Isomerization of ethylbenzene

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

Catalytic isomerizations of ethylbenzenes and xylenes over zeolites are commercial processes and have been used as test reactions of acid catalysts. Corma and Sastre26 have recently suggested that xylenes can form via transalkylation of trimethylbenzene which is believed to be an intermediate in the isomerization of p-xylene. A general scheme as that shown in Eq. 626 was proposed on the basis of kinetic and mass spectrometric data. The reactant p-xylene was believed to produce m-xylene as a primary product but also rearranges in the pores of ultrastable faujasite zeolites to form o-xylene which appears as a primaiy product. In addition, trimethylbenzenes were formed along with toluene. 

Xylenes might be produced through isomerization of ethylbenzene as follows ... 

Hence the isomerization of ethylbenzene is possible in the case of polyhmctionai catalysts controlled by a partial pressure of hydrogen. According to some authors, a similar mechanism is also involved for the mutual conversion of xylenes. 

One of the important features of the octafining catalyst is its ability to convert ethylbenzene to xylenes. The data of Table I demonstrate the selectivity level at high conversions achievable with the present commercial octafining catalyst for the isomerization of ethylbenzene. 

Mordenite has been used since the second half of the 1970s in industrial xylene and ethylbenzene isomerization proeesses, chiefly for the production of paraxylene. the isomer for whieh there is the highest demand (Table 2). First, the isomerization of ethylbenzene into xylenes implies a bifunetional aeid catalyst. Second, it requires a temperature of around 400°C and hydrogen pressure in the region of 1-1.5 MPa. The catalyst is. therefore, composed of acid mordenite associated with a strong hydrogenation function, supplied by Pt. 

It is believed that the isomerization of ethylbenzene to xylenes occur only on bifunctional catalysts, and hydrogenated intermediates are required (190). Selective catalysts for canying out this reaction have to maintain a good balance between the H-DM and the add function. Zeolites, with strong acid functions such as in mordenite or ZSM-5 actively isomerize cycloolefinic intermediates but also catalyze ring opening reactions which lead to a decrease in the formation of xylenes. However, the mild add nature of SAPOS make them specially useful for ethylbenzene isomerization. In this sense, 0.4-0.6 %wt Pt on SAPO-11 and SAPO-5 catalysts were used to isomerize mixtures of ethylbenzene and meta-xylene. Both catalysts produce near-complete xylene equilibration. SAPO-11 was more selective for producing xylenes than SAPO-5 (191). 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

To this point the presence of ethylbenzene in the mixed xylenes has been ignored. The amount can vary widely, but normally about 15% is present. The isomerization process must remove the ethylbenzene in some way to ensure that it does not build up in the isomerization loop of Figure 8. The ethylbenzene may be selectively cracked (40) or isomerized to xylenes (41) using a platinum catalyst. In rare cases the ethylbenzene is recovered in high purity by superfractionation. 

Another example of catalytic isomerization is the Mobil Vapor-Phase Isomerization process, in which -xylene is separated from an equiHbrium mixture of Cg aromatics obtained by isomerization of mixed xylenes and ethylbenzene. To keep xylene losses low, this process uses a ZSM-5-supported noble metal catalyst over which the rate of transalkylation of ethylbenzene is two orders of magnitude larger than that of xylene disproportionation (12). 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

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. 

We have shown that the high selectivity of ZSM-5 in xylene isomerization relative to larger pore acid catalysts is a result of its pore size. It is large enough to admit the three xylenes and to allow their interconversion to an equilibrium mixture it also catalyzes the transalkylation and dealkylation of ethylbenzene (EB), a necessary requirement for commercial feed but it selectively retards transalkylation of xylenes, an undesired side reaction. 

Since there are two different connections possible for n-octane, 1,6 or 3, 8, which could lead eventually to ethylbenzene, there is a statistical entropy factor involved here which is not part of the o-xylene route. Therefore, if both closures were equally possible from an enthalpy perspective, one would predict a 2 1 ethylbenzene to o-xylene ratio. The formation of the m- and p-xylene requires prior isomerization of n-octane to 2- and 3-methylheptane, respectively. 

In our calculations we will first discuss our results starting with both the 2-and 3- octyl cations (the 4- octyl cation cannot form a 1,6-p-H-structure). The n-octane conversion to aromatics, as described by Davis (8), is a good test of our proposed mechanisms, for several reasons (1) his experimental observation would require the formation of approximately equal amounts of 1,2-dimethylcyclohexane (o-xylene) and ethylcyclohexane (ethylbenzene), even though in our mechanism the structure of the needed 1,6-p-H cation intermediates are quite different, and (2) the formation of to- and p-xylene requires a prior isomerization of n-octane to 2- and 3- methylheptane, and this must be a faster reaction than the dehydrocyclization (or at least competitive with it). If our mechanisms are valid, we should be able to reproduce some aspects of the above results. 

Ethylbenzene Isomerization Isomerization of EB requires both metal and acid function. Hydrogenation results in an intermediate naphthene. The acid function is required to isomerize the naphthene to a methyl-ethyl-substituted five-mem-bered ring species that can further convert to a dimethyl-substituted six-membered ring naphthene. This can be dehydrogenated by the metal function to a xylene isomer, OX in the example shown in Figure 14.9. 

As already stated, isomerization on zeolite HY was always accompanied by disproportionation, even at 180 C. With time on stream, Yo,. Np increases, because these heavy products are most efficiently held by the fresh catalyst. It is an interesting result that, at 180 °C, the yield of naphthalene passed through a maximiun as well. Obviously, under appropriate reaction conditions, the disproportiotuOion of methylnaphthalenes in zeolite HY exhibits an induction period, as does the disproportionation of ethylbenzene in large pore zeolites [39,40]. 

There are also no experimental thermochemical studies113 of prismane (57), per se, but measurements of the enthalpy of isomerization of its hexamethyl derivative to hexam-ethylbenzene have been reported114. These experimentally measured isomerization enthalpies differ by some 40 kJ moT1. There are also measurements of the isomerization enthalpy for hexakis(trifluoromethyl)prismane to the corresponding benzene115. 

Laali et al.234 have developed a method to the highly selective pura-adamantylation of arenes (toluene, ethylbenzene, anisole) with haloadamantanes (1-chloro- and 1-bromoadamantane, l-bromo-3,5,7-trimethyladamantane) and 1-adamantanol promoted by triflic acid in butylmethylimidazolium triflate [BMIM][OTf] ionic liquid. In contrast to reactions mn in 1,2-dichloroethane, little or no adamantane byproduct was detected in [BMIM][OTf. Furthermore, no isomerization of para-tolyladamantane was observed supporting the intramolecular nature of the formation of meta isomers. In competitive experiments with benzene-toluene mixture (1 1 molar ratio), high substrate selectivities were found (kT/kB = 16-17) irrespective of the alkylating agent. This is in sharp contrast to values about unity measured in 1,2-dichloroethane. 

Recently, the results of the isomerization and transalkylation of isomeric diethylbenzenes with benzene in the presence of triflic acid have been reported. The aim is to find the best condition for the preparation of ethylbenzene.283 285ort/ta-Diethylbenzene and benzene reacting in 1 1 molar ratio at 35°C gave ethylbenzene in 49% yield in 6h 285 An even higher yield was obtained with /mra-diethylbenzene (51% at 22°C), whereas meta-diethylbenzene produced ethylbenzene only in 29% yield.283 Both decreasing temperature and decreasing diethylbenzene/benzene ratio resulted in decreasing yields. 

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