So you want to make methanol? Start with ruthenium and add some air

Imagine this solution to global warming: sucking all the anthropogenically created carbon dioxide, that chemical residue from fossil fuels sticking to our skies, and using it to create a fuel source! That's the dream partially realized by researchers at the University of Southern California.¹ They've found a way to nudge along a chemical reaction that transforms carbon dioxide from the air directly into methanol, a flexible fuel that can be used for applications from fuel cells to internal combustion engines.

Methanol (CH₃OH) is a common topic of conversation when discussing how to replace fossil fuels with a new form of energy storage. The molecule is versatile: it can be used as liquid fuel in internal combustion engines, it's an important starter for making chemical feedstock used to make plastics or other materials, and it can be produced through a simple reaction between carbon dioxide (CO₂) and hydrogen (H₂). All these advantages lead some experts to propose a methanol economy, in which methanol replaces fossil fuels as the primary transportation fuel or energy storage medium.

The problem is that burning methanol would still release greenhouse gases into the atmosphere (less than other current fossil fuels), unless we could create methanol using the carbon dioxide already in the atmosphere! Then all the carbon dioxide released during methanol combustion in an engine or power plant would not create any net gain of greenhouse gases in the atmosphere.

This idea of a human-made carbon cycle, mimicking how plants use carbon dioxide in photosynthesis, isn't new. Power plants in Reykjavik, Iceland already use geothermal energy to react carbon dioxide with hydrogen to create methanol and water. But in these cases, the carbon dioxide is not taken directly from the air. Instead, the geothermal plant first captures the CO₂, which is then funneled into the methanol production process.

To simplify this method, USC researchers have now developed the first technique to directly react CO₂ in air to create methanol. The secret lies in two major developments. First, researchers chose a new catalyst, the mysterious key to speeding up the rate of converting reactants to products in so many reactions. In this case, they tested several varieties of ruthenium complexes: molecules with a ruthenium atom at their center, surrounded by ligands made of phosphorous, nitrogen, and hydrogen. Second, the researchers used polyamines to capture CO₂ so the catalyst could do its work and foment the reaction to create methanol. Amines are derivatives of ammonia and contain high amounts of nitrogen, which are important sites to attract and absorb carbon dioxide.

With these advantages in place, the researchers injected air into a solution of the polyamine and catalyst (known as 'bubbling air'). After heating the solution up to about 125-165 degrees Celsius, they ended with a 79% yield of methanol. This percent is the amount of actual yield divided by the theoretical yield predicted by the stoichiometry of the reaction. This high yield should be seen as a success for a first attempt at direct CO₂ conversion to methanol!

Now that the proof-of-principle of this method has been established, next steps will involve improving the yield and understanding why the ruthenium catalyst works so well. This will require further experiments as well as computational modeling of the catalyst and its interaction with CO₂ and hydrogen. Such modeling provides an atomic view of how the carbon dioxide, hydrogen, catalyst, and amine all interact together to create a fast reaction. Changing certain components could reduce the reaction barrier and provide ways of improving the catalyst to further speed up the reaction rate.

These findings present a hopeful future of energy storage after the fossil fuel era. We could create our transportation fuel by essentially scrubbing the air for CO₂! Such technology provides a solution not only for the search for new liquid fuels but also to limit the amount of carbon dioxide we continue to release into our atmosphere.

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