(47b) Elucidating the Roles of An H-BEA Zeolite in Aqueous-Phase Fructose Dehydration

The main paths by which zeolites carry out the dehydration of fructose to 5-Hydroymethylfurfural (HMF) and the rehydration of HMF to levulinic acid in aqueous solutions are elucidated using an H-BEA zeolite with SiO₂/Al₂O₃ = 18 (H-BEA-18) as a representative solid acid catalyst.  Specifically, the relative roles of zeolite-induced homogeneous chemistry, the effect of external surface acid sites, and the effect of adsorption of products and reactants on the catalyst for these reactions are delineated. H-BEA-18 increases the conversion of fructose and HMF in part by catalyzing fructose isomerization to glucose and HMF rehydration to formic and levulinic acids, respectively.  Octahedral aluminum, which is formed during calcination of the zeolite, catalyzes both fructose isomerization and reactions to unknown products from both fructose and HMF. The zeolite is susceptible to partial dissolution under reaction conditions, producing homogeneous species that catalyze undesired side reactions.  These zeolite-derived homogeneous species contain both Si and Al, and the Al is octahedrally coordinated.  The dissolved species do not appear to be hydroxylate or carboxylate salts of Al or Si, but rather small fragments of dissolved zeolite.  H-BEA-18 more readily converts HMF, furfural, and levulinic acid compared to sugars, likely due to the relatively strong adsorption of furans and levulinic acid.  In contrast, changes in the initial pH due to the addition of the zeolite and catalysis by sites on the external surface of the zeolite contribute negligibly to the chemistry under the conditions investigated. Cumulatively, these experiments provide significant insights into catalyst behavior in aqueous-phase carbohydrate transformations and can help in the design of solid acid catalysts in the future.