MIT scientists have devised a cheaper way of synthesizing a biofuel that could revolutionize the way that biomass is used as a renewable carbon source in the 21st century.
Carbon fuels society; petroleum is widely used as a source of liquid fuels. However, as the industrialization of nations such as China and India increases demand for petroleum while the limited quantity of fossil-based fuels decreases, researchers increasingly turn to alternative sources of carbon (1).
Biomass in particular presents a tempting option; when properly processed, one can often blend the resulting fuel into existing fuel without undertaking costly infrastructure upgrades. Lignocellulosic biomass, derived from feedstock and similar agricultural waste, is produced on the scale of hundreds of billions of tons a year—only 3% of which is used (2).
This biomass, unlike ethanol, would thus not affect food prices like as corn-derived ethanol would. Historically, industry has dabbled infrequently in biomass-derived fuel, but the economic advantage that cheap, plentiful oil once held over biomass impeded the development of biomass-based fuel. However, with the rise in oil prices, lignocellulosic biomass now presents itself as an increasingly appealing option (2).
Inexpensive, environmentally benign, and readily renewable, biomass-derived fuel holds a variety of advantages over petroleum in many areas besides mature scientific development. However, it still requires considerable innovation to be produced and used on a larger scale (1). Difficulties still remain in converting lignocellulosic biomass into chemicals or fuels in an economical way at high yields (3).
Lignocellulosic biomass consists of three components: cellulose, hemicellulose, and lignin. Cellulose may be hydrolyzed to produce ethanol, platform chemicals such as levulinic acid, and various liquid fuels. Hemicellulose, on the other hand, is more reactive and is more easily extracted from biomass and may produce furfural and furfural derivatives. Finally, lignin possess a complex, non-uniform structure that makes processing more difficult.
Recently, researchers at the Massachusetts Institute of Technology have developed a method of improving the synthesis of a high-potential lignocellulosic biofuel—gamma-valerolactone (GVL) (3, 4). Higher-energy than ethanol, versatile as a fuel on its own or as an additive, stable, and useful as a solvent, GVL is tempting and yet cost-prohibitive for large-scale production (1, 3)
In traditional synthesis, the reaction starts with cellulose and hemicellulose in water, followed by conversion into levulinic acid, a five-carbon chain. The chain is then closed into a ring via hydrogenation; however, this step has proven extremely costly in two ways (3).
First, the reaction requires a precious metal catalyst such as palladium or ruthenium. Second, the addition of hydrogen atoms to the ring requires the dissolution of hydrogen gas into the solution—unfortunately, hydrogen gas is not very soluble in water. To dissolve enough hydrogen, the entire system must be placed under very high pressure using fairly expensive equipment (3, 4).
Researchers have attempted to circumvent these difficulties for some time now. However, less valuable metals such a copper fare poorly in catalyzing the reaction or are quickly deactivated in water. Formic acid, which releases hydrogen when dissolved in water, seems promising, but still requires a precious metal catalyst and harsh conditions (3, 5).
At MIT, researchers circumvented these two problems by using a series of cascading reactions different from the traditional pathway. Instead of converting hemicellulose into levulinic acid, they converted it into furfural, a molecule with a five-carbon ring. In the reaction cascade, the furfural ring is opened, hydrogenated, and closed into a new ring—GVL (3, 4).
Rather than using expensive precious metal catalysts, they used zeolite, a porous silicate mineral containing the more common zirconium and aluminum. And instead of using hydrogen gas dissolved in water, the researchers used 2-butanol. The process occurred at a relatively low temperature (120 °C) and required far less expensive equipment than the traditional process (3, 4)
Although the discovery has its merits, the MIT team is currently working on an economic analysis of the new process as well as an analysis of its potential applications to the production of similar fuels and fuel precursors. While the new process has a yield comparable to the old one (over 70%), they are investigating the effect of different solvents and acidity of the catalyst on the reaction (3,4).
Finally, the team is looking into coupling the synthesis of GVL to the synthesis of furfural from biomass, thus creating the reactant and the product of the reaction in a single elegant production process. Considering the vast quantities of lignocellulosic biomass available, every improvement in the process would yield vast dividends (3, 4).
1) D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Green Chem., 2013, 15, 584–595. Available at http://pubs.rsc.org/en/content/articlepdf/2013/gc/c3gc37065h (13 July 2013).
2) Massachusetts Institute of Technology, MIT discovery could lead to cheaper biofuels, biochemicals, Biomass Magazine (21 June 2013). Available at http://biomassmagazine.com/articles/9109/mit-discovery-could-lead-to-cheaper-biofuels-biochemicals (13 July 2013).
3) MIT Scientists Develop New Pathway to Cheaper Biofuel and Renewable Polymers, Macrocurrent (17 June 2013). Available at http://www.macrocurrent.com/mit-scientist-develop-new-pathway-to-cheaper-biofuel-and-renawable-polymers/ (14 July 2013).
4) N. Savage, Fuel Options: The Ideal Biofuel, Nature 474, S9-S11 (23 June 2011). Available at http://www.nature.com/nature/journal/v474/n7352_supp/full/474S09a.html (14 July 2013).