Preparation of NiOx/SiO2–Al2O3 catalysts by a homogenous precipitation method and their catalytic activity for ethylene oligomerization

Abstract

NiOx/SiO2–Al2O3 catalysts with different pore structures (BET specific surface area: 303–709 m2 g−1, average pore size: 2.2–5.1 nm), acid properties (total amount of acid sites: 0.99–1.66 mmol g−1), and NiO particle sizes (4.6–14.7 nm) were prepared, and the structural factors influencing their catalytic activity for ethylene oligomerization were studied. SiO2–Al2O3 support was prepared by means of a homogeneous precipitation method, and supports with different physicochemical properties were obtained by changing the preparation conditions. Ni was loaded by means of an impregnation method. Catalytic reactions were performed in a fixed-bed down-flow reactor operated at 300 °C and 0.1 MPa. NiOx/SiO2–Al2O3 catalysts prepared with the homogenous precipitation method had a higher surface area, more uniform pore size, and a larger amount of acid sites, and they showed a higher activity not only compared with NiOx/SiO2–Al2O3 catalysts prepared by means of our previous coprecipitation method but also compared with Ni-AlSBA-15, the best heterogenous catalyst for ethylene oligomerization reported to data. The catalytic activities of the NiOx/SiO2–Al2O3 catalysts strongly depended on the BET specific surface area and the amounts of acid sites. The optimal amounts of weak, medium-strength, strong, and total acid sites were 0.5, 0.3, 0.4, and 1.2 mmol g−1, respectively. Examination of the structure–activity relationships of the catalysts suggested that their catalytic activity was promoted by close contact between the Ni and acid sites.