Renewable Fuels Produced from Biomass Pyrolysis Vapors: Catalysis and Reaction Engineering

At present, lignocellulosic biomass is the only renewable and available resource capable of supplanting fossil fuel resources. Fast pyrolysis is the conversion of biomass at elevated temperature, high heating rate and under inert atmosphere to produce bio-oil. This bio-oil is a blend of oxygenated hydrocarbons, which must be upgraded prior to use as a transportation fuel. The focus of this thesis is the investigation of bio-oil upgrading reaction pathways and catalysts for the production of biofuels. There are two primary pathways for pyrolysis vapor upgrading: catalytic cracking and hydrodeoxygenation (HDO). During catalytic cracking, pyrolysis vapors react over the acid sites of ZSM-5 zeolite, which crack C-C bonds to release oxygen in the form of COx. Biofuel compound yield is greater when cracking occurs in-situ the pyrolysis reactor, but selectivity is greater when cracking occurs ex-situ. Because of its microporous structure, the ZSM-5 catalyst may exclude some of the bulky oxygenates formed during pyrolysis. This accessibility limitation can be alleviated through the introduction of mesoporosity in the ZSM-5 zeolite. Zeolite mesoporosity is beneficial for increasing aromatic yield and reducing coke on catalyst. While catalytic cracking is effective for removing oxygen from the pyrolysis vapors, this oxygen is removed as COx, reducing carbon return in the bio-oil. Moreover, the upgraded bio-oil is aromatic in nature, and aromatics in gasoline are limited. Incorporation of hydrogen into the pyrolysis reactor in the presence of Ni-ZSM-5 catalyst reduces char formation and substantially increases CH4 yield, but the bio-oil does not contain many alkanes. The high reaction temperature demanded by biomass volatilization thermodynamically limits hydrogenation reactions. Catalytic hydropyrolysis followed by secondary hydroprocessing produces a biofuel with heating value and aromaticity similar to gasoline. Liquid phase HDO is another pathway for removal of bio-oil oxygen. Catalytic HDO of anisole, 4-ethylphenol and benzofuran was performed with Ni, Ru and Pd supported on USY zeolite. Kinetic rate measurements show Pd is more effective than Ni and Ru in terms of reaction rate, deoxygenation activity and C-C coupling. Finally, controlled mesoporosity of the USY support is beneficial for enhancing access of oxygenates to the impregnated metal species for efficient hydrogenation