Enthalpic effects in the adsorption of alkylaromatics on the metal-organic frameworks MIL-47 and MIL-53

This work reports on the adsorptive separation of mono- and disubstituted alkylaromatics with various alkyl groups in the microporous metal-organic frameworks MIL-47 (VIVO{O2C–C6H4–CO2}) and MIL-53 (AlIII(OH){O2C–C6H4–CO2}). The mechanisms underlying the adsorption selectivity are investigated via determination of the adsorption enthalpies at low or high loading, either via pulse chromatographic or calorimetric methods. Results of both methods are in fair agreement. It is shown that the strong preference of MIL-47 for n-propylbenzene over cumene is due to a more favorable adsorption enthalpy. In addition, upon adsorbing n-alkylbenzenes with increasing alkyl chain length a stepwise increase in adsorption enthalpy is observed, which possibly indicates the importance of alkyl chain length in spanning the distance between adjacent, regularly spaced interaction sites in the crystal lattice. Finally, we also study the adsorption enthalpies of various xylene isomers and ethylbenzene on MIL-47 and MIL-53. It is found that enthalpic as well as entropic effects play an important role in determining the adsorption selectivity.

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► Adsorptive separation of mono- and disubstituted alkylaromatics in the microporous metal-organic frameworks MIL-47 and MIL-53.
► Separation mechanism investigated via determination of the adsorption enthalpies.
► Two methods for determining enthalpies, namely via pulse chromatography and calorimetry are in fair agreement.
► Extended to adsorption enthalpies of various xylene isomers and ethylbenzene on MIL-47 and MIL-53.