Effect of reaction pathway and operating parameters on the deoxygenation of vegetable oils to produce diesel range hydrocarbon fuels: A review

Growing demand for fossil fuels and related environmental issues have directed global attention towards development of alternative fuels from renewable sources. In this regard, biodiesel synthesized from vegetable oils and animal fats has shown potential as alternative to diesel fuel owing to its comparable fuel properties and combustion characteristics. However, higher oxygen content in biodiesel has raised some technical issues for its long term utilization in engines. Subsequently, the second generation liquid hydrocarbon fuels are being developed via catalytic deoxygenation of fatty acids present in vegetable oils. Presently, the research focus is on the pathways for catalytic deoxygenation like hydrodeoxygenation, decarboxylation, and decarbonylation. In hydrodeoxygenation, use of hydrogen gas and sulfided metal catalysts ensure higher conversion of vegetable oil into hydrocarbon fuel compared to the other two pathways. On the contrary, decarboxylation and decarbonylation are mostly hydrogen-free processes ensuring economical production of hydrocarbon fuel from vegetable oils. Hence, the techno-economical issues related to deoxygenation process need to be addressed for its commercial viability. Further, key operating parameters like nature of catalysts and supports, catalyst amount, reaction temperature, reaction atmosphere, hydrogen partial pressure, feed type, feed rate, type of solvent, H2/fatty acid molar ratio etc. are reported to have substantial influence on the hydrocarbon yield and selectivity. This review paper expounds a comparative assessment on the various deoxygenation pathways with their reaction mechanisms to opt for the suitable pathway for conversion of vegetable oils into hydrocarbon fuels based on yield and selectivity of the desired product, ease of use, economy etc. It also explicates the influence of various operating parameters to obtain optimum hydrocarbon conversion and selectivity during catalytic deoxygenation of vegetable oils and related feedstock.