Theoretical Insights into the Cooperative Catalytic Mechanism of a PW-Containing Keggin Heteropolyacid Anion and Ethanol toward Conversion of Fructose into 5‑Ethoxymethylfurfural in Ethanol Solution

Toward the conversion of fructose and ethanol into 5-ethoxymethylfurfural (EMF), the catalytic contribution of typical functional groups in PW-containing Keggin polyoxometalates has been theoretically studied at the M06/LANL2DZ,6-31++G­(d,p) level in ethanol solution. The [HPW12O40]2– anion is regarded as the model catalyst for the PW-containing polyoxometalates of both homogeneous and heterogeneous catalysts in ethanol solution, as it possesses basic functional groups, that is, [W–OcH–W], [WOdH], and [WOd]. The [HPW12O40]2– anion displays good catalytic activity, which originates from the contained proton H+, that is, both [W–OcH–W] and [WOdH] catalytically active sites. Herein, over both [W–OcH–W] and [WOdH], the rate-controlling steps are associated with both C2–OH and C1–H bond cleavages in fructose moiety, respectively. A mechanism of proton-induced C–H bond cleavage is proposed, in which the [HPW12O40]2– anion is responsible for both the C–OH bond cleavage and C–O–C ether–oxygen bond formation, while the ethanol solvent answers for the C–H activation of the protonated intermediates. The appropriate protonation capacity of a protic solvent plays an important role in the conversion of fructose and ethanol into EMF. Alternatively, the [HPW12O40]2– anion displays almost no catalytic activity toward the O–H bond cleavage through [2 + 2] addition, which comes from the very strong Lewis acidity of the W-site.