An in situ Fourier transform infrared spectroscopic investigation of coke formation on HY cracking catalysts was undertaken using NH4Y catalysts calcined between 300 and 538”C, with l- hexene and 4-methylcyclohexene used separately as reactants. Two types of active sites are proposed for the formation of coke. One site consists of the Bronsted acid hydroxyl groups present after low-temperature calcination. The second type of site involves a sort of synergism between stronger Bronsted acid hydroxyl groups and neighboring Lewis acid cationic sites, formed after high-temperature calcination. This dual-site suggestion for coking parallels similar suggestions made previously for cracking reactions on these catalysts. The composition of the coke on the catalysts exhibited a distinct lack of aromatic character, suggesting that the route to coke formation need not always proceed through aromatic precursors.
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