Recent review articles have highlighted the significant research and development (R&D) efforts that have been devoted to using and adapting petroleum hydrotreatment technology to convert triglycerides and fatty acids into hydrocarbons...The high H₂ consumption associated with these processes is their main drawback. H₂ is not currently available in large quantities from renewable resources and H₂ costs can be high. Moreover, H₂ is made primarily from steam reforming of natural gas and CO₂ is the byproduct. Thus, a near-term process for the production of fully renewable biofuels needs to operate without added H₂.

—Fu et al.

Several labs have been exploring decarboxylation chemistry as an alternative to hydrodeoxygenation, the authors note. Decarboxylation removes the O atoms in fatty acids as CO₂ rather than H₂O—i.e., stoichiometrically no H₂ is required for the reaction. Furthermore, they add, the loss of a CO₂ molecule does not lead to a loss in any of the chemical energy in the molecule; rather it produces a hydrocarbon molecule with an even higher energy density.

This path is an attractive one for making renewable hydrocarbon fuels, but aside from our recent report and a set of earlier catalyst screening experiments, all previous work on fatty acid decarboxylation has been done in organic reaction media, such as dodecane or mesitylene. In contrast, the focus herein is on fatty acid decarboxylation in water. This hydrothermal decarboxylation is technologically important, but the reaction pathways are essentially unexplored.

—Fu et al.

In a previous paper, the team reported that Pt/C and Pd/C catalysts are active for decarboxylation of a saturated fatty acid in an aqueous medium, with Pt/C showing higher activity with no added hydrogen required. In the current paper, they provide more detailed information about the hydrothermal reaction pathways and kinetics for this catalyzed transformation.

In general, they found that:

• The saturated fatty acids (stearic, palmitic, and lauric acid) gave the corresponding decarboxylation products (n-alkanes) with greater than 90% selectivity, and the formation rates were independent of the fatty acid carbon number.

• The unsaturated C₁₈ fatty acids (oleic and linoleic acid) underwent decarboxylation much more slowly, and hydrogenation was the main reaction pathway even though no H₂ was added to the reactor. The main pathway was hydrogenation to form stearic acid, the corresponding saturated fatty acid. This compound then underwent decarboxylation to form heptadecane. These results suggest that H₂ is formed in situ during this hydrothermal catalytic process.

Several general features noted herein for Pt/C-catalyzed hydrothermal decarboxylation (e.g., rate independent of fatty acid chain length, in situ H₂ formation, hydrogenation of unsaturated fatty acids precedes appreciable decarboxylation, high selectivity to the decarboxylation product) have also been noted previously for fatty acid decarboxylation in organic solvents. Thus, the identity or nature of the solvent does not appear to have a strong influence on these aspects. Other features of this reacting system, however, such as the catalyst activity, the deactivation rate, and the ease of in situ H₂ formation may yet be sensitive to the reaction medium. Additional research is required to address these issues.

—Fu et al.

Resources

• Fu, J., Lu, X. and Savage, P. E. (2011) Hydrothermal Decarboxylation and Hydrogenation of Fatty Acids over Pt/C. ChemSusChem doi: 10.1002/cssc.201000370

• J. Fu, X. Lu, P. E. Savage (2010) Catalytic hydrothermal deoxygenation of palmitic acid. Energy Environ. Sci. 3, 311 –317 doi: 10.1039/B923198F