A theoretical study of the dehydration and the dehydrogenation processes of alcohols on metal oxides using MOPAC

Abstract

Using a semi-empirical molecular orbital method, PM3, and 2-propanol as an example, the dehydration and the dehydrogenation processes of alcohol on oxide catalysts were studied. The catalysts addressed here were four kinds of oxides (Al 2O 3, SiO 2, ZnO, CdO) whose reaction selectivities had been experimentally determined. The usual models consisting of a surface metal ion, several oxide ions and an isopropoxy group were used in calculations. For the dehydration, heats of formation of the models were calculated at each point of the process where the distance between a β-hydrogen of the group and a basic site (i.e. oxygen of the group or a surface oxide ion) or a metal ion was gradually shortened, or where the length of the C α–O bond of the group was gradually increased. A reasonable dehydration mechanism was estimated by comparing activation energies calculated from the transitions of the heats of formation. The most probable dehydrogenation mechanism was also estimated in a similar way by gradually making an α-hydrogen close to a surface oxide ion, the metal ion or a surface proton. It was concluded that the dehydration proceeds by scission of the C α–O bond of the group after its oxygen was attacked by some electrophile on the surface and that the dehydrogenation proceeds by a mechanism in which an α-hydrogen of the group was extracted by the metal ion. Based on the dehydration mechanism thus deduced, alkoxy groups generated by adsorption of the primary, secondary and tertiary alcohols on SiO 2 were calculated in order to estimate the activation energies of their decompositions. In result, the order of the energies was found to be in good agreement with that of the decomposition rates experimentally determined by Kitahara. This agreement gives support to the validity of the mechanism deduced for the dehydration of alcohol.