Many zeolite catalysts currently used commercially are already hierarchical: FCC, hydroisomerization and hydrocracking being the most important examples in oil refining. In these cases, hierarchization is brought by steam and/or acid treatments where Al is selectively removed from the framework. Recenly, other methods have been rediscovered (caustic treatment where Si is removed to a greater extent than Al), studied in-depth by modern techniques and new methodologies created (fluoride treatment where Si and Al are removed at and equal rate). Moreover, the production of nanosized zeolite crystals is equivalent to hierarchization as a sizable (mesoporous) surface is developped on their external surface.<br/>This contribution focuses on the effect of hierarchization on coking and regeneration of ZSM-5 zeolite catalysts in hydrocarbon (methylcyclohexane, methanol and ethanol) transformations. Two parent ZSM-5, one made of micron-sized crystals and the other containing nano-sized crystals were studied as well as their offsprings produced by caustic and fluoride treatments.<br/>The time on stream behavior of the various catalysts shows the advantages of hierarchization and particle size. The deactivated catalysts are extensively characterized by their residual acidity and pore volumes, nature and quantity of the carbonaceous residues and ease of decoking.<br/>One important feature is that nanosized and hierarchical zeolite crystals produce less of the very toxic coke molecules (located at channels intersections) because most coke precursors diffuse efficiently to the external surface where their deleterious effect is minimal. Nanosized ZSM-5 are sytematically better than any hierarchical zeolites produced by post-syntesis treatment due, among other factors, to the much lower diffusion pathways molecules have to follow in the microporous channels.<br/>Another important feature of hierarchical and nanosized zeolites is a much easier removal of coke by combustion: temperatures required to completely remove coke from hierarchical or nanosized zeolite catalysts are up to 150 K lower than on their parent.

Coke management on zeolites: The benefits of hierarchization

249th National Meeting (2015)

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