Impacts of Ni-Loading Method on the Structure and the Catalytic Activity of NiO/SiO2-Al2O3 for Ethylene Oligomerization

Abstract

To clarify the Ni species of NiO/SiO2-Al2O3 catalysts that are active for ethylene oligomerization, 18 types of NiO/SiO2-Al2O3 were prepared using three Ni-loading methods (i.e., ion-exchange, impregnation, and homogeneous precipitation), with different Ni-loadings (1–20 wt%), and examined with respect to their structure and catalytic activity for ethylene oligomerization. Characterized by N2 adsorption, powder XRD, FE-SEM, H2-TPR, NH3-TPD, and C2H4-TPD showed that Ni species in the catalysts prepared by ion-exchange were mainly ion-exchanged Ni cations. In contrast, Ni species in the catalysts prepared by impregnation were a mixture of ion-exchanged Ni cations and NiO particles, and those in the catalysts prepared by homogeneous precipitation were all NiSiO3 particles. Catalytic-reaction tests at 300 °C and 0.1 MPa revealed the following: the ion-exchanged Ni cations showed the highest C2H4 conversion rate; the NiSiO3 particles showed a moderate reaction rate; and the NiO particles were not active for ethylene oligomerization. We concluded that the high catalytic activity of the ion-exchanged Ni cations was a result of their high dispersion and medium-strength acidity, which together promoted the adsorption and activation of ethylene on, and the desorption of oligomerization products from, the catalyst.