Hydroisomerization reaction, kinetics

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

F. Ribeiro, C. Marcilly, and M. Guisnet, Hydroisomerization of n-hexane on platinum zeolites. I. Kinetic study of the reaction on platinum/Y-zeolite catalysts influence of the platinum content, J. Catal., 78, 261-21A (1982). 

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