In situ solid-state NMR investigations of the vapor-phase Beckmann rearrangement of 15N-cyclohexanone oxime on MFI-type zeolites and mesoporous SBA-15 materials in the absence and presence of the additive 13C-methanol

The stepwise conversion of N-15-cyclohexanone oxime into 6-caprolactam via the vapor-phase Beckmann rearrangement on silicalite-1 (n(Si)/n(Al) = 1700), siliceous SBA-15 (nsi/nAl = 1800), H-ZSM-5 (nsi/nAl = 14), and [AIISBA-15 (n(Si)/n(Al)= 8.7) in the absence and presence of C-13-methanol was investigated by solid-state N-15 and C-13 CP/MAS NMR spectroscopy. The MFI-type zeolites and SBA-15 materials are characterized by different acid strengths and pore diameters. The surface CH concentrations in silicalite-1, SBA-15, H-ZSM-5, and [Al]SBA-15 materials were determined to 0.31, 2.81, 1.12, and 1.29 mmolg(-1), respectively. H-1, Si-29, and N-15 MAS NMR measurements revealed that the Bronsted acidic OH groups of H-ZSM-5 and [Al]SBA-15 and the large concentration of Q(3) silanol groups (Si(OSi)30H) in siliceous SBA-15 led to the formation of O-protonated epsilon-caprolactam. On loading and heating of N-15-cyclohexanone oxime on H-ZSM-5 in the presence of methanol, water was formed via dehydration of methanol into dimethyl ether. These water molecules promoted the conversion of O-protonated epsilon-caprolactam species into e-aminocapric acid. In addition, on both silicalite-1 and H-ZSM-5 catalysts, partial conversion of methanol into hydrocarbons, such as isobutane, was observed. Furthermore, isopropylamine was formed on silicalite-1 by the reaction of isobutane and byproducts of the Beckmann rearrangement, such as hydroxylamine. In contrast, no influence and conversion of methanol were found during the Beckmann rearrangement of N-15-cyclohexanone oxime on the mesoporous SBA-15 and [Al]SBA-15 materials. (c) 2008 Elsevier Inc. All rights reserved.