Transalkylation of aromatics

The synergism of a dual-catalyst system comprising of Pt/ZSM-12 and H-Beta aiming to improve the benzene product purity during transalkylation of aromatics has been studied. Catalyst compositions of the dual-catalyst system were optimized at various reaction temperatures in terms of benzene product purity and premium product yields. Accordingly, a notable improvement in benzene purity at 683 K that meets the industrial specification was achieved using the cascade dual-bed catalyst. 

Serra, J.M., Corma, A., and Guillon, E. (2008) Catalyst comprising a lOMR zeolite and a 12MR zeolite, and its use in transalkylation of aromatic hydrocarbons. US Patent 7419931. 

Merlen, E., Alario, F., Ferrer, N., and Martin, O. (2005) Catalyst comprising at least one zeolite with structure type NES and rhenium, and its use for transalkylation of aromatic hydrocarbons. US Patent 6864400. 

MOR Mordenite 7.0 X 6.5 P, N. xylenes. TMB Trialkylmonoaro Hydroisomerization of n-P Xylene isomerization, transalkylation of aromatics, cumene synthesis... 

Transalkylation is also catalyzed by acids, but requires more severe conditions than isomerization. As shown below, the methyl migration is intermolecular and ultimately produces a mixture of aromatic compounds ranging from benzene to hexamethylbenzene. The overall equiHbrium constants for all possible methylbenzenes have been deterrnined experimentally and calculated theoretically (Fig. 2 and Table 3). 

Table 4. Tatoray Transalkylation of Toluenes and Aromatic Compounds, Relative Wt Units...

Propylene feed, fresh benzene feed, and recycle benzene are charged to the upflow reactor, which operates at 3—4 MPa (400—600 psig) and at 200—260°C. The SPA catalyst provides an essentially complete conversion of propylene [115-07-1] on a one-pass basis. A typical reactor effluent yield contains 94.8 wt % cumene and 3.1 wt % diisopropylbenzene [25321-09-9] (DIPB). The remaining 2.1% is primarily heavy aromatics. This high yield of cumene is achieved without transalkylation of DIPB and is unique to the SPA catalyst process. 

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). 

In this section, the reactivities of organosilicon compounds for the Friedel-Crafts alkylation of aromatic compounds in the presence of aluminum chloride catalyst and the mechanism of the alkylation reactions will be discus.sed, along with the orientation and isomer distribution in the products and associated problems such as the decomposition of chloroalkylsilanes to chlorosilanes.. Side reactions such as transalkylation and reorientation of alkylated products will also be mentioned, and the insertion reaction of allylsilylation and other related reactions will be explained. 

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 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] ... 

Rozanska, X., Saintigny, X., van Santen, R.A., and Hutsdika, F.A. (2001) DFT smdy of isomerization and transalkylation reactions of aromatic spedes catalyzed by addic zeolites. /. Catal, 202,141-155. 

All, S.A., Al-Nawad, K., Okomoto, T., and Ishikawa, K. (2005) Transalkylation of heavy aromatics for enhanced xylene production. Proceedings of 15th Saudi-Japan Joint Symposium,... 

In model compounds, the aliphatic side chains on aromatic nuclei transalkylate without rearrangement s ll However, the reaction of aliphatic bridges is more complicated. For example, transalkylation of l-(4-methoxyphenyl)-3-(2-naphthyl)... 

In some cases, addition of selective poisons can increase the selectivity of catalytic processes. According to Williams et al. (85) the transalkylation of alkyl aromatic C8 hydrocarbons, occurring during isomerization, can be suppressed by adding (CH3)3CNH2 (only 5 ppm) to the reacting mixture. At the same time, isomerization is increased. 

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... 

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