Platinum-alumina catalyst coke deposition

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

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. 

Rapid deactivation occurs when n-dodecane is dehydrogenated over platinum-alumina without any diluent or with an inert diluent such as nitrogen. The rate of deactivation is decreased greatly when hydrogen is used as a diluent. However, even with hydrogen dilution, a slow deactivation (accompanied by carbonization of the catalyst) occurs. Eventually it is necessary to regenerate the catalyst by combustion of the coke deposits and reactivation in hydrogen. 

Influence of the Support on the Coke Formation. The reactions involved in coke formation are catalyzed by the acid function, for example, the oligomerization of olefins. Therefore, the acidity plays also a key role in the amount and nature of the coke deposits. It is well known that the amount of coke deposited on catalysts were the Pt is supported on nonacidic materials is very low. One of such examples is the Pt/KL-zeolite, were the amount of coke deposited during the reforming of ra-heptane is much lower than the corresponding deposit on Pt/Al203. Platinum supported on alumina that was neutralized with potassium hydroxide produces very small amount of coke. 

Platinum in the dual-function catalyst is important in reducing the steady-state olefin concentration, otherwise polymerization and cracking would lead to extensive coke formation. Some coke, however, does deposit slowly and the catalyst has to be regenerated at intervals. Mordenite supports are more stable than chlorided aluminas and can operate for longer at more than 60% conversion and about 97% selectivity. Catalysts are reported to operate for as long as seven years before being replaced. 

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