What we have done that no one else has been able to do is convert glucose directly in high yields to a primary building block for fuel and polyesters.

—Z. Conrad Zhang, PNNL Institute for Interfacial Catalysis

Zhang, lead author Haibo Zhao and colleagues John Holladay and Heather Brown, all from PNNL, were able to coax HMF yields upward of 70% from glucose and nearly 90% from fructose while leaving only traces of acid impurities.

Last year, researchers at the University of Wisconsin announced that they had developed a process to convert fructose to HMF with 80% HMF selectivity at 90% fructose conversion. (Earlier post.)

The PNNL team experimented with a novel non-acidic catalytic system containing metal chloride catalysts in an ionic liquid capable of dissolving cellulose. The solvent enabled the metal chlorides to convert the sugars to HMF. The ionic liquids also offer the additional benefit of being reusable, thereby eliminating the production of wastewater as in other methods.

Metal chlorides belong to a class of ionic-liquid-soluble materials called halides, which in general work well for converting fructose to HMF but not so well when glucose is the initial stock, according to Zhang. 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 with few impurities and at a relatively low temperature of 100°C.

This, in my view, is breakthrough science in the renewable energy arena. This work opens the way for fundamental catalysis science in a novel solvent.

—J.M. White, IIC director and Robert A. Welch chair in materials chemistry at the University of Texas

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.

The hydrogen-hydroxyl position-switch that allows the catalytic conversion was verified by nuclear magnetic resonance performed at the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility located at PNNL.

During the swap, the molecule opens, Zhang said, noting that the key is to take advantage of the open form to perform a hydride transfer through which glucose is converted to fructose.

The next step is experimentation with more ionic solvents and metal halides combinations to attempt to increase HMF yield further while reducing separation and purification cost.

Resources:

• “Metal Chlorides in Ionic Liquid Solvents Convert Sugars to 5-Hydroxymethylfurfural”; Haibo Zhao, Johnathan E. Holladay, Heather Brown, Z. Conrad Zhang; Science 15 June 2007: Vol. 316. no. 5831, pp. 1597 - 1600 DOI: 10.1126/science.1141199