A Survey of Applications of Polyoxometalates

Chem. Rev. 1998, 98, 359−387 359 A Survey of Applications of Polyoxometalates Dimitris E. Katsoulis Dow Corning Corporation, Central Research and Development, Midland, Michigan 48686 Received June 2, 1997 (Revised Manuscript Received November 8, 1997) Contents I. Introduction 359 II. Corrosion Resistant Coatings 362 III. POMs as Additives in Sol−Gel Inorganic Matrixes 364 IV. POMs as Additives in Organic Nonconductive 366 Matrixes V. Dyes/Pigments/Inks 366 VI. Recording Materials 367 VII. Electrophotography. Toners 367 VIII. Precursors of Oxide Films (Optoelectronics; 368 Optical Components) IX. Membranes. Sensors 370 A. Selective Electrodes 370 B. Gas Sensors 371 C. Solid-State Electrochromic Devices 372 Dimitris E. Katsoulis was born in Athens, Greece in 1955. He received D. Electrochemical (Fuel) Cells 372 a B.S. degree in Chemistry from University of Athens in 1977. He subsequently joined the research group of Prof. Michael T. Pope at X. Electrode Modification by POMs 374 Georgetown University and obtained a Ph.D. degree in 1985. He XI. Capacitors 375 continued with a postdoctoral assignment at Georgetown University, and XII. POMs as Dopants in Electrically Conductive 375 in 1988 he joined the Science and Technology function of Dow Corporation Polymers in Midland Michigan. He is currently an Associate Research Scientist in the Rigid Materials Science Expertise Center (Central R&D) in Dow XIII. Wood Pulp Bleaching (Oxidative Delignification) 376 Corning. His research interests include hybrid materials, with focus on XIV. Analysis 377 siloxane-polyoxometalate compositions, sol−gel chemistry, silsesquioxanes, XV. Protein Precipitation Agents 380 gel systems, polymer matrix composites, and nanocomposites. XVI. Staining Agents 380 XVII. Processing of Radioactive Wastes 380 exists based on publications, and Figure 3 lists the XVIII. Sorbents of Gases 381 10 largest patent assignee countries in the world. XIX. Miscellaneous Applications 381 Japan issues 40% of the worldwide patent literature ∼ XX. Conclusions 382 followed by the USA with 17%. The applications of POMs are based on combinations of so-called XXI. Acknowledgments 382 “value-adding properties” which are summarized in XXII. References 382 Table 1. From the above-listed properties, the applications I. Introduction of POMs are centered primarily on their redox The field of polyoxometalates (POMs), although a properties, photochemical response, ionic charge, mature field, continues to attract significant atten- conductivity, and ionic weights. The majority of the tion. The number of publications and patents con- patent and applied literature is devoted to the tinues to grow. New researchers are entering the applications of the Keggin type heteropolyacids (HPA) field. The scientific communities of fast-growing new and their salts. Primarily H3PMo12O40,H3PW12O40, economic/technology powers such as Peoples Republic H4SiMo12O40, and H4SiW12O40 are used as the main of China and India are becoming important contribu- examples for many applications. Their popularity tors of the total number of worldwide publications can be attributed to a large extent to the enormous on POMs. Figure 1 depicts the growth of the POM volume of literature over several decades that de- literature per year since 1966. In 1996, according scribes their fundamental chemistry and to their to Chemical Abstracts, nearly 600 refereed publica- commercial availability, which makes them conve- tions and over 120 patents were issued in reference nient starting materials. Approximately two-thirds to the POM chemistry and technology. Figure 2 of the applied chemistry/technology literature de- shows the countries where the most research activity scribes applications that are based on these POMs. S0009-2665(96)00398-6 CCC: $30.00 © 1998 American Chemical Society Published on Web 01/17/1998 360 Chemical Reviews, 1998, Vol. 98, No. 1 Katsoulis Figure 1. Publication and patent growth on POMs since 1966. Source: Chemical Abstracts. Figure 2. The 10 countries with the largest number of POM publications. Source: Chemical Abstracts. Given the enormous number of POM complexes obvious that plenty of room exists for expanding the available and their variability in properties, it is exploration of the opportunities for these materials. Applications of Polyoxometalates Chemical Reviews, 1998, Vol. 98, No. 1 361 Figure 3. The 10 largest POM patent assignee countries. Source: Chemical Abstracts. Table 1. “Value-Adding Properties”of Polyoxometalates 1. metal oxide like 2. stable (H2O/air, T): processing advantage 3. large size (diameter, 6-25 Å) 4. discrete size/discrete structure (confined geometric factors) 5. anions (charge from -3to-14) 6. high ionic weight (103 - 104) 7. fully oxidized compounds/reducible 8. variable oxidation numbers for the addenda atoms (E1/2 ) 0.5 to -1.0 V vs SCE) 9. color of oxidized forms different from color of reduced forms 10. photoreducible 11. arrhenius acids (pKa < 0) 12. incorporate over 70 elements and form large number of structures: processing advantage 13. acid forms very soluble in H2O & other oxygen carrying solvents (ethers, alcohols, ketones); also soluble or transferable into nonpolar solvents: processing advantage 14. hydrolyzable to form deficient structures: processing advantage As the multidisciplinary approach in science keeps as proven scientific events. Thus patents with spe- increasing, it is just a matter of time before the cific examples and data were sought for further widening of the scope of this field becomes a reality. analysis as being more reliable. Table 3 lists uses One course should always be to weigh the cost factors of specific POMs from illustrative examples from the against the value of the application. patent and refereed literature. The majority of the applications of POMs are found in the area of catalysis. About 80-85% of patent and The review is organized in 18 sections on the basis applied literature claims or investigates POMs for of the material form that POMs are found (or their catalytic activity. These applications are re- proposed to be) in applications, or on the basis of a viewed in two separate sections in this issue. The particular POM value-adding property (Table 1). Half remaining 15-20% of the applications can be grouped of the sections that follow describe applications where in the categories of Table 2. This review, which is based mainly on patent POMs function within coatings, membranes, or thin literature and on referee literature of applied re- films. These include corrosion resistant coatings, search, summarizes the developments in applications POMs in sol-gel matrixes and in conductive and of POMs other than catalysis and medicine. It is nonconductive polymer membranes, and POMs as well-known that claims in patent literature do not surface modifiers of substrates (i.e. carbon elec- necessarily represent commercial successes as well trodes). The remaining sections describe uses of 362 Chemical Reviews, 1998, Vol. 98, No. 1 Katsoulis Table 2. Categories of Applications for POMs inhibitors such as chromates, phosphates, or silicates. (Derived from Patent Literature; Excluding Catalysis Chromates, known as anodic passivating inhibitors, and Medicine) cause a large positive shift on the corrosion potential 1. coatings of a metal substrate such as steel during polarization 2. analytical chemistry measurements. Mechanistic studies suggest that 3. processing radioactive waste 4. separations metal protection is afforded by a combination of 5. sorbents of gases adsorption and oxide formation on the steel surface. 6. membranes The main concern with the use of chromate inhibitors 7. sensors is their toxic nature. 8. dyes/pigments Polyoxometalates have relatively low toxicity as 9. electrooptics compared to chromates; they accept electrons without 10. electrochemistry/electrodes 11. capacitors major changes of their structures and form insoluble 12. dopants in nonconductive polymers salts with large cations. These properties make them 12. dopants in conductive polymers attractive as oxidizing and film-forming corrosion 13. dopants in sol-gel matrixes inhibitors. 14. cation exchangers Thus a significant activity is encountered in the 15. flammability control 16. bleaching of paper pulp applied literature that claims known and unidentified 17. clinical analysis POM compositions as corrosion inhibitors as such 18. food chemistry and in formulations with suitable delivery systems. A recent study by Lomakina et al.1 investigated the POMs as pigments, toners, wood pulp bleaching corrosion inhibition of POMs for aluminum and its agents, reagents for chemical/biochemical analysis alloys under high-temperature aqueous conditions. and for nuclear waste processing, and some miscel- Such alloys are considered for use as construction laneous applications. materials in atomic industries, and thus a corrosion inhibitor must exhibit high radiation stability. - - II. Corrosion Resistant Coatings The corrosion rates of aluminum alloys (Al Ni Fe) as measured by the increase of sample weight Of the most widely used methods to combat corro- (due to formation of hydrous aluminum oxides) as a sion of metal surfaces is the utilization of corrosion function of time were reduced significantly in the Table 3. Applications of Specific POMs from Illustrative Examples in the Patent and Refereed Literature examples of POMs application reference H4PMo11VO40 modification of carbon electrodes, capacitors 188 KxH4-xPMo11VO40 H5PMo10V2O40 (NH4)6P2Mo18O62 H3PMo12O40 189 H3PMo12O40,H3PW12O40

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