Understanding the Thermodynamic and Kinetic Behavior of the CO₂/CH₄ Gas Mixture within the Porous Zirconium Terephthalate UiO-66(Zr): A Joint Experimental and Modeling Approach

A combination of experimental (gravimetry, microcalorimetry, and quasi-elastic neutron scattering) measurements and molecular modeling was employed to understand the coadsorption of CO 2 and CH 4 in the zirconium terephthalate UiO-66(Zr) material from both the thermodynamic and kinetic points of view. It was shown that each type of molecules adsorb preferentially in two different porosities of the material, that is, while CO 2 occupy the tetrahedral cages, CH 4 are pushed to the octahedral cages. Further, a very unusual dynamic behavior was also pointed out with the slower molecule, that is, CO 2, enhancing the mobility of the fast one, that is, CH 4, that contrasts with those usually observed so far for the CO 2 /CH 4 mixture in narrow window zeolites where the molecules are most commonly diffusing independently or slowing-down the partner species. Such behavior was interpreted in light of molecular simulations that evidenced a jump type mechanism involving a tetrahedral cages–octahedral cages–tetrahedral cages sequence that occurs more frequently for CH 4 when in presence of CO 2 . The consequences in terms of CO 2 /CH 4 selectivity and the possible use of this MOF-type material in a PSA process are then discussed. It is thus clearly emphasized that this MOF material combines several favorable features including a good selectivity, high working capacity, and potential easy regenerability that make it as a good alternative candidate of the conventional NaX Faujasite used in pressure swing adsorption.