Caprolactam alternative routes

Among the industrially produced lactams, e-caprolactam has by far the highest production capacity due to its important role as monomer in the polyamide business. There exist several synthetic routes to produce e-caprolactam. The most important one starts from benzene (Scheme 5.3.7). Benzene is hydrogenated in a first step to cyclohexane, followed by oxidation of the latter to a mixture of cyclohexanone and cydohexanol. This mixture is then reacted with NH2OH to give cyclohexanone oxime, which is converted under add catalysis in a so-called Beckmann rearrangement reaction to e-caprolactam. Alternative routes try to avoid the oxime intermediate (UCC peracetic add process via e-caprolactone), try to avoid the cydohexanone intermediate (e.g., DuPont process converting cydohexane directly into the oxime intermediate by reaction with nitric add), or start from toluene (Snia-Viscosa process). 

The alternative route involves the air oxidation of cyclohexane and proceeds via the production of a mixture of cyclohexanol and cyclohexanone often known as KA oil. It was in the cyclohexane oxidation section of the caprolactam plant of Nypro Ltd that the huge explosion occurred at Flixborough, England in 1974. 

Several oxidative routes are available to change cyclohexane to cyclohexanone, cyclohexanol, and ultimately to adipic acid or caprolactam. If phenol is hydrogenated, cyclohexanone can be obtained directly this will react with hydroxylamine to give cyclohexanone oxime that converts to caprolactam on acid rearrangement. Cyclohexane can also be converted to adipic acid, then adiponitrile, which can be converted to hexamethylenedi-amine. Adipic acid and hexamethylenediamine are used to form nylon 6,6. This route to hexamethylenediamine is competitive with alternative routes through butene. 

A Beckmann rearrangement of the oxime in hot concentrated sulfuric acid then gives the desired seven-membered cyclic amide, s-caprolactam. The crude product forms a separate oily phase which is separated from the reaction mixture and purified by distillation under reduced pressure (b.p. 136-138°C at 10 mm m.p. 72°C). There are also several alternative routes to produce caprolactam [25]. 

L-Lysine, an essential amino acid, is used in very large quantities to supplement human foods and animal feeds. Traditionally, L-Lysine is produced by fermentation processes. An alternative route developed by Toray Ind. involves the chemical synthesis of D/L-a-amino-s-caprolactam followed by the selective hydrolysis of the L-a-amino-s-caprolactam catalyzed by intracellular lactamase in Cryptococcus laurentii, to give L-Lysine. The process can be improved by adding a second micro-organism, Achromobacter obae, containing a-amino-E-caprolactam racemase.Thus, quantitative yields of L-lysine are obtained. 

Since its discovery some 55 years ago, the synthesis of caprolactam has been the subject of intense research and development. Interest in alternative routes continues today and current activities receiving a lot of attention are carbon monoxide-based routes under development by DSM, EniChem and DuPont [32]. Numerous routes using a variety of feedstocks have been patented and many have been piloted, however, only seven have actually been commercialized. The first was the process developed by I. G. Farben based on Schlack s chemistry known today as the Rashig or conventional route. Other commercial routes are the CAPROPOL process, the BASF process, the DSM-HPO process, the Allied process, the Toray PNC process, and the SNIA Viscosa process. 

An alternative route to cyclohexanone oxime developed in Italy by Enichem is shown in the following reaction. Cyclohexanone oxime is produced by the ammoxidation of cyclohexanone with ammonia and aqueous hydrogen peroxide in the presence of a solid, recyclable catalyst, titanium silicalite (TS-1). This reaction step eliminates approximately one-third of total salt formation. However, the oxime is still converted to caprolactam through the conventional route (Beckmann rearrangement), catalyzed by stoichiometric amounts of sulfuric acid, and produces ammonium sulfate salt. Therefore, this alternative process still leaves something to be desired. 

The TS-l catalyzed hydroxylation of phenol to a 1 1 mixture of catechol and hydroquinone has already been commercialized by Enichem. Another reaction of considerable commercial importance is the above mentioned ammoximation of cyclohexanone to cyclohexanone oxime66, an intermediate in the manufacture of caprolactam. It could form an attractive alternative to the established process that involves a circuitous route via oxidation of ammonia to nitric acid followed by reduction of the latter to hydroxylamine (figure 4). 

In classical processes cyclohexanone is converted to the corresponding oxime by reaction with hydroxylamine (see Fig. 2.27). The oxime subsequently affords caprolactam via the Beckmann rearrangement with sulphuric or phosphoric acid. Alternatively, in a more recent development, not yet commercialized, a mixture of cyclohexanone, ammonia and hydrogen peroxide is directly converted to cyclohexanone oxime over a titanium(IV)-silicalite (TS-1) catalyst. This route is more direct than the classical route and reduces the amount of salt formation but it involves the use of a more expensive oxidant (H2O2 rather than O2). 

It is important that the NHPI-catalyzed nitration is conducted under air, since NO generated in the course of the reaction can be readily reoxidized to NO2 by O2. In the absence of air, the yield of nitrocydohexane decreases to 43%. After the nitration, the NHPI catalyst can be separated from the reaction mixture by simple filtration and reused repeatedly. Nitrocydohexane is easily reduced to cyclohexanone oxime. Therefore, this nitration provides an alternative practical route to cyclohexanone oxime, which is a raw material for s-caprolactam leading to nylon-6 [150, 151]. ... 

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