Xylene, isomerization

Xylene isomerization Xylene isomers m-Xylenes [108-38-3] o-Xylenes [95-47-6] p-Xylenes [106-42-3] Xylenes... 

In many cases simple esters are more easily purified than their intractable parent acids. For example, /7-xylene is difficult to obtain in pure form because of the closeness of its boiling point to the other isomeric xylenes (o-144°C m- 138.8°C p- 138.5°C). It is thus difficult to produce terephthalic acid, which sublimes at 300°C, in a pure form. 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

Ethylbenzene, Thallium triacetate Ucmura, S. et al., Bull. Chem. Soc., Japan., 1971, 44, 2571 Application of a published method of thallation to ethylbenzene caused a violent explosion. A reaction mixture of thallium triacetate, acetic acid, perchloric acid and ethylbenzene was stirred at 65°C for 5 h, then filtered from thallous salts. Vacuum evaporation of the filtrate at 60°C gave a pasty residue which exploded. This preparation of ethylphenylthallic acetate perchlorate monohydrate had been done twice previously and uneventfully, as had been analogous preparations involving thallation of benzene, toluene, three isomeric xylenes and anisole in a total of 150 runs, where excessive evaporation had been avoided. 

From the solubilities of aromatic hydrocarbons in aqueous silver nitrate it is clear that there, is an interaction with the silver ion to give monoargentated and, to a lesser extent, diargentated hydrocarbons. The bond to silver ion is probably of the rc-type, since the differences in the association constants for the complex with the three isomeric xylenes are small and since a single methyl group stabilizes the complex almost as much as two.279... 

Isomar [Isomerization of aromatics] A catalytic process for isomerizing xylene isomers and ethylbenzene into equilibrium isomer ratios. Usually combined with an isomer separation process such as Parex (1). The catalyst is a zeolite-containing alumina catalyst with platinum. Developed by UOP and widely licensed by them. It was first commercialized in 1967 by 1992, 32 plants had been commissioned and 8 others were in design or construction. See also Isolene II. 

MLPI [Mobil low pressure isomerization] One of a family of processes developed by Mobil Corporation for isomerizing xylene mixtures, using a zeolite catalyst. This one was developed in 1977. See also LTI, MHTI, MVPI. 

Calculated Spin-Density Distribution (p,) in Xylene Cation-Radicals and Determined Partial Factors ((p,) of Ring-Attack Rates for Nitrations of Neutral Isomeric Xylenes with Nitric Acid in Acetic Anhydride... 

For nitration of aromatic hydrocarbons with acetylnitrate, there is a clear linear correlation between the IPs of these hydrocarbons and rate constants relative to benzene (Pedersen et al. 1973). Table 4.4 jnxtaposes spin densities of cation-radicals and partial rate factors of ring attacks in the case of nitration of isomeric xylenes with nitric acid in acetic anhydride. 

XIS [Xylene isomerization] A process for isomerizing />-xylene to the equilibrium mixture of Cg aromatic hydrocarbons. Developed by Marazen Oil in the United States. 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

Interconversion of isomeric xylenes is an important industrial process achieved by HF-BF3 or zeolite catalysts (see Section 4.5.2). Studies of xylenes and tri-and tetramethylbenzenes showed that the amount of catalyst used has a pronounced effect on the composition of isomeric mixtures.83 When treated with small amounts of HF-BF3, isomeric xylenes yield equilibrium mixtures (Table 4.2). Using a large excess of the superacid, however, o- and p-xylenes can be isomerized to m-xylene, which eventually becomes the only isomer. Methylbenzenes are well known to form stable a complexes (arenium ions) in superacids, such as HF-BF3. Since the most stable arenium ion formed in superacids is the 2,4-dimethylbenzenium ion (proto-nated m-xylene, 5), all other isomers rapidly rearrange into this ion. The equilibrium concentration of protonated m-xylene in the acidic phase, consequently, approaches 100%. 

Toluene disproportionation and transalkylation are important industrial processes in the manufacture of p-xylene. Toluene disproportionation [Eq. (5.73)] transforms toluene into benzene and an equilibrium mixture of isomeric xylenes. The theoretical conversion of toluene is 55%. Commercial operations are usually run to attain 42 18% conversions. In conventional processes308 309 324 325 alumina-supported noble metal or rare-earth catalysts are used in the presence of hydrogen (350-... 

In m-xylene the two methyl groups agree in activating the same positions, and this is the only one of the three isomeric xylenes which can be nitrated satisfactorily to yield a trinitro derivative. Since the three isomers occur in the same fraction of coal tar and cannot readily be separated by distillation, it is necessary to separate them by chemical means. When the mixed xylenes are treated with about their own weight of 93 per cent sulfuric acid for 5 hours at 50°, the o-xylene (b.p. 144°) and the m-xylene (b.p. 138.8°) are converted into water-soluble sulfonic acids, while the p-xylene (b.p. 138.5°) is unaffected. The aqueous phase is removed, diluted with water to about 52 per cent acidity calculated as sulfuric acid, and then heated in an autoclave at 130° for 4 hours. The m-xylene sulfonic acid is converted to m-xylene, which is removed. The o-xylene sulfonic acid, which remains in solution, may be converted into o-xylene by autoclaving at a higher temperature. The nitration of m-xylene is conveniently carried out in three steps. The effect of the two methyl... 

The alkylation of aromatic hydrocarbons with methyl alcohol over Nafion-H catalysts, including the mechanistic aspects, has been studied in detail. The degree of conversion of methyl alcohol was much dependent on the nucleophilic reactivity of the aromatic hydrocarbon. For example, the reactivity of isomeric xylenes was higher than that of toluene or benzene. 

For solvent purposes various grades of xylenes (the mixture of isomers and ethylbenzene) are available purification and drying procedures are similar to those used for benzene and toluene. For chemical purposes the commercially available pure isomeric xylenes are usually available in at least 99 per cent purity. 

The rationalizations of signs for HE of binary liquid mixtures presented in Sec. 16.7 apply approximately to the signs of S 2 for binary gas mixtures. Thus, positive Su is the norm for NP/NP, NA/NP, and AS/NP mixtures, whereas is usually negative for NA/NA mixtures comprising solvating species. One expects < 12 to be essentially zero for ideal solutions of real gases, e.g., for binary gas mixtures of the isomeric xylenes. 

Dobrianskii [19] tabulated the boiling points of the isomeric xylenes under various pressures (Table 91). 

Less information exists on the sorption behavior of aromatic hydrocarbons such as benzene, toluene, the three isomeric xylenes, mesitylene, and... 

Vinyl chloride monomer (VCM) Xylene isomerization Xylene isomers... 

---