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.