Role of p-Benzoquinone in the Photocatalytic Production of Solketal.

The role of p-benzoquinone (BQ) as a photocatalyst in the synthesis of solketal under UV irradiation has been studied, along with the combined use of BQ/TiO(2) P25 as a photocatalytic system for the process. The presence of the O(2)/O(2)(-•) redox couple is essential for the reaction to take place. However, experiments with p-benzoquinone as a superoxide radical scavenger failed, with the opposite effect of enhancing the reaction being observed. It was found that p-benzoquinone and oxygen compete for photogenerated electrons in the conduction band of titania. A redox equilibrium between p-benzoquinone and hydroquinone (H(2)Q), mediated by the O(2)/O(2)(-•) 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/H(2)Q redox system being pivotal in the formation of the hemiacetal intermediate. This intermediate is subsequently converted into the final acetal (solketal), with H(2)Q 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.