Hydrotreating of Lignocellulosic Bio-Oil (A Review)

This review discusses recent advances in catalytic hydrodeoxygenation of lignocellulosic biomass. Lignocellulosic biomass is the most promising plant-based raw material for the production of liquid engine fuels or individual petrochemical monomers. Among the several existing techniques for biomass processing, pyrolysis offers superior efficiency. Given that the bio-oil produced by biomass pyrolysis has unsatisfactory performance characteristics caused by the presence of oxygenates, this bio-oil cannot be used directly as a fuel. Hydrodeoxygenation using selective catalysts is able to reduce the oxygen content in bio-oil and to improve its performance characteristics. To this end, bifunctional catalysts that contain active metal sites on an acid support hold promise. Noble metals (e.g., Pt, Pd, and Ru) and/or transition metals (e.g., Ni, Co, and Mo), as well as sulfides and phosphides of transition metals, can be used as an active catalytic phase. Metal oxides (e.g., ZrO₂, CeO₂, Al₂O₃, and TiO₂), carbon, zeolites (e.g., ZSM-5, Y, Beta, and SAPO-11), and mesoporous silica-based materials (e.g., SBA-15 and MCM-41) have been most often used as supports in hydrodeoxygenation catalysts. However, the implementation and upscaling of the hydrodeoxygenation of biomass pyrolytic bio-oil is limited because of the rapid deactivation of the catalyst in the presence of water, due to sintering and leaching the active phase with acidic components of bio-oil. Therefore, the development of catalysts that would provide high activity and stability under bio-oil hydrodeoxygenation conditions has become one of the most pressing issues for the petrochemical industry.