Abstract
Metal oxides are widely utilized as heterogeneous catalysts and catalyst supports and have been used recently for the on-purpose production of commodity chemicals through the conversion of biorenewable oxygenated feedstocks such as ethanol and carboxylic acids. The first part of this work focuses on multifunctional Lewis acid catalysts as effective ethanol-to-butadiene catalysts. Promoted with Ag-SiO2 in a physical mixture, oxides of four elements, Ta, Y, Pr, and La were tested on two different supports, crystalline Beta zeolite (HBZ) and amorphous SiO2, in the cascade reaction of ethanol to butadiene. Results from diffuse reflectance UV-Vis spectroscopy showed zeolite-supported Ta and Pr catalysts have a smaller metal oxide cluster size relative to their SiO2 analogues. The oxidation states of the cations supported on HBZ, evaluated by X-ray photoelectron spectroscopy (XPS), were the same as their SiO2-supported analogues. Comparison of the performance of SiO2-supported catalysts revealed a greater distribution of butadiene among the C4 coupling products over stronger Lewis acids, such as Ta, which was evaluated by the 2-propanol decomposition reaction to propene and acetone. The use of Beta zeolite as a support for the Lewis acid cations significantly enhanced the rate of C-C coupling by order of magnitude greater than their SiO2 analogues.
The second part of the work focuses on supported tungsten oxide (WOx), a reducible metal oxide. When promoted with Pd, supported WOx catalysts can be used in the reduction of carboxylic acids with H2 to form aldehydes and alcohols. During this reaction, hydrogen spillover from Pd nanoparticles may participate in the reaction itself while also facilitating the reduction of the WOx species. The influence of hydrogen spillover on SiO2 and P25-TiO2 supported WOx species was studied through a variety of techniques. Results from H2 temperature-programmed reduction showed the presence of Pd on SiO2-supported WOx lowered the initial reduction temperature of the WOx species but did not affect the reduction of those species supported on TiO2. Highly isolated WOx species on acid-treated SiO2 were less reducible than larger WOx clusters on SiO2. High-angle annular dark-field scanning transmission electron microscopy showed nanometer-size WOx clusters on SiO2 and highly dispersed species on TiO2. In situ XPS showed SiO2-supported WOx species reduce from an initial +6 oxidation state to primarily +5 after thermal treatment in H2, while the fraction of +5 species detected on the P25-TiO2 support did not change, regardless of reducing environment or addition of Pd.
Silica and titania-supported Pd-W catalysts were then evaluated for carboxylic acid reduction via the gas-phase conversion of propionic acid to propanal and propanol with H2. High resolution STEM images confirmed the presence of nm-size Pd particles on both Pd-W-SiO2 and Pd-W-P25-TiO2 catalysts. During steady state conversion of propionic acid, the presence of Pd on both W-SiO2 and W-P25-TiO2 enhanced the selectivity and formation rate of propanal and propanol, with a combined selectivity of > 96 % at conversion levels of 1 % for Pd-W-SiO2 and 9.2 % for Pd-W-P25-TiO2. Over the P25-TiO2-supported Pd-W catalyst, the reaction orders were 0.3 and zero in H2 and propionic acid, respectively, while the apparent activation energy was 64 kJ∙mol-1. Although increasing the Pd loading on P25-TiO2-supported W catalysts increased the combined propanal and propanol formation rate, the same effect had marginal influence on the formation rates over SiO2-supported W catalysts. Most importantly, the P25-TiO2 supported W catalysts exhibited order of magnitude higher formation rates of propanal and propanol compared to the SiO2 analogs.
