Comparison of H3PW12O40 and H4SiW12O40 heteropolyacids supported on silica by 1H MAS NMR

Abstract

The H 3 PW 12 O 40 and H 4 SiW 12 O 40 heteropolyacids supported on silica were studied by 1 H MAS NMR as a function of both the coverage and the dehydroxylation temperature. The signal at ca. 8 ppm attributed to the highly acidic protons of the anhydrous form of the heteropolyacid was never observed in the case of the phosphotungstic polyanion supported on silica while it was clearly present in the case of the silicotungstic species. These results explain the different activities of the two supported heteropolyacids in acid reactions involving strong acid sites. To cite this article: A.

Trialkyl tin salts of polyoxometalates: Synthesis and characterization of [α-SiW12O40][(CH3)3Sn(DMSO)2]4·2DMSO

“…On faujasite, which is known to be very acidic (this zeolite can achieve alkane cracking reactions), the reaction is observed even at À100°C [12]. This reaction could then be of interest for the determination of highly acid sites on supported heteropolyacids and give another proof of the results obtained by solid-state 1 H MAS NMR [13]. It should also be interesting, from a chemical point of view, to determine how the tin complex interacts with the polyoxometalate and it is for this reason that we have studied the reaction of the anhydrous heteropolyacid with tetramethyltin.…”

“…1 H, 13 C and 119 Sn liquid NMR spectra were recorded on a Bruker AC-300 spectrometer operating at 300.13 MHz for proton. Anhydrous DMSO-d 6 was purchased from SDS and used as received.…”

“…The preparation is classically made by treatment under vacuum at 200°C of the solid hydrated heteropolyacid. The obtention of the anhydrous heteropolyacid can be checked by solid-state 1 H MAS NMR [13]. A lot of studies have described this preparation but there is no report of the solubility of this anhydrous in various solvents.…”

“…Anhydrous dodecatungstosilicic acid, prepared by treatment under vacuum at room temperature of the hydrated polyacid [13], was dissolved in anhydrous DMSO (1 g of heteropolyacid in 5 mL of solution). Tetramethyltin (0.8 mL corresponding to 16 equivalents per polyacid) was then added after freezing the solvent at 0°C.…”

Gas phase dehydration of glycerol catalyzed by rutile TiO2-supported heteropolyacids

“…The HSiW-TiO 2 catalysts had lower acid strength than the HPW-TiO 2 catalysts, but the catalytic activity of the HSiW-TiO 2 catalysts was higher than that of the HPW-TiO 2 catalysts. The reason why HSiW-TiO 2 catalysts had high catalytic activity in the gas phase dehydration of glycerol is probably due to that silicotungstic acid is more stable than phosphotungstic acid at high reaction temperature, supplying strong acid protons for the dehydration reaction [5,17]. In addition to the effect of high acid strength, another reason is probably due to that silicotungstic acid has a high resistance to water [5].…”