Tungstosilicic acids

Characterize your sample by one or more of the following techniques elemental analysis (Si and/or W), UV-vis spectroscopy (2x 10 M in 0.1 M HCl), FTIR 

The checkers collected crystals after 1 week and obtained a 53% yield. 

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Tokuyama A process for hydrating propene to propenediol, catalyzed by an aqueous solution of 12-tungstosilicic acid. Developed by Tokuyama (a Japanese company) and commercialized in Japan... 

Characterization of external surface properties of zeolite ZSM-5 modified by 12-tungstosilicic acid... 

We tried to monitor the acidic and catalytic properties of the external surface of a HZSM-5 sample after a wet impregnation with 12-tungstosilicic acid by probe molecules which are unable to access the pore system. 4-Methyl quinoline was used in order to characterize acidity... 

Argon adsorption proved the full accessibility of the ZSM-5 pore system after loading of 12-tungstosilicic acid and calcining (Table 1). 

Thermal analysis of the pure 12-tungstosilicic acid and sample lOW-ZSM-5 was performed on a SET ARAM TGDTA92 equipment using 34.3 mg and 31.6 mg of the sample, respectively, and a heating rate of 10 K/min. 

Figure 2. DTA/TG curves of unsupported 12-tungstosilicic acid (sample weight 34.3 mg, solid lines) and of sample 1 OW-ZSM-5 (sample weight 31.6 mg, dotted lines). Heating rate 10 K/min.

The process described by eq (3) actually means a decomposition of the acid reflected in the DTA curve by the exothermic peak without any mass loss as indicated by TG, in accordance with the stoichiometry of the oxide formation. A recent UHV-TG study of unsupported 12-tungstosilicic acid with respect to the process of water loss during activation revealed that already at about 432 K water is removed to an extent of approximately 76% [12]. DTA/TG measurements of supported 12-tungstosilicic acid (sample lOW-ZSM-5) could not resolve... 

DSC up to 875 K without pre-adsorption of 4-methyl quinoline did not resolve significant differences between sample OW-ZSM-5 and sample lOW-ZSM-5 (Figure 3, curve a, c). The DSC curve of the unloaded zeolite shows that water desorption proceeds in several steps characterized by three endothermic processes. The process at 560 K is most likely caused by condensation of hydroxyls preceding water desorption. These three endothermic processes are retained even after modification with 12-tungstosilicic acid (sample 1 OW-ZSM-5). 

It is concluded that the dispersion of the acid is fairly high and that larger crystallites of 12-tungstosilicic acid were formed neither on the external ZSM-5 surface nor as separate phase. 

According to results of Lefebvre et al. [11], a loading as high as 17 wt.% is necessary for supported 12-tungstosilicic acid on silica to yield X-ray diffraction lines of crystalline acid. 

This confirms our view that the loading of 12-tungstosilicic acid in sample 1 OW-ZSM-5 (corresponding to 13 wt.% 12-tungstosilicic acid) does not allow formation of a heteropolyacid bulk phase on the zeolite surface. 

Among the ethyltoluenes, the para isomer is considered to be preferably formed in the alkylation reaction inside the zeolite channel system [14], whereas or//jo-ethyltoluene is mainly formed on external surface sites [15]. This view is completely in agreement with our findings that or/Ao-ethyltoluene formation is negligible even on the unmodified H-ZSM-5 and that it is entirely suppressed after passivation of the external surface by 12-tungstosilicic acid. The formation of small amounts of or/Ao-ethyltoluene over lOW-ZSM-5 agrees with the decrease in / ara-selectivity as explained for this sample above. 

H4SiWi204o-a H20 Silico tungstic acid H4Si04l2W0s- rHjO 12-Tungstosilicic acid... 

These experiments are suitable for students working individually or in pairs. The preparations typically require the following amounts of laboratory time ammonium 12-molybdophosphate, 1 h to get to the start of the final air-drying step 12-tungstosilicic acid, 3.5 h to get to the start of the crystallization step 12-tungstophosphoric acid, about 2 h to get the start of the crystallization step (about 3h if the diethyl precipitation procedure is followed) 12-molybdophosphoric acid, about 2.5 h to get to the point of drying the solid. 

Klisch, M., 1998. 12-tungstosilicic acid (12-TSA) as a tungsten precursor in alcoholic solution for deposition of xW0(3)(l-x)Si02 thin films (x <= 0.7) exhibiting electrochromic coloration ability. 7 SoZ-Ge/ Sci Technol 12 21-33. 

Pielichowski and Flasik [22] studied the thermal properties of a new family of catalysts on the bulk doped and surface doped heteropolyanion-doped polyaniline by TGA and DTA. It was found that all systems exhibited good thermal stability at the temperature of their catalytic application (up to 250 "C) in both oxidative and inert reaction atmospheres. Thermal stability varied significantly with the dopant used, being highest for 12-tungstosilicic acid and the lowest for 12-molybdophosphoric acid. 

Bielanski A, Lubanska A, Pozniczek J, Micek-Ilnicka A (2003) Oxide supports for 12-tungstosilicic acid catalysts in gas phase synthesis of MTBE. Appl Catal A 238 239-250... 

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