Thu. Dec 1st, 2022
Dump truck full of coal drives through strip mining area.

Given all the celebrations, I think we all deserve some punishment. Why else would I force chemistry on you? Chemistry is like your car’s suspension system; it’s unnoticed and unloved, but it allows all the cooler parts to do their thing, as does the chemistry behind the development of a better carbon dioxide sponge.

One of the amazing (for me at least) developments in chemistry was metal-organic frameworks. These frameworks form both open spaces – they allow small molecules in – and closed spaces – the trapped molecules behave as if they were trapped in a crystal. This combination opens up a world of possibilities.

Through the current research I have been able to discover that the possibilities are also increased by the fact that you can also make these frameworks without the metal.

Metal-Organic Frameworks

The carbon chemistry that makes up life has a backbone of carbon-carbon bonds that are really strong. This is what makes plastic so hard to break down. But many of those bonds still have the freedom to rotate, and different parts of the molecule can be attracted to each other through things like charge differences. The end result is that organic molecules tend to fold up, creating quite closed structures.

Metal-organic frameworks helped keep them open. Essentially, each metal atom gives a fixed structure to the organic molecules, which have to bond in a specific way. The distance between the metal atoms is in turn determined by the length of the organic molecule. With the right organic molecule, these frameworks form very open structures.

The result is that metal-organic frameworks have an incredible amount of surface area for a given volume and a very low density. They are like no natural product on earth.

What I didn’t realize until recently was that, if you pay close attention, you could create these relatively open structures without the metal: a completely organic framework. The advantage of a purely organic framework is that you can use the variability of the shape and structure of organic molecules to change the shape of the internal structure. The researchers used a few organic molecules to create a crystal structure with a regular series of corkscrew-shaped tunnels running in parallel. These openings are quite large and can accommodate molecules such as carbon dioxide and nitrogen.

But when the researchers put the material in a mixture of carbon dioxide and nitrogen, 600 times more carbon dioxide entered the framework than nitrogen.

Charging is everything

To understand this difference, the researchers performed calculations on the molecular structure of the framework and on how carbon dioxide moves within the structure. They found that the organic molecules had charges at fixed locations in the spiral. A carbon dioxide molecule was just the right length to be held slightly in place by the charges. But the spiral also means that when the carbon dioxide shakes loose and spins (as it will when released), it can’t easily return to its previous location. Instead, it has to shuffle around the corkscrew to the next matching set of charges. On average, a carbon dioxide molecule will make about a million of these shear steps per second.

The direction in which the carbon dioxide ends up (on average) is determined by the concentration gradient: carbon dioxide goes to the place where there is less carbon dioxide. This causes the structure to absorb carbon dioxide.

Under ideal conditions, a single unit cell (which is the smallest unit that can be repeated to create the entire framework) can contain eight carbon dioxide molecules. The researchers also tested how quickly it absorbs carbon dioxide from flue gas. At the outlet of a furnace, the carbon dioxide concentration is typically about 15 percent, with the majority of the remaining gas being nitrogen. Under these conditions, the corkscrew frame can absorb 39 mg of carbon dioxide per gram of sponge. This is as good or better than most other materials available (including various metal-organic frameworks). However, if the total gas pressure is lowered further, the amount of carbon dioxide absorbed shoots up to 59 mg per gram.

Does this mean we are one step closer to efficient carbon capture? I honestly don’t know. The problem is probably one of scale. Coal plants burn hundreds of tons of coal per hour, which means it takes a huge amount of sponge to soak up all that carbon dioxide. And that must be done without killing the gas flow. Not only that, the sponge needs to be refreshed. That seems like a tough problem that requires an excellent sponge along with a lot of other technical development.

Chemical Sciences2018, DOI: 10.1039/C8SC04376K

By akfire1

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