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ContentsI. Introduction 3641 II. Zeolites 3642 1. Organic Reactions in Water Catalyzed by Zeolites 3642 2. Hydrophobicity of Zeolites 3646 3. Acidic Property of Zeolites 3647 III. Heteropolyacid 3648 1. Acidity of Liquid Heteropolyacids 3648 2. Aqueous Organic Reactions Catalyzed by Liquid Heteropolyacids 3649 3. Biphasic System Consisting of Aqueous Phase 3650 4. Solid Heteropoly Compounds That Are Active in Water 3651 i. Microstructure 3651 ii. Acidity of Solid Heteropoly Compounds 3652 iii. Hydrophobicity 3653 iv. Organic Reactions in Water 3653 IV. Oxides and Phosphates 3656 1. Niobic Acid 3656 2. MoO 3 −ZrO 2 3657 3. Zirconium Tungstate 3658 4. Zirconium Phosphate 3658 5. Lanthanum Phosphate 3659 6. Niobium Phosphate 3660 V. Organic−Inorganic Composites 3661 1. Polymer Resin−Silica Composites 3661 2. Mesoporous Organic Composites 3661 VI. Lewis Acids and Organometallic Catalysts 3662 VII. Closing 3663 VIII. Acknowledgments 3663 IX. References 3663

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A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs 2.5 H 0.5 PW 12 O 40 , has been attempted by combining the new results obtained with solid-state NMR, XRD, SEM, and N 2 porosimetry with the data reported previously. The precipitates of Cs 2.5 H 0.5 PW 12 O 40 just formed from aqueous solutions of H 3 -PW 12 O 40 and Cs 2 CO 3 consist of ultrafine crystallites in which the acid form, H 3 PW 12 O 40 , is epitaxially deposited on the surface of Cs 3 PW 12 O 40 crystallites. Calcination of the precipitates brings about the migration of H + and Cs + in the solid to form a nearly uniform solid solution in which protons distribute randomly through the entire bulk, as revealed by XRD and 31 P solid-state NMR. Impregnation of Cs 3 PW 12 O 40 with the aqueous solution of H 3 PW 12 O 40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the analysis of N 2 desorption isotherm showed that Cs 2.5 H 0.5 PW 12 O 40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH 3 sorption indicated the presence of very strong acid sites on Cs 2.5 H 0.5 PW 12 O 40 . High catalytic activity of Cs 2.5 H 0.5 -PW 12 O 40 reported for solid-liquid reaction systems is thus principally attributed to the strength and number of acid sites and the mesoporous structure appropriate for the rapid diffusion of molecules.

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