ε- has become a commodity chemical with basically a single outlet nylon 6.

A description of the classical manufacture of ε-caprolactam can be used to present not only the problems of this route but also its modern variations. The synthesis consists of three 'organic' and one 'inorganic' steps:
1. Manufacture of cyclohexanone
2. Oxime formation from cyclohexanone with
3. Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam
4. Manufacture of hydroxylamine

To 1:

Most cyclohexanone is made from cyclohexane. A second route starts with . Another route used only in the CIS (capacity 20000 tonnes per year) uses cyclohexylamine, which is catalytically dehydrogenated and then hydrolyzed with steam to cyclohexanone. In 1995 about 57% and 55% of the e-caprolactam production in the USA and Western Europe, respectively, was based on cyclohexane oxidation and most of the remainder (i. e., 43 and 45%, respectively) came from phenol hydrogenation. Typical ε-caprolactam processes with a phenol feedstock were developed by Allied Chemical, Montedipe, and Lmna-Werke.

A new process for the production of cyclohexanol from Asahi Chemical is at the pilot-plant stage. Here, benzene is partially hydrogenated on a ruthenium catalyst to cyclohexene, which is then hydrated to cyclohexanol under acid catalysis. Process conditions for the proposed 60000 tonne-per-year plant have not yet been disclosed.

In the first-mentioned cyclohcxanone oxidation route, cyclohcxanone is distillcd from the cyclohexanone/cyclohcxanol mixture and the cyclohexanol portion is catalytically dehydrogcnated at 400-450 °C and atmospheric prcssure over Zn or Cu catalysts:



The cyclohexanol conversion is about 90%, with a selectivity to cyclohexanone of 95%.

Earlier, phenol could only bc converted into cyclohexanone in a two-step process: after ring hydrogcnation with nickcl catalysts at 140-160°C and 15 bar, the dehydrogenation was conducted analogous to equation 18. Simplification of this route was made possible by selective hydrogenation with Pd catalysts:

Phenol is completely convcrtcd in the gas phase at 140-170°C and 1-2 bar using a supported Pd catalyst containing alkaline earth oxides (e. g., Pd-CaO/Al₂O₃). The selectivity to cyclohexanone is greater than 95%.

To 2:

Oxime formation with cyclohexanonc is done with a hydroxylamine salt, usually the sulfate, at 85 °C:

In order to displace the equilibrium, NH₃ must be continuously introduced to maintain a pH of 7. The first ammonium sulfate formation takes place at this stage in the process.

The same constant pH value can be attained without salt formation in the DSM/Stamicarbon HPO process (hydroxylamine-phosphate-oxime) with hydroxylamine in a buffer solution containing H₃PO₄. The buffer solution - "liberated" during oxime formation - is recycled to the hydroxylamine production.

Toray and Du Pont practice two other methods. Both involve cyclohexanone oxime, but avoid its manufacture from cyclohexanone.

Toray developed a commercial process from a known reaction, the photonitrosation of cyclohexane directly to cyclohexanone oxime (PNC process):

A gas mixture consisting of HCl and nitrosyl chloride (NOCl) is fed into cyclohexane at a temperature below 20°C. The reaction, which is initiated by Hg light, results in the formation of cyclohexanone oxime with 86% selectivity (based on C₆H₁₂). Toray uses this process in two plants (startups in 1963 and 1971) with a total production capacity of 174000 tonnes (1995). NOCl is obtained from the reaction of HCl with nitrosyl sulfuric acid:



H₂SO₄ and HCl go through the process without either salt formation or great losses.

The Du Pont process (Nixan process, process) was practiced for a time in a 25000 tonne-per-year oxime manufacturing plant. was introduced into cyclohexane by nitration with HNO₃ in the liquid phase, or NO₂ in the gas phase. The nitrocyclohexane was then catalytically hydrogenated to the oxime:

To 3:

All processes described so far proceed either directly or via intermediate stages to cyclohexanone oxime, which is then converted into ε-caprolactam by a rearrangement with H₂SO₄ or oleum discovered by E. Beckmann in 1886:

Commercial development of this process was done by BASE In the continuous process the oxime solution, acidified with sulfuric acid, is passed through the reaction zone which is kept at the rearrangement temperature (90 - 120 °C). The rearrangement is complete within a few minutes, and the resulting lactam sulfate solution is converted into the free lactam with NH₃ in a neutralization vessel. It separates from the saturated ammonium sulfate solution as an oily layer, which, after extraction with benzene, toluene, or chlorinated hydrocarbons and stripping with water, is further purified and then distilled. The selectivities amount to almost 98%.

To 4:

for the conversion of cyclohexanone into the oxime is usually manufactured in a modified four-step Raschig process. Essentially, it consists of the reduction of ammonium nitrite with SO₂ at about 5°C to the disulfonate, which is then hydrolyzed at 100 °C to hydroxylamine sulfate:

One mole (NH₄)₂SO₄ is formed for each mole of hydroxylamine sulfate. A further mole of ammonium sulfate is obtained from the oxime formation with cyclohexanone, where it is introduced to the reaction by hydroxylamine sulfate and NH₃ (to neutralize the oxime).

A basic improvement in hydroxylamine manufacture was achieved with the DSM HPO process. ions are reduced with hydrogen to hydroxylamine using a palladium-on-charcoal catalyst or Al₂O₃ with, e. g., promoters such as germanium compounds, suspended in a phosphate buffer solution:

The buffered hydroxylamine solution is then used, during the oximation of cyclohexanone in toluene, to dissolve and extract the oxime. After addition of HNO₃, the spent solution is recycled to nitrate hydrogenation. By this means, much less salt is formed than in the Raschig process.

Since NO₃^- is manufactured by oxidation of NH₃, the whole process can be regarded as an oxidation of NH₃ to NH₂OH. Overall selectivity to hydroxylamine from ammonia is 58%.

Other processes for the manufacture of hydroxylamine, such as the platinum- or palladium-catalyzed reduction of NO with hydrogen in dilute mineral acid solution developed by BASF, Bayer, and Inventa, do in fact result in less salt. However, the starting materials must be very pure and an involved catalyst recovery is necessary. Commercial use of these processes is increasing.