Xylenes ortho isomer

Poro-xylene is an industrially important petrochemical. It is the precursor chemical for polyester and polyethylene terephthalate. It usually is found in mixtures containing all three isomers of xylene (ortho-, meta-, para-) as well as ethylbenzene. The isomers are very difficult to separate from each other by conventional distillation because the boiling points are very close. Certain zeoHtes or mol sieves can be used to preferentially adsorb one isomer from a mixture. Suitable desorbents exist which have boiling points much higher or lower than the xylene and displace the adsorbed species. The boihng point difference then allows easy recovery of the xylene isomer from the desorbent by distillation. Because of the basic electronic structure of the benzene ring, adsorptive separations can be used to separate the isomers of famihes of substituted aromatics as weU as substituted naphthalenes. 

A particular selectivity was observed in the isomerization of m-xylene over MCM-41 mesoporous aluminosilicate. When compared to silica-alumina, MCM-41 exhibits very similar acidity characteristics and activities, and the disproportionation to isomerization ratios are not very different. However, there exists a large difference in the relative rates of formation of the para and the ortho isomers namely, the ortho isomer is preferentially formed (ortho para > 2.5). This was attributed to the presence of the regular, noninterconnected channels of MCM-41, in which xylene molecules undergo, before desorption, successive reactions of disproportionation and transalkylation. 

These isomers can be distinguished on the basis of their l3C-NMR spectra because they have different symmetries. The ortho-isomer has four different carbons, the meta-isomer has five, and the para-isomer has only three. The unknown must be orthoxylene. Note that this compound could be identified as one of the xylene isomers on the basis of its H-NMR spectrum, but it would be difficull to establish which isomer it is from just that information. 

Use and exposure There are three forms of xylene meto-xylene, ortho-xyltnt, and para-xylene (m-, o-, and p-xylene). These different forms are referred to as isomers. Xylene is a colorless, sweet-smelling liquid that catches tire easily. It occurs naturally in petroleum and coal tar. Chemical industries produce xylene from petroleum. Xylene is used as a solvent in the printing, rubber, and leather industries. It is also used as a cleaning agent, a thinner for paint, and in paints and varnishes. It is found in small amounts in airplane fuel and gasoline. Xylene is used extensively in the manufacture of many other chemicals, such as plastics, synthetic fibers, pesticides, insect repellents, and leather goods. ... 

Two alternative commercial operations have been developed to perform the separation. In one, a mixture of the isomers is contacted with a molecular sieve that has pores large enough to accommodate para-xylene but not the meta or ortho isomers. This operation is referred to as adsorption. In another process, the difference in freezing points of the three isomers (para-xylene freezes at 13.3°C, ortho at —25.2°C, and meta at -47.9°C) forms the basis of a crystallization operation. The mixture is cooled to a temperature at which para crystallizes and can then be separated physically from the remaining ortho and meta liquid. 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

Friedel-Crafts alkylation reactions are, in general, accompanied by isomerization processes. Olah et a/. reported the results of the water-promoted, AlCb-catalyzed isomerization of o-, m- and p-di-f-bu-tylbenzene. No ortho isomer was present in the equilibrium mixture. The isomerization of o-di-r-bu-tylbenzene was very rapid largely due to relief of steric strain. In these and other related sterically hindered arenes, intramolecular isomerization and not dealkylation was observed. Isomerization of di-and mono-methylnaphthalenes, catalyzed by HF-BF3, was also reported. Isomerization of /i-alkyl-toluenes and -xylenes, catalyzed by AICI3 at room temperature, afforded chiefly /n-/i-alkyltoluenes and /n-/i-alkylxylenes, respectively. The process leading to the meta isomer has a lower energy than the other processes. 

Xylenes. Mixed xylenes are generally obtained by the catalytic reforming of petroleum fractions (see Section 6.2.1.7, above) or by the recovery of the Cg fraction from an aromatic concentrate (pyrolysis gasoline) stream. An increasingly important source of mixed xylenes is from the disproportionation of toluene. The separation of mixed xylenes into para, meta, and ortho isomers can be accomplished by several methods (e.g., fractional crystallization). 

In all the above mentioned processes, the xylene fraction which constitute about 20-30% of the total aromatics, contain nearly thermodynamic equilibrium composition (23 53 24) of para, meta and ortho isomers [5]. Among the three xylene isomers, para has got better industrial importance due to its conversion to terephthalic acid, which is used in the manufacture of polyester fibre. Hence it was considered of interest to look into the aspect of xylene isomer distribution in the products of aromatization. ... 

The equilibration of the ring anions to the benzyl anion is the probable explanation, especially considering the recent work of Gau and our observation of an equilibration between m-xylene and toluene in the presence of (benzyllithium)2 TMEDA (20). (In fact, this type of system might be the basis of another route for a hydrocarbon acidity scale.) The more rapid disappearance of the ortho isomer, compared with the meta, may be the result of a possible intramolecular route for conversion to the benzyl anion. The meta and para isomers probably change to the benzyl anion by an intermolecular route that would be slower and agree with what was observed. Although complete resolution by GLC of the para and meta isomers was not done in this study, the para disappeared faster than the meta, but much slower than the ortho. In other time studies on the disappearance of the meta isomer at room temperature, about half the initial amount of this isomer was gone in three days and all of it in two to three months. 

One of the most important industrial alkylations is the production of 1,4-xylene from toluene and methanol (Reaction 2). ZSM-5, in the proton exchanged form, is used as the catalyst because of its enhanced selectivity for para substituted products. para-Xylene is used in the manufacture of terephtha-lic acid, the starting material for the production of polyester fibres such as Terylene. The selectivity of the reaction over HZSM-5 occurs because of the difference in the rates of diffusion of the different isomers through the channels. This is confirmed by the observation that selectivity increases with increasing temperature, indicating the increasing importance of diffusion limitation. The diffusion rate of para-xylene is approximately 1000 times faster than that of the meta and ortho isomers.14... 

Restricting a reaction to produce a specific product, when more than one product is possible, can also occur because of the differences in the relative diffusivities of the various products. In Figure 3.3b, para-xylene can emerge from the pore system much more rapidly than either the meta or the ortho isomer. A representation of para-xylene in a zeolite pore is shown in Figure 3.4. 

First, analyze a standard mixture that includes pentane, hexane (or hexanes), benzene, heptane, toluene, and xylenes (a mixture of meta, para, and ortho isomers). Inject a 0.5-yu,L sample or an alternative sample size as indicated by your instructor into the gas chromatograph. Measure the retention time of each component in the reference mixture on your chromatogram (see Technique 22, Section 22.7). The previously listed compounds elute in the order given (pentane first and xylenes last). Compare your chromatogram to the one posted near the gas chromatograph or the one reproduced in this experiment. 

The other silane formed, namely the methyibenzyitrimethylsilane is likely to be the ortho-isomer. Evidence for this is that it has a longer retention time than the para-isomer, the retention time of which is known. It would be expected to have a higher boiling point than the para-isomer (cf. o- and p-tolyltrimethylsilanes). This compound is probably formed by attachment by a trimethylsilyl radical on an o-xylene molecule. 

Orf/jo-xylene must be separated from the xylene streams, since only the ortho isomer (and not the meta or para) can he converted to phthalic anhydride. 

Within the aromatics e.g. benzene, toluene and xylene) the xylenes are used as a feedstock for the production terephthalic acid and dimethyl terephthalate, both monomers that are used for the production of polyethylene terephthalate (PET), which is the main constituent of plastic bottles and polyester clothing. There are three very close boiling isomers of xylene ortho-, meta- and para-) and it is mainly p-xylene that is used for PET production. The fractionation processes for all existing xylene isomers (crystallization or simulated moving bed) are expensive technologies and it is of interest to study the potentials of membranes for such a separation. 

Dimethyl derivatives of benzene are called xylenes There are three xylene isomers the ortho (o) meta (m) and para (p) substituted derivatives... 

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. 

Separation of a chemical species from a mixture of similar compounds can also be achieved by melt crystallization, which is, for example, an important means of separatingpara- s.yXen.e (p-xylene) from the ortho and meta isomers. -Xylene is crystallized at the top of a vertical column and crystals are moved downward countercurrentiy to Hquid. The Hquid flowing upward is generated by adding heat to melt the crystals at the bottom of the column a portion of the melt is removed as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother Hquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

If several groups are attached to the benzene ring, their names as well as their relative positions should be indicated. For example, dimethylbenzene or xylene, CgH (CH,)2, has three geometric isomers, with prefixes ortho-, meta-, and para-, indicating the relative positions of the two methyl groups. 

Commercial grades Xylenes are available as an isomer mixture (about 10% ortho-, 72% meta-, and 18% para-) and as the pure isomers. The mixed xylenes are priced by the gallon while the pure isomers are priced per pound. 

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