Cage of Cages: New Type of Porous Material Shows Potential in Trapping Carbon Dioxide and More Potent Greenhouse Gasses

Gasses in the atmosphere, like carbon dioxide, are known to trap heat similar to the glass roof of a greenhouse. Known as greenhouse gasses, these compounds have far-ranging environmental effects by causing extreme weather, increased wildfires, and disruptions in food supply.

Potent Greenhouse Gas

Carbon dioxide, water vapor, and methane are known to be the most significant greenhouse gasses in the Earth's atmosphere. There are many other greenhouse gasses and sulfur hexafluoride (SF6) is the one with a very high radiative forcing effect.

While carbon dioxide is known to linger in the atmosphere for 5 to 200 years, sulfur hexafluoride can stay there between 800 to 3,200 years. This means that even if the levels of sulfur hexafluoride in the atmosphere are much lower, its long lifetime gives a global warming potential that is about 23,500 times that of carbon dioxide when compared over a century.

Eliminating large amounts of sulfur hexafluoride and carbon dioxide from the atmosphere or stopping them from entering it in the first place is considered an important step in fighting climate change. It is estimated that experts need to absorb about 20 billion tons of carbon dioxide each year to reverse our carbon emission, which continues to trend upwards.

So far, strategies to remove carbon dioxide can only eliminate 2 billion tons per year, mostly done by trees and soils. New technologies, like direct air capture, remove about 0.1% of carbon or 2.3 million tons per year using porous materials that trap carbon dioxide from the air.

Scientists try to devise more materials to improve direct air capture and make it more efficient and less energy-intensive. However, in order to prevent the worst effects of climate change, we need to eliminate greenhouse gas emission faster than these technologies can. Creating materials with high structural complexity is not easy, even if their precursor molecules have the ability to assemble themselves technically.

First-of-Its-Kind Molecule

In a recent study, researchers from the UK and China synthesized the novel material, called the [4[2+3]+6]cage molecule, in two steps. The reactions assemble triangular prism building blocks into larger, more symmetrical tetrahedral cages. The scientists claim that they produced the first molecular structure of its kind.

Using a strategy called supramolecular self-assembly, the team created chemically interlocked structures from simpler building blocks. However, it needed to undergo a fine-tuning process since the best reaction conditions are usually not obvious. As the final molecules become more complex, it becomes more difficult for experts to synthesize them.

In order to handle the invisible molecular interactions, the research team used simulations to find out how the started molecules would assemble into a novel type of porous material. They considered several factors such as the chemical stability and rigidity of the final material as well as the geometry of potential precursor molecules.

The resulting material has abundant polar molecules which enable it to attract and hold greenhouse gasses with strong affinity. Additionally, it demonstrated excellent stability in water, a critical feature for its use in absorbing carbon in industrial settings.

The experiments also suggest that the new cage-like material had a high uptake of sulfur hexafluoride (SF6), although this claim is not tested at scale. According to the U.S. Environmental Protection Agency, sulfur hexafluoride is the most potent of all greenhouse gasses.