A DFT Study on the Mechanism of Active Species in Selective Photocatalytic Oxidation of Toluene into Benzaldehyde on (WO3)3 Clusters

Abstract

AbstractIt is of great significance to transform toluene into value‐added aldehydes under mild conditions, but the oxidation mechanism is still unclear, which seriously hinders the development of this direction. To reveal the nature of photocatalytic reaction, we studied the reaction mechanism of photocatalytic selective oxidation of toluene into benzaldehyde (BAD) on (WO3)3 clusters by density functional theory calculation, and analyzed the effect of active species on the reaction, including electrons, holes, dissolved oxygen (O2), superoxide radical anions (⋅O2−), hydroxyl radicals (⋅OH) and hydroperoxyl radicals (⋅OOH). The results show that ⋅OH and ⋅OOH play a major role in the reaction, from which we summarized two main pathways for BAD production. The direct oxidation of toluene to BAD is dominated by ⋅OOH and has higher selectivity. The oxidation process dominated by ⋅OH first converts toluene to benzyl alcohol (BA), and then BA continues to be oxidized to BAD. This path is more thermally advantageous because the dehydration energy barrier of C6H5CHOHOH (IM5) (22.3 kcal mol−1) is lower than that of C6H5CH2OOH (IM2) (42.9 kcal mol−1). However, the selectivity of this path is low because ⋅OH and holes can easily oxidize BAD to benzoic acid. Notably, water molecules reduced the energy barrier for dehydration of IM2 and IM5 by 12.4 kcal mol−1 and 13.0 kcal mol−1, respectively. The results elucidate the mechanism of free radicals involved in the photocatalytic reaction at molecular level.