Insights into the structure and reactivity of the biopolymer lignin using diffusion ordered spectroscopy and synthetic model studies

Abstract

This thesis concerns the use of diffusion ordered spectroscopy (DOSY) and synthetic model studies to further our understanding of the biopolymer lignin. Chapter 1 introduces the structure of lignin and describes the pre-treatment processes used for its isolation. A summary of the acidolysis of lignin into aromatic monomers is then given. Finally, analytical methods that are used to study lignin’s structure are discussed with a detailed description of DOSY-NMR.

Chapter 2 discusses the application of DOSY-NMR to estimate the molecular weight of lignin. This involved generating a calibration curve made up of over 150 different data points. By correlating the average diffusivity of each fraction with the estimated MW, a linear correlation was observed according to the Mark-Houwink-Sakurada power law. The use of this curve to estimate the molecular weight of freshly extracted lignin was then tested.

Chapter 3 investigates the potential to use 3D HSQC-DOSY to analyse specific structural features of lignin. Concatenation of the DOSY sequence with a HSQC pulse train generated the highest signal intensities. A 2-fold reduction in acquisition times was achieved using NUS with sine-weighted Poisson gap sampling. Three preliminary applications of this advanced NMR method were then investigated.

Chapter 4 investigates the acidolysis of acylated lignins. Protection of hydroxyl groups within lignin shut down the normally dominant C₂ pathway, producing a new aromatic monomer. Benzoyl protecting groups proved the most adept at achieving this switch. Unfortunately, this new aromatic monomer proved unstable, only isolated in low yield. However, this work illustrates how a simple modification can make a big difference.

Chapter 5 investigates the fate of the lignin backbone during the incorporation of alcohols using acidolysis conditions. An experiment was designed that used a ¹⁹F labelled alcohol and protecting groups. ¹⁹F DOSY-NMR was then used to show that the lignin backbone is not heavily degraded during this modification.

Date of Award	24 Jun 2020
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Tomas Lebl (Supervisor) & Nicholas James Westwood (Supervisor)