Anatase TiO2 Activated by Gold Nanoparticles for Selective Hydrodeoxygenation of Guaiacol to Phenolics

Gold nanoparticles on a number of supporting materials, including anatase TiO2 (TiO2-A, in 40 nm and 45 μm), rutile TiO2 (TiO2-R), ZrO2, Al2O3, SiO2, and activated carbon, were evaluated for hydrodeoxygenation of guaiacol in 6.5 MPa initial H2 pressure at 300 °C. The presence of gold nanoparticles on the supports did not show distinguishable performance compared to that of the supports alone in the conversion level and in the product distribution, except for that on a TiO2-A-40 nm. The lack of marked catalytic activity on supports other than TiO2-A-40 nm suggests that Au nanoparticles are not catalytically active on these supports. Most strikingly, the gold nanoparticles on the least-active TiO2-A-40 nm support stood out as the best catalyst exhibiting high activity with excellent stability and remarkable selectivity to phenolics from guaiacol hydrodeoxygenation. The conversion of guaiacol (∼43.1%) over gold on the TiO2-A-40 nm was about 33 times that (1.3%) over the TiO2-A-40 nm alone. The selectivity of phenolics was 87.1%. The products are mainly phenolic compounds with no aromatics and saturated hydrocarbons such as cyclohexane. The gold particle size ranging from 2.7 to 41 nm and water content were found to significantly affect the Au/TiO2-A-40 nm catalyst activity but not the product selectivity. The reaction rates of 0.26 and 0.91 (min-1 g-cat-1 cm3) were determined for guaiacol hydrogenation and catechol hydrogenation, respectively. Bimolecular methylation was established as the dominant mechanism for methyl group transfer among the phenolics. Two major pathways of guaiacol hydrogenation to phenolics over the 0.4Au-19 nm/TiO2-A-40 nm are proposed: (1) direct hydrogenation of guaiacol to form phenol and methanol, (2) hydrodehydroxylation of catechol intermediate from the transmethylation between guaiacol and phenol.