New approach for sustainable aviation fuel

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2020

The intermediate isophorone was used to produce a cyclobutane derivate through a photochemical reaction using ultraviolet light.

KEY CONCEPTS
• A new, sustainable, aviation fuel was prepared from the intermediate isophorone that can be derived from acetone produced from natural sugars.
• The key step in the process is the production of a cyclobutane derivative through a
photochemical reaction.
• When blended with the current petroleumbased aviation fuel (Jet-A) at a 30% treat rate, a 3.8% improvement in energy density was realized.

The push for sustainability is prompting researchers to evaluate alternative technologies in an effort to reduce carbon dioxide emissions and other pollutants. An example is the movement towards the use of sustainable aviation fuel globally. 

The ICAO (International Civil Aviation Organization) is developing a 2050 vision for sustainable aviation fuel.¹ The fuel used in civil aviation is a petroleum derivative known as Jet-A. Dr. Courtney Ford Ryan, Postdoctoral Fellow at the Los Alamos National Laboratory in Los Alamos, N.M., says, “Jet-A is a hydrocarbon-based material that contains hundreds of compounds based on aromatics, cycloalkanes, linear and branched alkanes. The composition of Jet-A varies depending upon the petroleum
source.”

One of the issues in using Jet-A is the concern about the flammability of mist that can easily be ignited, leading to the potential of a fire. In a previous TLT article,² researchers developed a new additive that can minimize the misting of aviation fuel. The additive is a type of polymer that can dynamically associate into megasupramolecules which inhibit elongation and break up of fuel droplets. Testing done by hurtling a 140-mile per hour projectile to disperse jet fuel mist over continuously burning propane torches led to no fireball when 0.3% by weight of the additive was added to Jet-A.

In an effort to develop a sustainable jet fuel, researchers have been looking for chemistries that can exhibit high energy density. Dr. Andrew Sutton, Team Leader at Los Alamos National Laboratory, says, “Optimizing energy density is an important factor in sustainable jet fuel winning acceptance with the aviation industry. Civilian airlines are looking for fuel with better energy density to extend the range of their airplanes and enable them to use less fuel.”

One structure that meets the characteristics of higher energy density is the cyclobutane ring. Ryan says, “Cyclobutanes make strained rings that add energy to molecules. Our approach was to determine if a sustainable aviation fuel can be developed based on this strained ring system.”

In evaluating approaches to make cyclobutene derivatives from a sustainable source, the solvent, acetone, was found to be an interesting starting material. Ryan says, “Acetone can now be produced through a fermentation process that converts sugars into this solvent, butanol and ethanol. Genetic engineering, combined with alternative fermentation methods, has improved the commercial viability of this pathway.”

Acetone itself is not suitable as an aviation fuel. Ryan says, “Acetone is volatile and flammable which presents significant hazards for use as an aviation fuel. As a strong solvent, acetone is also incompatible with aircraft engines including its ability to degrade seals.”

Ryan, Sutton and their colleagues (see Figure 3) have now led the development of a sustainable aviation fuel by using acetone as the starting material.


Figure 3. Researchers have developed a promising new sustainable aviation fuel using acetone as the starting material. Courtesy of Los Alamos National Laboratory.

Photochemical cycloaddition
Working backwards, the researchers determined that the intermediate isophorone (a cyclohexene derivative) can be used to produce a cyclobutane derivative through a photochemical (2+2) cycloaddition reaction using ultraviolet light. Isophorone can be synthesized from six equivalents of acetone with the loss of water.

The appealing nature of combining two equivalents of isophorone to form a cyclobutane derivative is that the process takes place under mild reaction conditions in an aqueous medium. Isophorone’s structure is amenable to absorbing light in the ultraviolet frequency, which means that a photosensitizer is not required.

Removal of the carbonyl oxygens was accomplished through a hydrodeoxygenation process to produce a completely saturated cyclobutane derivative. The overall yield of the process from acetone was 83%.

The strained cyclobutane ring means there may be concern about instability. Ryan says, “We feel the cyclobutane derivative produced is pretty stable. During the hydrodeoxygenation reaction, only a small percentage (less than 10%) of the final product mixture were ring-opened compounds.” The researchers blended the cyclobutene derivative with Jet-A at 10%, 20% and 30% treat rates. Upon analysis of key fuel properties such as energy density and viscosity and comparison with Jet-A, the results showed an improvement of 3.8% for the former parameter when the cyclobutene derivative is used at a 30% treat rate. The higher viscosity of the cyclobutene derivative pushed the viscosity of the 30% blend over the maximum limit for Jet-A.

Sutton says, “We have undertaken a proof of concept in showing that the cyclobutane derivative is a viable aviation fuel. To improve the viability of the process, the cycloaddition reaction needs to be conducted using visible light which necessitates the need for developing a suitable photocatalyst.”

Further long-term stability testing is required and the new cyclobutane derivative will need to meet ASTM aviation fuel specifications. Sutton says, “We have shown that cyclobutene derivatives are viable candidates for a sustainable aviation fuel. This research will hopefully inspire the design of different sustainable fuels that can be prepared with ultraviolet light and more closely match the performance of Jet-A.”

Additional information on this research can be found in a recent article³ or by contacting Sutton at adsutton@lanl.gov .

REFERENCES
1. Click here.
2. Canter, N. (2015), “Safer Jet Fuel,” TLT, 71(12), pp. 10-11.
3. Ryan, C., Moore, C., Leal, J., Semelsberger, T.,Banh, J., Zhu, J., McEnally, C., Pfefferle, L. and Sutton, A. (2020), “Synthesis of aviation fuel from bio-derived isophorone,” Sustainable Energy & Fuels, 4 (3), pp. 1088-1092
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net .