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<middle dot>6H2O (HSiW<middle dot>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<middle dot>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<middle dot>6H2O crystals. The state of absorbed molecules in HSiW<middle dot>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.