Lignin extraction, purification and depolymerization study

Resum

Petroleum reverses are depleting, fuel prices are increasing and the push of theenvironmental policies such as Kioto protocol are forcing the search for new biofuels,biomaterials and biochemicals. Lignocellulosic biomass seems to be a promising rawmaterial for its transformation into the desirable aforementioned bioproducts in theso-called Biorefineries. Bearing this in mind, an entire biorefinery process is proposedin this thesis in order to add value to lignin stream by transforming lignin into simplephenolic monomeric compounds. Agricultural residues were considered as interestingcandidates to be used as raw material in biorefinery processes because of theirabundance and their low price. In that sense, olive tree pruning was used as rawmaterial in the developed study.Two lignin revalorization processes were proposed in this thesis to produce highaddedvalue bulk chemicals. Revalorization process schemes were composed of threestages: lignin extraction, lignin purification and lignin depolymerization. Extractionand purification processes were the same for the two proposals. Lignin extractionwas studied in the sense of the enhancement of lignin production as well as trying toextract lignin with optimum physicochemical properties for its valorization. To thisend, different extraction methods were studied and the one that gave the bestresults was optimized. Purification process objective was to obtain specific ligninfractions with different characteristics so they can be used in their most adequateapplication. Different purification processes (differential precipitation andultrafiltration) were considered with this purpose. Ultrafiltration was selected as thepurification process that produced lignin fractions with different purity, specificmolecular weight and defined physicochemical properties.Finally, two catalytic pathways (hydrolysis and hydrogenolysis) were proposed forlignin revalorization into high value added bulk chemicals. Lignin hydrothermaldepolymerization was studied in terms of screening the best catalyst (homogeneouscatalysts) and of optimizing the reaction conditions (temperature and time).Comparatively, lignin hydrogenolytic depolymerization was also considered in orderto reach the same goal. In this sense, different heterogeneous metallic catalysts andhydrogen donating solvents were proposed so as to improve simple phenoliccompounds production. The optimized conditions of each depolymerization processwere applied to define lignin fractions in order to study the influence of ligninproperties on depolymerization reactions yields and to compare which catalyticproposal presented the best challenge for lignin revalorization.