Consequence of heterogeneity of active sites for reactivity mechanism of n-butane isomerization over SO42-/ZrO2-Al2O3 catalyst

The relationship between the heterogeneity of active sites and isomerization mechanism of n-butane over alumina-promoted sulfated zirconia (SZA) was elucidated by two series of catalysts with different surface properties. The incomplete removal of coke deposition leads to a decrease in overall activity without the reduction of the rate of isobutane formation. Subjecting the fresh SZA catalyst to the thermal treatment at 650 degrees C causes a striking decrease in activity, resulting from the decrease in the superacidity and oxidizability of the catalyst. While the thermal treatment at 600 degrees C only selectively suppresses the side reactions. It is found that the peaks in NH3-TPD profiles at 500-700 degrees C are caused by a redox reaction between strongly adsorbed ammonia and superacid sites by in situ NH3-TPD-MS measurements. Thus, this peak reflects the strength of superacidity and oxidizability. At the beginning of the reaction, the effective oxidative and protonative activation routes result in the higher concentration of intermediates, which facilitates the "dimerization-isomerization-cracking" reactions. The incomplete removal of coke and the appropriate decrease in the density of sulfate species on the surface selectively promoted the monomolecular pathway.