It Breathes but isn't Alive – MIL-53, a Flexible Framework

What does it look like?

The open wine-rack structure of MIL-53(Cr), with chromium atoms in blue, and benzenedicarboxylate linkers in brown and red (hydrogen atoms not shown). The crystal structure data can be found in the original paper describing the structure. Image generated using the VESTA (Visualisation for Electronic and STructural Analysis) software.

What is it? Where did the structure come from?

MIL-53(Cr) is a metal-organic framework made up of chromium atoms linked together by benzenedicarboxylate molecules, forming a three-dimensional wine-rack-like structure. MIL stands for Materials Institut Lavoisier, after the place where the material was first made by C. Serre et al. in 2002.

MIL-53 exhibits a 'breathing' effect, where the structure opens and closes depending upon what's inside the pores. When hydrated, rather than causing swelling of the framework, the water molecules inside have an attractive effect on the walls and actually hold the wine rack closed. Once the water is removed by heating, the flexible wine rack opens and the volume of the structure expands by an impressive 50%.

The '‘breathing' effect in MIL-53 (from C. Serre et al., JACS 2002, image © American Chemical Society).

Similar breathing phenomena have since been found in other frameworks, and can be used for selectively separating different gases, when the structure only opens up for a certain type of molecule.

Tags: MOF   flexible   framework