Transalkylation and Disproportionation

Xylenes Produetion Via Toluene Transalkylation and Disproportionation. The toluene that is produced from processes such as catalytic reforming can be converted into xylenes via transalkylation and disproportionation. Toluene disproportionation is defined as the reaction of 2 mol of toluene to produce 1 mol of xylene and 1 mol of benzene. Toluene transalkylation is defined as the reaction of toluene with or higher aromatics to produce xylenes ... 

Experiments carried out by feeding TBPE only over H-MWW, showed that the O-alkylated product do not rearrange to C-alkylated phenol derivatives in our conditions, but it is hydrolysed to phenol. So, TBPE is not a reaction intermediate and perforce O-and C-alkylation are parallel reactions. Also o-TBP and p-TBP were fed each of them alone over our catalysts. As it could be observed in Fig. lb, o-TBP convert to p-TBP (by transalkylation) and in minor extent 2,4-DTBP (by disproportionation), while p-TBP (results not shown here) convert to 2,4-DTBP (by disproportionation). Because the transalkylation and disproportionation are bimolecular reactions and need large spaces, it is plausible to suppose that the alkylation could not take place in the pores, but on the external surface of H-MWW zeolites. 

Due to the greater industrial importance of benzene and xylenes than toluene, a large number of studies have concentrated on the transalkylation and disproportionation of these two arenes. 

Studies already published in the literature report that a higher activity for platinum catalysts supported on zeolites compared with y-Al203 supported catalysts (Katzer, 1977). In addition, the reactions of ethylbenzene through routes of isomerization, transalkylation, and disproportionation are reported as preferred. 

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... 

Transall lation. Two molecules of toluene are converted iato one molecule of benzene and one molecule of mixed xylene isomers ia a sequence called transalkylation or disproportionation. Economic feasibiUty of the process strongly depends on the relative prices of benzene, toluene, and xylene. Operation of a transalkylation unit is practical only when there is an excess of toluene and a strong demand for benzene. In recent years, xylene and benzene prices have generally been higher than toluene prices so transalkylation is presendy an attractive alternative to hydrodealkylation (see also Btx... 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

The typical operating conditions of xylene and EB isomerization processes are shown in Table 9.3. These conditions minimize the above side reactions. Pressure, temperature and H2/HC ratio are key parameters that define the partial pressure of C8 naphthenes intermediates for EB isomerization. Naphthene cracking and disproportionation/transalkylation are responsible for the C8 aromatics net losses that affect the overall pX yield. The C8 recycled stream from the isomerization unit to the separation unit is three times higher than the fresh feed stream (since there cannot be more than 24% of pX in the C8 aromatic cut after isomerization). This means that each percent of loss in the isomerization unit will decrease the pX yield by 3%. For example, when standard mordenite-based catalysts lead to 4% of net losses, the overall pX yield is roughly 88%. 

Bifunctional catalysts (acid hydrogenation over supported metal) are required for the isomerization reaction. Under these conditions, reactions of hydrogenation of aromatics (reactants and products) and of alkene reactions occur simultaneously as well as secondary reactions, such as hydrocracking of naphthenic and hydrogenolysis. Due to the acidic character of the support, the transalkylation reactions, disproportionation, and isomerization of xylenes also occur in bifunctional catalysts. 

Reactions of p-DIPB isomer, benzene and triflic acid with different molar ratios of 1 1 1,1 3 1 and 1 6 1 respectively at room temperature caused mainly rapid isomerization as well as transalkylation to cumene at the first half an hour. Then there was rapid transalkylation and little disproportionation to mainly cumene and TIPB as shown in figures (1,2, and 3). 

The activity of triflic acid was similar to that found in our previous studies such as alkylation of benzene with ethene[6], isopropylation of heazsaae with propene[7], isomerization and disproportionation of diethylbenzene isomers[8], and isomerization and transalkylation of diethylbenzene isomers [9]. 

The chemistry of the o-complex is also important in transalkylations and related reactions. For example, monoalkylbenzenes undergo disproportionation reactions by transalkylation, which in the case of cumene provides diisopropylbenzene and benzene (Eq. 1.12) [75]. In this case, cnmene serves as a nucleophile, while the isopropyl cation is the likely electrophile. Shape-selective zeolite catalysts... 

There are several commercial processes that produce xylenes via disproportionation or transalkylation. These include UOP s Tatoray and PX-Plus,... 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

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). 

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. 

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 catalytic disproportionation of toluene (Figure 10-13) in the presence of hydrogen produces henzene and a xylene mixture. Disproportionation is an equilihrium reaction with a 58% conversion per pass theoretically possible. The reverse reaction is the transalkylation of xylenes with henzene ... 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

In the case of toluene disproportionation, reduction to benzene occurs when a methyl group pops off (hydrodealkylation takes place) and oxidation to xylene occurs as that methyl group that popped off attaches itself to another toluene molecule (a transalkylation 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%. 

It was concluded at this point that zeolites with a very spacious pore system, such as faujasites or ZSM-20, are inappropriate catalysts for the isomerization of 1-methyl-naphthalene. Subsequently, a zeolite with much narrower pores was tested, viz. HZSM-5. Pertinent results are shown in Fig. 2. At 300 C, the conversion is low and even a temperature increase of 100 °C does not bring about a considerable increase in conversion. We presume that the reaction of 1-methylnaphthalene in HZSM-5 is controlled by diffusioiL There were practically no side reactions such as cracking, dealkylation or transalkylation, in other words XjMHp Y2.M.NP "e identical. This is at variance with the results of Matsuda et al. [21] who did observe some disproportionation on their H2SM-5 sample at 300 °C. More work is needed to elucidate the reasons for this different catalytic behavior of various samples of HZSM-5. As a whole, zeolite ZSM-5 was discarded at this stage due to its too narrow pore system. 

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