Photocatalytic oxidative esterification of benzaldehyde by V2O5–ZnO catalysts

Abstract

Coupled V2O5–ZnO materials were prepared and characterized to be used as photocatalysts to propose a single step oxidative esterification process of benzaldehyde as an alternative route to current used process. The aim was to improve reaction efficiency, decrease reaction time, generate a new alternative method for direct esterification of aldehydes, and discuss the possible mechanism of coupled materials. V2O5–ZnO materials were prepared by precipitating of the individual oxides and annealed at 350 °C. The obtained materials were characterized by several physicochemical techniques (mainly by spectroscopy) and tested in the oxidative esterification of benzaldehyde. ZnO with hexagonal wurtzite and vanadium pentoxide were identified by X-ray in individual oxides as well as in combined oxides. However, the presence of small quantities of the Zn2(V3O8)2 was identified in coupled materials. ZnO and V2O5 phases identified by infrared and Raman spectroscopies were observed in combined catalysts. The scanning electron microscopy images revealed a fine fibrillary morphology for V2O5, particles agglomerates for ZnO and rods with variable length and size for V2O5–ZnO catalysts. ZnO showed a absorption band at 340–200 nm in the ultraviolet region while V2O5 and V2O5–ZnO samples showed absorption in both the visible region (800–400 nm) as well as in part of the UV (400–200 nm) region. Up to 99.5% of benzaldehyde to ethyl benzoate conversion was achieved when the V2O5–ZnO composites were irradiated with UV light. Analysis by infrared and 13C NMR spectroscopies of the irradiated solution showed that ethyl benzoate was the only reaction product identified and quantified. The improved benzaldehyde conversion using V2O5–ZnO catalysts was attributed to a synergy between the two components of the catalysts which suppresses the recombination of charge carriers.