Reverse shape selectivity in the adsorption of hexane and xylene isomers in MOF UiO-66

An adsorption study of hexane and xylene isomers mixtures was addressed in a rigid zirconium terephthalate UiO-66 (UiO for University of Oslo) with octahedral and tetrahedral cavities of free diameter close to 1.1 nm and 0.8 nm, respectively. Multicomponent equimolar breakthrough experiments show that the adsorption hierarchy of structural isomers in UiO-66 is opposite to the one observed in conventional adsorbents. For hexane isomers, it was found that the amount adsorbed increases with the degree of branching, being 2,2-dimethylbutane (22DMB) and 2,3-dimethylbutane (23DMB) the more retained molecules. Regarding the xylene isomers, the results show that the adsorption of the bulkier ortho-xylene (oX) is favoured compared to its homologues. The structural similarity between MOF UiO-66 and zeolite MCM-22 suggests that the reverse shape selectivity observed in the adsorption of hexane and xylene isomers might be attributed to the rotational freedom of the molecules inside the small cavities.

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► MOF UiO-66 possesses octahedral and tetrahedral cavities.
► Multicomponent fixed bed experiments with hexane and xylene isomers were addressed.
► Adsorption hierarchy in UiO-66 is opposite to the one observed in conventional adsorbents.
► Experimental data suggest that inverse selectivity might be attributed to the rotational freedom of the molecules inside the tetrahedral cavities.