Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently

Abstract

Here, we show that framework tin sites in pure silica zeolite Beta (Sn-Beta) can isomerize glucose to fructose by a Lewis acid-mediated intramolecular hydride shift in aqueous solvent, but not in methanol solvent. Mechanistic studies using isotopically labeled (^(2)H, ^(13)C) glucose reactants show that in methanol, Sn-Beta instead epimerizes glucose to mannose by a Lewis acid-mediated intramolecular carbon shift mechanism known as the Bilik reaction. We also provide evidence that extraframework tin sites located within the hydrophobic channels of zeolite Beta can isomerize glucose to fructose in both water and methanol solvent, but through a base-catalyzed proton-transfer mechanism. SnO_2 particles located at external zeolite crystal surfaces or supported on amorphous silica catalyze isomerization in methanol but not in water, suggesting that contact with bulk water inhibits isomerization at SnO_2 surfaces. ^(119)Sn MAS NMR spectroscopy was used to unambiguously identify framework Sn sites, which give resonances for octahedral Sn (−685 to −700 ppm) in hydrated Sn-Beta that disappear upon dehydration, with the concomitant appearance of resonances for tetrahedral Sn (−425 to −445 ppm). In sharp contrast, spectra of hydrated samples containing extraframework SnO_2 show resonances for octahedral Sn centered at −604 ppm that do not change upon dehydration. These findings demonstrate that aldose–ketose isomerization reactivity on Sn-zeolite samples cannot be ascribed to the presence of framework Sn sites in the absence of isotopic labeling studies. They also indicate that any Sn-zeolite samples that initially convert glucose to fructose, instead of mannose, in methanol solvent contain Sn species that are structurally different from framework Sn centers.

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