Researchers at the Naval Air Warfare Center, China Lake have synthesized a new class of renewable diesel fuels from a methyl ketone and diols including 2,3-butanediol, 1,2-propanediol, and ethylene glycol. These fuels exhibit comparable net heats of combustion (NHOCs) to conventional biodiesel, while maintaining derived cetane numbers between 82-91. These values are 20–30 units higher than conventional biodiesel and 40–50 units higher than petroleum-derived diesel fuel.
The short combustion delays of the new fuels make them compelling blendstocks to enhance the combustion efficiency of petroleum-derived diesel fuel; further, careful selection of the renewable diol allows for custom tailoring of viscosity and freezing point. In addition, methyl ketones can be efficiently generated from sugar feedstocks or CO2/H2 with metabolically engineered microorganisms, while the diols can be readily obtained from biomass sugars via fermentation or chemical methods.
In a paper in the RSC journal Sustainable Energy & Fuels, the team suggests that this biosynthetic approach may allow for the generation of these fuels on industrially relevant scales while eliminating competition with food sources and promoting responsible use of land resources.
The fermentation of sugars with metabolically engineered organisms to generate methyl ketones has recently been the subject of intense interest. Medium chain methyl ketones, e.g. 2-undecanone and 2-tridecanone, have been proposed as diesel fuel blending agents and mixtures of medium chain methyl ketones have been produced from glucose at titers of up to 3.4 g L-1 in E. coli, while Ralstonia eutropha was shown to produce up to 180 mg L-1 from CO2 and H2. With the preparation of methyl ketones from sustainable carbon sources firmly established, it is of significant interest to develop new methods for the conversion of methyl ketones into fuels that can be used directly in conventional diesel engines. Although medium chain methyl ketones with straight chains have high cetane numbers, the saturated chains also result in high melting points that are unsuitable for diesel fuel. For example, 2-tridecanone has a melting point of 24–27 ˚C.
One potential route to upgrade the fuel properties of methyl ketones would be to form dioxolanes through condensation with diols under acidic conditions. This approach has already been investigated for the production of gasoline range fuels and solvents, and diesel fuel additives. The first advantage of this approach is the ability to use bio-derived diols to form the dioxolanes. These diols could include ethylene glycol, propylene glycol, and 2,3-butanediol which can all be made from biomass sugars via fermentation. The second advantage is the potential to lower the freezing points of the fuels by incorporating subtle chain branching, while maintaining the high inherent cetane number of the parent methyl ketone. Finally, the elimination of water during formation of the dioxolane represents a low energy pathway for deoxygenation that does not rely on energy intensive hydrogen production.—Harrison and Harvey
The researchers synthesized three dioxolane compounds using equimolar amounts of a given diol and 2-tridecanone and a low-cost acid catalyst. After establishing the purity of the dioxolanes, the team measured the key fuel properties, including densities, net heats of combustion (NHOC), viscosities, freezing/melting points, and derived cetane numbers (DCN).
Among the findings:
The densities of the dioxolane fuels are in the range of 0.87–0.88 g mL-1 and they all have similar NHOCs around 115 kBtu gal-1.
The derived cetane numbers for all three dioxolanes were above 80.
In their paper, the researchers note that use of lignocellulosic biomass as a feedsstock could enable large-scale production of these fuels with concomitant significant reductions in greenhouse gas emissions and the potential for eventual cost parity with petroleum-derived fuels.
Another intriguing approach would be to couple methyl ketone production to solar powered water hydrolysis for the storage of solar energy in the form of alkyl dioxolanes. Work by Nocera has shown that photosynthetic systems based on Ralstonia eutropha can be used for the production of fuels and chemicals from CO2 and sunlight. Nocera’s system is capable of up to 10% CO2 reduction energy efficiency, greatly outperforming natural photosynthesis. Further, Beller has recently reported that methyl ketones can be generated by Ralstonia eutropha from CO2 and H2. The use of Beller’s organisms in Nocera’s system would be a compelling route to the sustainable production of dioxolane fuels.—Harrison and Harvey
Kale W. Harrison and Benjamin G. Harvey (2017) “High cetane renewable diesel fuels prepared from bio-based methyl ketones and diols” Sustainable Energy & Fuels doi: 10.1039/c7se00415j