Immobilization of Co-containing polyoxometalates in MIL-101(Cr): structural integrity versus chemical transformation

Abstract

The encapsulation of three different cobalt substituted polyoxometalates (POMs) within the mesoporous chromium(III) terephthalate MIL-101(Cr) metal–organic framework (MOF) was studied by a simple and green impregnation method using water. The POM@MIL composite materials were fully characterized by EDX, XRPD, IR, MAS NMR and N₂ porosimetry measurements. The encapsulated POMs were then extracted by an exchange procedure using a LiCl solution. ³¹P NMR spectroscopy is the key experiment which indicates that the monosubstituted Keggin anion [PW₁₁CoO₃₉(H₂O)]⁵⁻ (PW₁₁Co) and the sandwich-type anion [(PW₉O₃₄)₂Co₄(H₂O)₂]¹⁰⁻ (P₂W₁₈Co₄) can be encapsulated and extracted without degradation, neither of the POM nor of the MOF, while the hybrid sandwich-type POM [(PW₉O₃₄)₂Co₇(OH)₂(H₂O)₄(O₃PC(O)(C₃H₆NH₃)PO₃)₂]¹⁴⁻ (Co₇-Ale) evolves into P₂W₁₈Co₄ inside the cavities of the mesoporous material. The PW₁₁Co Keggin anion is the most quantitatively uploaded and the most easily extracted anion. ³¹P MAS-NMR spectroscopy further suggests that this anion is more mobile inside the cavities of the MOF than the P₂W₁₈Co₄ POM.