Hydrodeoxygenation of waste fat for diesel production: Study on model feed with Pt/alumina catalyst

Hydrodeoxygenation of waste fats and oils is a viable method for producing renewable diesel oil. In this study a model feed consisting of oleic acid and tripalmitin in molar ratio 1:3 was hydrotreated at 325 °C with 20 bars H2 in a stirred batch autoclave with a 5 wt% Pt/γ-Al2O3 catalyst, and samples were extracted periodically and analyzed on GC. Despite the significant hydrogen pressure hydrogenation of both reactants were limited and decarboxylation or decarbonylation of the ester and carboxylic acid functionalities were highly favored, yielding carbon chain lengths of odd numbers. Moreover, Pd/γ-Al2O3 was observed to be slightly more active than Pt/γ-Al2O3 and had a higher ratio of decarboxylation and decarbonylation to hydrogenation, while Ni/γ-Al2O3 was substantially less active than Pt and also showed a markedly lower ratio of decarboxylation and decarbonylation to hydrogenation. Variation of the temperature showed that triglycerides as well as free fatty acids were converted at all investigated temperatures, but the conversion of oleic acid increased from 6% to 100% when the temperature was increased from 250 °C to 325 °C. The tripalmitin reacted via a palmitic acid intermediate, and its conversion was limited by formation of this free fatty acid.