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 B 2 O 3 −SiO 2 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-Al 2 O 3 . No coordination of carbon monoxide is observed on B 2 O 3 −SiO 2 even at low temperatures, whereas strongly coordinated CO species are formed on γ-Al 2 O 3 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 B 2 O 3 −SiO 2 than on γ-Al 2 O 3, 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 Al 3+ and B 3+ Lewis acid sites adequately reproduce experimental observations. The main difference between Al 3+ and B 3+ results from the higher energy required to convert boron from a trigonal planar conformation to a tetrahedral conformation upon adsorption of the probe molecule. Despite strong charge transfer, this leads to a weaker adsorption of pyridine and acetonitrile on B 3+ as compared to Al 3+ Lewis acid sites. Carbon monoxide is not basic enough to compensate for the energy required for the conformational change of the B 3+ Lewis acid center.