Researchers find flexible solutions for gas separation

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 carbon capture or removal process. of impurities from natural gas.

Separating the gases, however, can be energy intensive and expensive.

“For example, when you separate oxygen and nitrogen, you have to cool the air to very low temperatures until they liquefy. Then, by slowly increasing the temperature, the gases will vaporize at different points, allowing one to return to gas and split,” explains Wei Zhang, a professor of chemistry at the University of Colorado Boulder and chair of the Department of Chemistry. “It’s very energy intensive and expensive.”

Most gas separation relies on porous materials through which the gases pass and separate. This too has long presented a problem, because these porous materials are generally specific to the types of gases that are released. Try running any other type of gas through them and they don’t work.

However, in a research published today in the journal scienceZhang and his co-researchers detail a new type of porous material that can accommodate and separate many different gases and is made from common, readily available materials. Further, it combines rigidity and flexibility in a way that allows size-based gas separation to occur at a greatly reduced energy cost.

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

Adding flexibility

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

For several years, Zhang and his research group worked to develop a porous material that introduces an element of flexibility into a joint in an otherwise rigid porous material. This flexibility allows the molecular bonds to oscillate, or move back and forth at a regular rate, changing the accessible pore size in the material and allowing it to accommodate multiple gases.

“We found that at room temperature, the pore is relatively the largest and the flexible linker barely moves, so most of the gases can get inside,” Zhang says. “When we raise the temperature from room temperature to about 50 degrees Celsius), the wobble of the binder becomes larger, causing the effective pore size to decrease, so larger gases cannot enter. If we continue to raise the temperature, there will be more gases returned due to increased oscillations and further reduction in pore size, finally, at 100 degrees, only the smallest gas, hydrogen, can pass through.

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

“It’s a porous material that has a lot of very ordered pores,” he says. “You can imagine it as a honeycomb. Most of it is solid organic material with these regular-sized pores that line up and form channels.”

The researchers used a fairly new type of dynamic covalent chemistry that focuses on the boron-oxygen bond. By using a boron atom with four oxygen atoms around it, they took advantage of the reversibility of the bond between boron and oxygen, which can be broken and reformed again and again, enabling self-correcting, foolproof behavior and leading to the formation of ordered structural frames.

“We wanted to build something adaptable, responsive, adaptable, 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,” says Zhang.

Sustainable solutions

The development of this new porous material took time.

Zhang says, “Making the material is easy and simple. The difficulty was from the beginning, when we took the material and had to understand or clarify its structure—how do the bonds form, how do the angles form within this material, are there two— “Dimensional or three-dimensional. We had some challenges because the data looked promising, we just didn’t know how to explain it (X-ray diffraction), but we couldn’t immediately understand what kind of structure those 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 small molecule model system containing the same reactive sites as those in their material to understand how the molecular building blocks are packed in a solid state and this helped explain the data.

Zhang adds that he and his co-researchers considered scalability in developing this material, since its potential industrial use would require large quantities, “and we believe this method is very scalable. The building blocks are available on the market and are not expensive, so it may be adopted by the industry when the time is right.”

They have applied for a patent on the material and are continuing research with other building block materials to learn the substrate scope of this approach. Zhang also says he sees the potential to collaborate with engineering researchers to integrate the material into membrane-based applications.

“Membrane separations generally require much less energy, so in the long term they may be more sustainable solutions,” says Zhang. “Our goal is to improve technology to meet industry needs in sustainable ways.”