Mechanism of Vanadium-Catalyzed Selective C–O and C–C Cleavage of Lignin Model Compound

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Abstract

Efficient depolymerization methods are critical to the sustainable production of fuels and chemicals from biomass. Ligand-controlled selective C(sp3)–O and Ar–C(sp3) cleavages of β-O-4 lignin model compounds were realized with vanadium catalysts under redox-neutral conditions or air atmosphere. To clarify the mechanism and the origin of selectivity, a joint theoretical and experimental study was performed herein. First, with the aid of density functional theory (DFT) calculations, an updated mechanism involving VV, VIV, and VIII complexes was discovered for the C(sp3)–O cleavage process catalyzed by the Schiff base vanadium complexes with an overall free energy barrier of 34.9 kcal/mol. Meanwhile, a detailed catalytic cycle involving novel stepwise O–O/Ar–C(sp3) cleavage was clarified for the Ar–C(sp3) cleavage process catalyzed by the bis(8-oxyquinolate) coordinated vanadium complexes, having an overall free energy barrier of 28.8 kcal/mol. Further analysis based on the energetic span model revealed that the switchable selectivity results from the different T1 (ground triplet state)–HOMO separation/charge dispersion effects of ligands and the different formal oxidation states of the TOF-determining transition state (TDTS) in the C(sp3)–O and Ar–C(sp3) cleavage processes. Finally, control experiments of base and oxygen pressure were conducted to validate the conclusions from DFT studies regarding the role of bases and the TDTS step in the Ar–C(sp3) cleavage process.

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