15 June 2007
Glucose Converted Directly To Faux Fossil Fuel
by Kate Melville
A big step towards viable biofuel refining was taken this week, with scientists claiming to have directly converted the natural sugars in plant matter into a promising surrogate for petroleum-based chemicals.
The faux fossil fuel is hydroxymethylfurfural (HMF), which the researchers say can be derived from glucose and fructose. Importantly, the HMF from glucose contains very little of the residual impurities that have made the quest for a viable glucose biorefining method so daunting in the past.
"What we have done is convert glucose directly in high yields to a primary building block for fuel and polyesters," said research leader Z. Conrad Zhang, a scientist at Pacific Northwest National Laboratory. While glucose - found in plant starch and cellulose - is nature's most abundant sugar, Zhang explained that getting a commercially viable yield of HMF from glucose was very challenging. "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," he said.
Impressively, the researchers were able to coax HMF yields upward of 70 percent from glucose and nearly 90 percent from fructose while leaving only traces of acid impurities. To achieve this, they experimented with a novel non-acidic catalytic system containing metal chloride catalysts in a solvent capable of dissolving cellulose. The solvent, called an ionic liquid, enabled the metal chlorides to convert the sugars to HMF. Ionic liquids provide an additional benefit: re-use, thus producing none of the wastewater in other methods that convert fructose to HMF.
The metal chlorides belong to a class of ionic-liquid-soluble materials called halides, which work well for converting fructose to HMF, but less effectively with glucose. Working with a high-throughput reactor capable of testing 96 metal halide catalysts at various temperatures, Zhang discovered that a particular metal - chromium chloride - was by far the most effective at converting glucose to HMF with few impurities and at a low temperature of 100 degrees centigrade.
Zhang speuclates 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. Zhang explained that during the swap, the molecule opens, which allows a hydride transfer through which glucose is converted to fructose.
Zhang's next step is to tinker with ionic solvent and metal halide combinations to see if he can increase HMF yield from glucose while reducing separation and purification cost. "The opportunities are endless," Zhang said, "and the chemistry is starting to get interesting."
Source: DOE/Pacific Northwest National Laboratory