The mechanism of coke formation on silica-alumina cracking catalysts was studied by analysis of cracking experiments with seven pure hydrocarbons at 445' C. and n-hexadecane at 500" C. under a variety of process conditions. Coke deposits were isolated from selected catalyst samples for infrared spectral examination. Aromatic skeletal vibrations were observed a t 1580 to 1590 cm.-' Although the cokes were low in hydrogen content, it was possible to identify -Ctlz, -CH3, and aromatic -CH groups by infrared absorption in the C-H stretch region. Mechanism of coke formation involves initial adsorption of hydrocarbons followed by chemical reactions of adsorbed materials. The latter include condensation reactions followed b y hydrogen elimination either by direct dehydrogenation to form hydrogen gas or by a hydrogen transfer process. Coke formation in fixed beds is shown to b e a complex function of length of

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A high-temperature infrared cell for recording spectra of solids and adsorbed species at temperatures up to 650°and pressures from 10~5 to 760 mm is described. Thus, spectra recorded at 427°show that hydrogen faujasite (HY) produced by calcining NH4Y under vacuum to 427°contains three different hydroxyl groups having absorption frequencies of 3740, 3635, and 3540 cm-1. These groups readily exchange with deuterium gas to form their OD analogs. When hexene-1 is adsorbed on HY at 93°, the double-bond character disappears. At 150°, polymerization and dehydrogenation processes begin to occur to form a conjugated polyene type of structure as evidenced by a band at 1600 cm-1. Upon heating to 260°, cyclization occurs to form a hydrogen-deficient aromatic ring structure characterized by a band at 1580 cm-1. With the exception of ethylene, which formed no adsorbed species capable of detection, similar results were observed with other low molecular weight olefins, although differences in extent of reaction were found. The condensed-ring structure was not produced on other ion-exchanged forms of zeolite Y with the possible exception of AgY in which partial reduction could have occurred to form some HY. The main reaction on other ion-exchanged forms involved the loss of double-bond character.

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