The dynamic nature and flexibility of molecular sieves is crucial to understanding their performance for the transport of small molecules. Image: University of Liverpool.
The dynamic nature and flexibility of molecular sieves is crucial to understanding their performance for the transport of small molecules. Image: University of Liverpool.

Researchers at the University of Liverpool in the UK and the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have reported some exciting findings relating to metal-organic frameworks (MOFs), a class of porous materials, that could benefit a wide range of important gas separation processes. They report these findings in two research papers.

Metal-organic frameworks (MOFs) are a relatively new class of porous, crystalline materials with a broad range of applications. Some MOFs can act as a molecular sieve, allowing one type of gas molecule from a mixture to pass through while blocking the others. For example, it is known that some MOFs can separate propylene from propane, an important process in the manufacture of polypropylene plastics, for which high purity propylene is required.

In the first paper, published in Nature Communications, the researchers report that, unlike a kitchen sieve, these three-dimensional molecular sieves can change their pore shape and that their flexibility is vital for this performance.

The researchers uncovered these findings using computational modelling supported by experimental X-ray data. For one high-performing MOF, called KAUST-7, they found that structural changes in the MOF triggered by the presence of the propylene and propane gas molecules are qualitatively different. This results in stronger adsorption and faster transport of propylene, thus essentially sieving propane molecules out.

However, it is hard to predict which other kinds of MOFs possess this functional flexibility and therefore might also be good for a given gas separation. This is because their performance is controlled by specific molecular interactions that are hard to anticipate or identify experimentally.

In a second paper, published in Physical Chemistry Chemical Physics, the researchers focus on this challenge. They developed a computational screening approach to assess over 4000 previously reported MOFs for their flexibility when acting like a molecular sieve. Using this approach, they identified the top four MOFs with potential for separating propylene from propane – two of them are already known to have a good separation ability while the other two have not yet been tested experimentally for this application.

"MOFs have attracted considerable interest in recent years and there are great hopes for technical applications, especially for flexible MOFs," said Matthew Dyer, a lecturer in chemistry and part of the University of Liverpool's Leverhulme Research Centre for Functional Materials Design. "Our research adds to our knowledge of MOFs, why some are able to act as sieves and which ones show flexibility.

"Using a computational approach, we are able to identify flexible MOFs and these findings have the potential to make the process of purifying gases more energy efficient. This is important in terms for the manufacture of high-quality plastics which need pure starting compounds that are commonly extracted from gaseous by-products in petrochemical processing.

"Such high-throughput screening approaches can be applied to many different materials with varying potential applications. They have the potential to change the way that we find materials to meet technological challenges."

This story is adapted from material from the University of Liverpool, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.