Cu- and Li-ion-exchanged ZSM-5 for nitrogen adsorption: Reduced Li usage and water vapor adsorption

Li-X zeolites have been widely used as nitrogen-selective adsorbents in oxygen concentrators. However, the synthesis of Li-X zeolites requires an excessive amount of Li for ion exchange, and Li-X zeolites are vulnerable to humid conditions because of their high affinity for water vapor. To solve these problems, a ZSM-5 zeolite exchanged with both Cu and Li (CuLi-ZSM-5) was developed for N₂ adsorption. CuLi-ZSM-5 achieved a high Li content (Li/Al =0.84) despite the use of a smaller amount of Li in its ion-exchange procedure and exhibited higher N₂ uptake (0.81 mmol/g) at 298 K than the Li-X zeolite (0.69 mmol/g). This is possibly because the Li⁺ in CuLi-ZSM-5 is effectively distributed at sites accessible for N₂ adsorption. In addition, CuLi-ZSM-5 had higher N₂/O₂ selectivity in the low-pressure range and a much lower affinity for water vapor than the Li-X zeolite, resulting in significantly lower H₂O/N₂ selectivity. CuLi-ZSM-5 has the potential to be a new economical N₂-selective adsorbent with low interference from water vapor.