Unraveling the real active sites of an amorphous silica–alumina-supported nickel catalyst for highly efficient ethylene oligomerization

Abstract

Ethylene oligomerization is a versatile method for higher olefin production that can be utilized in a broad range of industries and has a large global economic impact. Nickel (Ni) aluminosilicates have been widely investigated in heterogeneous oligomerization processes since they show comparable performance to homogeneous catalysts. However, an understanding of the nature of Ni active sites remains elusive, which hinders the rational design of improved catalysts. Herein, we applied in situ EPR, DRIFTS, and XPS techniques to disentangle the fundamental issues using amorphous silica–alumina (ASA) supported Ni as the catalyst. By varying the pretreatment atmosphere, the oxidation state of Ni was regulated. The majority of the Ni species were found to exist in Ni²⁺, Ni⁺, and Ni⁰ states when pretreated in air, N₂, and H₂ respectively. Among these, the Ni⁺ sites showed unprecedented activity in ethylene oligomerization. On the basis of our results, it is proposed that Ni⁺ directly takes part in the formation of active sites for ethylene oligomerization.