Deactivation and Regeneration of Alumina Catalysts for the Rearrangement of Cyclohexanone Oxime into Caprolactam

Abstract

Abstract Modified alumina catalysts have been studied for the Beckmann rearrangement of cyclohexanone oxime to caprolactam in the temperature range 250-35°C. Boron oxide supported on alumina was the most successful catalyst preparation used. Good initial activities and high selectivities to the lactam were observed, other products observed included cyclohexanone, 5-cyanopent-1-ene and aniline. Coke formation was noted from the commencement of reaction which eventually led to loss in activity, followed by a lessening in selectivity to caprolactam and a lowering of surface areas. The loss in activity was related to the cumulative amount of cyclohexanone oxime converted in the reactor, and selectivity to caprolactam diminished as the oxime partial pressure increased. There was a direct relationship also between the amount of coke which formed on the catalysts and the decline in catalytic activity. Coke formed readily on the more basic catalysts. It is postulated that two reactions which occur on surface basic sites are responsible for coke formation. The first of these is polymerization of caprolactam. The second is the formation of aniline; this by-product can condense with cyclohexanone to form a Schiff base and further condensation of this base with cyclohexanone, via the Mannich reaction, represents a pcssible starting point for coke formation. Evidence is presented that surface acidic sites were responsible for caprolactam formation. Treating the deactivated alumina supported boron oxide catalysts in air at 500°C did not regenerate the original catalytic activity. although the surface area was fully regenerated. X-ray diffraction, infrared, and X-ray photoelectron spectroscopies as well as chemical analysis indicated that a proportion of the boron had entered into the alumina carrier to form an amorphous phase during the deactivation process, and temperature-programmed desorption of ammonia indicated that this process was accompanied by an irreversible decline in the surface concentration of acidic sites.