Hydrodeoxygenation of 4-Methylphenol over Unsupported MoP, MoS2, and MoOx Catalystst

A study of the hydrodeoxygenation (HDO) of 4-methylphenol over unsupported, low-surface-area MoS2, MoO2, MoO3, and MoP catalysts is reported. With the exception of MoO3, the catalysts had the same physicochemical properties before and after the 5 h reaction at 623 K and 4.40 MPa H-2. The used MoO3 was partially reduced to a mixed oxide containing Mo4O11, MoO2, and Mo. Compared to the unused MoO3, the used MoO3 CO uptake increased by a factor of 100 following the reaction. The partially reduced Mo oxide catalyst had a high conversion for the HDO of 4-methylphenol because of Bronsted acid sites and the formation of anionic vacancies. The catalyst turnover frequency (TOF) based on CO uptake for the HDO of 4-methylphenol decreased in the order MoP > MoS2 > MoO2 > MoO3, while the activation energy increased in the order of MoP < MoS2 < MoO2 < MoO3. The activity trends correspond to the increased electron density of the Mo among the catalysts. Two primary reactions, C-O hydrogenolysis to yield toluene and saturation of 4-methylphenol followed by rapid dehydration to produce 4-methylcyclohexene, were identified. The catalysts differed in their hydrogenation and isomerization capabilities. The MoP catalyst displayed the highest selectivity toward hydrogenated products, suggesting that the rate-limiting step over MoO3, MoO2, and MoS2 was the saturation of 4-methylphenol to produce 4-methylcyclohexanol.