Characterisation of supported and unsupported 12-heteropolyacids

Abstract

Phase transformations of H4[SiW12O40].ᵪH2O in the temperature range 25 - 900°C have been investigated and characterised by differential thermal analysis (DTA), on thermogravimeteric analysis (TGA), X-ray powder diffraction (XRD), 29Si MAS NMR and FT-IR spectroscopy. Dehydration occurs between 25 - 300°C to give three crystalline hydrated states (ᵪ = 24,14 and 6) and finally an anhydrous form. Silica-supported H4[SiW12O40] with 25 wt % loading has been studied using DTA, TGA, XRD, FT-IR, 29Si MAS NMR and X-ray photoelectron spectroscopy (XPS). It is found that the Keggin anion remains unchanged and is well dispersed on the support. A chemically distinct interfacial H4[SiW12O40] species is identified by XPS and is attributed to perturbed tungsten atoms within Keggin anions in direct contact with the silica-support. Used catalysts have been characterised by DTA (under nitrogen), TGA, XRD, 29Si MAS NMR, FT-IR and XPS : the basic Keggin cage structure is unaffected by standard reactor conditions and lifetimes. TGA and DTA studies carried out under flowing air revealed two forms of coke deposited on the catalyst: hard and soft coke. Extraction with dichloromethane allowed the removal of soft coke (TGA removal range 170-370°C) but was unable to remove hard coke (TGA removal range 370-570°C). Removal of hard coke from industrial catalysts was attempted using aerobic oxidisation at 400°C, the complete oxidisation of the coke occurred after 6 hours. The acidic sites of H4[SiW12O40], H3[PW12O40] and H4[SiW12O40]/H3[PW12O40] (1:1 molar ratio), both silica-supported and non-supported, were characterised using solid state 13C NMR experiments for adsorbed acetone. For all three solid acids CP/MAS NMR experiments revealed the presence of isolated and hydrated acidic protons. The isolated acidic protons are restricted in mobility compared to the hydrated acidic protons. A reduction in the acidity of the hydrated acidic proton was observed when the 12-heteropolyacid compounds were supported on silica.