Transalkylation ethylbenzene

An early liquid phase process used an aluminum trichloride catalyst at 85°- 95°C at pressures just above atmospheric. A low ethylene/benzene ratio was used to limit the formation of diethylbenzene and other polyethylbenzenes. By-products could, however, be recycled with benzene and were recovered as ethylbenzene by transalkylation. Ethylbenzene selectivity was about 94% based on benzene and higher on et lene. The catalyst that formed in solution was thought to be HAlCLt-w-C 5C2H5, which gradually deactivated and was replenished as required. Other acid catalysts such as boron trifluoride can be used in the hquid phase process, which is still widely used in older plants. 

Other species that are also present in the feed, such as ethylbenzene and methylethylbenzenes will also undergo transalkylation reactions. These reactions tend to approach an equiUbrium that depends on the operating conditions. 

Fig. 3. Unocal—Lummus—UOP ethylbenzene process AR = alkylation reactor TR = transalkylation reactor BC = benzene column ...

The transalkylation reaction is essentiaHyisothermal and is reversible. A high ratio of benzene to polyethylbenzene favors the transalkylation reaction to the right and retards the disproportionation reaction to the left. Although alkylation and transalkylation can be carried out in the same reactor, as has been practiced in some processes, higher ethylbenzene yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. 

Ethyltoluene is manufactured by aluminum chloride-cataly2ed alkylation similar to that used for ethylbenzene production. All three isomers are formed. A typical analysis of the reactor effluent is shown in Table 9. After the unconverted toluene and light by-products are removed, the mixture of ethyltoluene isomers and polyethyltoluenes is fractionated to recover the meta and para isomers (bp 161.3 and 162.0°C, respectively) as the overhead product, which typically contains 0.2% or less ortho isomer (bp 165.1°C). This isomer separation is difficult but essential because (9-ethyltoluene undergoes ring closure to form indan and indene in the subsequent dehydrogenation process. These compounds are even more difficult to remove from vinyltoluene, and their presence in the monomer results in inferior polymers. The o-ethyltoluene and polyethyltoluenes are recovered and recycled to the reactor for isomerization and transalkylation to produce more ethyltoluenes. Fina uses a zeoHte-catalyzed vapor-phase alkylation process to produce ethyltoluenes. 

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). 

Ethylbenzene can also be produced by catalytic alkylation of benzene with ethylene. Benzene is alkylated with ethylene in a fixed bed alkylator. An excess of benzene is used to suppress the formation of di- and triethyl- benzenes. The excess benzene is removed from the alkylate by fractionation and recycled to the alkylator. The ethylbenzene is separated from the polyalkylated benzenes which are in turn fed to a separate reactor. Here benzene is added to convert the polyalkylated benzenes to monoethylbenzene by transalkylation. 

Karge and Ladebeck (90) studied the alkylation of benzene with olefins over aluminum-deficient, beryllium exchanged mordenite and found a considerable extension of the lifetime of the catalyst, as compared to H-mordenite. The authors were able to carry out quite efficiently the alkylation reaction as well as the transalkylation of ethylbenzene at relatively low temperatures. 

Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise it would build up in the recycle stream. With dual-functional catalysts, ethylbenzene is converted partly to xylenes and is partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene is converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity is produced as a valuable by-product. 

Figure 5. Transalkylation reactions in the ethylbenzene-xylene system.

This may be partly the result of increased steric crowding in the transition state of transalkylation. Another contributory factor to the increased selectivity in ZSM-5 is the higher diffusion rate of ethylbenzene vs m-/o-xylene in ZSM-5 and hence a higher steady state concentration ratio [EB]/[xyl] in the zeolite interior than in the outside phase. Diffusional restriction for xylenes vs ethylbenzene may also be indicated by the better selectivity of synthetic mordenite vs ZSM-4, since the former had a larger crystal size. 

In commercial xylene isomerization, it is desirable that the necessary ethylbenzene conversion is accompanied by a minimum conversion (transalkylation) of xylenes, since the latter constitutes a downgrading to less valuable products. The ability of ZSM-5 to convert ethylbenzene via transalkylation in high selectivity, as shown in Table II, leads to high ultimate p-xylene yields in a commercial process. With a simulated commercial feed containing 85% m- and o-xylene and 15% ethylbenzene, we have obtained the data shown in Table III. It is seen that for a given ethylbenzene conversion, the xylene loss... 

Figure 6. Transalkylation of an ethylbenzene-xylene feed over HZSM-4. TMB = trimethylbenzene, DMEB = dimethylethyl-benzene, DEB = diethylbenzene, and ETol = ethyltoluene. Feed 16% EB, 62% m-xylene, 22% o-xylene. Temperature ...

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. 

In the chapter on benzene and in Figure 2—7, you saw that toluene disproportionation yielded both benzene and mixed xylenes. When the catalyst-prompted methyl group removes itself from the toluene, it usually attaches itself to another toluene molecule in a way that it forms xylene. That s transalkylation. The freed methyl group might attach itself momentarily to another free benzene molecule, or it might attach itself to the methyl group of another toluene, forming ethylbenzene. However, the creation of benzene and xylenes predominates, and the combined yields of the two are 92-97%. 

The reaction of ethylbenzene with five equivalents of Ic under the same alkylation conditions used for toluene, gives pentakis- (25%), tetrakis- (9%), tris- (4%), and bis[2-(dichloromethylsilyl)ethyl]ethylbenzene (1%) as well as a mixture of many transalkylated products (44%). It is of interest that longer alkyl-substituted benzenes exhibited different behavior in peralkylations with Ic. The transalkylation of ethylbenzene is responsible for the significantly low yield (25%) of peralkylation product in comparison with yields obtained from the alkylation of benzene " or toluene. Peralkylation of K-propylbenzene and K-butylbenzene gives similar results to those of ethylbenzene. 

The catalytic activity Z -5 -Jype. zeolitesmodified by polyvalent cations (Ca, Mg, x, In, Dy, So, Ga, A1, Be ), were investigated in reactions of toluene al lation by ethylene and transalkylation of ethylbenzene. The presence in these samples of aprotic acid centres of different strength and absence of prot-ic centres were established by IR spectroscopy technique of adsorbed CO. The strength of aprotic centres was characterized by the heat of CO adsorption and was shown to be a main factor determining the selectivity of catalytic action of the systems studied. 

In this work acidic and catalytic properties of ZSM-5 type zeolites containing polyvalent cations in exchange positions wera Investigated. Toluene alkylation by ethylene and ethylbenzene transalkylation were studied as model reactions. 

Transalkylation of ethylbenzene temperature Inside catalyst bed - 385-395 0 starting reagent- ethylbenzene WHSV to ethylbenzene - 3 f 4-3,6 h . ... 

Pig. 2 Scheme of main reaction routes of ethylbenzene transalkylation (DEB=diethylbenzene). 

Table 2. Values of para-seleotivlty (pS) to different dialkyl-benzenes in reactions of toluene alkylation by ethylene and transalkylation of ethylbenzene.

Alkylation of benzene with ethylene gives ethylbenzene,283,284,308,309 which is the major source of styrene produced by catalytic dehydrogenation. High benzene ethylene ratios are applied in all industrial processes to minimize polyethylation. Polyethylbenzenes formed are recycled and transalkylated with benzene. Yields better than 98% are usually attained. Reactants free of sulfur impurities and water must be used. 

Friedel-Crafts catalysts (corrosion, continuous catalyst makeup). Although it gives a broader spectrum of alkylated products, recycling and transalkylation ensure high ethylbenzene yields. Steamed ZSM-5 and chrysozeolite ZSM-5 were shown by Union Carbide to afford ethylbenzene with high selectivity in the alkylation of benzene with ethanol.317... 

In the Mohil-Badger vapor-phase process, fresh and recycled benzene are vaporized and preheated to the desired temperature and fed to a multistage fixed-bed reactor. Ethylene is distributed to the individual stages. Alkylation takes place in tile vapor phase. Separately, file polyethylbenzene stream from the distillation section is mixed with benzene, vaporized and heated, and fed to the transalkylator, where polyethylbenzenes react with benzene to form additional ethylbenzene. The combined reactor effluent is distilled in the benzene column. Benzene is condensed in the overhead for recycle to the reactors. The bottoms from the benzene column are distilled in the ethylbenzene column to recover the ethylbenzene product in the overhead. The bottoms stream from the ethylbenzene column is further distilled in the polyefitylbenzene column to remove a small quantity of residue. The overhead polyethylbenzene stream is recycled to the reactor section for transalkylation to ethylbenzene. 

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. 

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. 

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