Hydrogenation of carboxylic acids to the corresponding aldehydes

Abstract

In this article, the key issue of the catalysts for hydrogenation of aromatic and aliphatic carboxylic acids to the corresponding aldehydes was reviewed. For the hydrogenation of aromatic carboxylic acids, metal oxides, such as ZrO 2, CeO 2, ZnO and MnO, show high activity and selectivity to aldehydes. On the other hand, for aliphatic carboxylic acids which have two α-hydrogen atoms, these metal oxides show low selectivity for hydrogenation, because undesirable ketonization occurs. For the hydrogenation of aliphatic carboxylic acids, Cr 2O 3 and the partially reduced Fe 2O 3 show high selectivity. Especially, the highly pure Cr 2O 3 exhibits superior chemo-selectivity to aldehyde without CC bond migration or ketonization. The chromium-modified ZrO 2 catalyst for manufacturing aromatic aldehydes and the highly pure Cr 2O 3 catalyst for manufacturing aliphatic aldehydes have been commercially used in industrial processes. A new aspect of direct hydrogenation catalysts is briefly reviewed, and the reaction mechanism for hydrogenation of carboxylic acids is also discussed.