Evidence for Discrepancy between the Surface Lewis Acid Site Strength and Infrared Spectra of Adsorbed Molecules: The Case of Boria−Silica

Abstract

The acidity of amorphous B2O3−SiO2 has been investigated by infrared spectroscopy using the following three probe molecules presenting a wide range of basic strength: pyridine, acetonitrile, and carbon monoxide. The results are compared to those obtained on γ-Al2O3. No coordination of carbon monoxide is observed on B2O3−SiO2 even at low temperatures, whereas strongly coordinated CO species are formed on γ-Al2O3 under such conditions. Coordinated pyridine and acetonitrile show important infrared frequency shifts on both metal oxides, indicating strong charge transfer from the probe molecules to the surface Lewis acid centers. However, the thermal stability of coordinated species is much lower on B2O3−SiO2 than on γ-Al2O3, which suggests that there is no direct correlation between charge transfer and adsorption energy. Density functional theory (DFT) calculations on the interaction of these probe molecules with simple models representing Al3+ and B3+ Lewis acid sites adequately reproduce experimental obser...