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Catalytic Conversion of Lignocellulosic Biomass to Carboxylic Acids and Derivatives
Chemical and Materials Engineering
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The transition from today’s fossil fuel based economy to a sustainable economy that is grounded in renewable biomass is driven by concern of climate change and anticipation of dwindling fossil fuel resources. Biofuel is a central theme of the sustainable transition due to the large market demand in the transportation sector. However, transformation of biomass into high value added chemicals is advantageous to secure optimal use of the biomass resources from economical and ecological perspectives. Therefore, it is important for a modern biorefinery to integrate fuel and chemical production from biomass derived platform chemicals. The aim of this dissertation is to design and develop a novel and environmentally benign process to produce carboxylic acids and their derivatives, which are the building blocks for biofuels and biobased chemicals, from various lignocellulosic biomass types using recyclable heterogeneous catalysts in water and/or alcohol solutions. In this process, wet lignocellulosic biomass is pre-treated by low-temperature partial oxidation that selectively converts lignin to carboxylic acids or phenolics in aqueous-phase solutions. The remaining structured hemicellulose and cellulose are catalytically converted to lactic acid or its esters which can be further upgraded to liquid hydrocarbon fuels or value added chemicals.Although a variety of lignocellulosic biomass can be used as feedstocks in designed conversion processes, the arid-land adapted, highly water-use efficient CAM (crassulacean acid metabolism) species, (e.g., Agave tequilana and Opuntia ficus-indica) which are unique biomass species compared to other C3 and C4 plants, are especially promising. In Chapter 2, Agave and Opuntia were characterized by a series of standard biomass analytical procedures wherein both species were found to contain high water contents which would be potentially useful for aqueous phase processing of biomass, as well as low lignin and high para-crystalline cellulose contents, making them more amenable to deconstruction. In Chapter 3, a lignin model compound, guaiacol, was selectively oxidized with H2O2 as the oxidant and titanium silicate (TS-1) catalyst at low temperatures. The value-added dicarboxylic acid, maleic acid, was obtained in this TS-1/H2O2 catalytic aqueous-phase partial oxidation reaction system. In Chapter 4, hemicellulosic biomass, xylan, and xylose can be converted to lactic acid using ZrO2 catalyst in pH-neutral hydrothermal solutions. The yields of lactic acid, up to 25% and 18%, were produced from xylose and xylan, respectively. The Lewis acid property of ZrO2 was demonstrated to effectively facilitate the retro-aldol condensation of xylose, which is the initial step of the conversion of xylose to lactic acid. In Chapter 5, the mesoporous Zr-SBA-15 materials with increased Lewis acidity compared to ZrO2 were synthesized in order to increase the yields of lactic acid esters. Alkyl lactates were produced from sugars and cellulose with this catalyst in relevant alcohols in a one-pot reaction. The yields of methyl lactate at up to 41 % and 44% were produced from pentose and hexose, respectively, with methanol solvent, while the yield of up to 31% ethyl lactate was directly produced from cellulose in ethanol-water solutions with the Zr-SBA-15 catalyst. Lastly, in Chapter 6, the conclusion remarks and future research directions are given.