ON THE RELATIONSHIP BETWEEN COKE FORMATION CHEMISTRY AND CATALYST DEACTIVATION

A link has been observed between the decline in catalyst activity and coke formation in the cracking of n-nonane. The link involves the chemistry and kinetics of the reaction, as well as the chemical structure of coke as observed by 13C CP/MAS-NMR spectroscopy. We find that the chemical structure of surface species normally analyzed as coke plays an important role in the activity of the catalyst. This we believe reflects the influence of the structure of surface carbenium ions on their reactivity in chain propagation and in olefin-forming, site-releasing desorption reactions. Both these processes are vital to the maintenance of catalyst activity. As the average surface species becomes more dehydrogenated, it also becomes less reactive both in bimolecular chain propagation with gas phase reactant molecules and in olefin formation by desorption. This time-dependent loss of reactivity of the average carbenium ion manifests itself in a decline in total catalyst activity with time on stream. © 1992.