Ionic Liquid-Stabilizing Vanadium Oxo-Cluster Catalysts for One-Step Selective Oxidative Cleavage of β–O–4 Lignin Model Compounds

Abstract

An easily accessible and efficient catalyst is highly promising for the cleavage of the C–C bond and C–O bond of β–O–4 lignin model compounds. In this work, ionic liquid [Bim][Pic] (Bim = 1-butyl imidazolium, Pic = picolinate anion)-stabilizing vanadium oxo-cluster catalysts were constructed conveniently for selective aerobic oxidation to cleave the β–O–4 linkages into phenols, eaters and acids. The as-synthesized vanadium oxo-cluster catalysts Bim-V-n (n = 1–3) were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), electron paramagnetic resonance (EPR), mass spectrometry (MS), and elemental analysis. The vanadium in the catalyst exists stably in a mixed-valence state of V4+ and V5+. Molecular oxygen was activated through the mutual transformation of V4+ and V5+ to form superoxide free radicals, which were captured by the EPR spectrum. Especially, Bim-V-2 afforded superior catalytic activity under aerobic conditions (10 h, 120 °C). The highly reversible interconversion of V4+ and V5+ species in vanadium oxo-cluster allowed the coexistence of the mixed-valence vanadium species, which was responsible for oxidizing β–O–4 alcohols to β–O–4 ketones. Moreover, superoxide free radicals can facilitate the cleavage of C–C and C–O bonds in β–O–4 ketones. Especially, 1-butyl imidazolium also played a positive role in the cleavage of C–O bonds by the action of the cation protons. No appreciable loss of catalytic activity was observed, and the catalyst could be recovered and reused up to five times, reflecting the excellent recyclability of the present vanadium oxo-cluster catalysts. The catalytic system can be extended for the cleavage of the C–C bond and C–O bond of β–O–4 of the other lignin model compounds, and the reaction could still be completed efficiently. This provides a new perspective for the selective aerobic oxidation of β–O–4 lignin without co-catalysts.