Effect of the N-based ligands in copper complexes for depolymerisation of lignin

Several organic soluble N-based ligands and their copper complexes were firstly investigated as catalysts to depolymerise organosolv lignin in the organic solvent, dimethylformamide (DMF) and an ionic liquid (1-ethyl-3-methylimidazolium xylenesulfonate, [emim][ABS]). The results of screening depolymerisation reactions in DMF and [emim][ABS] showed that all the copper–amine complexes catalysed lignin depolymerisation more efficiently in ionic liquids than in DMF. Among the seven types of ligands, copper complexes with two types of ligands (E)-N-(pyridin-2-ylmethylene)aniline and (E)-4-methoxy-N-(pyridin-2-ylmethylene)aniline depolymerised the lignin more efficiently than the others. These two copper complexes with the N-based ligand were further studied to determine the most efficient conditions for the depolymerisation of the lignin. The most effective depolymerisation by conditions involved treatment at 180 °C for 12 h in [emim][ABS]. Cyclic voltammetric studies were carried out to investigate the reversible potential associated with the copper centers of their complexes with these N-based ligands. The results suggest that two types of ligands have more positive reversible potentials than those of other copper complexes

Lignin chemical degradation using redistribution mechanism and its biomass applications

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

Lignin Depolymerization with Phenol via Redistribution Mechanism in Ionic Liquids

Nowadays, ionic liquids (ILs) have been recognized as a promising way to fractionate biomass. In this study, we discuss how lignin depolymerization was achieved via the redistribution mechanism with phenols in ionic liquids. Two ionic liquids (1-ethyl-3-methylimidazolium xylenesulfonate [emim]­[ABS] and 1-butyl-3-methylimidazolium methylsulfate [bmim]­[MeSO₄]) were selected as a solvent for lignin depolymerization. Before embarking on lignin depolymerization, two selected lignins (organosolv lignin and Klason lignin) were characterized based on their chemical structure and molecular weight. The depolymerized results show that the two lignins (organosolv lignin and Klason lignin) can be extensively depolymerized under oxidative conditions using a Cu/EDTA complex in the presence of a monomeric phenol (4-<i>tert</i>-butyl-2,6-dimethylphenol) that is blocked from oxidative coupling in the <i>ortho</i> and <i>para</i> positions at temperatures of 180 °C

Poly(N-4-vinylbenzyl-1,4,7-triazacyclononane) Copper Complex Grafted Solid Catalyst for Oxidative Polymerization of 2,6-Dimethylphenol

A new solid phase catalyst, poly(N-4-vinylbenzyl-1,4,7-triazacyclononane) copper(I) complex, grafted onto polystyrene particles, has been employed for the oxidative polymerization of 2,6-dimethylphenol using an aqueous biphasic (water/toluene) solvent system. The solid catalyst was synthesized by first grafting N-(4-vinylbenzyl)-1,4,7-triaza-cyclononane onto polystyrene particles using a radical mediated polymerization method and next by creating the polymer-metal complex of copper-triazacyclononane with these modified particles. Poly(2,6-dimethyl-1,4-phenylene oxide) was successfully obtained from the polymerization of 2,6-dimethylphenol using this new metal-organic solid phase catalyst

Immobilized Horseradish Peroxidase (I-HRP) as Biocatalyst for Oxidative Polymerization of 2,6-Dimethylphenol

An enzyme, horseradish peroxidase (HRP), was immobilized on silica nanorods and employed as a catalyst for the oxidative polymerization of 2,6-dimethylphenol. With this catalytic system, the polymer, poly­(2,6-dimethyl-1,4-phenylene oxide), was successfully obtained from a water–acetone solvent system. The immobilized HRP exhibited substantially enhanced enzymatic activity toward oxidative polymerization as well as some degree of reusability compared with free HRP

Two-Photon-Induced [2+2] Cycloaddition of Bis-thymines: A Biocompatible and Reversible Approach

Despite having great value across a wide variety of scientific fields, two-photon polymerizations currently suffer from two significant problems: the need for photoinitiators, which generate toxic side products, and the irreversibility of the process. Hence, the design of a versatile approach that circumvents these issues represents a major scientific challenge. Herein, we report a two-photon absorption strategy where reversible [2 + 2] cycloaddition of bis-thymines was achieved without the need for any photoinitiator. The cycloaddition and cycloreversion reactions could be induced by simply changing the irradiation wavelength, and repeated writing and erasing cycles were performed. The simplicity, reversibility, and biocompatibility of this strategy open up a whole new toolbox for applications across a wide variety of scientific fields.status: publishe

Two-Photon-Induced [2+2] Cycloaddition of Bis-thymines: A Biocompatible and Reversible Approach

Despite having great value across a wide variety of scientific fields, two-photon polymerizations currently suffer from two significant problems: the need for photoinitiators, which generate toxic side products, and the irreversibility of the process. Hence, the design of a versatile approach that circumvents these issues represents a major scientific challenge. Herein, we report a two-photon absorption strategy where reversible [2 + 2] cycloaddition of bis-thymines was achieved without the need for any photoinitiator. The cycloaddition and cycloreversion reactions could be induced by simply changing the irradiation wavelength, and repeated writing and erasing cycles were performed. The simplicity, reversibility, and biocompatibility of this strategy open up a whole new toolbox for applications across a wide variety of scientific fields