Characterization of H4SiW12O40 supported on mesoporous silica (SBA-15), non-structured amorphous silica and γ-alumina

Abstract

H4SiW12O40 (HSiW) was supported on mesoporous silica (SBA-15), non-structured amorphous silica (AEROSIL) and γ-alumina (Al2O3) by impregnation. The supported HSiW samples were characterized by X-ray diffraction (XRD), N2 adsorption-desorption isotherms, infrared (IR) spectroscopy and ethanol dehydration reaction as a model reaction. HSiW was not highly dispersed even at 29 wt% loading, but the hexahydrate (HSiW·6H2O) was stable under evacuation below 150 °C on silica supports, while destruction of Keggin units occurred on Al2O3. IR spectra of HSiW·6H2O loaded SBA-15 and AEROSIL were analyzed in detail in combination with changes in surface area and XRD patterns. The loaded HSiW·6H2O crystals possessed weak secondary interaction with adjacent silanol groups. The crystal size was increased in three dimensions on AEROSIL by increasing loading amount, while the extension of the size to the pore direction was found on SBA-15. The number of protons in supported HSiW·6H2O was quantified by the integrated intensity of the key band of pyridinium cation (C5H5N+H) at 1538 cm−1 in IR spectra. At high loading (>29 wt%) four hydrogen atoms per Keggin unit in HSiW·6H2O were detected as proton on AEROSIL and SBA-15 with negligible amount of Lewis acid sites, while less than a half on Al2O3 with a majority of Lewis acid sites. Thus, IR method using pyridine as a probe can quantify Brønsted and Lewis acid sites in the absence of thermal decomposition of loaded HSiW·6H2O. It was also found that all the protons detected by pyridine adsorption could not catalyze the reaction over HSiW/Al2O3, suggesting the necessity of a caution to relate the acid amount measured by probe molecules to catalysis.