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The synergy between four laboratories (the LCS of Caen, the ILV / IMAP of Versailles / Paris, the ICGM of Montpellier and the KRICT of Korea) with unique skills in synthesis, advanced characterization and modeling of materials, made possible to design a organic-inorganic hybrid porous system, commonly known as the Metal Organic Framework (MOF), capable of preferentially fixing nitrogen gas, even in the presence of other gases.
The MOFs have a zeolite structure, composition and structural entities similar to those of an organometallic compound, but are solid, stable up to relatively high temperatures and with spatial control of the active entities, A very important accessible porosity for adsorbing reagents or molecules then confined within their pores.
Flow tests (operando conditions) have shown that some of these solids (especially if they are composed of chromium-based active centers) are very efficient in the selective adsorption of nitrogen in the presence of a gas mixture, in particular methane and oxygen. DFT calculations on the active sites for nitrogen fixation allowed us to understand that the Cr (III) oxoclusters, stabilized by a hybrid structure based on tricarboxylate ligands, exhibit a specific (and unusual, interaction with respect to other metal cations such as Fe (III) or Al (III)) with the nitrogen molecules. The reasoned choice of the best solid corresponding to these characteristics led to the synthesis of a crystallized porous material containing a very high density of Cr (III) sites, the total evacuation of the water of which allows access to unsaturated metallic sites, i.e., available for the adsorption of nitrogen molecules, as confirmed by in situ spectroscopic analyzes.
These results are of major importance because they made possible to identify, for the first time, an easily recyclable material capable of purifying natural gas (mainly composed of methane, but contaminated by high concentrations of nitrogen) or separating oxygen from the air, at low pressure and temperature, without using the processes commonly employed, which are often energy-intensive. No porous solid had so far been able to carry out this type of separation by thermodynamic effect.
From a more general point of view, this is a brilliant demonstration that it is possible to shape porous hybrid solids of the MOF type in order to adapt them to particular chemical reactions. The prospects are numerous on the one hand from the industrial point of view to develop more efficient processes of separation of the gases and on the other hand it opens the way to other applications, especially in catalysis of the nitrogen products.
See Nature Materials (2016)