Selective nitrogen adsorption via backbonding in a metal–organic framework with exposed vanadium sites

Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations1–4. Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N2, through backbonding interactions5–7, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates8. Here, we report a metal–organic framework featuring exposed vanadium(ii) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N2 capacities and selectivities for the removal of N2 from mixtures with CH4, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents. Nitrogenases use transition metals to selectively capture weak π acids such as N2 by employing backbonding interactions. Here, a metal–organic framework with exposed vanadium sites is presented that uses this approach for selective capture of N2 from CH4, with impressive selectivity and capacity.