Kinetic Study on Gas-Phase Beckmann Rearrangement of Cyclohexanone Oxime to ε-Caprolactam over a Silica-Supported Niobia Catalyst

Reaction kinetics of the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam was studied over a NbO x /SiO 2 catalyst in a fixed-bed reactor. Kinetic measurements were carried out by variation of cyclohexanone oxime partial pressure and reactor temperature in the range of 5 to 80 hPa and 360 to 420 °C, respectively. It was found that ethanol, used as solvent in addition to toluene, is essential for high catalytic performance. Reaction rates as a function of cyclohexanone oxime partial pressure display Langmuir-type behavior, but the results could not be interpreted satisfactorily on the basis of a Langmuir–Hinshelwood mechanism. Another kinetic scheme is proposed, involving a kinetic adsorption step for cyclohexanone oxime instead of an adsorption equilibrium. Activation energies for the adsorption step and for the Beckmann rearrangement reaction were found to be 154 and 68 kJ·mol –1, respectively. Substitution of ethanol by alcohols with longer chain length (e.g., n -hexanol) resulted in significantly higher ε-caprolactam selectivities.