Comparison of the different promoting effects of Fe-Mn, Ni and Pt on the n-butane isomerization activity of sulfated zirconia catalysts

The n-butane isomerization has been studied on a series of transition metal-promoted sulfated zirconia catalysts. In support of the bimolecular mechanism that involves the participation of olefins and oligomeric carbenium ions on the surface, it was found that the activity of the catalysts under continuous flow operation was much higher than that obtained in the pulse mode. However, when small quantities of l-butene were added to the pulses, the activity was greatly enhanced. This enhancement significantly varied with the type of metal promoter present in the SZ catalyst. A strong influence of the purity of the n-butane used on activity and sensitivity to hydrogen was observed. When pure n-butane feed was used, the presence of hydrogen had a strong inhibiting effect on activity at low temperatures, but a promoting effect at high temperatures. This promoting effect was particularly prominent on PtSZ and NiSZ catalysts and is primarily due to a decreased deactivation. When n-butane with olefin impurities was used as a feed, the effect of hydrogen was only evident on the PtSZ catalyst. The NiSZ catalyst did not show the inhibition at low temperatures nor the enhanced stability at high temperatures exhibited when n-butane of high purity was used. It was found that the formation of coke has a direct relationship with the low temperature activity. A comparison of the different catalysts investigated under various conditions reflected a trend: the lower the temperature at which n-butane can be converted, the more rapid deactivation and the higher the amount of coke formed. (C) 1997 Elsevier Science B.V.