Enhanced CO₂/CH₄ Separation Performance of a Mixed Matrix Membrane Based on Tailored MOF-Polymer Formulations

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.