Influence of the nature and porosity of different supports on the acidic and catalytic properties of H3PW12O40

Abstract

In order to obtain highly dispersed heteropolyacid (HPA) species, H3PW12O40 was supported on various supports exhibiting different porosities and surface chemical properties. Amorphous and monoclinic amphoteric zirconias, activated montmorillonite (AC) and hexagonal silica (HMS) were chosen as supports. It was observed that the zirconia support partly decomposed HPA at low coverage, giving the lacunary anion PW11O39 7−, due to the reaction with basic hydroxyl groups, whereas montmorillonite and HMS did not. Catalytic properties for n-butane to isobutane isomerisation at 473 K and propan-2-ol decomposition at 353 K were compared for all samples as a function of HPA loadings and compared to data already published on bulk H3PW12O40 and Cs1.9H1.1PW12O40 samples. It was found for HPA/AC samples that, although their activity for propan-2-ol decomposition varied linearly with HPA loading, their activity for n-butane isomerisation was very weak, which indicates a weaker acid strength by supporting the HPA. This is probably due to the exchange of protons from the HPA by the exchangeable cations of the montmorillonite, the new protons associated with the clay being much less acidic. It also appeared, when comparing with catalytic data already published on for HPA/HMS, that HMS was the best support for HPA without modifying appreciably its catalytic and thus their acid properties, the HPA being certainly bonded to the HMS walls by hydrogen bonding. Assuming a diameter of 1.2 nm for each Keggin anion, the turnover frequency (TOF) values for n-butane isomerisation at 473 K were calculated per surface Keggin species for HPA/HMS, bulk H3PW12O40 and Cs1.9H1.1PW12O40, and were found to be equal to 155, 113 and 100×10−5 s−1, respectively, and thus to be very close, showing that the acid strength of the three samples was comparable. It was found to equal only 3.4×10−5 s−1 for the HPA/AC sample, in agreement with a weaker acid strength. At last, mesoporosity of the support was found to favour n-butane isomerisation reaction.