Oxidative pretreatment and biocatalytic valorization of lignocellulosic biomass
- Oxidative Vorbehandlung und biokatalytische Wertschöpfung von lignocellulosehaltiger Biomasse
Wiermans, Lotte; Leitner, Walter (Thesis advisor); Pohl, Martina (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2016)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2015
In this PhD thesis, several novel (catalytic) approaches for lignocellulose valorization have been developed, using either lignocellulose, lignin, or other relevant biomass-derived platform chemicals. In the first section, the oxidative delignification of lignocellulose - using enzymatically in situ formed peracid - was studied on different natural resources in dimethyl carbonate (DMC) as solvent. Through that processing the formation of a yellowish lignin oil was observed, together with a delignified white polysaccharide fraction, which was highly accessible to cellulases to afford fermentable sugars. The characterization of the novel lignin oil revealed that during the delignification also a significant dearomatization of lignin took place. Thus, a novel dearomatized lignin-based material could be achieved.In a subsequent section of this PhD thesis only the lignin fraction of beech wood - obtained through the in-house developed OrganoCat pretreatment - instead of lignocellulose was used to study this more in-depth. This lignin was fully soluble in DMC, and consequently reactions could be conducted in a homogeneous reaction system, significantly accelerating the oxidative processing. The dearomatization of lignin in this homogeneous reaction system took also place without any catalyst. Herein, the degree of lignin dearomatization depended strongly on the oxidant concentration, and to a lesser extent on temperature. Taking advantage of this fact, a catalyst-free oxidative lignin dearomatization was developed yielding lignin products with different features, ranging from a solid dark brown powder to a yellowish lignin oil. Several attempts were conducted to elucidate the mechanism and the reactive species of this catalyst-free dearomatization, yet relevant aspects of the mechanism still remain elusive. In the last section, the lignin dearomatization was performed using a novel enzyme from Mycobacterium smegmatis, which catalyzes (trans)esterification in buffer under conditions where other hydrolases can only conduct hydrolysis. One of these reactions is perhydrolysis, thus forming the organic peracid in aqueous media, which is needed for dearomatization. However, the dearomatization in aqueous systems catalyzed by this enzyme turned out unsuccessful. Other synthetic reactions - like transesterification and MsAcT mediated oxidation of 5-hydroxymethyl furfural (HMF) and furfural respectively - were performed successfully. This opens several options for downstream processing, enantioselective (multistep) aqueous syntheses, and peracid-mediated epoxidations and oxidations.