Origin of Water-Induced Bronsted Acid Sites in Sn-BEA Zeolites

Transformation of the Sn-BEA site structure during the interaction with water has been investigated by means of Fourier transform infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and catalytic experiments. It is shown that the Lewis and Bronsted acid properties of Sn-BEA zeolite before and after the adsorption of water change significantly. New surface OH groups exhibiting different structures are observed after adsorption, whereas tin oxide supported on Si-BEA is inactive in this transformation. It is demonstrated that the formed bridged OH groups possess strong Bronsted acidity, thus enabling the protonation of pyridine. It is suggested that the adsorption of water occurred over tin Lewis acid sites followed by the hydrolysis of the Si-O-Sn bonds and the formation of Si-OH and Sn-OH surface species. In this process, the tin atoms change their coordination number from 4 to 6, possessing different kinetics for the different types of Sn sites observed by NMR spectroscopy. The formation of additional catalytically active acid sites through water adsorption on Sn-BEA is demonstrated in situ in the course of isobutene dimerization reaction.