Study advances understanding of membrane permeability

By John Butcher

Undergraduate student Yue Yu, first author of the research paper (left) and Kai Zhang, assistant professor of chemistry at Duke Kunshan (right)

A Duke Kunshan professor-student team has published research into the permeability of gases through membranes, which could have implications for a range of industrial processes, including removing pollution from factory emissions and purifying oxygen from the air.

The theoretical work, conducted by professor Kai Zhang and undergraduate student Yue Yu (first author), produced a simplified model and simulation in order to understand the mechanisms by which a gas passes through a membrane. It is a step towards improving the design of membrane materials that allow certain gases through rapidly while blocking others, such as carbon dioxide.

“Membrane separation is an energy-efficient way to extract some substances from others with a multitude of industrial applications,” said Zhang, an assistant professor of chemistry at DKU. “Our goal was to maximize permeability at the highest possible level of purity by filtering out undesired gas particles.”

Commencing in 2019, the pair started the research process on paper, with Yu developing a simplified model of spherical gas particles passing through a porous membrane, which they later coded into a supercomputer at Duke Kunshan and Duke University in the United States. Yu, who described being first author of the research paper as a “milestone” in his life, then ran a set of simulations to test how the model worked, before discussing the results with Zhang.

“The membrane has pores in it for gas to pass through, in this case cylindrical pores. Interaction between the spheres and membrane is collision driven, like billiard balls going into table pockets,” said Zhang.

The researchers were interested in two factors: permeability, which is how fast gas particles pass through a membrane; and selectivity, which is the purity of those particles. Their aim was to get purity as high as possible (allowing only the desired gas through) while losing as little permeability as they could.

“Often there in is a trade-off between permeability and selectivity,” said Zhang. “If we make the pores large any gas can go through easily. If we make them small, we slow down the permeability but gain more purity.”

Using the model they calculated quantitatively the permeability and selectivity of the membrane for the separation of gas mixtures in order to understand its qualities and create a benchmark for the design of other isoporous materials.

Zhang and Yu’s research is theoretical and computational but could provide a better understanding of how to improve the design of real membranes used in industries where gases or other substances need to be separated. Examples include the separation of carbon dioxide from other gases emitted in an industrial process to prevent it being released into the atmosphere or in water purification.

“Industry is trying to build improved membranes as this will be energetically more efficient and economically more profitable,” said Zhang. “Our work is trying to show the fundamental mechanisms underlying permeability and selectivity, as well as to provide certain guiding principles for membrane design.”

The next stage in Zhang’s research will be to move beyond the basic model to one that is more true to life.

“We have been looking at the simplest model, spherical gases and cylindrical pores, and only collision interactions, but in reality the interactions are more complicated. Most gas molecules are not spheres,” he said.

“For example, for oxygen or carbon dioxide, they are more rod-like, so they have certain rotational effects. The next step is to make the model more realistic, to integrate more features that could affect the results.”

For Yu, the next step is postgraduate study, taking with him the technical and practical skills he learned during the research project.

“I learned a lot of technical skills, but also how to use them in practical scientific research, and how to combine computer programming, mathematics and physics, this interdisciplinary combination,” he said.

Through long discussion with Zhang, he also picked up “critical thinking skills,” he added.

“To be a scientist you really have to be able to think about solutions yourself, to get around issues on your own, that is something he repeatedly taught me, especially later on in the experiment.”

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