Trimethylbenzene isomerization

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. 

When 2-isopropyl-l,3,5-trimethylbenzene is heated with aluminum chloride (trace of HCl present) at 50°C, the major material present after 4 h is l-isopropyl-2,4,5-trimethylbenzene. Suggest a reasonable mechanism for this isomerization. 

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). 

Side Reactions One of the major side reactions that occurs during isomerization of Cg aromatics is transalkylation. This reaction produces species such as toluene, trimethylbenzene, methylethylbenzene, dimethylethylbenzene, benzene and diethylbenzene. The types and specific isomers of transalkylated products formed depend on the acidity and spatial constraints of the zeolitic catalyst used. These reactions can be controlled through modification of catalyst properties, especially pore size and external acidity, though these reactions are still among the major contributors to xylene losses. 

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). 

A variation on the aryne mechanism for nucleophilic aromatic substitution (discussed above, Scheme 2.8) is the SrnI mechanism (see also Chapter 10). Product analysis, with or without radical initiation or radical inhibition, played a crucial role in establishing a radical anion mechanism [21]. The four isomeric bromo- and chloro-trimethylbenzenes (23-X and 25-X, Scheme 2.9) reacted with potassium amide in liquid ammonia, as expected for the benzyne mechanism, giving the same product ratio of 25-NH2/23-NH2 = 1.46. As the benzyne intermediate (24) is unsymmetrical, a 1 1 product ratio is not observed. 

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. 

Catalytic isomerizations of ethylbenzenes and xylenes over zeolites are commercial processes and have been used as test reactions of acid catalysts. Corma and Sastre26 have recently suggested that xylenes can form via transalkylation of trimethylbenzene which is believed to be an intermediate in the isomerization of p-xylene. A general scheme as that shown in Eq. 626 was proposed on the basis of kinetic and mass spectrometric data. The reactant p-xylene was believed to produce m-xylene as a primary product but also rearranges in the pores of ultrastable faujasite zeolites to form o-xylene which appears as a primaiy product. In addition, trimethylbenzenes were formed along with toluene. 

Further isomerization of m-xylene as well as transalkylation of trimethylbenzene and toluene to form m-xylene can occur. Evidence for the bimolecular transalkylation mechanism was provided by observation of a peak at m/e 109 in the mass spectra for CD3 substitution of toluene. These data rule out unimolecular 1,2-methyl shifts as the sole means of formation of xylenes. The higher the A1 content of the ultrastable faujasite the greater the extent of bimolecular transalkylation. These observations have significant implications for unimolecular kinetic models that have been proposed as well as reported activation energies and turnover frequencies. 

As with HZSM5-la, we attribute the initial deactivation to blocking of catalytically active sites by adsorbed xylene molecules preventing toluene methylation to occur at these sites. The longer residence time of the bulkier xylene isomers in the larger crystals of HZSM5-2 (see Table 1) seems to favour further alkylation of m- and o-xylene to trimethylbenzenes over isomerization to p-xylene Once trimethylbenzene is formed, dealkylation is rather difficult at 573 K and its rate of transport is too low to be able to diffuse out of the zeolite pores. It forms, thus, a dead end product that decreases the availability of active sites and reaction intermediates (leading to slow deactivation). 

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). 

In their studies of conductance of the methylbenzenes in anhydrous hydrofluoric acid, Kilpatrick and Luborsky (96) found that the specific conductance of solutions of prehnitene and durene changed with time, and additional experiments indicated this was due to rearrangement toward isodurene. It should be noted that the symmetrical configuration 1,2,4,5- is the strongest base of the three tetramethylbenzenes. The concentration and stability of the ArH+ ions are the important factors in the isomerization. With the xylenes and trimethylbenzenes in anhydrous hydrofluoric acid, no reaction was observed at 20°, but isomerization did take place upon addition of boron trifluoride. This is interpreted to be due to an increase in the concentration of ArH+ by the reaction... 

The photolysis of perfluoro(l,3,5-trimethyl)benzene yields perfluorotrimethyl-prismane 19 The latter was considered to be formed via a Dewar benzene. The prismane was isomerized to another Dewar compound which is an intermediate of perfluoro(l,2,4-trimethylbenzene) (see Eq. 16). There are many reactions where prismanes are postulated as intermediates. Some of them are described in the review of Bryce-Smith and Gilbert 29). 

There are only three isomeric trimethylbenzenes. Write their structural formulas, and name them. 

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... 

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... 

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... 

This can be done by Friedel and Crafts catalysts, and indeed classical ones as AlClj and BFj-HF were primarily used (172). The isomerization is accompanied by a transalkylation reaction giving toluene and trimethylbenzenes ... 

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. 

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