Direct observation of tin sites and their reversible interconversion in zeolites by solid- state NMR spectroscopy

Metal-substituted zeolites are an important type of solid Lewis acid with a wide range of applications. Despite the importance of this type of catalyst, identifying active sites can be challenging because different types of metal sites experience similar environments in zeolites. Here we show direct observation of different tin sites in Sn-beta zeolite. Two types of open tin sites are unambiguously identified via correlating the hydroxyl groups to Sn atoms with one- and two-dimensional proton-detected H-1/Sn-119 correlation solid-state NMR spectroscopy, which only amounts to ca. 17% of the total tin content. A reversible transformation between the open and closed tin site is observed. The results provide valuable insights into the nature of tin sites in Sn-beta zeolite and open an avenue for the use of proton-detected solid-state NMR methods for characterization of metal sites in zeolite catalysts.