NMR spectroscopy and theoretical calculations demonstrate the nature and location of active sites for the Beckmann rearrangement reaction in microporous materials

15N solid-state NMR and theoretical calculations were combined to investigate the Beckmann rearrangement of 15N-cyclohexanone oxime and 15N-cyclododecanone oxime over two MFI-type zeolites as catalysts, H-ZSM-5 and silicalite, containing Brönsted acid sites and silanol groups, respectively. The results demonstrated that the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam occurs in the interior of the pores of MFI zeolites and that O-protonated ε-caprolactam is formed over the Brönsted acid centers. The acid groups placed at the outer shell of the crystals, probably at the pore mouths, are also active; meanwhile, external silanol groups are significantly less active and less selective than the internal ones.