Changes in Surface Acidity of Silica-Supported Dodecatungstosilicic Acid in Relation to the Loading Amount

Changes in surface acidity of H4SiW12O40/SiO2 in relation to the loading amount were investigated by using temperature-programmed desorption (TPD) of benzonitrile (BN), BN infrared (BN-IR) spectroscopy, and test reactions, including skeletal isomerization of n-butane and double-bond shift of 1-butene. From BN-TPD profiles and BN-IR spectra, H4SiW12O40/SiO2 had medium strength Bronsted (Mid-B site) and strong Bronsted acid sites (Strong-B site) on its outermost surface, whereas there was no evidence for Lewis acid sites regardless of the loading amount. The number of Strong-B sites correlated with the catalytic activity for the skeletal isomerization of n-butane and was a maximum when the loading amount was 50 wt %. On the other hand, the number of Mid-B sites was a maximum for 30 wt % H4SiW12O40/SiO2, which had the highest activity for the double-bond shift reaction of 1-butene. Mid-B sites of H4SiW12O40/SiO2 were present in the starting H4SiW12O40, whereas the Strong-B sites formed when H4SiW12O40 was supported on SiO2. There were no Strong-B sites on H4SiW12O40/SiO2 with low loadings, including 5 and 10 wt %. Thus, it was concluded that the direct interaction of H4SiW12O40 with the surface of SiO2 caused the Strong-B sites to form in the second-layer of H4SiW12O40 exposed on the surface.