Lignin valorisation using supercritical water

Resumen

Limited resources of fossil fuels and environmental concerns arose from their exhaustive use make us to move to a new sustainable approach for energy production and use. The development of biorefineries that use biomass for the production of fuels and chemicals is the sustainable substitution of petroleum refineries on which rely our society. Lignocellulosic biomass is the most abundant and easiest grown kind of biomass. This biomass, composed of three main constituents, cellulose, hemicellulose and lignin could be a good source for fuel and a wide range of chemicals. However, in order to use this biomass in an effective way and make biorefinery feasible and competitive different challenges need to be faced. One of the technologies that can face these challenges is the technology that uses supercritical water (SCW) as solvent and reaction media. One of those important changes that we need to face is biomass recalcitrance due to its very complex and rigid structure. Lignin fraction contributes significantly to biomass recalcitrance. Chapter 1 of this thesis discusses hydrolysis of the defatted grape as high lignin content biomass in supercritical water at 380 °C and 260 bar from 0.18 s to 1 s. The focus is on the potential recovery of sugars and detailed characterization of the remaining solid phase enriched in polyaromatics. After the longest reaction time of 1 s, 56 % of carbohydrates could be recovered in the liquid phase, as a result of carbohydrate hydrolysis. Total extraction of carbohydrates from biomass could not be obtained as after the longest reaction time carbohydrates still composed 10 % of the solid phase. Milled wood lignin was extracted from the original biomass and compared to dioxane extract of the solid phase, in order to understand the main structural changes during the SCW hydrolysis process. Analysis showed dioxane extract of solids phase as a very complex mixture of lipophilic extractives, flavonoids, and lignin with a certain amount of chemically linked carbohydrates. 2D NMR analysis of compared extracts indicates very subtle changes in the amounts of main lignin moieties (β-O-4’, β-β’ (resinol) and β-5 (phenylcoumaran)) after the SCW process. This subtle change of the main lignin structures is an important feature in the further valorization of this sulfur-free lignin residue. Lignin is a by-product of different biorefineries and its valorization is an important contribution to their economic development. The production of aromatic compounds is the most attractive way for lignin valorization. However, the depolymerisation reaction of depolymerization products was reported to be the main challenge for better recovery of aromatic compounds from lignin using different technologies. In Chapter 2 of the thesis, this repolymerisation reaction is studied following the reactivity of lignin depolymerization products among each other and with lignin. Sulfonated kraft lignin (SKL) was used together with four model compounds: vanillin, vanillic acid, vanillyl alcohol and acetovanillone. As the structure of SKL is less known, Indulin as kraft lignin with a well-known structure was also employed as the reference for a better understanding of the repolymerisation reaction. 2D-NMR analysis of the solid residue from different samples showed the presence of diarylmethane structures. Those structures could be responsible for the repolymerisation reaction. However, the study shows that the formation of diarylmethene structures from lignin and model compounds follows a different mechanism, including lignin released fragments with free phenolic β-O-4 structures and monomeric product vanillyl alcohol. Understanding of repolymerisation process is crucial for the development of strategies for better recovery of aromatic compounds from lignin Properties of water vary significantly over a wide temperature and pressure range. When crossing its critical point fluids exist in a unique supercritical phase, where no boundary between liquid and vapor phase exists. Chapter 3 of the thesis follows exactly fading of this boundary. Interfacial tension of water was measured in two systems: liquid water/gas water system and water/nitrogen system. The measurements were performed using a pendant drop method. In water/water system measurements follows the saturation line of water up to the temperature of 350 °C, while in the water/nitrogen system measurements were performed under the pressure of 10 MPa, 15 MPa, and 18 MPa and temperature up to 325 °C. The experimental data were correlated using the Macleod-Sugden equation. Good agreement of equation with measurements result was found, showing an AAS of 3.52 % for water nitrogen system and 2.98 % for the water/water system.