Borate-containing layered double hydroxide composites: synthesis, characterization and application as catalysts in the Beckmann rearrangement reaction of cyclohexanone oxime

Boron-containing CaAl-layered double hydroxides (LDHs) were prepared by milling, as well as the dehydration-rehydration and the anion-exchange methods. The substances obtained were structurally characterized by powder X-ray diffraction measurements, scanning electron microscopy and infrared spectroscopy. Various borate species could be introduced among the layers and/or the outer surface of the CaAl-LDH. The materials were tested as catalysts in the Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam in the liquid phase, at 373 K. Two of them, the samples prepared by dehydration-rehydration and anion exchange, proved to be active, selective and recyclable catalysts, while the one made by the mechanochemical method was practically inactive. The catalytic activity was tied to intercalated and/or surface-bonded triborate and tetraborate species.