Propene aromatization over alkali-exchanged ZSM-5 zeolites

The transformation of propene was carried out over alkali-exchanged ZSM-5 zeolites. Compared with H-ZSM-5, the alkali-exchanged ZSM-5 showed lower activity but higher selectivity to benzene among aromatics. Furthermore, they displayed activity for dehydrogenation of cyclohexane to form benzene. Infrared results using both pyridine and pyrrole as probe molecules revealed that these alkali-ZSM-5 samples do not contain Brønsted acid sites, however strong Lewis acid sites and also weak Lewis basic sites are present. The oligomerization of propene and the further dehydrocyclization are suggested to comprise the reaction route for benzene formation, the active sites for these reactions are Lewis acid sites, which should be regarded as acid-base pairs of alkali—cation and the neighbouring basic oxygen. A mechanism involving these Lewis acid—base pairs is proposed for the observed products of methylpentene.