Xylenes meta isomer

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. 

Properties There are three xylene isomers, commonly known as orr/zo-xylene, meta-xylene, and para-xylene. They are all colorless liquids. The orr/zo-isomer boils at 144°C, the meta- at 139.1°C, and the para-at 138.5°C. 

The isomers, called ortho-xylene, meta-xylene, and para-xylene, each have unique properties. Two such properties are the freeze points, at which xylenes turn from liquid to crystals, and the boiling points, at which xylenes turn from liquid to vapor. These two properties figure importantly in the apparatus used to separate xylene isomers from each other. Mixed xylenes, a commonly traded commodity, is a combination of the three isomers. 

Lithiation study of xylenes indicates a much higher reactivity of the meta isomer (m- o >p-). On the other hand, only the meta isomer of methylacetophenone... 

More than 70% of /neto-xylene absorbed was excreted into the urine as metabolites. A minor portion was exlialed unchanged (lARC, 1989). Elimination is rapid, with a biological half-time of 1 h for a rapid phase after 6-16 h of exposure and of about 20 h for a slow phase. Removal of industrial xylene from subcutaneous adipose tissue, however, is slow, with a half-time of 25-128 h for the meta isomer (lARC, 1989). 

Another illustration of the application of the additivity procedure is the scheme employed to determine mp for non-catalytic bromination (Brown and Stock, 1957a). Direct analysis of the products of bromination of toluene indicates only 0.3% m-bromotoluene providing mfe is 5.5 (Table 2). The difficulties in the analysis of the products for the small quantity of meta isomer introduce considerable uncertainty in the partial rate factor. As a test of the observation, the rate of bromination of p-xylene was examined. The additivity treatment (40) yields expression (41),... 

Another possibility for separating the para-isomer involves selective adsorption on zeolites, then desorption after the ortho and meta isomers have been separated. The slightly different boiling point of the o-xylene is the basis for separation from the other two isomers through an elaborate column. 

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. 

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 three isomers of dimethylbenzene are commonly named ort/zo-xylene, meta-xylene, and para-xylene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable using 13C NMR. 

An offering by Cytec Specialty Chemicals, the meta isomer of tetramethyl xylene diisocyanate (TMXDI) is interesting because it contains an aromatic ring, but the NCO groups themselves are aliphatic isocyanates and have reaction characteristics typical of aliphatic diisocyanates. It reacts even more sluggishly than the more standard aliphatic isocyanates because of steric interactions, making the reactions easier to control. Compounds such as dimethyl tin dilaurate, lead octoate, or tetrabutyl diacetyl distannox-ane have been shown to be effective catalysts for the isocyanate-hydroxyl reaction. The manufacturer claims that it is less toxic than many other isocyanates. 

Interestingly, the evaluation of vdw for all our systems gives within few kJ/mol the same value, and we could assume as observed from ref. 60 that the dispersion contribution is a constant contribution for a given adsorbate size. For xylene isomers, this Van der Waals correction is. vdw = - 95 kJ/mol. Then, the adsorption energies for para-xylene, meta-xylene, and ortho-xylene are - 132 kJ/mol, - 125 kJ/mol, and - 128 kJ/mol respectively. 

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... 

A marked contrast is observed in the behaviour of the simplest aromatic hydrocarbon-air mixtures at high pressures. No cool-flame phenomena or an ignition peninsula in the (p-Ta) diagram are observed. These are found only when sufficiently reactive aliphatic side-chains are associated with the aromatic ring. Burgoyne et al. [129] showed this to be the case for n-propylbenzene in a closed vessel (Fig. 6.18). The ortho- and meta-isomers of the xylenes also showed a similar reactivity. Benzene, toluene and ethylbenzene were found to undergo spontaneous ignition at temperatures only above 700 K. 

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. 

Sudhakar and Katz have devised an improved route to [7] helicene. The bromine atom is found to direct the stilbene cyclization away from occupied positions and those ortho to the substituent so that with (153) the bromo [7] helicene (154) is formed in 75% yield on irradiation of benzene solutions containing iodine, and less than 10% of the planar arene (155) is observed. Such results compare very favourably with those from irradiation of the hydrocarbon (156) when equal quantities of the [7] helicene and (157) are formed.Irradiation of (158), the meta isomer of (156), cyclizes selectively without bromine direction and (159) is formed in 75% from xylene solution in the presence of oxygen and iodine The origins of this selectivity may not be readily evident and it is difficult to appreciate why of the four possible conformers of the first intramolecular photo-oxidative cyclization product only (160) undergoes further reaction. The authors suggest that this feature reflects that of the four conformers (160) has the smallest degree of steric interactions between the two aryl moieties. The photo-cyclizations of halostilbenes have been examined by other workers who report that various o-chloro and bromo-compounds (161) undergo... 

The distribution among aromatic products was also altered by the presence of molybdenum (Fig. 1). The relative yields of para and meta isomers of xylenes and ethyltoluenes were increased over Mo exchanged ZSM-5 and over the physical mixture of MoO and H-ZSM-5 (no aromatics were detected over the impregnated catalyst), but this effect was not found for the partially poisoned catalyst. 

The significant finding was that the xylene fraction was 99% para. The other fractions are not lost. Toluene can be disproportionated to p-xylene and benzene with H-ZSM-5 treated with a little hexamethyldisiloxane to give 99% p-xylene, so that the usual separation of the ortho- and meta isomers with another zeolite would not be required.177 Benzene can be transalkylated with the higher aromatics to give toluene. Ethylbenzene can be isomerized to p-xylene. Ethylbenzene can be alkylated with ethanol in the presence of a modified ZSM-5 catalyst to produce p diethylbenzene with 97% selectivity.178... 

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. 

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. ... 

Pure silica microporous solids show no such strong affinity for polar molecules, and are of interest to separate hydrocarbon molecules of different shapes. In particular, the separation of xylene isomers is of great industrial significance, and since the relative diffusivities of the para isomer are known to be much higher than those of the ortho (and meta) isomers within silicalite, the preparation of silicalite membranes is an attractive target. If the aluminium content can be made vanishingly small, the membranes can be used for hydrocarbon separation at elevated temperatures without the effects of coke formation due to catalytic reaction. 

In a somewhat similar manner i-xylene can be separated from a mixture of m- and /i-xylene this binary system forms a eutectic. Carbon tetrachloride produces an equimolecular solid compound with /i-xylene, but not with 0- or m-xylene. Egan and Luthy (1955) reported on a plant for the production of pure -xylene by crystallization meta-para- xylene mixtures in the presence of carbon tetra-chloride. Up to 90 per cent of the para- isomer was recovered by distillation after splitting the separated solid complex. The meta- isomer was recovered by fractionally crystallizing the CCU-free mother liquor. Perfect separation of /i-xylene is not possible, because the ternary system CCU/m-xylene/CCU -xylene forms a eutectic, but fortunately the concentration of the complex CCI4 /i-xylene in this eutectic is very low. Several commercial clathration processes for the separation of m-xylene from Cg petroleum reformate fractions using a variety of complexing agents have been operated (Sherwood, 1965). 

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