Mechanistic Pathways for Olefin Hydroisomerization and Aromatization in Fluid Catalytic Cracking Gasoline Hydro-upgrading

The reactivities of model hydrocarbons (n-paraffins, i-paraffins, olefins, naphthenes, and aromatics) typical for fluid catalytic cracking (FCC) gasoline were investigated over a Ni-Mo/modified HZSM-5 + Al2O3 catalyst. Olefins had the highest reactivity and were unidirectionally converted into molecules of the other groups, especially i-paraffins and aromatics. On the basis of the results, a reaction mechanism was proposed for olefin hydroisomerization and aromatization in the presence of excess hydrogen. During the olefin hydroisomerization and aromatization, the initial adsorption sites of the olefins are the acid sites rather than the metal sites of the bifunctional catalyst. The olefin hydroisomerization reaction is in accordance with the hydrogen spillover concept. The function of the metal sites is to dissociate hydrogen molecules into active H ions, and the acid sites are the reaction sites. The olefin aromatization in the presence of hydrogen occurs through diene and cyclo-olefin intermediates by hydrogen transfer/dehydrogenation and cyclization. The results obtained form the fundamental basis for the olefin conversion in the presence of hydrogen and shed a light on the correlation between the reactivity of model compounds and that of real gasoline with complex compositions.