The atomic structure of… holes

The “porous solids” group at the Institut Lavoisier (Versailles) is developing methods of synthesis and is studying the properties of metal-organic frameworks ( MOFs); these so called hybrids are composed of a skeleton combining organic (essentially carboxylate) and inorganic (metallic ion) components, connected three-dimensionally, synthesized by trial and error. Among this class of porous material, Nathalie Guillou studies more specifically magnetic porous materials based on nickel and cobalt, thus combining electronic interactions and controlled porosity.
Using the CRISTAL beamline, this research group has sought to determine the crystalline structures of several materials with a high resolution powder diffractometer.

Already marketed by the German company BASF under the name “Basolite”, MIL*-53, based on aluminium, can absorb, at room temperature, gases such as O₂ or CO₂, captured in its pores. In MIL-53 the skeleton is formed of octahedral chains in which metals (Cr³⁺, Al³⁺, Fe³⁺) are connected by sharing corners. These chains are then linked in the other two directions by terephtalate ions to form a three-dimensional skeleton with lozenge-shaped tunnels.
This capacity to store, not only gas molecules, but also, for example, the active constituents of drugs (thus making them medical applications), is linked to the specific internal surface of the porous solid. A material can have pores without any capacity for absorption.
To obtain information on a newly synthesized material, or study the evolution of a known compound in relation to temperature or degree of hydration, the “porous solids” group uses X-ray diffraction techniques.

A “classic” diffractometer can be used when the sample studied is monocrystalline or in powder form with a fairly simple atomic structure. With increasing complexity, and if the unit cell becomes too big, researchers have to turn to centres of synchrotron radiation. The quality of the incident X-ray beamlines at SOLEIL (very parallel, monochromatic), but also the precision of the instruments (detectors, goniometers) gives greater resolution: it becomes possible to separate the numerous peaks in the powder patterns and thus give access to the structure of the material.
In addition, the photon flux allows, in two hours of analysis, the acquisition of a greater quantity of information than is possible in a month of measurements in the laboratory.

The solids studied by Nathalie Guillou are in powder form and placed in capillaries of less than one millimetre diameter, i.e. a few milligrams of powder. The sample is positioned in the middle of the CRISTAL diffractometer, equipped with 21 detectors, and the recording of the diffraction pattern can begin.

The aim of the first experiments carried out on CRISTAL was initially the resolution of the structure of new unknown compounds, but also the study of the gallium analogue of the already well-known MIL-53, notably for a follow-up on the dehydration of the material, its behaviour in relation to temperature or its interaction with different solvents.
These experiments showed that, as for its iron analogue, the water molecules arrange themselves in the tunnels of MIL-53-Ga, also leading to a superstructure. This hydrate is stable until about 60°C and leads to the formation of a “closed” anhydrous compound, which progressively opens at 220°C or above, eventually reaching a so-called “open” phase^[1].

Fig. 1 : Evolution of tunnel opening in relation to temperature for the different analogues of MIL-53 (Al, Ga or Fe)

This first series of measurements were followed by further experiments aimed at completing the data obtained on the solids being tested. A mixed succinate containing both cobalt and nickel has recently been synthesized [2], which also contains water coordinated to the metal. The data recorded at SOLEIL showed that the transformation to the anhydrous phase was accompanied by changes to the coordination number of a metal site, the movement of certain organic molecules, but especially the totally unexpected reduction in the dimensionality of the oxide sub-network. In fact, the three-dimensional, inorganic sub-network in the initial compound becomes two-dimensional in the anhydrous compound: the Metal-Oxygen-Metal connection is no longer infinite in the three dimensions of space.
In addition, the splitting in two of a few reflections observed in the pattern of the initial phase, discovered by means of the synchrotron radiation, gives rise to the supposition that this phase is not a solid solution, as the researchers thought, but a mixture of phases with very similar compositions. 

Fig. 2 : Study of dehydration of the nickel-cobalt mixed succinate leading to the reduced dimensionality of the inorganic sub-network. On the left is represented the initial compound, on the right, the anhydrous form. A break is clearly visible in the polyhedra connection.

Existing or potential applications for these porous materials: storage of hydrogen for fuel cells, CO₂ storage to limit its discharge into the atmosphere or to purify other gases, drug delivery, production of nanoparticles of the same size inside the pores... making them strategic materials.
Scientists at SOLEIL will, without doubt, have other opportunities to welcome members of this “porous solids” research group from the Institut Lavoisier.


* MIL : Material from the Institut Lavoisier

Référence : « Les nouveaux solides poreux ou le miracle des trous », G. Férey (2007), L’actualité chimique, 304, p.III-XV.

^[1] C. Volkringer, T. Loiseau, N. Guillou, G. Férey, E. Elkaim & A. Vilmont, Dalton Trans. (2009), 12, 2241-2249.
^[2] C. Livage, P.M. Forster, N. Guillou, M.M. Tafoya, A.K. Cheetham & G. Férey. Angew. Chem. Int. Ed. (2007), 46(31), 5879-5879.