Reverse-selective polymer-ionic liquid membranes for light paraffin gas separations

Davila Labastida, Jose Carlos
Journal Title
Journal ISSN
Volume Title

Natural gas plays a major role as both a relatively clean-burning fossil fuel and a precursor to hydrogen, ethylene, and numerous chemical commodities. With advancements in horizontal drilling and hydraulic fracturing, abundant shale resources are a growing piece of the global energy transition puzzle. Shale gas is richer in C₂₊ hydrocarbons than conventional natural gas which presents challenges and opportunities to centralized gas processing infrastructure. Membranes offer an energy efficient alternative separation technology for fractionating the light C₂₊ hydrocarbons at modular scales. Reverse-selective membranes may offer an economic advantage given their typically high productivities. Reverse-selective polymers, much like ionic liquids and other solvents, discriminate between gas molecules based primarily on solubility differences. Unlike traditional solvents, however, ionic liquids are negligibly volatile and can be combined with polymers into stable membrane platforms, albeit with lesser understood transport properties. In this thesis, a comprehensive study of the light paraffin gas transport properties of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, or [hmim][Tf₂N], ionic liquid supported in poly(1-trimethylsilyl-1-propyne), or PTMSP, membranes is presented, along with preliminary work on some alternative polymer-ionic liquid combinations. The thermal, mechanical, and swelling properties of these supported ionic liquid membranes (SILMs) were initially characterized for various ionic liquid loadings. The pure-gas permeation, sorption, and diffusion properties of SILMs were then determined and compared with those of the neat parent materials. While prior work on SILMs for gas separations is widespread, few studies focus on light paraffin separations, and fewer still evaluate mixed-gas transport properties or pronounced plasticizing effects of the polymer support on the gas transport properties. A mixed-gas permeation system was built and validated to probe the more industrially relevant mixture properties. Unlike in the neat PTMSP, pronounced permeability increases with rising C₂₊ activities were noted in SILMs. A plasticization model was fit to the diffusion data of the three lightest paraffin gases to better understand the impact of ionic liquid content on plasticization. With increasing ionic liquid content, a clear transition in the C₂₊ sorption behavior was observed from a rigid, open glass (dual-mode) to a dense, softer material (Flory-Huggins). A new hybrid model was proposed to fit sorption isotherms in all regimes of this transition. Lastly, an atypical volume expansion behavior was observed in preliminary SILM dilation studies and an optical transition matching the inflection in the C₂₊ sorption isotherms and permeability curves was identified