By developing routes to convert both xylose and glucose with the robustness to handle very crude biomass in the form of a potato, we are now well positioned to apply this to real hydrolysates derived from switchgrass or corn stover, wherein the main two constituents are xylose and glucose.

—Sutton et al.

The basic approach to converting biomass sugars (xylose and glucose) to drop-in hydrocarbon fuels is through chain extension steps and hydrodeoxygenation to remove the oxygen atoms abundant in biomass sugars. An overabundance of oxygen is detrimental to high energy density fuels because (a) they are over-oxidized from a combustion standpoint, and (b) they exhibit reactivity that is often difficult to predict and control.

Reducing the oxygen content helps to confer stability and, more importantly for fuels, in-creased energy density by reducing C–O bonds to C–H bonds, while at the same time, minimizing the yield of CO₂ as a by-product. In contrast to thermochemical routes, a catalytic approach, using selective functional group transformations could facilitate lower energy routes to access hydrocarbons.

… To eliminate some processing steps, and to move towards “raw biomass” as the primary process input, rather than commodity chemicals produced from biomass (i.e., 5-HMF), we began to investigate the potential of using sugars directly.

—Sutton et al.

The Los Alamos team used an adaption of the Garcia–Gonzalez (GG) reaction, which converts glucose to furan rings. In a subsequent step, ring opening and hydrodeoxygenation produces the desired branched alkanes under relatively mild conditions.

Conversion of a Russet potato into G-GG and the conversion of G-GG to 3-ethyloctane and 7-ethyloctyl acetate using GG chemistry. Sutton et al. Click to enlarge.

The GG reaction typically uses Lewis acidic metal catalysts, such as CeCl₃. The Los Alamos team developed routes that utilize a non-precious metal catalyst (NPMC) in the form of iron triflate [Fe(OTf)₃].

This approach can be applied to monomeric sugars (glucose and xylose), oligosaccharides (starch), and potentially to hydrolyzed dedicated energy crops to allow the conversion of real biomass into fuel type molecules.

—Sutton et al.

Resources

• Sutton, A. D., Kim, J. K., Wu, R., Hoyt, C. B., Kimball, D. B., Silks, L. A. and Gordon, J. C. (2016) “The Conversion of Starch and Sugars into Branched C₁₀ and C₁₁ Hydrocarbons.” ChemSusChem doi: 10.1002/cssc.201600669