A Comprehensive Review of Experimental Parameters in Bio-oil Upgrading from Pyrolysis of Biomass to Biofuel Through Catalytic Hydrodeoxygenation

Fossil fuel reserve depletion and environmental concerns have spurred substantial research to find alternative energy sources. Bio-oil derived from biomass pyrolysis has great potential to substitute fossil fuels. However, bio-oil physicochemical properties are far below the requirements for biofuels due to several issues such as low heating value, and high-water content, acidity, and viscosity. Bio-oil is unstable and tends to polymerize due to the high content of reactive oxygenates and molecular compounds, even during storage. Therefore, bio-oil without quality upgrading is not suitable for use as a fuel. A promising method to improve bio-oil properties is through catalytic hydrodeoxygenation (HDO) — a hydrogenolysis process for removing oxygen from the oxygen-containing compounds. However, the complex mixture of organic components in bio-oil renders the complexity of HDO, and the significant issues in HDO are coking and decreasing catalyst performance. Therefore, various approaches to overcome these issues have been developed. The final product distribution of HDO can be customized by tuning the experimental parameters such as catalyst acidity, pressure, temperature, types of solvents, and even reaction duration. In this review, the parameters of catalytic HDO are elaborated as functions to provide comprehensive options for constructing the strategy in practicing HDO of bio-oil.