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Issue 3, 2011
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In situ spectroscopic investigation of the cobalt-catalyzed oxidation of lignin model compounds in ionic liquids

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Abstract

The cobalt-catalyzed oxidation of lignin and lignin model compounds using molecular oxygen in ionic liquids proceeds readily under mild conditions, but mechanistic insight and evidence for the species involved in the catalytic cycle is lacking. In this study, a spectroscopic investigation of the complexes involved during this process was conducted using in situ ATR-IR, Raman, and UV-Vis spectroscopy. A plausible mechanism was proposed that explains the role of the ionic liquid and the other reaction conditions necessary to achieve high catalytic activity. Direct spectroscopic evidence for the species involved in the catalytic cycle was obtained. In addition, substrate consumption and product formation during the oxidation of several lignin model compounds, such as veratryl alcohol, cinnamyl alcohol, and a model compound with a β-O-4 linkage, was directly monitored. The reaction proceeds via the coordination of alcohol-containing substrates to the Co followed by formation of a Co-superoxo species. The presence of hydroxide is necessary for coordination of the alcohol to occur. Hydrogen peroxide that forms as a reaction by-product underwent rapid disproportionation to yield water and molecular oxygen. Involvement of the various intermediates was further confirmed by 18O2 labeling studies. The properties of the ionic liquid greatly influence the catalytic activity both by stabilizing reactive intermediates and by favoring the coordination of the substrate to the cobalt over the direct oxidation of the cobalt without substrate.

Graphical abstract: In situ spectroscopic investigation of the cobalt-catalyzed oxidation of lignin model compounds in ionic liquids

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Publication details

The article was received on 13 Aug 2010, accepted on 14 Dec 2010 and first published on 04 Feb 2011


Article type: Paper
DOI: 10.1039/C0GC00437E
Citation: Green Chem., 2011,13, 671-680

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    In situ spectroscopic investigation of the cobalt-catalyzed oxidation of lignin model compounds in ionic liquids

    J. Zakzeski, P. C. A. Bruijnincx and B. M. Weckhuysen, Green Chem., 2011, 13, 671
    DOI: 10.1039/C0GC00437E

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