The Mechanism of Glucose Isomerization to Fructose over Sn‐BEA Zeolite: A Periodic Density Functional Theory Study

Abstract

The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.

Competitive Adsorption of Substrate and Solvent in Sn‐Beta Zeolite During Sugar Isomerization

“…This could be attributed to a confinement effect. The increased concentration of one of the anomers in the zeolite micropores should not affect the catalytic performance, as the barrier for mutarotation on Sn sites is much lower than the barrier for the H‐shift reaction 34, 35…”

Dehydration of Glucose to 5‐Hydroxymethylfurfural Using Nb‐doped Tungstite

“…Based on earlier work,19, 20 we explored a mechanism involving three steps: (i) α‐ d ‐glucopyranose ( Glu ) ring opening to the acyclic form of glucose ( o‐Glu ), (ii) hydride shift from the C2 to the C1 atom in glucose, and (iii) ring closure of acyclic fructose ( o‐Fru ) to form α‐ d ‐fructofuranose ( Fru ). Glucose coordinates through its O1 atom sites to a surface tungsten site in a planar adsorption model (Figure 4).…”

“…Recently, important contributions from computational chemistry have shed new light on this topic 19, 20, 21, 22. The aldose–ketose isomerization is thought to occur either by proton transfer or by intramolecular hydride transfer.…”

“…In addition to fructose, this also yields mannose as the product of epimerization. Lewis acids catalyze the intramolecular hydride shift that transforms the open form of glucose to the open form of fructose 20. Dehydration of fructose is facile and removes three water molecules.…”

Early Transition Metal Doped Tungstite as an Effective Catalyst for Glucose Upgrading to 5-Hydroxymethylfurfural

“…Mechanistically, it is commonly assumed that the conversion of glucose proceeds via fructose as an intermediate. Lewis acids play an important role in isomerizing glucose to the more reactive fructose by stabilizing glucose in its acyclic form, facilitating a 1,2-hydride transfer and releasing fructofuranose (fructose) [21][22][23][24]. Subsequent dehydration then removes three water molecules to form HMF (Scheme 1).…”