Researchers at MIT have devised a simple, soluble metal oxide system to capture and transform CO2 into useful organic compounds. More work is needed to understand and to optimize the reaction, but this approach could offer an easy and inexpensive way to recapture some of the carbon dioxide emitted by vehicles and power plants, says Christopher Cummins, an MIT professor of chemistry and leader of the research team.
The new reaction, described in an open access paper in the RSC journal Chemical Science, transforms carbon dioxide into a negatively charged carbonate ion, which can then react with a silicon compound to produce formate, a common starting material for manufacturing useful organic compounds. This process relies on the simple molecular ion molybdate: an atom of the metal molybdenum bound to four atoms of oxygen.
Metal oxide catalysts for CO2 transformations are advantageous based on considerations of cost, ease of re-use, and stability, but these advantages come at the expense of our ability to readily characterize such systems at a molecular level of detail. Intrigued by the paucity of soluble transition-metal oxide systems known to react with CO2 in a well-defined manner … we decided to investigate salts of the molybdate dianion in this respect, in order to determine the behavior and mode of reaction (if any) of a simple oxoanion with carbon dioxide as either the potential basis for a new homogeneous catalytic system or as a soluble model for known heterogeneous oxide catalysts.
Accordingly, herein we report the finding that molybdate absorbs not just one but two equivalents of CO2 (the second, reversibly) together with complete characterization including single-crystal X-ray diffraction studies of the resulting mono- and dicarbonate complexes.—Knopf et al.
Scientists have long sought ways to convert carbon dioxide to organic compounds. Noble metals such as ruthenium, palladium, and platinum, which are relatively rare, have proven effective catalysts, but their high price makes them less attractive for large-scale industrial use.
|“Ideally we’d like to develop carbon-neutral cycles for renewable energy, to get carbon dioxide out of the atmosphere and avoid pollution. In addition, since producers of oil have lots of carbon dioxide available to them, companies are interested in using that carbon dioxide as an inexpensive feedstock to make value-added chemicals, including things like polymers.”|
As an alternative, chemists have tried to make abundant metals, such as copper and iron, behave more like one of these powerful catalysts by decorating them with molecules that alter their electronic and spatial properties. These molecules—ligands—can be very elaborate and usually contain nonmetallic atoms such as sulfur, phosphorus, nitrogen, and oxygen.
With most of those catalysts, the carbon dioxide binds directly to the metal atoms. Cummins was curious to see if he could design a catalyst where the carbon dioxide would bind to the ligand instead.
After finding some success with metal complexes consisting of either niobium or titanium bound to ligands consisting of large organic molecules, Cummins decided to try something simpler, without unwieldy ligands.
Molybdate is relatively abundant and stable in air and water. A simple tetrahedron with four atoms of oxygen bound to a central molybdenum atom, molybdate is commonly used as a source of molybdenum, which can catalyze many types of reactions. Until now, no one had studied its interactions with carbon dioxide.
Working with molybdate dissolved in an organic solvent that also contained dissolved carbon dioxide, the researchers found that the ion could bind to to two molecules of carbon dioxide. The first carbon dioxide attaches irreversibly to one of the oxygen atoms bound to molybdenum, creating a carbonate ion.
A second molecule of carbon dioxide then binds to another oxygen atom, but this second binding is reversible, which could enable potential applications in carbon sequestration, Cummins says.
Tetrahedral [MoO4]2− readily binds CO2 at room temperature to produce a robust monocarbonate complex, [MoO3(κ2-CO3)]2−, that does not release CO2 even at modestly elevated temperatures (up to 56 °C in solution and 70 °C in the solid state). In the presence of excess carbon dioxide, a second molecule of CO2 binds to afford a pseudo-octahedral dioxo dicarbonate complex, [MoO2(κ2-CO3)2]2−, the first structurally characterized transition-metal dicarbonate complex derived from CO2.
The monocarbonate [MoO3(κ2-CO3)]2− reacts with triethylsilane in acetonitrile under an atmosphere of CO2 to produce formate (69% isolated yield) together with silylated molybdate (quantitative conversion to [MoO3(OSiEt3)]−, 50% isolated yield) after 22 hours at 85 °C. This system thus illustrates both the reversible binding of CO2 by a simple transition-metal oxoanion and the ability of the latter molecular metal oxide to facilitate chemical CO2 reduction.—Knopf et al.
In theory, the system could allow researchers to create a cartridge that would temporarily store carbon dioxide emitted by vehicles. When the cartridge is full, the carbon dioxide could be removed and transferred to a permanent storage location.
Another possible application would be transforming the carbon dioxide to other useful compounds containing carbon. Cummins and his colleagues showed that the trapped carbon dioxide could be converted to formate by treating silicon-containing compounds called silanes with the molybdate complex.
More research is needed before the reaction can become industrially useful, Cummins says. In particular, his lab is investigating ways to perform the reaction so that molybdate is regenerated at the end, allowing it to catalyze another reaction.
This is a really elegant addition to the carbon dioxide fixation literature because it shows that some really beautiful transformations are achievable without an elaborate ligand system.—Christine Thomas, associate professor of chemistry at Brandeis University, who was not involved in the research
The paper’s lead author is graduate student Ioana Knopf; other authors are former visiting student Takashi Ono, former postdoc Manuel Temprado, and recent PhD recipient Daniel Tofan. The research was funded by the Saudi Basic Industries Corporation; the Spanish Ministry of Education, Culture and Sport; the Spanish Ministry of Economy and Competitiveness; and the National Science Foundation.
Ioana Knopf, Takashi Ono, Manuel Temprado, Daniel Tofan and Christopher C. Cummins (2014) “Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation,” Chem. Sci. doi: 10.1039/C4SC00132J