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 bimolecu lar mechanism that involves the participation of olefins and oligomeri c carbenium ions on the surface, it was found that the activity of the catalysts under continuous flow operation was much higher than that o btained in the pulse mode. However, when small quantities of l-butene were added to the pulses, the activity was greatly enhanced. This enha ncement significantly varied with the type of metal promoter present i n the SZ catalyst. A strong influence of the purity of the n-butane us ed on activity and sensitivity to hydrogen was observed. When pure n-b utane 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 PtS Z and NiSZ catalysts and is primarily due to a decreased deactivation. When n-butane with olefin impurities was used as a feed, the effect o f hydrogen was only evident on the PtSZ catalyst. The NiSZ catalyst di d not show the inhibition at low temperatures nor the enhanced stabili ty at high temperatures exhibited when n-butane of high purity was use d. It was found that the formation of coke has a direct relationship w ith the low temperature activity. A comparison of the different cataly sts investigated under various conditions reflected a trend: the lower the temperature at which n-butane can be converted, the more rapid de activation and the higher the amount of coke formed. (C) 1997 Elsevier Science B.V.