Influence of Palladium Precursors on Structural Properties and Phenol Hydrogenation Characteristics of Supported Palladium Catalysts

Abstract

Pd(OOCCH3)2, PdCl2, and Pd(NH3)4Cl2 were used to study the influence of palladium precursors on structural properties and phenol hydrogenation characteristics of Al2O3- and MgO-supported palladium catalysts. Palladium dispersion as well as phenol conversion is higher over MgO-based catalysts with the respective palladium precursors. Pd(OOCCH3)2 shows good metal dispersion and phenol hydrogenation activity over both of the supports. Pd(NH3)4Cl2 leads to lower dispersion of Pd, probably due to the presence of inherent NH3 (a reducing agent) and facile surface mobility of the precursor. The presence of chlorided Pd species in reduced Pd/Al2O3, originating from precursor PdCl2, 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(OOCCH3)2 and Pd(NH3)4Cl2; the activity shows a negative deviation over the catalysts originating from PdCl2. Palladium precursors do not have a significant influence on the distribution of products. Al2O3-based catalysts are totally selective for cyclohexanone production, whereas MgO-based catalysts, along with 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. Al2O3-based catalysts show initial deactivation, while MgO-based catalysts show strong resistance to deactivation.