Lewis acidic Sn sites in Beta zeolites catalyze selective reactions of oxygenates, with each reaction responding differently to the structural heterogeneities prevalent among binding sites and their surrounding environments. Specifically, non-polar Si-O-Si linkages and polar Si-OH defects surround binding sites to provide solvating environments that influence the stability of polar molecules. Here, we study how these structural features in Sn-Beta affect stability during aqueous-phase glucose isomerization, and reactivity during gas-phase ethanol dehydration. Hydrophobic, non-polar Sn-Beta-F zeolites were exposed to hot liquid water (373 K) for increasing time (0–24 h), which caused hydrolysis of siloxane bridges to form Si-OH defects. After 24 h, Si-OH densities and first-order glucose isomerization rate constants (373 K, per open Sn) were commensurate with those of hydrophilic, polar Sn-Beta-OH zeolites, because the Si-OH defects formed stabilize extended H2O structures, which entropically destabilize glucose-fructose isomerization transition states. Yet, shorter (&lt;1 h) water treatments cause first-order fructose formation rate constants (373 K, per open Sn) on Sn-Beta-F to increase by 2×, because Si-OH groups at lower density provide binding sites for H2O molecules that enthalpically stabilize glucose-fructose isomerization transition states. These data highlight the dynamic polarity of confining environments that surround Sn sites in water, which lowers reactivity when they stabilize extended solvent structures. Gas-phase bimolecular ethanol dehydration turnover rates (404 K, per Lewis acidic Sn) were measured on ten Sn-Beta zeolites with either polar or non-polar frameworks. A mechanism-derived rate equation allowed quantifying the equilibrium constants to form reactive (ethanol-ethanol) and inhibitory (ethanol-water) dimer species, which were 3–4× higher within polar frameworks because intraporous hydroxyl groups preferentially stabilize polar dimeric intermediates over monomeric ethanol intermediates. In contrast, intrinsic dehydration rate constants were independent of framework polarity, and thus differences in gas-phase reactivity solely reflect different surface coverages of reactive and inhibitory dimeric intermediates at Sn sites. These findings provide design strategies to mitigate reversible inhibition by H2O by controlling Si-OH density, and to prevent irreversible siloxane bridge hydrolysis that forms Si-OH groups.

Effects of confining environment polarity on reactivity and stability of Sn-Beta zeolites during gas-phase and liquid-phase catalysis

2019 Spring National Meeting

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