Influence of palladium precursors on structural properties and phenol hydrogenation characteristics of supported palladium catalysts

摘要

Pd(OOCCH_3)_2, PdCl_2, and Pd(NH_3)_4Cl_2 were used to study the influence of palladium precursors on structural properties and phenol hydrogenation characteristics of Al_2O_3- and MgO-supported palladium catalysts. Palladium dispersion as well as phenol conversion is higher over MgO-based catalysts with the respective palladium precursors. Pd(OOCCH_3)_2 shows good metal dispersion and phenol hydrogenation activity over both of the supports. Pd(NH_3)_4Cl_2 leads to lower dispersion of Pd, probably due to the presence of inherent NH_3 (a reducing agent) and facile surface mobility of the precursor. The presence of chlorided Pd species in reduced Pd/Al_2O_3, originating from precursor PdCl_2, was confirmed by UV-visible spectroscopy. The initial activity of phenol hydrogenation is directly proportional to the palladium area over the catalysts prepared with Pd(OOCCH_3)_2 and Pd(NH_3)_4Cl_2; the activity shows a negative deviation over the catalysts originating from PdCl_2. Palladium precursors do not have a significant influence on the distribution of products. Al_2O_3-based catalysts are totally selective for cyclohexanone, produce cyclohexanol as the minor product. A comparison of the product distributions and the overall conversion of phenol over the MgO-based catalysts reveals that changes in product selectivity cannot be attributed merely to a change in the conversion level and also that one product does not form at the expense of another. Al_2O_3-based catalysts show initial deactivation, while MgO-based catalysts show strong resistance to deactivation.