Hydrodeoxygenation of Fatty Acids in High-Temperature Water Using Molybdenum Carbide-Based Catalysts

The hydrolysis of triglycerides produces fatty acids, which can then be upgraded to produce hydrocarbons, direct replacements for petroleum-derived liquid fuels. To date, most work has focused on the decarboxylation of fatty acids in organic solvents or no solvent, using noble metals or alkali hydroxide catalysts [1-3]. Early transition metal carbides have been reported to have catalytic properties that are similar to those of platinum group metals for several reactions [4] and are known to be active for hydrodeoxygenation (HDO) [5-6]. In this paper we report the rates and selectivities for a series of molybdenum carbide-based catalysts and correlate these properties with pertinent structural and compositional properties.

The molybdenum carbide catalysts were found to be active and highly selective for the hydrodeoxygenation of palmitic acid in high temperature water. The major product was hexadecane and minor products included C₁₆ alkanes, alcohols, and aldehydes. Smaller hydrocarbons were also observed suggesting the presence of hydrogenolysis sites or thermal cracking. X-ray diffraction of the spent catalyst revealed formation of some MoO₂, indicating oxidation of the carbide catalysts. Deposition of metals onto the carbide surface via wet impregnation resulted in a shift in selectivities and stability.  These and other results will be described in this paper.

In addition to the use of batch reactors to screen the catalyst formulations, a flow reactor was used to elucidate kinetics, reaction pathways, and behavior above and below the supercritical point of water (374^oC, 22MPa). Results suggest that there is a shift in kinetics and activity when in the supercritical vs. subcritical regime, as seen in Figure 1. These results are correlated with high temperature water properties, including dielectric constant, ionic product, and density. These properties are highly tunable as a function of temperature and pressure, as seen in Figure 2, and can lead a change in reactivity of the medium.

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