Silicotungstic acid

The molybdenum trioxide in the anhydrous material is volatilized in a stream of dry hydrogen chloride. The residue is weighed as impure silica. The silica is then determined by the loss in weight when volatilized with hydrofluoric and sulfuric acids. The molybdenum trioxide is calculated by difference. 

Titration of the acid using methyl orange as the indicator gives an equivalent weight of 485. 

For a complete bibliography consult Mellor A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, Green Company, New York, Vol. VI, p. 890. 

Silicotungstic acid is prepared by the hydrolysis of a mixture of sodium tungstate and sodium silicate. This reaction goes to completion only in the presence of acid. Sulfuric, acetic, and nitric acids all have been used,1,213 but none of them is completely satisfactory. Sulfuric acid, since it cannot be volatilized from the silicotungstic acid, is apt to cause decomposition. It also tends to precipitate 

In this preparation, hydrochloric acid is used because it can be expelled readily from the partially dehydrated crystals by heating. 

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Sellaite, see Magnesium fluoride Senarmontite, see Antimony(III) oxide Siderite, see Iron(II) carbonate Siderotil, see Iron(II) sulfate 5-water Silica, see Silicon dioxide Silicotungstic acid, see Silicon oxide—tungsten oxide—water (1/12/26)... 

Papovic et al. [14] used a conductometric method for the determination of primaquine phosphate. This method is recommended with perchloric acid, silicotungstic acid, and sodium hydroxide as titrants. Conductometric curves obtained with... 

Scheme 3 Acrolein can be obtained by dehydration of glycerol. The reaction was reported many years ago using powdered KHSO4/K2SO4 as catalyst. Recently, the use of silica-supported heteropolyacids has also been described, notably with silicotungstic acid as catalyst.

The liquid phase Friedel-Crafts acylation of thioanisole with iso-butyric anhydride to produce 4-methyl thiobutyrophenone has been studied using supported silicotungstic acid catalysts. Reaction is rapid, giving the para-acylation product in high yield. Reactions have been performed in both batch slurry and trickle bed reactors. In both reactors catalyst deactivation due to strong adsorption of product was observed. 

In this paper we report the use of supported heteropoly acid (silicotungstic acid) and supported phosphoric acid catalysts for the acylation of industrially relevant aromatic feedstocks with acid anhydrides in the synthesis of aromatic ketones. In particular, we describe the acylation of thioanisole 1 with iso-butyric anhydride 2 to form 4-methyl thiobutyrophenone 3. The acylation of thioanisole with acetic anhydride has been reported in which a series of zeolites were used as catalysts. Zeolite H-beta was reported to have the highest activity of the zeolites studied (41 mol % conversion, 150°C) (2). 

The activity and selectivity of silicotungstic acid as catalyst has been evaluated in the acylation of thioanisole with iso-butyric anhydride. Reactions were performed using a 3 fold molar excess of thioanisole. The performance of silicotungstic acid was evaluated In both supported (42%STA/slllca) and bulk form. The results are shown in Table 1. 

Catalysts were prepared by impregnation of the appropriate support material with aqueous solutions of silicotungstic acid using JM proprietary methods. The concentration of STA in the impregnation solution was such as to achieve the desired loading on the catalyst support. Thereafter the impregnated material was dried in air (LOD <5%). Catalysts were used as prepared. Details of the catalyst supports used in this study are given in Table 6. 

In Refs. 3-5, potassium antimonyl tartrate was used as a source of Sb. While the deposition conditions appeared to be almost identical in all cases (other than the silicotungstic acid added in some cases in the latter two studies), there were some unexplained differences in structural and electrical properties (the latter shown in the table). The lower resistivities obtained in the first study were mirrored by higher carrier concentration (2 X 10 cm vs. 10 cm ) and slightly higher mobility (14 vs. 10 cm V sec ). 

In Ref. 57, a small concentration ( 10 M) of silicotungstic acid (H4SiWi204o) was added to a triethanolamine/ammonia/selenosulphate bath. 

Silicotungstic acid (tungstosilicic acid H4SiWi204o) [12027-38-2] M 2914.5. Extracted with ethyl ether from a solution acidified with HCI. The ethyl ether was evaporated under vacuum, and the free acid was crystallised twice [Matijevic and Kerker JPC 62 1271 1958],... 

The catalytic activity of these oxometalates is well documented [360, 361]. An inactive surface of Ti02 becomes an efficient catalyst for H2 evolution as it is derivatized with silicotungstic acid [362, 363]. However, while real electrocatalytic effects seem likely for a pigmented Ni surface in view of the lower Tafel slope observed (which can also be due to some activation of the Ni itself), these are not completely established for the surface of pure oxometalates surface area effects could be entirely responsible for the apparent activation. The real surface state of these electrodes deserves to be further investigated since these materials might fall into the category of amorphous phases. 

The preparation of silicomolybdic acid is based on a hydrolytic reaction similar to that used in the preparation of silicotungstic acid. (See synthesis 48.) Only a few modifications are necessary to avoid the precipitation or reduction of molybdic acid. 

The properties of silicomolybdic acid are like those of silicotungstic acid. Toward indicators having a pH of 5 to 6 it behaves as a tetrabasic acid. With phenolphthal-ein as the indicator it requires eight equivalents for titration in cold solution but twenty-four equivalents for titration at 100°C. With tertiary organic bases it is tetrabasic. 

The acid prepared by the following directions has about six molecules of water of hydration. Illingsworth and Keg-gin1 believe that the X-ray photographs show the penta-hydrate cubic packing. As in the case of silicotungstic acid, the amount of water of hydration is usually not important. The concentration of a solution of silicomolybdic acid can be determined by titration with a standard alkali or by evaporation followed by careful ignition. 

When silicotungstic acid is titrated with a base using methyl orange or chlorophenol red as the indicator, an end point is obtained when four equivalents have been added. The end point with chlorophenol is sharper because this indicator does not form a precipitate with the acid.4 When the acid is titrated using phenolphthalein as the indicator, the end point is not permanent. However, when the titration is carried out in a hot solution, an end point is reached when twenty-four equivalents of base have been added. The end products are sodium tungstate and silicic acid. 

Silicon, higher chlorides of, 42 Silicon tetrabromide, 38, 40 Silicon tetrachloride, 44 Silicopropane, octachloro, 44 Silicotungstic acid, 129 analysis, 131 ether complex, 131 Silver, metallic, 4 Silver chloride, reduction of, 3 Silver cyanamide, 98 Silver residues, purification of, 2 Sodium amalgam, 10 Sodium amide, 74 Sodium azide, purification of, 79 Sodium azidodithiocarbonate, 82 Sodium butoxide, 88 Sodium hypochlorite (solution), 90 Sodium iodate, 168 Sodium metaperiodate, 170 Sodium paraperiodate, chlorine method, 169 persulfate method, 170 Strontium amalgam, 11 Sulfur hexafluoride, 121 Sulfuryl chloride, 114... 

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