Rare-Earth Metal Organic Framework Materials for Hydrogen Storage.

As the world grapples with the rising cost and diminishing reserves of fossil based fuels, there is an urgent need to resolve the issues surrounding the use of drogen as a fuel. Storage is one of the issues which limit full exploitation of hydrogen in this regard. While it can be stored in pressurized cylinders as a gas and as a liquid under cryogenic conditions, the volume restrictions associated with on-vehicle use requires that hydrogen be stored at densities higher that its liquid density. To this end various chemical storage methods are being explored. The high porosity and large surface area inside the open framework structures of metal-organic framework materials (MOFs) (Figure 1) make them suitable as molecular containers for trapping and storing small guest molecules such as hydrogen. MOFs which are based on rare-earth metals offer advantages such as the ability to trap and simultaneously detect the type of guest that is present in the pores by producing a light signal which is specific to the guest.

Figure 1: Diagrammatic representation of a Metal-Organic Framework Material. Note the open spaces in the MOF which can be used to trap and store H2.

We have used hydrothermal synthesis to prepare rare-earth containing MOFs whose structures display the potential for hydrogen storage.

 H2(g)

Figure 2: Example of a rare-earth containing Metal Organic Framework Material made in SinghWilmot’s Group. The rare-earth metals are in pink. Note the pores (spaces) on the inside of the frame-like structure which can now be exploited for chemical storage

Figure 3: Another MOF made in Singh-Wilmot’s group. Rare-Earths are shown in yellow and organic linkers in grey. The red balls in the spaces represent water molecules which were removed by heating and thereafter the frameworks remains stable up to 600 ⁰C.

Rare-earth Nanoclusters

In recent years investigations into the chemistry of rare earths and the synthesis of novel complexes from them have boomed significantly. This is because scientists all over the world have come to appreciate the unique intrinsic properties of these elements as well as realized their true potential in various applications. Thus far rare earths have been employed in organic synthesis, as luminescence sensors, catalysis and magnetic functional materials.

Although it is possible to reap many benefits of lanthanides from mononuclear complexes, it has been shown that the unique properties of these ions are in fact enhanced when presented in polynuclear clusters whose dimensions are in the nanometer range. Having polynuclear lanthanide nanoclusters enhances the unique properties of the rare earth ions through intra or intercluster metal-metal interactions and allows researchers to manipulate or tune the behaviour of the clusters by varying the metal composition.

 In addition, as a consequence of their small size, nanoclusters have the potential to give new behaviour, with improved features which can lead to the development of new materials with unique and exciting applications. For this reason there is a much activity in the synthesis and characterization of Ln(III) polynuclear nanoclusters such as the one shown in figure 1 below which consists of twenty-six dysprosium(III) ions.  The literature has shown many other reports on nanoclusters containing even more metal ions. However scientists are yet to develop a rational synthetic route for producing these clusters and further, there is little data available on the luminescent properties and metal-metal interactions in rare earth nanoclusters. Such information in critical if we are to take advantage of the promised exciting properties of these nanoclusters.

 Xiaojun Gu and Dongfeng Xue Inorg. Chem., 2007, 46 (8), pp 3212–3216

Our research goal is to study the dynamics of cluster formation by synthesizing and

characterizing novel rare earth nanoclusters. We also investigate the photophysical properties

including luminescence and decay dynamics in an effort to understand the behavior of

nanoclusters and to explore their potential applications as functional materials including as

precursors to the formation of MOFs.

Some examples of recently synthesised nanoclusters are given below.



Synthetic Scheme

Tetranuclear cluster

green‐erbium, black‐carbon, red‐oxygen, blue‐nitrogen. M.A. Singh-Wilmot et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.028