Jeremy G. Immer1, M. Jason Kelly1, Benjamin D. Estabrook2, and H. Henry Lamb3. (1) Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, (2) Middlebury University, 2885 Middlebury College, Middlebury, VT 05753, (3) NC State University, Chemical Engineering, Raleigh, NC 27695-7905
Catalytic deoxygenation of C18 free fatty acids (FFAs) can be used to produce C17 hydrocarbons as an alternative to conventional fatty acid methyl ester (FAME) biodiesel [1]. FFA deoxygenation occurs via two pathways: decarbonylation and decarboxylation. In this work, deoxygenation of oleic and stearic acids over a 5 wt.% Pd/C catalyst was investigated to determine the effects of unsaturation of the alkyl chain and the feasibility of using the hydrocarbon reaction product as solvent. FFA deoxygenation was conducted in a semi-batch autoclave at 300ºC and 15 atm using either n-dodecane or n-heptadecane as solvent with a constant flow of 60 mL/min of He or 10% H2 in He. Deoxygenation of stearic acid occurs primarily via a decarboxylation pathway in either He or 10% H2 flow. The initial decarboxylation rate under He is much higher than under 10% H2, but the reaction reaches completion more quickly in the latter case because H2 inhibits catalyst deactivation. Deoxygenation of oleic acid is strongly inhibited under He, achieving a conversion of only 12% after 3 h with less than 10% selectivity to heptadecane. In contrast, the oleic acid deoxygenation goes to completion in <1 h in the presence of H2 yielding n-heptadecane in essentially 100% yield. Additional experiments established that the overall reaction for oleic acid is a stepwise process: initial hydrogenation of the double bond to form stearic acid and subsequent decarboxylation to n-heptadecane. Using n-heptadecane as solvent, the initial rate of stearic acid decarbonylation under 10% H2 was equivalent to that observed in n dodecane. The initial rate of decarboxylation, however, was substantially lower due to reversibility making decarbonylation a more significant pathway in the overall reaction. Stearic acid deoxygenation nears completion in n-heptadecane in ~2 h aided by removal of the CO2 reaction product.