Production of anthracene

Anthracene was first discovered in coal tar by Jean B.A Dumas and Auguste Laurent in 1832. The importance of anthracene for industrial aromatic chemistry began with the synthesis of the dyestuff alizarin by Carl Graebe and Carl Th. Liebermann, as well as by William H. Perkin in 1868, replacing the natural dye produced from madder. Anthraquinone dyestuffs have remained the most important class of dyes, alongside azo-dyes, since the beginning of the chemistry of synthetic dyestuffs. 

From the time of the first alizarin synthesis, anthraquinone has been the most versatile anthracene derivative. This applies both for dye production and for the more recent applications of anthraquinone, as an additive (redox-catalyst) in wood pulping, and as a hydrogen carrier in H2O2 production.

Production of anthracene

While it is possible to recover the bicyclic naphthalene from pyrolysis products of coal and petroleum-derived raw materials, the tricyclic anthracene can be isolated only from high-temperature coal tar, since anthracene is present only in small amounts in other pyrolysis products and in the residues of catalytic hydrocarbon conversion.
Synthetic production of anthracene has gained no industrial significance so far, since sufficient quantities are present in coal tar; in theory, supplies of around 150,000 tpa of anthracene are available to meet a demand of some 20,000 tpa.
The first stage in the production of anthracene is the recovery of a 25 to 30% anthracene concentrate by crystallization, which can be carried out in two stages to increase the yield. The crystallizate, known as 'anthracene cake', is generally concentrated to around 50% by vacuum distillation. The main co-boiling compound of '50's anthracene' is henanthrene while carbazole is reduced to below 2%. Subsequent refining to yield 'pure anthracene', containing over 95%, is normally achieved by recrystallization in polar solvents, such as acetophenone, mixtures of cyclohexanol/ cyclohexanone or N-methylpyrrolidone; in addition, distillation or azeotropic distillation with ethylene glycol can be used for purification.

The quality of anthracene obtained by this method is sufficient for the manufacture of anthraquinone by the usual processes. Because of hydrogen transfer during coal tar refining, the fore-runnings which arise during the distillation of the anthracene cake are enriched with 9,10-dihydroanthracene, which can be converted into anthracene by oxidation with air. This anthracene, recoverable by subsequent crystallization, is distinguished by an exceptionally low nitrogen content.

Figure below shows the flow diagram for recovery of pure anthracene from anthracene oil, by the combined application of crystallization and distillation.

Figure: Flow diagram for production of anthracene from anthracene oil

In spite of the adequate availability of anthracene from coal tar, processes have been developed to synthesize anthracene from phenanthrene, which is present in coal tar in higher concentrations, or from diphenylmethane.
In the Bergbau-Forschung process, phenanthrene is first hydrogenated to sym-octahydrophenanthrene, then converted to sym-octahydroanthracene, and finally dehydrogenated.

This synthesis, in common with the production of anthracene based on diphenylmethane by Friedel-Crafts reaction and dehydrogenation, has had no industrial significance to date.

World production of anthracene from coal tar is around 20,000 tpa. RillgerswerkelWest Germany, with around 8,000 tpa and Nippon Steel Chemical/Japan, with around 3,000 tpa are the major producers.
The industrial importance of anthracene consists mainly in its use as a raw material in the production of anthraquinone; small amounts are used in production of the tranquilizer benzoctamine, which is produced from anthracene and acrolein in a Diels-Alder reaction, followed by reductive amination of the aldehyde with methylamine.

A new application for anthracene could develop with the increasing importance of hydrogen as a fuel, in the form of magnesium hydride. Magnesium hydride contains 150 g hydrogen per liter, which is released on heating to 250 to 300 DC. A metalorganic complex of magnesium with anthracene is an intermediate stage in the synthesis of magnesium hydride. This complex is produced by the reaction of pure magnesium with anthracene in tetrahydrofuran, at ambient temperature.

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