Researchers have found a flexible solution for gas separation

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

However, separation gases can be energy intensive and expensive.

“For example, to separate oxygen and nitrogen, you need to cool the air to very low temperatures until it liquefies. Then, by slowly increasing the temperature, the gases will vaporize at different points, allowing one to become a gas again and separate.” ” explains Wei Zhang, professor of chemistry at the University of Colorado Boulder and chair of the chemistry department. “It’s very energy-intensive and expensive.”

Much of gas separation depends on the porous materials through which the gases pass and are separated. This has also long been a problem because these porous materials are generally specific to the types of gases being separated. Try running any other type of gas through them and they don’t work.

However, in research published today in the journal ScienceZhang and his co-researchers detail a new type of porous material that can hold and separate many different gases and is made from common, readily available materials. Furthermore, it combines stiffness and flexibility in a way that enables size-based gas separation at significantly lower energy costs.

“We’re trying to improve the technology,” 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 were rigid and affinity-based – specific to the types of gases being separated. The stiffness allows the pores to be well defined and helps direct the gases during separation, but also limits the number of gases that can pass through the different size molecules.

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

“We found that at room temperature, the pores are relatively largest and the flexible linker barely moves, so most of the gases can get in,” Zhang says. “When we increase the temperature from room temperature to about 50 degrees (Celsius), the oscillation of the linker increases, which causes the effective pore size to decrease, so larger gases cannot get in. If we keep increasing the temperature, more gases are produced. He turned away because of the increased oscillation and further reduced pore sizes. Finally, at 100 degrees, only the smallest gas, hydrogen, can pass through.”

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

“It’s a porous material that has a lot of highly ordered pores,” he says. “You can think of 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 targets the boron-oxygen bond. Using a boron atom with four oxygen atoms around it, they took advantage of the reversibility of the boron-oxygen bond, which can break and reform again and again, enabling error-resistant self-repairing behavior and leading to the formation of structurally ordered frameworks.

“We wanted to create something with tunability, with responsiveness, with adaptability, and we thought that the boron-oxygen bond might be a good part 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 says, “Making the material is easy and simple. The difficulty was at the very beginning when we first got the material and we needed to understand or clarify its structure—how the bonds are formed, how the angles are formed in this material, it’s two-dimensional or three-dimensional We had some problems because the data looked promising, we just didn’t know how to explain it, certain peaks (X-ray diffraction), but we couldn’t immediately tell 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 a model system of small molecules containing the same reactive sites as in their material to understand how the molecular building blocks are packed in the solid state, and this helped explain the data.

Zhang adds that he and his co-researchers considered scalability when developing this material because its potential industrial use would require large quantities, “and we believe this method is highly scalable. The building blocks are commercially available and inexpensive, so it could be adopted industry when the time is right.”

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

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