Visible Light-Driven Selective Aerobic Oxidation of Benzylalcohols to Benzaldehydes by a Cu(acac)2-BiVO4-Admicelle Three-Component Heterosupramolecular Photocatalyst

Abstract

A hybrid photocatalyst consisting of bis(acetylacetonato)copper(II) and monoclinic sheelite bismuth(III) vanadate (Cu(acac)2/ms-BiVO4) has recently been shown to fulfill multiple O2 reduction, exhibiting a high visible-light activity for the oxidation of amines without sacrificial agents (Naya et al. Angew. Chem., Int. Ed. 2014, 53, 13894.). This study shows that the addition of trimethylstearylammonium chloride (C18TAC) in the reaction system drastically promote the Cu(acac)2/ms-BiVO4-photocatalyzed oxidation of benzylalcohol analogues to the corresponding aldehydes with selectivity greater than 99% under visible-light irradiation (λ > 430 nm) at 298 K. The photocatalytic activity in the presence of C18TAC is quite sensitive to the stirring conditions. Under vigorous stirring conditions (rotation speed of stirrer, v = 1200 rpm), the activity increases with increasing C18TAC concentration (Csurf) to go through a maximum at Csurf = 0.2 mM. Under mild stirring conditions (v = 600 rpm), the activity significantly decreases as compared to that for the vigorous stirring system, and also, it decreases with an increase in Csurf at 0 < Csurf < 0.2 mM to reach almost constant at Csurf ≥ 0.2 mM. The dispersion examination of the ms-BiVO4 particles in water suggested that the C18TAC monolayer grows on the ms-BiVO4 surface at 0 < Csurf < 0.2 mM to form an adsorption bilayer (or admicelle) at Csurf ≥ 0.2 mM. The remarkable enhancing effect by the C18TAC admicelle originates from the concentration of the reaction substrate near the ms-BiVO4 surface by incorporating it into the hydrophobic nanospace and the good particle dispersibitily in water due to the electrostatic repulsion between the surface charges. Also, the high selectivity can be attributed to the cooperative effect of the moderate oxidation ability of the ms-BiVO4 valence band holes, the 4-electron reduction of O2 to H2O, and the spontaneous transport of the intermediates from the reaction field to the water phase.