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

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.