Bio-oil upgrading through hydrogen transfer reactions in supercritical solvents

The increase in energy demands and problems related to fossil fuels have encouraged the development of suitable renewable energies as petroleum alternatives. Fast pyrolysis is a thermochemical process that can produce liquid bio-oil, which has the potential to alter transportation fuel. However, liquid bio-oil has disadvantages such as instability and low calorific value. Hence, various upgrading methods have been proposed to enhance its quality and stability. One such method is the transfer hydrogenation of bio-oil in supercritical fluids, which could convert it into a stable and low acidic fuel with notably reduced oxygen content even without external hydrogen. However, the bio-oil thus derived consists of a number of compounds, the composition and reaction mechanisms of which have not been accurately determined. Some researchers have suggested a mechanism using model compounds, but this is insufficient to commercialize the upgrading process. Therefore, this study reviews the transfer hydrogenation method to upgrade bio-oil, focusing on the major factors affecting the process. It concludes by identifying future challenges and the requirements to make the transfer hydrogenation upgrading process commercially available.