Photoxidation of Benzyl Alcohol with Heterogeneous Photocatalysts in the UV Range: The Complex Interplay with the Autoxidative Reaction

In conventional studies of heterogeneous photocatalytic oxidation, it is crucial to evaluate by means of control experiments the relevance of the “non-catalyzed” autoxidation reaction under irradiation. If the autoxidation is found to be negligible, it is usually considered that it can be safely disregarded. However, in the case of aromatic aldehydes’ synthesis such as benzaldehyde, irradiating with UV light may lead to a more complex mechanism than previously thought. Herein, we prove that neglecting the autoxidative reaction can lead to a misinterpretation of the mechanism as well as an overestimation of the catalyst's efficiency, even if the control experiment exhibits zero yield. As an example, we studied the synthesis of benzaldehyde from benzyl alcohol by UV-A irradiation with and without TiO₂ and CdS nanoparticles; without catalyst, after a long induction time, the aldehyde enhances its own production through an autocatalytic reaction activated by the same light wavelengths utilized for the nanocatalysts. The long induction time leads to considering the autocatalysis to be deceptively negligible. However, the nanoparticles act as a fast initiator of the benzaldehyde autocatalysis, in such a way that the oxidation rate reached a similar performance with nanoparticles and after removing them. This suggests that the commonly ignored autocatalysis of benzaldehyde indeed is a relevant parallel pathway to the heterogeneous catalysis mechanism.