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京都大学 / 新制・課程博士 / 博士(工学) / 甲第24106号 / 工博第5028号 / 新制||工||1785(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The conversion of biomass to biouel has received considerable attention as a sustainable way to produce energy. As worldwide fossil fuels become depleted these efforts grow in importance. The overall strategy is to transform the parent biomass feedstock to increase C-C bonds while reducing oxygen in the final products. A catalytic approach is often used to achieve good yields of transportation grade liquid hydrocarbons from biomass. Development of novel catalyst systems to aid in the thermochemical conversion of biomass to biouel is the focus of this thesis. Gasification of biomass produces synthesis gas (CO and H2). Synthesis gas can be converted to liquid hydrocarbons using Fischer-Tropsch (FT) synthesis. Mo/ZSM-5 FT catalysts with a potassium (K) promoter are introduced to enhance liquid hydrocarbon production and CO conversion of synthesis gas. Liquid products and CO conversion were determined using GC-MS analysis with respect to changes in K loading from 0-2%. The highest liquid product selectivity (21.7%) was found with 1.0% K loading while largest CO conversion (63%) was found with 1.2% K loading. This catalyst work was extended by introducing Ni and Co into the Mo/ZSM-5 catalysts. A copper based water gas shift catalyst (WGS) was also used in concert with the FT catalyst to improve product selectivity. This WGS catalyst promotes the in-situ production of H2 while decreasing water content. The FT+WGS catalyst were used to convert both 1:1 CO: H2 syngas and bio-syngas at 280 °C and 350 °C. The liquid hydrocarbon selectivity was significantly changed and the CO conversion was remarkably increased compared to the reactions without the dual catalyst at both temperatures. In the fourth chapter, FT+WGS catalysts were studied for upgrading bio-oil model compounds. Guaiacol and furfural were used as the model compounds and upgrading reactions were done under H2, syngas and bio-syngas at 200, 250 and 300 °C. Significant conversion of both guaiacol (85%) and furfural (100%) occurred with syngas at 300 °C. Products upgraded from syngas had a higher combined heat of combustion than the products with pure H2. This suggests the incorporation of some C from CO with model compound upgrading reactions with syngas.

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Hydrogen is a vital component in several different chemical reactions as well as a potential fuel source for the future. The water gas shift (WGS) reaction converts CO and water to hydrogen and CO2. The objective of this work is to first, characterize the potential benefits of the addition of lanthanum oxide (lanthana) to the iron-chromium-copper (Fe-Cr-Cu) oxide catalysts industrially used in high temperature water gas shift processes, and second, analyze these catalysts using in-situ UV-Visible spectroscopy. The benefits of each component in the catalyst are discussed as well as potential benefits from the addition of lanthana. Lanthana is a rare earth oxide that dramatically increases the surface area of the iron based WGS catalysts, and small concentrations of other rare earth oxides (i.e. cerium) have been shown to increase the rate of desorption of CO2 from iron surfaces (Hu Yanping 2002). Lanthana has similar chemical properties to other rare earth oxides tested and has not been previously tested as an additive to the WGS catalyst. Therefore catalysts with 0, 1, 2, 5, 10, and 20 wt% lanthana were made via a co-precipitation method in order to measure changes in activity, physical stability, and thermal stability. Catalyst characterization techniques utilized include electron dispersive X-ray spectroscopy (EDX), temperature programmed reduction (TPR) with hydrogen, and nitrogen physisorption (BET). The kinetic analysis was performed utilizing both mass spectroscopy (MS) and gas chromatography (GC). The addition of 1 wt% lanthana to the Fe-Cr-Cu catalysts increases WGS reaction rates of the catalyst at 425°C and 350°C, however the 0% La catalyst has the highest rates at 375°C and 400°C. The 0% La catalyst shows significant drop off in rate at 425°C, suggesting that the lanthana provides a small thermal stabilizing, i.e. the addition of lanthana prevents catalyst sintering at higher temperatures. Traditionally, chromia acts as the sole thermal stabilizer in these catalysts. The addition of large amounts of lanthana inhibits the chromia stabilizing effect, however small additions of lanthana appear to have an additional catalyst promotional effect without interfering with the chromia thermal stabilization. The increased WGS reaction rates at higher temperatures could allow for greater throughput of reactants in industrial settings. Higher concentrations of lanthana decrease the activity due to what is believed to be disruption of the chromia stabilizing effect as well as reduced amount of the active phase of catalyst. In-situ UV-Visible analysis shows that the oxidation state of the iron in the catalyst has a direct correlation to the UV-Visible light absorbance of the surface of the iron catalyst. Extent of reduction is traditionally measured with a synchrotron which is significantly more expensive than UV-Vis spectroscopy. This study uses the more economical UV-Vis spectrometer to determine similar information. The lanthana doped catalysts show an over-reduction of iron during WGS conditions (i.e. rapid reduction of Fe2O3 to Fe3O4 and FeO).

The structural and functional roles of lanthana in unsupported iron-based catalysts for the high-temperature water-gas shift reaction and Fischer-Tropsch synthesis were investigated. The performance of the catalysts with varying lanthana contents was based on their activity, selectivity, and stability. With regard to the former reaction, extent of reduction of the iron in Fe2O3/Cr2O3/CuO/La2O3 water-gas shift catalysts is a key parameter that was characterized using UV-visible spectroscopy. Minor addition of lanthana (<0.5 wt%) produces more active and stable catalysts apparently because it stabilizes the iron-chromium spinel, increases the surface area of the reduced catalysts, enhances the reduction of hematite to the magnetite active phase, and facilitates the adsorption of CO on the surface of the catalyst modeled by an adsorptive Langmuir-Hinshelwood mechanism. Statistical 95% confidence contour plots of the adsorption equilibrium constants show that water adsorbs more strongly than CO, which inhibits the reaction rate. A calibration curve that correlates the oxidation state of surface iron domains to normalized absorbance of visible light was successfully generated and applied to the water-gas shift catalysts. UV-visible studies indicated higher extent of reduction for surface Fe domains for the catalysts promoted with 1 wt% of lanthana and showed potential to be a more convenient technique for surface chemistry studies than X-ray absorption near edge spectroscopy (XANES). Lanthana addition to iron-based Fischer-Tropsch catalysts enhances the olefin-to-paraffin ratio, but decreases their activity, stability, and selectivity to liquid hydrocarbons. Adding lanthana at the expense of potassium reduces the water-gas shift selectivity and enhances the activity and stability of the catalysts. Finally, a model that simulates heat and mass transfer limitations on the particle scale for the Fischer-Tropsch reaction applicable at lab-scale suggests optimal operating and design conditions of 256°C, 30 bar, and 80 mirons are recommended for higher selectivity to liquid hydrocarbons. The model considers pressure drop, deactivation, pore diffusion, film heat transfer, and internal heat transfer when solving for the optimal conditions, and maps them as functions of design variables. This model can be up-scaled to provide guidance for optimal design of commercial-size reactors.

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