Selective isomerization of n-butenes to isobutene on high Si/Al ratio ferrierite in the absence of coke deposits: implications on the reaction mechanism Original Research Article

High Si/Al ratio ferrierite (Si/Al=59) synthesized using pyridine as structure-directing agent was found to show a high selectivity from very short times on stream (5 min TOS) during the skeletal isomerization of n-butenes. Such a high initial selectivity is accompanied by an unusually high stability with TOS even under reaction conditions that favor the oligomerization of the olefins (low temperatures and high n-butenes partial pressure). In contrast to what has been previously established for ferrierite with lower Si/Al ratios, the isomerization reaction occurs with a high selectivity without appreciable formation of carbonaceous deposits, as determined by elemental and thermogravimetric analysis of the zeolite after reaction. Thus, at 350°C and 0.1 atm partial pressure only about 0.3 wt% carbon was formed after 3.5 h on stream on the high-silica ferrierite (FER59 sample), whereas it amounted to ca. 5 wt% for a commercial low Si/Al ratio ferrierite (Si/Al=8.5, FERTOS sample) under the same reaction conditions. As a result, the acidity and porosity of the FER59 ferrierite remained unaltered after 3.5 h on stream under the above conditions, while both parameters significantly decreased for the FERTOS catalyst. These results, together with the analysis of the initial selectivity with temperature and partial pressure, indicate that isobutene is selectively formed in the ferrierite structure through a monomolecular reaction mechanism without the need of invoking the presence of coke deposits. It appears that reducing the density of acid sites in ferrierite by increasing the framework Si/Al ratio to the required levels is the key parameter to control the stability and selectivity of the zeolite during n-butenes isomerization.