Catalytic Hydrodeoxygenation of Guaiacol over Palladium Catalyst on Different Titania Supports

Pd catalysts supported on TiO2 with different crystalline phases were prepared with formaldehyde as reducing agent and examined for hydrodeoxygenation (HDO) of guaiacol. Their properties were characterized by N2 adsorption, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Compared to the carbon-supported Pd catalysts, TiO2-supported Pd catalysts exhibited higher C–O bond scission ability, which may be attributed to the presence of partially reduced titanium species originating from the reduction of Ti4+ by spillover hydrogen from Pd at 200 °C on the surface of TiO2. Guaiacol was hydrogenated on Pd sites to give 2-methoxycyclohexanol, which diffused to partially reduced titanium species and subsequently reacted with hydrogen from Pd to generate cyclohexane. Anatase TiO2-supported Pd catalyst gave the highest HDO activity of guaiacol among the Pd catalysts supported on three types of TiO2 (anatase, rutile, and their mix, P25), suggesting that more partially reduced titanium species are in favor of the HDO reaction because anatase is facile to reduce by H2 at 200 °C. Higher selectivity of cyclohexane for Pd/TiO2 reduced at 500 °C than that reduced at 200 °C further confirmed that the enhanced C–O bond scission ability of Pd/TiO2 is mainly attributed to the partially reduced titanium species on the surface of TiO2.