Meta-Xylene Isomerization

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

Meta-xylene isomerization to ortho- and para-xylene over 10- and 12-MR zeolites is another illustration of product shape selectivity effects [13]. The two products are essentially equally favorable from the standpoint of thermodynamics. With decreasing pore size, however, kinetics come into play and the selectivity to para-xylene increases, as illustrated in Figure 13.37 for results obtained at 317-318°C, 0.5 kPa meta-xylene pressure (in the presence of He carrier gas) and 10% conversion [64]. While the para ortho ratio is typically 1.0-1.5 with multi-dimensional... 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

Let us now consider the methyl shift isomerizations that lead from para-xylene to meta-xylene (see Figure 15). 

Pseudo-cumene from Brom para-Xylene and Brom meta-Xylene. Bl om para-xylene, as will be recalled from the discussion of the constitution of para-xylene, exists only in one form as no isomeric compounds are possible. This substance, by means of Fittig s synthesis, yields pseudo-cumene, the constitution of which, therefore, can only be I-3-4-tri-methyl benzene. 

Brom meta-xylene exists in three isomeric forms (p. 473) and one of... 

Di-nitro Benzene.—Of the three isomeric di-nitro benzenes the meta is the one formed by direct nitration with fuming nitric acid. The proof of its meta constitution is its transformation into meta xylene. 

Of the three isomeric xylenes, each of which yields nitro products, it is the meta-xylene or i-3-di-methyl benzene which is most easily nitrated. The number of isomeric nitro xylenes possible has been previously explained (pp. 472 and 482). In the case of meta-xylene three such nitro compounds are possible but only one is readily obtained. It is I-3-di-methyl 4-nitro benzene that is, the nitro group enters the ring ortho to one methyl group and para to the other. This is just what we should expect from the influence of the methyl group upon subsequent substitution (p. 506). The nitro xylenes are not so important as nitro benzene or the nitro toluenes, but have some use in dyestuff manufacture. 

P4-14(- A total of 2500 gal/h of metaxylene is being isomerized to a mixture of orthoxylene, metaxylene, and paraxylene in a reactor containing I0(X) ft- of catalyst. The reaction is being carried out at 750°F and 300 psig. Under these conditions, 37% of the metaxylene fed to the reactor is isomerized. At a flow rate of 1667 gal/h. 50% of the meta.xylene is isomerized at (he same temperature and pressure. Energy changes are negligible. 

Ortho-xylene is readily separated from the isomeric mixture by fractional distillation and was the first of the isomers to achieve commercial importance as a raw material for oxidation to a dicarboxylic acid, phthalic anhydride. Methods have been perfected by the petroleum industry for separation of the para and meta isomers. Para-xylene is important as the raw material for terephthalib acid, and meta-xylene has recently entered the large commercial scale as raw material for oxidation to isophthalic acid. ... 

Application GT-lsomPX is GTC s xylene isomerization technology available in two versions ethylbenzene (EB) isomerization and EB dealkylation. Both versions gain high ethylbenzene conversion rates while producing equilibrium mixed xylenes—meta-xylene (MX), ortho-xylene (OX) and paraxylene (PX). 

During the early 1970s, Clarence D. Chang, Anthony J. Silvestri, and William Lang, at Mobil Research and Development, discovered Zeolite ZSM-5, one of the most important catalysts ever produced. Zeolite ZSM-5 is a superacidic substance that catalyzes isomerization of ortho-and meta-xylenes to para-xylene the latter s cylindrical shape helps it to diffuse much more rapidly through the zeolite matrix. Quite by accident, they discovered that ZSM-5 catalyzes the conversion of methanol to... 

Xylene isomerization. It is commercially much incentive for isomerizing the less valuable meta-xylene to the ortho and para isomers, and specially, to the more valuable para-xylene. 

It is believed that the isomerization of ethylbenzene to xylenes occur only on bifunctional catalysts, and hydrogenated intermediates are required (190). Selective catalysts for canying out this reaction have to maintain a good balance between the H-DM and the add function. Zeolites, with strong acid functions such as in mordenite or ZSM-5 actively isomerize cycloolefinic intermediates but also catalyze ring opening reactions which lead to a decrease in the formation of xylenes. However, the mild add nature of SAPOS make them specially useful for ethylbenzene isomerization. In this sense, 0.4-0.6 %wt Pt on SAPO-11 and SAPO-5 catalysts were used to isomerize mixtures of ethylbenzene and meta-xylene. Both catalysts produce near-complete xylene equilibration. SAPO-11 was more selective for producing xylenes than SAPO-5 (191). 

The isomerization of a-complexes does have significant commercial importance. For example, this chemistry is used to prepare isomeric xylenes— important feedstock chemicals for aromatic dicarhoxylic acids [74]. The proportion of xylene isomers depends on the conditions nsed, as strongly acidic conditions (i.e., HF—BFj) are employed to favor the meta isomer and shape-selective zeohtes are used to favor the para isomer (the isomer with the smallest cross-sectional area). The thermodynamically most stable a-complex for the protonated xylenes is the areninm ion 89, which leads to the meta xylene product (Scheme 1.25). 

Zhang, C., Hong, Z., Gu, X.H., Zhong, Z.X., Jin, W.Q. and Xu, N.P. (2009) Silicalite-1 zeolite membrane reactor packed with HZSM-5 catalyst for meta-xylene isomerization. Industrial and Engineering Chemistry Research, 48, 4293 299. 

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. 

Cohn points out that position isomerism is of the greatest importance as regards the odours of isomerides, this is strikingly instanced in the case of the tri-nitro tertiary butyl xylenes since the only one possessing the powerful musk odour is that in which the nitro groups are situated each in the meta position to the two others again the ortho-amido-benzaldehyde has a strong odour but the meta and para isomerides are odourless. 

The meta-selectivity for toluene activation, observed for both systems, is very unusual (Fig. 5). Also remarkable is the switch in selectivity from aryl C-H activation to benzylic activation inp-xylene, just by changing the chelate ligand from the diimine equipped with trifluoromethyl substitutents in the meta-positions of the phenyl residue to the diimine bearing methyl substituents in the ortho-positions (Fig. 5). The authors suggested that the C-H bond activation is reversible and the isomeric a-methane complexes are in equilibrium prior to the substitution of... 

When there are two or more substituents on a benzene ring, structural isomerization can result. For instance, there are three possible isomers of xylene (dimethylbenzene). The isomers are commonly designated as ortho, meta, and para. A letter (o, m, or p) designates the isomeric position. 

Synonym dimethyl benzene Formula CSH10 Structures Xylene occurs in three isomeric forms as ortho-, meta-, and para isomers. 

A further improvement of the approach of Wei [107] was reported in 1989 by Hashimoto et al. [42], which considered not only adsorption effects, but also the nonselective reactions occurring at the outer surface of the crystallites. The nonselective influence of these reactions has also been recognized by Fraenkel [35] in 1990, who studied the formation of xylene from toluene on a HZSM-5 catalyst. Fraenkel assumed that inside the crystallite only />-xylene is formed, whereas the ortho and meta isomers are sterically inhibited there. Hence, he concluded that the amount of o- and m-xylene observed during his experiments must be due to the isomerization of p-xylene at the outer surface of the crystallites. This two-step mechanism was first suggested in 1987 by Paparetto et al. [82] for the ethylation of toluene. It may also be worth noting that Fraenkel s model took into account not only the isomerization but also the nonselective alkylation at the outer crystallite surface. 

The spectra of the intermediate transients formed in the reaction of OH with dichloro- and dibromo-benzenes and chloro- and bromo-toluenes exhibited absorption maxima around 325-330 nm with both ortho- and w to-isomers of dichlorobenzenes, dibromobenzenes, and bromotoluenes (Fig. 6). The transients were assigned to the isomeric OH adducts formed from the addition of OH to the benzene ring. A blue shift was observed in the absorption maximum of -bromotoluene (315 nm) when compared to its ortho- and meta-isomers which have maxima at 330 nm. Such a behavior was also seen in the absorption spectra of OH adducts of 0- and w-chlorotoluenes (325-330 nm) and w-xylenes (326-328 nm) as compared to their... 

Since commercial xylol is a mixture of the three isomeric xylenes, ortho, meta and para, the method of bromination mentioned above produces a mixture of the three derivatives ... 

---