Carbon dioxide trap and drop

The reduction of greenhouse gas carbon dioxide to a useful chemical industry feedstock material, carbon monoxide, can be catalysed by a ruthenium-substituted polyoxometalate according to a new study. The work holds the promise of our developing a carbon-neutral energy platform.

Alexander Khenkin, Irena Efremenko, Jan Martin, Ronny Neumann, and Lev Weiner of the Weizmann Research Institute in Rehovot, Israel, explain how there are essentially three approaches to delivering a carbon-neutral energy platform nuclear energy, carbon capture, and renewable energy sources. In the latter area, solar energy is the most abundant resource and alternatives to photovoltaic that mimic photosynthesis are a keen focus of much research.

Neumann and colleagues are specifically interested in how carbon dioxide might become a source of chemical energy. Until recently, most research has looked at activating carbon dioxide by hydrogenation or the reaction with nucleophiles to amines and epoxides, which can then serve as industry feedstocks as alternatives to cracked oil. However, the Weizmann team has investigated how the catalytic conversion of carbon dioxide to an "activated" form, carbon monoxide, might work.

In the earlier work on carbon dioxide activation, a wide range of transition-metal coordination compounds based on iron, cobalt, ruthenium, and rhenium have been investigated as photocatalysts for the energy-efficient conversion. The team explains that commonly these reactions not only require a photosensitizer but also suffer from catalyst degradation. As such, polyoxometalate catalysts might offer a more tenable alternative.

Previous researchers have discussed the binding of polyoxometalates to carbon dioxide in other reactions, which suggested to the team that it might be possible to develop this type of catalyst for carbon dioxide activation.

The team has now studied a polyoxometalate with a Keggin structure substituted with ruthenium(III). The structure ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] where ⁶Q is (C₆H₁₃)₄N⁺ can catalyse the photoreduction of carbon dioxide to carbon monoxide with a tertiary amine, such as triethylamine, as a reducing agent. Progress and mechanistic insights were obtained through UV/Vis and nuclear magnetic resonance, and electron spin resonance spectroscopy as well as computational methods using Gaussian 03.

The combined spectroscopic experiments suggest that upon dissolution of the complex in organic solvent, toluene, a dehydrated compound, containing a penatacoordinated centre is formed. When carbon dioxide is added to this species the gas becomes reversibly coordinated and can be removed again by bubbling argon through the solution. Perhaps more importantly, the spectra also reveal the underlying mechanism of how this coordinated species then interacts with triethylamine to reduce the carbon dioxide to carbon monoxide. Details were published online in Chemistry - A European Journal this month.

The team points out that follow-up research will be directed to improving the efficiency of the new reaction system and to substituting sacrificial amines for hydrogen donors that can be regenerated, thus making the process sustainable.