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Staff Accomplishments

It Isn't Easy Being Green

July 2007
Fuel, polyester and other chemicals from biomass get an important boost, PNNL team reports in the journal Science Replacing crude oil as the source for plastic, fuels and other industrial and household chemicals with inexpensive, nonpolluting renewable plant matter is a critical goal in reducing carbon emissions. Scientists reported in the journal Science that they have directly converted glucose, ubiquitous in nature, as an alternative source for those products. The scientists have converted glucose into HMF, which stands for hydroxymethylfurfural. It is viewed as a promising surrogate for petroleum-based chemicals. Glucose, in plant starch and cellulose, is nature's most abundant sugar. "But getting a commercially viable yield of HMF from glucose has been very challenging," said Z. Conrad Zhang, senior author who led the research and a scientist with the PNNL-based Institute for Interfacial Catalysis (IIC). "In addition to low yield until now, we always generate many different byproducts," including levulinic acid, making product purification expensive and uncompetitive with petroleum-based chemicals. Zhang, lead author and former post-doc Haibo Zhao, and colleagues John Holladay and Heather Brown, all from PNNL, were able to achieve HMF yields of about 70 percent from glucose and about 90 percent from fructose while leaving only traces of levulinic acid impurities. To achieve this, they experimented with a novel non-acidic catalytic system containing metal chlorides in a solvent capable of dissolving cellulose. The solvent, called an ionic liquid, enabled the metal chlorides to convert sugars to HMF. Ionic liquids provide an additional benefit: They are reusable and thus produce none of the wastewater in other methods that convert fructose to HMF. Metal chloride in ionic-liquids "in general work well for converting fructose to HMF," Zhang said -- but not so well when glucose is the initial stock. In fact, attempts at direct glucose conversion created so many impurities that it was simpler to start with the fructose, less common in nature than glucose. Zhang and his team, working with a high-throughput reactor capable of testing 96 metal halide catalysts at various temperatures, discovered that a particular metal -- chromium chloride -- was by far the most effective at converting glucose to HMF at low temperature, 100 degrees centigrade. "This, in my view, is breakthrough science in the renewable energy arena," said Mike White, IIC director and Robert A. Welch chair in materials chemistry at the University of Texas. "This work opens the way for fundamental catalysis science in a novel solvent." The chemistry at work remains largely a mystery, Zhang said, but he suspects that metal chloride catalysts work during an atom-swapping phase that sugar molecules go through called mutarotation, in which an H (hydrogen) and OH (hydroxyl group) trade places. During the swap, the molecule opens, Zhang said. "The key is to take advantage of the open form to perform a hydride transfer through which glucose is converted to fructose." Zhang's next step is to tinker with ionic solvents and metal halides combinations to see if he can increase HMF yield from glucose while reducing separation and purification cost. Contact: Walter Weimer, Process and Measurement Technology Product Line.

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