Sustainable formation of tricarballylic acid from citric acid over highly stable Pd/Nb2O5·nH2O catalysts

Recently, a reaction sequence was developed for the production of tricarballylic acid, an interesting plasticiser precursor, from citric acid by using a H-Beta zeolite and Pd/C catalyst. Although yields of 85% of tricarballylic acid were obtained, citric acid elicited Al leaching from the zeolite framework, resulting in loss of activity. In this work, Nb2O5·nH2O was found to be a stable and performant catalyst for this reaction, and a strong involvement of the hydroxyl group of citric acid with the Nb2O5·nH2O surface was observed by 13C MAS NMR. Next, Pd/Nb2O5·nH2O catalysts were synthesized via a low temperature reduction method to preserve the acidity of the Nb2O5·nH2O support; the nature of the Pd phase was examined by XAS. In presence of a 0.6 wt% Pd/Nb2O5·nH2O catalyst, yields over 90% of tricarballylic acid were obtained over multiple runs. Finally, this catalyst was also suitable for the dehydration-hydrogenation of other monohydroxy carboxylic acids.