(594d) Characterization of the Structural Changes of Tin Sites throughout the Life Cycle of Sn-ß Prepared By Post-Synthetic Methods

Lewis acidic catalysts have emerged at the forefront for important biomass transformations, oxidations, and sugar isomerization/upgrading [1]. In the material space of solid acid catalysts, Sn-β zeolites have shown remarkable catalytic activity for carbonyl group activation, which stems from the strong Lewis acid strength of Sn^IV metal ions substituted in the zeolite framework [2]. However, differences in the synthesis protocols of hydrothermal and post-synthetic strategies have led to a wide distribution in Sn^IV site coordination and dispersion in the final material, complicating structure-activity relationships [3,4]. Therefore, in this work, we aim to contribute to the characterization of Sn-β materials (>5 wt%) that contain sites that undergo structural changes throughout their life cycle: synthesis preparation, storage, reaction, and regeneration. Our approach involves synthesizing Sn-β through reflux grafting [5] and solid-state ion exchange (SSIE) [6] under controlled conditions (i.e., in the absence of solvents), characterizing Sn speciation, and correlating structural changes to catalytic performance. After Sn incorporation and oxidative pretreatments, moisture-free Sn-β materials showed isomorphous substitution of Sn^IV (UV-vis DRS) and the presence of a mixture of open (FTIR 3668 cm^-1; [7]) & closed Sn for SSIE procedure in contrast to solely closed species for the grafted catalyst as evidenced by FTIR, and ¹¹⁹Sn ssNMR. Adsorption of deuterated acetonitrile coupled with FTIR supported the presence of these sites and the stronger interaction with open Sn, as evidenced by acetonitrile desorption kinetics at 25^oC under vacuum (10^−4Torr). Different extents of reversible changes in Sn coordination between IV & VI with hydration and dehydration treatments were observed for synthesized Sn-β materials with UV-vis DRS, FTIR, and ¹¹⁹Sn ssNMR. Through this work, we offer insights into the use of detailed catalyst characterization to obtain information on potential synthetic knobs for developing more defined solid acids for biomass conversion and improved anchoring sites for metal incorporation.