Understanding The Chemistry of Hydrothermal Liquefaction of Different Lignins

The valorisation of waste biomass is essential for sustainable global development and particularly crucial for the production of renewable carbon-based chemicals. Lignins are a major structural constituent of plants and some algae, and form a large proportion of agricultural and horticultural waste biomass. Thus, waste lignins could be a valuable source of renewable carbon-based chemicals. Notably, most waste lignins are produced by technical processes in the paper and pulp industry, and the structural characteristics of these so-called technical lignins depend on the type of pulping process that is used.

Hydrothermal liquefaction (HTL) is a promising method for the conversion of waste lignins into valuable chemical compounds. Investigations of the HTL of a variety of technical lignins have been performed, but the fundamental chemistry of lignin HTL remains poorly understood. Furthermore, there is little understanding of the variance in the reactivities of different lignins under HTL conditions. These aspects must be understood to enable HTL processes to be optimised on an industrial scale.

In this proposed project, we will extend our understanding of the HTL chemistry of lignins by investigating the hydrothermal processing of lignin model compounds and real lignins. Thus, we will synthesise and hydrothermally process model compounds possessing the structural characteristics of native (i.e. unprocessed) lignin and technical lignins (e.g. sulfonated lignin). The results of these model compound studies will provide an understanding of HTL mechanisms and reaction rates, and how these are influenced by the structural substituents of lignins, catalysts and process conditions. Subsequently, HTL of native lignin and three types of technical lignins derived from the same feedstock will be performed, and optimal HTL conditions will be determined. The results of these native and technical lignin HTL studies will be correlated with those from model compound studies to reveal the impact of structures and reaction pathways on selectivities and yields of the final products. These studies will also shed light on the variability in the reactivities of different lignins under HTL conditions. The new knowledge generated by this proposed project will greatly aid the optimisation of HTL as an industry-ready process for the valorisation of waste lignins.