Energy storage materials under pressure

Surprising discovery at BESSY II: the adsorption capacity of MOFs does not rise automatically with increasing pressure

Metal-organic frameworks (MOFs) can store gases such as methane in their surface interstices, or pores. Now teams from the Technische Universität Dresden and Helmholtz-Zentrum Berlin (HZB) have precisely observed the process of gas absorption into these pores under positive pressure at BESSY II for the first time. They discovered a surprising effect: for MOFs reaching a specific pressure level, the gas already adsorbed eruptively escapes because the pores suddenly contract. This suggests many new applications. These observations were possible because the scientists have developed a specialised sample environment. It allows them to adjust the temperature and gas pressure as well as determine the quantity of gas adsorbed during the X-ray studies conducted with the KMC-2 beamline at BESSY II. The results have now been published in Nature.

Methane is considered an ecologically friendly alternative to petrol and diesel fuel, especially if it is able to be produced from solar energy in the future. To fill automobile tanks with methane, suitable materials must be developed that can retain the gas without leakage. Being able to adsorb and store gases in their pores means metal-organic frameworks (known as MOFs) are candidates for this purpose. Now a team from Technische Universität Dresden has developed a MOF by the name of DUT-49. The structure of DUT-49 contains large spaces with diameters of 1.0 – 2.4 nanometres and can therefore adsorb extremely large amounts of methane, more than 300 g of methane per kilogramme of DUT-49 at room temperature. As a result, DUT-49 is being considered for methane storage in automobiles being operated with natural gas or biogas.

Crystal structure of MOFs and gas adsorption under positive pressure investigated at BESSY II

In order to improve this material, the TU Dresden team headed by Professor Dr. Stefan Kaskel has now analysed the pressure and temperature dependence of gas adsorption and release together with the associated structural changes. Working together with experts headed by Dr. Dirk Wallacher (User Platform/Sample Environments) and Dr. Daniel Többens (Energy Materials/Structure and Dynamics) at Helmholtz-Zentrum Berlin, they developed a sample environment that enables the temperature and gas pressure to be adjusted during X-ray studies at BESSY II as well as being able to determine the quantity of gas that has been adsorbed.
They were able to shed light on the crystal structure of the material using X-ray diffraction and X-ray absorption spectroscopy (EXAFS) at the BESSY II KMC-2 beamline, showing where the gas molecules are embedded in the pores of the crystal and how the framework deforms as a result. The sample environment utilised here, which made possible controlled loading of the samples with various gases during measurements (in situ), was specially developed for the KMC-2 beamline under German Federal Ministry of Education and Research (BMBF) Project 05K13OD3. This current BMBF project is a joint effort between TU Dresden, the HZB Sample Environment group, and the HZB Structure and Dynamics of Energy Materials department.

Eruption of gas from the contracting pores

It was discovered that DUT-49 behaves itself far more unusually than expected. When the pressure of the externally fed methane or butane gas is gradually increased, more and more gas molecules are initially adsorbed into the crystal and fill the tiny pores. However, if the gas pressure exceeds a threshold of 10 kilopascals for methane or 30 kilopascals for butane, the material’s structural form closes off. The organic molecules that have stretched the framework become twisted and kinked, causing the pores of the structure to contract. The gas is then eruptively expelled from the material, and the crystal structure shrinks to less than half its volume. The volume of the pores is reduced even more, by about 61 per cent. The structure only gradually re-opens at still higher pressure, with pores of all sizes again filling completely with gas molecules. If the pressure is reduced once again, then the opposite process occurs and the open-pored structure is restored. However, this occurs only at very low pressures, an effect referred to as hysteresis.

Theory provides the answers

Quantum mechanical calculations by two french teams in Paris and Montpellier show that the different shape of the small pores in the closed form is especially favourable for deposition of methane molecules. At very high gas pressure, it is energetically more favourable if more methane is deposited into the large pores. At lower pressures, there is not enough methane present to close the pores.

New potential applications as micro-pneumatic components

Above a pressure threshold, the pores contract so quickly that the gas already adsorbed into the MOF is explosively expelled, raising the gas pressure even higher. After the pores have closed, the structure contains less gas then before, although the gas pressure is higher. This kind of “negative gas adsorption” is quite rare; the behaviour has never been observed in metal-organic frameworks before. It suggests new potential applications such as for the design of micro-pneumatic components in rescue systems, microengineering, and separation processes that react sensitively to changes in environmental pressure.

The results have now been published in Nature: "A pressure-amplifying framework material with negative gas adsorption transitions". Simon Krause, Volodymyr Bon, Irena Senkovska, Ulrich Stoeck, Dirk Wallacher, Daniel M. Többens, Stefan Zander, Renjith S. Pillai, Guillaume Maurin, François-Xavier Coudert & Stefan Kaskel   
Nature (2016), doi:10.1038/nature17430