Transalkylation and Dealkylation

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)  

Transalkylation86 206 207 can be effected by Lewis and Brpnsted acids and zeolites. Methylbenzenes exhibit exceptional behavior since they undergo isomerization 

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 

This then readily undergoes cleavage to produce benzene and a dialkylbenzene. The initial R+ cation initiating the reaction might arise from some impurity present in the reaction mixture. Consistent with this mechanism is the observed very low reactivity of methylbenzenes due to the necessary involvement of the primary benzyl cations. At the same time 1,1-diarylalkanes undergo cleavage with great ease. 

The transalkylation of fm-alkylbenzenes follows a different route, since they have no abstractable benzylic hydrogen. They were shown to transalkylate by a dealkylation-transalkylation mechanism with the involvement of free ferf-alkyl cations. The exceptional ability of fcrt-alkyl groups to undergo transalkylation led to the extensive utilization of these groups, especially the fm-butyl group, as positional protective groups in organic synthesis pioneered by Tashiro.223 

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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. 

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. 

These data expose some pertinent new facts concerning porphyrin diagenesis as well as shedding hght on the origin of petroporphyrins which have long been believed to be the products of chlorophyll diagenesis (24,25). Temperature is the promoter of both transalkylation and dealkylation reactions which can occur in deeply buried sediments. However, because of the extreme temperatures (300°-1000°C) to which the sediments in this case were exposed, dealkylation was favored over transalkylation. Under milder conditions (50°-150°C) the transalkylation... 

Zeolites are integral components of petrochemical refineries that produce benzene, xylene isomers, ethylbenzene and cumene. These aromatics must be high in purity for downstream conversion to polyesters and styrenic or phenolic based plastics. Catalytic processes for producing aromatics employ zeolites for isomerization, disproportionation, transalkylation, alkylation, and dealkylation. 

The following reactions occur isomerization, disproportionation, transalkylation, ring saturation, and dealkylation/cracking. The low yields of methane and ethane indicate that methyl groups are removed and combined to form higher paraffins by what is known as... 

Dealkylation, transalkylation and isomerization of the alkylaromatics. occurring on the zeolite component. 

Reaction Mechanisms, In catalytic cracking, the basic reaction mechanisms involve the formation of carbonium ions and include beta scission cracking, olefin isomerization, dealkylation, transalkylation and hydrogen transfer (Venuto and Habib, 1979). The rate of cracking of paraffins increases with increasing carbon number although the olefins (formed from... 

The most common by-product losses are due to transalkylation, dealkylation, saturation and cracking. Transalkylation results in toluene, trimethylbenzenes, methylethyl benzenes, benzene and ClOAs. These are the best by-products to have, because they are the easiest to react back into C8A in a transalkylation unit (if the aromatics complex is so equipped) without any loss of carbon atoms [59-61]. Dealkylation results in benzene, toluene, methane and ethane. The benzene and toluene are aromatics and represent valuable by-products, but the C1-C6 nonaromatics represent carbons that are lost from the complex as less valuable LPG and fuel gas. 

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. 

Serra, J.M., Guillon, E and Corma, A. (2004) A rational design of alkyl-aromatics dealkylation-transalkylation catalysts using C8 and C9 alkyl-aromatics as reactants. J. Catal., 227, 459-469. 

It is noteworthy at this point that methane was not found as a product, although it was ascertained that it could have been detected if it had formed. This indicates unambiguously that naphthalene is formed by transalkylation rather than by dealkylation, at variance with the conclusions of Solinas et al. [18,19]. Indeed, dimethylnaphthalenes were always present in the product, although the yield ratio Y m-np/Ynp never reached unity, even not at mild temperatures (e. g., 180 °C) where consecutive transalkylations into trimethylnaphthalenes were absent. This is an interesting finding which indicates that some dimethylnaphthalenes accumulate on the catalyst, in other words they are held strongly and not displaced efficiently from the zeolite surface by fresh methylnaphthalenes. Deactivation at mild temperatures. 

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. 

The final inspection of Figure 3.8 shows that the separation objectives have been fulfilled. The insertion of chemical transformation stages allows the operation on market demand. Dealkylation or transalkylation may convert a surplus of toluene to more valuable benzene and xylenes, while the isomerization improves the yield in p-xylene to 100%, if needed. 

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... 

As mentioned earlier, transalkylation, measured both by band width and A.I., was most extensive in the sample from —4.93 m (Table IV). No kinetic data for the transalkylation reaction are available however, intuitively a maximum in the range of 100°-150°C seems reasonable. Below that range the alkylation reaction is noncompetitive with other diagenetic reactions and above that range dealkylation and destruction predominate. 

Other Transformations of Alkylaromatics. An extensive amount of work has been done in the area of alkylaromatic isomerization-transalkylation, dealkylation, and related areas (7, 8,13, 32, 34, 35, 36, 41, 42, 60, 61, 62). Ward 67, 68, 69) also has conducted a series of very thorough studies into the relationship of activity in catalytic reactions, such as cumene dealkylation and alkylaromatic isomerization, and structural properties in synthetic faujasites, using IR spectroscopy among other... 

With alkyl aromatics, precious-metal H-mordenite catalysts are active for hydrogenation at low temperatures and hydrocrack at higher temperatures. Certain metal exchanged mordenites are effective for hydrogenation (30), dealkylation (7), transalkylation, disproportionation (31,38), and isomerization reactions (23). 

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]. 

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