A mini review on catalytic hydrodeoxygenation for biofuels production: catalyst, mechanism and process

As the effects of environmental problems and energy crises become apparent, zero carbon routes are vital to achieve large-scale production of fuels, such as efficient utilization of biomass to produce biofuels. This critical review provides a systematic summary and guidance on the catalytic conversion of waste biolipids and lignocellulose into biofuels via hydro-conversion in respect of metal catalysts and reaction mechanisms. First, five typical metal catalysts are reviewed, including reducible metal oxides, metal sulfides, metal phosphides, metal carbides, and metal nitrides. Among them, reducible metal oxides and metal sulfides are widely reported due to their efficient hydrodeoxygenation (HDO) activity. The design and preparation of catalysts from noble and non-noble metals, especially the rational design of catalyst active sites, are explored. For metal sulfides, novel in situ synthesis methods are summarized, especially in HDO reactions using oil-soluble precursors, such as ionic liquids with highly designable moieties. The importance of support is then discussed, particularly the positive effects of morphology, synergistic metal-support interactions, and surface acidity on HDO performance. Subsequently, the reaction mechanism of biomass hydrogenation treatment to produce biofuels was explored in detail using methyl palmitate as a biolipid model compound and cellobiose as cellulose and hemicellulose model compound. We also introduce two typical processes for hydro-conversion of biomass to biofuels involving three general units: pre-treatment unit, HDO unit, and separation unit. Finally, this review provides a summary and insights into catalyst design, one-pot isomerization, operando characterization, and reaction process development.