Looped-oxide catalysis: a solar thermal approach to bio-oil deoxygenation

Abstract

A critical step in the conversion of pyrolysis and liquefaction bio-oils to transportation fuels is deoxygenative upgrading. With this perspective we introduce a two-step thermochemical cycle which harnesses concentrated solar radiation to drive bio-oil deoxygenation. In this cycle, which we have termed “looped-oxide catalysis” (LOC), a metal oxide is reduced in a high-temperature solar thermal reactor and subsequently reacted with bio-oil, reforming the original metal oxide and yielding a deoxygenated biofuel product. By augmenting the chemical energy stored in bio-oil with solar thermal energy, LOC may increase fuel yields for a given quantity of biomass beyond what is currently achievable with standalone biomass-to-fuel pathways. We identify five promising candidate LOC materials based on equilibrium thermodynamics, diffusion kinetics and catalytic performance in hydrodeoxygenation (HDO) reactions. Additionally, we present new experimental results and mechanistic implications from an investigation of the LOC upgrading of acetic acid. The formation of acetaldehyde with comparable selectivity in LOC with zinc metal as well as HDO with zinc oxide suggests that surface oxygen vacancies are the catalytically relevant sites in both processes.