Elsevier

Journal of Catalysis

Volume 125, Issue 2, October 1990, Pages 311-324
Journal of Catalysis

Coke and deactivation II. Formation of coke and minor products in the catalytic cracking of n-Hexene on USHY zeolite

https://doi.org/10.1016/0021-9517(90)90306-5Get rights and content

Abstract

The minor products formed during the conversion of n-hexene on USHY zeolite were examined. GC-MS analyses of the concentrated products show the presence of alkylated monocyclic and polycyclic olefins, alkylated mononuclear and polynuclear aromatics up to and including C 18 species, and acyclic paraffins and olefins up to C21. Relative to the corresponding cycloolefins, only trace amounts of acyclic diolefins were detected, suggesting that dehydrogenation of the surface species is normally preceded by a cyclization reaction. The spectrum of species detected presents a cohesive picture of their development from the linear feed (n-hexene) to mono- and polycyclic species and finally to mono- and polynuclear aromatics. The presence of acyclic C21 species indicates that tetramers of the feed are formed, although the tetramers themselves were not detected. The largest linear molecule detected was pentadecane. 13C CP/MAS-NMR of the coked catalyst indicates that the irreversibly adsorbed surface species consist of aromatic and aliphatic structures. Some highly deshielded structures with chemical shifts up to 150 ppm have been observed as well as structures in the -2-ppm range. Spectra taken at various times on stream indicate that the structure of the irreversibly adsorbed “coke” reaches a steady state within the first 20 s of reaction and does not change subsequently. The loading of carbon in the coke per gram of catalyst follows a similar pattern: a rapid increase in the first 20 s of reaction followed by very little subsequent growth with increasing time on stream. The 29Si MAS-NMR spectra indicate a variation in the Si(lAl) adsorption peak linewidth with time on stream. The pattern of broadening observed suggests that coke is formed in localized regions and distributes itself over the rest of the catalyst surface on a time scale of minutes. This long period of redistribution, coupled with the rapid decay of the catalyst in this system, suggests that both coking and reactant conversion occur mainly near the surface of the zeolite crystallites. After the catalyst is essentially deactivated the coke continues to migrate over the catalyst surface.

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