The nature of the active site for the isomerization of 1-butene over magnesium oxide

Abstract

The nature of the reaction of 1-butene over MgO has been investigated. The isomerization rate at 26°C was measured in a flow system as a function of the catalyst pretreatment temperature which was varied from 300° to 900°C. The activation energy and the initial selectivity (cis/trans ratio) for the reaction at each pretreatment temperature was determined. The concentration of V[subscript OH], V, and S center surface defects and the concentration of Mg⁺² and strongly basic O⁻² ions were also measured as a function of the pretreatment temperature. The results showed that the sites on MgO that are responsible for the isomerization reaction are dependent upon the pretreatment temperature and pass through a maximum concentration on MgO pretreated at 700°C. Constant activation energies and turnover rates indicated that the isomerization site did not change in type as the pretreatment temperature was increased. The initial cis/trans ratios of the product 2-butene were found to be dependent upon the pretreatment temperature and reaction temperature. As the reaction rate increased, the initial selectivity decreased. The cis/trans ratios that were observed indicated that the isomerization reaction proceeds via a carbanion intermediate. Results from infrared studies indicated that butene was physically adsorbed on MgO and remained olefinic in character. No evidence was found for the existence of adsorbed π-complexes or π-allylic species. A correlation was found to exist between the sites on MgO that adsorb TCNE and the 1-butene isomerization rate. The data indicated that the O⁻² ions that are located at the comers of the cubic oxide lattice are possibly the sites that adsorb TCNE, as well as the sites that are active in the isomerization of 1-butene. No correlation was found to exist between the isomerization rate and the concentration of surface defects and exposed Mg⁺² ions.