Selective toluene disproportionation model

Impregnation of ZSM-5 with different compounds produce small changes in the zeolite pore dimensions which increases the selectivity to para-xylene (168,169) as can be seen from the results in Table 12 taken from (169). Iliese results can be explained on the bases of a model which considers the interplay between the relative rates of the toluene disproportionation, product diffusion, and xylene isomerization (170), as it is schematized in Figure 32, taken from (170), in where D is the diffusion coefficient, K is the rate constant, and r is the crystal radius. The reactions considered are Isomerization "(I) and Disproportionation (D), while the reaction products are benzene (Bz), toluene (I and xylenes (P,M,0). Initial products (i), primary products (o), and secondary products (P,M,0) arc considered in this scheme. 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Benzyl alcohol is a typical model substrate to test a chosen heterogeneous catalyst and the reaction conditions. Apart form the formation of benzalde-hyde, disproportionation, and dehydration can occur to give toluene or dibenzyl ether (Scheme 52). Therefore, selective, active, and recycla-... 

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