Selective Oxidation of Lignin Models and Extracts with Earth-Abundant Transition Metals and Hypervalent Iodine

Abstract

As a significant component of lignocellulosic biomass, lignin represents a potential source of value-added aromatic chemicals. In this thesis, catalytic systems with earth-abundant metal catalysts such as molybdenum(VI) and hypervalent iodine complexes were developed to selectively break down lignin models into lower molecular weight chemicals under mild conditions. Due to the complexity of lignin, simple lignin model substrates (A to E), representing common linkages in lignin, were used to investigate the catalytic activity/selectivity of these catalysts. With the molybdenum catalysts [7– 11]/SPC/Adogen®464 system (SPC = sodium percarbonate), oxidation of simple β-1 model compound A in acetonitrile showed primarily C-H bond cleavage to form the ketone product, benzoin methyl ether, whereas the Cα-Cβ bond cleavage product, methyl benzoate, was obtained by switching the reaction solvent to benzonitrile. Preference for generating the Cα-Cβ bond cleavage product, i.e. benzaldehyde, can also be achieved with other early to middle transition metal catalysts using H2O2(aq) as the terminal oxidant. Stoichiometric amounts of hypervalent iodine/Lewis acid systems [20a-c] were able to selectively cleave Cα-Cβ bonds to aldehydes with both simple β-1 model compound A and β-O-4 model compound C. In contrast, other lignin model compounds with different linkages were unable to be oxidized to a great extent using these Mo- or iodine-based complexes. The catalytic activity and selectivity of the reported vanadium complexes, copper salts and non-metal system 1-5 on non-volatile organosolv (NVO) lignin was investigated under basic condition. Details of the depolymerisation of lignin were determined by using Gel Permeation Chromatography (GPC) and the two-dimensional NMR technique, quantitative HSQC (q-HSQC) spectroscopy. Vanadium [2] and copper systems were found to be the most active for depolymerization of NVO lignin.