Cracking experiments using 2,2,4-trimethylpentane as a model component have been performed on five FAU and three MFI zeolites. In addition to these eight commercially available catalysts, two newly developed zeotype materials with bimodal pore structure, BIPOMs, have been investigated. Both BIPOMs possess an MFI ultramicropore (<1 nm) network but a different ordered supermicropore (1.5-2.0 nm) network. Site time yields are lower on MFI than on FAU because of the slower diffusion of the reactant inside the pores. The site time yield obtained on the BIPOMs is comparable to commercial MFI with similar Al content. Within one framework type, the zeolite acid properties determine its activity in catalytic cracking of 2,2,4trimethylpentane, while the framework topology controls its selectivity. The main reaction route on FAU is hydride transfer followed by β-scission leading to mainly C 4 species, while on MFI protolytic scission is responsible for the formation of high amounts of C 1 -C 3 species. This points to the presence of transition state shape selectivity in MFI. These features allow to distinguish between FAU and MFI type catalytic behavior and to locate the active sites of BIPOM1 in the supermicropores and those of BIPOM3 in both micropore networks.
Single‐event microkinetic (SEMK) modeling is applied to catalytic cracking of 2,2,4‐trimethylpentane on a series of faujasites with Si/Al ratio ranging from 2.6 to 30. Standard activation entropies of the various elementary reaction families are calculated a priori from transition state theory and statistical thermodynamics, while activation energies are estimated on a reference faujasite by regression to experimental kinetic data. The SEMK model is then extended with two acidity descriptors. The concentration of active sites is available from independent NH3‐TPD measurements, while the change in standard protonation enthalpy, relative to the reference faujasite, is obtained by regression to experimental kinetic data. The latter parameter accounts for the effect of the zeolite average acid strength both on the stability of the intermediates and on the activation energies of the protonation and protolytic scission reactions. For these five commercially available faujasites, a variation in standard protonation enthalpy of 29 kJ mol−1 was found. © 2012 American Institute of Chemical Engineers AIChE J, 2012
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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