Mitigating coking during methylcyclohexane transformation on HZSM-5 zeolites with additional porosity

by F. Ngoye, L. Lakiss, Z. Qin, S. Laforge, C. Canaff, M. Tarighi, V. Valtchev, K. Thomas, A. Vicente, J.P. Gilson, Y. Pouilloux, C. Fernandez, L. Pinard

just appeared in the Journal of Catalysis (Volume 320, December 2014, Pages 118–126)

DOI: 10.1016/j.jcat.2014.10.001

This work is one the result of a 3 years collaborative work between our lab (LCS) and the IC2MP institute (University of Poitiers).  The research was funded by the ANR, French Research Agency (Project HIZECOKE, ANR BLANC 2010).

Abstract of the work:

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.