Researchers from the Dalian Institute of Chemical Physics and the University of Chinese Academy of Sciences have developed a photocatalyst for the selective decarboxylation of fatty acids to produce diesel- and jet-range molecules under mild conditions (30 °C, H₂ pressure ≤0.2 MPa).

A photocatalytic decarboxylation strategy for the production of alkanes from bio-derived fatty acids. The photocatalytic decarboxylation procedure is highlighted, and the interactions between photogenerated carriers (holes and electrons) and fatty acids are schematically shown in the photoredox cycle. Huang et al.

Industrial low-value fatty acid mixtures—e.g., soybean and tall oil fatty acids)—can be transformed into alkane products in high yields (up to 95%). A paper on their work appears in Nature Catalysis.

Long-chain alkanes are among the most important chemicals in modern society as they are the major components in diesel and jet fuel and can be used as feedstocks for olefin and aromatic production. The production of such alkanes from renewable biomass instead of fossil resources is very attractive and important for sustainable energy and chemical supply. In this context, bio- derived fatty acids are promising candidates, owing to their inherent structural similarities to diesel-type hydrocarbons, inedible nature, abundance and low cost. A substantial amount of fatty acids are produced as low-value by-products in fat and oil processing and the pulp industry.

Fatty acids can be transformed into alkanes via hydrodeoxygenation; however, most of the established catalytic systems require harsh conditions (reaction temperatures ≥250 °C, H₂ pressures ≥2 MPa) that lead to intensive energy input and excessive H₂ consumption (≥3 molar ratio of H₂ per reactant). Alternatively, hydrodecarbonylation and decarboxylation of fatty acids consume less or no H₂, but elevated temperatures (≥300 °C) are required for C–C bond scission and are accompanied by unwanted cracking and coke deposition. Such harsh reaction conditions dramatically challenge the overall energy efficiency of the proposed alkane synthesis, so a method capable of efficiently converting fatty acids into alkanes under mild conditions (ambient temperature, low reaction pressure) with minimal H2 consumption is greatly desired.

… Herein we report a photocatalytic decarboxylation route for alkane production from bio-derived fatty acids under mild conditions (ambient temperature, p_H2 ≤0.2MPa). Long-chain alkanes can be obtained in ≥90% yield from fatty acids by using a Pt/TiO₂ catalyst under illumination. Interaction between the catalyst and H₂ forms a hydrogen-rich surface that promotes rapid radical termination with surface hydrogen species, thus efficiently inhibiting radical dimerization and oxidation reactions. The consumption of H₂ during this decarboxylation process is negligible owing to its integrated photoredox cycle wherein photogenerated holes convert fatty acids into C_n–1 alkyl radicals that are subsequently combined with the hydrogen generated through electron-mediated reduction of the carboxyl protons, yielding C_n–1 alkanes.

—Huang et al.

Resources

• Huang, Z., Zhao, Z., Zhang, C. et al. (2020) “Enhanced photocatalytic alkane production from fatty acid decarboxylation via inhibition of radical oligomerization.” Nat Catal 3, 170–178 doi: 10.1038/s41929-020-0423-3