Vapor phase Beckmann rearrangement using high silica zeolite catalyst

Abstract

Vapor phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam has been studied using high silica zeolite catalysts. Catalysts with different crystal sizes and gel-ageing times have been activated by ionic exchange in different conditions by means of a highly basic solution and a nearly neutral solution both containing ammonium salts. Samples have been calcined at different temperatures in order modify the number of defective sites. We observed that samples exchanged by means of a highly basic solution (pH > 10) and calcined at a relatively lower temperature (450 °C) show the most interesting catalytic results. X-ray powder diffraction patterns of these samples show retention of the unit cell symmetry (orthorhombic cell) if compared to the dried sample. NH3-TPD confirms the low acidity of high silica zeolites, however a higher amount of desorbed ammonia is observed for the samples exchanged at higher pH and calcined at 450 °C. Due to silanol nests the IR spectra of the same samples show the formation of Si–NH2 bonds which are absent in the same material exchanged by other methods. Such sites seem to promote the high stability of the high silica zeolite catalysts also to the regeneration which is needed to remove the heavy carbonaceous compounds from the catalyst surface.