Award Address (Gabor A. Somorjai Award for Creative Research in Catalysis sponsored by the Gabor A. and Judith K. Somorjai Endowment Fund). New heterogeneous catalysts for converting sugars in aqueous media

The isomerization of glucose into fructose is a large-scale reaction for the prodn. of high-fructose corn syrup, and recently, is being considered as an intermediate step in the possible route of biomass to fuels and chems. Here, it is shown that a large pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aq. media with high activity and selectivity. Specifically, a 10 wt% glucose soln. contg. a catalytic amt. of Sn-Beta (1:50 Sn:glucose molar ratio) gives product yields of approx. 46% (wt./wt.) glucose, 31% (wt./wt.) fructose, and 9% (wt./wt.) mannose after 30 and 12 min of reaction at 383 K and 413 K, resp. This reactivity is achieved also when a 45wt% glucose soln. is converted. The Sn-Beta catalyst can be used for multiple cycles, and the reaction stops when the solid is removed, clearly indicating that the catalysis is occurring heterogeneously. With isotopically labeled glucose, it is demonstrated that the isomerization reaction catalyzed by Sn-Beta in water proceeds by way of an intramol. hydride shift, confirming that framework tin centers in Sn-Beta act as Lewis acids in aq. media. The active site is shown to be Sn that has three bonds to framework oxygen atoms, and reaction rates are strongly dependent on the hydrophobicity of the mol. sieve. The Sn-Beta catalyst is able to perform the isomerization reaction in highly acidic, aq. environments with equiv. activity and product distribution as in media without added acid. This enables Sn-Beta to couple isomerizations with other acid-catalyzed reactions, including hydrolysis/isomerization or isomerization/dehydration reaction sequences, including starch to fructose and glucose to 5-hydroxymethylfurfural (HMF). Modifications of Sn-Beta (and Ti-Beta) have expanded the types of reactions that can be catalyzed. Some of those reactions include the conversion of glucose to mannose, glucose to sorbose and lactose to lactulose