Towards Sustainable Hydrocarbon Biorefineries: Deoxygenation of Biomass Oxygenates to Hydrocarbons via Methane
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One of the most important technical challenges in the realization of sustainable hydrocarbon (HC) biorefineries is the removal of oxygen from the feedstock, which typically constitutes up to 35 wt% of lignocellulosic biomass. The closest technologies for deoxygenation, hydrodeoxygenation and hydrocracking, are estimated to use approximately 66 kg of hydrogen per metric ton of biomass. This hydrogen gas need affects the long-term sustainability of HC biorefineries. Consequently, there is need for an effective and a sustainable mechanism to remove oxygen from biorenewable feedstocks. Methane is a hydrogen-rich gas which can be produced by sustainable processes. The overall objective of this proposal is to understand the fundamental catalytic processes that occur when methane contacts biomass-derived oxygenates on bi-functional catalysts under pyrolysis conditions. Intellectual Merit The possibility of kinetically coupling a hydrogen-consuming reaction with a concurrent hydrogen-producing reaction is the basis for oxygen-rich biomass feedstock deoxygenation. Specifically, aromatization of biomass-derived oxygenates is the hydrogen consuming reaction. During this process, benzene, toluene and xylene (BTX) are typically formed by dehydration reactions, which produce water as byproduct. The hydrogen producing reactions are methane steam reforming and methane aromatization. It is hypothesized that methane can be directly coupled with biomass-derived pyrolytic vapors (oxygenates) and deoxygenated into hydrocarbons over an appropriate metal supported ZSM-5 bi-functional catalyst via two pathways: (1) methane steam reforming and oxygenate aromatization, or (2) methane aromatization and oxygenate aromatization. In the first coupling reaction, hydrogen gas formed during methane steam reforming is used for the aromatization of oxygenates. Water formed during oxygenate aromatization via dehydration is used for methane steam reforming. The ultimate products of this coupling reaction are hydrocarbons and carbon dioxide. In the second coupling reaction, methane is aromatized to produce BTX (benzene, toluene, xylene) and hydrogen gas. Oxygenates utilize the hydrogen formed and aromatize by dehydration, removing oxygen as water. In the proposed research, the reactions that occur between methane and the model oxygenate glucose in the presence of selected bi-functional catalysts that contain metals (Sn, Ni, Ce, Ru or Mo2C) on ZSM-5 catalysts will be elucidated. Candidate bi-functional catalysts identified by this model study will then be used to explore the spectrum of hydrocarbon products generated when when biomass constituents (cellulose, lignin, solid biomass and bio-oil) are catalytically pyrolyzed in the presence of methane. This research will provide fundamental understanding on the chemistry that occurs at a catalyst surface when an oxygen-rich substrate and methane - an oxygen-deficient, hydrogen-rich substrate that is historically difficult to activate - are brought together. The research is potentially transformative because it suggests a new route to produce hydrocarbons from biomass that does not require hydrogen. The research has practical implications, because existing petroleum refinery infrastructure could be used to formulate commercially valuable hydrocarbons from biomass. Broader Impacts In addition to the training of graduate students, the education and outreach plan focuses on undergraduate student teaching and learning in a hands-on research environment. Specifically, the proposed education activities feature the development of the REACH program (Research Experiences for the Academically Challenged). In the proposed REACH program, students from under-represented groups who are academically at risk (<2.5 GPA) are paired with students who are not at risk. This team is provided with hands-on research experiences early in their undergraduate program. The academic progress of these students will be tracked during the course of their undergraduate career to evaluate whether or not exposing them early on to hand-on research in this team environment helps stimulate their excitement towards learning and promotes their retention in engineering programs.