A fundamental study of selective oxidation reactions on supported vanadium oxide catalysts: Reaction mechanism and structure -activity relationships

Supported vanadium oxide catalysts consist of a molecularly dispersed vanadia phase supported on another metal oxide. The primary goal of this thesis was to systematically study the mechanism of selective oxidation of alcohols on supported vanadia and determine how the support affects the energetics of the elementary steps involved in this reaction. One of most significant contribution of this research was to identify the transition state of the β-hydrogen elimination from alkoxide groups on supported vanadia catalysts. Activation energies for β-C-H bond cleavage reaction from a series of fluorinated ethoxide adsorbed on a supported vanadia catalyst were measured using the TPD technique. Results for TiO2- and CeO2-supported monolayer vanadium oxide catalysts both showed an increasing aldehyde peak temperature with increasing fluorination of the methyl group in the ethoxide. Microcalorimetry studies showed that fluorination has little effect on stabilization of the adsorbed ethoxide and the fluorinated ethoxide. Due to the high electronegativity of fluorine, fluorination of the methyl group in the ethoxide would increase the field substituent constant of the methyl group. Primarily using TPD and Microcalorimetry techniques, we were able to measure energetics for elementary reaction steps of methanol oxidation, i.e. methanol adsorption, hydride elimination from methoxide and the replenishment of gaseous oxygen for the surface oxygen vacancies, independently. Using energetic analysis, it was found that more electronegative supports, i.e. ZrO2 , TiO2 and CeO2 make the methanol adsorption, the hydride transfer for methoxide and the reoxidation of surface vanadia following methanol oxidation energetically more favorable. In contrast, less electronegative supports, such as Al2O3 and SiO 2 hinder the occurrence of above individual reaction steps. Although the identity of the supports have a influence on the energetics of each reaction step, the compensation between the energetics for each reaction step offset the support effect on individual step, which leads to constant apparent activation energy for the reaction. This thesis work also addressed some other fundamental research issues regarding supported vanadia catalysts, i.e. the determination of the surface active site density which is critical to the calculation of the turn over frequency of selective oxidation product; the examination the effect of oxidation states on the activity of the catalysts and how different pretreatment and reaction conditions will change the oxidation states distribution of surface vanadia, which are also important to comprehensively understand the catalysts. (Abstract shortened by UMI.