Favorable Intercalation of Nitrate Ions with Fluorine-Substituted Layered Double Hydroxides

Understanding and controlling confined nanospace to accommodate substrates and promote high ion conduction are essential to various fields. Layered double hydroxides (LDHs) have emerged as promising candidates for anion exchangers using the interlayer nanospace in their crystal structures. Miyata reported in 1983 that the affinity of anions for intercalation with most major Mg-Al LDHs increased in the following order: NO₃^- < Br^- < F^- < SO₄^2- < HPO₃^2-. Attempts to alter the affinity with different metal cations (M²⁺ and M³⁺) have been unsuccessful. Analyses of the crystalline structures of LDHs, positively charged host layers, interlayer anions, and interlayer water molecules indicate that they inevitably interact through hydrogen bonding. In other words, the affinity of LDHs for anions is controlled by tuning the hydrogen bonding. In this study, we prepared fluorine-substituted LDHs (F-LDHs) with different Mg/Al ratios by partially replacing the OH structural groups, which originated from the host layer, with fluorine atoms; the resulting change in affinity was investigated. The distribution coefficient, which is a useful indicator of the affinity of an LDH for a particular anion, was examined. The results showed that only F-LDHs with Mg/Al ratios of 3.5 exhibited high affinity, especially for NO₃^- ions, and the affinity increased in the following order: HPO₄^2- < SO₄^2- < F^- < Br^- < NO₃^-. The separation factors of these specific F-LDHs with respect to both NO₃^-/F^- and NO₃^-/SO₄^2- were higher than that of LDHs with other compositions by 1 order of magnitude. Raman spectroscopy above 3000 cm^-1 revealed that the fluorine substitution of LDHs significantly changed the hydrogen bonding nature in the interlayer space. Highly electronegative fluorine atoms significantly decrease the extent of hydrogen bonding interactions between OH structural groups and both interlayer water molecules and anions, wherein steric effects are induced by the shrunken interlayer space, and van der Waals forces are revealed to be the predominant interaction with anions. Therefore, the highest affinity was observed for NO₃^- ions in F-LDHs.