Structural studies of high dispersion H3PW12O40/SiO2 solid acid catalysts

Abstract

Highly dispersed H₃PW₁₂O₄₀/SiO₂ catalysts with loadings between 3.6 and 62.5 wt% have been synthesised and characterised. The formation of a chemically distinct interfacial HPW species is identified by XPS, attributed to pertubation of W atoms within the Keggin cage in direct contact with the SiO₂ surface. EXAFS confirms the Keggin unit remains intact for all loadings, while NH₃ adsorption calorimetery reveals the acid strength >0.14 monolayers of HPW is loading invariant with initial ΔH_ads = ∼−164 kJ mol⁻¹. Lower loading catalysts exhibit weaker acidity which is attributed to an inability of highly dispersed clusters to form crystalline water. For reactions involving non-polar hydrocarbons the interfacial species where the accessible tungstate is highest confer the greatest reactivity, while polar chemistry is favoured by higher loadings which can take advantage of the H₃PW₁₂O₄₀ pseudo-liquid phase available within supported multilayers.