Oligomerization of n-butenes over Ni/SiO2–Al2O3: influence of support modification by steam-treating

Abstract

The acidic properties of a selected SiO₂–Al₂O₃ support material have been modified by steam-treating to study the influence on the formation of active nickel sites for butene oligomerization. For this purpose, the steam-treated supports were impregnated in situ with Ni (Cp)₂. FTIR spectroscopy of pyridine adsorption revealed that the steaming procedure provokes changes of Brønsted as well as Lewis acid sites. In dependence on intensity of the steam-treatment, the amount of Brønsted acid sites decreases and weaker Al³⁺ Lewis acidic sites are created possibly in alumina-like agglomerates. Changes of the Al coordination sphere by steam-treatment was also confirmed by ²⁷Al MAS NMR spectroscopy. The decrease of Brønsted acidity of the SiO₂–Al₂O₃ support lowers its catalytic activity for the proton-catalyzed butene oligomerization, but the formation of active nickel sites on the differently treated supports under reaction conditions is not affected, as reflected by comparable activity of the respective Ni catalysts. As demonstrated by pyridine adsorption studies, mainly the weak Al³⁺ Lewis acidic sites (fivefold coordinated Al) created by steam-treatment were covered by Ni(Cp)₂ impregnation, resulting in the formation of new, medium strong Ni²⁺ Lewis acidic sites which are active in coordinative catalysis. Although the Brønsted sites are only partly replaced by Ni, the remaining Brønsted sites have no influence on octene isomer distribution. It seems that the coordinative mechanism of oligomerization is generally preferred on Ni-impregnated samples.