the PTMSP support, thus exacerbating swelling due to highly condensable NGLs. Ionic liquid and NGL-induced swelling behavior is elucidated via density and ellipsometry measurements.