The Ordering and Arrangement of Intercalated Carboxylate Ions in Anion-Exchangeable Layered Yttrium Hydroxides

Abstract

The intercalation chemistry is essential in the application of anion-exchangeable layered metal hydroxides. However, much of the key information regarding intercalated anions, such as the degree of ordering and arrangement is still not clear or missing to date, due to the difficulty in probing the local environment of anions by routine characterization techniques. Herein, we employed solid-state NMR (ssNMR) spectroscopy to offer valuable complementary insights into the ordering and arrangement of a series of monocarboxylate anions (RCOO⁻: R=HO, H, CH₃, C₂H₅, and C₃H₇) within the interlayer space of two layered yttrium hydroxides (LYH-Cl and LYH-Br). The results imply that the two smaller carboxylate anions (R=HO and H) are disordered after intercalation, while the larger carboxylate anions (R=CH₃, C₂H₅, and C₃H₇) are relatively ordered. We further explored if the intercalated anions are parallel arranged as those in sodium salts or antiparallel arranged as those predicted in the majority of previous reports. We uncovered that the intercalated anions are antiparallel arranged based on the formation energy obtained in density functional theory calculations and ssNMR data. Our results therefore contribute to a deeper and comprehensive understanding of the intercalation chemistry of layered rare earth hydroxides.