Structures of a common alkenone produced by Isochrysis sp. (a) and common FAME methyl linoleate (b). Nomenclature for both compounds is number of carbons:number of double bonds; however, note that the configuration of double bonds is different. Credit: ACS, O’Neil et al. Click to enlarge.

These compounds [alkenones] are unlike the cis-unsaturated methylene interrupted fatty acid components of triacylglycerols (TAGs) as they typically have two to four trans alkenes occurring at 7-carbon intervals.

… Alkenones are thought to reside in cytoplasmic lipid bodies and can be more abundant than TAGs especially in the stationary growth phase. Under N or P limitation, up to 10−20% of cell C in the stationary phase is accumulated as alkenones. Evolutionarily, alkenones may have been favored over TAGs because their trans double bond geometry provides a more photostable form of energy storage.

… We argue that alkenones represent a potentially fruitful and as yet unexplored renewable carbon source with structures particularly well suited for a number of catalytic processes. Specifically, alkenones feature long olefinic carbon-chains, favorable carbon-to-hydrogen ratios, and few heteroatoms (i.e., no sulfur or nitrogen and C:O ∼ 37−40:1). Key differences relative to fatty acids include a much longer hydrocarbon backbone, more widely spaced trans double bonds, and a ketone functional group. Alkenones are also fundamentally different than the terpenoid botryococcenes that have received significant attention as a potential algal biofuel source, despite the noted slow growth habit of B. braunii, which poses concerns about its suitability as a biofuel feedstock. PULCAs [haptophyte algae (Isochrysis, Emiliania, Gephyrocapsa, Chrysotila) that produce a suite of polyunsaturated long-chain (C₃₇-C₃₉) alkenes, alkenones, and alkenoates] thus represent an unexplored source of renewable carbon biosynthesized by robust algal species that could provide access to a unique suite of products unobtainable from other oil feedstocks.

—O’Neil et al.

The study in Energy & Fuels is a proof-of-concept for the use of olefin metathesis as an alkenone-conversion strategy to produce a hydrocarbon mixture in the kerosene/jet fuel boiling range as a coproduct with algal biodiesel.

The researchers freeze-dried wet Isochrysis biomass; algal oil was extracted from the solid biomass with hexanes. Saponification and separation resulted in two streams: free fatty acids (60%) and neutral lipids (40%). The neutral lipids were dissolved in dichloromethane and flushed through silica gel; the resulting solid left after solvent removal was recrystallized in hexanes to give pure alkenones, which then underwent butenolysis. The FFA underwent esterification to produce biodiesel (fatty acid methyl esters).

Routine yields for the isolated alkenones and biodiesel were routinely 3.5 and 12 % (w/w) with respect to the starting dry Isochrysis biomass.

The researchers converted the isolated alkenones to smaller jet-fule range hydrocarbon fragments by cross-metathesis with 2-butene (butenolysis) using several commercial ruthenium-based metathesis initiators.

Butenolysis with a second-generation catalyst was rapid at 4 ˚C, yielding near quantitative conversion within 30 minutes to a mixture containing mostly 8-decen-2-one (C₁₀), 2,9-undecadiene (C₁₂), and 2-heptadecene (C₁₇).

The proof-of-concept showed that alkenones are reactive toward olefin metathesis, with a jet-range product. The butenolysis reaction yields a predictable mixture of hydrocarbons, uses commercially available catalysts, and occurs rapidly at low temperatures. However, a key obstacle is the cost of the Isochrysis biomass.

It is known that alkenone unsaturation is sensitive to algae growing conditions. Other more sophisticated techniques like viral infection might also prove capable of controlling the alkenone unsaturation profile to ultimately fine-tune the products obtained by metathesis. There is no prior work on optimizing alkenone production from Isochrysis sp. However, as a coproduct of an industrial biofuel process, even at 5% (or 3.5% pure from our method) of the biomass, this would represent a significant amount of alkenones.

For instance, in the US, consumption of petroleum-derived products is approximately 10 L per person per day (20 million bbl/day, 319 million people, 159 L/bbl). Replacing 10% of that quantity with an Isochrysis-derived biodiesel, thus, would require producing about 1 kg of algal biofuel/person/day. We typically obtain 12% (w/w) of a pure biodiesel (FAME) from dry Isochrysis biomass, meaning one would need 8.33 kg of Isochrysis/person/day.

Using our total and purified alkenone yields, this would correspond to potentially 0.3−0.4 kg of alkenones/person/day, or 30−40% of the biodiesel production volume. Utilization of this material in some capacity (e.g., fuels or specialty chemicals), therefore, could significantly increase the overall value of the algal biomass. The current price of Isochrysis is approximately $400/kg, sold by a handful of vendors and controlled by the economics of its current market as shellfish feed. Using this value, the Isochrysis lipid-derived fuels we have described are far from cost-competitive (> $10,000/gal). Future work is aimed at assessing the commercial viability of these and other haptophyte-based biofuels.

—O’Neil et al.

Resources

• Gregory W. O’Neil, Aaron R. Culler, John R. Williams, Noah P. Burlow, Garrett J. Gilbert, Catherine A. Carmichael, Robert K. Nelson, Robert F. Swarthout, and Christopher M. Reddy (2015) “Production of Jet Fuel Range Hydrocarbons as a Coproduct of Algal Biodiesel by Butenolysis of Long-Chain Alkenones” Energy & Fuels doi: 10.1021/ef502617z