| dc.description.abstract | Energy conversion using renewable biomass has significant environmental impact. To acquire high value products using preliminary processed biomass, considerable amount of upgrading reactions has been conducted in liquid-phase conditions. Addition of solvents could interact with all component within reaction system. In order to influence heterogeneous catalytic reactions, solvents have to affect the kinetically relevant steps such as adsorption, surface reaction, and desorption. The following chapters investigate kinetics and solvent effects for biomass conversion.
In Chapter 4, cyclohexene hydrogenation has been carried out over 0.25 wt.% Pd/α-Al2O3 in heptane, methylcyclohexane and decalin with a wide range of reaction conditions, from 30 psi to 600 psi of hydrogen pressure under 40 °C – 100 °C. Those inert solvents demonstrate competitive adsorption with reactants cyclohexene and lead to its decrease in surface coverage. Decalin competes the strongest and results the lowest hydrogenation rate followed by methylcyclohexane and then heptane. Heptane illustrates negligible surface competitive adsorption. Additionally, the non-ideality of reaction mixtures also alters the surface concentration of the solvent molecules compared to the bulk phase. Four kinetic models have been established based on different surface scenarios, and the cyclohexene hydrogenation has been found to be the first hydrogenation as the rate-determining step with cyclohexene and hydrogen absorbed on different Pd sites. Combined with statistical thermodynamic analysis, surface cyclohexene and hydrogen retain a certain amount of entropy based on their degree of freedom(DOF) of translation, rotation and vibration modes. ∆Sads for cyclohexene is -143 ± 19 J/mol·K, which loses around 2/3 of total entropy of its liquid state. Hydrogen losses more entropy upon adsorption and ∆Sads is -108 ± 12 J/mol·K. Meanwhile, heat of adsorption decreases with surface coverage. ∆Hads for cyclohexene on a clean Pd surface is -75 ± 14 kJ/mol, while the ∆Hads for hydrogen is -41 ± 7 kJ/mol. On a fully covered Pd surface, ∆Hads for cyclohexene becomes -53 kJ/mol, while ∆Hads for hydrogen reduces to -33 kJ/mol.
In Chapter 5, the protic solvent, water, participates in the kinetically relevant step, surface reaction step, and influences the reaction rate and product distributions. Furfural hydrogenation in cyclohexane and water over 3% Pd/α-Al2O3 presents distinctive reactivity and product distribution. In the presence of water, hydrogenation of the carbonyl group of furfural is favored and therefore furfuryl alcohol is the main product. While the furanyl ring hydrogenation is favored in solvent cyclohexane and the reactivity in cyclohexane is much less. DFT calculations demonstrate that water stabilizes the intermediate and final product significantly by forming hydrogen bonds and provides an alternative hydrogenation pathway through water H-shuttling mechanism. Water-assist hydrogenation decreases the overall energy at the transition state from 46 kJ/mol to -7 kJ/mol. Kinetic studies in water and cyclohexane further unveil that reaction orders with respect of H2 pressure also differ. 0.8th order has been found when using water, which indicates the second hydrogenation step is the rate-determining step. 0.3th order in cyclohexane manifests the first hydrogenation step is the rate-determining step. The kinetic results are consistent with DFT calculations. When water shuttles the surface H proton to the oxygen of the carbonyl group through the H-bonding networks, the energy at the transition state for this step decreases from 46 kJ/mol to -16 kJ/mol, making the second hydrogenation of the carbon the rate-determining step.
In Chapter 6, water serves as a solvent and the critical reactant that converts surface species to desired products. Aqueous phase reforming (APR) of ethanol over various Pt and Ru catalysts have been carried out in ethanol water mixture. Upon adsorption, ethanol decomposes by sequential dehydrogenation starting from Cα followed by dehydrogenation on Cβ until surface ketene CH2CO species are achieved. The C-C cleavage happens afterwards and surface CO species then react with water to produce more H2 and CO2 through the water-gas-shift (WGS) reaction. The presence of water drives the equilibrium of WGS reaction and removes the surface CO species. Therefore, negligible amount of CO has been detected in products. Serious Ru catalysts with different particle sizes have also been explored. Ru tends to favor the C-C cleavage compared to Pt. However, smaller Ru particles present strong adsorption of surface CO species and cause site poisoning. Larger Ru particles greatly favors the methanation reaction that consumes H2 in the product and CO, CO2 to form CH4. To optimize the catalyst, achieve desired product H2 and inhibit CH4 production, bimetallic Ru-Pt catalysts have been investigated. The co-impregnated 1%Ru-2%Pt/TiO2 displays the synergistic performance, with increased reaction rate for H2 production but lower rate for CH4. | en_US |
