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.

Cellulosic ethanol from corn - a quick route to hundreds of millions of gallons

Cellulosic ethanol production in the US is far behind the ambitious targets set in the Renewable Fuel Standard, and one of the reasons for the delay is the enormous capital investment needed to build out a significant cellulosic fuels industry. Given today's challenging investment climate, first-of-a-kind risks in cellulosic technologies, and relatively large capital investment needed per gallon of production, cellulosic ethanol facilities have been slow to arrive.

But what if cellulosic technology could be deployed across the existing corn ethanol fleet with almost no capital investment required? What if cellulosic ethanol could be produced side-by-side with corn ethanol and increase the capacity of corn ethanol plants? What if hundreds of millions of gallons of cellulosic ethanol were produced in the US as soon as this year?

A recently published patent application from DuPont Industrial Biosciences suggests that near zero-capex cellulosic ethanol could become a reality. In US Patent Application 20120276593 Mian Li and Colin Mitchinson describe a process in which a standard cellulase enzyme package is added to the dry grind corn ethanol process. In the normal dry grind process, ground corn is heated with alpha-amylase enzymes to liquefy the starch component. This turns the corn into a mash, which is then digested into sugars by glucoamylase enzymes (saccharification) and fermented into ethanol by yeast. However, corn grain contains around 8-10% of cellulosic fiber which is not digested by alpha-amylases or glucoamylases. Because the enzymes aren't able to break this fiber down into sugars, it sticks around and is not made into ethanol. Typically, this fiber is recovered along with protein and oil and used in animal feed.

While the standard corn ethanol enzymes can't break down fiber, the cellulases which have been developed for breaking down biomass in a cellulosic biorefinery are quite capable of doing this. Recognizing the opportunity, the DuPont researchers added their cellulase package, Accellerase 1500, to the saccharification step along with glucoamylase. They observed as much as 3% more ethanol produced than in the normal process in a number of different trials reported in the patent application. The increase in yield may come from sugar released from the cellulose in the corn or from starch becoming more accessible due to the breakdown of fiber which entraps it. Importantly, they did not have to make major modifications to the rest of the process in order to see the yield boost.

Considering the best (optimistic) case yield increases and some of the enzyme loadings described in the application, one can compare the enzyme cost against the ethanol yield boost. One ton of dry corn produces about 118 gallons of ethanol, so this technology could contribute an extra 3.5 gallons. This might be worth about $10 or more, depending on whether it gets credit as cellulosic fuel. At the lowest useful enzyme dosage of 0.08%, 1.6 lbs of cellulase enzyme would be required per ton of corn. The most recent NREL process design costed cellulase enzymes at $1.90/lb, hence the enzyme cost would be $3 to get $10 worth of additional ethanol yield. Even if the enzyme cost tripled, the cellulase yield boost may still be attractive, especially if the ethanol gets a cellulosic credit.

If it is possible to scale these results from the lab into commercial production, then cellulases could be used to unlock cellulosic ethanol from corn in existing biorefineries, just by sprinkling in some additional enzymes. Corn ethanol production capacity in the US is over 14 billion gallons, suggesting that it would be possible to produce as much as 420 million more gallons through deployment of cellulases in the process (although some modifications would be needed to apply this technology to wet mill ethanol facilities). At least some of this increased ethanol would likely be derived from cellulose so that it could legally supply the volumes of cellulosic fuel mandated by RFS2, and it would greatly increase the supply beyond the 10 million gallons in the standard in 2012. Finally, this could be a large-scale, cost effective use of cellulase enzymes which would get them out into the market and encourage producers to drive their costs down. Lower enzyme costs would then make cellulosic ethanol from other sources such as ag wastes and energy crops much more affordable and enable the cellulosic industry to grow faster.

P.S. This simple enzyme addition concept is different from the "bolt-on" technologies being pursued by NCERC, NEAtech, and Quad County. These require new equipment to be added to the corn ethanol plant to convert the cellulose, while the DuPont concept uses cellulase enzymes to break down the corn fiber within the existing plant's equipment.

“You can make anything from lignin” . . . a selective oxidation for lignin from researchers at the University of Wisconsin

Lignin is one of the main components of plant biomass (~40% of the energy content) and is a gooey aromatic polymer that encases the sugar polymers in plant structures. It is usually an impediment to productive uses of biomass. It hinders digestion of grass by animals (and may make it taste gross!), its removal from wood is a major step in many papermaking processes, and it is a barrier to the enzymatic breakdown of biomass for cellulosic fuels. Most old and wise wood scientists like to say “You can make anything from lignin except money!”

Recently, Shannon Stahl, John Ralph, and coworkers at the University of Wisconsin-Madison published a selective method to oxidize lignin as a first step toward breaking it down into useful pieces ("Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin"). Non-selective oxidation has already been used for making valuable products from lignin; vanillin, the source of vanilla flavor, can be produced in low yield by oxidation of the lignin liquor waste from sulfite pulping of wood. Nonetheless, the poor vanillin yield and large amounts of waste produced in this process have made it unattractive and led to its demise in the US and Canada (for this interesting story, see this paper by Martin Hocking). Noting the low selectivity of previous methods of lignin oxidation, Stahl and coworkers performed a careful screen of a number of different oxidation methods on a lignin model compound and found that TEMPO-based oxidants and co-catalysts performed well, achieving up to 94% isolated yield for oxidation of a benzylic secondary alcohol to a ketone (see scheme below).

Here, oxygen is used as the terminal oxidant and the nitrogen oxides (from HNO3) and TEMPO derivative serve as co-catalysts. The oxidative chemistry performed well with a range of other lignin model compounds. However, it is common for attempts at lignin chemistry to be quite successful with model compounds but fail miserably with real lignin. To their credit, the Wisconsin team tested their chemistry with an isolated aspen lignin and examined its conversion by 2D NMR. They found that the benzylic alcohols were oxidized to ketones on essentially all of the guaiacyl units and the majority of the syringyl units according to the NMR. The mass recovery in the lignin reaction was slightly more than 80%, which could indicate that some of the lignin was broken down into soluble organics as well.

There are some important limitations to this small scale lignin study, though. According to the supplemental procedures and references, the particular lignin sample used in this work was prepared by ball milling of wood, enzymatic removal of the associated cellulose and hemicellulose, and extraction of the lignin into dioxane. This process leads to a soluble and easy to handle form of lignin. Practical processes using lignin are likely to have to work with poorly soluble, impure, and intractable lignins.

As a final note, Stahl and coworkers showed that the main oxidized lignin model compound which they produced could be broken into vanillic acid and guaiacol with alkaline hydrogen peroxide in moderate yield. It is unfortunate, though, that the control reaction of the non-oxidized lignin model compound was not also included in the discussion. Alkaline hydrogen peroxide is a widely used condition for breaking down lignin, and it has been previously reported that this condition can be used to cleave a lignin model compound very similar to the one used by Stahl and coworkers. Is there a clear difference in lignin deconstruction due to the selective oxidation? Hopefully this topic as well as the depolymerization of real lignin will be addressed in future publications. It is exciting to see lignin chemistry receiving more attention from chemists like Stahl, and this may someday lead to chemistry for making money from lignin.

Hydroxymethylfurfural, A Versatile Platform Chemical

For my first post on the Biobased Chemicals blog, it is my pleasure to highlight an excellent new review on 5-hydroxymethylfurfural (HMF),one of the most tantalizing renewable chemicals sought by chemists. This six-carbon furan with several versatile handles for manipulation is a starting point for multiple products such as 2,5-furandicarboxylic acid (polymers), 5-alkoxymethylfurfurals and 2,5-dimethylfuran (fuel components), pharmaceuticals, and flavors, and it can be made in high yield from fructose - the sugar which is already mass-produced in high fructose corn syrup. Despite all of these attractive qualities, HMF has lagged behind while other bio-based chemicals like lactic acid, 1,3-propanediol, and even its cousin furfural have been scaled up, commercialized, and transformed into myriad consumer products. Practical and economical production of HMF and its derivatives remains a challenge which is being attacked by many researchers and companies.

Scientists at Avantium, one of the leaders in the drive toward economical HMF, the University of Groningen, and DSM have now published a comprehensive review in Chemical Reviews on the chemistry of HMF and its derivatives. In doing so, they provide a much needed update to earlier perspectives from Lewkowski, Lichtenthaler, and others - as they note, the literature on HMF has exploded since 2000, with some 200+ publications and patents now being published annually. Moreover, HMF has been one of the hottest research areas in bioenergy, with a 2009 Binder & Raines paper on production of HMF from biomass considered the hottest recent research publication in the field.

Heeres, de Jong, de Vries and the other authors provide a wide-ranging look at HMF, covering topics such as toxicological aspects, dehydration mechanisms, process chemistry for HMF production, reactor systems, and attempts to scale up HMF production. They also take a look at HMF's position as a platform chemical and its derivatives such as FDCA, adipic acid, fuel components, and fine chemicals. Throughout, the authors take a critical look at the topic while providing some humor along the way to keep the nearly 100 pages of dense chemistry interesting. For instance, they note that "ever since mankind started heating its food, furan compounds have been part of the human diet, as HMF is formed during the thermal decomposition of carbohydrates."

One highlight of the publication is the discussion of the potential mechanisms by which HMF is formed from sugars, in which the authors pull together insights and proposals from more than 50 years of research on this topic. Despite many studies on this topic, it is difficult to draw many conclusions. One is that HMF is much more readily formed from fructose than from other sugars, and another is that a cyclic mechanism from fructose is more likely than an acyclic mechanism. Finally, "the literature strongly indicates that the first dehydration of fructose is the rate determining step in the formation of HMF, with all subsequent steps proceeding much faster. This makes it almost impossible to determine the reactive intermediates by analysis. A second complicating factor is the inherent reactivity of sugars, facilitating a wide array of side-reactions of the sugar."

These side reactions have plagued efforts toward practical production of HMF, and the review discusses the many variations on process chemistry used to avoid side reactions and increase yields. "Until the 1980s, research in this field almost exclusively focused on the use of homogeneous acids as the catalyst in water, a common solvent for traditional sugar chemistry. The past three decades have seen a shift from water-based chemistry to alternative solvent systems to improve the overall yield of HMF." The three major alternative solvent systems have been organic solvents such as DMSO, room temperature ionic liquids, and mixed solvents such as biphasic water/organic systems. In some solvents, derivatives of HMF such as ethers and esters, are formed, and Avantium itself has focused on the production of HMF ethers in alcohol solvents such as methanol.

A further complication to HMF production is its isolation from the reaction mixture, and the review provides an overview of the more promising methods (extraction and adsorption) and the less promising methods (distillation: "the distillation of crude HMF is recognized as troublesome due to thermal degradation of HMF associated with the formation of tarry carbonaceous materials.")

The review closes with a perspective on the challenge and promise of HMF:

It has always been a challenge to obtain HMF in an efficient way. Although a lot of improvements have been made in understanding the mechanism and kinetics of the dehydration process, significant challenges still remain in transferring it to an industrial scale. The vast majority of the research summarized in this review has been performed on lab-scale in batch reactions without a real focus on finding an efficient and economically viable process. Improvements have been made in recent years by applying different solvent types and extraction methods, and by applying bifunctional catalyst systems. The use of biphasic systems appears to be straightforward; good yields of HMF have been obtained, and such processes would appear to be scalable although they require relatively large amounts of extracting solvents. There has been little focus on the purification of HMF and the efficient recycling of reaction- and extraction solvents or catalysts . . . .

The biggest challenge still to overcome is the use of glucose and glucose-based polymers, like starch and cellulose, as the carbohydrate feedstock because of the unstable nature of HMF and the severe reaction conditions required. Key to the success of HMF will be an economical production process. So far two pilot plant efforts have not been successful in the direct production of HMF. Recently, Avantium Chemicals started operating a pilot plant for production of furan-based chemicals and plastics in which HMF is in situ converted to its ether to help improve yield and purification. A similar strategy is followed by Mascal in a process that produces 5-chloromethyl-furfural, another more stable derivative of HMF to obtain high yields in combination with efficient separation . . . .

This review underlines the progress that has been made toward an economic production of furan derivatives from biomass; a development which will open the way to a wealth of interesting materials all the way from fuels and bulk chemicals to fine chemicals and pharmaceuticals applications.

I believe that this review will guide and encourage further progress toward that goal.