ABSTRACT

The Beckmann rearrangement of ketoximes to amides is a fundamental step in the industrial production of (-caprolactam, the precursor in the manufacture of Nylon-6, starting from cyclohexanone oxime. The large-scale, classical industrial process is performed in liquid phase using sulphuric acid as catalyst. In order to avoid the use of this acid, an intensive research has been carried out to develop a suitable solid that catalyse the Beckmann rearrangement.

We have investigated the Beckmann rearrangement of different type of oximes, linear as acetophenone oxime, and cyclic as cyclohexanone or cyclododecanone oximes, over zeolites and mesoporous materials as catalysts, varying pores sizes and nature of the active sites. This study has been tackled by combining ´in situ´ spectroscopic techniques, infrared and solid-state NMR, and theoretical calculations, in order to study in depth the reaction mechanism as well as the nature, Brönsted acids or silanol groups, of the active sites in the Beckmann rearrangement present in zeolites and mesoporous materials, and their location.

Combination of ´in situ´ solid NMR and theoretical calculations of the oxime-zeolite interaction models indicates that the first step in the Beckmann rearrangement is the protonation of the oxime on the nitrogen atom, when catalyst-containing Brönsted acid sites are used. The active silanol groups present in zeolites and mesoporous materials are not sufficiently acid to completely transfer the proton to the oxime, but they are interacting through hydrogen bonds.
The results obtained show that both Brönsted acids and silanol groups, placed inside or at the pore mouths of zeolites, or in MCM-41 mesoporous materials, are also active in the Beckmann rearrangement. Isolated hydroxyl groups or weakly hydrogen bonded silanols groups are not active in the Beckmann rearrangement reaction. The study of cyclic oximes of different ring sizes suggest that the active centers placed at the outer shell of the crystals of zeolites are less selective to amide formation that the internal ones.


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Abstract