Low-temperature, selective catalytic deoxygenation of vegetable oil in supercritical fluid media

The effects of supercritical fluids on the production of renewable diesel-range hydrocarbons from natural triglycerides were investigated. Various supercritical fluids, which included CO₂ (scCO₂), propane (scC₃H₈) and n-hexane (scC₆H₁₄), were introduced with H₂ and soybean oil into a fixed-bed reactor that contained pre-activated CoMo/γ-Al₂O₃. Among these supercritical fluids, scC₃H₈ and scC₆H ₁₄ efficiently allowed the reduction of the reaction temperature by as much as 50C as a result of facilitated heat and mass transfer and afforded similar yields to reactions in the absence of supercritical fluids. The compositional analyses of the gas and liquid products indicated that the addition of scC₃H₈ during the hydrotreatment of soybean oil promoted specific deoxygenation pathways, decarbonylation and decarboxylation, which consumed less H₂ than the hydrodeoxygenation pathway. As a result, the quantity of H₂ required to obtain a high yield of diesel-range hydrocarbons could be reduced to 57% if scC ₃H₈ was used. As decarboxylation and decarbonylation are mildly endothermic reactions, the reduced heat transfer resistance in scC ₃H₈ may drive the deoxygenation reaction to thermodynamically favourable pathways.