Ethylbenzene xylene isomerization

In shape-selective catalysis, the pore size of the zeoHte is important. For example, the ZSM-5 framework contains 10-membered rings with 0.6-nm pore size. This material is used in xylene isomerization, ethylbenzene synthesis, dewaxing of lubricatius oils and light fuel oil, ie, diesel and jet fuel, and the conversion of methanol to Hquid hydrocarbon fuels (21). 

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. 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

Historically, the isomerization catalysts have included amorphous silica-alumina, zeolites, and metal-loaded oxides. All of the catalysts contain acidity, which isomerizes the xylenes and if strong enough can also crack the ethylbenzene and xylenes to benzene and toluene. 

The feed to an aromatics complex is normally a C6+ aromatic naphtha from a catalytic reformer. The feed is split into Cg+ for xylene recovery and C7 for solvent extraction. The extraction unit recovers pure benzene as a product and C7+ aromatics for recycling. A by-product of extraction is a non-aromatic C6+ raffinate stream. The complex contains a catalytic process for disproportionation and transalkylation of toluene and C9+ aromatics, and a catalytic process for isomerization of C8 aromatics. Zeolitic catalysts are used in these processes, and catalyst selectivity is a major performance factor for minimizing ring loss and formation of light and heavy ends. The choice of isomerization catalyst is dependent on whether it is desired to isomerize ethylbenzene plus xylenes to equilibrium or to dealkylate ethylbenzene to benzene while isomerizing the xylenes. Para-selectivity may also be a desired... 

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 ... 

Since the complexes fonned with p-xytene and o-xylene are less stable than that obtained with m-xyiene. they are salted out in the organic phase when m-xylene b to excess, facilitating their separation. However, these catalysts present the drawback of not isomerizing ethylbenzene which yidds an unstable complex, and of hivoiing certain side reactions, particularly dismutadon to benzene and dietbylbenzenes. 

Ethylbenzene, too, may be isomerized to xylenes. Xylenes are normally recovered from a mixed aromatics stream by extraction with sulfolane or a glycol. 

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)... 

For 1-hexene isomerization and for acid catalyzed Cg aromatic reactions all molecular sieves were evaluated in their calcined, powdered state. For the study of Cg aromatics, selected SAPO molecular sieves were aluminum exchanged or steam treated as noted in Table IV. For bifunctional catalysts used in paraffin cyclization/isomerization and ethylbenzene-xylene interconversions, the calcined molecular sieve powder was mixed with platinum-loaded chlorided gamma alumina powder. These mixtures were then bound using silica sol and extruded to form 1/16" extrudates which were dried and calcined at 500°C. The bifunctional catalysts were prepared to contain about 0.54 platinum and about 40 to 504 SAPO molecular sieve in the finished catalysts. 

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. 

Borosilicate catalysts provide high approach to thermodynamic equilibrium of the xylenes, and offer high selectivity in the conversion of ethylbenzene (8.12.22.50 ). In addition, they have been shown to be less prone to the effects of thermal and steam treatments than corresponding aluminosilicate zeolite catalysts (51). The catalytic activity of borosilicate catalysts was demonstrated to be a function of the structural boron content of the molecular sieve (22.36,50). In addition, the by-product distribution obtained from a borosilicate catalyst in a xylene isomerization/ethylbenzene conversion process was found to be distinctive (50), with high transethylation reactivity relative to transmethylation. 

In 195932, the heats of sulphonation of the isomeric ethylbenzene and the three dimethylbenzenes (xylenes) were reported by Leitman and Pevzner. After correcting for heats of solvation and dilution, these authors found the following exothermicities for ethylbenzene (37), 5.1 0.2 o-xylene (38a), 5.5 0.2 m-xylene (38b), 3.9 0.1 p-xylene (38c), 4.1 0.2 kcal mol-1. No product analysis was reported in this study so that we cannot ascertain the site(s) of sulphonation or even the possibility of rearrangement and/or transalkylation reactions. However, that 37,38a-38c have comparable heats of formation [AHf(lq) = — 2.9, — 5.8, — 6.1, — 5.8 kcal mol - A], and that their heats of sulphonation are comparable, suggests that all of these results are consistent with each other. The heat of formation of any of the solid isomeric ethyl- or dimethylbenzenesulphonic acids are thus ca — 136 kcal mol-1 with an anticipated few kcal mol-1 spread of values33. 

As mentioned earlier, benzene adsorption into molecular sieves, especially into faujasite-type zeolites, was extensively studied via IR spectroscopy by Barthomeijf and colleagues [792,793]. IR investigations of adsorption of benzene and especially simple benzene derivatives (toluene, ethylbenzene, xylenes) on zeolites were largely related to problems of diffusion (cf. Sect. 5.6.4) and catalytic reactions such as alkylation and isomerization (see Sect. 5.6.3). 

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. 

The experimental unit for isomerization of xylenes has the function of increasing the content of ortho-xylene in the xylenes stream from the ethylbenzene conversion. 

Extraction of C-8 Aromatics. The Japan Gas Chemical Co. developed an extraction process for the separation of -xylene [106-42-3] from its isomers using HF—BF as an extraction solvent and isomerization catalyst (235). The highly reactive solvent imposes its own restrictions but this approach is claimed to be economically superior to mote conventional separation processes (see Xylenes and ethylbenzene). 

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). 

Ethylbenzene, Thallium triacetate Ucmura, S. et al., Bull. Chem. Soc., Japan., 1971, 44, 2571 Application of a published method of thallation to ethylbenzene caused a violent explosion. A reaction mixture of thallium triacetate, acetic acid, perchloric acid and ethylbenzene was stirred at 65°C for 5 h, then filtered from thallous salts. Vacuum evaporation of the filtrate at 60°C gave a pasty residue which exploded. This preparation of ethylphenylthallic acetate perchlorate monohydrate had been done twice previously and uneventfully, as had been analogous preparations involving thallation of benzene, toluene, three isomeric xylenes and anisole in a total of 150 runs, where excessive evaporation had been avoided. 

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. 

Parex (1) [Para extraction] A version of the Sorbex process, for selectively extracting p-xylene from mixtures of xylene isomers, ethylbenzene, and aliphatic hydrocarbons. The feedstock is usually a C8 stream from a catalytic reformer, mixed with a xylene stream from a xylene isomerization unit. The process is operated at 177°C the desorbent is usually p-diethylbenzene. The first commercial plant began operation in Germany in 1971 by 1992, 453 plants had been licensed worldwide. Not to be confused with Parex (2). 

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. 

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... 

Table ill. Selectivity in Xylene Isomerization Feed 15% Ethylbenzene, 85% Xylene (63% m, 22% o)... 

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