Electrochemical Route Improves Acetic Acid Synthesis

Researchers at Rice University, Houston, have come up with an energy efficient way to directly convert carbon monoxide (CO) into acetic acid with higher levels of purity. Their electrochemical process also reduces the steps required to purify the product. In addition, it may suit production of other compounds from CO and carbon dioxide (CO₂).

“People traditionally produce acetic acid in liquid electrolytes, but they still have the issue of low performance as well as separating the product from the electrolyte,” notes Haotian Wang of Rice’s Brown School of Engineering. He, along with colleague Thomas Senftle, developed a continuous reactor that uses renewable energy and nanoscale cubes of copper (Cu) as the primary catalyst coupled with a porous solid-state electrolyte (PSE).

The catalyst’s ability to maintain high performance is of great importance for practical applications, note the researchers. In 150 hours of continuous lab operation, the device produced up to 2% acetic acid solution in water. The acid component was up to 98% pure, an improvement compared to earlier attempts to catalyze CO into liquid fuel, the researchers say. Furthermore, a post-catalysis scanning-electron-microscopy characterization revealed that the Cu nanucube structure is well maintained. “A 150-hr CORR [carbon monoxide reduction reaction] electrolysis was continuously and stably operated under 150 mA cm-2 current, maintaining an acetate relative purity of above 90 wt.% with negligible degradations in selectivity and activity,” adds Wang.

Carbon Monoxide Conversion

Figure 1. Rice University engineers have developed a reactor to produce liquid acetic acid directly from carbon monoxide. The reactor uses a catalyst of copper nanocubes and a solid-state electrolyte. Source: Peng Zhu, Rice University.

An article in the Proceedings of the National Academy of Sciences contains more detail.

The next step is to develop a scalable system, but creating a larger PSE reactor first requires addressing a sealing problem. “It is necessary to ensure there is no leakage, otherwise it may result in [a] short circuit,” explains Wang. “Another problem may be the heat exchange. The reactor will generate resistance heat in the production process, so heat exchange equipment is needed to reduce the influence of heat on the membrane and catalysts.”

On the other hand, scaling-up the catalyst should be fairly straightforward, believe the researchers. “The synthesis of the copper nanocube catalyst is ligand-free and controllable. It took around an hour for the reaction process, followed by the further centrifuge process. And the yield-to-feed ratio of the catalyst was about 80%, so it could be easily produced on a large scale,” elaborates Wang.

While the team did not focus on the catalyst’s susceptibility to poisoning, Wang points to a previous study that indicates negligible long-term impact of nitrogen oxides on catalytic performance. “The selectivity will recover once a pure carbon dioxide feed is restored. However, sulfides oxides can permanently poison the catalyst and affect its performance,” he cautions.

Because the PSE membranes are commercially purchased, their stability is very high and guaranteed under current conditions, the researchers report. “We have successfully demonstrated an over 100-hr stability test to generate 11,000 ppm H₂O₂ using solid electrolyte. Even under some extreme conditions, for example, directly using humidified N₂ instead of DI [deionized] water, it also works very well. More efforts are needed towards improved chemical stability of catalysts and the contact of anion exchange membrane (AEM) and solid electrolyte, especially producing acids,” they say.

The approach also holds promise for producing pure streams of other C₂ liquids. “Methanol and ethanol are also high-value and high-energy-density liquid fuel and could be very attractive target products using our PSE reactor to generate. How to syntheses electrocatalysts that can highly selectively reduce CO₂ into methanol or ethanol is our further works. We have already started working on this project. On the other hand, further work should also focus on optimizing the solid electrolyte, such as further improving the performance like crossover efficiency and energy efficiency,” concludes Wang.