N₂ Capture Performances of the Hybrid Porous MIL-101(Cr): From Prediction toward Experimental Testing

The purification of nitrogen-containing gas mixtures, natural/shale gas, and dry air calls for economically viable adsorptive separation processes involving an adsorbent with a higher affinity for N₂ over hydrocarbons and oxygen. This led to the discovery of a new class of unprecedented N₂-selective metal-organic frameworks (MOFs) with coordinatively unsaturated chromium(III) sites, e.g., MIL-100(Cr) (MIL: Materials of Institut Lavoisier). Following this preliminary study, here grand canonical Monte Carlo simulations identified MIL-101(Cr), an analogue of MIL-100(Cr), as another N₂-selective adsorbent from mixtures of both CH₄-N₂ (natural gas purification) and O₂-N₂ (air purification). This prediction was further compared to single gas adsorption and breakthrough separation experiments. It was evidenced that only the more energetic coordinatively unsaturated chromium sites released using an activation temperature of 523 K are responsible for the N₂-selective behavior of MIL-101(Cr). The separation mechanisms were then elucidated at the molecular-level, and this emphasized the central role played by the concentration of coordinatively unsaturated chromium(III) sites in MIL-101(Cr) that can be controlled by the activation temperature of the sample.