Biodiesel as a substitute of traditional petroleum-derived liquid fuels has been put into use in recent years. However, the first generation biodiesel (fatty acid methyl esters) with high oxygen content is inconvenient for large-scale use due to its incompatible nature with fossil fuels. As a result, green biodiesel (the second generation biofuel) prepared from catalytic hydrodeoxygenation (HDO) of the first generation biodiesel has been gradually developed. The biodiesel after deoxygenation which has a great number of advantages is similar to petroleum fuel in composition, so it can be employed directly in fuel industry. To obtain the expected products, accessible production processes and suitable catalyst systems are needed. In this review, we first make an analysis on the pathways and processes of deoxygenation reactions including hydrodeoxygenation, decarboxylation and decarbonylation. Selectivity to reaction pathways has a close relationship with raw materials, catalysts or reaction conditions. The special goal of this review is to highlight the advances in the heterogeneous sulfur-free catalysts used for deoxygenation, including the sulfur-free noble metals, non-noble metals, metal phosphides, metal carbides and metal nitrides. We thoroughly discussed the different performances of these developed catalysts in the deoxygenation reactions, such as activity, selectivity and stability. Fundamental mechanisms over sulfur-free catalysts, including experimental comparison of different active phases and calculations by Density Function Theory (DFT), were also addressed. This review also involved effects of different support materials, composition, structure optimization, water and H₂ pressure on the HDO activity and silectivity, and detailed information about catalyst deactivation. It is expected that this review can provide some new design and modification strategies for fabricating highly active,selective and durable earth-abundant HDO catalysts for the substainable production of green bio-diesel.

近年来，生物柴油替代了传统的石油液体燃料。然而，具有高氧含量的第一代生物柴油（脂肪酸甲酯）由于其与化石燃料的不相容性而不适用于大规模使用。结果，由第一代生物柴油的催化加氢脱氧（HDO）制备的绿色生物柴油（第二代生物燃料）已经逐渐发展。脱氧后的生物柴油具有很多优点，其组成与石油燃料相似，因此可以直接用于燃料工业。为了获得预期的产品，需要可及的生产工艺和合适的催化剂体系。在这篇评论中，我们首先分析脱氧反应的途径和过程，包括加氢脱氧，脱羧和脱羰。对反应途径的选择性与原料，催化剂或反应条件密切相关。这篇综述的特殊目的是强调用于脱氧的非均相无硫催化剂的研究进展，包括无硫贵金属，非贵金属，金属磷化物，金属碳化物和金属氮化物。我们彻底讨论了这些开发的催化剂在脱氧反应中的不同性能，例如活性，选择性和稳定性。还探讨了无硫催化剂的基本机理，包括不同活性相的实验比较和密度函数理论（DFT）的计算。₂对HDO活性和介电常数的压力，以及有关催化剂失活的详细信息。预期这次审查可以为制造可持续生产绿色生物柴油的高活性，选择性和耐用的富含地球的HDO催化剂提供一些新的设计和改性策略。