Base-catalyzed depolymerization of lignin and hydrodeoxygenation of lignin model compounds for alternative fuel production
Olarte, Mariefel Valenzuela
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This study considered the potential use of lignin as possible renewable fuel and chemical feedstock source. Among the various polymers present in lignocellulosic biomass, the polyaromatic lignin is the one component that is most chemically similar to petroleum. However, it still contains a much larger amount of oxygen compared to crude oil. As such, two strategies were employed in this study: (1) studying the lignin depolymerization in the presence of high temperature and base catalysts; and, (2) employing hydrodeoxygenation as a means to decrease the O/C ratio in lignin-derived model compounds. The base-catalyzed depolymerization (BCD) of organosolv lignin was done in a 500-mL Monel Parr reactor at temperatures ranging from 165°C to 350°C. Complete solubilization of lignin derivatives was possible in the presence of NaOH and KOH, except at 350°C. NMR experiments revealed formation of oxidized groups (carboxylic and hydroxyl groups) as well as alkyl groups. On the other hand, the use of NH4OH showed N incorporation. Identified and quantified DCM-soluble monomeric compounds were at most 6% of the starting material and are mainly phenolic. This study revealed the apparent susceptibility of syringyl units over guaiacyl units in BCD. This could in turn guide the choice of substrate on which base-catalyzed depolymerization could be applied. Syringaldehyde was used as the starting material to study batch hydrodeoxygenation (HDO) using several non-cobalt/molybdenum based catalysts. A 50-ml Parr reactor was used, pressurized by 1000 psig of H2 and heated to 300°C. Nickel based catalysts (nickel phosphide, nickel oxide and nickel phosphate) as well as supported precious metals (Pt and Pd) were tested as HDO catalysts. Of the three O-containing functional groups of syringaldehyde, the aldehydic group was found to be the most susceptible. In the presence of the Al2O3-supported catalysts, the methyl groups liberated were found to be incorporated back into the aromatic ring, forming alkylated compounds. In the last section of this dissertation, hydrothermally synthesized supported Ni on mesoporous silica (MCF) and acid catalysts (HY and H-Al-MCF) were used for probing the effect of bifunctional metal-acid catalysis on phenol hydrodeoxygenation/hydrogenation. Catalyst configurations were varied from the previously studied wet-impregnated Pt/HY catalyst. Based on a hypothesis that coking catalyzed by the acidic zeolite in the wet impregnated Pt/HY catalyst was the main cause of catalyst deactivation and decreased phenol conversion, separately synthesized metal and acid catalyst systems were tested. Complete phenol conversion was sustained for at least three times longer in a continuous flow reactor operated at 200°C and 0.79 MPa of flowing H2. The separation of the metal and acid sites generated a tunable system capable of producing cyclohexanol, cyclohexane or cyclohexene at very high selectivities, even achieving 99% selectivities for cyclohexane.