Catalytic Oxidation of Benzoins by Hydrogen Peroxide on Nanosized HKUST-1: Influence of Substituents on the Reaction Rates and DFT Modeling of the Reaction Path

Abstract

In this research, the oxidation of a series of benzoins, R-C(=O)-CH(OH)-R, where R = phenyl, 4-methoxyphenyl, 4-bromophenyl, and 2-naphthyl, by hydrogen peroxide in the presence of nanostructured HKUST-1 (suspension in acetonitrile/water mixture) was studied. The respective benzoic acids were the only products of the reactions. The initial average reaction rates were experimentally determined at different concentrations of benzoin, H2O2 and an effective concentration of HKUST-1. The sorption of the isotherms of benzoin, dimethoxybenzoin and benzoic acid on HKUST-1, as well as their sorption kinetic curves, were measured. The increase in H2O2 concentration expectedly led to an acceleration of the reaction. The dependencies of the benzoin oxidation rates on the concentrations of both benzoin and HKUST-1 passed through the maxima. This finding could be explained by a counterplay between the increasing reaction rate and increasing benzoin sorption on the catalyst with the increase in the concentration. The electronic effect of the substituent in benzoin had a significant influence on the reaction rate, while no relation between the size of the substrate molecule and the rate of its oxidation was found. It was confirmed by DFT modeling that the reaction could pass through the Baeyer–Villiger mechanism, involving an attack by the HOO– anion on the C atom of the activated C=O group.

Advances and Challenges in Development of Transition Metal Catalysts for Heterogeneous Hydrogenation of Organic Compounds

“…The majority of commercial carbons are microporous ones, however, microporosity is not required for improvement of the catalytic activity and can even lead to unwanted adsorption of reagents and products. [90] In some cases decrease of the metal content in the catalyst leads to improved dispersion (and higher activity), but at the same time it results in increase of the quantity of adsorbed organic compounds and their loss. In this view, mesoporous supports have good potential, [91] but their use is generally restricted by higher cost.…”

“…Activated carbons of different grades are the most popular support materials due to their low cost and high specific surface. The majority of commercial carbons are microporous ones, however, microporosity is not required for improvement of the catalytic activity and can even lead to unwanted adsorption of reagents and products [90] . In some cases decrease of the metal content in the catalyst leads to improved dispersion (and higher activity), but at the same time it results in increase of the quantity of adsorbed organic compounds and their loss.…”