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

Abstract

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.