Titolo della tesi: Valorization of lignocellulosic biomass to produce fuels and chemicals through hydrothermal liquefaction
Lignocellulosic biomass is an important renewable energy resource which is regarded as a promising substitution of fossil fuel. Hydrothermal liquefaction (HTL) is the most promising technique for the conversion of wet biomass into liquid bio-fuel. However, the yield and quality of bio-crude obtained from lignocellulosic biomass is poor and the searching of high efficiency catalysts which could improve its yield and quality, and the design of suitable upgrading process are urgent issues to be addressed. In this thesis, oak wood as a typical woody biomass in Europe, was selected as feedstock. The work focused on the fundamental mechanism occurring in the HTL biomass process in presence of heterogeneous hydrogen producers and catalysts to improve the quality of the oil produced. Firstly, Zero-valent metals (ZSMs): Zn, Fe, Ni and Co were adopted in the oak wood HTL process. Different functional mechanisms were observed: Ni and Co worked as catalyst, while Zn and Fe worked as hydrogen producer through the redox reaction with subcritical water. The active hydrogen released from metal oxidation stabilizes the reactive oak wood degradation intermediates, preventing the formation of char by repolymerization and condensation reactions. The functions of ZVMs are related with their oxidability, and Fe shows better performance than Zn due to its slow oxidation process. The synergic effect between the hydrogen producers (Fe and Zn) and the catalysts (Ni and Co) was demonstrated. Best result is obtained combining Fe and Ni at reaction condition of 330 °C for 30 min with a bio-crude yield of 48.2%, which is an extremely high value for a woody biomass, and high heating value (HHV) of 30.2 MJ/kg. Some hydrogenation extent was reflected on the bio-crude chemical composition: increasing of 2-cyclopenten-1-ones at the expense of furan derivatives and the conversion of oxidized aromatics into reduced compounds. Then, a novel unsupported Ni catalyst with flower-like superficial nano-spikes was synthesized through electroless plating method and applied on oak wood HTL. The formed Ni nano-spike increased notably the catalytic activity of the Ni powders, increasing the active surface exposed to the reaction environment. The bio-crude yield consequently raised from 35.1% to 36.6%, which demonstrates the applicability of electroless plating method in catalyst preparation. However, the nano-spikes collapsed during the utilization due to the mechanical solicitation which impacts its reusability and further investigation is required to improve the mechanical properties.
To better understand the interaction between the ZVMs hydrogen producers, the catalyst and the biomass, tests using as feedstock cellulose and lignin as main constituent of the biomass are performed. The liquefaction of cellulose and lignin waste are conducted with the presence of ZVMs and Al2O3-supported metal catalysts. As to the cellulose, Fe showed the best performances, by increasing the bio-crude yield from 17.4 % of the blank test to 26.5 % and by raising its HHV from 27.0 MJ/kg to 29.7 MJ/kg. Using Zn an increase of the water-soluble products was obtained but only a slight increase in the bio-crude amount was observed. Fe and Zn reduced furan derivatives and increased the 2-cyclopenten-1-ones and aromatic compounds in the bio-crude. As to the lignin enriched waste obtained from bioethanol factory, the functions of ZVMs (Ni, Co, Fe and Zn) and Al2O3-supported metal catalysts (Ni/Al2O3, Co/Al2O3, Fe/Al2O3 and Cu/Al2O3) were tested. The highest bio-crude yields were obtained from HTL at 330 °C for 60 min with the combination of Zn/Fe with Ni/Al2O3 catalyst and the HHV of the bio-crude was also slightly higher.
Finally, the bio-crude upgrading investigation was done on the model compound (guaiacol) and real oak wood HTL bio-crude. The hydrotreating of guaiacol at aqueous environment was studied using bifunctional catalysts constituted of Ni supported on chemically modified zeolites (ZSM-5, Beta, Y) with increased mesoporosity and Zn-H2O redox system as hydrogen source. The alkaline treatment notably increased the mesopore volume of Ni/HZSM-5 and Ni/HBeta by 5.6 and 3.8 times, respectively. Excellent guaiacol hydrodeoxygenation results are obtained with Ni/HY and desilicated Ni/HBeta zeolite catalysts with the guaiacol conversion of 89.8% and 97.5%, hydrodeoxygenation efficiency of 44.5% and 69.4%, and hydrodearomatization efficiency of 72.4% and 62.8%, respectively. The main guaiacol conversion products are cyclohexane, cyclohexanone, cyclohexanol, benzene and phenol. Low-cost Ni supported Y zeolite was synthesized from rice husk using TMAOH template and was employed into the hydrotreating of oak wood HTL bio-crude with Zn as hydrogen producer. The H/C ratio and HHV were significantly enhanced from 1.02 and 30.1 MJ/kg of the non-upgraded oil to 1.08 and 33.1 MJ/kg of catalytic upgraded oil. The area percentages of hydrogenation products: cyclopentanones and hydrogenated benzene derivatives increased in the catalytic upgraded oil samples consuming furan derivatives and aromatics.
In this work we comprehensively studied the reaction schemes and application prospect of ZVMs hydrogen producers, supported/unsupported transition metal catalysts and aqueous reaction environment on the liquefaction of lignocellulosic biomass and upgrading of bio-crude. ZVMs-H2O redox system was proven to be effective on inhibiting the repolymerization reaction and enhancing the bio-crude yield and quality, and also a good substitution of gaseous hydrogen in the bio-crude hydrotreating upgrading process. High efficiency catalysts including unsupported Nano-Ni catalyst, Ni supported on chemically modified zeolite catalysts, and Ni/Y zeolite catalyst from rice husk silica are synthesized showing excellent performance on biomass liquefaction and bio-crude hydrodeoxygenation.