DISPROPORTIONATION OF TOLUENE

The catalytic disproportionation of toluene (Figure 1.1) in the presence of hydrogen produces benzene and a xylene mixture. Disproportionation is an equilibrium reaction with a 58% conversion per pass theoretically possible. The reverse reaction is the transalkylation of xylenes with benzene:

Typical conditions for the disproportionation reaction are 450–530°C and 20 atmospheres. A mixture of CoO-MoO₃ on aluminosilicates/alumina catalysts can be used. Conversions of approximately 40% are normally used to avoid more side reactions and faster catalyst deactivation. The equilibrium constants for this reaction are not significantly changed by shifting from liquid to vapor phase or by large temperature changes.

Currently, zeolites, especially those of ZSM-5 type, are preferred for their higher activities and selectivities. They are also more stable thermally. Modifying ZSM-5 zeolites with phosphorous, boron, or magnesium compounds produces xylene mixtures rich in the p-isomer (70–90%). It has been proposed that the oxides of these elements, present in zeolites, reduce the dimensions of the pore openings and channels and so favor formation and outward diffusion of p-xylene, the isomer with the smallest minimum dimension.

Figure 1.1. The Mobil Oil Corp., IFP process for the disproportionation of toluene to mixed xylenes.