Role of p-Benzoquinone in the Photocatalytic Production of Solketal

The role of <i>p</i>-benzoquinone (BQ) as a photocatalyst in the synthesis of solketal under UV irradiation has been studied, along with the combined use of BQ/TiO₂ P25 as a photocatalytic system for the process. The presence of the O₂/O₂^-• redox couple is essential for the reaction to take place. However, experiments with <i>p</i>-benzoquinone as a superoxide radical scavenger failed, with the opposite effect of enhancing the reaction being observed. It was found that <i>p</i>-benzoquinone and oxygen compete for photogenerated electrons in the conduction band of titania. A redox equilibrium between <i>p</i>-benzoquinone and hydroquinone (H₂Q), mediated by the O₂/O₂^-• system, was identified as a key factor in enabling the reaction. Furthermore, EPR spin-trapping experiments confirmed the presence of the carbon-centered radical 2-hydroxypropan-2-yl, which was determined to be the main radical species involved in the process. Either acetone or 2-propanol can generate this radical, with the BQ/H₂Q redox system being pivotal in the formation of the hemiacetal intermediate. This intermediate is subsequently converted into the final acetal (solketal), with H₂Q acting as a photoacid through an excited-state proton transfer (ESPT) mechanism. The photoacid behavior of hydroquinone was confirmed using pyridine as a basic probe, as the formation of hydroquinone-pyridine adducts was detected by Raman spectroscopy.