Preferential Intercalation of Pyridinedicarboxylates into Layered Double Hydroxides

Abstract

Intercalation of 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, or 3,5-pyridinedicarboxylate into the layered double hydroxide (LDH), [Mg0.73Al0.27(OH)2](CO3)0.14·1.34H2O was carried out by the reconstruction method in the molar ratio of organic acid:calcined LDH=3:8, in 80% alcoholic aqueous solution at 70 °C. Selective reaction was observed in competitive experiments involving an equal concentration pairs of acids. The preference order of the organic acids intercalated into the Mg-Al-LDH was found to be in the order of 2,3-pyridinedicarboxylate>2,5-pyridinedicarboxyate>2,4-pyridinedicarboxylate>3,5-pyridinedicarboxylate>3,4-pyridinedicarboxylate>2,6-pyridinedicarboxylate. The structures of the intercalates formed by the reaction of six guests with Mg-Al-LDH were characterized by X-ray diffraction, infrared and thermogravimetry techniques. And the charge density on the oxygens of each of the carboxylate groups for the six anions was investigated utilizing ab initio (HF/6-31G) method by G98w. From the X-ray diffraction data, the guest size and the charge density of the oxygens of the guest, the orientation of 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, or 3,5-pyridinedicarboxylate anions between the layers was determined and the preferential intercalation mechanism was discussed. These results indicate the possibility of a molecular recognition ability of LDH and it would be exploited for the chemical separation of some anions from solution.