Metal-Organic Frameworks (MOFs) can be considered to be molecular sponges. In the same way that the pores in a bathing sponge absorb lots of water, these materials use their much tinier pores to absorb vast amounts of gas phase molecules into a relatively small amount of material.
A schematic model of a MOF.
The pores are simply empty space within the MOF structure. A metal organic framework is made of linkers (or spacers) which are carbon (organic) based and cornerstones which are metal based. In the figure shown, the spacers are represented by blue cylinders and the cornerstones by red cubes. The spacers link between the cornerstones in three dimensions but the arrangement leaves a high volume of empty space for the adsorption of gases, observable in the figure.
The ease in which the spacer and cornerstone of the material can be changed makes the total amount of different compounds with different properties nearly infinite. For example, simply increasing the length of the spacer would result in more empty space for the uptake of gas molecules. The connectivity of the linker may be changed e.g. a trigonal linker will connect 3 cornerstones while a linear linker connects only 2. The metal on the cornerstone may also be changed which may result in the cornerstone changing its shape. Using the figure as an example, the cube may become a tetrahedron or indeed any other polyhedron. It is also easy to adjust the chemical groups on the linker so that they interact more strongly with a particular gas, the green balls on a stick are a representation of a functional chemical group on a linker.
It is for this reason that these materials have attracted a lot of attention for their potential application as gas adsorbants to store environmentally or energetically strategic gases. Examples may include the adsorption of carbon dioxide (carbon capture) as a measure to combat global warming and the adsorption and storage of hydrogen for use as an ideal clean energy source.
Our interest lies in the use of MOFs as catalysts. MOFs are commonly referred to for their potential in this application, however the science is young and the number of studies is relatively low. The main problem with MOFs in this application is that their stability under the harsh conditions of catalytic reactions is insufficient. We have made a milestone to overcome this by synthesising one of the most stable MOFs which we named UiO-66. Represented in a similar simple figure as the one shown, UiO-66 would have a dodecahedral (12 sided) cornerstone. One of our main goals is to make this material into an active catalyst by introducing chemical groups on which are known to be catalytically active on to the linkers.
We are also making a similar approach in order to make UiO-66 efficient in absorbing light for use in photocatalysis. The linkers may be modified in such a way that they act as antennae for the harvesting of light. When light is absorbed by a photocatalyst, electrons and “holes” are formed which are available to react with adsorbed molecules. One possible application is in the decomposition of environmental pollutants in the incidence of a chemical spill, for example.
Representations of the UIO-66 framework, shown in different projections.