Dealkylation of ethylbenzene

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 a zeolite catalyst sample, which was coked via dealkylation of ethylbenzene at reaction temperatures somewhat higher than those of the sorption experiments, the diffusion coefficient of ethylbenzene remained essentially unchanged even though the sorption capacity significantly decreased due to deposition of carbonaceous material. 

Application of the IR method proved to be also suitable for the measurement of diffusivities in coking porous catalysts. This was deihonstrated by uptake experiments with ethylbenzene where the sorbent catalyst, H-ZSM-5, was intermittently coked in-situ via dealkylation of ethylbenzene at temperatures (465 K) somewhat higher than the sorption temperature (395 K). Coke deposition was monitored in-situ via the IR absorbance... 

Deposition of heteropolyacids on ZSM-5 and its utilization for alkylation of toluene was found to enhance the selectivity of /wrra-ethyltoluene [8] whereas effects on the isomerization of weto-xylene and dealkylation of ethylbenzene were reported to be rather small [9]. The modification of the external zeolite surface contributing only a small fraction to the overall surface area is difficult to characterize. 

The isomerization of xylenes and the dealkylation of ethylbenzene into benzene are other possibilities that have been industrially exploited on a large scale since the mid 1980s. This requires a catalyst that is more effective than mordenite which is not very active in dealkylation. The best-suited catalyst is undeniably ZSM-5 (Table 2). If this zeolite is used in its purely acid form, the ethylene produeed by dealkylation of ethylbenzene at around 350°C is alkylated on another ethylbenzene moleeule mainly to form paradiethylbenzene. some of which is produced industrially by this method. In order to avoid the alkylation reaction in the dealkylation of ethylbenzene, it is necessary to operate under hydrogen pressure ( 1.5 MPa) and to associate a small amount of Pi to the ZSM-5 zeolite, which hydrogenates the ethylene into ethane as it is produced. 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

The activity of ZSM-5 increases linearly with the Al-concentration varying from 10 to 10,000ppm [161,162]. This correlation has also been observed with the conversion of MeOH and the dealkylation of ethylbenzene [163,63]. This one-to-one relationship between the activity of the zeolite and the amount of Al-atoms suggests that each active site contributes equally to the observed activity, whatever is the total concentration. 

Catalytic methods of dehydrogenation have received attention because Nager, M., Dealkylation of Ethylbenzene, Ind. Eng. Chem., 49, 39 (1957). 

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. 

As mentioned earlier, at higher temperature the selective conversion of ethylbenzene is further enhanced by opening an additional pathway, i.e., dealkylation, that yields increased amounts of benzene of high purity ... 

The rather low concentration of the desired p-xylene component in the Parex unit feed means a large fraction of the feed stock contains other A8 components that are competing for adsorption sites in the adsorbent zeoHte cages. Due to this typically lean feed, a significant hike in the Parex unit capacity can be obtained by even a small increase in the composition of the p-xylene. Techniques to increase the p-xylene feed concentration include greater dealkylation of the ethylbenzene in the Isomar unit by converting from an ethylbenzene isomerization catalyst to... 

In the alkylation of ethylbenzene with ethylene, with conventional acid catalysts under usual conditions,. veobutyl benzene is a byproduct. rec-Butylbenzene was detected when the reaction was carried out over catalysts such as supported phosphoric acid,189 ferric phosphate,189 or AICI3—NiO—Si02-190 When Nafion-H or AICI3 are used, no such byproduct is detected, probably due to fast dealkylation of sec-butylbenzene under the more acidic conditions. 

The mechanism of ethylbenzene disproportionation depends on the zeolite pore structure (3). With large pore zeolites, this reaction occurs mainly through the carbocation chain mechanism proposed for xylene disproportionation (Figure 9.4) which involves benzylic carbocations and diarylmethane intermediates. With MFI zeolites in the pores of which steric constraints limit the formation of the bulky diarylmethane intermediates, ethylbenzene disproportionation occurs mainly through a successive dealkylation-alkylation process ... 

This difference in mechanism is clearly demonstrated by substituting bifunctional catalysts for acidic catalysts (29). The introduction of platinum in MFI catalysts leads to a large decrease in the rate of ethylbenzene disproportionation (divided by 6), which is due to a large consumption of ethylene by hydrogenation as shown by the large increase in the rate of dealkylation. On the other hand, the introduction of platinum in MOR catalysts leads to a limited change in the rates of disproportionation and dealkylation. 

MFI zeolites seem to be the most efficient for EB dealkylation, in terms of activity, selectivity and stability. In the 70s, on metal-free MFI catalysts, EB was disproportionated into benzene and diethylbenzenes. As indicated above, with MFI catalysts, ethylbenzene disproportionation occurs through a deethylation-ethylation mechanism, with ethylene as desorbed intermediate. The addition of a metal (carried out early 80s) allows a rapid and irreversible conversion of ethylene into ethane with a consequent shift of ethylbenzene transformation from disproportionation to hydrodealkylation. The selectivity is highly sensitive to temperature that must be in the range 380°C-460°C to limit both alkylation and naphthene cracking. 

Galich el al. 136) showed that, within an alkali metal-exchanged X series (Table XIX), as cationic radius increased, cumene conversion decreased. Also, the products contained larger amounts of l-methyl-3-ethylbenzene and less toluene, ethylbenzene, and propenylbenzene. The dealkylation of lower temperature (260°) over REX catalyst than did other related dealkylations. The major liquid product was benzene, with small amounts of toluene, ethylbenzene, and cumene. Isobutane was the major gaseous product, and no olefins were observed. 

Catalysts which promote carbonium ion reactions have long been used for hydrocarbon isomerization (2, 6). Although such catalysts promote aromatics isomerization (3), in the case of ethylbenzene, the predominant reactions are disproportionation and dealkylation (5). Pitts, Connor, and Leum (7) demonstrated that hydrogenated intermediates were required to isomerize both ethylbenzene and cumene over platinum-... 

Conversion of alkylbenzenes over zeolite catalysts. I-Dealkylation and disproportionation of ethylbenzene over... 

Dealkylation (or hydrogenolysis) of ethylbenzene was largely and disproportionation almost completely suppressed. Similarly, dehydrogenation of cyclohexane over Pd,Ca,H-ZSM-5 prepared via successive SSIE proceeded with high selectivity and very low catalyst deactivation (cf. Fig. 63). 

Benzene was first isolated by Faraday in 1825 from the liquid condensed by compressing oil gas. It is the lightest fraction obtained from the distillation of the coal-tar hydrocarbons, but most benzene is now manufactured from suitable petroleum fractions by dehydrogenation (54%) and dealkylation processes. Its principal industrial use is as a starting point for other chemicals, particularly ethylbenzene, cumene, cyclohexane, styrene (45%), phenol (20%), and Nylon (17%) precursors. U.S. production 1979 2-6 B gals. 

After passing through the reaction chamber the products are cooled and the aluminium chloride, which is in the form of a complex with the hydrocarbons, settles out. The ethylbenzene, benzene and polyethylbenzenes are separated by fractional distillation, the ethylbenzene having a purity of over 99%. The polyethylbenzenes are dealkylated by heating at 200°C in the presence of aluminium chloride and these products together with the unchanged benzene are recycled. 

Xyloflning [Xylol refining] A process for isomerizing a petrochemical feedstock containing ethylbenzene and xylenes. The xylenes are mostly converted to the equilibrium mixture of xylenes the ethylbenzene is dealkylated to benzene and ethylene. This is a catalytic, vapor-phase process, operated at approximately 360°C. The catalyst (Encilite-1) is a ZSM-5-type zeolite in which some of the aluminum has been replaced by iron. The catalyst was developed in India in 1981, jointly by the National Chemical Laboratory and Associated Cement Companies. The process was piloted by Indian Petrochemicals Corporation in 1985 and commercialized by that company at Baroda in 1991. 

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. 

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

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