Kinetic model of 2-cyclohexenone formation from cyclohexanol and 2-cyclohexenol dehydrogenation

In this study, a kinetic model (including reaction scheme and the kinetic parameters) for formation reactions of dehydrogenation impurities (2-cyclohexenone and phenol) was developed for the cyclohexanol and 2-cyclohexenol dehydrogenation processes. 2-Cyclohexenol used to be an impurity contained in commercial cyclohexanol. Runs were carried out at 250 and 290 °C in a fixed bed reactor using a Cu–Zn catalyst. It was found that 2-cyclohexenol quickly produces 2-cyclohexenone and other dehydration impurities, and 2-cyclohexenone rapidly reverts to cyclohexanone in the presence of hydrogen. Phenol formed from high amounts of cyclohexanone and hydrogen is mainly produced by the direct reaction of cyclohexanone.In the caprolactam manufacturing process, cyclohexanone is mostly produced by cyclohexanol dehydrogenation. Using Cu–Zn catalysts, as in this study, phenol is obtained as the main impurity, at high concentrations and formed from cyclohexanone. However, 2-cyclohexenone was also found as a dehydrogenation impurity. This compound can produce unsaturated lactam in the caprolactam production from cyclohexanone that considerably decreases the quality of nylon. Therefore, 2-cyclohexenone can be produced from cyclohexanone or 2-cyclohexenol. In turn, phenol can originate directly from cyclohexanone dehydrogenation or from 2-cyclohexenone dehydrogenation in a serial reaction process.

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► The main impurities obtained in the cyclohexanol dehydrogenation process are 2-cyclohexenone and phenol.
► Small amounts of 2-cyclohexenone have dramatically negative effects on caprolactam quality.
► 2-Cyclohexenone was found to be formed from cyclohexanone in a reversible reaction with a low equilibrium constant value.
► Phenol was obtained to be formed from both cyclohexanone and 2-cyclohexenone.
► 2-Cyclohexenol quickly produces 2-cyclohexenone.