Toward a sustainable society in the 21st century, the development of truly powerful catalysts is indispensable. By using the excellent properties of polyoxometalates (POMs), Prof. Noritaka Mizuno has successfully developed a number of highly efficient catalysts with precisely controlled active sites at atomic and/or molecular levels. He has synthesized lacunary POMs as "molecular motifs" starting from monomeric metal oxide units, followed by introduction of metal species into their vacant sites to design "molecular catalysts" with precisely controlled active sites. In addition, he has developed "molecular heterogeneous catalysts" by "immobilization" or "solidification" of POM-based molecular catalysts. His major achievements are briefly summarized below.

1. Development of Highly Efficient Heterogeneous Catalysts by Precise Design of Metal Oxide Clusters
Prof. Mizuno has synthesized organic solvent-soluble [WO₄]^2-, which for the first time chemically fixed atmospheric carbon dioxide with diamines to produce the corresponding cyclic urea derivatives via activations of both diamines and carbon dioxide. By this specific bifunctionality, the chemical fixations of carbon dioxide to amines, aminonitriles, and propargylic alcohols have also been achieved.
By the use of a monomeric metal oxide unit, e.g., [WO₄]^2-, he has successfully synthesized various kinds of lacunary POMs. In addition, he has precisely tuned up their acid and base properties as well as their abilities to activate oxidants. For example, a tetra-protonated di-vacant lacunary POM [γ-SiW₁₀O₃₄(H₂O)₂]^4- could efficiently activate hydrogen peroxide and showed 100% selectivities and 100% utilizations of hydrogen peroxide for epoxidation of simple alkenes including industrially important propylene.
Lacunary POMs could be utilized as "molecular motifs", and various kinds of metal-substituted POMs, i.e., "molecular catalysts", have successfully been synthesized by introduction of metal species, such as iron, manganese, vanadium, copper, titanium, palladium, and rare-earth metals, into their vacant sites in not only water (conventional procedures) but also in organic solvents by strict control of amounts of protons and water and use of organic solvent-soluble lacunary POMs (novel procedures developed by him). The resulting POMs could act as powerful catalysts for various kinds of functional group transformations. For example, a bis-μ-hydroxo bridged di-vanadium-substituted POM [γ-PW₁₀O₃₈V₂(μ-OH)₂]^3- showed high activities for oxygenation of alkenes using hydrogen peroxide, while the other mono- and tri-vanadium-substituted POMs as well as di-vanadium-substituted POMs with different coordination environments were completely inactive, clearly showing the importance of control of compositions and structures of active sites. The di-vanadium-substituted POM regioselectively epoxidized non-conjugated dienes at the terminal double bonds. Such specific regioselectivities have never been reported previously and are likely resulting from the POM's bulky framework. This POM efficiently catalyzed oxygenations of inactive C-H bonds of alkanes and aromatics to form the corresponding alcohols and phenols, respectively. In addition, a di-iron-substituted POM [γ-SiW₁₀O₃₈Fe₂(μ-OH)₂]^8- and a di-copper-substituted POM [γ-H₂SiW₁₀O₃₆Cu₂(μ-1,1-N₃)₂]^4- could act as effective catalysts for highly difficult aerobic epoxidation of alkenes and aerobic oxidative homocoupling of terminal alkynes, respectively. Besides the above-mentioned POMs, He has also succeeded in syntheses of POMs with various active sites, e.g., multi-homometallic, multi-heterometallic, and metal cluster-encapsulating POMs, and developments of highly efficient and green functional group transformations with these POMs.

2. Development of Highly Efficient Heterogeneous Catalysts by Precise Design of Metal Oxide Clusters
For practical applications, the development of easily recoverable and recyclable POM-based catalysts is highly desirable. Prof. Mizuno has developed "molecular heterogeneous catalysts" by "immobilization" or "solidification" of POM-based molecular catalysts.
He has synthesized a dihydroimidazolium-based ionic liquid and then modified the surface of metal oxides, e.g., SiO₂, with the ionic liquid. By using the modified metal oxides as anion-exchangers, the catalytically active POM-based molecular catalysts anion could be immobilized with keeping their structures of active sites as well as their intrinsic catalytic performance. Moreover, he has developed a novel zinc-modified tin oxide anion-exchanger and successfully created highly active dioxo tungstate species on the support. This catalysts could act as a truly powerful heterogeneous catalyst for oxygenations of alkenes, amines, hydrosilanes, and sulfides using hydrogen peroxide as the oxidant.
With regard to "solidification", he has found that complexation of POM-based molecular catalysts with organometallic cations with appropriate charge, size, shape, and hydrophobicity lead to the formation of insoluble fine particles in any solvents. Alkali metal and alkyl ammonium cations could also be utilized for solidification of POM-based molecular catalysts. These catalysts have been utilized for the above-mentioned liquid-phase functional group transformations.

During the course of the investigations for the design of POM-based molecular catalysts, he has found that various functional group transformations should be promoted by the concerted activation of substrates by the Lewis acid and Brø nsted base pair sites of metal hydroxide species. Firstly, he has succeeded in the design highly dispersed ruthenium hydroxide species on inexpensive supports such as Al₂O₃ and TiO₂ (Ru(OH)_xsupport) by the precise control of the solution species and immobilization of the controlled solution species onto supports, followed by base treatment. As expected, the Ru(OH)_x/support catalysts could act as efficient reusable heterogeneous catalysts for various kinds of functional group transformations. In addition, he could successfully develop several novel catalytic transformations with Ru(OH)_x/support. Besides ruthenium-based catalysts, he has shown that the generality of hydroxide-based catalyst design is very wide and developed many efficient hydroxide-based catalysts for green functional group transformations.

In summary, Prof. Mizuno has carried out pioneering works in the field of catalyst design and succeeded in developments of truly powerful catalysts with precisely designed active sites and efficient functional group transformations with them. Besides catalytic applications, he has applied the above-mentioned POM-based materials to adsorption, separation, and molecular recognition. He has authored 287 original papers, 52 reviews, 32 chapters in books, and 9 patents. These accomplishments are published in top journals with high impact factors and highlighted in many scientific journals, magazines, and news papers. In particular, "Adv. Catal., 41, 113 (1996)" (Times cited: 1224) and "Chem. Rev., 98, 199 (1998)" (Times cited: 1216) received high acclaim, and became "the Bible" of catalyst design using metal oxide clusters. In addition, there are 19 papers reporting more than 100 citations, and the total number of citations that the candidate reported so far is 12574 with the h-index of 54. Accordingly, it was approved that this achievements merit the Chemical Society of Japan (CSJ) Award.