Gated pores for gas storage

A major programme of research into metal-organic frameworks (MOFs) as materials for hydrogen and fuel gas storage is underway at the University of Nottingham. Safe, efficient hydrogen storage is a pre-condition for powering cars using this clean fuel, which has no carbon emissions at the point of use. In order to understand how our MOFs can store hydrogen, we need to determine their three-dimensional crystal structures. This is often not possible to achieve in-house because the crystals are small, weakly diffracting or have high solvent content. Access to Beamline I19 at Diamond allows us to successfully determine these structures, explain how the current MOFs work, and help us design the next generations of MOFs.

Theoretical studies have suggested that doping lithium into these materials can enhance the hydrogen binding energy, leading potentially to materials that can store hydrogen at more ambient temperature and lower pressures [1-4]. In practice, doping with lithium is difficult because of its high reactivity. Our group has successfully developed a unique methodology for introducing lithium ions into anionic framework materials by ion exchange under mild conditions. We have carried out the first systematic study of lithium doping correlated with enhancements in hydrogen storage. The lithium-loaded materials have been analyzed by X-ray diffraction using synchrotron radiation, and the nature and positions of the lithium centres were found to correlate well with the hydrogen adsorption properties.

We have therefore demonstrated that charged MOFs can be modified through ion-exchange to incorporate various counter ions as “pore gates”. The pore environment is tunable by careful choice of counter-ion and organic linker, allowing us to manipulate these materials to achieve controllable and desirable gas storage properties via selection of a suitable “pore gate”.

We used the standard configuration of experimental hutch 1 on beamline I19, with sample cooling provided by an open-flow nitrogen cryostat, and using the Rigaku Saturn 724+ CCD detector.

Charged porous framework materials have long been dismissed as viable candidates for gas storage on the reasonable grounds that any charged framework must have counter-ions within the pore channels and this would inevitably reduce or even eliminate porosity of the framework. Attempting to remove or extract the counter ions and/or organic template from the synthesised network solid often leads to decomposition and amorphisation of the local framework, thereby losing the desired microporosity [5][6]. Here we show that the presence of distinct cations within anionic framework materials can provide positive enhancement to materials properties and function, and by post-synthetic ion-exchange one can create a range of materials with the same framework structure but incorporating different cations as “pore gates”, leading to different functionalities in these materials.

 
Figure1: X-ray crystal structures of tri-porous units in NOTT-201; Indium (green), carbon(grey), oxygen (red), lithium (purple). (a, top left) and NOTT-207 (b, top right) Li+ ion coordination environment in NOTT-201 (c, bottom left) and NOTT-207 (d, bottom right).

We synthesised complex MOFs from tetracarboxylate ligands and In(III) cations to afford anionic frameworks that contain organic ammine cations - the gates that can reversibly block pore channels. Studies of two MOFs (NOTT-201, Fig.1a, and NOTT-207, Fig.1b) at Diamond confirmed these materials to be isostructural and the corresponding Li+ ions (introduced via cation exchange) have been located in the smallest channel, but with different coordination geometries in each case. In NOTT-201, the Li+ is tetrahedrally coordinated by four oxygen atoms, two from carboxylate groups and two others from coordinated water molecules (Fig.1c). In contrast, the Li+ in NOTT-207 is octahedrally coordinated by six oxygen atoms, all from carboxylate groups (Fig.1d). Although both materials show enhanced hydrogen uptake and adsorption isosteric enthalpy compared with the organic-cation framework analogues, the isosteric enthalpy of NOTT-201 is ca. 3 kJ/mol higher than that of NOTT-207, a result which correlates with the difference in the Li+ coordination environments in these two structures. In NOTT-201, the Li+ can be partially exposed to the hydrogen molecules, while in NOTT-207 it is completely surrounded by oxygen atoms and therefore not accessible to hydrogen molecules.

Our structural results from Diamond have allowed us to establish that doping of Li+ into framework materials can indeed enhance the hydrogen binding energy and this can be modulated further by different coordination at Li+. On this basis, we anticipate that more effective and efficient doping of Li+ into such frameworks is possible, with the aim of achieving hydrogen storage by nanoporous framework materials at room temperature. The coordination environments of Li+ ions in the original and activated framework materials have been precisely interrogated by X-ray crystallography and also by 7Li solid-state NMR spectroscopy, providing important evidence for the observed enhanced hydrogen storage property, consistent with the theoretical predictions [1-4].

We intend to pursue the synthesis of these new materials, and to characterise them fully in order to further enhance the application of MOFs as gas storage agents.

References
[1] A. Blomqvist, C.M. Araújo, P. Srepusharawoot, and R. Ahuja, Proc. Natl. Acad. Sci. USA, 104, 20173 (2007).
[2] S.S. Han, W. A. Goddard, J. Am. Chem. Soc, 129, 8422 (2007).
[3] S.S. Han, W. A. Goddard, J. Phys Chem. C,112, 13431 (2008).
[4] E. Klontzas, A. Mavrandonakis, E. Tylianakis, and G.E. Froudakis, Nano Letters, 8, 1572 (2008).
[5] J. Patarin, Angew. Chem. Int. Ed, 43, 3878 (2004).
[6] M.E. Davis, Nature, 417, 813(2002).

Principal Publications and Authors
X. Lin, I. Telepeni, A.J. Blake, A. Dailly, C.M. Brown, J. M. Simmons, M. Zoppi, G. S. Walker, K.M. Thomas, T.J. Mays, P. Hubberstey, N.R. Champness, and M. Schröder. High Capacity Hydrogen Adsorption in Cu(II) Tetracarboxylate Framework Materials: The Role of Pore Size, Ligand Functionalization and Exposed Metal Sites, J. Am. Chem. Soc. 131, 2159 (2009).

S. Yang, X. Lin, A.J. Blake, G.S. Walker, Peter Hubberstey, N. R. Champness, and M. Schröder. Cation-induced Kinetic Trapping and Enhanced Hydrogen Adsorption in a Modulated Anionic Metal Organic Framework, Nature Chemistry, 1, 487 (2009).

Y. Yan, I. Telepeni, S. Yang, X. Lin, W. Kockelmann, A. Dailly, A. J. Blake, W. Lewis, G. S. Walker, D. R. Allan, S. A. Barnett, N. R. Champness, and M. Schröder. Metal-Organic Polyhedral Frameworks: High H2 Adsorption Capacities and Neutron Powder Diffraction Studies, J. Am. Chem. Soc. 132, 4092 (2010).

S. Yang, G. S. B. Martin, J. J. Titman, A. J. Blake, D. R. Allan, N. R. Champness, and M. Schröder. Gated Pores for Gas Storage, to be submitted.

Funding Acknowledgement
Engineering and Physical Sciences Research Council, UK.
European Research Council