Vanadium-Catalyzed Aerobic Oxidation of Diols and Lignin Models/Extracts
As the world moves forward to the development of biorefineries, the interest to replace chemicals and materials derived from petroleum is increasing exponentially. Lignin is a macromolecular by-product derived from the wood pulping industry, mainly used for heating purposes. The development of new processes to produce high value-added lignin products such as multifunctional aromatic chemicals and high-tech carbon materials are required to fulfill the needs for biorenewable feedstocks. Such processes are likely to include selective oxidation catalysis. The aim of this Thesis is to advance the state-of-the-art for the oxidation of lignin models and lignin extracts using homogeneous catalysts based on vanadium, an inexpensive and abundant transition metal, using air as the only oxidant. Lignin models containing the most important features of lignin (e.g., β-O-4 and β-1 linkages) were initially used to assess the catalytic potential. Previously reported (HQ)2V(O)(OiPr) and (dipic)V(O)(OiPr) catalysts (dipic = dipicolinate and HQ = 8-oxyquinoline) displayed different selectivity for C-H, C-O and C-C bond cleavage upon varying of the solvent, the lignin model or the catalyst. Moreover, these catalysts cleave the C-H bond of secondary alcohols through a two-electron oxidation process and the C-C bond cleavage of the oxidation product ketone in the presence of exogenous base. Several amine bis(phenolate) oxovanadium(V) catalysts were synthesized and fully characterized, and demonstrated very good activity for the oxidation of lignin models and the depolymerization of organosolv lignin. These new catalysts overcome the need for added base, display higher reaction rates of oxidation, and improve the selectivity for the disassembly of lignin models. The different selectivities involving C-H vs. C-O vs. C-C bond cleavage are discussed together with a novel redox-neutral C-C bond cleavage of lignin model 1,2-diphenyl-2-methoxyethanol. The oxovanadium(V) catalysts, along with a metal-free variant and other transition metal catalysts, were employed to assess their performance for the oxidation and depolymerization of organosolv lignin. Although most catalysts oxidized the lignin extracts, the oxovanadium(V) complexes demonstrated the highest degree of lignin depolymerization as shown by Gel Permeation Chromatography (GPC), quantitative-Heteronuclear Single Quantum Correlation (q-HSQC) and quantitative 31P NMR spectroscopy of derived phosphite esters. In a complementary study, oxovanadium(V) catalysts also established their utility for the valorization of cellulose-derived substrates (e.g., diols). Two trialkoxyamine oxovanadium(V) complexes bearing a triethoxyamine and tris[2-(3,5-di-tert-butyl-phenoxy)methyl]amine ligand respectively, selectively cleaved the C-C bond in 1,2-diols with excellent rates and using air as the only oxidant. In a stoichiometric investigation of this reaction, it was determined that the transformation proceeds through an unusual direct oxidative two-electron cleavage of diols, affording a non-oxo monometallic V(III) intermediate. DFT calculations support a single-site proton-coupled electron-transfer of the hydroxyl hydrogen to the V oxo orbital. In summary, this Thesis describes new developments in vanadium catalysis such as mechanistic implications and catalyst optimization for the valorization of lignocellulosic biomass utilizing air as an oxidant.
Lignin, Vanadium, Catalysis, Inorganic chemistry
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Université d'Ottawa / University of Ottawa