Ba-Doped Pd/Al2O3 for Continuous Synthesis of Diphenylamine via Dehydrogenative Aromatization

A series of alkaline-earth-metal-modified Pd/Al2O3 nanocatalysts were prepared by an impregnation and kneading method. It was found that all of the catalysts doped by alkaline metals exhibit better catalytic performance in the continuous dehydrogenative aromatization of the Schiff base derived from cyclohexanone and aniline to diphenylamine (DPA) than that of Pd/Al2O3, in which, Pd1/Al3BaO presents a remarkable catalytic performance. X-ray diffraction, CO chemisorption, transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed hydrogen reduction, NH3-temperature-programmed desorption, Fourier transform infrared spectroscopy of pyridine adsorption, and a N2 adsorption–desorption method were employed to explore their structure–activity relationship. It was found that the doped Ba in Pd1/Al3BaO could promote the dispersion of Pd nanoparticles and reduce its acidity compared with that of Pd1/Al2O3. Therefore, the better Pd dispersion facilitated the dehydrogenation of Schiff base to DPA and the lower acidity of catalysts restrained the hydrolysis of the Schiff base and other side reactions, achieving good DPA selectivity. Moreover, Pd1/Al3BaO exhibited good stability. This work provides a promising strategy for the highly selective continuous synthesis of diarylamine or its derivatives from nonaromatic compounds.