A review on the catalytic hydrodeoxygenation of lignin-derived phenolic compounds and the conversion of raw lignin to hydrocarbon liquid fuels

Petroleum, one of the fossil fuels, is still the main source for liquid fuel production. Lignin derived from renewable biomass has the potential to replace petroleum. The abundance of aromatic units in lignin makes it potential to produce high-value liquid fuel. This review offers a summary of the extensive study that has been devoted to the catalytic hydrodeoxygenation (HDO) of lignin-derived phenolic compounds and the conversion of raw lignin into hydrocarbon liquid fuels. Based on the product classification (cyclohexanes and arenes), different catalytic systems, mainly including catalyst species, solvents and reaction conditions, are analyzed in detail. A model study of lignin-derived compounds (phenolic monomers and dimers) is usually carried out to elaborate on the HDO reaction mechanism. 100% yield of hydrocarbon liquid fuels can be achieved in most tests. With respect to the real lignin-derived bio-oil, however, hydrocarbon yields only reach 16.2%-62.8% due to the various components and high instability of the substrate. The technical barriers and challenges in this part are highlighted throughout. Moreover, the conversion of raw lignin is also comprehensively summarized, which actually combines the depolymerization of lignin and HDO of lignin-derived bio-oil in one pot, and its hydrocarbon yields are generally lower than those of lignin-derived bio-oil. In light of this, the important features of raw lignin that influence the production of hydrocarbon liquid fuels are adequately addressed. Overall, this paper focuses on the scientific and technological advances of hydrocarbon liquid fuel production from lignin, and the potential strategies to produce renewable fuels from lignin are discussed.