No abstract

NbOx/SiO2 with a very high catalytic activity for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, was investigated by different spectroscopic methods in order to obtain new insights in the formation and nature of the active sites. FT-IR spectroscopy in combination with pyridine adsorption measurements revealed that the catalyst material contains Lewis-acidic sites, most probably related to the Nb[double bond, length as m-dash]O groups of isolated tetrahedral NbO4 surface species, whereas no Brønsted-acidic sites were observed. Results from in situ Raman and complementary FT-IR measurements strongly suggest that Brønsted-acidic Nb-OH sites can be generated from Nb[double bond, length as m-dash]O groups by reaction with ethanol. This is in agreement with the observation that ethanol is essential for obtaining a very good catalyst performance. However, the Brønsted-acidic sites can be detected in significant amounts in particular in the presence of a Lewis-base, e.g. pyridine, most probably because the formation and/or the stability of these Brønsted-acidic sites are enhanced by a basic molecule. Assuming that cyclohexanone oxime, being a base, can play a similar role as pyridine, we propose on the basis of the spectroscopic findings obtained in this work and our kinetic results published recently, a reaction scheme for the formation of the active site at the Nb[double bond, length as m-dash]O group as well as for the recovery of the Nb[double bond, length as m-dash]O site during the final stage of the gas-phase Beckmann rearrangement.

Reaction kinetics of the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam was studied over a NbO x /SiO2 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.