Adsorption of thiophene and model hydrocarbons on MoO3/γ-Al2O3 catalysts studied by gas–solid chromatography

Abstract

A number of MoO₃/γ-Al₂O₃ catalysts of varying Mo content have been prepared and the adsorption of some probe molecules (thiophene, benzene, cyclohexane, cyclohexene, pentane, hexane, heptane and octane) has been studied using the gas–solid chromatographic technique. Isotherms for the adsorption of thiophene at 250 °C as well as enthalpies of adsorption of the probe molecules in the range 140–300 °C have been determined. Thiophene is adsorbed on three distinct kinds of site. On the first kind it is reversibly adsorbed. There was no preference for the development of this type of site on a particular Mo supported species. On the second type of site, located on octahedral Mo^VI ions of the polymeric Mo, reversible adsorption takes place, but the molecules adsorbed on these sites were desorbed very slowly. It was concluded that the third type of site, on which thiophene is irreversibly adsorbed, is generated exclusively on the bulk MoO₃ supported on the carrier. Only reversible adsorption of instantaneous desorption type of the probe hydrocarbons was observed on the catalysts prepared. On the basis of linear relations between the enthalpy of adsorption and polarizability of the non-polar hydrocarbons used it was inferred that the MoO₃/γ-Al₂O₃ catalysts exert non-specific (London or electrostatic) forces on these non-polar molecules. In the case of benzene, specific interactions resulting in the formation of a surface π-complex are involved in addition to the non-specific ones. These specific interactions are promoted by the supported Mo^VI.