Study on the deactivation of Ni-based catalyst in the hydrotreating process of waste plastic pyrolysis oil

The deactivation mechanism of Ni-based catalyst in the hydrotreating process of waste plastic pyrolysis oil (WPPO) was investigated. The accelerated deactivation experiment of catalyst was performed with model oil containing different impurities, and results indicated that the deactivation of catalyst was related to chlorides, nitrides, and sulfides in WPPO. By changing the concentration of chloride, the interactions between the three impurities were studied. Combined with the characterization results of fresh and spent catalysts, chloride was the direct cause of the deactivation of the catalyst. Chlorine atoms in chlorides were converted into HCl gas in hydrotreating, which reacted with Ni active sites, resulting in the sintering of the average Ni particle size from 52.62 nm to 139.63 nm, and the loss of Ni active components by 3–8 wt%, especially inhibiting the hydrodesulfurization activity of the catalyst. In addition, HCl reacted with NH3 (byproduct of nitride hydrogenation) to generate NH4Cl. The high concentration of HCl in the reaction system increased the crystallization temperature of NH4Cl from 210 °C to 294 °C, which led to the crystallization of ammonium salts in the catalyst bed. NH4Cl crystals adsorbed on the catalyst surface, resulting in a decrease in specific surface area and pore volume, and an increase in the average pore diameter of the catalyst, which was the main reason for the rapid deactivation of the catalyst. Furthermore, based on the deactivation mechanism, several appropriate solutions were proposed.