Boosting the Visible-Light-Driven Selective Toluene Oxidation via Synergistic Effect Between Nanoparticulate Pd/BiVO₄ Photocatalyst and a Cyclic Nitroxyl Redox Mediator

Selective oxidation of organic compounds using semiconductor photocatalysts has been focused on the viewpoint of sustainable synthesis processes. Especially, the selective oxidation of C(sp³)-H bonds in toluene, which is one of the C-H activation reaction, is an attractive reaction because the desired oxidation product, benzaldehyde, is extensively used as an indispensable intermediate in the synthesis of drugs, perfumes, and fine chemicals. Conventional synthesis of benzaldehyde involves TiO₂-based photocatalysts, however, they have brought the drawback of absorbing only UV light and the low selectivity due to its large bandgap. Therefore, developing visible-light responsible photocatalyst system for efficient toluene oxidation has been strongly desired to construct an alternative way to realize sustainable organic synthesis.<br/>Recently, we reported photo-oxidation of toluene to benzaldehyde over visible-light responsible Pd-loaded BiVO₄ (Pd/BiVO₄) photocatalyst. However, it remains challenging due to its low efficiency. In this work, we introduce a novel strategy to apply a cyclic nitroxide compound as an efficient redox mediator along with Pd/BiVO₄ for toluene oxidation. Addition of a cyclic nitroxide significantly improved the toluene oxidation activity on Pd/BiVO₄ photocatalyst under blue-to-green light irradiation. In particular, <i>N</i>-hydroxyphthalimide (NHPI) boosted the benzaldehyde production up to ca. 28 times compared to without NHPI. The optimized conditions realized a toluene conversion in high efficiency (apparent quantum yield = 10.8 and 5.4 % at 420 and 520 nm, respectively) and with excellent selectivity. The superior conversion yield was realized by the synergistic effect between the presence of NHPI and the use of nanoparticulate Pd/BiVO₄ with a relatively large surface area.