Heating hosts and guests: how guest molecules affect the thermal expansion of its host

What is it about?

Metal-organic frameworks (MOFs) consist of metal ions and organic linkers in a scaffolding-like arrangement. They are heavily researched for their porosity, and ability to soak up small molecules---like a sponge---which leads to applications within catalysis or for the removal of harmful chemicals. But these so-called 'guest' molecules can also change the properties of the its host MOF. This is particularly true for the mechanical properties, i.e. the expanding or shrinking of a materials as a result of changes in temperature or pressure. In this work, we studied the thermal expansion of the MOF PCN-222 with varying amount of guest molecules. How much the MOF expands when temperature is changed depends strongly on the number and possibly also the location of the guests within the pores. We believe that if the guests move from one position to another in the MOF, this can make the MOF contract rather than expand as temperature is changed. Such counterintuitive negative thermal expansion is generally rare, but is fundamentally very interesting.

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Photo by Aaron Burden on Unsplash

Photo by Aaron Burden on Unsplash

Why is it important?

In order to use metal-organic frameworks for applications, it is important to know how they behave as conditions (such as temperature or the amount of guest molecules in the pore) are varied. Moreover, the link between thermal expansion and pore content shows that the flexibility of the MOF is affected by how many guest molecules that are present, and also where they are located!

Perspectives