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 the catalytic activity of these materials as a function of time on stream exhibits a typical shape containing an induction period during which the activity increases. The induction period is followed by a rapid deactivation and then a slower deactivation. The shape of the conversion-time curves contains important information about the way these catalyst operate. Slight changes in the preparation and pretreatment of the catalysts result in changes in the overall activity and shape of the conversion-time curves, The results of this study suggest that the induction period is due to the formation and accumulation of reaction intermediates on the surface. These intermediates participate in the reaction as part of an inter-molecular mechanism. The n-butane molar fraction, the presence of olefins or hydrogen in the feed, and the reaction temperature strongly affect the induction period. A simple mathematical model that successfully describes the behavior of the conversion-time curves has been developed. This model is consistent with the existence of two types of sites with different reactivity and stability. One type is responsible for most of the activity observed during the first few minutes on stream, but it is rapidly deactivated. This type of sites is easily deactivated by simple exposure to hydrogen, so it is consistent with an oxidized species recently proposed. The second type of sites is more resistant to deactivation.