Spectroscopic study on the active site of a SiO2 supported niobia catalyst used for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

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.