Catalysts toluene transalkylation

The Xylene Plus process of ARGO Technology, Inc. (95,96) and the FINA T2BX process (97) also use a fixed-bed catalyst in the vapor phase for transalkylation of toluene to produce xylenes and benzene. The Mobil low temperature disproportionation (LTD) process employs a zeoHte catalyst for transalkylation of toluene in the Hquid phase at 260—315°C in the absence of hydrogen (98). 

By considering these aspeots, this work deals with the study of coke formation on H-mordenite during the benzene transalkylation with C9 aromatics, under several reaction conditions, in order to state the best condition to reduce catalyst deactivation in industrial processes. Although there are several publications and patents about the toluene transalkylation [10,13], there are very few works concerning the benzene transalkylation [1, 14]. Several industrial processes uses mordenite for toluene transalkylation with high performance [2]. 

Naflon-H is a very useful catalyst for transalkylation reactions. Transfer of a t-butyl group occurs very easily over Nafion-H at temperatures as low as 330 K. For example, 2,6-di-f-butyl—p-cresol is dealkylated in 0.5 h top-cresol. Toluene acts as abetter acceptor than benzene. ... 

The Tatoray process, which was developed by Toray Industries, Inc., and is available for Hcense through UOP, can be appHed to the production of xylenes and benzene from feedstock that consists typically of toluene [108-88-3] either alone or blended with aromatics (particularly trimethylbenzenes and ethyl-toluenes). The main reactions are transalkylation (or disproportionation) of toluene to xylene and benzene or of toluene and trimethylbenzenes to xylenes in the vapor phase over a highly selective fixed-bed catalyst in a hydrogen atmosphere at 350—500°C and 1—5 MPa (10—50 atm). Ethyl groups are... 

Tetralin, hydrogenation of, 12 Titanium compounds as catalysts, 188 Titanium dichloride, 192, 193 number of propagation centers, 198-200 Titanium trichloride, 193, 194 Toluene in exhaust gases, 67 Transalkylation, 141, 142 Transalkylidenation, 142 Transition metal compounds as catalysts, 174... 

The catalytic performances obtained during transalkylation of toluene and 1,2,4-trimethylbenzene at 50 50 wt/wt composition over a single catalyst Pt/Z12 and a dualbed catalyst Pt/Z 121 HB are shown in Table 1. As expected, the presence of Pt tends to catalyze hydrogenation of coke precursors and aromatic species to yield undesirable naphthenes (N6 and N7) side products, such as cyclohexane (CH), methylcyclopentane (MCP), methylcyclohexane (MCH), and dimethylcyclopentane (DMCP), which deteriorates the benzene product purity. The product purity of benzene separated in typical benzene distillation towers, commonly termed as simulated benzene purity , can be estimated from the compositions of reactor effluent, such that [3] ... 

Table 1. Product yields of transalkylation reaction of toluene and 1,2,4-trimethylbenzene (at 623 K) over Pt-supported single- and dual-bed catalysts.

Tatoray [Transalkylation aromatics Toray] A process for transalkylating toluene, and/or trimethylbenzenes, into a mixture of benzene and xylenes. Operated in the vapor phase, with hydrogen, in a fixed bed containing a zeolite catalyst. Developed jointly by Toray Industries and UOP and now licensed by UOP. First operated commercially in Japan in 1969 as of 1992, 23 units were operating and 6 more were in design and construction. 

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%. 

Side Reactions One of the major side reactions that occurs during isomerization of Cg aromatics is transalkylation. This reaction produces species such as toluene, trimethylbenzene, methylethylbenzene, dimethylethylbenzene, benzene and diethylbenzene. The types and specific isomers of transalkylated products formed depend on the acidity and spatial constraints of the zeolitic catalyst used. These reactions can be controlled through modification of catalyst properties, especially pore size and external acidity, though these reactions are still among the major contributors to xylene losses. 

After the separator, the liquid product is sent to a deheptanizer to remove toluene, benzene and other lighter products. If this is an EB isomerization-style process, the deheptanizer operation may be constrained by the need to send the C8N to the bottoms, which also results in more toluene in the bottoms than would be present in an EB dealkylation system (which does not require C8N recirculation). The elevated toluene is not generally detrimental to catalyst performance, primarily acting as a diluent, although in some cases it may actually be beneficial, by pushing the toluene -i- C9A transalkylation equilibrium back toward C8A. 

There are two methods of manufacture of the xylenes. The major one is from petroleum by catalytic reforming with a platinum-alumina catalyst. The second method (which has been developed recently) is by processes involving the disproportionation of toluene or the transalkylation of toluene... 

Toluene disproportionation and transalkylation are important industrial processes in the manufacture of p-xylene. Toluene disproportionation [Eq. (5.73)] transforms toluene into benzene and an equilibrium mixture of isomeric xylenes. The theoretical conversion of toluene is 55%. Commercial operations are usually run to attain 42 18% conversions. In conventional processes308 309 324 325 alumina-supported noble metal or rare-earth catalysts are used in the presence of hydrogen (350-... 

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. 

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... 

An interesting way to increase pX production of an aromatics complex is to transform less valuable aromatics such as toluene and C9+ aromatics into xylenes. This is achieved either by toluene disproportionation (TDP) or by toluene C9+ aromatics transalkylation. There are two types of disproportionation processes Normal TDP produces a xylene cut where pX content is at thermodynamic equilibrium, while with STDP pX content is much higher (para-selectivity). This is possible when a selectivated zeolite (generally MFI) is used. For normal TDP as well as for transalkylation processes, mordenites doped with a hydrogenating metal to limit coking are used as catalysts. 

The catalytic reactions occurring in C8 aromatics isomerization and toluene disproportionation or transalkylation with heavy aromatics are given in the following paragraphs. The catalysts used for these reactions and the processes are also described. 

ATA [Advanced TransAlkylation] A process for converting aromatic hydrocarbons, whose molecules contain more than nine carbon atoms, to a mixture of benzene, toluene, and xylenes. The catalyst is a zeolite promoted by a platinum metal. Developed by Zeolyst International and SK Corporation operated in Korea by SK Corporation since 1999. 

Blocking the pore mouth and reducing the diffiisivities of the xylenes does not change this overall picture for toluene methylation, but enhances the p- selectivity [258]. As a negative side effect the catalysts deactivate and this has to be balanced with higher reaction temperatures. The higher reaction temperatures are required to open new reaction channels (dealkylation, transalkylation, disproportionation) to drain products fi om the pores as the longer residence times lead to polymethylated products that are unable to leave the zeolite pores and would eventually block all acid sites [258]. 

Wu and Leu, ° and Bursian et have carried out disproportionation and transalkylation of toluene with pseudocumene, using various modifications of mordenite zeolite catalysts. The disproportionation... 

De-aluminated mordenites were claimedto give more active and stable catalysts for toluene disproportionation than conventional H-mordenite. Becker, Karge, and StreubeP studied the alkylation of benzene with ethene and propene over an H-mordenite catalyst. Shape-selective catalysis was found because only ethylbenzene, w-diethylbenzene, p-diethylbenzene, cumene, p-di-isopropylbenzene, and m-di-isopropylbenzene were detected in the products neither o-diethylbenzenes nor higher alkylated products were found. The results are in agreement with earlier transalkylations over H-mordenite. Catalyst aging was caused by olefin polymerization. The selectivity of Be-mordenite... 

The process converts toluene to benzene and xylenes by disproportionation (also called transalkylation). The xylenes also disassociate to form toluene and trimethyl-benzene. These reactions occur with a catalyst in the presence of hydrogen, with a typical ratio of 7 1 hydrogen to toluene feed to the reactor. The hydrogen also reacts with toluene to form benzene and methane or with xylenes to form benzene and methane. The molar consumption of hydrogen is about 10 percent of that of toluene. 

The significant finding was that the xylene fraction was 99% para. The other fractions are not lost. Toluene can be disproportionated to p-xylene and benzene with H-ZSM-5 treated with a little hexamethyldisiloxane to give 99% p-xylene, so that the usual separation of the ortho- and meta isomers with another zeolite would not be required.177 Benzene can be transalkylated with the higher aromatics to give toluene. Ethylbenzene can be isomerized to p-xylene. Ethylbenzene can be alkylated with ethanol in the presence of a modified ZSM-5 catalyst to produce p diethylbenzene with 97% selectivity.178... 

Transalkylation is a more recent entry into this field and uses an acidic catalyst. The scheme is illustrated in Figure 8. As presently designed, it is used to convert toluene to benzene plus xylenes, but Co and higher aromatics could be introduced. 

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