Organic heteropolyacids

The scientific interests of Anatoly K. Babko ranged widely, especially in regard to fundamental aspects of analytical chemistry, applications of organic reagents in inorganic analysis, chemistry of complex compounds (including heteropolyacids), analytical applications of complex compounds in photometry, luminescence and chemiluminescence, ion chromatography, and liquid-liquid extraction. 

Heteropolyacids (HPA) are the unique class of inorganic complexes. They are widely used in different areas of science in biochemistry for the precipitation of albumens and alkaloids, in medicine as anticarcinogenic agents, in industry as catalysts. HPA are well known analytical reagents for determination of phosphoms, silica and arsenic, nitrogen-containing organic compounds, oxidants and reductants in solution etc. 

There also exists interference from diphosphoric acid, other more highly condensed phosphoric acids, and their organic derivatives. The free phosphoric acid can be determined as a heteropolyacid complex of phosphoric acid and ammonium molybdate. Afterward the complex is reduced by stannum II chloride to molybdenum blue. The amount of this dye can be measured photometricly at 625 nm. Organic derivatives of phosphoric acid and condensed phosphoric acids do not interfere with this method. 

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

As was stated above, the very strong acidity (and probably together with the organophilicity of the pore wall) makes these salts very active catalysts in liquid-solid organic reaction systems. We wish to emphasize that this is the first example for the shape selective catalysis of heteropolyacids at least to our knowledge. 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Other materials that have been investigated include sulfated zirconia, Br0nsted and Lewis acids promoted on various supports, heteropolyacids, and organic resins, both supported and unsupported. On the whole, these materials also deactivate rapidly, and some of them also exhibit environmental and health hazards. 

Izumi, Y., Urabe, K., and Onaka, M., Zeolite, Clay, and Heteropolyacids in Organic Reactions, Kodansha, Tokyo, VCH, Weinheim, 1992. 

Heteropolyacids are frequently used to modify proton-conducting composites,or they are just dispersed in inert matrixes.However, because the proton conduction mechanism of such hydrated salts is similar to those of hydrated polymeric sys-tems, these composites show qualitatively similar transport properties. The same is true for organically modified inorganic layered compounds such as titanium phosphate sulfophenylenphosphonate, the conductivity of which is dependent on the RH value, in a manner similar to that observed with Nafion. ... 

Heteropolyacids are more active catalysts for various reactions in solution than conventional inorganic and organic acids, and they are used as industrial catalysts for several liquid-phase reactions (6, 12). Important characteristics accounting for the high catalytic activities are the acid strength, softness of the heteropolyanion, catalyst concentration, and nature of the solvent (6, 7, 9, 116, 160, 161). 

The use of heteropolyacids as catalysts for fine organic synthetic processes is developing. Syntheses of antioxidants, medicinal preparations, vitamins, biologically active substances, etc., have been reported and some are already applied in practice (10, 160). 

The Wacker PdCl2-CuCl2 catalyst is a highly corrosive one, and its use requires special vessels and apparatus, such as titanium or tantalum alloys. Heteropolyacids have been used as alternative noncorrosive reoxidants of palladium for a variety of organic transformations,410 for example in reaction (155).411... 

Heteropolyacids are much more active than mineral acids for several types of homogeneous reactions in both organic solvents and aqueous solution [4, 8]. The enhancement is generally greater in organic solvents. For the hydration of isobutene in a concentrated aqueous HPA solution (above 1.5 mol dm-3), the reaction rate is about 10 times greater than for mineral acids [21]. This rate enhancement is attributed to the combination of stronger acidity, stabilization of protonated intermediates, and increased solubility of alkenes [21]. In this case, the selectivity is also much improved with HPA catalysts. 

Heteropolyacids and their salts have been studied as catalysts for oxidation both in the liquid and in the gas phase of several organic saturated and unsaturated substrates (44-50). The main features of these systems, which make them suitable for application as heterogeneous catalysts, can be summarized as follows (51-53) ... 

Y. Izumi, K. Urabe, and M. Onaka, Zeolite, Clay and Heteropolyacid in Organic Reactions, VCF1, Weinheim, 1992, p 157. 

The heteropolyacids are very soluble in water. They can form sparingly soluble or insoluble salts with ions such as ammonium, cerium, cesium, potassium, silver, and such. The acids are often soluble in organic solvents, such as alcohols, ketones, carboxylic acids, and carboxylic esters. Long-chain tetraalkylammonium salts can also be soluble in organic solvents. In a sense, heteropolyacids are soluble versions of insoluble metal oxide catalysts. They can be used as catalysts both in solution and as solids. A catalyst that is soluble in water would be a solid if used alone or in a hydrocarbon medium. They can also be placed on insoluble supports. In the insoluble forms at least, they offer the advantages of easy separation and recovery for reuse. 

Compare clays, heteropolyacids, and zeolites as catalysts for organic reactions. 

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