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

NbOx/SiO2 in the gas-phase Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam: Influence of calcination temperature, niobia loading and silylation post-treatment

NbOx/SiO2 catalyst materials prepared by the incipient wetness impregnation method were studied in the industrially relevant gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. The catalytic experiments were carried out in a fixed bed reactor system at atmospheric pressure. Results have been complemented with Raman and FT-IR spectroscopy as well as N2 physisorption measurements. Optimal catalytic results were observed for a catalyst calcination temperature of 600 °C and a niobia loading of 0.3 wt.%. Raman spectroscopy revealed that isolated tetrahedral mono-oxo NbO4 surface species most probably play a key role in the catalytic reaction. Very positive effects were achieved by applying a catalyst silylation post-treatment. Firstly, the catalytic long-term stability increased very substantially as a consequence of a decreased coke deposition on the catalyst surface. Secondly, the harmful effect of water on catalytic performance was strongly suppressed even to such an extent that water could be added to the feed to enhance catalytic long-term stability. Cyclohexanone oxime conversion and ε-caprolactam selectivity could be maintained at constant high values >99% and around 95% respectively, for 26 h time-on-stream. Finally, considering the Sumitomo gas-phase Beckmann rearrangement process as a benchmark, we made a rough comparison between the catalytic performances of our niobia/silica catalyst and the silicalite catalyst used by Sumitomo