Porous Chromium Terephthalate MIL-101 with Coordinatively Unsaturated Sites: Surface Functionalization, Encapsulation, Sorption and Catalysis

A metal-organic framework (MOF), chromium(III) terephthalate (MIL-101), possesses several unprecedented features such as a mesoporous zeotype architecture with mesoporous cages and microporous windows, a giant cell volume, huge surface area, and numerous unsaturated chromium sites. The presence of unsaturated Cr(IIII) sites in MIL-101 provides an intrinsic chelating property with electron-rich functional groups. This feature offers a powerful way to selectively functionalize the unsaturated sites in the MOFs, differing in many respects from the functionalization of mesoporous silica and other porous hybrids reported so far. This work demonstrates that the surface amine-grafting undoubtedly provides a general way of selective functionalization of porous MOFs besides that with unsaturated metal sites. The present approach indeed ensures the development of functionalized MOFs for the immobilization and encapsulation of organic molecules and metal components that are useful for catalysis, adsorption, molecular recognition, and so on. Here some of the recent ideas concerning site-selective functionalization of MIL-101 are examined and explained, focusing on the utilization of unsaturated Cr(III) sites. Recent advances in synthesis, selective surface functionalization, outstanding sorption properties, encapsulation of nano-objects, and catalytic applications in MIL-101 are also described.