Preferential intercalation of pyridinedicarboxylates into layered double hydroxides

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).1.34H(2)O 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 V. 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-pyridinedicarboxylate>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.