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

The amount of incorporation of linear alcohols and ethers in HSiWO·6HO (HSiW·6HO, 50 wt %) supported on silica (SiO) 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, which was observed for the water of crystallization of HSiW·6HO, to HO 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 DO. The 2200 cm 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 band was employed to evaluate the extent of interaction of linear alcohols and ethers with HO 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 HO species consumed, the ratio of interacted molecules and HO species was estimated as HO: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 HO species in HSiW·6HO crystals. The state of absorbed molecules in HSiW·6HO crystals was presumed by IR spectra: the presence of Fermi resonance of the hydrogen-bonded OH stretching band of HO species with CH deformation bands of interacted molecules implied the equilibrium state of as-interacted molecules and protonated species (transition state) at room temperature.