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Issue 32, 2016
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Spectroscopic study on the active site of a SiO2 supported niobia catalyst used for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

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Abstract

NbOx/SiO2 with a very high catalytic activity for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, was investigated by different spectroscopic methods in order to obtain new insights in the formation and nature of the active sites. FT-IR spectroscopy in combination with pyridine adsorption measurements revealed that the catalyst material contains Lewis-acidic sites, most probably related to the Nb[double bond, length as m-dash]O groups of isolated tetrahedral NbO4 surface species, whereas no Brønsted-acidic sites were observed. Results from in situ Raman and complementary FT-IR measurements strongly suggest that Brønsted-acidic Nb–OH sites can be generated from Nb[double bond, length as m-dash]O groups by reaction with ethanol. This is in agreement with the observation that ethanol is essential for obtaining a very good catalyst performance. However, the Brønsted-acidic sites can be detected in significant amounts in particular in the presence of a Lewis-base, e.g. pyridine, most probably because the formation and/or the stability of these Brønsted-acidic sites are enhanced by a basic molecule. Assuming that cyclohexanone oxime, being a base, can play a similar role as pyridine, we propose on the basis of the spectroscopic findings obtained in this work and our kinetic results published recently, a reaction scheme for the formation of the active site at the Nb[double bond, length as m-dash]O group as well as for the recovery of the Nb[double bond, length as m-dash]O site during the final stage of the gas-phase Beckmann rearrangement.

Graphical abstract: Spectroscopic study on the active site of a SiO2 supported niobia catalyst used for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

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Publication details

The article was received on 04 May 2016, accepted on 21 Jul 2016 and first published on 27 Jul 2016


Article type: Paper
DOI: 10.1039/C6CP03014A
Citation: Phys. Chem. Chem. Phys., 2016,18, 22636-22646
  • Open access: Creative Commons BY license
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    Spectroscopic study on the active site of a SiO2 supported niobia catalyst used for the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

    M. M. Maronna, E. C. Kruissink, R. F. Parton, F. Soulimani, B. M. Weckhuysen and W. F. Hoelderich, Phys. Chem. Chem. Phys., 2016, 18, 22636
    DOI: 10.1039/C6CP03014A

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