10 Vapor phase beckmann rearrangement over a high silica MFI zeolite

Abstract

A catalyst that is mainly composed of a high silica MFI zeolite (Silicalite-1) has been developed for the vapor phase Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam. This catalyst does not possess acidity that can be detected by ammonia TPD. Methanol fed into the reactor with the oxime improves the yield of caprolactam. Methanol reacts with terminal silanols on the zeolite surface at an elevated temperature and converts them to methoxyl groups. The modification of the catalyst by methanol plays an important role in the Beckmann rearrangement reaction. We consider that the reaction mechanism of Beckmann rearrangement under vapor phase conditions is the same as in the liquid phase, namely the alkyl group in the anti position against the hydroxyl group of the oxime migrates to the N atom position. Nest silanols located just inside the pore mouth of the MFI zeolite are assumed to be the active sites of the catalyst. We propose that the NOH group of cyclohexanone oxime molecule adsorbed on the zeolite surface is coordinated by OH groups of the nest silanols with hydrogen bonding. This coordination seems to play an important role in the reaction.