119Sn MAS NMR Study of the Interaction of Probe Molecules with Sn-BEA: The Origin of Penta- and Hexacoordinated Tin Formation

Sn-119 CPMG MAS NMR was applied to study the adsorption of acetonitrile, methanol, isopropyl alcohol, isobutyl alcohol, and water over Sn-BEA enriched with 119Sn isotope. Two signals observed at ca. -422 and -443 ppm over dehydrated samples were attributed to tetracoordinated framework tin sites with strong and weak Lewis acidity, respectively. The adsorption of acetonitrile and methanol resulted in the observation of pentacoordinated tin species, due to the formation of 1:1 adsorption complexes over both Sn sites. Water adsorption led first to formation of pentacoordinated tin species, which were further converted into hexacoordinated species at longer reaction times. The latter transformation was found to be kinetically limited and was attributed to chemical interaction of tin sites with water, such as hydrolysis of Si-O-Sn bonds. The adsorption of isopropyl alcohol and isobutyl alcohol was accompanied by the formation of pentacoordinated Sn species in the case of weak sites and hexacoordinated Sn over sites with strong Lewis acidity, pointing to the possibility of dissociative adsorption of secondary alcohols over strong Sn sites.