Selective Hydrogenation Of Phenol Over Heterogenous Catalysts

Cyclohexanone is an important organic compound in the manufacture of nylon, and billions of kilograms of cyclohexanone are produced annually. Until to now, cyclohexanone is popularly produced via the selective oxidation of cyclohexane over cobalt catalysts in air. Alternatively, cyclohexanone can also be synthesized via the selective hydrogenation of phenol, but hydrogenation of phenol requires high temperature and several byproducts, such as cyclohexanol and cyclohexane, formed along with the production of cyclohexanone. How to increase the selectivity of cyclohexanone became a hot topic in recent years. It was concluded that the product distribution during phenol hydrogenation depends strongly on both the type of active metal and on the properties of supports, and the co-added acid and/or the acidity of catalysts can improve the conversion of phenol. However, achieving a high selectivity of cyclohexanone (>90%) at elevated phenol conversions (>80%) over a single catalyst remains a challenging work.First, a series of Pd catalysts were prepared on different supports (Fe2O3, SiO2, ZnO, MgO, Al2O3, carbon and Amberlyst-45) and used in the selective hydrogenation of phenol to cyclohexanone in water. The Amberlyst-45 supported Pd catalyst(Pd/A-45) was highly active and selective under mild conditions (40-100℃, 0.2-1MPa), giving a selectivity of cyclohexanone higher than 89% even at complete conversion of phenol. Experiments with different Pd loadings (or different particle sizes) confirmed that the formation of cyclohexanone was a structure-sensitive reaction, and Pd particles of 12-14 nm on Amberlyst-45 gave better selectivity and stability.At the same time, a series of core-shell structured Ru@G-CS catalysts with very fine dispersed Ru nanoparticles encapsulated in N-doped graphene layers were prepared via a simple pyrolysis of the mixture of RuCl3, glucose and melamine. Characterizations indicated the morphologies and properties of Ru@G-CS nanocomposites depended strongly on pyrolysis temperature (500-800℃). it seemed that the pyrolysis temperature of around 700℃ was critical, it was found Ru@G-CS-700 sample showed small particle-size cystalline Ru NPs encapsulated ultrathin graphene layers (onlyl-2 layers) highly anchored on CS. And the as-formed Ru@G-CS-700 catalyst achieved the maximum catalytic hydrogenation activity of phenol to cyclohexanol than that of other catalysts under 20℃, and also exhibited an excellent stability. Moreover, this Ru@G-CS-700 catalyst was also active for the hydrogenation of other phenolic derivatives and aldehdye derivatives, which provided a promising approach to prepare high-grade diesel fuels from renewable resources.