Looped Oxide Catalysis: The Prospect of Bio Oil Deoxygenation over Reduced Metal Oxides

One of the key challenges in the production of liquid transportation fuels from cellulosic biomass is the upgrading of primary bioliquids to reduce the total oxygen content. This upgrading step is necessary to generate fuels with higher combustion energy, volatility and thermal stability and lower viscosity and molecular weight. Hydrodeoxygenation (HDO) of biomass fast-pyrolysis and liquefaction bio-oils with renewably-sourced H₂ represents a promising carbon-neutral upgrading pathway. However, HDO processing is limited by requisite high pressures and by the destruction of chemical exergy during the reaction of H­₂ with easily-reduced oxygenate compounds. As an alternative to HDO, we propose a two-step thermochemical cycle wherein a metal oxide is reduced to its metal or to a lower-valence oxide in a high-temperature solar furnace and subsequently reacted with a bio-oil feedstock, reforming the higher oxide and yielding a deoxygenated fuel product. This process, which we have termed “looped oxide catalysis” (LOC), synthesizes concepts from the fields of solar thermochemical hydrogen production, chemical looping, and hydroprocessing and may allow for low-pressure deoxygenation of bio-oil while also achieving high exergy efficiency. From a set of eighty possible LOC catalysts we have identified five most-promising candidates on the basis of known diffusion kinetics, catalytic activity in HDO reactions, and metal-oxygen bond strength. Oxides with low metal-oxygen bond strength will be unable to abstract oxygen from bio-oil oxygenates, while oxides with high metal-oxygen bond strength will require excess energy to reduce. Therefore oxides with intermediate metal-oxygen bond strength are most desirable; this behavior is illustrated in a thermodynamic “volcano plot.” An exergy analysis of the LOC cycle is developed for the five candidate oxides to identify inherent thermodynamic irreversibilities. Additionally, the experimental performance of the five candidate LOC catalysts in a continuous-flow reactor will be discussed in comparison with traditional HDO catalysts.