Study of the mechanism of the cracking of n-hexane over zeolites I. The cracking chain length of n-hexane over zeolites

The effects of zeolite structure on the mechanism of n-hexane cracking have been studied. The results can be explained in terms of the chain mechanism of cracking reaction through the integration of mono- and bimolecular reaction mechanisms. Based on this mechanism, a special parameter called cracking chain length (CCL)″ has been proposed. A high value of CCL means that bimolecular reaction of chain propagation predominates, such as the w-hexane cracking over large-pore Y and β zeolites. Whereas, a low value of the paramete indicates that the monomolecular route is the main cracking pathway, for instance, n-hexane cracking over small-pore ferrierite zeolite. Moreover, the minimum value of CCL has been obtained by theoretical derivation. Comparing with other parameters which characterize zeolite pore structure or relative contribution of reaction pathways, CCL not only can be employed to express the share of mono- and bimolecular reactions, but also tightly correlates to zeolite pore structure. Hence, CCL is an important parameter for researchers to study paraffin cracking mechanism and relating product distribution of cracking to zeolite structure.