Maximizing Ether Oxygen Content in Polymers for Membrane CO₂ Removal from Natural Gas

Abstract

Membrane materials for CO₂ removal from natural gas are based on glassy polymers with a high CO₂/CH₄ diffusivity selectivity. However, these polymers suffer from competitive sorption by heavy hydrocarbons that decreases CO₂ permeability and physical aging that reduces gas permeability with time. We circumvent these issues by designing rubbery, solubility-selective polymers with a ratio of ether/ester oxygen to carbon as high as 0.8 through the use of 1,3-dioxolane and 1,3,5-trioxane. The ether/ester oxygen groups interact favorably with CO₂ but do not interact with CH₄, leading to a high CO₂/gas solubility selectivity that is unaffected by heavy hydrocarbons in the raw natural gas. These polar groups are incorporated in short branches to yield an amorphous and rubbery nature, leading to high gas permeability that is stable over time. A polymer with an O/C ratio of 0.71 (P71) shows a mixed-gas CO₂ permeability of 320 Barrers and a CO₂/CH₄ selectivity of 21 in the simulated natural gas at 50 °C, which is independent of the hexane content and above the upper bound for CO₂/CH₄ separation at 50 °C.