Tandem Lewis acid/Brønsted acid-catalyzed conversion of carbohydrates to 5-hydroxymethylfurfural using zeolite beta

• Glucose isomerization and fructose dehydration to HMF in a single catalyst/single pot.
• First time obtained intrinsic kinetics in a bifunctional H-BEA zeolite.
• There is an optimum ratio of Lewis/Brønsted that optimizes HMF rate and yield.
• The tandem reaction shows multiple kinetic regimes as varying active site composition.

We conduct a combined experimental and computational study to reveal the kinetics of tandem glucose isomerization and fructose dehydration to 5-hydroxymethylfurfural (HMF) over a bifunctional zeolite H-BEA-25 in water. The model accounts for multicomponent adsorption, homogeneous Brønsted acid catalyzed chemistry of fructose, intrinsic heterogeneous Lewis acid catalyzed isomerization, Brønsted acid catalyzed fructose dehydration, HMF rehydration, and humin formation chemistry. The octahedrally coordinated extra-framework Al sites catalyze glucose to fructose isomerization effectively. The activation energy for the isomerization in H-BEA-25 is between those reported for Ti-BEA and Sn-BEA. We reveal that tandem reactions exhibit multiple kinetic regimes. When a bifunctional catalyst with a fixed total number of acid sites is used (e.g., aluminosilicate zeolites), the HMF formation rate exhibits a volcano type curve vs. the Lewis to Brønsted acid site ratio. On the other hand, when the two types of sites are varied independently (e.g., in Sn-BEA and HCl), the HMF formation rate increases and then approaches a plateau with increasing Brønsted acid site density. These appear to be generic features of tandem reactions catalyzed by multiple or multifunctional catalysts. We show that materials with stronger sugar adsorption would produce HMF in significantly higher yields and higher rate than H-BEA. When HMF degradation reactions are suppressed, a ratio of Lewis to Brønsted acid sites of ∼0.3 maximizes the HMF rate produced from glucose and the HMF yield (which is predicted to be ∼60% at 130 °C). These predictions provide a framework for understanding and improving tandem reactions catalyzed by heterogeneous catalysts.

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