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

摘要

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 nmol g~(-1) per cat) than on Pd/AC (12.2 nmol 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.