The effect of surface acidic and basic properties on the hydrogenation of aromatic rings over the supported nickel catalysts

Ni/Al2O3, Ni/MgAlO and Ni/MgO catalysts containing about 60 wt% of nickel were prepared by the co-precipitation method. The textural and structural properties were characterized and it was found that the dispersion of nickel was high in these catalysts. The active nickel surface area was found to be high (78 m2/gcat.) in Ni/MgAlO, corresponding to the average nickel particle size of 3.5 nm. Microcalorimetric adsorption of NH3 and CO2 showed that the Ni/Al2O3 and Ni/MgO exhibited strong surface acidity and basicity, respectively, while the Ni/MgAlO possessed both surface acidity and basicity. In addition, both the initial heat and coverage were higher on Ni/Al2O3 than on Ni/MgO for the adsorption of toluene, indicating the strong interaction between the aromatic rings in toluene (that may act as a Lewis base due to the enriched electron densities) and the surface acidic sites on the support. The adsorption of toluene on the metallic nickel surface produced higher heat, indicating the strong interaction of the π electrons in aromatic rings of toluene with the d orbitals of surface nickel atoms. Although the adsorption of H2 showed the higher active surface nickel area in Ni/MgAlO than in Ni/Al2O3, the activities of hydrogenation of toluene and phenol were significantly higher on Ni/Al2O3 than on Ni/MgAlO, indicating the important effect of surface acidity on the hydrogenation of aromatic rings.