Hydrogenolysis, coke effects

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

Effects of Potassium on the Catalytic Behavior of Coked Nickel Catalysts in Hydrogenation and Hydrogenolysis Reactions... 

EFFECTS OF POTASSIUM ON THE CATALYTIC BEHAVIOR OF COKED NICKEL CATALYSTS IN HYDROGENATION AND HYDROGENOLYSIS REACTIONS... 

On the other side, the addition of a second metal like Re, Ir or Sn, which increases the stability of Pt/Al203 catalyst, has been studied extensively in the literature. In summary, the effect of Sn is ascribed to i) a reduction of the number of Lewis sites [17] ii) an increase of selective hydrogenation of coke precursors (dienes) [18] Hi) a suppression of hydrogenolysis reactions [19]. Moreover, Sn can promote alkane oligomerization or even Diels-Alder type reactions [20]. 

In conclusion, bimetallic Pt-Sn/alumina catalysts prepared by successive impregnations with an intermediary reduction step and introduction of the tin salt (SnCU) under hydrogen are less sensitive to coke deactivation than catalysts prepared by coimpregnation. This behavior probably results from a more effective interaction between the two metals, leading to smaller platinum ensembles, as evidenced by the low hydrogenolysis activity. However, the amount of coke deposited on the whole catalyst depends on the nature of the feed and therefore on the nature of the dehydrogenated species which are more or less active precursors for coke deposition on the support. 

The effect of chloride on the activity and selectivity is mainly through the support. It has been noted earlier that, by changing support-metal interactions, chloride minimizes sintering. The formation of large particles would favor hydrogenolysis. In addition, chloride increases the acidity of the support, which improves isomerization and, in some cases, cracking reactions. Increasing the support acidity would also increase the ability of the support to adsorb coke. Hence, there is likely to be more spillover of coke from metal to support, and consequently less coke... 

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