Towards a cost-competitive biorefinery: fractionation of willow with low-cost ionic liquids and synthesis of lignin-based copolymers
Creators: name: Weigand, Lisa
Publisher: name: Imperial College London
Year: 2018

description: Lignocellulosic biomass is considered as a promising alternative bio-based renewable resource that can be utilized for the production of energy, fuels, chemicals and materials. In order to realise its full potential, a pretreatment step which breaks up the biomass matrix and separates the lignocellulosic biopolymers is required. Research in this field has been ongoing for several decades, however, the development of cost efficient pretreatment processes and novel materials derived from lignocellulose is still a challenging and important task. The work described in this thesis focuses on two main aspects, namely biomass pretreatment using ionic liquids and synthesis of value added materials based on lignin, to aid the advancement of a biobased economy. Firstly, pretreatment of the hardwood willow was undertaken using the low-cost ionic liquid solution triethylammonium hydrogensulfate [N2220][HSO4]80%. The effect of the pretreatment severity (described by the pretreatment temperature, residence time and ionic liquid acid/base ratio) on the pulp composition and digestibility as well as the lignin properties was studied. The pretreatment conditions significantly affected the pulp properties, lignin molecular structure and molecular weight. High pulp digestibility and glucose yields were achieved for the optimised pretreatment severity. The hardwood willow is a potential feedstock for a biorefinery and characterized through its genomic diversity. The effects of ionic liquid pretreatment using the ionic liquid solution triethylammonium hydrogensulfate [N2220][HSO4]80% on 14 willow varieties was investigated to understand which lignocellulose and pulp property influences the glucose release. The lignin content of the biomass and pulp did not significantly influence the glucose release, but pulp properties such as available surface area, pore size and cellulose degree of polymerization did play a role in pulp digestibility. The use of lignin as a macromonomer for the synthesis of materials increases the cost efficiency of a potential biorefinery and the development of lignin based copolymers was investigated in the second part of this thesis. An ionic liquid mediated pathway for the synthesis of lignin-poly(furfuryl alcohol) copolymers was discovered in which poly(furfuryl alcohol) is grafted from the lignin polymer. The effect of different reaction conditions and lignin-to-fufuryl alcohol ratios on the properties of the synthesized copolymers was investigated. The material properties strongly depended on the reactant ratio. These copolymers display a high carbon content and thus are promising candidates for the production of carbon fibres or glassy carbon. Furthermore, protic ionic liquids were used as a combined solvent and catalyst system for the addition of aromatic compounds to the lignin polymer. Aromatic compounds such as 6-bromo-2-naphthol and 6,6’-dibromo-1,1'-bi-2-naphthol were successfully incorporated in the lignin polymer. This introduces a bromine functionality to the lignin structure which can be used as a reactive functionality to graft polymers from lignin via cross-coupling reactions offering a novel and simple synthesis pathway for the versatile functionalization of lignin and the production of lignin based materials. The implementation of a lignocellulosic biomass based biorefinery is still faces challenges such as high enzyme and high investment costs amongst others, thus limiting the number of the already existing biorefineries. The development of new technologies for biomass fractionation (e.g. using low-cost, recyclable ionic liquid solutions) and the utilization of lignin for the production of value added chemicals and materials are step stones to aid the realisation of a biobased economy.