In Situ FTIR Studies of the Protonic Sites of H₃PW₁₂O₄₀ and Its Acidic Cesium Salts M_xH_3-xPW₁₂O₄₀

Author： Essayem N. Holmqvist A. Gayraud P.Y. Vedrine J.C. Ben Taarit Y.

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.197, Iss.2, 2001-01, pp. : 273-280

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Abstract

The dehydration mechanism of 12-tungstophosphoric acid H₃PW₁₂O₄₀ and of its acidic cesium salts Cs_xH_3-xPW₁₂O₄₀, have been investigated by in situ FTIR spectroscopy at 240 K. Both protonic species and vibrational modes of the Keggin anions have been followed in situ as a function of the extent of dehydration. After being outgassed in vacuo at ambient temperature, di-aqua protons, H₅O₂⁺, and/or hydroxonium ions, H₃O⁺, species were detected for H₃PW₁₂O₄₀ and Cs₂HPW₁₂O₄₀ samples. These species are characterized by a δ(H₂O) vibration band at 1708 cm^-1 and a ν(OH) vibration band at 3355 cm^-1. The intensity of these absorption bands progressively decreased as the protons were substituted by Cs⁺. Further dehydration leads to the decomposition of the former protonic species with the formation of hydroxyl groups, characterized by a broad absorption band centered at ∼3209 cm^-1. After degassing at 673 K, the heteropolycompounds were completely dehydroxylated. The decomposition of protonic water clusters induced strong modifications of the vibration bands of the Keggin anion ascribed to changes in bond lengths and in the local symmetry of the anion. The extent of these modifications is closely connected to the proton content of the heteropolyacid. The Cs_2.7H_0.3PW₁₂O₄₀ sample showed only minor changes with dehydration.