Efficient and ecofriendly route for the solvent-free synthesis of piperonal and aromatic aldehydes using Au/CeO2 catalyst

In this work, an eco-friendly process for piperonal synthesis from piperonyl alcohol oxidation was developed. Piperonal was obtained with 100% selectivity under solvent free conditions, using air as oxidant at atmospheric pressure. The preparation of high surface area CeO2 using a surfactant-template method allowed the preparation of a catalyst with highly dispersed Au nanoparticles having an average diameter of about 1 nm. The obtained results, using Au/CeO2 catalysts, suggested that the reaction network consists in the rapid oxidation of piperonyl alcohol to piperonal. Increasing reaction time to promote alcohol conversion a very small amount of formed aldehyde undergoes to further oxidation to the acid. Subsequently, piperonylic acid can easily react with the alcohol, leading to the formation of the ester that can be adsorbed on the catalyst surface, deactivating the materials. Nevertheless, the fine-tuning of the process, using solvent free conditions, air and low reaction time, demonstrated the possibility to control catalyst deactivation. Indeed, gold supported on nano-CeO2 were able to catalyse the production of piperonal without by-product formation, with 35% of piperonyl alcohol conversion after 45 min of reaction and the catalyst can be recycled without any activity loss, demonstrating the possibility of scaling up a low conversion continuous process or low reaction time batch process. No toxic reagents were used neither for catalysts preparation nor for catalytic tests and the use of air as oxidant improved the safety of the process. The latter, together with the absence of a solvent, can led to important advancements in the industrial applications reducing the costs of the purification section. This versatile approach, interesting from both the economic and the environmental point of view, was also applied in the synthesis of other aromatic aldehydes, such as benzaldehyde, vanillin and anysil aldehyde, highlighting the importance of this process.