A FT-IR study of deactivation phenomena during methylcyclohexane transformation on H-USY zeolites: Nitrogen poisoning, coke formation, and acidity–activity correlations

The present work discloses some of the less clear aspects regarding the deactivation of acidic USY zeolites due to coke formation and poisoning with basic nitrogen molecules (3-methylpyridine, 2,6-dimethylpyridine, and quinoline). Coke formation occurs essentially in the supercages; consequently, the Brønsted acid sites located in these structures lose their activity quite rapidly. Nitrogen-containing molecules also interact preferentially with the Brønsted acid sites located in the supercages. 2,6-Dimethylpyridine has a higher poisoning ability than the other two bases, probably due to steric hindrance, which limits the interaction with the nonactive Lewis sites. (Only Brønsted acid sites are poisoned, and hence deactivation is more pronounced.) This work also demonstrates that quantification of the PyH+ band on pyridine adsorption is an inaccurate method for determining the decrease in the number of Brønsted acid sites during the deactivation of catalytic cracking catalysts. Indeed, only the surface of the hydroxyl vibration bands can be used to determine the decrease in the number of accessible active sites. Acidity–activity correlations show that only the Brønsted acid sites located in the supercages are active in methylcyclohexane transformation; as a result, an average turnover frequency for these sites can be determined with the slope of the activity versus acidity curve (0.33 s−1).