DOI: 10.1002/ijch.201000082 MIL-101 Supported Polyoxometalates: Synthesis, Characterization, and Catalytic Applications in Selective Liquid-Phase Oxidation Nataliya V. Maksimchuk, [a] Oxana A. Kholdeeva,* [a] Konstantin A. Kovalenko, [b] and Vladimir P. Fedin [b] 1. Introduction Catalytic oxidation in the liquid phase is widely used in the manufacture of bulk and specialty chemicals, [1] and is becoming important in the synthesis of fine chemicals under increasing environmentalist pressure to replace tra- ditional processes which employ stoichiometric inorganic oxidants. [2] Large-scale liquid-phase catalytic oxidations generally employ soluble metal salts or complexes in combination with environmentally benign and inexpen- sive oxidants such as O 2 ,H 2 O 2 , or ROOH. [1, 3] Moreover, the use of heterogeneous catalysts has obvious benefits of facile recovering and recycling. [2, 4] Polyoxometalates (POMs) are early-transition-metal oxygen anion clusters that have been widely applied to various fields, such as structural chemistry, analytical chemistry, surface science, medicine, electrochemistry, photochemistry, and catalysis. [5] POMs have several ad- vantages as oxidation catalysts: (i) their redox and acid– base properties, as well as solubility, can be controlled by changing the chemical composition, (ii) they are not prone to oxidative and thermal degradation in compari- son with organometallic complexes, and (iii) the catalyti- cally active sites can be designed and characterized at the atomic and/or molecular levels. The development of strat- egies for converting soluble POMs to solid materials that would be stable in the liquid phase is of primary impor- tance, and many research groups are involved in this ac- tivity. [5, 6] In the past decade, metal-organic frameworks (MOFs) have attracted great attention as prospective materials for gas storage, liquid phase separations, drug delivery, and heterogeneous catalysis, due to their unique combination of properties, such as crystalline open structures, varying pore volumes and surface areas, tunable functionality, etc. [7] Until recently, a serious shortcoming of MOFs that limited their application in catalysis was low thermal, chemical, and hydrolytic stability as compared to com- pletely inorganic materials, for example, zeolites. In 2005, FØrey and co-workers reported the discovery of the meso- porous chromium terephthalate MIL-101 which showed good resistance to water, common solvents, and tempera- ture (up to 300 8C). [8] MIL-101 has a rigid zeotype crystal structure consisting of quasi-spherical cages of two modes (2.9 and 3.4 nm) accessible through windows of ca. 1.2 and 1.6 nm. All these allow to view MIL-101 as a pro- spective support for immobilization of catalytically active complexes, specifically POMs. [7l, 8, 9] Here we summarize our progress in the preparation and characterization of hybrid materials based on catalyt- ically active POMs incorporated within the cages of MIL- 101, and their application for liquid-phase selective oxida- tion of organic compounds. Special attention is paid to [a] N. V. Maksimchuk, O. A. Kholdeeva Boreskov Institute of Catalysis Lavrentiev Avenue 5, Novosibirsk 630090, Russia phone: + 7 (0)383 3269433 fax: + 7 (0)383 3309573 e-mail: khold@catalysis.ru [b] K. A. Kovalenko, V. P. Fedin Nikolaev Institute of Inorganic Chemistry Lavrentiev Avenue 3, Novosibirsk 630090, Russia Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ijch.201000082. Abstract : In this short review paper, we survey our recent achievements in the preparation of heterogeneous catalysts via incorporation of polyoxometalates (POMs) within cages of the mesoporous coordination polymer MIL-101, their physicochemical characterization, and application for liquid- phase selective oxidation of organic compounds with green oxidants — O 2 and aqueous H 2 O 2 . Special attention is paid to analyze a manifestation of confinement effects and to ad- dress the issues of catalytic activity and selectivity after im- mobilization and after recycling, catalyst resistance to POM leaching, and the nature of catalysis. The scope and limita- tions of POM/MIL-101 catalysts are discussed. Keywords: heterogeneous catalysis · metal-organic framework · MIL-101 · selective oxidation · supported polyoxometalate Isr. J. Chem. 2011, 51, 281 – 289  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 281 Review