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

Abstract

The stepwise conversion of 15N-cyclohexanone oxime into ε-caprolactam via the vapor-phase Beckmann rearrangement on silicalite-1 ( n Si / n Al = 1700 ), siliceous SBA-15 ( n Si / n Al = 1800 ), H-ZSM-5 ( n Si / n Al = 14 ), and [Al]SBA-15 ( n Si / n Al = 8.7 ) in the absence and presence of 13C-methanol was investigated by solid-state 15N and 13C CP/MAS NMR spectroscopy. The MFI-type zeolites and SBA-15 materials are characterized by different acid strengths and pore diameters. The surface OH 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 mmol g −1, respectively. 1H, 29Si, and 15N MAS NMR measurements revealed that the Brønsted acidic OH groups of H-ZSM-5 and [Al]SBA-15 and the large concentration of Q 3 silanol groups (Si(OSi) 3OH) in siliceous SBA-15 led to the formation of O-protonated ε-caprolactam. On loading and heating of 15N-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 ε-caprolactam species into ε-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 15N-cyclohexanone oxime on the mesoporous SBA-15 and [Al]SBA-15 materials.