Kinetic compensation effects and activated states for alcohol decomposition over alkali-modified vanadium oxides

Abstract

The rate of thermal decomposition of ethanol, isopropanol and t-butanol over well-defined alkali vanadates (trivanadate, MV 3O 8, M = Li, Na, and K; and hexavanadate, M 2V 6O 16, M = Rb and Cs) were precisely measured in a closed cyclic reaction system at constant reactant vapor pressures between temperatures of 190 and 370 °C. A steady rate was evident in the initial 20 min for most of the experiments. The initial mean rate decreased in the order Li → Na → K for the trivanadate. However, the initial mean rate increased in the order Rb → Cs for the hexavanadate. The plot of rate versus alkali ion size was a U-shaped curve for each alcohol run. The activation energies ( E a) for the hexavanadates were much larger than those for the trivanadates with any alcohol tested. Despite the kinetic differences between the tri- and hexavanadates, a linear relationship was found between the preexponential factor (log V 0) and the activation energy ( E a) through the series of alkali-modified vanadates for any alcohol. The data depended on the alkali ion size. The slope of the plot increased as the size of the alkyl group on the reactants increased. This compensation effect can be explained by assuming that the activation entropy is proportional to the excitation of the internal frictional rotation of an alkyl group in the reactants. In other words, the entropy term is directly related to the strength of the interaction energy, when based on a model where the alkyl and hydroxyl groups of the reactant molecule interact with the surface VO group and the alkali metal ion, which is placed on the VOV interchains.