Research progress in catalytic conversion of lignin to produce liquid fuels

Abstract

Lignin is recognized as the most abundant renewable aromatic polymer in nature with the highest concentration of benzene ring structures, and it stands out for its eco-friendly, sustainable, and biodegradable properties. The valorization of lignin through biorefinery strategies to produce aromatic compounds, which are subsequently converted into tailored lignin-derived liquid fuels via catalytic hydrogenation, deoxygenation, and other upgrading processes, has become a central focus in comprehensive biomass utilization research. This article systematically reviews the fundamental aspects and technological advancements in lignin-to-fuel conversion. Initially, it elaborates on the basic structural units of lignin—primarily the three phenylpropane monomers (guaiacyl, syringyl, and p-hydroxyphenyl units)—and their diverse interunit linkages, including β-O-4, α-O-4, 4-O-5, β-β, β-5, and 5-5 bonds, which collectively contribute to lignin's structural complexity and recalcitrance. These linkages dictate the depolymerization challenges and influence the selection of conversion pathways. Subsequently, it comprehensively reviews the principal technical pathways for manufacturing bio-based liquid fuels from lignin, particularly detailing three strategic approaches for producing liquid-phase products: lignin gasification-Fischer-Tropsch synthesis by gasifying lignin into syngas (CO/H₂) and then catalytically reassembling into liquid hydrocarbons, lignin pyrolysis liquefaction involving thermal decomposition at 400–800 °C under inert conditions to yield bio-oil, and lignin liquid-phase catalytic conversion by employing solvents and catalysts to depolymerize lignin into monomers under milder conditions. Special emphasis is placed on analyzing key technologies in fuel production, including catalytic hydrodeoxygenation (HDO) and carbon-carbon coupling reactions. The discussion critically evaluates current technological limitations and challenges in lignin-to-fuel conversion, highlighting two pivotal technical bottlenecks requiring urgent resolution: the design and optimization of more stable and efficient catalytic systems, and the improvement of separation/purification processes for lignin depolymerization products. Furthermore, building upon contemporary research trends in bio-liquid fuels, the paper proposes that future investigations should prioritize the development of high-carbon-number cyclic aviation biofuels and high-density biodiesel derived from lignin. These directions are particularly promising given lignin's unique aromatic structure and high energy density characteristics, which align well with the stringent performance requirements of advanced transportation fuels. The analysis underscores the necessity for interdisciplinary collaboration between catalysis science, process engineering, and materials chemistry to overcome existing barriers. Specifically, the development of multifunctional catalysts capable of simultaneously activating multiple reaction pathways while resisting deactivation under harsh processing conditions represents a critical research frontier. Additionally, the integration of novel separation technologies such as membrane filtration and ionic liquid extraction with traditional distillation methods may revolutionize the purification of complex lignin-derived mixtures. From an industrial application perspective, the paper emphasizes the importance of establishing techno-economic models and lifecycle assessments to evaluate the commercial viability and environmental benefits of various lignin conversion routes. The potential synergies between lignin valorization and existing petroleum refining infrastructures are also discussed as a strategic approach to accelerate technology deployment. In conclusion, while significant progress has been made in understanding lignin's conversion mechanisms, the transition from laboratory-scale achievements to industrial implementation demands sustained efforts in catalyst innovation, process intensification, and system integration. The realization of economically competitive lignin-derived liquid fuels could fundamentally transform the renewable energy landscape by providing sustainable alternatives to fossil-based transportation fuels while fully utilizing lignocellulosic biomass resources.