Abstract

Facing climate change and the near end of the oil age led us to rethink our consumption patterns and energy models, to insure a sustainable future for human beings. In this way, the biorefinery plays an important role and is a key concept to bring a part of the solution, exploiting the potential of biomass, which is renewable and widely available, for energy production and chemicals, materials, human health, and agri-food products manufacturing.
Hence, this project aims at valorizing wood hemicelluloses in the frame of a lignocellulosic biorefinery integrated in a kraft pulp mill. These are complex heteropolysaccharides accounting for one third of wood and ones of the most abundant biopolymers on earth. Both softwood and hardwood chips industrial mixtures are used for hemicelluloses oligomers extraction prior to the kraft process through a low-tech and green process (without using any chemicals) named autohydrolysis. This pretreatment step allows for the solubilization of oligosaccharides, monosaccharides, lignin fragments and other small molecular weight organic compounds and is the only one used at an industrial scale to produce hemicelluloses from wood. The chemical composition of these wood hydrolysates depends much on wood species and autohydrolysis severity.
The main objective of the thesis is to design a sort of universal scheme of analytical methods that can be applied to any kind of hydrolysate in order to identify, quantify and isolate its hemicelluloses families.

The study of the molecular diversity of soluble oligosaccharides containing liquors produced at high and low temperature (170 and 150°C) is based on several purifications’ steps before analysis.
First of all, membrane filtration is used with different molecular weight cut-offs to classify and separate oligomers and impurities according to their size. In a second part, soluble lignin and sugar degradation products generated during autohydrolysis are removed by adsorption on activated charcoal. Less polydisperse and 99% pure oligosaccharides mixtures are thus obtained thanks to the optimization and combination of these two techniques of purification. Nevertheless, big losses of hemicelluloses are triggered  by these treatments between 70 and 80% compared to the raw hydrolysate. In the end, the last chapter of experiments deals with the final purification and characterization steps of these purified samples which are performed by size exclusion liquid chromatography and mass spectrometry analyses.
This chromatographic technique operating at low pressure is an interesting tool for the separation of neutral oligomers from hexuronic acid substituted oligomers in both hardwood and softwood extracts. Also, elimination of recalcitrant lignin carbohydrates complexes was achieved for some fractions as well as the isolation of specific bioactive molecules of low degree of polymerization (potential nutraceutical applications).