Mitigating coking during methylcyclohexane transformation on HZSM-5 zeolites with additional porosity
Résumé
The effects of two different hierarchization procedures (alkaline and fluoride leaching) on the performances of ZSM-5 catalysts in the transformation of methylcyclohexane at 723 K are highlighted and discussed in relation to their porosities. The hierarchical catalysts exhibit different porosities; namely, the fluoride treatment leads to a zeolite combining micropores and macropores, while alkaline leaching adds mesopores interconnected with the native micropores. While the initial activities and selectivities of catalysts derived from the three zeolites are very similar in the conversion of methylcyclohexane, the presence of mesopores (alkaline leaching), close to the active sites, greatly improves the stability of such a hierarchical catalyst by favoring the desorption of products. This behavior is similar to a reduction in zeolite crystal size. This increased stability is not due to a decrease of the coke toxicity, but rather to an inhibition of the growth of coke precursors, in turn related to the shorter diffusion paths of reactants and products. Two types of coke are present on the meso-/micro-porous zeolite: (i) a "light coke" composed of alkylbenzenes strongly adsorbed on Lewis acid sites and silanols, (ii) a "heavy coke" (alkylphenanthrenes and alkylpyrenes) trapped at the intersection of the zeolite channels. While the light coke has no impact on the catalyst stability, the heavy coke poisons active sites, most probably remote from the mesopores