Understanding coke formation and deactivation mechanism on zeolite based acid catalysts

Abstract

Zeolite, a crystalline microporous aluminosilicate, is one of the most important solid acid catalysts applied in oil refinery, petrochemistry, and fine chemical synthesis due to its shape selectivity, strong acidity, and high thermochemical stabilities. Like other solid acid catalysts, zeolites suffer from deactivation resulting from coke formation during various hydrocarbon conversions. It serves as a major drawback in the application of the zeolite materials in high-demand catalytic processes. In this study, coke formation and deactivation mechanism of zeolite were rigorously investigated in the various acid-catalyzed reactions by analyzing the amount, composition, chemical nature, and location of coke generated on the zeolites having different crystallite sizes, secondary mesoporosity, and defect sites. Regardless of the types of the reaction or the reaction conditions, mass transfer of hydrocarbons within the zeolite crystals significantly affected the location of coke, whereas the amount of defect sites determined the total amount of coke deposited on the zeolite catalyst. By combining a variety of complementary analytic techniques, it is possible to propose that the internal coke (i.e. coke deposited within the zeolite micropore) is likely to have the polymeric structures of the methylated acenes connected via methylene bridges. On the other hand, the external coke (i.e. coke deposited on the external surface of the zeolite) is highly polyaromatic with many fused rings. In terms of catalyst deactivation, the internal coke proved to be more detrimental than the external coke. Overall, zeolite having a thin zeolite framework and minimized defect sites can hinder deposition of both internal and total coke amount, thus can serve as a long-lived solid acid catalyst. The present work would provide an important insights for understanding the deactivation behaviors of the zeolite catalysts and improving the eÿciency in various zeolite-based acid-catalyzed reactions.