Abstract
Membrane‐based separations offer great potential for more sustainable and economical natural gas upgrading. Systematic studies of CO
2
/CH
4
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 CO
2
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 CO
2
/CH
4
selectivity (≈130) for the hybrid Y‐
fum
‐
fcu
‐MOF/6FDA‐DAM membrane, about four‐fold enhancement, with an associated CO
2
permeability above 1000 barrers. At subambient temperatures, the pronounced CO
2
/CH
4
diffusion selectivity dominates the high permeation selectivity, and the enhanced CO
2
solubility promotes high CO
2
permeability. The differences in adsorption enthalpy and activation enthalpy for diffusion between CO
2
and CH
4
produce the observed favorable CO
2
permeation versus CH
4
. Insights into opportunities for using mixed‐matrix membrane‐based natural gas separations at extreme conditions are provided.