Insight into the reaction mechanism of photocatalytic production of solketal

Abstract

In this work, the behavior of a commercial TiO2 (Evonik P25) and synthesized CeO2 and Bi2WO6 photocatalysts in the photoacetalization of glycerol with acetone to obtain solketal under simulated sunlight was studied. The solids were characterized by X-ray diffraction (XRD), N2 adsorption desorption isotherms, Raman spectroscopy and UV–Vis spectroscopy techniques. The influence of the oxygen content in the reaction atmosphere (1–40% O2) was analyzed. It was concluded that oxygen is essential for the reaction taking place, suggesting the participation of the superoxide radical as an intermediate in the transfer of electrons. To confirm this point, p-benzoquinone, a known superoxide radical scavenger, was used to evaluate the role of O2. However, two opposite effects were observed for p-benzoquinone depending on the amount added to the reaction medium. Thus, for small amounts of p-benzoquinone a negative effect on the yield of solketal was observed, in agreement to its superoxide radical scavenger character. On the contrary, at high concentrations, p-benzoquinone enhanced the solketal yield obtained in the process. A mechanism is proposed by which the semiquinone radical formed acts as an intermediate capable of facilitating the transfer of electrons in the process, thereby improving the overall process. The catalyst showing the lowest catalytic activity was CeO2, whereas Bi2WO6 had a higher reaction rate and activity than TiO2 (P25), with both reaching approximately 78% conversion under optimum conditions. These results are promising because the conversion achieved is comparable to that obtained by acid catalysis, using sunlight as energy source.