FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions

Abstract

The acidity of two dealuminated HY zeolites, obtained either by isomorphous substitution or by hydrothermal treatment at 550°C followed by acid leaching, was characterized by IR spectroscopy in dynamic or static modes. The probe molecules used were 2,6-dimethylpyridine, pyridine and deuterated acetonitrile. They showed that the steamed sample presented very strong acidic Brønsted sites, characterized by OH groups giving rise to a ν (OH) band at 3600 cm −1, and Lewis acidity. The steamed sample was found active for n-hexane cracking at 400°C. Use of an IR cell working as a flow reactor and specific poisoning of hydroxyl groups by 2,6-dimethylpyridine evidenced the catalytic role played by the 3600 cm −1 OH groups. On the other hand, cyclohexene conversion, a much less demanding reaction, occurred on both types of zeolite. All the accessible acidic hydroxyls appeared to be active and results were discussed according to the various reactions observed: cyclohexene isomerization, hydrogen transfer and cracking. The intensity of a band at 1586 cm −1, assigned to carbonaceous deposits, was correlated to catalyst deactivation. The results showed that the 3600 cm −1 OH groups were first consumed under cyclohexene flow, due to a rapid poisoning by coke. Adsorption of probe molecules on deactivated samples evidenced that the Lewis acid sites were almost unaffected whatever the reaction, suggesting that they do not play a significant role. This was confirmed by comparison of activity measurements on both types of zeolite towards cyclohexene conversion.