Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH₄) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N₂), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N₂ removal technologies is hampered by N₂'s inertness and its resemblance to CH₄ in terms of kinetic size and polarizability. N₂-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N₂ from CH₄. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N₂. Particularly, Li⁺-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N₂ adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li⁺, and this Li⁺ can interact with multiple N₂ molecules but only one CH₄, revealing the mechanism for the high capacity and selectivity of N₂. This work suggests promising adsorbents to enable N₂ rejection from CH₄ in the gas industry without going for energy-intensive cryogenic distillations.