In the Lab
Progress to a pathway: PNNL process holds promise for biobased chemicals
"HMF is a chemical platform that can be made from abundant cellulosic-based glucose," Zhang says. "HMF can be made into other monomers to make polymeric materials such as polyester, polyurethane and polyamide, and those are very big markets. We can make those polymers from biomass."
Zhang's process relies on a compound called ionic liquids. Ionic liquids are basically salts that have a melting point below 212 degrees Fahrenheit. A family of catalysts, called metal halides, will dissolve in ionic liquids and can be used to promote a variety of interesting chemical reactions, including the conversion of carbohydrates into HMF. Current technology relies on acid catalysts that are limited to using fructose as a byproduct and that also produce unwanted impurities that are difficult to separate from HMF. These processes also take a lot of energy, raising the cost of making HMF.
Zhang's research involved evaluating various metal halides as catalysts. Zhang and his team developed a high-throughput reactor that was able to test 96 catalysts under different temperatures. One in particular, chromium dichloride (CrCl2), proved especially interesting. Not only did it efficiently convert fructose to HMF at relatively low temperatures (less than 212 degrees Fahrenheit), it also converted up to 70 percent of glucose into HMF. With the current technology, glucose lowers the production of HMF and adds several unwanted byproducts. "This is the first time anyone has used this process," Zhang says. "It is not optimized for yield. We are looking to improve the yield, but 70 percent (glucose conversion) is already probably better than people have reported for fructose. So, this process is already quite attractive."
This reaction takes place under relatively mild conditions, which means a production facility wouldn't need equipment designed for high pressures and temperatures. Lower temperatures also mean lower energy costs. The ionic liquid and catalyst can also be recovered after the reaction is complete, making it a cleaner process. "It will minimize the use of water and the generation of waste water," Zhang says.
A particular problem with using acid to make HMF is that the reaction also produced levulinic acid. Zhang's process produced less than a tenth of a percent of levulinic acid, so the HMF was much more pure. Levulinic acid was a particular problem, Zhang says, because HMF decomposes below the temperature needed for distillation to separate the two compounds. "Once you have levulinic acid mixed with HMF, it is difficult because there is no good way to remove it," Zhang says.
While ionic liquids may be exotic to the general public, they are fairly common industrial chemicals. Zhang says if his process is successfully commercialized, he doesn't think there will be any problems with supplies. "Recently, one company came to us and said they are not concerned at all because they produce such a large amount of this material," Zhang says. "So, they said that was not the issue for them."
Another interesting property of ionic liquids is that they will dissolve cellulosic material. Zhang's current research is examining how cellulosic material reacts in ionic liquid-catalyst systems. "Since we published our work on glucose, we have been working on cellulose because it is more abundant and cheaper," Zhang says. "We are also continuing to work with glucose, making separation cheaper and improving the process' economics."