Caging greenhouse gases

“One kind of porous material is made up of metal-organic polyhedra (MOPs), which are large molecules shaped like small cages. The cages can be used to trap smaller molecules, and can be tweaked in various ways. Shuhei explains how the arrangement of cages within the material can affect how good they are at storing and releasing other molecules.

"Broadly speaking, when we make these cages there are two different types of bonds at work holding our solids together", he says. "One set of bonds is strong and we use them to build our individual cages – this keeps the cage intact and allows us to exploit the pores. The second set of bonds is much weaker and forms when the cages come into close contact with each other, like Velcro®. This Velcro®-like behaviour has a big advantage: in the same way that you can pull Velcro® apart and stick it together again, we can separate our individual molecules and stick them back together again into different arrangements."

"In our paper, we show that how we stick the cages back together is very important for how they capture gas molecules. For one single cage, we can make two different arrangements. One is rigid, and carbon dioxide molecules simply enter the material until it is full. The other is flexible, and the material actually responds to the entry of carbon dioxide – once we reach a certain pressure of carbon dioxide, the cages rearrange to provide a lot more space for the incoming gas, and the material has a much higher capacity for gas storage. This type of rearrangement and increase in the capacity of the porous material is known as gate-opening behaviour.

"Our paper is significant for two main reasons. Firstly, because it is the first time that this behaviour has been observed for MOPs, and secondly because we show that one type of cage can show both rigid and flexible behaviour. This means that gate-opening behaviour could be widespread amongst MOPs and is waiting to be revealed."