Processing and upgrading of lignin for high-value applications

Abstract

Lignin is a complex, heterogeneous biopolymer composed of aromatic subunits connected through a variety of interunit linkages. It is the second most abundant natural biopolymer behind cellulose. Lignin represents a renewable source of aromatic monomers and a substrate for renewable materials generation. The continued development of lignin-derived alternatives to current crude oil products is therefore of high importance and is the motivation behind the work in this thesis.

Lignin can be isolated from lignocellulosic biomass using a variety of methods. In this thesis, a butanosolv pretreatment is used with a variety of biomass sources. Detailed analysis of the resulting lignins using a variety of techniques (nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy) led to novel insights into the structure and properties of butanosolv lignins. Optimisation of the butanosolv pretreatment led to beneficial structural changes to the isolated lignin that guided the choice of subsequent application. Butanosolv lignins isolated from different components of the same biomass were found to have different structural properties. Further fractionation of isolated lignins identified the influence of underreported contaminants on bulk lignin properties.

Butanosolv lignin was used to prepare novel biomass-derived materials that showed potential flame-retardant properties. Flame-retardant compounds were synthesised from lignin-derived monomers and used to modify lignins in a successful attempt to source as much carbon as possible in these potential commercial products from biomass.

Last, the chemistry of lignin-derived monomers was explored in the synthesis of the natural product Linderuca C and in the attempted synthesis of bioactive indanone compounds reported to have anti-microbial properties. The chemistry of mixtures of lignin-derived monomers was explored with the reaction progress monitored using NMR spectroscopy without the need for purification.