Lignin chemical degradation using redistribution mechanism and its biomass applications

Abstract

Lignin is one of the most abundant renewable raw materials available on earth and it has the potential to yield valuable low molecular weight aromatic compounds if it can be depolymerized selectively. Despite its unique characteristics as a natural product with multiple chemical and biophysical functionalities, it is largely under-exploited, because of the lack of available methods that effect depolymerization in a selective manner. One of the dominant linkages in lignin has a similar aryl ether structure to poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). There is evidence that the formation of PPO by an oxidative coupling polymerization of 2,6-dimethyl phenol (DMP) involves a redistribution mechanism. The most likely pathway involves the formation of a quinone ketal intermediate. This ketal unit can be either be redistributed (dissociated) or undergo an intramolecular rearrangement. Considering the reversibility of the redistribution mechanism in PPO polymerization and the existence of the same structural moiety in lignin, selective depolymerization of lignin may be effectively carried out under conditions favouring the redistribution mechanism. The focus of this thesis is to exploit PPO as a model polymer for lignin depolymerization and repolymerization in water and ionic liquids. The same methodology will be applied to lignin using a range of reaction conditions in an attempt to effect depolymerization to provide useful oligomers and monomers, which can be subsequently repolymerized to produce biomass derived plastics. In this research PPO was synthesized using different chemical and enzymatic catalysts and depolymerization of the PPO was achieved in water and ionic liquid with 4-tert-butyl-2,6-dimethylphenol (TBDMP) and 2,6-dimethylphenol (DMP) under oxidative conditions. Applying the same olptimized conditions used for PPO, lignin depolymerization was achieved, involving the redistribution mechanism under controlled mild conditions. Furthermore, several lignin-based copolymers (polyester-based and polyurethane-based) were then synthesised using depolymerized oligomeric lignins. The molecular weight of the all depolymerized products and lignin based copolymers was determined by gel permeation chromatography (GPC). The depolymerized products and newly formed lignin based copolymers were further analyzed by nuclear magnetic resonance (NMR) and infrared (IR). Thermal properties of lignin based copolymers were also evaluated using thermoanalytical techniques including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Overall these studies demonstrated that PPO can be successfully depolymerized in water and ionic liquid. The potential for controlling the extent of depolymerization and the molecular weight of the depolymerized oligomers by using solvents where the solubility of PPO is limited was demonstrated. Moreover, it has been demonstrated that lignins can be extensively depolymerized under oxidative conditions using Cu(II) complexes and a monomeric para-blocked phenol (TBDMP). Novel lignin based thermoplastic copolyester was successfully synthesised by the esterification of oligomeric lignin with sebacoyl chloride in the presence of Triethylamine (TEA) as a base. Furthermore, several attempts were made to successfully synthesise lignin based polyurethane via a reaction of depolymerized lignin with aliphatic diisocyanate.