Structure and activity of fluorinated alumina. 2. Nature of the active site for 2-methylpropene oligomerization

Abstract

The oligomerization of 2-methylpropene catalyzed by fluorine-containing aluminas is studied in pulse, flow, and static reactors. The number of active sites of two series of fluorinated aluminas was measured by poisoning using 2,6-dimethylpyridine and n-butylamine. This number is compared with (i) the number of hydroxyl groups, measured by quantitative reaction of hexamethyldisilazane and the surface hydroxyl groups, and (ii) the number of Brønsted sites measured by infrared spectroscopy. It is concluded that the active sites are Brønsted sites which are formed by part of the hydroxyl groups. The influence of preparation methods on the structure of the catalysts is given. It is shown that catalysts which were prepared by impregnating alumina with an aqueous solution of aluminum nitrate and ammonium fluoride have a relatively higher surface area than catalysts prepared by other methods. A surface model for fluorinated alumina is presented. The reaction products are analyzed and identified. Mainly dimerization to trimethylpentenes occurs. It is shown that the products which are formed can be explained by a reaction mechanism based on the formation of a classical carbonium ion. The role of irreversibly adsorbed surface polymers as active centers is discussed.