Stefania Bertella
Lignin is a renewable aromatic polymer that due to its abundance and unique chemical structure is a promising candidate to replace aromatic materials that are currently sourced from fossil oil. The same structure of lignin poses however drawbacks for its valorization. The high temperature and strong pH that are generally used to isolate lignin, make this polymer prone to reactions of condensation during its extraction. This translates into isolated lignins where most of their native functional groups are eliminated in favor of the formation of interunit carbon-carbon linkages. The lack of control on lignin's chemical functionalities ultimately hinders the use of this polymer for the development of new materials. Many research groups have been trying to tackle these challenges by developing new processes that could allow for the efficient isolation of lignin without compromising on its functional groups. Amongst these, the Aldehyde-Assisted Fractionation (AAF), which relies on the formation of acetal groups between lignin's ß-O-4 units and an added aldehyde, was proven to be efficient in isolating high yields of lignin, that could be then quantitatively depolymerized into small aromatic monomers. Inspired by this research, the aim of my work is to take advantage of the AAF to quantitatively isolate lignin from lignocellulose, at the same time introducing on the biopolymer scaffold new functional groups that could allow for more effective development of lignin-based materials. My work is based on a Multifunctional-AAF, in which the multifunctional aldehydes can react with the lignin via the formation of acetals, at the same time leaving on the biopolymer additional functionalities. In the first part of my thesis, I show how the extraction and simultaneous functionalization of lignin can be achieved by using terephthalic aldehyde (TALD). In addition to successfully functionalizing the lignin, I demonstrate how the degree of functionalization can be precisely controlled and monitored via 1H NMR, yielding a material with high reactivity towards phenolation. In the second part, I show how the isolation and controlled functionalization of lignin can be translated to the use of other aldehydes, namely glyoxylic acid (GA), to functionalize lignin with carboxylates, quantified via 31P NMR. The isolated lignin offers in this case unique avenues for the development of more sustainable surfactants. In the last chapter of the thesis, I show that the Multifunctional-AAF can be expanded to isolate and functionalize lignin with multiple functionalities by using mixtures of aldehydes (TALD and GA), demonstrating again that each functional group can be easily quantified. These lignins, extracted with different ratios of the two functional groups, showed dissimilar properties in terms of solubility and reactivity towards gelatin. We used these lignins to develop lignin-gelatin based hydrogels, showing how the bi-functionalization was pivotal for the formation of hydrogels with improved rheological, mechanical, self-healing and adhesive properties. In summary, this thesis shows the development of a new approach for lignin extraction and simultaneous valorization. The versatility of this process enables more efficient functionalization, allowing the obtainment of lignins which are "tailor-made" for their end use. The control over lignin's chemical structure and functionalization will pave the way for the use of lignin in high-end application.
EPFL2022
