Industrial synthesis caprolactam

Fig. 14.42. Industrial synthesis of caprolactam via the Beckmann rearrangement of cyclohexanone oxime.

Caprolactam, a white solid that melts at 69°C, can be obtained either in a fused or flaked form. It is soluble in water, ligroin, and chlorinated hydrocarbons. Caprolactam s main use is to produce nylon 6. Other minor uses are as a crosslinking agent for polyurethanes, in the plasticizer industry, and in the synthesis of lysine. 

Apart from the uses in the production of e-caprolactam, the Beckmann rearrangement has been used industrially in the synthesis of various APIs (Active Pharmaceutical Ingredient) or other compounds with economical value (essentially monomers for the production of polymers). A survey of the bulk reaction scaled in the GMP facilities at... 

Photonitrosylations show an extremely high tendency for filming, and the difficulties in implementing the caprolactam synthesis on an industrial scale are also linked to reactor designs not taking into account the competitive secondary reactions leading to polymerized material. 

Catalytic synthesis of hydroxylamine from nitrogen oxide and hydrogen is widely used in industry as a constituent part of caprolactam production. The reaction is conducted in aqueous sulfuric acid solution saturated with NO and H2 at 40°C and a pressure of approximately 1 atm. Platinum supported on porous graphite, in the form of fine particles suspended in the intensely stirred solution, is used as a catalyst. The main direction of the reaction is... 

The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications. One 10,000 ton/year plant for the production of catechol and hydroquinone has been operating since 1986 with excellent results. Moreover, successful tests conducted on a 12,000-ton/year pilot plant for cyclohexanone ammoximation (Notari, 1993b) could be followed soon by an industrial-size plant that would greatly simplify the synthesis of caprolactam. Both these examples are clear indications of the potentials of the new oxidation chemistry made possible by the new materials. 

Homolytic liquid-phase processes are generally well suited to the synthesis of carboxylic acids, viz. acetic, benzoic or terephthalic acids which are resistant to further oxidation. These processes operate at high temperature (150-250°C) and generally use soluble cobalt or manganese salts as the main catalyst components. High conversions and selectivities are usually obtained with methyl-substituted aromatic hydrocarbons such as toluene and xylenes.95,96 The cobalt-catalyzed oxidation of cyclohexane by air to a cyclohexanol-cyclohexanone mixture is a very important industrial process since these products are intermediates in the manufacture of adipic acid (for nylon 6,6) and caprolactam (nylon 6). However, the conversion is limited to ca. 10% in order to prevent consecutive oxidations, with roughly 70% selectivity.97... 

Tetrahydroindole (1) was isolated by extraction of the reaction mixture with organic solvents (Et20, benzene) and purified by distillation or recrystallization. This process for preparing 4,5,6,7-tetrahydroindole is simple enough, industrially feasible, safe, and based on the cheap and accessible raw materials. Cyclohexanone oxime is an inexpensive large-scale commercial product (caprolactam synthesis intermediate), vinyl chloride being one of the cheapest commercial vinyl compounds. 

Both industrial demand and academic interest stimulated detailed research of the synthesis of lactam polymers and elucidation of the polymerization mechanisms. Until recently, nylon 6 has been the only industrially produced lactam polymer and, therefore, most of the extensive investigations are dealing with the polymerization of caprolactam [2]. The mechanisms of the main elementary reactions derived for caprolactam are applicable to other lactams as well, except to the / /-substituted ones. 

Aliphatic and alicyclic molecules such as cyclohexane undergo photosubstitution with nitrosyl chloride (Pape, l%7). The reaction is of considerable industrial importance in the synthesis of 6-caprolactam, an intermediate in the manufacture of polyamides (nylon 6). (Cf. Fischer, 1978.) At long wavelengths a cage four-center transition state between alkane and an excited nitrosyl chloride molecule is involved, as indicated in Scheme 63. In contrast to light-induced halogenation, photonitrosation has a quantum yield smaller than unity, and is not a chain reaction. 

Chromium substituted aluminophosphate-5 is an active and recyclable catalyst for the selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one. The product is of potential industrial interest for the synthesis of caprolactam. 

One of the important processes in the petrochemical industry is the production of phenol. Phenol is an important raw material for the synthesis of petrochemicals, agrochemicals, and plastics. Examples of employment of phenol as an intermediate are production of bisphenol A, phenolic resins, caprolactam, alkyl phenols, aniline, and other useful chemicals. Current worldwide capacity for phenol production is nearly 7 milUon metric tons per year. More than the 95% of phenol is produced by the common industrial process known as Hock or cumene process involving three successive reaction steps ... 

Still in the phenol industrial cycle, after its reduction to cyclohexanone, a real breakthrough was achieved in the synthesis of s-caprolactam, an intermediate produced at ca. 4 Mt/y which, in turn, is mainly used in nylon-6 production. 

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