Researchers find flexible solution to separate gases

For a wide range of industries, gas separation is an important part of the process and product: from separating nitrogen and oxygen from air for medical purposes to separating carbon dioxide from other gases in the process of carbon capture or removing impurities from natural gas.

Gas separation, however, can be both energy-intensive and costly.

“For example, to separate oxygen and nitrogen, you have to cool air at a very low temperature until they liquefy. Then, as you slowly increase the temperature, the gases evaporate at different points, allowing one of them to become a gas again and separate,” says Wei Zhang, a professor of chemistry at the University of Colorado Boulder and chair of the chemistry department. “It’s a very energy-intensive and expensive operation.”

Much of gas separation relies on porous materials through which gases pass and are separated. This, too, has long been a problem, because these porous materials are generally specific to the types of gases separated. Try sending other types of gas through them and they don’t work.

However, in a study published today in the journal ScienceZhang and his fellow researchers have described a new type of porous material that can accommodate and separate many different gases and is made from common, readily available materials. Moreover, it combines rigidity and flexibility in a way that allows the separation of gases based on their size, at a significantly reduced energy cost.

“We’re trying to improve the technology,” Zhang says, “and improve it in a way that’s scalable and sustainable.”

Add flexibility

For a long time, porous materials used in gas separation have been rigid and based on a specific affinity to the types of gases being separated. The rigidity allows the pores to be well defined and helps direct the gases into the separation, but also limits the number of gases that can pass through due to the different molecule sizes.

For several years, Zhang and his research group have been working to develop a porous material that introduces an element of flexibility into a binding node in an otherwise rigid porous material. This flexibility allows the molecular linkers to oscillate, or move back and forth at a steady rate, changing the size of the accessible pores in the material and allowing it to accommodate multiple gases.

“We found that at room temperature, the pores are relatively larger and the flexible linker barely moves, so most gases can penetrate,” says Zhang. “As we increase the temperature from room temperature to about 50 degrees (Celsius), the oscillation of the linker becomes greater, which causes the effective pore size to decrease, so that larger gases cannot enter . If we continue to increase the temperature, more gases are released.” diverted due to increased oscillation and further reduced pore size. Eventually, at 100 degrees, only the smallest gas, hydrogen, can pass through.

The material developed by Zhang and his colleagues is made of small organic molecules and is very similar to zeolite, a family of porous crystalline materials composed primarily of silicon, aluminum and oxygen.

“It’s a porous material that has lots of very ordered pores,” he says. “You can think of it like a honeycomb. Most of it is solid organic material with these regularly sized pores that line up and form channels.”

The researchers used a relatively new type of dynamic covalent chemistry focused on the boron-oxygen bond. Using a boron atom surrounded by four oxygen atoms, they took advantage of the reversibility of the bond between boron and oxygen, which can break and reform again and again, enabling self-correcting behavior and error-free and leading to the formation of structurally ordered frameworks.

“We wanted to build something with tunability, responsiveness and adaptability, and we thought the boron-oxygen bond might be a good component to integrate into the framework we were developing, because of its reversibility and flexibility,” Zhang says.

Sustainable solutions

The development of this new porous material took time.

Zhang explains: “Making this material is easy and simple. The difficulty came at the very beginning, when we first got the material and had to understand or elucidate its structure: how bonds are formed, how angles are formed within this material, whether it is two-dimensional or three-dimensional. We ran into some difficulties because the data looked promising, but we didn’t know how to explain it. It showed certain peaks (X-ray diffraction), but we couldn’t immediately tell what kind of structure these peaks corresponded to.”

So he and his research colleagues took a step back, which can be an important but little-discussed part of the scientific process. They focused on the model system of small molecules containing the same reactive sites as those in their material to understand how the molecular building blocks are stacked in a solid state, which helped explain the data.

Zhang adds that he and his fellow researchers considered scalability in developing this material because its potential industrial uses would require large quantities, “and we believe this method is highly scalable.” The basic elements are commercially available and inexpensive, so it could be adopted by industry when the time is right. »

They have filed a patent on this material and are continuing their research with other basic materials to discover the scope of this approach in terms of substrate. Zhang also says he sees potential in partnering with engineering researchers to integrate the material into membrane-based applications.

“Membrane separations typically require much less energy, so they could be more sustainable solutions in the long term,” Zhang says. “Our goal is to improve the technology to meet the industry’s needs in a sustainable way.”