Reaction cavity disproportionation products

A third type of control, called spatiospectffeity, occurs when both reactants and products pass the opening but reaction intermediates or transition states are restricted by the size of the cavity. In xylene isomerization processes, selectivity is lost through disproportionation to toluene and trimethylbenzene. Diphenylroethane intermediates are too large for ZSM-5... 

With medium-pore zeolites (e.g. ZSM-5) very little disproportionation occurs, because the smaller cavities of ZSM-5 cannot easily accommodate this bulky intermediate. Much more of the product is formed through the isomerization reaction proceeding via a mechanism in which only a single molecule of xylene is involved. In this unimolecular xylene undergoes a 1,2-methyl shift... 

An example is the disproportionation of /w-xylene to toluene and trimethyl-benzenes in the wide-pored zeolite Y (Fig. 7-5 c). In the large zeolite cavity, bulky diphenylmethane carbenium ion transition states can be formed as precursors for methyl group rearrangement, whereby the less bulky carbenium ion B is favored. Thus the reaction product consists mainly of the imsymmetrical 1,2,4-trimethylben-zene rather than mesitylene (case A). In contrast, in ZSM-5, with its medium sized pores, monomolecular xylene isomerization dominates, and the above-mentioned disproportionation is not observed as a side reaction. 

The isomerisation of xylenes within H-ZSM-5 provides the textbook example of transition state shape selectivity (Table 8.2). Xylenes isomerise on zeolite H-Y to an equilibrium distribution of o-, m- and p-xylenes within the large cavities. Under the same conditions they also undergo disproportionation to give toluene and a mixture of trimethylbenzenes. This (unwanted) side reaction proceeds via a bimolecular reaction. Over the medium-pore H-ZSM-5, however, only xylenes are observed, enriched in the pnra-isomer by product shape selectivity. Little or no disproportionation is observed. This is a result of transition state selectivity, since there is no space within the ZSM-5 structure for the bimolecular transition state of the disproportionation reaction. ... 

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