Solvent effects in the liquid-phase Beckmann rearrangement of oxime over H-Beta catalyst II: adsorption and FT-IR studies

Solvent effect has been investigated in the liquid-phase Beckmann rearrangement of cyclohexanone oxime over H-Beta catalyst. Co-adsorption of substrate and solvent suggests that the facility of protonation of oxime is primarily dependent upon the competitive adsorption between substrate and solvent. On the other hand, the result of IR study indicates that the N-protonation of oxime is largely preferred to the O-protonation and the 1,2-H-shift is possibly more energy-demanding than the subsequent rearrangement. In addition, it is revealed that there is an interaction between the protonated oxime and a polar solvent. This implies that there exists a quite stable five-membered ring complex which may decrease the energy barrier of the 1,2-H-shift significantly. In this regard, a solvent making a balance between these two competitive factors is considered suitable for the liquid-phase Beckmann rearrangement of oxime over solid-acid catalyst. (C) 2001 Elsevier Science B.V. All rights reserved.