257704 Effects of Zeolite Framework and Active Site Structures On the Rates and Mechanisms of Glucose Isomerization
Previous studies have shown that tin and titanium atoms incorporated into the framework of zeolite catalysts behave as Lewis acid sites for the isomerization of glucose to fructose. Yet, glucose isomerization rates over Lewis acidic zeolite catalysts are much higher in aqueous media than the low and often undetectable rates observed in organic media. Here, we report on recent efforts to understand the mechanistic origin of these phenomena and their dependence on zeolite hydrophobicity and active site structure.
Beta zeolites were synthesized with varying amounts of defective hydroxyl groups and with Ti or Sn present in framework locations or in extra-framework locations. The structures of metal active sites were characterized by diffuse reflectance UV-Visible, infrared and solid-state nuclear magnetic resonance spectroscopies. The hydrophobic properties of zeolite channel environments were characterized by water and methanol adsorption isotherms and by measurements of competitive adsorption between glucose and solvent molecules. Kinetic studies demonstrated when rates are controlled kinetically or by the transport of reactants to active sites, and mechanistic studies using isotopically-labeled glucose reactants were used to discriminate among mechanisms prevalent on Sn or Ti sites of different structure. Taken together, these data help clarify the effects of zeolite framework and active site structures on the rates and mechanisms of glucose isomerization with Sn and Ti-containing zeolite beta catalysts.
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