Isomerization of aromatics

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. 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

MF1 type zeolites are known to show enhanced selectivity to p-substituted products in alkylation and isomerization of aromatic molecules [e.g. 1,2,3,4]. This shape selectivity is more pronounced with larger zeolite crystals and can be further enhanced by modification of the parent zeolites through post-synthesis treatments like impregnation with basic oxides, metal salts or the deposition of silica or coke [3,5,6,7]. The gain in selectivity is, however, usually accompanied with loss in catalytic activity and in some cases more rapid deactivation [8,9]. Despite the large number of patents and reports in open literature, the reasons for the enhancement of shape selectivity of MFI zeolites by post-synthesis treatment and the limits of the severity of this treatment are not unequivocally explained to date. 

It was only recently that these phenomena were detected photochemically using photochromic molecules whose chromophores undergo a reversible photoisomerization reaction. The photochemical reactions involved in these processes are mainly the trans-ds isomerization of aromatic azo compounds and stilbene derivatives as well as the ring opening/dosure reaction of spirobenzopyran derivatives . ... 

Most elaborated studies were based on the reverable isomerization of aromatic azo compounds (Eq. (1)) and the ring opening/dosure of sjMobenzopyrans (Eq. (2)). While in the first case, isomerization can proceed without important steric requirement, in the second case photodiromism involves deavage of the C-O-pyran bond followed by rotation of one part of the molecule so as to approach coplanarity the change of conformation is therefore more pronounced. 

Cimiraglia, R., and Hoftnann, H.-J. (1994). Rotation and inversion states in thermal E/Z isomerization of aromatic azo compounds. Chem. Phys. Lett. 217, 430-435. 

The expert system we are developing concerns several aromatic reactions of great industrial interest [12-23], e.g., alkylation and isomerization of aromatics such as benzene, toluene, and xylene, catalyzed by ZSM-5, mordenite, and Y... 

This section covers the isomerizations of aromatic derivatives bearing oxygen- and nitrogen-containing functional groups which lead to phenols. Because these reactions are widely known, only a brief survey will be presented here concerning the most typical examples of these transformations. More detailed information can be found elsewhere. ... 

The Af-aryl-Af-acylhydroxylamines 289 and 290 rearrange to aminophenol derivatives in the presence of sulfonyl chlorides (equation 138) as well as of iodonium salts in a reaction similar to the benzidine rearrangement (equation 139). The A-aryl-N,0-diacylhydroxylamine 291 undergoes isomerization on heating to produce dibenzoylated aminophenol 292 (equation 140) . The Wallach rearrangement consists of isomerization of aromatic azoxy compounds 293 to form the hydroxyazobenzenes 294 on heating in... 

Eisenbach, C.D. (1980) Isomerization of aromatic azo chromophores in poly(ethyl acrylate) networks and photomechanical effect. Polymer, 21, 1175—1179. 

Zeolites function as Bronsted or Lewis acid catalysts or (less frequently) as basic catalysts. Examples of the former are alkylation and isomerization of aromatics, such as the isomerization of xylenes an example of the latter is the use of cesium zeolite in the synthesis of the key intermediate, 4-methylthiazole, used in the preparation of the anthelmintic, thiabendazole. 

The isomerization reactions shown in equation (6.42) can proceed selectively depending on the catalyst used in the process. Aluminosilicates are typical for selective isomerization of aromatic compounds, because they have well defined pore sizes depending on the type of aluminosilicate. The mechanism of selective isomerization can be explained based on Figure 6.9. 

From Figure 6.9, it is obvious that a strictly defined size of catalyst pores allows only the molecules with size less than the pore size to leave the catalyst pore. This leads to the selective isomerization of aromatic compounds as shown in the example in Figure 6.9. 

The fundamental role of the kinetic characteristics of degenerate processes is demonstrated by the results of using Marcus Eq. (1) in the research on isomerization of aromatic compounds... 

The intramolecular isomerization of aromatic compounds involving a change of position in the aromatic ring for the primary alkyl groups, aryl fragments, chlorine atoms and CH3SO2 group is similar... 

There is another mechanism for the acid-catalysed isomerization of aromatic compounds connected with the reversibility of some types of electrophilic substitutions ... 

A majrked shift in favour of 1,3-isomer upon isomerization under complete protonation must also be observed with other disubstituted benzenes with substituents capable of taking part in delocalizing the positive charge of benzenium ions. Similarly, in the case of tri- and tetra-substituted benzenes the 1,3,5- and 1,2,3,5-isomers must be sharply predominant in the equilibrium. Some examples of such isomerization of aromatic compounds are of synthetic interest (see and Table 46). 

Eisenbach CD. 1980a. Cis trans isomerization of aromatic azo chromophores, incorpo rated in the hard segments of poly(ester urethane)s. Macromol Rapid Commun 1(5) 287 292. 

Stable nano/mesoporous materials with mixed oxides such as zeolites, clays, and other minerals are widely used in various fields catalysis, adsorption, ion-exchange, separation, etc. because of their catalytic activity in acid/base and redox (e.g., materials with titania phase) reactions, ability to sorb selective molecules of diverse types, participate in ion-exchange reactions, providing sieve effects, etc. (Tanabe 1970, Grandjean and Laszld 1989, Rocha and Anderson 2000, Cundy and Cox 2005, Tao et al. 2006). The most important processes that utilize the selective properties of these materials are alkylation and isomerization of aromatic hydrocarbons as well as conversion of methanol to hydrocarbons, and some other reactions. Silicalite is an extreme type of the materials with the ZSM-5 zeolite structure but whose aluminum content is negligible. Therefore, unlike conventional zeolites, silicalite does not possess ion-exchange properties, and its surface has a weak affinity to water. 

Aromatics are stable species and are relatively inert. Reactions of substituted aromatic involve isomerization, hydrodealkylation, or disproportionation. Isomerization of Aromatics. 

Tabata, M. Tanaka, Y Sadahiro, Y Sone, T. Yokota, K. Miura, 1. Pressure-induced cis to trans isomerization of aromatic polyacetylenes. 2. Poly((e-ethoxyphenyl)acetylene) stereoregularly polymerized using a Rh complex catalyst. Macromolecules 1997, 30, 5200-5204. 

Paik, C.S. and Morawetz, H. (1972) Photochemical and thermal isomerization of aromatic residues in the side chains and the backbone of polymers in bulk. Macromolecules, 5, 171-177. 

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