Microwave-assisted, Hydrogen-free Routes for Biomass Pyrolysis Oil Upgrading: A Literature Review

Bio-oil, derived from abundant lignocellulosic biomass, represents a promising feedstock for sustainable fuels and chemicals in a Net Zero economy. Its direct application is limited by its acidity, high oxygen and water content, and the presence of up to 30 wt% high-molecular-weight pyrolytic lignin. The conventional method for upgrading, catalytic hydrodeoxygenation (HDO), is effective but energy-intensive and relies on high-pressure hydrogen gas. This review explores a sustainable, hydrogen-free upgrading strategy with catalytic transfer hydrogenation (CTH) for in situ transfer of hydrogen from donor solvents directly to bio-oil molecules. Microwave (MW) heating is explored to enable process electrification, providing enhanced energy efficiency, shorter reaction times, and selective heating of MW-absorbing catalysts and solvents. Whilst the review confirmed the benefits of this approach for various feedstocks and model compounds, the MW-assisted catalytic upgrading of lignocellulosic bio-oil remains a largely unexplored area.