Quantitative Estimation of Effective Brønsted Acid Sites of Silica-Supported H₄SiW₁₂O₄₀ Heteropoly Acid for Adsorption of Various Molecules

The amount of incorporation of linear alcohols and ethers in H₄SiW₁₂O₄₀·6H₂O (HSiW·6H₂O, 50 wt %) supported on silica (SiO₂) was estimated by a conventional volumetric method and infrared (IR) spectroscopy, and the state of involved molecules was elucidated. First, the attribution of the key IR band at 2200 cm^-1, which was observed for the water of crystallization of HSiW·6H₂O, to H₅O₂⁺ species (protons) was verified by coincident observation of thermogravimetric-differential thermal analysis, X-ray diffraction (XRD), and IR spectroscopy during thermal treatment in addition to the isotope exchange with D₂O. The 2200 cm^-1 band was gradually decreased in intensity by increasing the amount of adsorption of pyridine and was totally consumed at saturation, while the volumetric method provided the accurate number of included pyridine molecules. Thus, the decrease of the integrated intensity of the 2200 cm^-1 band was employed to evaluate the extent of interaction of linear alcohols and ethers with H₅O₂⁺ species in equilibrium and irreversible conditions at room temperature. Linear alcohols (C1-C5) consumed almost all protons in equilibrium and about 50% in evacuation at room temperature, while about a half of protons were interacted in equilibrium and a quarter in evacuation in the case of dimethyl and diethyl ethers. Knowing the number of molecules incorporated and the amount of H₅O₂⁺ species consumed, the ratio of interacted molecules and H₅O₂⁺ species was estimated as H₅O₂⁺:molecules = 1:2-3. XRD patterns indicated the copresence of molecule-incorporated and molecule-free domains under irreversible conditions. Therefore, the incorporated molecules were concentrated around the H₅O₂⁺ species in HSiW·6H₂O crystals. The state of absorbed molecules in HSiW·6H₂O crystals was presumed by IR spectra: the presence of Fermi resonance of the hydrogen-bonded OH stretching band of H₅O₂⁺ species with CH deformation bands of interacted molecules implied the equilibrium state of as-interacted molecules and protonated species (transition state) at room temperature.