Relation between properties and performance of zeolites in paraffin cracking

The effects of zeolite structure, acid site density, and reaction temperature on the mode of n-hexane cracking have been studied. The results can be consistently explained in terms of the occurrence of two different cracking routes, viz. the classical β-scission route and the monomolecular protolytic pathway. The relative contribution of each of the two cracking routes is expressed by the “cracking mechanism ratio” (CMR): a high value of this index points to a relatively high contribution of the protolytic cracking route, whereas a low value indicates that the classical β-scission route is the main cracking pathway. With increasing temperature, decreasing aluminium content, and decreasing pore dimensions the relative contribution of the (monomolecular) protolytic cracking route increases as compared to the (bimolecular) classical route. On the basis of these results the variation of the constraint index (i.e., the ratio of the first-order rate constants for conversion of n-hexane and 3-methylpentane) with aluminium content and reaction temperature can be easily rationalized. Furthermore, the present results also provide an explanation for the change in the activation energy of n-hexane cracking with aluminium content in zeolite ZSM-5 (MFI). Finally, it is tentatively concluded that the classical cracking route is favoured by the presence of two adjacent acid sites and by low reaction temperatures. Consequently, the relation between the reaction rate constant and the lattice aluminium content changes with the reaction temperature; whereas at 811 K the cracking constant varies linearly with the aluminium content, at 623 K the order in the aluminium content increases to about 2.