Ion-exchange properties of NASICON-type phosphates with the frameworks [Ti2(PO4)3] and [Ti1.7Al0.3(PO4)3]

Abstract

Ion-exchange properties in aqueous A⁺ solution (A⁺= Li⁺, Na⁺, K⁺, Ag⁺, Rb⁺, NH₄⁺) are investigated for the title materials at room temperature. In LiTi₂(PO₄)₃ and Al-containing Li_1.3Ti_1.7Al_0.3(PO₄)₃, Li⁺ is exchanged rapidly and selectively by Na⁺ and Ag ⁺. The exchange rate of Li_1.3Ti_1.7Al_0.3(PO₄)₃ is faster than that of LiTi₂(PO₄)₃; this is probably related to a larger diffusion coefficient of Li⁺ in the former due to its high Li⁺ conductivity. Large ions such as K⁺ are not readily accepted by the anion frameworks. Accordingly, Li_1.3Ti_1.7Al_0.3(PO₄)₃ has excellent Na⁺ selectivity with fast exchange rate to extract even a very small amount of Na⁺ in reagent-grade KOH. The Na⁺-exchanged product of Li_1.3Ti_1.7Al_0.3(PO₄)₃ can exchange Na⁺ for Ag⁺ selectively. The reverse exchange also occurs easily. This material can be used to recover Ag⁺ in waste solutions.