Trimethylbenzene disproportionation

In this case the feed is a mixture of toluene and higher aromatics, preferably trimethylbenzenes. Disproportionation and transalkylation occur simultaneously, permitting the manufacture of benzene and xylenes. 

Fig. 1. Disproportionation of trimethylbenzenes. Composition cited in text derived by differences between curves at given temperature.

The Tatoray process, which was developed by Toray Industries, Inc., and is available for Hcense through UOP, can be appHed to the production of xylenes and benzene from feedstock that consists typically of toluene [108-88-3] either alone or blended with aromatics (particularly trimethylbenzenes and ethyl-toluenes). The main reactions are transalkylation (or disproportionation) of toluene to xylene and benzene or of toluene and trimethylbenzenes to xylenes in the vapor phase over a highly selective fixed-bed catalyst in a hydrogen atmosphere at 350—500°C and 1—5 MPa (10—50 atm). Ethyl groups are... 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

This side reaction leads to undesirable losses of xylenes. With REHY zeolite as catalyst, disproportionation occurs at a rate comparable to that of isomerization of m-xylene (8), e.g., 14% disproportionation at 16% isomerization. In fact, the product, trimethylbenzene, is postulated as an important intermediate in isomerization (8). 

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

Figure 13.46 Mechanism for disproportionation of meto-xylene to toluene and trimethylbenzene via diphenlymethane intermediate.

The disproportionation and isomerization of trimethylbenzene(TrMB) were studied at 200°C using a continuous fixed bed reactor. The reactant TrMB was diluted with nitrogen in a molar ratio of 1 9. The cracking of cumene was carried out at 400 C using a pulse reactor. The catalyst was treated in a stream of nitrogen for 1 h at a desired temperature in the range 400-600°C prior... 

Applicability of Monomolecular Rate Theory to Xylene Isomerization Selectivity Kinetics over Fresh AP Catalyst. The kinetics of liquid-phase xylene isomerization over fresh zeolite containing AP catalyst are effectively interpreted by pseudomonomolecular rate theory. The agreement between the experimental data (data points) and predicted reaction paths (solid lines) for operation at 400° and 600°F is shown in Figure 2. The catalyst used was in the form of extrudates comprised of the zeolite component and an A1203 binder. Since xylene disproportionation to toluene and trimethylbenzenes was low, selectivity data were obtained by mere normalization of the xylene compositions (2 axyienes = 1.0). 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

Disproportionation of 1,2,4- trimethylbenzene Al-B, A1-M Al-S NH3 treatment reduced pillar density wdiich can be increased by re-intercalation. Catalytic activity increased with the number of pillars, so was the selectivity for 1,2,4,5-tetramethylbenzene. Al-M have less acidity than Al-S hence greater disproportionation activity. 49 65 66... 

In Table 6.15 the reaction data of toluene disproportionation on the CVD-modified mordenite zeolite catalyst at 400°C are presented.1611 As the Si02 deposition amount increases, the toluene conversion decreases gradually. From the product distribution it is seen that the amount of nonaromatics increases slightly whereas the amount of trimethylbenzene is reduced to zero, and the xylene/benzene ratio drops as well,... 

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... 

The lifetime of the carbenium ion formed will be limited by transferring a proton back to the zeolite, thus completing the dehydrogenation ofthe hydrocarbon. Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig. 22.9) ofthe carbenium ion formed (6) has such a high proton affinity (1031 kj mohh Table 22.1) that proton transfer back to the zeolite does not occur at all. However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alk-oxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mofi more stable than the carbenium ion [9]. 

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. 

In the presence of solid acid catalysts, m-xylene undergoes isomerization forming o-xylene and p-xylene, and disproportionation to produce toluene and a mixture of trimethylbenzene isomers (Fig. 5). 

Time on Stream = Ih TMB = Trimethylbenzene " p-xylene/o-xylene = Isomerization/Disproportionation... 

Figure 13. Reaction pathway in the disproportionation of 1,2,4-trimethylbenzene using Al-pillared clays [79],...

As mentioned earlier in section 2.2, a two-step mechanism via intermediate formation of methanol has been proposed by Adebajo et al, [21-23, 26, 36] for the oxidative mcthylation of benzene with methane over acidic zeolites in a high-pressure batch reactor. In view of this mechanism, a preliminary investigation has been carried out by these workers [24] on the reaction of toluene with methane over acidic ZSM-5 catalysts in a batch reactor containing residual air to determine the actual contribution of direct mcthylation (via intermediate methanol formation) to the observed reaction products. The reactions were carried out at 400 C and 6.9 MPa pressure. The major reaction products obtained by these wwkers were benzene and xylenes. Smaller amounts of ethylbenzene, trimethylbenzene and other higher aromatics were also produced. Over acidic catalysis, the conversion of toluene can, in principle occur through two different reaction pathways mcthylation by methane via methanol (as in the case of benzene mcthylation) and disproportionation, as shown in equations (4) and (S) below ... 

The transalkylation of toluene with trimethylbenzenes (TMB) takes place on solid acid catalysts and it is part of a complex network of reversible reactions. The transalkylation reaction is mainly in equilibrium with the disproportionation reaction and the isomerization of polymethylbenzenes also takes place [2,3]. The distribution of the several isomers is governed by kinetics factors like the reactivity of the... 

The next question was how to minimize xylene loss by disproportionation to toluene and trimethylbenzenes according to the 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. ... 

In a more general way, the changes in the isomerization/ disproportionation ratio on zeolite Y with different Si/Al ratio have been related with changes in the zeolite adsorption capacity occurring during dealumination (182,183). The xylene isomerization on other 12 MR zeolites h been reported (184,185), showing that depending on the size and structure of channels different isomerization/disproportionation ratios are obtained, as well as different distribution in the trimethylbenzenes formed. 

While it was found by means of isotopic studies than on amorphous silica-alumina the reaction proceed by an intramolecular mechanism (194), in zeolite Y, the distribution of isomers in the trimethylbenzene fraction indicates that some of the isomers could be obtained by a bimolecular mechanism (172,175). In a very recent work (196,197) it has been demonstrated by means of isotopic studies, that on some 12 MR zeolites such as Y, and mordenite, xylenes are isomerized by both uni and bimolecular transalkylation mechanism. The ratio of the uni to bimolecular increases when increasing the Si/Al ratio, and decreases when increasing the reaction temperature, the partial pressure of the feed, and the contact time. Another 12 MR, Beta zeolite, while being able to disproportionate xylene, does not isomerize via the bimolecular mechanism. This was explained by space constraints to accommodate a xylene and a trimethylbenzene as a bimolecular intermediate in the channels of the zeolite. A medium pore zeolite (ZSM-5) does isomerize only through a unimolecular 1,2 methyl-shift mechanism. 

The disproportionation of trimethylbenzene is a possible route to produce durene (1,2,4,5-tetramethylbenzene). PILCs have been reported to be catalysts for this reaction, and the shape selective effect was discovered in this reaction from the product distribution with durene being the major product. This disproportionation reaction is catalyzed by the PILC Lewis sites, while the isomerization of trimethylbenzene (side reaction) is well correlated to the Bronsted acidity. 

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