Artificial photosynthesis: Conversion of carbon dioxide to carbon monoxide

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2012

A new ionic liquid is capable of facilitating the conversion of solar energy into hydrocarbons.

KEY CONCEPTS
• Progress has been slow in developing a process to emulate photosynthesis. One step that has proven to be very difficult is converting carbon dioxide to carbon monoxide.
• An ionic liquid known as EMIM-BF₄ forms a complex with carbon dioxide that facilitates the conversion to carbon monoxide.
• The researchers envision this step as becoming part of a process that involves conversion of solar or wind energy into a liquid hydrocarbon fuel.

A LONG-STANDING GOAL IN DEVELOPING AN EFFICIENT PATHWAY FOR PREPARING BIOFUELS is to emulate photosynthesis, which is nature’s way of efficiently converting sunlight and carbon dioxide to sugars and other hydrocarbons. Attempts to develop an artificial version of photosynthesis have involved obtaining energy from a solar collector or a wind turbine and then using it to convert carbon dioxide into simple hydrocarbons.

Other strategies for developing biofuels have included taking advantage of biosynthetic pathways used by microbes such as bacteria. In a previous TLT article, the mechanism for the first step of a process bacteria used to convert fatty acyl groups to long chain, monounsaturated olefins was discussed (1). The first step involves the condensation of two Coenzyme A fatty acid groups.

Paul Kenis, professor of chemical and biomolecular engineering at the University of Illinois in Champaign, Ill., says, “Progress has been made on the development of solar cells to more effectively capture solar energy, but using that energy to efficiently convert carbon dioxide is still very hard.”

The theoretical potential to reduce carbon dioxide to carbon monoxide is 1.33 volts. Kenis indicates that the typical potential to do this reaction is between 2.1 and 2.2 volts. He adds, “This huge overpotential is due to the inertness of carbon dioxide.”

One of the problems is that the intermediate carbon dioxide radical anion is not very stable and is hard to form. Kenis states that even when it forms, there is a very good chance for a rapid fallback to carbon dioxide.

One option for facilitating the conversion of carbon dioxide is the use of ionic liquids. This class of compounds is defined as a salt with a melting point below 100 C. They are being looked at in a variety of applications including lubrication. In a previous TLT article, silver ionic liquids are discussed as solvents for efficiently separating olefins from paraffin-rich petroleum waste streams (2).

Kenis says, “A seminar on the use of ionic liquids for carbon dioxide capture triggered our thoughts that the chances of forming a complex between the cation of the ionic liquid and carbon dioxide are quite good, thereby possibly stabilizing the radical intermediate.”

A new process using ionic liquids to convert carbon dioxide to carbon monoxide may help accelerate development of artificial photosynthesis. Such a process has now been developed.

EMIM-BF₄
In collaboration with Dr. Rich Masel, who recently retired from the University of Illinois, Kenis and his associates have determined that an ionic liquid known as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄) can be used to facilitate the conversion of carbon dioxide to carbon monoxide. He says, “We chose to work with EMIM-BF₄ because the imidazoliumbased cation component of this ionic liquid forms a complex with the otherwise unstable intermediate radical of carbon dioxide that is involved in this reaction.”

The researchers conducted experiments with a flow cell containing a platinum anode and a silver cathode. The anode compartment contained an aqueous solution of sulfuric acid, and the cathode was operated in the presence of an aqueous solution of EMIM-BF₄. Figure 1 shows an image of the reactor used.


Figure 1. The ionic liquid, EMIM-BF₄ is used in this reactor to effectively convert carbon dioxide to carbon monoxide, which is an important step in the goal of developing an artificial photosynthesis pathway to convert solar energy to hydrocarbons. (Courtesy of the University of Illinois)

As the researchers applied a voltage to the system, they analyzed the gaseous stream flowing away from the flow cell by gas chromatography. Carbon monoxide was detected at an applied potential of 1.5 volts, which means that an overpotential of only 0.17 volts is needed to drive the reaction.

In the absence of EMIM-BF₄, carbon monoxide was not detected until a cell potential of 2.1 volts was reached. The Faradaic efficiency of the process, which is a measure of what percentage of electrons move to carbon monoxide as opposed to the byproduct hydrogen, was measured. Kenis adds, “We found a Faradaic efficiency that is over 96%. Hydrogen formation for the electrolysis of water was found to account for less than 3% of the Faradaic efficiency.

Kenis says, “Overall, EMIM-BF₄ works very well, but there were some problems that needed to be overcome. This ionic liquid was very viscous, so it needed to be diluted with water to a concentration of 18%. A screen needed to be added to the flow cell to protect the platinum anode from being poisoned by the carbon monoxide formed at the cathode.”

Future work will focus on development of a viable approach to scale-up this process. Kenis says, “The current process shows a turnover rate of only 1 per second. Typical commercial processes run at turnover rates between 1 and 10 per second. We would like to see a turnover rate of 50 per second or more.”

Kenis says, “To drive the process, we envision the use of renewable energy from a solar cell or a wind turbine to power artificial photosynthesis. The formed carbon monoxide can be converted into higher hydrocarbons such as liquid transportation fuels or into other building blocks for the chemical industry.”

“Instead of carbon monoxide, key products that can be generated include formic acid or methanol, which are excellent fuels for fuel cells. One can envision converting solar or wind energy, when these sources exceed demand, into these two liquid fuels, and then use these fuels when solar and wind energy do not meet demand to provide additional power. This scenario would start to address the issues related to the intermittent nature of solar and wind energy.”

This work is being done in collaboration with the start-up company, Dioxide Materials, started by Dr. Rich Masel upon his retirement from the University of Illinois. Further information on this research can be found in a recent publication (3) or by contacting Kenis at kenis@illinois.edu.

REFERENCES
1. Canter, N. (2011), “Enzymatic Production of Renewable Fuels,” TLT, 67 (10), pp. 15-16.
2. Canter, N. (2011), “Silver Ionic Liquids: Potential New Solvents,” TLT, 67 (8), pp. 12-13.
3. Rosen, A., Khojin, A., Thorson, M., Zhu, W., Whipple, D., Kenis, P. and Masel, R. (2011), “Ionic Liquid-Mediated Selective Conversion of CO₂ to CO at Low Overpotentials,” Science, 334 (6056), pp. 643-644.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.