Mechanistic Study of Catalytic Partial Oxidation of Cellulose in Aqueous Solutions
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Biomass, the only sustainable source of organic carbon, is regarded as an ideal renewable energy source to replace fossil fuels, chemicals and carbon-based materials from petroleum. Lignocellulosic biomass is the platform building block for hydrocarbon advanced biofuels. Cellulose is the most abundant component in green plants and is considered to be a good feedstock to produce various biomass-derived chemicals and fuels. Through aqueous phase conversion method, such as in high pressure and high temperature water, this solid biomass can be converted into carboxylic acids that can be further upgraded to fuels and chemicals. Levulinic acid, a short chain fatty acid, is a versatile platform chemical with numerous potential applications, such as hydrocarbon liquid fuels or valeric esters as fuel additives, as well as a variety of chemicals. There are a number of approaches have been studied for the synthesis of levulinic acid from cellulose. The most accepted synthesis method of levulinic acid is the acid-catalyzed hydrolysis of cellulose using sulfuric acid. It is not suitable to use homogenous acid catalyst to produce high yields of levulinic acid on the industrial scale because of both the high cost on equipment and environment impacts. Heterogeneous catalysis shows excellent activity and selectivity for the oxidation of carbohydrates. The development of heterogeneous catalysts with high activity is necessary. Our group developed a new approach, aqueous phase partial oxidation (APPO), using heterogeneous catalyst to deconstruct cellulose directly into organic acid products. Among different kinds of solid acid catalysts, such as noble metal loaded metal oxides and metal oxides with acid or base properties, zirconium oxide, ZrO2 (surface area 108 m2g-1), showed the best catalytic effects and gave the highest yield of levulinic acid (50%) at 240 ºC and 2.8% O2 partial pressure. This result is the highest reported levulinic acid (LA) yield directly converted form cellulose by a heterogeneous catalyst, to the best of our knowledge.This thesis aims to further investigate the detailed mechanism of this APPO reaction. The new APPO conversion pathway compared with the conventional acid hydrolysis way. The identification of reaction intermediate, the generation of superoxide radicals and the effects of different reaction conditions were studied.