Abstract
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-β 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 1H/119Sn 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-β zeolite and open an avenue for the use of proton-detected solid-state NMR methods for characterization of metal sites in zeolite catalysts.
Highlights
Metal-substituted zeolites are an important type of solid Lewis acid with a wide range of applications
Tin-substituted Sn-β zeolite represents a breakthrough in the exploitation of atom-efficient solid Lewis acid catalyst for green and sustainable production of chemicals and fuels[2] because of its unparalleled catalytic performance in transformation of biomass and biomass-derived feedstocks[7,8,9,10,11,12,13,14,15,16] such as isomerization of glucose[8,9] and conversion of sugars to lactic acid[7] and furan derivatives[11] as well as in the important reactions of Baeyer−Villiger oxidations[2] and Meerwein−Ponndor−Verley reduction[3]
With the help of DFT calculations, hydrated closed and open Sn sites were proposed in postsynthetic Sn-β32, while the Sn species was claimed to exist mainly in closed state in Sn-β zeolite synthesized by the direct hydrothermal method[27]
Summary
Metal-substituted zeolites are an important type of solid Lewis acid with a wide range of applications. Two types of open tin sites are unambiguously identified via correlating the hydroxyl groups to Sn atoms with oneand two-dimensional proton-detected 1H/119Sn correlation solid-state NMR spectroscopy, which only amounts to ca. The results provide valuable insights into the nature of tin sites in Sn-β zeolite and open an avenue for the use of proton-detected solid-state NMR methods for characterization of metal sites in zeolite catalysts. We show that proton-detected 1H/119Sn double-resonance correlation solid-state NMR spectroscopy can be used to unambiguously characterize the Sn active sites in Sn-β zeolites. Two types of open Sn sites containing Sn–OH groups are directly identified by one-dimensional (1D) and two-dimensional (2D) 1H{119Sn} dipolar-mediated (D-) HMQC NMR spectroscopy at high field and fast MAS speed, which only amounts to ca. In combination with 2D 1H {29Si} D-HMQC NMR experiment, a reversible transformation between the open and closed Sn sites is ascertained
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