Some innovative organic syntheses

Aniline

Aniline is the precursor of classic dyes. Before the Second World War it was imported from the UK and Germany and later, in 1942, Timbrol produced war-time requirements by the traditional process of hydrogenation of nitrobenzene, using hydrochloric acid and iron filings. Experiments at ICI Billingham had shown promise for an alternative route, reduction of nitrobenzene over a copper chromium catalyst with synthesis gas for ammonia (N2 : H2 ratio 1 : 3). A similar process was said to have been tried in Germany. A full scale plant (16,000 tpa) was built in Lancashire, but operated only for a few months. Subsequently one reactor (1000 tpa) was obtained for Australia, where it was needed for the production of 'Carbamite', used as an anti-flash additive to 'Cordite', in the manufacture of ammunition for field and naval artillery. Additional aniline was required for diphenylamine, an intermediate to the newly discovered anthelmintic phenothiazine. Diphenylamine was also an intermediate in the manufacture of sulphur containing antioxidants for the rubber compounding industry.

The Australians decided to manufacture aniline employing the new process, although it had not yet been in production in the UK. The nitrobenzene required was made in a small plant at Deer Park, using benzene from Port Kembla (AIS) and mixed sulphuric/nitric acid from the nearby ICI Australia explosives plant. Nitrobenzene was distilled and desulphurised (sulphur poisons the catalyst), by boiling with aqueous caustic soda. This was an economic simplification of the UK process involving methanolic caustic. The process was further simplified in Australia because pure hydrogen was available from the Yarraville electrolytic caustic/chlorine cells. The UK plant arrived in Australia in 1942, and was in production in Australia early in 1944. It operated for 20 years, without problems, but eventually, once again, the scale of manufacture and demand for the product were too small for the plant to survive.

Diphenylamine

Although diphenylamine had some use in explosives, its main potential in Australia was for a specialised, particularly Australian purpose, the synthesis of phenothiazine. It was, by the standard of the time, a reasonably complex heterocyclic molecule, made by reacting diphenylamine and sulphur. The standard diphenylamine process was expensive and yields were poor. An elegant new process had been evolved on laboratory scale (1 inch diameter tube) at Winnington in the UK, but there were not even pilot plant data for the next step. Nevertheless, there was an urgent demand and ICI Australia decided to adopt the process and, in one step, to scale it up by a factor of over 1000. The plant design was carried out wholly in Australia (A. W. Hamer, F. Lamont, I. J. Robinson). It included an ingenious method of utilising the heat in the vapour from the catalyst chamber to fractionate the diphenylamine from the excess aniline (conversion was only 20 per cent), which was recycled directly to the vaporiser. The temperature in the reactor had to be maintained at over 450°. This involved heating the highly reactive aniline to over 500°. To minimise carbonisation of aniline at this temperature, an ingenious pre-heater was invented which served the dual purpose of heating aniline and nitrogen used to bring the catalyst to reaction temperature.

Australian Academy of Technological Sciences and Engineering