Bermejo-Deval, Ricardo and Gounder, Rajamani and Davis, Mark E. (2012) Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently. ACS Catalysis, 2 (12). pp. 2705-2713. ISSN 2155-5435. http://resolver.caltech.edu/CaltechAUTHORS:20130124-084012360
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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.
|Additional Information:||© 2013 American Chemical Society. Received: July 16, 2012; revised: October 23, 2012; published: October 29, 2012. This work was financially supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DESC0001004. R.B.D. acknowledges the Obra Social “la Caixa” for a graduate fellowship. We thank Dr. Son-Jong Hwang for the solid-state 119Sn MAS NMR spectra, Dr. David VanderVelde for assistance with the liquid 1H and 13C NMR spectra, Joshua Pacheco for the SEM images, and Carly Bond for experimental assistance with N2 physisorption measurements.|
|Subject Keywords:||Bilik reaction; epimerization; glucose; isomerization; mannose; methanol; tin-Beta; water|
|Official Citation:||Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently Ricardo Bermejo-Deval, Rajamani Gounder, and Mark E. Davis ACS Catalysis 2012 2 (12), 2705-2713|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||24 Jan 2013 21:14|
|Last Modified:||24 Jan 2013 21:14|
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