Evaluating the preferential adsorption of N₂ from a binary mixture of N₂/O₂ on extra-framework cations of zeolites: a computational and experimental study

Separation of N₂ from a N₂/O₂ gas mixture is critical for various industrial/medical applications. Temperature/pressure swing adsorption is the top-notch industrial technology used for this separation, where zeolites are the materials used for adsorption. Zeolite X/Y with Li⁺ as an extra-framework cation is the best-known sorbent for N₂ gas molecules. However, the present net zero emission scenario has made lithium a critical element, making it imperative to implement its alternative in various other technologies. In this context, the present work is a computational evaluation to identify a cation that can replace Li⁺ for preferential adsorption of N₂ over O₂. The DFT study, based on parameters such as selective adsorption energies of N₂ over O₂ and IR stretching frequencies of the adsorbed N₂ and O₂ molecules, identifies Mg²⁺, Ca²⁺, Sr²⁺, Co²⁺ and Zn²⁺ as potential cations. These cations have preferential adsorption for N₂ over O₂ by 10 kJ mol^-1 or more. However, BOMD simulations reveal that only Mg²⁺, Ca²⁺, Co²⁺ and Zn²⁺ keep the N₂ molecule bound at 300 K and the O₂ molecule gets desorbed from these frameworks. The desorption temperature of N₂ on Ca²⁺ and Zn²⁺ is 350 K and on Mg²⁺ is 400 K. These observations are corroborated by electronic charges on cations and molecular orbitals. Significantly, Ca²⁺ is identified to adsorb up to 2 N₂ molecules, making it an ideal candidate for N₂/O₂ separation in place of Li⁺. To validate this, we have carried out an experimental study that showed a good N₂ adsorption capacity of 2.1 mmol g^-1 for Ca²⁺.