Product Inhibition in Glycerol Oxidation over Au/TiO2 Catalysts Quantified by NMR Relaxation

Liquid-phase catalytic oxidation of glycerol in aqueous solutions using porous solid catalysts represents a viable strategy for the sustainable production of fine chemicals from renewable resources. Various aspects of these novel types of reactions are still under investigation. Catalyst deactivation is one of those issues that need to be understood and addressed in order to make these processes commercially viable. In a previous study, it has been reported that the catalytic activity of Au/TiO2 catalysts for the oxidation of glycerol with O-2 under basic conditions can be severely inhibited by some reaction intermediates or products. It was suggested that the presence of certain species blocks the active sites of the catalyst, preventing the adsorption of glycerol, which in turn results in a decrease of reaction rate. In this work, we used NMR relaxation time measurements in order to assess surface interactions of glycerol in a Au/TiO2 catalyst pretreated with aqueous solutions of various oxygenates, including intermediates and products of glycerol oxidation, under basic conditions, in particular, evaluating changes in glycerol adsorption properties. The NMR T-1/T-2 ratio of glycerol, which is indicative of the strength of interaction of glycerol with the catalyst surface, traces out well the trend in catalytic activity in the presence of different additives, suggesting that adsorption of glycerol onto the catalyst surface plays a crucial role in the reaction, which supports the hypothesis previously made in the literature. This experimental approach and the related results represent a significant advance in the understanding of liquid-phase catalytic reactions occurring over solid surfaces, which can be used to understand and optimize catalytic processes and the effect of intermediate and product inhibition.