8 Polyalkylated benzenes - production and uses

Petroleum- and coal-derived heavy gasoline fractions with a boiling range of around 160 to 220 DC contain polymethylated benzenes, such as trimethylbenzenes (pseudocumene, mesitylene and hemimellitene), together with the tetramethylated benzenes durene, isodurene and prehnitene. Indane and indene compounds, penta- and hexamethylbenzene and cumene, are also present in these heavy gaso­line fraction. (Cumene is predominantly converted to phenol as described in Chapter 5.2).

~CH' CH3

Pseudocumene Mesityiene

H3CIQYCH3

H3C~CH3 ~CH3

H3C~CH3

Durene

~CH3 H3C¥CH3

CH3

Pentamethyibenzene

Isodurene

CH3

H3C~CH3 H3C¥CH3

CH3

Hexamethylbenzene

Hemimellitene

r§-C~ ~CH3

CH3

Prehnitene

Table 8.1 shows the composition of C9-aromatics fractions from pyrolysis gas­oline and catalytic reforming.

H.-G. Franck et al., Industrial Aromatic Chemistry© Springer-Verlag Berlin Heidelberg 1988

292 Polyalkylated benzenes - production and uses

Table 8.1: Composition of y-aromatics from pyrolysis gasoline and cata­lytic reforming (in percentage)

y-aromatics from pyrolysis from catalytic benzene reformer

Cumene 4.2 0.6 n-Propylbenzene 12.3 5.2 0-Ethyltoluene 11.8 9.1 m-Ethyltoluene 24.0 17.4 p-Ethyltoluene 11.5 8.6 Mesitylene 5.6 7.4 Pseudocumene 14.6 41.3 Hernimellitene 3.3 8.2 Indane 12.7 2.0

Of the polymethyl benzenes, only pseudocumene, mesitylene and durene have any noteworthy industrial significance.

8.1 Pseudocumene

Pseudocumene (1,2,4-trimethylbenzene) is produced by fractional distillation of the trimethylbenzene cut from the heavy gasoline residues of catalytic reforming; because of the very slight differences in boiling point of their constituents, effi­cient fractionation is required in distillation columns with up to 300 trays.

The most important derivatives of pseudocumene are trimellitic anhydride and 2,3,5-trimethylaniline, an intermediate in the production of vitamin E (see Chapter 5.3.4.3.2). In addition, durene can be produced by methylation of pseudocumene.

The oxidation of pseudocumene to trimellitic anhydride can be carried out in the liquid-phase using cobalt/manganese salts and bromine compounds as cata­lysts in acetic acid (Amoco process).

o

" ~o HOOC~

o

Trimellitic anhydride

A process developed by Bergbau-Forschung recommends dilute nitric acid as oxidizing agent (Figure 8.1).

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294 Polyalkylated benzenes - production and uses

The reaction is carried out at 170 to 190°C, under a pressure of 20 bar with 7% nitric acid.

Trimellitic anhydride is used as a raw material for plasticizers, as a component in polyesterirnides and as a hardener for epoxy resins. High-temperature resistant and high-strength polyirnides are produced by the reaction of trimellitic acid chlo­ride with an aromatic diarnine, such as 4,4' -diarninodiphenylmethane (e. g. Torian, Amoco).

+ n H2N-@-CH2-@-NH2

o II

CJQr)-@-CHa@-NH II \\ o 0

n

Torian

- 3n Hel

Nitration and reduction of pseudocumene yields 2,3,5-trimethylaniline, which is used as a raw material in the production of vitamin E, via the corresponding trimethylhydroquinone.

8.2 Mesitylene

~CH3 H3CJQ,lCH3

2,3,5-Trimethylanilin.

Mesitylene can be recovered by distillation of the C9-aromatics of reformer resi­dues; separation from the co-boiling component, o-ethyltoluene, is, however, extremely intricate. Mesitylene is oxidized in small quantities to trimesic acid (1,3,5-benzenetricarboxylic acid). The oxidation can be carried out in either the gas-phase or the liquid-phase.

Polyalkylated benzenes - production and uses 295

The nitration and reduction of mesitylene yields mesidine (2,4,6-trimethylani­line), which is used as an intermediate in the production of dyestuffs.

Trimesic acid Mesidine

8.3 Durene

Durene is recovered from reformer residues by low-temperature crystallization; recovery by distillation is not possible, because of the virtually identical boiling point of isodurene. Furthermore, durene occurs in gasoline produced by the recently developed Mobilprocess (see Chapter 3.4.1); high concentrations in these methanol-derived gasolines can lead to blockages in the carburetor, a result of the tendency of durene to crystallize.

Durene is predominantly oxidized to pyromellitic dianhydride; this anhydride can also be produced by oxidation of the corresponding triisopropyltoluenes and diisopropylxylenes. The favored process is gas-phase oxidation with V20 5 as a catalyst, at temperatures from 400 to 600°C.

Pyromellitic dianhydride is mainly used in the production of polyimides, e. g. by reaction with an aromatic diamine, such as 4,4'-diarninodiphenyl ether; the high­temperature resistant plastic Kapton (Du Pont) is obtained by this method.

o 0

~ n~o II \\

+ n H2N-@-O-@-NH2

o 0

Pyromellitic dianhydride 4,4'-Diaminodiphenylether

o 0 \\ 1/

(:rgQ-@-o 1/ \\ o 0

n

Kaptan

-2n H2 0

296 PolyaIkylated benzenes - production and uses

8.4 Other cumene derivatives

8.4.1 Nitrocumene and isoproturon

Next to phenol the herbicide isoproturon is one of the few cumene derivatives with any large-scale importance. Nitration of cumene yields 2-/4-nitrocumene in the ratio 35 :65. The 4-isomer is recovered by vacuum distillation and is then reduced to cumidine (4-isopropylaniline). (Cumidine can also be produced by ammonolysis of 4-isopropylphenol, which arises as a by-product during the oxida­tion of 1,4-diisopropylbenzene to produce hydroquinone). Cumidine is reacted with phosgene to give 4-isopropylphenyl isocyanate. Reaction of 4-isopropyl­phenyl isocyanate with dimethylamine yields isoproturon, which in Western Europe is produced in quantities of around 6,000 tpa.

CH3 -@-'cH 0 N=C=O + CH3 -NH-CH3

CH/

Isoproturon

8.4.2 Cumenesulfonic acid

By the reaction of cumene with sulfuric acid, in a manner similar to toluene sul­fonation (see Chapter 6.4), cumenesulfonic acid is obtained, which, after neutrali­zation with sodium hydroxide in aqueous solution, is used extensively as a surfac­tant (hydrotrope).

Cumenesulfonic acid

Polyalkylated benzenes - production and uses 297

8.5 Indan and indene

Indan can be recovered by distillation of the heavy gasoline from coal tar refining. Indene, which is present in pyrolysis gasoline and in coal tar heavy gasoline, is of particular technical importance. It is polymerized with coumarone and other ole­fins to produce indenel coumarone resins.

Indan lndene Coumarone

Indenel coumarone resins find extensive application, especially in the produc­tion of adhesives, as reinforcers and tackifiers in the production of commerical rubber products, and in paint manufacture. Production of indene-derived resins in Western Europe is around 110,000 tpa.