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.

Ionic liquid-stabilized metal oxoclusters: from design to catalytic application

“…63 Similarly, imidazolium IL [BIm][Pic] (BIm = 1-butyl-imidazolium)-stabilized vanadium oxocluster catalysts were prepared as well. 64…”

“…63 Similarly, imidazolium IL [BIm][Pic] (BIm = 1-butyl-imidazolium)-stabilized vanadium oxocluster catalysts were prepared as well. 64 Ti oxoclusters stabilized by carboxylic acid-functionalized imidazolium-based IL were also fabricated via a solvothermal method. 65 Characterization indicated that Ti oxoclusters (Ti oxo-HSO 4 ) existed in the form of subnanosized structure, uniformly dispersed with an average particle size of ca.…”

“…Hou et al used imidazolium IL [BIm][Pic] to stabilize the vanadium oxocluster catalysts, and the catalyst showed superior activity for one-step selective oxidation to cleave the β-O-4 linkage of lignin model compounds to phenols, eaters, and acids. 64 It was found the presence of mixed-valence state of V 4+ and V 5+ in V oxocluster catalysts was crucial for enhancing the catalytic activity, and the oligomeric states of V species could be tuned by adjusting the molar ratio of IL [Bim][Pic] to V. Especially, 1-butylimidazolium also played a facilitating role in the cleavage of C-O bonds by the action of the protonated IL cations. Notably, the catalyst can be recycled with the catalytic activity did not decrease significantly even after five cycles (Fig.…”

Ionic Liquid-Regulated Titanium Oxo Clusters Catalyze Selective Oxidation of Anilines to Azoxybenzenes

“…Therefore, IL has become an important substitute for traditional volatile solvents in the field of chemical reaction and separation. , More importantly, IL with different functions can be formed by special design and regulation of hydrogen bonds, coordination, and electrostatic interactions, and different combinations of cations and anions can achieve different properties, which has attracted wide attention and has been widely used in catalysis. , For example, in our previous reports, α-hydroxy carboxylate-based ILs as an indispensable agents can stabilize Nb oxo clusters and showed superior activity of catalytic oxidation . Besides, we uncovered that IL [Bim]­[Pic]-stabilizing vanadium oxo-cluster catalysts were constructed and utilized for selective aerobic oxidation to cleave the β–O–4 linkages …”

“…35 Besides, we uncovered that IL [Bim][Pic]-stabilizing vanadium oxo-cluster catalysts were constructed and utilized for selective aerobic oxidation to cleave the β−O−4 linkages. 36 As a continuation of our efforts to design easily accessible, highly efficient, and stable metal oxo clusters for catalytic oxidation, in this work, we attempted to develop IL-stabilizing Ti oxo-cluster catalysts for the oxidation of aromatic amines to azoxy compounds in the presence of H 2 O 2 since Ti can be regarded as an earth-abundant transition metal. As a transition metal, titanium species can form highly active sites for catalytic oxidation.…”