Tuning the Ni site location of bifunctional Ni-based catalysts for improving the performance in ethylene oligomerization

Abstract

The location of Ni sites in a Ni-based catalyst is very important for ethylene oligomerization. In this work, a series of bifunctional Ni/Al-MCM-41 samples with carefully controlled location of Ni sites were successfully prepared, by using incipient wetness impregnation (IWI), ion exchange (IE) and physical mixing (PM) methods. Various characterization techniques such as XRD, BET, SEM, TEM, Py-IR, XPS, and NMR were applied to reveal the Ni site distributions and the properties of the prepared catalysts. The catalytic performance of these samples was investigated via ethylene oligomerization in a continuous fixed-bed reactor. The experimental results showed that the Ni sites were distributed mainly on the external surface of the catalyst by the IWI method, while Ni sites were located in the holes by the IE method. In ethylene oligomerization, the IWI catalyst with moderate proximity between the Ni active sites and acid sites demonstrate an enhanced catalytic performance, and shows higher activity with a conversion rate of 95% and a longer catalytic lifetime than IE and PM catalysts. Moreover, the possible relationship between the Ni site location and the catalytic performance of bifunctional Ni/Al-MCM-41 is proposed in detail.