Beckmann rearrangement over solid acid catalysts

Beckmann rearrangement of cyclohexanone oxime to caprolactam has been investigated by passing the oxime vapour in helium over a series of modified alumina or zeolites held between 250 and 380°C in a continuous flow fixed bed micro-reactor. The range of solid acids studied included alumina modified with boron, sodium, phosphorus, sulphate or chloride, and the zeolites H-mordenite, H-A, H-Y and HZSM-5. Each catalyst studied exhibited an initiation period, a period of steady state activity and a deactivation period. The duration of each phase of activity depended on experimental conditions but for the boron modified alumina operating at 300°C the initiation period was ca. 1 hour. This phase was characterised by a high conversion of the oxime but very poor selectivity to caprolactam and was accompanied by a build up of coke on the surface of the catalyst. The steady state period was ca. 8 hours and featured high conversion and selectivity. The final phase featured a slow irreversible lowering of conversion and selectivity and was accompanied by the formation of a boron-alumina glassy phase. The boron modified alumina features a high concentration of acidic sites of intermediate acid strength, which are associated with high selectivity to caprolactam. Deactivation and loss of selectivity are associated with the loss of boron through its incorporation into the alumina solid support.