Mechanistic Insights into the Selective Oxidation of 5-Hydroxymethylfurfural over Au/TiO2 Catalysts under Photoexcited and Dark States

The 5-hydroxymethylfurfural oxidation reaction (HMFOR) is regarded as a promising approach to produce biomass-derived highly valuable chemical products. 5-Hydroxymethyl-2-furancarboxylic acid (HMFCA) as a typical product via selective oxidation of the aldehyde group in HMF was obtained on Au/anatase and Au/rutile catalysts under photoexcited and dark states. The results show that the HMF oxidation reactivities of Au/rutile and Au/anatase were improved under visible light irradiation compared with those under dark conditions, and the increase of the activity is more obvious as the reaction temperature decreases. The kinetic studies showed that visible light irradiation reduced the energy barrier of the HMF oxidation reaction and promoted the reaction activities. The radical capture and EPR experiments were further used to explore the mechanisms of the HMF oxidation reaction under both conditions over Au/TiO2 catalysts. The evidence indicated that the main active species were 1O2 and trace amounts of •O2– under dark conditions, while the main active species were •O2–, 1O2, and h+ under visible light irradiation conditions for both Au/rutile and Au/anatase catalysts.