Researchers Develop Two-Step Chemical Process to Take Untreated Biomass to Furans for Fuels and Chemicals
|Yields of bio-based chemicals from a variety of feedstocks, including untreated corn stover. Conditions are optimized for the conversion of carbohydrates into HMF (1 step) and DMF (2 steps). Credit: ACS. Click to enlarge.|
Researchers at the University of Wisconsin-Madison have developed a two-step, low-temperature, non-enzymic pathway from lignocellulosic biomass to furans, specifically 2,5-dimethylfuran (DMF), a promising alternative biofuel. DMF has an energy content (31.5 MJ/L) similar to that of gasoline (35 MJ/L) and 40% greater than that of ethanol (23 MJ/L). With a boiling point of 92-94 °C, DMF is also less volatile than ethanol (bp 78 °C) and is immiscible with water.
The key to the new process is the first step, in which a novel solvent system converts cellulose into the renewable platform chemical 5-hydroxymethylfurfural (HMF), from which a variety of valuable commodity chemicals and fuels can be made. A paper describing the process was published in the 11 Feb issue of the Journal of the American Chemical Society.
Professor Ronald Raines and graduate student Joseph Binder, a doctoral candidate in the chemistry department, developed the unique solvent system—N,N-dimethylacetamide (DMA) containing lithium chloride (LiCl)—that enables the single-step synthesis of HMF with “unprecedented yield” from untreated lignocellulosic biomass, as well as from purified cellulose, glucose, and fructose.
Other work in the production of HMF and subsequent fuel derivatives have primarily started with fructose or glucose. (Earlier post.) The Raines process appears to be the first that can take untreated lignocellulosic biomass to HMF and then DMF.
|“This solvent system can dissolve cotton balls, which are pure cellulose. And it’s a simple system—not corrosive, dangerous, expensive or stinky.”|
The solvent system, for which a patent is pending, has an extraordinary capacity to dissolve cellulose. The conversion of cellulose into HMF via the solvent is unabated by the presence of other biomass components, such as lignin and protein. The solvent can slip between the lignin molecules, where it works to dissolve the cellulose, cleave it into its component pieces, and then convert those pieces into HMF. The team determined that loosely ion-paired halide ions in DMA-LiCl are critical for the remarkable rapidity (1-5 h) and yield (up to 92%) of this low-temperature (≤140 °C) process.
In step two, Raines and Binder convert HMF into DMF. Starting by applying the solvent to corn stover, the team then removed the chloride ions from the resulting crude HMF by ion-exclusion chromatography in water. This separation step prevented the chloride from poisoning the copper hydrogenolysis catalyst. They then subjected the crude HMF from corn stover to hydrogenolysis in 1-butanol with a carbon-supported copper-ruthenium catalyst and obtained a 49% molar yield of DMF, similar to that obtained by Dumesic and his colleagues using HMF that contained trace chloride.
The overall molar yield of DMF based on the cellulose content of the stover was 9%. Raines and Binder expect that optimization of the process could improve upon this result. In addition to corn stover, Raines and Binder have tested their method using pine sawdust, and they’re looking for more samples to try out.
Under our best conditions, we transform 42% of the dry weight of cellulose into HMF and 19% of the dry weight of corn stover into HMF and furfural in one step. For comparison, cellulosic ethanol technology, which has been optimized extensively, enables the conversion of 24% of the dry weight of corn stover into ethanol in a complex process involving multiple chemical, biochemical, and microbiological steps.
Our process is also competitive on the basis of energy yield. The HMF and furfural products contain 43% of the combustion energy available from cellulose and xylan in the corn stover starting material, whereas ethanol from corn stover preserves 62% of the sugar combustion energy. Biomass components that cannot be converted into HMF, such as lignin, could be reformed to produce H2 for HMF hydrogenolysis or burned to provide process heat.—Binder and Raines (2009)
Areas under investigation for improving the process include addressing the high loading of the chromium catalyst and the toxicity of this metal, which could be barriers to its large-scale use. Raines and Binder have already found that decreasing chromium loading by two-thirds decreases the yield of HMF from glucose only slightly.
The research was supported by the Great Lakes Bioenergy Research Center, a US Department of Energy bioenergy research center located at the UW-Madison. Additional support was provided through a National Science Foundation Graduate Research Fellowship awarded to Binder.
Joseph B. Binder and Ronald T. Raines (2009) Simple Chemical Transformation of Lignocellulosic Biomass into Furans for Fuels and Chemicals. J. Am. Chem. Soc., 131 (5), 1979-1985 doi: 10.1021/ja808537j