Navy researchers produce high-performance renewable fuels by combining heterogeneous catalysis with biosynthesis
A team from the Naval Air Warfare Center, Weapons Division (NAWCWD) at China Lake, with colleagues from the National Institute of Standards and Technology (NIST), have demonstrated that renewable high density fuels with net heats of combustion ranging from ~133,000 to 141,000 Btu gal-1—up to 13% higher than commercial jet fuel (~125,000 Btu)—can be generated by combining heterogeneous catalysis with multicyclic sesquiterpenes produced by engineered organisms. A paper on their work is published in the RSC journal Physical Chemistry Chemical Physics.
This advance has the potential to produce a range of higher-density biofuels to improve the range of aircraft, ships, and ground vehicles without altering engine configurations, they suggested.
A number of renewable jet and diesel fuels have been now developed that can be broadly classified as synthetic paraffinic kerosenes (SPKs). (Earlier post.) These SPK fuels are composed of linear and branched alkanes, and are of moderate densities due to their lack of aromatics or cyclic hydrocarbons (napthenes). While these fuels are excellent for diesel or jet propulsion, their lower densities and lack of aromatics has required blending of SPK fuels with conventional jet fuel to meet specifications, the researchers note.
An alternate approach is the alcohol-to-jet pathway (earlier post, earlier post), in which fuels are synthesized from renewable alcohols that can be produced from lignocellulosic biomass by dehydrating the cellulosic alcohols to olefins and then selectively oligomerizing the olefins to generate fuels. (The China Lake team has also worked on butanol-to-jet fuels.)
Regardless of the process, these fuels have similar properties to other SPK fuels and represent a bottom-up approach in which organisms generate small molecules that are then deoxygenated and combined to produce longer chain fuel molecules.
In contrast to the bottom-up approach represented by Fischer–Tropsch and ATJ fuels, a number of research groups have developed biosynthetic approaches in which microorganisms directly generate larger hydrocarbons required for jet and diesel fuel. After direct fermentation to generate a fuel-like molecule, these hydrocarbons are then converted to stable, high-performance fuels through straightforward chemical processes including hydrogenation, isomerization, and distillation. This approach has the potential to reduce capital costs by removing much of the chemical processing required for bottom-up methods, while utilizing either CO2 or biomass-derived sugars as the carbon feedstock to produce fuels. Remarkable progress has been made in the direct production of renewable fuels and oils by organisms including cyanobacteria, bacteria, algae, and yeast. Much of this work has focused on long chain linear alkanes and alkenes.
One of the most promising approaches to high-performance biosynthetic fuels is to utilize the tools of metabolic engineering to overproduce specific molecules with structures of interest. Terpenoid structures are an obvious choice based on the vast number of naturally occurring terpenoids (~50,000) and the structural diversity of these molecules including branched chain, cyclic, and multicyclic hydrocarbons. In particular, monoterpenes (C10) and sesquiterpenes (C15) are of interest for both jet and diesel fuels.
… to develop renewable fuels that have the potential to outperform conventional petroleum fuels in regard to density and net heat of combustion, the current work explores the synthesis and fuel properties of both pure hydrogenated multicyclic sesquiterpenes and complex mixtures of isomerized sesquiterpenes.—Harvey et al.
In the study, the researchers hydrogenated and isomerized three multicyclic sesquiterpenes: valencene and premnaspirodiene biosynthesized from glucose, and natural caryophyllene. While the first two can be produced biosynthetically (the researchers acquired the biosynthetics from Allylix, Inc.), caryophyllene is a prime candidate for biosynthesis.
Hydrogenating the sesquiterpene molecules produced saturated hydrocarbons with net heat of combustion (NHOC) “significantly higher” than Diesel #2 and Jet A. However, the viscosities are higher, and out of spec for Jet A. (Although blending with petroleum fuels is a possibility.) Their cetane numbers were much lower than diesel (e.g., 23 to 29, as opposed to >41.)
The researchers then became interested in evaluating other structures that might have improved performance, and could be produced by the isomerization of the parent sesquiterpenes with the heterogeneous acid catalyst Nafion SAC-13.
This resulted in high density fuels with the net heats of combustion ranging from ~133,000– to 141000 Btu gal-1.
In addition to high-performance fuels, the coupling of metabolic engineering, catalytic isomerization, and conventional synthetic organic chemistry has the potential to uncover new routes for the preparation of fine chemicals. For example, although the isomerization of valencene and premnaspirodiene did not result in structures that would likely lead to improved fuel properties, it did result in a formal synthesis of δ-selinene. This same type of strategy applied to other biosynthetic sesquiterpenes will allow for the synthesis of a wide variety of functional hydrocarbons that are rare and prohibitively difficult to isolate from plant extracts or prepare through a purely biosynthetic approach.—Harvey et al.
Benjamin G. Harvey, Heather A. Meylemans, Raina V. Gough, Roxanne L. Quintana, Michael D. Garrison and Thomas J. Bruno (2014) “High-density biosynthetic fuels: the intersection of heterogeneous catalysis and metabolic engineering,” Phys. Chem. Chem. Phys. doi: 10.1039/C3CP55349C