Catalytic Hydration of Cyclohexene to Cyclohexanol for Nylon-6 Production: A Review

Nylon-6 (PA6) is a widely used engineering plastic, with cyclohexanol as the key intermediate of the benzene–cyclohexene–caprolactam chain. Cyclohexene hydration is considered a green alternative to cyclohexane oxidation owing to its high atom economy and selectivity. This review compares direct and indirect hydration within a four-level framework (catalyst, process, system and industrial chain). Direct hydration is constrained by thermodynamic equilibrium and oil–water interfacial mass transfer, with an industrial single-pass conversion of about 10–13% under excess-water conditions. Reported intensification routes include synergistic acidity–hydrophobicity–diffusion design of HZSM-5, cosolvent screening, and RD → HPRD → RDWC integration. Indirect hydration reconstructs the reaction network via esterification–hydrogenation (or hydrolysis/transesterification), can reach conversions above 99%, but adds process complexity, catalyst-stability demands and hydrogen consumption. Benchmarking the schemes on a per-ton basis, route preference is discussed within a three-dimensional framework (plant scale, hydrogen price and carbon constraints). The data required for a fully quantitative ranking are identified. The framework may aid process selection under given hydrogen and carbon-market conditions.