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Production of Green Aromatics and Olefins from Lignocellulosic Biomass by Catalytic Fast Pyrolysis: Chemistry, Catalysis, and Process Development

Diminishing petroleum resources combined with concerns about global warming and dependence on fossil fuels are leading our society to search for renewable sources of energy. In this respect, lignocellulosic biomass has a tremendous potential as a renewable energy source, once we develop the economical processes converting biomass into useful fuels and chemicals. Catalytic fast pyrolysis (CFP) is a promising technology for production of gasoline range aromatics, including benzene, toluene, and xylenes (BTX), directly from raw solid biomass. In this single step process, solid biomass is fed into a catalytic reactor in which the biomass first thermally decomposes to form pyrolysis vapors. These pyrolysis vapors then enter the zeolite catalysts and are converted into the desired aromatics and olefins along with CO, CO2, H2O, and coke. The major challenge with the CFP process is controlling the complicated homogeneous and heterogeneous reaction chemistry. The focus of this thesis is to study the reaction chemistry, catalyst design, and process development for CFP to advance the CFP technology. To gain a fundamental understanding of the underlying chemistry of the process, we studied the reaction chemistry for CFP of glucose (i.e. biomass model compound). Glucose is thermally decomposed in a few seconds and produce dehydrated products, including anhydrosugars and furans. The dehydrated products then enter into the zeolite catalyst pore where they are converted into aromatics, CO, CO2, H2O and coke. The zeolite catalyzed step is far slower than the initial decomposition step (>2 min). Isotopic labeling studies revealed that the aromatics are formed from random hydrocarbon fragments composed of the dehydrated products. The major competing reaction to aromatic production is the formation of coke. The main coking reaction is the polymerization of the furan intermediates on the catalyst surface. CFP is a shape selective reaction where the product selectivity is related to the zeolite pore size and structure. The shape selectivity of the zeolite catalysts in the CFP of glucose was systematically studied with different zeolites. The aromatic yield is a function of the pore size and internal pore space of the zeolite catalyst. Medium pore zeolites with pore sizes in the range of 5.2 to 5.9 √Ö and moderate pore intersection size, such as ZSM-5 and ZSM-11 produced the highest aromatic yield and least amount of coke. The kinetic diameter estimation of the aromatic products and the reactants revealed that the majority of these molecules can fit inside the zeolite pores of the medium pore zeolites. The ZSM-5 catalyst, the best catalyst for aromatic production, was modified further to improve its catalytic performance. These modifications include: (1) adjusting the concentration of acid sites inside the zeolites catalyst; (2) incorporation of mesoporosity into the ZSM-5 framework to enhance its diffusion characteristics, and (3) addition of Ga to the ZSM-5. Mesoporous ZSM-5 showed high selectivity for heavier alkylated monoaromatics. Ga promoted ZSM-5 increased the aromatic yield over 40%. A process development unit was designed and built for continuous operation of the CFP process in a pilot scale. The effects of process variables such as temperature, biomass weight hourly space velocity, catalyst to biomass ratio, catalyst static bed height, and fluidization gas velocity were studied to optimize the reactor performance. It was demonstrated that CFP could produce liter quantities of aromatic products directly from solid biomass.
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