Emerging catalysis in solvent-free hydrodeoxygenation of waste lipids under mild conditions: A review

Waste lipids, which are generated in enormous quantities worldwide each year (over 10 million tons), are a type of green and sustainable energy source. The production of renewable fuels from these waste lipids is therefore of great interest due to their contribution to carbon neutrality. The advancement of hydrodeoxygenation (HDO), which enables the production of fossil-like hydrocarbon fuels, offers tantalizing prospects. However, realistically scaling up HDO is still a challenge due to high production costs. Designing a catalyst that enables efficient HDO under mild and solvent-free conditions would significantly contribute to reducing production costs. In this context, the new catalysts, and the corresponding strategies for catalytic HDO processing of waste lipids were reviewed and summarized. First, the HDO reaction pathways, products, and solvation effects were presented. The development of state-of-the-art HDO catalysts with a focus on alleviating the reaction conditions was summarized. Mechanistic insights into HDO conversion based on density functional theory (DFT) calculations were also reviewed. Discussions of other promising deoxygenation strategies such as electro-/photo-catalysis were included. Finally, future developments and directions on the catalytic strategy for mild HDO conversion were suggested.