INDUCTION OF ACTIVITY AND DEACTIVATION OF FE, MN-PROMOTED SULFATED ZIRCONIA CATALYSTS

The isomerization of n-butane on sulfated zirconia catalysts promoted by the addition of Fe and Mn ions has been studied. The evolution of t he catalytic activity of these materials as a function of time on stre am exhibits a typical shape containing an induction period during whic h the activity increases. The induction period is followed by a rapid deactivation and then a slower deactivation. The shape of the conversi on-time curves contains important information about the way these cata lyst operate. Slight changes in the preparation and pretreatment of th e catalysts result in changes in the overall activity and shape of the conversion-time curves, The results of this study suggest that the in duction period is due to the formation and accumulation of reaction in termediates on the surface. These intermediates participate in the rea ction as part of an inter-molecular mechanism. The n-butane molar frac tion, the presence of olefins or hydrogen in the feed, and the reactio n temperature strongly affect the induction period. A simple mathemati cal model that successfully describes the behavior of the conversion-t ime curves has been developed. This model is consistent with the exist ence of two types of sites with different reactivity and stability. On e type is responsible for most of the activity observed during the fir st few minutes on stream, but it is rapidly deactivated. This type of sites is easily deactivated by simple exposure to hydrogen, so it is c onsistent with an oxidized species recently proposed. The second type of sites is more resistant to deactivation.