By dotting the pores of a zeolite with individual rhodium atoms, researchers have come up with a catalyst that couples methane, carbon monoxide, and oxygen to produce acetic acid at low temperature. The findings, which the team reported last week at the ACS national meeting in New Orleans, could lead to a low-cost, low-energy catalytic process for manufacturing the organic acid.
The chemical industry produced about 14 million metric tons of acetic acid in 2017 for use as a reagent to make monomers and other compounds, as well as for use in the medical and food industries.
Manufacturers produce acetic acid through a number of commercial processes, including methanol carbonylation, which couples CO to the alcohol. Methanol is typically produced from a mixture of CO and hydrogen known as synthesis gas, which in turn is made from coal or methane. Some of those industrial processes run at temperatures as high as 1,000 °C.
Franklin (Feng) Tao of the University of Kansas reported that he and his coworkers used a wet chemical process and ion-exchange chemistry to deposit isolated rhodium cations inside the micropores of ZSM-5, an aluminosilicate zeolite. On the basis of X-ray absorption spectroscopy and other methods, the team concluded that the catalyst takes the form of anchored RhO5 species.
The team evaluated catalysts with a range of rhodium concentrations by first dispersing them in water and then delivering the suspensions to a reactor charged with methane, CO, and oxygen at various partial pressures. The reactions produced a mixture of acetic acid, formic acid, and methanol. By optimizing the conditions, the team boosted the selectivity for acetic acid to 70% at just 150 °C. Under those conditions, the anchored catalyst was more than 1,000 times as active as solution-phase rhodium cations.
To understand how the catalyst does its job, the team used isotope labeling and other experimental methods coupled with computational analysis. The researchers proposed a complex 18-step mechanism, which generated a lively discussion at the presentation.
“The mechanism is quite convincing and seems consistent with the data,” remarked Aditya (Ashi) Savara, a symposium coorganizer and catalysis specialist at Oak Ridge National Laboratory. “The selectivities, activities, and reaction rates under those conditions are surprising and impressive.”
Georgia Tech’s Meilin Liu also praised the study, noting that rationally designed catalysts like the ones described by Tao may benefit other catalytic processes by considerably reducing energy consumption and cost.