Deoxygenation of fatty acid for the production of renewable diesel

Abstract

The deoxygenation of oleic acid and palmitic acid to diesel-like hydrocarbons was studied over commercial NiMo/Al2O3, NiMoS/Al2O3, and Pd/C catalysts. In the deoxygenation of oleic acid over NiMo/Al2O3 under hydrogen atmosphere, n-octadecane (n-C18) and n-heptadecane (n-C17) was observed as main products with small amounts of stearic acid, octadecanol, CO, and water. The reaction pathway has been proposed; that is, oleic acid is hydrogenated to stearic acid, followed by the hydrogenation of stearic acid to octadecanol. After that, octadecanol can undergo either hydrodeoxygenation via dehydration to octadecene and hydrogenation to n-C18, or hydrodecarbonylation via decarbinylation to heptadecene and hydrogenation to n-C17. The deoxygenation of oleic acid over NiMo/Al2O3 is more selective to hydrodeoxygenation path as higher yield of n-C18 was observed. Due to the higher acidity, the dehydration was enhanced over NiMoS/Al2O3, resulted in higher n-C18 selectivity compared to that obtained with unsulphided catalyst. In contrast, Pd/C gave only n-C17 hydrocarbon resulted from hydrodecarbonylation path. Similarly, the preferred reaction for palmitic deoxygenation over NiMo/Al2O3 and NiMoS/Al2O3 is hydrodeoxygenation while the preferred reaction over Pd/C is hydrodecarbonylation. The optimum condition for deoxygenation of both oleic acid and palmitic acid is at 325°C, 500 psig.