Carbon-carbon bond formation is highly topical for the biorefinery since small, oxygenated molecules have to be joined to bigger ones. Ketones are potential target products that can be obtained from aldehydes. This reaction involves an oxidation into carboxylic acids and the ketonic decarboxylation of the latter. This mechanism has been under discussion and different pathways were considered, owing to the manifold types of materials used as solid catalysts. Herein, the transformation of heptanal into 7-tridecanone is evaluated experimentally over zirconium and cerium oxide materials in the gas phase. Mechanisms and kinetics are elucidated. The reaction occurs smoothly in the presence of both materials. For ZrO2, water has to be co-fed to suppress the aldol condensation product as side product. With CeO2, co-feeding water improves primarily catalyst stability with time on stream whereas the influence on selectivity was smaller. Cross-coupling experiments of aldehydes and carboxylic acids of different chain length were carried out and these confirmed unequivocally that the carboxylic acid is the reaction intermediate for both materials. As working hypothesis, it was considered that the oxidation of the aldehyde into the carboxylic acid occurs by a hydride shift to the oxide surface. This might happen from a tetrahedral configuration of the aldehyde, adding a surface hydroxy group to the carbonyl group, i.e. by chemisorption of the aldehyde. Isotopic exchange when passing hydrogen and deuterated water supported the occurrence of surface hydride species. Although similar experimental results were achieved with both materials, kinetics were different. In both cases, water is a reaction partner of the aldehydes, the terminal “oxidant” that provides the oxygen incorporated into the organic molecule and a proton for the formation of molecular hydrogen. Water decreased the reaction rate at any partial pressure for ZrO2, probably because water was always present on the surface due to the aldol side reaction. In contrast, with CeO2, low partial pressures facilitate the reaction, but at higher partial pressure, the same deceleration of the reaction rate was observed as for ZrO2. As a conclusion it can be stated that CeO2 and ZrO2 are efficient catalysts for the ketonization of aldehydes, but on molecular scale, the surfaces involve their own particularities.

Ketonization of aldehydes on zirconium and cerium oxide surfaces

2019 Spring National Meeting

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