Influence of Supports on Structure and Performance of Nickel Phosphide Catalysts for Hydrodechlorination of Chlorobenzene

SiO2, TiO2, gamma-Al2O3, and HY zeolite supported phosphide catalysts were prepared by the hydrogen temperature-programmed reduction method from phosphate precursors. The physicochemical properties of the catalysts were characterized by means of N-2 adsorption-desorption, hydrogen temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, hydrogen temperature-programmed desorption, inductively coupled plasma atomic emission spectroscopy, energy-dispersion X-ray spectroscopy, and thermal gravimetric analysis. The catalyst performance in the hydrodechlorination of chlorobenzene was evaluated in a fixed-bed reactor at atmospheric pressure. It has been found that the support property remarkably affects the formation of nickel phosphides. With the same Ni/P molar ratio (about 0.7) in the precursors, Ni2P is prepared on SiO2 and TiO2; however, Ni and Ni3P form on gamma-Al2O3 and Ni and Ni12P5 form on HY. This phenomenon is attributed to some phosphorus reacting with gamma-Al2O3 and HY to form AIM, and the phosphorus reacting with nickel is scarce. Under identical reaction conditions, the hydrodechlorination performance of the catalysts decrease in the order of SiO2-supported N2P, gamma-Al2O3-supported Ni-Ni3P, TiO2-supported N2P, and HY-supported Ni-Ni12P5. The catalyst performance is closely related to the properties of active phases and hydrogen species. Nickel phosphides have better performance than metallic nickel due to the electron deficiency of nickel, and the spilt-over hydrogen species also contribute to the hydrogenolysis of C-Cl bond. The chlorobenzene conversion exceeds 99% over SiO2-supported Ni2P during 130 h at 573 K. The excellent performance is ascribed to the strong poison resistance of Ni2P to chlorine and the abundant hydrogen species. TiO2-supported N2P and HY-supported Ni-Ni12P5 have good initial activities; however, their deactivation is remarkable, especially HY-supported Ni-Ni12P5. Their deactivation is mainly owing to the carbonous deposition.