Catalytic Guaiacol Hydrodeoxygenation in Supercritical Fluids: A Review

Upgrading bio-oil is crucial for the use of renewable fuels in the transportation sector. Hydrodeoxygenation (HDO) is a key technology for upgrading bio-oil and removing oxygen in the form of water, typically under pressurized hydrogen. However, the use of high-pressure hydrogen is limited by several factors (e.g., storage safety issues). This review primarily discusses the substantial advantages of catalytic HDO in supercritical fluids, which overcome the need for external H₂ while providing a wider range of products. In particular, this study focuses on ethanol and methanol as effective H-donor solvents for HDO processes due to their ability to provide active hydrogen under relatively mild conditions, along with their multifunctionality as both solvents and H-donors and their potential to enhance mass- and heat-transfer efficiencies. Guaiacol was chosen as the HDO feedstock because it can be found abundantly in bio-oil. Furthermore, the effects of the catalyst properties (e.g., pore size, acidity, and acidic and metallic site distribution) and reaction conditions on the reaction efficiency and product distribution are discussed. This review also describes technical challenges, proposes solutions, and outlines future research directions for advancing the supercritical HDO processes with the aim of guiding future studies toward more effective and sustainable HDO-based biofuel-upgrading technologies.