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

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. (31)P 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. (31)P MAS-NMR spectroscopy further suggests that this anion is more mobile inside the cavities of the MOF than the P2W18Co4 POM.