Résumé
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 piesence of two sets of cages (approximate to 24 angstrom and 29 angstrom) and windows (approximate to 5 angstrom and 9 angstrom) 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 (approximate to 27 angstrom and 34 angstrom) and windows (approximate to 12 angstrom and 16 angstrom). 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.