The role of acid site strength in the Beckmann rearrangement

A range of catalysts including β-zeolite, mordenite, ZSM-5, Y-zeolite, mesoporous alumino silicate, and amorphous silica - alumina with varying Si:Al ratios have been tested in the range 250-350°C for the Beckmann rearrangement of cyclohexanone oxime into caprolactam. All catalysts studied exhibited some level of deactivation which was characterized by a loss in conversion of cyclohexanone oxime with time on stream. There were considerable differences in resistances to deactivation through coke formation. Thus, mesoporous alumino silicate showed little decay in conversion with up to 16 wt % coke, whereas 6% coke on ZSM-5 caused a loss of 78% of the initial conversion. In general, the best catalysts were those which operated well in the presence of coke. Selectivity to caprolactam was in the range of 30-75% but did not change in a systematic fashion with time on stream. The number and strength of Brönsted and Lewis acid sites on each catalyst were determined from FTIR analysis of pyridine adsorption. These data show that a low ratio of strong/weak acid sites is important in achieving a high yield of caprolactam. A low acid site strength combined with a high resistance to deactivation by coke formation are two necessary parameters for good catalyst performance.