Defence of doctoral thesis: Jenevieve G. Yao – Microwave-assisted transformations of lignin for value-added material applications

The chemical diversity and structural complexity of lignin, an abundant biopolymer found in vascular plants, present a multitude of opportunities for the modification and fine-tuning of its properties to suit downstream demands. In this thesis, microwave-assisted strategies were explored as efficient and environment-friendly pathways for lignin valorization towards value-added material applications. First, the mild microwave-assisted organosolv extraction of lignin from spruce wood was demonstrated. The effective deconstruction of lignocellulosic structures by microwave irradiation led to the rapid extraction of structurally intact lignin with preserved β-aryl ether linkages and minimal condensation. The high structural quality of the obtained lignin was visibly manifested in its significantly lighter color relative to most technical lignins, hence improving its suitability for material applications wherein the color of the end product is important. Next, a green approach to convert lignosulfonate to carbonaceous products was implemented via microwave-assisted hydrothermal carbonization. This resulted in the synthesis of carbon spheres that served as precursors for nanographene oxide (nGO)- type carbon dots, which were characterized as having abundant oxygen- containing functional groups. The nGO-type carbon dots were utilized as building blocks in the development of porous composites for the adsorption of metal ions and cationic dye pollutants. Lastly, microwave-assisted esterification was employed to both hydrophobize alkali lignin and to functionalize the microwave-extracted organosolv lignin with methacrylate units to facilitate their utilization for 3D printing applications. Through this microwave-assisted approach, high degrees of substitution were achieved after a short reaction duration without the need for additional solvents or catalysts. Effective hydrophobization was exemplified in the enhanced thermal stability and compatibility of the hydrophobized lignins in polylactide (PLA), thereby enabling the melt-processing of up to 50 wt% of lignin in PLA thermoplastic blends for fused filament fabrication. Also, successful functionalization of microwave-extracted organosolv lignin with reactive methacrylate moieties enabled it to partake in network formation within a photocurable resin for digital light processing. This ultimately resulted in 3D printed thermosets with improved tensile strength (by 15%) and elongation at break (by 79%) relative to unfilled resin, after the incorporation of just 1 wt% lignin.

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