Alcohol Organosolv Pretreatment: Toward an Efficient Lignin Extraction

Context: For several decades now that an increasing amount of research has emrged from a common interest to face the fossil-fuels reserves depletion and the greenhouse gas emission increase. This context has given rise to a growing interest to move from an oil economy towards a sustainable and circular bio-based economy. One of the solutions is to replace part of the fossil resources by renewable ones based on lignocellulosic biomass. The development of biorefineries, using plant material that does not compete with food/feed (lignocellulose) to produce bio based products (biofuels and biomaterials), is in constant evolution. In order to break the complex structure and allow the use of lignocellulosic biomass, an additional fractionation step called pretreatment is necessary before valorizing its constituents. Organosolv treatment, fractionation processes using organic solvent, are among the most promising strategies for valorising lignocellulosic biomass and could facilitate the transition towards efficient utilization of renewable feedstocks.
Scientific research: The present thesis aims at broadening the knowledge and opening new perspectives in terms of Organosolv treatment with alcohol in order to extract a lignin with high valorization potential. In particular, it allowed to study an emerging strategy constituted by a sulfuric acid (H2SO4) catalyzed process in a biphasic system using a hydrophobic alcohol (n-butanol) and water.
Therefore, the first exploratory work focuses on a conventional organosolv using high ratio of ethanol to dissolve lignin from a crop dedicated for biofuel production: tall fescue. This “lignin-first” approach was limited as it seriously degraded cellulose, produced non-fermentable sugars derivatives and required large amount of water to recover lignin.
The second work opens perspectives for improvements in terms of efficiency of lignocellulosic fractionation on the one hand, and on the other hand on the possibility of recycling the solvent and the isolation of the products solubilized in the "liquor". The butanol/water biphasic system was proposed and studied on six different biomasses in order to assess to what extend the process was suitable on a wide variety of heterogeneous biomasses as versatility of process being of great importance. The process is efficient in term of delignification and improves enzymatic saccharification significantly, however, Japanese cedar was too recalcitrant.

Furthermore, the lack of knowledge regarding lignin partition in butanol/water biphasic systems constituted a major limitation for further optimization. The third work is focused on the study of the partition in “aqueous/hydrophobic organic solvent” of three technical lignins originating from various biomasses (grass, hardwood, softwood). They were performed in aqueous/alcohol systems of varying pH and organic phase. Partition coefficients and analyzes of recovered fractions were investigated. Among the investigated alcohols, n-butanol is viewed as a promising solvent for lignin separation considering its ability to solubilize a wider range of lignin polymer. Results provide valuable information regarding the chemical structure and polymer size of recovered lignin based on the setting of medium parameters. This will be helpful for both separation of homogenous lignin fractions from pulp & paper industries and biorefineries byproducts and to understand solubility and partition mechanisms of lignin in pretreatment performed in biphasic systems.
A fourth and final work explores the possibilities of optimizing butanol treatment on Japanese cedar. The recovery of butanol after treatment as well as several parameters (solid/liquid ratio, butanol/water ratio, temperature, time, type of acid catalyst...), were investigated. The solid/liquid and butanol/water ratios appear to be the most important factors in the opitimization of the treatment. Moreover, the addition of an acid hydrolysis catalyst (Lewis acid) FeCl3 gives promising results.
Conclusion and perspectives: The development of a biphasic process allowed the separation of high purity cellulose, the hydrolysis of hemicellulose and the recovery and isolation of lignin, in three distinct phases. The “in-situ” homogenization of the lignin structure by varying the extraction/solubilization conditions appears promising. The treatment has good perspectives in terms of solvent recycling and is effective in destabilizing the biomass for subsequent saccharification (and potential valorization into biofuel). Several improvements can be envisaged, namely: the implementation of a (semi-)continuous process and further optimization of the softwood fractionation to increase its versatility, in particular by investigating Lewis acids (e.g. FeCl3). Finally, the economic viability of the process should be thoroughly evaluated, by promoting the cascade valorization of the whole plant and maximizing the potential of each component (cellulose, furanic derivatives, lignin, secondary metabolites...).