Restructuring of\nthe Lewis Acid Sites in Y‑Modified\nDealuminated Beta-Zeolite by Hydrothermal Treatment

Yttrium-modified dealuminated Betazeolite (Y-BEA) represents\na\ntype of Lewis acid zeolite that has gained attention for its potential\nto efficiently catalyze the conversion of biomass-derived oxygenates.\nThe structure of the Y active sites and their dynamics during biomass\nconversion reactions, which normally involve substantial amounts of\nwater, necessitate thorough investigation for the rational design\nof more active and stable catalysts. Here, we conducted a study where\na series of Y-BEA catalysts with different yttrium loadings (1–7\nwt.%) were subjected to hydrothermal treatment (450 °C, 20% water)\nand investigated for their structural and catalytic activity changes\nthrough a combination of multiple characterizations and kinetic measurements.\nThe number of acid sites of Y-BEA decreased without a change in acid\nstrength following the hydrothermal treatment, which was confirmed\nby the results of acid site titration, infrared spectroscopy of probe\nmolecules, and kinetic measurements for probe reactions (acetone aldol\ncondensation). Structural analysis using X-ray diffraction (XRD),\nspecific surface area measurement, X-ray absorption spectroscopy (XAS),\nand X-ray photoelectron spectroscopy (XPS) demonstrated that both\nthe zeolite structure and the isolation status of the Y site remain\nintact after hydrothermal treatment. Further, the Diffuse Reflectance\nInfrared Fourier Transform Spectroscopy (DRIFTS) spectra, thermogravimetric\nanalysis (TGA), and operando <sup>1</sup>H and <sup>29</sup>Si magic-angle\nspinning (MAS) nuclear magnetic resonance (NMR) revealed the dehydroxylation\nof Y-BEA induced by hydration-rearrangement-condensation restructuring\nduring the high-temperature steam treatment. Dehydroxylation affects\nthe structure of Y sites by reducing their vicinal silanol sites.\nThis conversion of Lewis acidic Y sites into nonacidic sites is the\nprimary factor behind the change in acid site quantity and catalytic\nactivity on Y-BEA.