Recently, Prof. Makoto M. Watanabe and his team at the University of Tsukuba discovered the heterotrophic alga Aurantiochytrium 18W-13a strain (left) which very rapidly produces squalene (right) from organics in water.

Typically, biohydrocarbons, in particular algae-derived ones, are large molecules with many branches. While some amount of squalene (derived from sharks) has been used in cosmetics, biohydrocarbons need to be refined into smaller molecules formost other uses such as biofuel. Conventional methods for re-fining large hydrocarbons typically use solid acids in combination with noble-metal catalysts, and many side reactions can occur such as isomerization or coke formation. Although isomerization is beneficial for fuel production from linear-alkane-based feedstock such as petroleum, isomerization is un-desirable in the case of branched algal hydrocarbons and only complicates the reaction mixture.

Herein, we show that a Ru/CeO₂ catalyst can produce small alkanes from biohydrocarbons by regioselective C–C hydrogenolysis, without isomerization and coke formation. By using this catalyst and molecular hydrogen, internal CH₂–CH₂ bonds located between branches are preferably dissociated to give branched alkanes with very simple distributions of isomers.

—Oya et al.

The catalyst was prepared by mildly decomposing the ruthenium precursor at 300 °C under inert atmosphere after impregnation. This procedure led to sub-nanometer-sized ruthenium particles supported on cerium oxide.

Squalane, obtained by the hydrogenation of squalene, was treated with this catalyst and hydrogen at 60 atm and 240 °C to produce smaller hydrocarbons. The C-C bonds located between the methyl branches were selectively dissociated, and branched alkanes were produced without the loss of branches.

Distribution of products in carbon number from squalane hydrogenolysis over ruthenium supported on cerium oxide catalyst. Click to enlarge.

Branched hydrocarbons are good components for gasoline and jet fuels because of the high octane number, low freezing point and good stability. Other noble metal catalysts were also tested, but the results were inferior to the sub-nanometer-sized ruthenium on cerium oxide catalyst in terms of activity and selectivity.

A conventional catalyst—a combination of platinum and strong solid acid—produced a very complex mixture of products because of acid-catalyzed isomerization. In this new catalyst system, the deposition of carbonaceous solid on the catalyst (coking) was negligible, while it is often problematic in many catalytic reactions in petroleum refinery. The catalyst was reusable 4 times without loss of performance.

This catalytic system makes good use of the squalene’s branched structure, while conventional methods are suitable to straight-chain molecules in petroleum. In the future, this catalytic conversion method could be applied to real wastewater samples and other important algal hydrocarbons, such as those from Botryococcus braunii, the researchers suggested.

Resources

• Oya, S.-i., Kanno, D., Watanabe, H., Tamura, M., Nakagawa, Y. and Tomishige, K. (2015), “Catalytic Production of Branched Small Alkanes from Biohydrocarbons” ChemSusChem doi: 10.1002/cssc.201500375