Catalytic Hydroliquefaction of Lignin to Value-Added Chemicals

Abstract

Despite the high potential of lignin as a source of renewable aromatics, it is mainly treated as a low value product due to its recalcitrant character. The goal of this PhD study was to degrade lignin to fuels and higher value chemicals. To this end, lignin was dispersed in a solvent and treated at elevated temperature in the presence of a catalyst and under hydrogen pressure. Mainly conversion of lignosulfonate, as one of the major types of commercial lignin, was investigated. Lignosulfonate was provided by its world leading supplier, Borregaard A/S. The role of solvent in degradation of lignosulfonate was evaluated in ethylene glycol and ethanol media at 250 ˚C using Ni based catalysts and 50 bar hydrogen. A similar yield of liquefied products was obtained by catalytic conversion in ethanol and ethylene glycol, being 31 and 32 wt%, respectively. It was observed that the solvent clearly affects the products of the degradation. The oil fractions from depolymerization in ethanol had lower molecular weight compared to the oil products obtained in ethylene glycol medium, indicating higher degree of degradation of liquefied products in ethanol. On the other hand, ethylene glycol showed superior activity in inhibiting condensation reactions and char formation; 16 and 46 wt% THF soluble and THF insoluble fractions were obtained from catalytic conversion of H-LS in ethanol, while those numbers from conversion in ethylene glycol medium were 45 and 23 wt%, respectively. Formation of char was only observed in ethanol medium. No effect was observed by changing the support material, which indicates that only Ni sites are active in hydrogenolysis of lignosulfonate. The presence of NiS in the spent catalyst was confirmed by X-ray crystallography and ion coupled plasma (ICP). It was suggested that Ni/NiS sites may catalyze the reaction via a sulfur removal cycle. Lignosulfonate was further subjected to a reductive catalytic degradation over an alumina supported NiMo catalyst (provided by Haldor Topsøe A/S) in ethanol medium at 310 ˚C. A high oil yield and low char yield of 88 and 15 wt% were obtained, respectively, with catalyst: lignin: solvent ratio of 2 g: 10 g: 100 ml. The role of ethanol was prominent for the stabilization of reactive intermediates, which was catalyzed in the presence of NiMo. Simultaneous deoxygenation and desulfurization reactions took place in the presence of catalyst. The oxygen and sulfur content in the oil fraction obtained after 4 hours reaction time were 11.2 and 0.1 wt%, indicating considerable deoxygenation and desulfurization compared to the lignosulfonate feed (O: 30.8 wt%, S: 3.1 wt%), suggesting that this oil fraction can be used as a fuel additive or further be upgraded. It was noticeable that despite the high yields of degraded compounds in the presence of catalyst, the liquefied fractions were mainly within the range of dimers and oligomers. The reusability of the catalyst without any pretreatment was confirmed for at least two times. In a further series of experiments, direct conversion of beech wood was targeted over a sulfided NiMo/Al2O3 catalyst (provided by Haldor Topsøe A/S) in ethanol medium at 300 ˚C. Biomass was converted into monomers and dimers, derived from lignin, and light hydrocarbons, originating from cellulose and hemicellulose. The main identified monomers were 4-propyl guaiacol (PG) and 4-propyl syringol (PS) with total monomer yield of 18.1 wt% based on the Klason lignin content in beech wood. Studies of the influence of reaction temperature indicated that at 200 ˚C, the process targets only the lignin with a monomer yield of 12.1 wt% based on the Klason lignin content, while the holocellulose is conserved. The highest monomer yield of 20.0 wt% based on the Klason lignin content was obtained at 260 ˚C, indicating that the optimum temperature required for degradation of lignin fractions to monomers is within 200-260 ˚C. The direct conversion of biomass with high yield of lignin monomers showed promise compared to a two-step procedure involving isolation of lignin by the organosolv method and subsequent conversion of organosolv lignin. Here, a monomer yield of only 4.3 wt% was detected from conversion of organosolv lignin at 300 ˚C. Moreover, the oil from direct biomass conversion possessed a lower molecular weight compared to the oil from conversion of organosolv lignin.