Support Effects in Gold Catalyzed Oxidation Reactions
Authors:   , de Boed Ewoud Jacob Jan 
Published: May 2024

This thesis aims to extend the knowledge on the role of the support in gold catalyzed oxidation reactions. One of these reactions is the selective insertion of oxygen into the double bound of alkenes, as in the epoxidation of propylene using gold supported on titania-silica (Ti-SiO2) as catalyst.

We discovered that the Ti-SiO2 support itself was active in catalyzing the isomerization of propylene oxide (PO) and thus lowering the selectivity of the catalyst. The use of an alkali base during the gold deposition step resulted in residual Na+ and neutralized Ti-sites on the final Au/Ti-SiO2 catalysts. These catalysts demonstrated high PO selectivity. Catalysts without Na+ showed high selectivity towards propanal, formed from isomerization of PO. Of the studied alkali cations, Na+ is most beneficial for the highest epoxide productivity. In addition, an alternative strategy to lower the isomerization activity is lowering the Ti-loading.

Besides titania-silica based supports, we investigated the role of other group 4-6 transition metals in two types of epoxidation reactions. Firstly, in liquid phase epoxidation reactions with hydrogen peroxide as the oxidant, the observed epoxidation activity is correlated to the strength of Lewis acidity of catalysts. Secondly, after depositing gold nanoparticles on selected supports, the catalysts were tested in the gas phase epoxidation of propylene. The observed productivity is opposite of what was observed for the liquid phase tests, as the initial epoxidation activity was higher for the weaker Lewis acid Ti, compared to stronger Lewis acids (Ta and Nb).

Furthermore, the role of a selection of metal oxide supports in the gold catalyzed propene epoxidation was investigated. It was observed that metal oxides with a strong acidic character catalyze the isomerization of PO to propanal and acetone. Readily reducible supports show combustion of PO. A requirement for the metal oxide supports to achieve high epoxide selectivity is hence the absence of surface sites that can catalyze isomerization, a low specific surface area and/or low density of surface -OH groups. Ti4+-based supports react with PO, but their use is necessary to achieve high epoxidation activity, selectivity, and stability.

The application of gold catalysts is not only limited to epoxidation reactions. Included is a study on the synergistic effects of bimetallic AuAg catalysts in the two- step oxidation of 5-hydroxymethylfurfural (HMF), via 5-hydroxymethylfuran carboxylic acid (HMFCA) to 2,5-furan dicarboxylic acid (FDCA) in the aqueous phase is described. The addition of Ag to Au lowered the activity for the first step, the oxidation of HMF to HMFCA, probably by alteration of the electronic properties of the Au-Ag nanoparticles. Oxidation of the intermediate product HMFCA was accelerated significantly by adding 10-20% Ag, thereby yielding the highest amounts of FDCA. We propose that the synergistic effect in the second step is the enhanced HMFCA adsorption by the Au-Ag nanoparticles. In addition, the bimetallic nanoparticles were much more stable than their monometallic counterparts.