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Isomerization of small alkanes is compared over two bifunctional catalysts with similar concentrations of metal and acid sites and similar strenths of Bronsted acid sites. The catalytic performance and the kinetic parameters are significantly different for Pt/H-Mor and Pt/ZrO 2 SO 4 catalysts. Under conditions where n-butane isomerization is rapid over Pt/ZrO 2 SO 4 , it is undetectable over Pt/H-Mor. The apparent activation energy of n-pentane isomerization is significantly higher over the zeolite based catalyst. These differences can be rationalized in terms of different pore geometries: in the one-dimensional pore system of mordenite the rates of monomolecular reactions are single file diffusion controlled; as a result, bimolecular processes such as isomerization via intermediate dimer formation are all but impossible. Likewise, hydride ion transfer, which is the chain propagation step in alkane reactions via carbenium ions, is sterically inhibited in micropores and replaced by a process involving Pt-proton adducts. |
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