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Résumé :
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Catalytic cracking of methylcyclohexane has been studied on eight commercially available zeolites, five FAUs and three MFIs, and on two newly developed zeotype materials with bimodal porous structure, BIPOMs. Both BIPOMs are composed of an MFI ultramicropore (<1 nm) network and a different supermicropore (1.5−2.0 nm) network. Site time yields obtained on FAU and MFI zeolites with varying acid properties are in the same range, showing that mass transfer limitations inside the pores of both zeolite frameworks are absent. Site time yields obtained on BIPOM3 are comparable to those on commercial MFI with similar Si/Al ratio, while BIPOM1 is significantly less active. Within a given framework type, the zeolite acid properties determine its activity in methylcyclohexane cracking, while the pore topology controls its selectivity. On FAU, methylcyclohexane isomerization, followed by ring opening and subsequent cracking, is the main reaction pathway, while on MFI, protolytic scission, followed by cracking, is predominant. This is explained by the occurrence of transition state shape selectivity in MFI, hampering the bimolecular hydride transfer reaction. These typical features allow one to distinguish between FAU- and MFI-type catalytic behavior and to locate the active sites of BIPOM1 mainly in the supermicropores and those of BIPOM3 in both micropore networks but to a greater extent in the ultramicropores.
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