Al-MCM-41: its acidity and activity in cyclohexene conversion

Abstract

Investigation of cyclohexene conversion over Al-MCM-41 has shown that the concentrations of Brønsted and Lewis acid sites increase with the Al content of the materials. The conversion follows mainly two mechanisms: cyclohexene skeletal isomerisation (CSI) and hydrogen transfer (HT). The products with 6 carbon atoms in a molecule prevail in all cases. Some amounts of products with 1 to 8 carbon atoms are also formed as a result of cracking or alkylation. All these reactions can occur at the acidic centres of the catalysts. The process of conversion, proceeding over the Brønsted acid sites, results in the formation of coke deposits in addition to volatile products. The amount of coke decreases with increasing reaction temperature and decreasing Al content of the material. The deposits cause a decrease in the effective concentrations of both the Brønsted and Lewis acid sites. Thermodesorption of pyridine has shown that (i) the concentrations of the Brønsted and Lewis acid sites before and after the conversion processes differ only slightly and (ii) the acidic strength of the Brønsted sites is almost independent of their concentration and the Si/Al ratio.