Membrane-based separation of combined acid gases carbon dioxide and hydrogen sulfide from natural gas streams has attracted increasing academic and commercial interest. These feeds are referred to as “sour,” and herein, we report an ultra H₂S-selective and exceptionally permeable glassy amidoxime-functionalized polymer of intrinsic microporosity for membrane-based separation. A ternary feed mixture (with 20% H₂S:20% CO₂:60% CH₄) was used to demonstrate that a glassy amidoxime-functionalized membrane provides unprecedented separation performance under challenging feed pressures up to 77 bar. These membranes show extraordinary H₂S/CH₄ selectivity up to 75 with ultrahigh H₂S permeability >4000 Barrers, two to three orders of magnitude higher than commercially available glassy polymeric membranes. We demonstrate that the postsynthesis functionalization of hyper-rigid polymers with appropriate functional polar groups provides a unique design strategy for achieving ultraselective and highly permeable membrane materials for practical natural gas sweetening and additional challenging gas pair separations.

Membrane‐based separations offer great potential for more sustainable and economical natural gas upgrading. Systematic studies of CO₂/CH₄ separation over a wide range of temperatures from 65 °C (338 K) to as low as −40 °C (233 K) reveals a favorable separation mechanism toward CO₂ by incorporating Y‐fum‐fcu‐MOF as a filler in a 6FDA‐DAM polyimide membrane. Notably, the decrease of the temperature from 308 K down to 233 K affords an extremely high CO₂/CH₄ selectivity (≈130) for the hybrid Y‐fum‐fcu‐MOF/6FDA‐DAM membrane, about four‐fold enhancement, with an associated CO₂ permeability above 1000 barrers. At subambient temperatures, the pronounced CO₂/CH₄ diffusion selectivity dominates the high permeation selectivity, and the enhanced CO₂ solubility promotes high CO₂ permeability. The differences in adsorption enthalpy and activation enthalpy for diffusion between CO₂ and CH₄ produce the observed favorable CO₂ permeation versus CH₄. Insights into opportunities for using mixed‐matrix membrane‐based natural gas separations at extreme conditions are provided.