Ethylene oligomerization over NiO /SiO2-Al2O3 catalysts prepared by a coprecipitation method

Thirty-three types of NiOx/SiO2-Al2O3 with different pore structures (BET specific surface area: 32–565 m2 g−1), acid properties (total acid sites: 0.32–1.50 mmol g−1), and NiO crystallite sizes (3.8–20.6 nm) were prepared and evaluated as catalysts for ethylene oligomerization. SiO2-Al2O3 support was prepared by a coprecipitation method, and Ni was loaded on it by an impregnation method. Catalysts were characterized by nitrogen adsorption, powder X-ray diffraction, scanning electron microscopy, temperature-programmed reduction under hydrogen, and temperature-programmed desorption of ammonia. Catalytic reactions were performed in a fixed-bed, down-flow reactor operated at 300 °C and 0.1 MPa. The preparation conditions of the SiO2-Al2O3 support (i.e., type of precipitant, Al2O3 content, and aging temperature) greatly influenced the structure and activity of the resulting NiOx/SiO2-Al2O3 catalysts. The highest activity was obtained when SiO2-Al2O3 was prepared using aqueous ammonia as the precipitant, the Al2O3 content was 15%, and the aging temperature was 70 °C. The activity of this NiOx/SiO2-Al2O3 catalyst was comparable with that of Ni-AlSBA-15, which was the highest activity heterogenous catalysts reported to date. The activity of NiOx/SiO2-Al2O3 increased linearly with increasing BET specific surface area and number of acid sites of the catalyst, but it decreased with increasing Ni surface density. We speculate that highly dispersed Ni species and (weak) acid sites together improve the activity of the developed catalysts.