A team of researchers from Oregon State University (OSU) and the University of Delaware (UDel) have discovered a more-efficient and less-expensive chemical process that uses highly selective metal oxides as catalysts.
“This work was inspired by our research on the conversion of biomass, such as wood and agricultural residues, into fuels and commodity chemicals. We wanted to understand the principles of biomass conversion using oxide-based catalysts, which previous studies had suggested were selective catalysts,” says Konstantinos Goulas, assistant professor of chemical engineering in the OSU College of Engineering.
Because oxides are abundant and relatively inexpensive, the team compared how fast specific chemicals can be made on a variety of metal oxide catalysts. The results provided insight into what properties generate the best metal-oxide catalysts. Their findings have been published in Nature Catalysis.
“We measured the activity of a series of oxides for the hydrodeoxygenation (HDO) reaction and found that this trends with Gibbs free energy of formation of the oxide (ΔGfo — a property tabulated in various databases),” notes Goulas. “This trend looks like a so-called volcano curve: it rises to an apex at approximately ΔGfo = -1 eV, corresponding to IrO2, and then falls down as the oxides become completely reduced to the unselective metallic form due to the presence of hydrogen in the reaction. This tells us that if you want to run an HDO reaction efficiently, your catalyst needs to be an oxide that’s reducible (i.e. has a ΔGfo that is not too negative), but not too reducible (so it does not become a metal during reaction). In some ways, this is analogous to dependence of the hydrodesulfurization reaction over sulfides to the properties of the sulfide.”
“Regarding the opportunities this offers us, one could think of many reactions,” he adds. “One example could be methane and alkane oxidation in general (for miniature power devices). Another one is the oxidation of CO and volatile organic compounds in the context of pollution abatement from stationary and mobile sources. Last but not least, this could apply to systems like the oxidative dehydrogenation (ODH) of hydrocarbons, such as propane and propylene, for the production of propylene and acrolein, respectively. It all boils down to ease of removal of oxygen atoms from a catalyst surface!”
“When we started this investigation, we had a good understanding of the mechanism of the HDO reaction over RuO2. However, we weren’t sure if the mechanism would be the same over other oxides. Moreover, we wanted to discover new catalysts comprised from elements cheaper and more earth-abundant than Ru. I should emphasize here that this was a team effort, in the context of a DOE EFRC, comprising researchers from many institutions. Based on our investigations, our colleagues from University of Pennsylvania discovered that NiCu alloy catalysts are very selective and active for the HDO reaction. This was important, because Ni and Cu are both earth-abundant elements. Our subsequent investigations, using X-ray absorption spectroscopy, showed that this is due to the formation of an oxide!”
The team hasn’t yet determined commercial viability: “While I would love to see this system commercialized, the techno-economic analysis questions are beyond my expertise. The ultimate goal of this process is the production of biomass-derived para-xylene (pX) and terephthalic acid,” says Goulas.
Goulas’ colleagues have been investigating these issues and have recently published papers comparing favorably the production of pX from biomass to its production from petroleum.
“Regarding the further development of these systems, I would say that answering questions related to reaction engineering and process intensification is the way to go right now. I believe there are efforts underway at UDel to address these questions. My own research seeks to apply lessons from this study to the decomposition of volatile organic compounds and other atmospheric pollutants. We are currently ramping up efforts with a graduate student and also collaborating with Greg Herman, a surface analysis expert, here at Oregon State,” he says.