Elucidating the Roles of Zeolite H-BEA in Aqueous-Phase Fructose Dehydration and HMF Rehydration

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Abstract

The main paths by which zeolites carry out the dehydration of fructose to 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 role of homogeneous chemistry (both solvent- and zeolite-induced), the effect of external surface acid sites, and the effect of adsorption of products and reactants on the catalyst for these reactions is delineated. We found that 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. The glucose-to-fructose isomerization is caused by octahedral aluminum atoms that act as Lewis acid sites as shown by 1H and 13C NMR. These Lewis sites are formed during calcination and are stable under reaction conditions. They also catalyze reactions to unknown products from both fructose and HMF. The acids produced from HMF rehydration dissolve aluminosilicate species from the zeolite, which also catalyze some of the undesired side reactions. We show that the decrease of the initial pH due to the addition of the zeolite and the catalysis by sites on the external surface of the zeolite have a negligible contribution to the chemistry under most conditions investigated. H-BEA-18 more readily converts HMF than fructose, due to strong preferential adsorption of HMF, furfural, and levulinic acid compared to sugars. Under mildly acidic conditions (without the addition of inorganic acids) that are environmentally preferred, zeolites can increase the conversion of HMF and the selectivity to levulinic acid many-fold. This provides an indication that heterogeneous materials may be superior in the production of levulinic acid from HMF.

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