12-Tungstophosphoric acid

In a 100 mL beaker, sodium tungstate dihydrate Na2W04 2H2O (16.2 g, 

1 mmol) and disodium hydrogen phosphate hydrate Na2HP04 H2O (2.6 g, 

2 mmol) are dissolved in boiling water (25 mL) with stirring. Then, eoneen-trated hydrochlorie acid (13 mL, 156 mmol) is added dropwise with stirring. The product begins to precipitate as a white sohd when most of the acid is added. The suspension is cooled in an ice bath, and the sohd is collected by filtralion and washed quickly with cold water. Yield 3.0 g (30%). 

H3[PWi204o] is a white crystalline solid that, as isolated, possesses about 20-29 water molecules per Keggin unit. In order to reach the hexahydrate state, the final product is dried over CaCl2 in a desiccator for several days. It is highly soluble in water, ethanol, ether, acetonitrile, and acetone, but is insoluble in chloroform and hydrocarbons, and decomposes in basic aqueous solution (e.g., 0.5 M NaOH). IR (cm ) 1080 pJP-O), 990 p(W=0), 890 uJW-O-W), 804 uJW-O-W). Powder XRD (20 in degrees, Cu Ka radiation, 2 = 1.5418 A) 10.2, 14.5, 17.8, 

The yield can be increased to 50-80% with some difficulty as follows. After the acid addition step, the resulting suspension is cooled and transferred to a 250 mL separatory funnel. Diethyl ether (2 mL) is added and the mixture is shaken. Additional diethyl ether is added in 2 mL aliquots, followed by shaking each time, until 10 mL of ether has been added in total. At this point, if solid is present in addition to the liquid layers, water (20 mL) is added and the mixture is vigorously shaken until three liquid layers are present. The lowest layer is an oil and contains the heteropoly acid as an etherate complex, the middle layer is water, and the upper layer is diethyl ether. The lowest layer is separated and the other two layers are discarded. The lowest layer is washed twice with water (20 mL each time). Then, the lower layer is separated, placed in a Petri dish, and evaporated to dryness under a flow of air inside a fume hood. Crystals form in about 2 days. The product may be recrystallized from a minimum amount of water and dried at 150-200°C in air to remove acid and ether residues. 

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The selectivity for acetic anhydride in the catalytic dehydration of acetic acid could be controlled by the pore size of pure mesoporous silica SBA-15. New acid catalyst comprising Keggin-type heteropoly acid supported on SBA-15 enhanced the activity etfectively when tungstophosphoric acid was highly dispersed on the silica substrate. 

Apart from a few reports" on solid acid catalyzed esterification of model compounds, to our knowledge utilization of solid catalysts for biodiesel production from low quality real feedstocks have been explored only recently. 12-Tungstophosphoric acid (TPA) impregnated on hydrous zirconia was evaluated as a solid acid catalyst for biodiesel production from canola oil containing up to 20 wt % free fatty acids and was found to give ester yield of 90% at 200°C. Propylsulfonic acid-functionalized mesoporous silica catalyst for esterification of FFA in flotation beef tallow showed a superior initial catalytic activity (90% yield) relative to a... 

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. 

TABLE 13.3 Isobutane/2-Butene Alkylation over Acidic Salts of 12-Tungstophosphoric Acids... 

In the case of heteropolyoxometalates, the compositions can also be tailored in order to achieve good catalytic properties. Table 13.3 shows the variation of the catalytic performance of acidic salts of 12-tungstophosphoric acids during the iso-butene/2-butene alkylation. ... 

Figure 13.7 shows an effective confrontation between the catalytic results observed on nafion/Si02, sulfated zirconia, and 12-tungstophosphoric acid supported on MCM-41 (HPW/MCM) at a reaction temperature of 50°C. Under these conditions, the HPW/MCM catalyst presents the better yield of TMP. However, similar productivities to that could be obtained on nafion/Si02, or sulfated zirconia, at higher or lower temperatures, respectively, than those optimized for the HPW/MCM-41 catalyst. ... 

Figure 13.7 Conversion of 2-butene and the selectivities to cracking products, TMP, and C9+ hydrocarbons during the isobutane alkylation at 50°C on nafion/Si02 (NS-1), sulfated zirconia (SZ), and MCM-41-supported 12-tungstophosphoric acid (HPW/MCM). Experimental conditions T = 32 C TOS = 1 min molar ratio of 15.

Figure 13.8 Catalyst decay during the isobutane alkylation on nafion/Si02, sulfated zirconia, beta-zeolite, and MCM-41-supported 12-tungstophosphoric acid.

The tungstophosphoric acid shows high activity, close to H2SO4 used as a benchmark. Regrettably, this acid is soluble in water and hence not usable as a solid catalyst. However, the corresponding cesium salt (Cs2.5) is also super acidic and its mesoporous structure has no limitations on the diffusion of the reactants. Cs2.5 exhibits low activity per weight, hence it is not suitable for industrial applications. 

Further research has shown that bifunctional acid-base catalysts such as WO3 on AI2O3 or tungstophosphoric acid, lacking shape-selective nature can also bring about the methyl alcohol-hydrocarbon conversions.79 Shape selectivity of the catalyst thus is important in controlling product distributions and also to limit coking over the catalysts. 

Several compounds of tungsten, which is a transition metal closely related to molybdenum, are able to catalyze the selective oxidation of secondary alcohols with hydrogen peroxide as secondary oxidant. These include tris(cetylpyridinium) 12-tungstophosphate,33 peroxotungstopho-sphate (PCWP)34 and Na2W04 in the presence of a phase transfer catalyst.35 Tungstophosphoric acid is able to catalyze the selective oxidation of secondary alcohols in the presence of ferric nitrate as secondary oxidant.36... 

Name Ammonium tungstophosphate on silica Structure Tungstophosphoric acid with ammonium nitrate... 

Tungstophosphoric acid (20% in ethanol) Variety of colored spots after heating 71... 

The reaction between a,(3-unsaturated aldehydes and thiophenols is catalyzed by tungstophosphoric acid and in the absence of solvent at room temperature thiochromans are formed rapidly and in good yield (Equation 161). The heteropolyacid is recoverable and reusable <2005TL7567>. 

These principles are illustrated in the crystal structure of 12-tungstophosphoric acid hydrate, as determined by X-ray diffraction (Fig. 9)81,82. Of the 29 water molecules, 17 are held together in a well-defined group by hydrogen bonding. The... 

Vinu and others219 exploited 31P CP/MAS NMR spectroscopy together with other instrumental techniques to describe physicochemical characterisation of a catalyst consisting of tungstophosphoric acid supported on zirconia and meso-porous silica MCM-41 and MCM-48. 

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