Recent advances in hydrodeoxygenation catalysts for upgrading the renewable bio-oils to second-generation biodiesel

In recent years, the rising energy crisis and greenhouse effects have driven wide spread adoption of biofuels as promising alternatives to fossil fuels. However, first-generation biodiesel produced by transesterification exhibit inherent limitations, including high oxygen content, poor stability, and low calorific value. In this context, second-generation biodiesel produced through hydrodeoxygenation processes demonstrate a distinct advantage of being drop-in ready for existing fuel infrastructure. The recent research focuses on hydrodeoxygenation (HDO) catalysts. This review systematically examines recent advances in HDO catalysts for bio-oil upgrading, focusing on three key aspects of the catalysts: the active phase, the support, and the promoter. We critically evaluate traditional sulfide catalysts and emerging sulfur-free alternatives, highlighting their deoxygenation efficiency, and stability challenges. In addition, prospects on catalyst design strategies, such as MOFs as supports and defect engineering in catalyst design, are discussed to guide the development of cost-effective and sustainable HDO systems.