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

Separation of N from a N/O 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 N 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 N over O. The DFT study, based on parameters such as selective adsorption energies of N over O and IR stretching frequencies of the adsorbed N and O molecules, identifies Mg, Ca, Sr, Co and Zn as potential cations. These cations have preferential adsorption for N over O by 10 kJ mol or more. However, BOMD simulations reveal that only Mg, Ca, Co and Zn keep the N molecule bound at 300 K and the O molecule gets desorbed from these frameworks. The desorption temperature of N on Ca and Zn is 350 K and on Mg is 400 K. These observations are corroborated by electronic charges on cations and molecular orbitals. Significantly, Ca is identified to adsorb up to 2 N molecules, making it an ideal candidate for N/O separation in place of Li. To validate this, we have carried out an experimental study that showed a good N adsorption capacity of 2.1 mmol g for Ca.