(52d) Quantification of the Active Site Distribution in Zeolites and Its Implications in Brønsted & Lewis Acid Catalyzed Reactions

From the gamut of solid catalysts, zeolites are widely considered materials uniform in structure with repeating porous environments and distinct crystallographic tetrahedral (T) sites. [1,2] Yet, for defined catalysts such as zeolites, molecular-level descriptions of reaction turnovers are still limited by imprecise knowledge of active site distribution and structure at the micro- and mesoscopic scales. [3,4]. Site heterogeneity and distribution are further exacerbated by differences in synthesis protocols of hydrothermal and post-synthetic strategies that lead to unwanted formation of framework associated and extraframework sites that impact catalytic performance. Therefore, this work aims to contribute to the characterization and titration of the active site distribution in zeolitic materials by developing quantitative strategies for a set of carefully prepared materials of interest, stannosilicates (Sn-β via post-synthetic incorporation) and aluminosilicates (H-ZSM-5). In the material space of Lewis acidic catalysts, we show through UV-vis DRS, titration FTIR, and quantitative ¹¹⁹Sn ssNMR that the active site distribution of Sn-β zeolites is more accurately determined through characterization of an undried sample, as materials exhibit site condensation and SnO_x formation upon sample dehydration. During Meerwein−Ponndorf−Verley reduction and Oppenauer oxidation (MPVO) transfer hydrogenation reaction between 2-butanol and cyclohexanone, the presence of water led to rate irreproducibility that is ascribed to dynamic water induced changes in Sn site coordination that caused catalyst inhibition. In the case of aluminosilicates, base probe analysis of a set of carefully dried catalyst (in the absence of moisture) demonstrate that specific bases are more suitable to determine active site distribution. In terms of reaction performance, rate correlations for propane cracking show that rate enhancement occurs for an optimum amount of framework associated sites that subsequently drops with the presence of extraframework species, where these sites are rarely distinguished in the literature.