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

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.