A Golden Opportunity

How gold is valued in biomass catalysis
By Bryan Sims | December 02, 2010

A team of chemical engineers at the University of Virginia in Charlottesville have discovered gold. Although neither the type or amount that one might associate with Hollywood films like “The Pirates of the Caribbean,” the findings from their research could potentially unravel a new layer of value specific to the biochemical industry.

UVA chemical engineer Robert Davis, along with colleague Matthew Neurock, found that gold and other precious metal nanoparticles exhibit high catalytic reactivity when placed in alkaline water. Armed with their expertise in catalytic chemistry, they studied the mechanism for oxidizing ethanol and glycerol, which coproduced intermediate chemicals in their salt states, such as acetic and glyceric acids. The researchers submitted a paper on the subject, titled, “Reactivity of the Gold/Water Interface During Selective Oxidation Catalysis,” which appeared in the October issue of Science.

According to Davis, the team observed about a 60 percent yield of glyceric acid and 70 percent yield of acetic acid in their salt forms from glycerol and ethanol, respectively. “However, we never ran the reactions to complete conversion of glycerol or ethanol, so the yields are conservative,” he says.
Acetic acid, one of the simplest carboxylic acids, is a valuable chemical reagent and industrial building block that can be used for production of polyethylene terephthalate, glues and as a food additive. Glyceric acid, a derivative of glycolic acid, is often used as a facial peel and chemical exfoliant.
Prior to the completion of their project, it wasn’t fully understood how water could play a role in the oxidation catalysis of alcohols, according to Davis. He adds that because petroleum and many petroleum-derived products aren’t water-soluble, water wasn’t generally considered to be an effective solvent. With that in mind, Davis and his team embarked on a mechanistic study to understand and demonstrate the role of hydroxyl groups in water for the oxidation process.

“We saw the important role water had as a contributor to the oxidation reaction, specifically oxidation of carbon monoxide to form carbon dioxide via a gas phase reaction,” Davis says. “Since water played a key role, we decided to start looking at the role of gold in liquid water. That’s important for biorefining because the molecules from renewable resources are water soluble, which is different than standard petroleum refining processes that aren’t water soluble.”

Davis says the research team has studied the oxidation reaction in semi-batch and fixed-bed reactors. With the ability to recover the catalyst, one might think it would be easily scalable, but there are a few glaring issues. “The capital cost of using gold is very high,” Davis says. “The other issues are that the acids are actually salts of the acid because they’re high pH, so the recovery of the acid isn’t trivial either because you have to neutralize the solution.”

As for further research, Davis says the team intends to refine the process with the absence of a base “because that’s going to be a real cost issue if we have to consume base to do this, and then neutralize with other acids to recover it,” he adds.

Another possibility would be to use air instead of pure oxygen, which he doesn’t foresee being an issue. “Using air in the absence of base would be our future goals and improving selectivities,” he says.

—Bryan Sims