Xylenes para 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. 

It has been found that the disproportionation of toluene over ZSM-5 catalyst can be directed such that p-xylene is the predominant xylene isomer (14-17). This reaction, designated STOP, is one of several in which disubstituted aromatics rich in the para isomer are produced. Others are the alkylation of toluene with methanol to produce p-xylene (15,18) and with ethylene to produce p-ethyltoluene (19,20), as well as the aromatization of olefins (20), paraffins (20) and of methanol... 

The general characteristics of toluene disproportionation are summarized by the data presented in Figure 8. With standard HZSM-5 catalyst, as indicated by the lowest curve, the xylenes produced contain essentially an equilibrium concentration of the para isomer (24%) and exceed it only slightly at low conversion. The other curves result from a variety of HZSM-5 catalysts modified in different ways and to different degrees. It is apparent that a wide range of para-selectivities can be obtained. At increasing toluene conversions, the para-selectivity decreases for all catalysts. 

The primary product will be rich in the para isomer if initial m-and o-xylene diffuse out of the zeolite crystal at a lower rate (Dm q/t2) than that of their conversion to p-xylene (kj) and the latter s diffusion (Dp/r2). Conversion of the para-rich primary product to secondary product low in p-xylene is minimized when the actual, observed rate of isomerization (kj g is lower than the rate of toluene disproportionation (kD). 

The research gnomes at the lab benches have now developed the catalysts, and companies have commercialized plants to shift a mixture of mixed xylenes and ethylbenzene towards the para- isomer and away from the meta-. 

Because of the large demand for p-xylene, another method is now being used by Amoco to increase the percentage of the para isomer in mixed xylenes. They are heated at 300°C with an acidic zeolite catalyst, which equilibrates the three xylenes to an o,m,p ratio of 10 72 18%. The para isomer is separated by fractional crystallization, whereas the o,m mixture is reisomerized with the catalyst to produce more para product. Theoretically, all the xylenes could be transformed into the desired para isomer. The zeolite catalyst has the following structure. 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

In a kinetic study of the acylation of toluene, with p-xylene and the corresponding perdeutero compounds with aroyl triflates, correlation was found between the primary kinetic isotope effect and the ortho para ratio.35 Different conformations of the bent cr complexes36 for the two isomers resulted in a much higher rate of deprotonation and rearomatization for the para isomer. By appropriately selecting reaction conditions and thereby affecting the ratio of the two conformations, unusually high amounts of ortho products may be obtained.37,38... 

The classical argument concerting the equivalence of the positions on the benzene ring is based on the existence of three (ortho, meta. para) isomers of xylene (dimethylbenzene). How many isomers ore there of dimelhylhorazine ... 

The Grignard reagent is prepared from 0.5 mole of methyl iodide in a total of 200ml of sodium-dried di-n-butyl ether as detailed in No 157. The tin (IV) chloride is added in 100ml of dry xylene (the commercial mixture of ortho, rneta, and para isomers is quite satisfactory). One hour reflux is required. The product is isolated by fractional distillation, b.p, 78°C air-stable liquid. 

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

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. 

Nitration of monosubstituted aromatics, toluene in particular, has been extensively studied using zeolites in order to direct the reaction towards the formation of the desired para-isomer. Toluene has been nitrated para-selectively with benzoyl nitrate over zeolite catalysts.[14,15] For example, when mordenite is used as a catalyst, MNTs are formed in almost quantitative yields, giving 67 % of the para-isomer in 10 min, but tetrachloromethane is required as solvent. However, the main problems associated with the use of benzoyl nitrate are handling difficulties due to its sensitivity toward decomposition, and the tendency toward detonation upon contact with rough surfaces. Nagy et a/.[19 21] reported the nitration of benzene, chlorobenzene, toluene and o-xylene with benzoyl nitrate in the presence of an amorphous aluminosilicate, as well as with zeolites HY and ZSM-11, in hexane as a... 

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. 

Before spectroscopy was invented, Komer s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Komer s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed. 

Structural Isomers. Chromatograms illustrating the separation of ortho, meta and para isomers of cresol (22) and and xylene ( O)on RP columns are shown in Figures 4 and 5. They enable a comparison of the chromatographic properties and selectivities due to <. - and -CD complexation between positional isomers of the above compounds.Similar behaviour was observed for ortho,meta and para isomers of fluoronitrobenzene, chloronitrobenzene, iodoni-trobenzene, nitrophenol, nitroaniline, dinitrobenzene (22), nitrocinnamic acid (22) some mandelic acid derivatives (19,21,34) and ethyltoluene (28). Both [Pg.225]

Benzene, toluene and / -xylene are the most industrially important aromatic hydrocarbons. Their relative proportions must be adjusted according to the needs of the market, which presently requires more benzene and xylene. Dealkylation of toluene can be carried out by hydrogenolysis with H2 using Cr, Mo or Pt oxides as catalyst precursors (Equation 4). Methyl redistribution in toluene is best carried out around 420°C on ZSM-5 zeolite. Under these conditions, xylene is mainly present as the para isomer (90% selectivity. Equation 5) due to the shape-selectivity of the zeolite. 

Chronologically, the effon was im tially dirreted towards a better petrochemical upgrading of toluene and m-xylene, which continue to be used as solvents and in the gasoline pool. Toluene is thus converted to benzene by hydrodealkylation. Through i omerizatioo,. m-.xyiene yields the onho and para isomers in proportions corresponding to thermodynamic equilibrium in the reaction conditions, namely a mixture of Cg from which the... 

The conversion of ortho- to meta- and para-xylene was carried out on a series of decationated catalysts which were subjected to thermal treatment at various temperatures. 10-/xl pulses of o-xylene were used, the catalyst amount was 300-350 mg, and the flow rate of the helium carrier gas was 50-100 ml/min. The products were analyzed on 7.8-benzoquinoline on Chromosorb W gas-chromatographic column. The m-xylene predominated over the para isomer. A small toluene production seemed to parallel that of isomerization. The conversion of xylene X... 

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