ABSTRACT
The depolymerization of the biopolymer lignin can give pure aromatic monomers but selective catalytic approaches remain scarce. Here, an approach was rerouted to deliver an unusual phenolic monomer. This monomer's instability proved challenging, but a degradation study identified strategies to overcome this. Heterocycles and a useful synthetic intermediate were prepared. The range of aromatics available from the ß-O-4 unit in lignin was extended.
ABSTRACT
As societal challenges go, the development of efficient biorefineries as a means of reducing our dependence on petroleum refineries is high on the list. One of the core strengths of the petroleum refinery is its ability to produce a huge range of different products using all of the components of the starting material. In contrast, the target of using all the biopolymers present in lignocellulosic biomass is far from realised. Even though our ability to use the carbohydrate-based components has advanced, our plans for lignin lag behind (with the notable exception of vanillin production). One approach to lignin usage is its controlled depolymerisation. This study focuses on an increasingly popular approach to this challenge which involves highly selective lignin oxidation to give a material often referred to as ligninOX. But what do we mean by ligninOX? In this study we show that it is possible to form many different types of ligninOX depending on the oxidation conditions that are used. We show that variations in the levels of processing of the ß-O-4, the ß-ß and a third linkage occur. Through use of this information, we can form a well-defined ligninOX from six different hardwood lignins. This process is reproducible and can be carried out on a large scale. With a source of well-defined ligninOX in hand, we show that it can be converted to simple aromatic monomers and that any remaining ligninOX is sufficiently soluble for further processing to be carried out.
ABSTRACT
One key challenge hindering the valorization of lignin is its structural complexity. Artificial lignin-like materials provide a stepping stone between the simplicity of model compounds and the complexity of lignin. Here, we report an optimized synthesis of an all-G ß-O-4 polymer 1 designed to model softwood lignin. After acetylation, the polymer Ac-1(V) was fractionated using a protocol that involved only volatile organic solvents, which left no insoluble residue. Using diffusion ordered spectroscopy NMR in combination with gel permeation chromatography, it was revealed that this fractionated material behaved like a flexible linear polymer in solution (average α > 0.5). Acetylated kraft lignin was subsequently processed using the same fractionation protocol. By comparison with the model polymer, we propose that the acetylated kraft lignin is composed of two classes of materials that exhibit contrasting physical properties. One is comparable to the acetylated all-G ß-O-4 polymer Ac-1, and the second has a significantly different macromolecular structure.
ABSTRACT
Understanding the structure of technical lignins resulting from acid-catalysed treatment of lignocellulosic biomass is important for their future applications. Here we report an investigation into the fate of lignin under acidic aqueous organosolv conditions. In particular we examine in detail the formation and reactivity of non-native Hibbert ketone structures found in isolated organosolv lignins from both Douglas fir and beech woods. Through the use of model compounds combined with HSQC, HMBC and HSQC-TOCSY NMR experiments we demonstrate that, depending on the lignin source, both S and G lignin-bound Hibbert ketone units can be present. We also show that these units can serve as a source of novel mono-aromatic compounds following an additional lignin depolymerisation reaction.
