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Zeolite supported/non-noble metal catalysts for the deoxygenation of fatty acids to transportation fuels
Crawford, James M.
Crawford, James M.
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2022
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Abstract
Legislative and industrial momentum are swinging in favor of renewable energy resources, with ambitions to reach carbon neutrality in the U.S. by 2050. To successfully meet this challenging target, continued research efforts emphasizing the conversion of biomass to transportation fuels are essential. Emissions from the transportation sector (~1/3 of U.S. total) contribute to elevated atmospheric CO2 giving increased global temperatures, weather phenomena, sea level rising, and oceanic acidification. While decarbonization of passenger vehicles has been aided by green power and battery storage, heavy cargo and air traffic cannot accommodate these resources. Chemicals and fuels derived from biomass hold immense potential in mitigating carbon emissions and generating sustainable domestic products without depleting edible vegetal supplies. Industrial vegetable oil hydrotreating has been demonstrated, but heterogeneous feedstocks limit the process to specific reactant compositions. Non-edible plant oils available for fuel upgrading are largely composed of saturated and unsaturated fatty acids. By selective deoxygenation, these oils can be converted to diesel or gasoline range hydrocarbons over heterogeneous catalysts. Active areas of development in the valorization of fatty acids include: one-pot continuous conversion of both saturated and unsaturated fatty acids, employing non-noble metals as cheaper catalysts, and decreasing the intensity of reaction conditions.
To address these points, low-cost and scalable catalyst are required. Previously, noble metal catalysts, including platinum and palladium, have shown high activity, but are highly sensitive to the recyclability and recoverability of the precious metal components. Also, sulfided metal hydrodeoxygenation catalysts have been tested, but present low stability in humid environments. Thus, researchers have searched for non-noble metal catalysts employing carbon, silica, alumina, titania, and zeolites as supports. Zeolites are of particularly high interest due to their scalability, stability, and functionality. Herein, we report the use of zeolite supported non-noble metals in the hydrodeoxygenation of fatty acids.
In this thesis work non-noble metal catalyst systems were synthesized, characterized, and tested for the deoxygenation of bio-derived model compounds. Catalyst systems included metallic combinations of Ni, Co, Cu, and Pt either self-supported or templated on zeolites mordenite, NaX, or 5A. Model compounds included linoleic, oleic, stearic and acetic acid. Catalysts were tested for activity, yield, recyclability, and scalability with the goal of continuous production of liquid range hydrocarbon fuels. Fundamental insights are provided relating to the location and habit of metal active sites confined within the zeolite channel. Additionally, mono, bi, and trimetallic Ni-Cu-Co catalysts were studied providing benchmark performance for non-noble metal systems. Our results indicate that non-noble metal systems are a promising candidate for industrial operation. Future studies are proposed with an emphasis on metal site stabilization to avoid sintering and coking under deoxygenation reaction conditions.
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