Heterogeneous sulfur-free hydrodeoxygenation catalysts for selectively upgrading the renewable bio-oils to second generation biofuels

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.