Acidity of Heteropolyacids

In the solid state, the primary Keggin stmctures pack into secondary and tertiary 

Thermogravimetric analysis indicates that the waters of hydration are removed at temperatures above around 500 K. This can lead to a dehydrated secondary form. Ab initio calculations indicate that the loss of secondary water molecules will begin to isolate protons on the bridging positions between two Keggin units (Fig. 5.16), which for the most part will be inaccessible for reaction. 

From Table 5.4, one can see that the whereas the proton prefers the bridging oxygen atoms, the difference between the bridge and atop sites is fairly small, which would suggest that the proton may be quite mobile, having the ability to diffuse between different sites. 

The acidity of the PW12O40 is such that it can stabilize the adsorption of an alkene. Janik et alJ l have shown that the barriers for the protonation of ethylene, propene, 2-butene and isobutene are 69, 50.5, 51 and 14 kJ/mol, respectively. The results indicate that the formation of the carbenium ion from the n adsorbed state is favored on the Keggin structure over the similar corresponding state in the chabazite (see Chapter 4). For example, the barrier for the protonation of propene in chabazite was calculated to be -1-56 kJ/mol in comparison with the value of 50.5 kJ/mol calculated on the phosphotungstic (HPW) Keggin structure. The barriers, however, for the reaction of a surface alkoxy to the carbenium ion state are all higher on phosphotungstic acid than on chabazite. 

The barriers for formation of ethylene, propene, 2-butene and isobutene from the alkoxy HPW states were calculated to be 114, 93.7, 87.8 and 64 kJ/mol, respectively, as compared with the value of 83 kJ/mol for propene over chabazite. Cleavage of the alkoxide bond initiates consecutive reaction steps necessary for many hydrocarbon conversion reactions. 

---

Janik, M.J., Davis, R.J. and Neurock, M. (2005) The relationship between adsorption and solid acidity of heteropolyacids. Catal. Today, 105, 134. 

The heteropolyanion stabilizes protonated intermediates by coordination in solution and the pseudoliquid phase as well as on the surface, thus lowering the activation energy and accelerating reactions. Several protonated intermediates including the protonated ethanol dimer and monomer [18], the protonated pyridine dimer [12, 19], and protonated methanol [20] have been detected in the pseudoliquid phase directly by use of X-ray diffraction (XRD), IR or solid-state NMR. In solid-state H NMR, the chemical shift for the protonated ethanol dimer, (C2HsOH)2H+ is 9.5 ppm down-field from tetramethylsilane, which lies in the range of supcracids reported by Olah et al. [18]. This fact also supports the strong acidity of heteropolyacids. 

The strong acidity of heteropolyacids (HPA) make them suitable as catalysts for many acid-catalyzed reactions. Since HPA are less corrosive and produce lower amount of waste than conventional acid catalysts, as sulfuric acid, they can be used as replacement in environmentally benign processes. 

Isopropanol was used as a probe molecule to characterize the acidity of heteropolyacid compounds because the product s distribution upon reaction depends on the nature of the surface active sites. Strong Brdnsted (H ) and Lewis acid sites catalyze the dehydration of isopropanol to propylene (di-isopropyl ether over weak Lewis acid sites), and redox/basic sites lead to the dehydrogenation of the alcohol to acetone. 

Sodium tungstate is used in the manufacture of heteropolyacid color lakes, which are used in printing inks, plants, waxes, glasses, and textiles. It is also used as a fuel-ceU electrode material and in cigarette filters. Other uses include the manufacture of tungsten-based catalysts, for fireproofing of textiles, and as an analytical reagent for the deterrnination of uric acid. 

Hori H, E Hayakawa, K Koike, H Einaga, T Ibusuki (2004b) Decomposition of nonafluoropentanoic acid by heteropolyacid photocatalyst H3PWJ2O4Q in aqueous solution. J Mol Cat A 211 35-41. 

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. 

Cesium salts of 12-tungstophosphoric acid have been compared to the pure acid and to a sulfated zirconia sample for isobutane/1-butene alkylation at room temperature. The salt was found to be much more active than either the acid or sulfated zirconia (201). Heteropolyacids have also been supported on sulfated zirconia catalysts. The combination was found to be superior to heteropolyacid supported on pure zirconia and on zirconia and other supports that had been treated with a variety of mineral acids (202). Solutions of heteropolyacids (containing phosphorus or silicon) in acetic acid were tested as alkylation catalysts at 323 K by Zhao et al. (203). The system was sensitive to the heteropoly acid/acetic acid ratio and the amount of crystalline water. As observed in the alkylation with conventional liquid acids, a polymer was formed, which enhanced the catalytic activity. 

Two types of insoluble triarylcarbonium compounds are used industrially as pigments. Both are salts of these basic dyes. So-called Alkali Blue type triarylcarbonium pigments are inner salts of sulfonic acids, while the second group comprises salts of complex inorganic anions of heteropolyacids. 

They present strong acidities (the pH values of aqueous solutious of heteropolyacids indicate that they are strong acids) both in solid and in liquid solution (Figure 13.2). In addition, they can be prepared in an wide range of surface areas (partially salified heteropolyoxometalates permit the modification of the surface areas of these materials) or be supported in metal oxides. 

Heteropolyacids are prepared in solution by acidifying and heating in the appropriate pH range (], 49-54). For example, 12-tungstophosphate is formed according to Eq. (1). Free acids are synthesized primarily by the following two methods (1) by extraction with ether from acidified aqueous solutions and (2) by ion exchange from salts of heteropolyacids. Dawson-type heteropolyanions,... 

The strength and the number of acid centers as well as related properties of heteropolyacids can be controlled by the structure and composition of heteropolyanions, the extent of hydration, the type of support, the thermal pretreatment, etc. 

Tokuyama Soda commercialized a catalytic process for propylene hydration catalyzed by aqueous solutions of heteropolyacids such as H3PW12O40 (165). In Table XV, the activities of various acids are compared at a constant proton concentration. H3PW12O40 is two or three times more active than H2S04 or H3PO4. The reason for the high activity is assumed to be the stabilization of intermediate propyl cations by coordination. 

The data of H2S04, HC1, HNO3, and HC104 also fit this equation. On this basis, they suggested that the hydration of isobutylene in the presence of heteropolyacids and inorganic acids proceeds via a common mechanism, in which the rate-limiting step is the conversion of the 7t-complex into a carbenium ion (Scheme 3). The complexation effect as described above is possibly included in the value of Ho according to this explanation. 

The activities of heteropolyacids for the decomposition of isobutyl propionate were found to be 60-100 times higher than those H2SO4 and p-toluenesulfonic acid 63). The activity increases with increasing acid strength of heteropolyacids. 

In catalytic dehydration of alcohols, pseudoliquid phase behavior (bulk type I reaction) of heteropolyacids has been demonstrated (Section VI). The high catalytic activity is associated with this behavior and the strong acidity. Unique pressure dependences of the catalytic activity and selectivity are found for H3PW1204o due to the pseudoliquid phase (Fig. 40). 

The activity of H3PW12O40 is greater by four orders of magnitude than that of sulfuric acid. The use of heteropolyacids instead of H2S04 not only eliminated the formation of waste water containing toxic cresol sulfate, but also reduced the loss of p-cresol during neutralization and washing. 

The hydration rates of isobutylene in concentrated aqueous solutions of heteropolyacids (HPA) such as H3PM012O40 and H3PW12O40 have been found to be about 10 times higher than those in aqueous mineral acids. This acceleration was attributed to better solubility of isobutylene in concentrated HPA and stronger acidity of concentrated aqueous HPA, as... 

Recently, various kinds of solid superacids have been developed. The first group is metal oxides and mixed oxides containing a small amount of sulfate ion, and those modified with platinum. The second group is metal oxides, mixed oxides, graphite, metal salts, etc. treated or combined with antimony fluoride or aluminum chloride. The third group is perfluorinated polymer sulfuric acid (Nafion-H). The fourth and fifth groups are H-ZSM-5 and a type of heteropolyacids, respectively. The last group is simply mixed oxides. 

---