(652b) A Highly Active and Stable Silica-Carbon Supported Palladium Catalyst for Decarboxylation of Oleic Acid to Produce Green Diesel
A large amount of waste cooking oil and grease (495 million gallon/year) is produced in the U.S. every year. This waste oil/grease which is a non-food competing, inexpensive and high energy value containing feedstock can be exploited for liquid biofuel generation. In particular, brown grease, which contains mainly oleic acid, can be a potential source to obtain straight chain hydrocarbons in the diesel fuel range (green diesel). Hydrodeoxygenation (HDO) is the currently available process for green diesel production from oil/grease. However, this technique requires high pressure and excess H2, therefore leading to high production cost. On the other hand, in a decarboxylation pathway, oxygen is eliminated as carbon dioxide without the need of additional H2 in its stochiometry.
There has been considerable study of the decarboxylation of free fatty acids (FFA) to hydrocarbons. However, there is not yet an active and selective catalyst that can be used for decarboxylation of unsaturated FFAs to hydrocarbons in the absence of H2. Due to the competitive adsorption and reaction of active C=C bonds on the catalyst surface, the decarboxylation yield decreases while yield of the side reactions increases over Pd/C catalyst. Although Pd/C exhibit high activity and selectivity for FFA decarboxylation, it readily deactivates due to coking and adsorbance of unsaturated products and reactants.
In this study a new catalyst that can improve the selectivity to green diesel in the absence of additional H2 was explored for conversion of oleic acid as a model unsaturated FFA molecule. Also the correlation of catalyst structure and acidity on the decarboxylation activity was investigated. Additionally, a procedure to maintain high catalytic activity was developed.
5% Pd on silica-carbon supports with various Si:C weight ratios were prepared and oleic acid decarboxylation activity tests were conducted in a 100 ml batch reactor in the absence of H2 at 300 °C and 1.5 MPa. A significant correlation between the Pd metal particle size and the catalytic activity was observed. Pd/Si-C-4 catalyst exhibited the highest selectivity to n- heptadecane. High catalytic activity of Pd/Si-C-4 was attributed to the accessible, small and well-distributed metallic Pd nanoparticles inside the hybrid mesopores of this catalyst. It was also observed that the Pd on Si-C supported catalyst favors a direct decarboxylation of oleic acid unlike a Pd/C catalyst which catalyzes C=C double bond hydrogenation prior to the oleic acid decarboxylation. Furthermore, the novel catalyst was highly stable for oleic acid conversion selectively for green diesel production in the absence of H2.