Synthesis of ε-caprolactam from ε-caprolactone and ammonia in supercritical water

Synthesis of ε-caprolactam from caprolactone with ammonia was carried out in supercritical water (SCW : T>374°C, P>22.1 MPa). A main reaction pathway for the formation of ε-caprolactam was elucidated on the basis of a series of reactions of 1) ε-caprolactcne, 2) hexanol + ammonia, 3) hexanoic acid + ammonia, and 4) hexanoic acid + hexanamide in SCW. The results clearly showed the following three steps of reactions for the formation of ε-caprolactam. The first step is the ring cleavage of ε-caprolactone to yield 6-hydroxyhexanoic acid. The second step is the dehydration between 6-hydroxyhexanoic acid and ammonia to form 6-hydroxyhexanamide. The final step is the intramolecular dehydration of 6-hydroxyhexanamide to produce ε-caprolactam. The effects of reaction temperature, water density, and ammonia concentration were also studied. At 380°C and 38 MPa (water density 0.5 g/cm³), the yield of ε-caprolactam from ε-caprolactone and ammonia at a ratio 1:5 increased with increasing reaction time and reached 79.2% in 60 min.