Toluene disproportionation thermodynamic equilibrium

At temperatures above 450°C ZSM-5 is a very effective catalyst for the disproportionation of toluene. A process has been developed and put into commercial practice (2). The thermodynamic equilibrium composition (11) is listed in Figure 7. The product obtained with ZSM-5 contains less of the highly substituted aromatics, as a result of diffusion and transition-state inhibition, such that the process can be approximated by the equation ... 

Figure 7. Thermodynamic equilibrium, in mole percent, for toluene disproportionation at 327°C and 523°C.

The main source of pX is the mixed xylenes cut, containing the four C8 aromatics including EB. The mixed xylenes are produced by catalytic reforming (82%), steam cracking (9%) and toluene disproportionation or toluene-heavy aromatics transalkylation (9%). The xylenes distribution is generally close to thermodynamic equilibrium values, while EB content depends on the origin of the cut (Table 9.1). 

An interesting way to increase pX production of an aromatics complex is to transform less valuable aromatics such as toluene and C9+ aromatics into xylenes. This is achieved either by toluene disproportionation (TDP) or by toluene C9+ aromatics transalkylation. There are two types of disproportionation processes Normal TDP produces a xylene cut where pX content is at thermodynamic equilibrium, while with STDP pX content is much higher (para-selectivity). This is possible when a selectivated zeolite (generally MFI) is used. For normal TDP as well as for transalkylation processes, mordenites doped with a hydrogenating metal to limit coking are used as catalysts. 

The CLD modification of zeolites can also effectively enhance the shape selectivity of catalytic reactions. Gao and coworkers used this method to adjust the pore size of HZSM-5 and effectively enhanced the shape selectivity of the zeolite for toluene-disproportionation reaction (see Table 6.18).[67] It is seen that as the SiC>2 deposition amount is increased, the pore size of the zeolite decreases gradually, and the toluene conversion reduces slightly but the p-X/XX value increases from 0.28 to 0.41, that is, the para-selectivity is enhanced by 46% the p-X concentration exceeds the thermodynamic equilibrium value. 

Under these conditions all molecular sieves evaluated give essentially complete isomerization of m-xylene feed to a thermodynamic equilibrium mixture of xylene isomers, while the disproportionation activity to toluene and trimethylbenzenes varies significantly. The results of this study are summarized in Table III and in Figure 3, where xylene disproportionation activity is plotted as a function of molecular sieve pore size. In general, a rough trend can be seen in which the molecular sieves with larger pore sizes are more active for this undesirable side reaction. 

Axens/ExxonMobil Mixed Xylenes Toluene In the MTDP-3 process, the feed is a mixture of fresh and recycled toluene. The mixed feed is combined with hydrogen-rich recycle gas, preheated and passed through the reactor where disproportionation reactions are promoted. Benzene and equilibrium xylenes are produced as a result of the thermodynamics equilibrium. 3 2007... 

The thermodynamic equilibria are illustrated in Figures 1 and 2. Figure 1 shows the resulting composition after pure pseudocumene or a recycle mixture of C9 PMBs is disproportionated with a strong Friedel-Crafts catalyst. At 127°C (400 K), the reactor effluent contains approximatdy 3% toluene, 21% xylenes, 44% C9 PMBs, 29% C10 PMBs, and 3% pentamethylbenzene. The equilibrium composition of the 44% C9 PMB isomers is shown in Figure 2. Based on the values at 127°C, the distribution is 29.5% mesitylene, 66.0% pseudocumene, and 4.5% hemimellitene (Fig. 2). After separating mesitylene and hemimellitene by fractionation, toluene, xylenes, pseudocumene (recycle plus fresh), C10 PMBs, and pentamethylbenzene are recycled to extinction. 

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