Transalkylation

This section is limited to simple systems in which an alkyl group moves from one metal ion to another. Examples in which transalkylation occurs from a nonmetal to a metal can be found in other sections, particularly that on oxidative addition (Chapter 8). Kochi s review mentioned earlier contains a section on transalkylation. 

There have been two studies of transalkylation between cobalt centers. The rates of reversible transmethylation between dimethylcobalt (III) systems have been studied by measuring rates of scrambling of C0-CH3 and C0-CD3 groups between compounds of the type [CpCo(phosphine) (CH3)2] (see Table 7.5). The reaction is first order in each reagent. 

Endicott s group has measured and made an extremely detailed analysis of rate and equilibrium constants for transmethylation between various cobalt N4-macrocyclic systems (see Table 7.6 and accompanying structures). The reactions are first order in each reagent. Rates can vary by 10 (even for the back reactions where AG 0). AG is analyzed into components, intrinsic free energy barriers to transmethylation being small for cobalt corrin and large for sterically hindered neutral macrocyclic complexes. Estimates for C0-CH3 bond energies are between 33 and 48 kcal mol the bond is stabilized by unsaturation in the N4 macrocycle, but is also very sensitive to stereochemistry. 

7 a 5-alkylation from organochromium systems has been studied by Samuels and Espenson.  

Reactions (21) and (22) are first order in each reagent and appear to proceed through an 5e2 mechanism, as in many related reactions. Increasing bulkiness in R leads to large decreases in rate constant and entropy of activation (see Table 7.7). Presumably adamantyl has to react by front-side attack, but what of cy- 

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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). 

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 ... 

Other species that are also present in the feed, such as ethylbenzene and methylethylbenzenes will also undergo transalkylation reactions. These reactions tend to approach an equiUbrium that depends on the operating conditions. 

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

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

The solvent is 28 CC-olefins recycled from the fractionation section. Effluent from the reactors includes product a-olefins, unreacted ethylene, aluminum alkyls of the same carbon number distribution as the product olefins, and polymer. The effluent is flashed to remove ethylene, filtered to remove polyethylene, and treated to reduce the aluminum alkyls in the stream. In the original plant operation, these aluminum alkyls were not removed, resulting in the formation of paraffins (- 1.4%) when the reactor effluent was treated with caustic to kill the catalyst. In the new plant, however, it is likely that these aluminum alkyls are transalkylated with ethylene by adding a catalyst such as 60 ppm of a nickel compound, eg, nickel octanoate (6). The new plant contains a caustic wash section and the product olefins still contain some paraffins ( 0.5%). After treatment with caustic, cmde olefins are sent to a water wash to remove sodium and aluminum salts. 

Additioaal uses for higher olefias iaclude the productioa of epoxides for subsequeat coaversioa iato surface-active ageats, alkylatioa of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomeriza tion to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from a-olefias either by direct hydroalumination or by transalkylation. In addition, a number of heavy olefin streams and olefin or paraffin streams have been sulfated or sulfonated and used in the leather (qv) iadustry. 

Fig. 3. Unocal—Lummus—UOP ethylbenzene process AR = alkylation reactor TR = transalkylation reactor BC = benzene column ...

Fig. 5. UOP cumene process Rx = reactor R = rectifier BC = benzene column CC = cumene column T = transalkylation.

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. 

The cumene product is 99.9 wt % pure, and the heavy aromatics, which have a research octane number (RON) of 109, can either be used as high octane gasoline-blending components or combiaed with additional benzene and sent to a transalkylation section of the plant where DIPB is converted to cumene. The overall yields of cumene for this process are typically 97—98 wt % with transalkylation and 94—96 wt % without transalkylation. 

Zeolite Catalysts. Uaocal has iatroduced a fixed-bed fiquid-phase reactor system based oa a Y-type zeofite catalyst (62). The selectivity to cumene is geaeraHy betweea 70 and 90 wt %. The remaining components are primarily polyisopropylbenzenes, which are transalkylated to cumene ia a separate reactioa zoae to give an overall yield of cumene of about 99 wt %. The distillation requirements iavolve the separation of propane for LPG use, the recycle of excess benzene to the reaction zones, the separation of polyisopropylbenzene for transalkylation to cumene, and the production of a purified cumene product. 

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 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... 

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. 

Alkylator Feed heater Transalkylator Benzene column... 

Ethyltoluene is manufactured by aluminum chloride-cataly2ed alkylation similar to that used for ethylbenzene production. All three isomers are formed. A typical analysis of the reactor effluent is shown in Table 9. After the unconverted toluene and light by-products are removed, the mixture of ethyltoluene isomers and polyethyltoluenes is fractionated to recover the meta and para isomers (bp 161.3 and 162.0°C, respectively) as the overhead product, which typically contains 0.2% or less ortho isomer (bp 165.1°C). This isomer separation is difficult but essential because (9-ethyltoluene undergoes ring closure to form indan and indene in the subsequent dehydrogenation process. These compounds are even more difficult to remove from vinyltoluene, and their presence in the monomer results in inferior polymers. The o-ethyltoluene and polyethyltoluenes are recovered and recycled to the reactor for isomerization and transalkylation to produce more ethyltoluenes. Fina uses a zeoHte-catalyzed vapor-phase alkylation process to produce ethyltoluenes. 

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... 

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). 

Benzimidazole, 2-alkoxy-l-methyl-transalkylation, 5, 443 Benzimidazole, 1-alkyl-metal derivatives, 5, 448 reactions... 

Separation of close-boihng nieta and para y lenes Transalkylation... 

Ethylbenzene can also be produced by catalytic alkylation of benzene with ethylene. Benzene is alkylated with ethylene in a fixed bed alkylator. An excess of benzene is used to suppress the formation of di- and triethyl- benzenes. The excess benzene is removed from the alkylate by fractionation and recycled to the alkylator. The ethylbenzene is separated from the polyalkylated benzenes which are in turn fed to a separate reactor. Here benzene is added to convert the polyalkylated benzenes to monoethylbenzene by transalkylation. 

In the UOP process (Figure 10-5), fresh propylene feed is combined with fresh and recycled benzene, then passed through heat exchangers and a steam preheater before being charged to the reactor.The effluent is separated, and excess benzene recycled. Cumene is finally clay treated and fractionated. The bottom product is mainly diisopropyl benzene, which is reacted with benzene in a transalkylation section ... 

Figure 10-5. A flow diagram of the UOP cumene process " (1) reactor, (2,3) two-stage flash system, (4) depropanizer, (5) benzene column, (6) clay treatment, (7) fractionator, (8) transalkylation section.

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 ... 

There are two plausible reactions which lead to the observed products of the metathesis of alkenes. The first possibility involves cleavage of a carbon-carbon single bond adjacent to the double bond the second involves cleavage of the double bond itself. The following transalkylation... 

Mol et al. (61) analyzed the products formed in the heterogeneously catalyzed metathesis of propene labeled with 14C. With propene labeled in the 2-position there will be, in the case of a transalkylation reaction, an equal distribution of the radioactivity over the products ... 

In experiments with [2- 4C] propene in the presence of a Re207-Al2C>8 catalyst, the ethene formed showed no radioactivity, while the butene showed a specific radioactivity twice as high as that of the starting material. This result is completely consistent with a transalkylidenation scheme, and excludes a transalkylation reaction. Similar results were obtained by Clark and Cook (62) in experiments with radioactive propene on a M0O3-C0O-AI2O3 catalyst. 

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