Catalytic Evaluation of Reduction Amination of Phenol to Aniline over Palladium Supported Zirconia Catalysts

A series of palladium catalysts supported on zirconia, with varying Pd loadings (0.5–6 wt.%), were synthesized via the impregnation method and characterized using XRD, surface area and pore analysis, UV–DRS, XPS, H₂-TPR, CO₂-TPD, and CO chemisorption. XRD patterns revealed the formation of a crystalline PdO phase for Pd loadings above 2 wt.%. Pore size distribution analysis indicated a reduction in both pore volume and diameter with increasing Pd content. Furthermore, the intensity ratio of the Pd 3d _5/2 and Zr 3d _5/2 XPS peaks correlated well with Pd dispersion values obtained from CO chemisorption. UV–DRS and TPR analyses identified two distinct types of palladium species on the ZrO ₂ support: highly dispersed PdO observed at lower reduction temperatures and bulk PdO forming at higher temperatures. Catalysts basicity increased with Pd loading, then declined at higher loadings. The results demonstrate that the number of exposed surface Pd active sites increases with Pd loading, reaches a plateau at 2 wt.% Pd, and remains essentially constant at higher loadings. Catalytic activity in the vapor-phase reductive amination of phenol to aniline increased up to 2 wt.% Pd and declined at higher loadings, consistent with trends in active Pd sites and basicity. The higher TOF for smaller Pd particles, which decreases with increasing particle size up to 3 nm and remains nearly constant for larger particles. The catalytic activity was found to depend strongly on Pd crystallite size, confirming that phenol reductive amination is a structure-sensitive reaction.