Vapor-phase Beckmann rearrangement over silica-supported boria catalyst prepared by vapor decomposition method

Abstract

The present work introduced a new method of catalyst preparation which involved the vapor decomposition of B(OEt) 3 onto silica gel to produce a silica-supported boria ( B 2O 3 SiO 2 ). A B 2O 3 SiO 2 catalyst prepared by this method (B 2O 3 content 34 wt%) showed high catalytic efficiency for the vapor-phase Beckmann rearrangement of cyclohexanone oxime (oxime conversion: 98%, ε-caprolactam selectivity: 96 mol%, at 250 °C). The vapor decomposition B 2O 3 SiO 2 was more active and selective at any B 2O 3 content than the B 2O 3 SiO 2 obtained by the ordinary impregnation method using H 3BO 3. The amounts of the acid sites whose acid strengths exceeded 80 kJ/mol in terms of the differential heat of adsorption of ammonia (DHA) were 0.7 and 0.4 mmol/g for the most active vapor decomposition B 2O 3 SiO 2 and the most active impregnation B 2O 3 SiO 2 , respectively. The vapor decomposition effected uniform deposition of B 2O 3 on SiO 2, and produced a solid acid with relatively uniform distribution of acid strength. It is suggested that the acid strength ( H 0) of an effective catalyst pertinent to the vapor-phase Beckmann rearrangement should be less than −5.6 which approximately corresponds to a DHA value of more than 80 kJ/mol.