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.