Isomerization of xylene

Although the demand for p-xylene to produce terephthalic acid for fibers, and for o-xylene to obtain phthalic anhydride for the manufacture of plasticizers has risen considerably in recent decades, demand for m-xylene for the production of iso-phthalic add is relatively small. m-Xylene and ethylbenzene are therefore isomer-ized to o-and p-xylene. 

Although xylenes can be converted through an acid-catalyzed carbonium ion mechanism, the presence of hydrogen is necessary to convert ethylbenzene into xylenes. 

When catalysts other than noble metals are used, such as AICI3 (Isomar, UOP) and AIF3 (Isarom, IFF), their service lives are shorter reaction temperatures are usually between 370 to 540 °C, with pressures from 15 to 30 bar. 

In the early 1970 s Mobil O//developed zeolite catalysts for the isomerization of xylene without the use of hydrogen (LTI method). This process is carried out in the liquid phase at a pressure of 30 bar and temperatures of from 200 to 260 °C. The service life of the catalyst is generally over 2 years. Regeneration (coke removal) is carried out from a reaction temperature of around 275 °C. 

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Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

As discussed previously for the isomerization of butanes, differentiation must be made between thermodynamic equilibria of hydrocarbons and their derived carbo-cations. The acidic isomerization of xylenes further emphasizes this point. 

Isomerization of xylenes is always coupled with separation processes. In most cases, p-xylene is removed from the reaction mixture by crystallization or selective adsorption. The recovered o-/m-xylene mixture, in turn, is usually recycled for reequilibration. Because of cost of separation, the highly selective carbonylation of toluene to p-tolualdehyde gained significance. Subsequent reduction gives p-xylene. 

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... 

Effect of Zeolite Crystallite Size on the Selectivity Kinetics of the Heterogeneous Catalyzed Isomerization of Xylenes... 

Using the monomolecular rate theory developed by Wei and Prater, we have analyzed the kinetics of the liquid-phase isomerization of xylene over a zeolitic catalyst. The kinetic analysis is presented primarily in terms of the time-independent selectivity kinetics. With the establishment of the basic kinetics the role of intracrystalline diffusion is demonstrated by analyzing the kinetics for 2 to 4 zeolite catalyst and an essentially diffusion-free 0.2 to 0.4 m zeolite catalyst. Values for intracrystalline diffusivities are presented, and evidence is given that the isomerization is the simple series reaction o-xylene <= m-xylene <= p-xylene. 

The procedure used for the kinetic analysis is that described by Wei and. Prater (f), and it has been applied in the following manner. The isomerization of xylenes is assumed to be kinetically first order and can be described by the following apparent reaction scheme ... 

Based upon the organic chemistry of the isomerization of xylenes, the probability of a direct isomerization from o-xylene to p-xylene is quite small. Furthermore, in homogeneously catalyzed liquid-phase reaction the simple series reaction scheme has been demonstrated (7,8,9,10,11,12). 

Let us now assume that isomerization of xylenes is a simple series reaction. Starting with pure o-xylene feed, and in the absence of any diffusional resistance, most molecules of o-xylene which enter the zeolite crystallites leave after, at most, one reaction step (to m-xylene), yielding primarily a single-step product. When substantial diffusion-resistance exists, most molecules remain in the crystallite long enough for several reaction steps to occur (to m-xylene and p-xylene), and the products are mostly multistep products. Wei (4) has shown that in such a case the apparent kinetics—i.e., those determined from the respective reactant product concentrations in the bulk phase—change from that of a simple series reaction... 

Nafion-H appears to be a very useful catalyst for transalkylation reaction as indicated in these studies. Transalkylation of benzene with diethylbenzenes, as well as with diisopropylbenzene, is efficiently catalyzed by Nafion-H in a flow system. The efficiency of the catalyst is, however, more limited when the transferring group is a methyl group.268 Beltrame and co-workers have also carried out269 detailed mechanistic studies on the isomerization of xylenes over Nafion-H. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

Whereas the mutnal isomerization of xylenes appears to take place by the transfer of the CH3 group according to a conventioiial carbonium ion mechanism, ethylbenzene isomerization is more com to and requires the presence of hydrogen. To interpret this factor, it is assumed that the conversion comprises the production of C5 and Q naph> thenes as intermediates, according to the following reaction ... 

We will use now the same method and Mordenite zeolite model as in the previous part, and investigate the isomerization of xylene isomers. As described in the previous part, this reaction can proceed via two alternative routes, viz. a methyl shift isomerization, and disproportionation reactions. Moreover, we observed than in the case of toluene isomerization, the location of toluene with respect to the Br0nsted acidic site for the shift isomerization was of no consequence for the activation energy barrier. We will check these mechanisms for the three xylenes. 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

Desulfurization of petroleum feedstock (FBR), catalytic cracking (MBR or FI BR), hydrodewaxing (FBR), steam reforming of methane or naphtha (FBR), water-gas shift (CO conversion) reaction (FBR-A), ammonia synthesis (FBR-A), methanol from synthesis gas (FBR), oxidation of sulfur dioxide (FBR-A), isomerization of xylenes (FBR-A), catalytic reforming of naphtha (FBR-A), reduction of nitrobenzene to aniline (FBR), butadiene from n-butanes (FBR-A), ethylbenzene by alkylation of benzene (FBR), dehydrogenation of ethylbenzene to styrene (FBR), methyl ethyl ketone from sec-butyl alcohol (by dehydrogenation) (FBR), formaldehyde from methanol (FBR), disproportionation of toluene (FBR-A), dehydration of ethanol (FBR-A), dimethylaniline from aniline and methanol (FBR), vinyl chloride from acetone (FBR), vinyl acetate from acetylene and acetic acid (FBR), phosgene from carbon monoxide (FBR), dichloroethane by oxichlorination of ethylene (FBR), oxidation of ethylene to ethylene oxide (FBR), oxidation of benzene to maleic anhydride (FBR), oxidation of toluene to benzaldehyde (FBR), phthalic anhydride from o-xylene (FBR), furane from butadiene (FBR), acrylonitrile by ammoxidation of propylene (FI BR)... 

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. 

Isomerization of xylene is also a very attractive application for MFI zeolite membranes (Fig. 10). The commercialisation of a new zeolite-based membrane is being planned by NGK Insulators for /i-xylene production from other xylene isomers, p-xylene molecules which are smaller than those of m- or o-xylene, are sieved from the mixture using the membrane, thus cutting significantly the production costs [157]. 

Xvlene Isomerization. The first reported use of borosilicate containing catalysts was for xylene isomerization (12,16). In this application, the purpose is to isomerize a reaction mixture which is lean in p-xylene to an equilibrium mixture from which the p-xylene can then be removed. In addition to the isomerization of xylenes, the catalyst also must convert a portion of the other components present in the feed to allow easier separation of p-xylene from the product mixture. The primary contaminant in the feedstock is ethylbenzene, which is converted via transalkylation to higher molecular weight compounds, which are valuable as gasoline blending components, and benzene. 

Harbison, K. G. 1970. Isomerization of xylenes. Experiment for the organic or instrumental laboratory. /. Chem. Educ. 47 837-839. 

Thus, Table XI shows some results on the isomerization of m-xylene under reforming conditions. Again, there is little reaction at the low pressure, high space velocity conditions. At higher pressures there is not only isomerization to o- and p-xylene, but also considerable dealkylation to toluene. This confirms that the isomerization of xylenes occurs via hydrogenation to naphthenes, at least some of which can isomerize to gem-dimethyl derivatives as shown in Figure 6. 

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