Artistic depiction of guest molecules trapped within the pores of a MOF. Image: Institute of Industrial Science, The University of Tokyo.
Artistic depiction of guest molecules trapped within the pores of a MOF. Image: Institute of Industrial Science, The University of Tokyo.

The ‘nothing’ (empty space) feature of soft porous materials is central to their properties and applications, and has been a focus of study for many years. Now, researchers from Japan have solved a long-standing puzzle regarding these materials that will speed up research and development, and improve their utility in practical devices.

The researchers from the Institute of Industrial Science (IIS) at the University of Tokyo have revealed the importance of a physical property – elastic heterogeneity – for tuning the molecular adsorption/desorption properties of a common class of soft porous materials known as metal–organic frameworks (MOFs). They report their findings in a paper in the Proceedings of the National Academy of Sciences.

MOFs, a focus of research since the 1990s, are sponge-like materials. They are highly tunable, meaning their size, shape, and composition can be tailored by changing the metal ions and organic linkers used to build them. This tuning can ultimately alter their properties, such as their mechanical flexibility. Furthermore, upon adsorbing molecules (known as guests), the crystal structure of the MOFs (known as hosts) can undergo nonuniform shape changes, termed elastic heterogeneity. The resulting stimuli-responsiveness and guest-adsorption selectivity of MOFs has led to their use for applications such as sensors, supercapacitors and drug delivery.

To improve the design of advanced MOFs, researchers have long sought to understand the link between the macroscopic property of elasticity and the corresponding microscopic host–guest interactions. The IIS researchers looked to computational simulations to provide this understanding.

"Our simplified statistical mechanical model reveals the role of guest adsorption and desorption in elastic heterogeneity," explains Kota Mitsumoto, lead author of the paper. "We mathematically link lattice expansion and contraction to the energetics and thermodynamics of the host–guest interactions."

The simulations focused on two different compositional regions, or domains, within MOFs: guest-adsorbed, which are harder; and guest-desorbed, which are softer. The researchers' main finding was that the shape of these domains depended on the difference in the elastic stiffness between the domains. Guest adsorption corresponded to compact domains, whereas guest desorption corresponded to flattened domains.

"We deduce the entropic and energetic contributions to the transition between guest adsorption and desorption," says Kyohei Takae, senior author of the paper. "Thus, we provide physicochemical insight into the origin of elastic heterogeneity within MOFs and analogous materials."

This work has implications for conferring targeted properties on soft porous materials. For example, compact domains can facilitate strong guest confinement, and thus could prove especially useful for gas storage. Alternatively, flattened domains can increase the surface area of MOFs, and could thus facilitate chemical reactions.

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