Effect of structural properties on catalytic performance in citral selective hydrogenation over carbon–titania composite supported Pd catalyst

Abstract

A novel C/TiO₂ composite supported palladium catalyst (Pd/C/TiO₂) was prepared. The effects of the structural properties on the catalytic performance in citral hydrogenation were estimated from calculations and compared with a commercial activated carbon supported palladium catalyst (Pd/AC). The results showed that although the reaction rate was comparatively low over Pd/C/TiO₂, which took approximately 24 h to reach 90% citral conversion, fourfold the time of Pd/AC, the selectivity for citronellal in citral hydrogenation remained high (approximately 85%) at the same citral conversion, while decreasing to 40% over Pd/AC. The comparatively lower reaction rate over Pd/C/TiO₂ was attributed to the fewer surface Pd sites (3.94 μmol g⁻¹ per cat) than on Pd/AC (12.2 μmol g⁻¹ per cat). Further calculations discovered similar initial turnover frequency values over the two catalysts (approximately 0.1 s⁻¹), which implied that citral hydrogenation is structure-insensitive over Pd catalysts and that crystallite size effects have little influence on the differences in the kinetics between the two catalysts. The high selectivity for citronellal over Pd/C/TiO₂ was due to the negligible internal diffusion limitation inside the catalyst, which was proved by calculating the Weisz–Prater numbers (less than 0.3 of each reactant). In contrast, the pore structures, mainly composed of micro pores, caused serious internal diffusion limitation over Pd/AC, which finally led to the increase of the selectivity to the deeply hydrogenated product, 3,7-dimethyloctanol.