Tungstated Zirconia Catalysts for Liquid-Phase Beckmann Rearrangement of Cyclohexanone Oxime: Structure-Activity Relationship

The performance of tungstated zirconia catalysts with different tungsten loadings and calcination temperatures for liquid-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam was studied and the relation of their activity with the structure of tungsten species on zirconia surface was investigated. Cyclohexanone was the major product when zirconia was used as the catalyst while ε-caprolactam was formed in major amounts with tungstated zirconia. The relative amounts of these products depended on the W surface density, and the maximum selectivity to ε-caprolactam was observed at tungsten loadings near that required for the formation of a monolayer. UV−visible absorption edge energies suggested that the surface contains polytungstate species at these loadings. These polytungstate domains create Brønsted acid centers on the surface, as suggested by acidity measurements using NH3 adsorption microcalorimetry and FTIR of adsorbed pyridine, thereby making the tungstated zirconia catalysts active for the formation of ε-caprolactam.