Factors affecting selectivity in the rearrangement of cyclohexanone oxime to caprolactam over modified aluminas

The Beckmann rearrangement of cyclohexanone oxime to caprolactam has been studied over a range of aluminas modified by the addition of chloride, phosphate, sulphate, sodium or boron oxide. The catalysts were characterized by temperature-programmed desorption of ammonia and carbon dioxide and by pyridine adsorption monitored by infrared spectroscopy. Conversion of the oxime declined with time-on-stream for all catalysts studied, but selectivity to caprolactam remained constant during the first five hours of operation. In this period a relationship was observed between the selectivity to cap- rolactam and the surface concentration of intermediate strength acid sites, wherein the selectivity increased as the surface concentration of these sites increased. The boron oxide modified alumina presented the largest concentration of intermediate strength acid sites, and the low level of coke formation on this material was associated with the total absence of surface basicity. A mechanism for the rear- rangement reaction on the modified aluminas is presented.