Pyrolysis is considered the most promising way to convert biomass to fuels. Upgrading biomass pyrolysis oil is essential to produce high quality hydrocarbon fuels. Upgrading technologies have been developed for decades, and this review focuses on the hydrodeoxygenation (HDO). In order to declare the need for upgrading, properties of pyrolysis oil are firstly analyzed, and potential analysis methods including some novel methods are proposed. The high oxygen content of bio-oil leads to its undesirable properties, such as chemical instability and a strong tendency to re-polymerize. Acidity, low heating value, high viscosity and water content are not conductive to making bio-oils useful as fuels. Therefore, fast pyrolysis oils should be refined before producing deoxygenated products. After the analysis of pyrolysis oil, the HDO process is reviewed in detail. The HDO of model compounds including phenolics monomers, dimers, furans, carboxylic acids and carbohydrates is summarized to obtain sufficient information in understanding HDO reaction networks and mechanisms. Meanwhile, investigations of model compounds also make sense for screening and designing HDO catalysts. Then, we review the HDO of actual pyrolysis oil with different methods including two-stage treatment, co-feeding solvents and in-situ hydrogenation. The relative merits of each method are also expounded. Finally, HDO catalysts are reviewed in order of time. After the summarization of petroleum derived sulfured catalysts and noble metal catalysts, transitional metal carbide, nitride and phosphide materials are summarized as the new trend for their low cost and high stability. After major progress is reviewed, main problems are summarized and possible solutions are raised.

Iron carbide catalysts encapsulated in graphene-like carbon were synthesized via a facile method by pyrolysis of an iron-glucose precursor. Different amounts of potassium (0–5 wt%) were in situ doped into the catalyst simultaneously. Glucose played a role both as the precursor to form a carbon support and a reducing agent that reduced iron oxides to θ-Fe₃C during the catalyst preparation. θ-Fe₃C underwent a phase transformation to χ-Fe₅C₂ as the active phase in Fischer–Tropsch synthesis. Characterization of the structural and chemical properties of the prepared catalysts revealed a core–shell structure with iron carbides enwrapped by several graphene-like layers. The addition of a potassium promoter increased the amount of defects on graphene-like layers and facilitated the formation of iron carbides during the catalyst preparation. Fischer–Tropsch synthesis under typical reaction conditions (320 °C, 20 bar, H₂/CO = 1, GHSV = 15 000 ml g_cat ⁻¹ h⁻¹) was carried out in a fixed bed reactor. A higher light olefin selectivity was obtained than that on common iron catalysts, probably because of the electron-rich surfaces of the prepared catalysts that made it hard for hydrogen to hydrogenate the unsaturated intermediates. A volcano-like evolution of light olefin selectivity was observed on the catalysts with different contents of K, and the highest olefin selectivity reached 41.9% on the 2K-Fe₃C@C catalyst (i.e., doped with 2 wt% of K). The induction period of the catalyst was shortened by K addition. No drastic changes in the catalyst morphology and performance during 100 h time on stream can be ascribed to the protection of graphene-like carbon layers that prevented the supported iron particles from migration and aggregation under harsh conditions in Fischer–Tropsch synthesis.

The conversion of lignin into aromatics as commodity chemicals and high-quality fuels is a highly desirable goal for biorefineries. However, the presence of robust inter-unit carbon–carbon (C–C) bonds in natural lignin seriously impedes this process. Herein, for the first time, we report the selective cleavage of C–C bonds in β-O-4 and β-1 linkages catalyzed by cheap copper and a base to yield aromatic acids and phenols in excellent yields in water at 30 °C under air without the need for additional complex ligands. Isotope-labeling experiments show that a base-mediated C_β–H bond cleavage is the rate-determining step for C_α–C_β bond cleavage. Density functional theory (DFT) calculations suggest that the oxidation of β-O-4 ketone to a key intermediate, i.e., a peroxide, by copper and O₂ lowers the C_α–C_β bond dissociation energy and facilitates its subsequent cleavage. In addition, the catalytic system could be successfully applied to the depolymerization of various authentic lignin feedstocks, affording excellent yields of aromatic compounds and high selectivity of a single monomer. This study offers the potential to economically produce aromatic chemicals from biomass.