Effect of Solvent on Heterogeneously Catalyzed Reactions: The Case of Hydrogenation of Carbonyl Compounds for Production of Fine Chemicals

NICOLÁS M. BERTERO; ANDRÉS F. TRASARTI; CARLOS R. APESTEGUÍA; ALBERTO J. MARCHI

Solvent Effects in Chemistry. Advances in Applications and Research.

Nova Science Publishers

Lugar: New York; Año: 2020; p. 1 - 100

The solvent effect on the rate and selectivity of different chemical reactions in liquid phase has been extensively documented, explored and interpreted in the case of homogeneous phase reactions. However, the solvent effect on heterogeneously catalyzed reactions has only been partially studied and a rigorous analysis to interpret the results was carried out in very few cases. The main difficulty to reach satisfactory explanations on the solvent effect lies in the solid catalyst complex interactions with both reagents/products and the solvent, the latter generally being the main component in the reaction medium. Therefore, a comprehensive approach must consider all possible interactions that may take place: solvent-reagents and solvent-products interactions in homogeneous phase, and the reagents-catalyst and solvent-catalyst interactions on the solid surface.A type of reactions where this effect has been widely observed but deeply analyzed only in few cases are hydrogenation reactions, where a substrate undergoes reduction by the action of dissociatively chemisorbed hydrogen on the surface of a metal catalyst. In particular, the chemo-selective hydrogenation of carbonyl group (-C=O) of compounds having other potentially reducible functional groups is an important application of heterogeneous catalysis in fine chemistry. The selective hydrogenation of unsaturated aldehydes, ketones and ketoesters produces valuable alcohols of high commercial value that find application in pharmaceuticals, fragrances and flavors, food additives, intermediates for agrochemicals, etc.The majority of fine chemicals are obtained in multipurpose batch reactors at relatively small scale. Most of these processes are carried out using a solvent that accomplish at least one of the following functions: (a) dissolution of solid reagents; (b) control of conversion rate by dilution effect in very fast reactions; (c) dissipation of heat generated in highly exothermic reactions. Although the use of solvent has a negative environmental impact, in many situations its presence is absolutely necessary. In these cases, solvent selection is critical, since it can strongly influence the productivity of the process by modifying catalytic activity and/or selectivity. Moreover, an inappropriate solvent choice can lead to the failure of the catalytic process.In this work, a critical review of the state of the art about the solvent influence on the performance of metal catalysts in the hydrogenation of carbonyl compounds, specifically unsaturated aldehydes, ketones and ketoesters, is carried out. The study of the solvent effect on the selective hydrogenation of C=O group of the above mentioned carbonyl compounds is by far the most explored. However, there are many aspects that still need a deeper discussion to elucidate the solvent influence. Due to the nature of the carbonyl C atom in aldehydes, the solvent effect on activity and chemoselectivity is discussed. Instead, in the case of ketones and ketoesters, whose carbonyl C is prochiral, an additional effect on the enantioselectivity is also considered. The use of different physicochemical characterization methods and experimental laboratory techniques is discussed along the text in order to provide useful pieces of information that help to explain this phenomenon, a clearly challenging task at the light of its complexity.