Selective photo-oxidation of lignin-based alcohols to aldehydes catalyzed by gold nanoparticles supported on titanate nanotubes and nanowires

Abstract

Photocatalytic conversion of biomass and its derivatives is a promising strategy for producing energy and products of industrial interest. In this study, the selective photooxidation of lignin-derived alcohols (vanillyl and veratryl) to their respective aldehydes was investigated using gold nanoparticles (AuNPs) supported on hydrogen titanate nanotubes (HTNTs) and nanowires (HTNWs) under visible light and O₂ at room temperature and pressure. The results show that the AuNPs/HTNTs and AuNPs/HTNWs systems are highly active and selective in the photooxidation of vanillyl and veratryl alcohol, with conversions above 90 % and selectivity above 95 % toward the formation of vanillin and veratraldehyde, respectively. The preparation of the photocatalytic systems by functionalization with (3-aminopropyl)trimethoxysilane and subsequent deposition–reduction with NaBH₄ of Au in successive cycles allowed us to obtain metal loadings of 1, 3 %, and 5 % by mass, controlling the size, dispersion, and stability of the nanoparticles AuNPs. Physicochemical characterization of the solids showed that the incorporation of AuNPs into HTNT and HTNW supports led to an increase in oxygen vacancies, a decrease in the charge transfer resistance of the material, and thus a lower recombination rate of electron–hole pairs. Further studies in the presence of scavenger molecules and electron paramagnetic spectroscopy allowed us to conclude that under visible light, the AuNPs generate photoelectrons that move toward the conduction band of the support, where the reduction of O₂ occurs to form the superoxide radical anion, the main reactive oxygen species causing the oxidation of alcohols.