Base-catalyzed depolymerization of lignin and hydrodeoxygenation of lignin model compounds for alternative fuel production

The depleting amount of fossil fuel resources throughout the world has encouraged countries to expend research on alternative fuels. Biomass is considered as one of the most promising sources of alternative liquid fuels as well as of renewable chemical feedstock. The challenge lies in depolymerizing this very complex heterogeneous macropolymer and then upgrading the fragments into compounds that approximate crude oil properties. This characteristic would enable biomass-derived liquids to be compatible with the existing petroleum infrastructure.;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 results generated by this study can be divided into three parts: (a) the base-catalyzed depolymerization of organosolv lignin; (b) batch hydrodeoxygenation of syringaldehyde in the presence of bulk and supported nickel phosphide; and, (c) continuous flow hydrodeoxygenation/hydrogenation of phenol utilizing the bifunctional catalyst system of metal-acid active sites.;The base-catalyzed depolymerization of organosolv lignin was done in a 500-mL Monel Parr reactor at temperatures ranging from 165°C to 350°C. Water was used as a solvent while the base catalysts tested included NaOH, KOH and NH4OH. Analyses by 13C NMR, elemental analysis, gel permeation chromatography and GC analysis of dichloromethane solubles were done. Complete solubilization of lignin derivatives was possible in the presence of NaOH and KOH, except at 350°C. Identified and quantified DCM-soluble monomeric compounds were at most 6% of the starting material and included phenol, syrngaldehdye, catechols and guaiacols. NMR experiments revealed formation of carboxylic and alcoholic groups in these samples. On the other hand, the use of NH4OH showed N incorporation. An interesting finding of this part of the study revealed the apparent susceptibility of syringyl units over guaiacyl units. This could in turn guide the choice of substrate on which base-catalyzed depolymerization could be applied.;Syringaldehyde was used as the starting material in the batch hydrodeoxygenation (HDO) part of this research. 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 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. Additionally, the C-O bond in the methoxy substituents was seen to be labile at longer reaction times. These two reactions occur even in the presence of the bulk catalysts. In the presence of the Al2O 3-supported catalysts, the methyl groups liberated were found to be incorporated back into the aromatic ring, forming alkylated compounds. Supported Ni12P5/Al2O3 showed preference over Pt/ Al2O3 and Pd/Al2O3 for the presence of these compounds in the product mixture. The average TOF for syringaldehyde conversion by Ni12P5/Al2O 3 was higher compared to the supported noble metal catalysts though kinetic studies at lower conversions are necessary for direct activity comparisons between these catalysts.;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.