The effect of guest cations on proton conduction of LTA zeolite

Zeolites as important crystalline microporous materials have been widely used as catalysts, sorbents and ion-exchangers. In such materials, the guest ions in the pores can not only balance the charge of the framework but also make a difference to the pore environment and the resulting performance. In this work, we focus on the proton conduction properties of zeolites, and have comprehensively studied for the first time the effect of guest ions on proton conduction. To this end, aluminosilicate NaA zeolite (LTA) and ion-exchanged NaA zeolites by different guest ions (i.e. Li⁺, K⁺, Mg²⁺, Ca²⁺ and Sr²⁺) have been successfully synthesized. The study indicates that the guest ions could affect the proton conduction properties through the synergistic effect between the pore features (e.g. pore size, pore polarity, etc.) and guest ions, the ionic concentrations, and the interference between different ions. Among various guest ions, the existence of Na⁺ ions can greatly promote the proton conduction properties. The proton conductivity of NaA can reach 1.98 × 10^-3 S cm^-1 (100% RH) at room temperature and 9.12 × 10^-3 S cm^-1 (80 °C) under the condition of 100% RH. In addition, guest monovalent ions (Li⁺, Na⁺ and K⁺) exhibit better proton conductivity than divalent ions (Mg²⁺, Ca²⁺ and Sr²⁺). The distinct effect of these guest ions enables zeolites with tunable proton conductivity, which will provide more opportunities to design zeolitic proton conducting materials.