Adsorption of C5–C9 hydrocarbons in microporous MOFs MIL-100(Cr) and MIL-101(Cr): A manometric study

Abstract

The adsorption of n-alkanes ( n-pentane to n-nonane) on the mesoporous MIL-100(Cr) and the MIL-101(Cr) rigid solids has been performed with a view of evaluating the influence of the alkyl chain length in terms of sorption capacity and sorbate/sorbent affinity. MIL-100(Cr) exhibits at first sight a classical type I behaviour towards the sorption of n-alkanes. These sorption isotherms have proven to be fully reversible. A direct relationship between amounts of alkane adsorbed at saturation and number of carbon atoms present in the adsorbed species has been found. At very low pressure ( p/ p 0 < 0.025), sub-steps are present in the isotherm curve, due to the presence of two sets of cages (≈24 Å and 29 Å) and windows (≈5 Å and 9 Å). In the case of MIL-101(Cr), the sub-steps are shifted to higher partial pressures ( p/ p 0 < 0.10) due to the larger size of the cages (≈27 Å and 34 Å) and windows (≈12 Å and 16 Å). For the same apolar sorbates, this outlines the effect of pore dimensions of the two MOFs, which are larger in the case of the MIL-101(Cr) structure, and gives rise to important differences in terms of affinity or sorption mechanisms. This is confirmed by the determination of the enthalpy of adsorption of n-hexane or n-heptane much lower in the case of MIL-100 as compared to MIL-101, at low coverage, −65 kJ mol −1 and −30 kJ mol −1, respectively. The kinetics of the sorption process has also been discussed by comparing the equilibration times of the different sorbates during their adsorption. n-Nonane exhibited much longer equilibration times as to compared to shorter n-alkanes.