A closer look at the latest game-changing route to drop-in biofuels

 In a Nature Chemistry article rather modestly entitled "The hydrodeoxygenation of bioderived furans into alkanes," a team from Los Alamos National Lab (Sutton, Waldie, Wu, Silks, and Gordon) and the University of Guelph (Schlaf) made the latest splash in chemical routes to biofuels. A number of different news sites published notes on the paper (for example phys.org and biodieselmagazine.com), while Biofuels Digest attracted readers' attention with an article discussing catalytic promiscuity and touting the world-changing potential of the work. However, none of these sources really provided enough information to appreciate the content of the paper, so I looked it up to have a closer look for myself.

It turns out that the Nature Chemistry paper is part of a larger body of work from LANL, Guelph, and Proctor & Gamble, including several publications and patent applications:

Keith, J. M. et al. Aqueous organocatalysis for the carbon chain extension of carbohydrate derivatives: application to the production of transportation fuels. Curr. Org. Chem. In press.

Waidmann, C.R. et al. Functional group dependence of the acid catalyzed ring opening of biomass derived furan rings: an experimental and theoretical study. Catal. Sci. Technol., 2013, 3, 106.

Method Of Carbon Chain Extension Using Novel Aldol Reaction. US 20120289720 A1 (LANL)

Method of producing saturated alkyl esters/acids. US 20130066089 A1 (P&G)

COMPOUNDS AND METHODS FOR THE PRODUCTION OF LONG CHAIN HYDROCARBONS FROM BIOLOGICAL SOURCES. WO2013040311 (LANL & P&G & Guelph)

Piecing together the contents of the published work, the overall route to fuels envisioned is as follows: sugars are dehydrated into furans such as furfural and HMF, then the furans are condensed with ketones such as acetone to make larger molecules, and finally the condensed product is hydrogenated into an alkane. Sound familiar? The Dumesic lab has been working on similar routes since 2006. Ryan West, who is now at P&G, did some of this work. What distinguishes the new approach from LANL et al. is that it uses relatively mild conditions. The following reaction sequence pulled from one of the patent applications provides an example:

Rather than using heterogeneous acid/base catalysts as previously used for the initial aldol condensation, the LANL team used a secondary amine salt as a catalyst. This type of catalyst is commonly used in synthetic organic chemistry but is not so typical in routes to fuels or commodity chemicals. For the hydrogenation of the condensed furan product, they used strong acids in combination with a noble metal catalyst to produce the alkane product, with these catalyst and solvent choices enabling the use of lower temperatures (200 C vs. 300 C) and hydrogen pressures (20 bar vs. 55 bar) than previously used.

The authors note that some care is needed to obtain the alkane product selectively. If the exocyclic alkenes are not saturated first, the condensation product may undergo a retro-aldol reaction and return to starting materials. In addition, if an acidic solvent is not used for the deoxygenation reaction, then it is possible to saturate the furans before they ring-open, leading to a fairly stable tetrahydrofuran product which is difficult to deoxygenate fully. Finally, a strong acid such as triflic acid or HCl is required to deoxygenate all the way to the alkane.

While this is an interesting route to alkanes and the LANL team clearly learned some basic chemistry lessons along the way, it is somewhat baffling that they chose to look at a rather impractical route to fuels. While the low temperature and pressure conditions which they used are easier for laboratory explorations, they are probably less attractive for fuel production than some of those used in the earlier work by Dumesic and coworkers. Homogenous catalysts, organic solvents, triflic acid, and long reaction times are all unlikely to be popular in a biorefinery.

The involvement of P&G provides a possible clue into the rationale for exploring this route. The P&G patent application includes some examples in which bio-derived furans are condensed with bio-derived ketones, such as ethyl levulinate (a possible byproduct of HMF synthesis). Though this sequence of reactions, ethyl levulinate plus furfural would give ethyl decanoate in just two steps in high yield. This ester, as well as homologues or branched isomers produced from similar starting materials, might be able to meet the price points for bio-based starting materials for P&G products, as noted in the application:

Current methods of processing furan materials result in the production of alkanes and only trace amounts of oxygen-containing species. The method of the present invention encompassing hydrogenating and hydrodeoxygenating furan compounds allows for the retention of ester or acid functionality. The saturated unbranched ester or acid products of the present invention are direct renewable replacements for esters/acids currently used in manufacturing. The saturated branched ester or acid products are novel structures with the potential for widespread use as esters, alcohols and surfactants. The method of the present invention produces ester or acid products that, unlike the alkanes formed using other methods, may be further processed into other additives, such as surfactants, thereby increasing their overall utility.

 Fuels by this new route? Not likely. But alternative supplies of long-chain esters and acids for personal care products? These may be showing up in a P&G product in your home someday.