Hydrocarbon adsorption in the isostructural metal organic frameworks MIL-53(Cr) and MIL-47(V)

The adsorption of n-hexane and n-nonane in MIL-53(Cr) and MIL-47(V) was first investigated by gravimetric measurements. While MIL-53(Cr) exhibits a sub-step during the sorption process induced by the breathing ability of this solid, the adsorption isotherm for MIL-47(V) is of type I. This different shape of the so-obtained isotherms was then interpretated in light of complementary experimental tools including thermogravimetry and X-ray thermodiffraction. We thus evidenced the occurrence of different structures in presence of n-hexane or n-nonane in the case of MIL-53(Cr) identified as the narrow pore (NP) phase and large pore (LP) phase. One noticed the existence of a swollen NP form which spans in a broad range of temperature. A computational assisted structure determination allowed us to elucidate the structures for both NP and LP forms in presence of each investigated alkane. From these structures, the adsorption enthalpies of n-alkanes over MIL-53(Cr) have been computed. It was shown that the NP phase leads to higher enthalpy of adsorption as compared to the LP phase which is related to a higher degree of confinement.

A comparative study of the adsorption of n-alkanes in the isostructural metal organic framework (MOFs) MIL-53(Cr) and MIL-47 (V) is presented. The role of the length of the alkyl chain length (n = 5–9), the nature of the metal (V, Cr) were investigated. The shape of the adsorption curves is driven by the occurrence of a phase transition in the case of MIL-53(Cr) whilst traditional type I isotherms are observed in the case of MIL-47(V). Enthalpies of adsorption were found proportional with the number of carbons in the sorbate molecules.Figure optionsDownload as PowerPoint slideResearch highlights
► MIL-53(Cr) undergoes a phase transition under pressure stress.
► The isostructural MIL-47 (V) retains an open structure at any pressure.
► n-Alkanes exhibit similar interaction with MIL-53(Cr)-lp and MIL-47 (V).