Promotion catalytic role of ethanol on Brønsted acid for the sequential dehydration-etherification of fructose to 5-ethoxymethylfurfural

- Ethanol does not show any catalytic activity toward the fructose dehydration.
- Proton H+ is solvated on ethanol, resulting in [C2H5OH2]+ as the active species.
- [C2H5OH2]+ exhibits better catalytic performance than the proton H+.
- [C2H5OH2]+ shifts the intramolecular H-shift to the intermolecular H-shift.
- Ethanol promotes the H+ catalytic effect toward the fructose dehydration.

The function of ethanol and Brønsted acid for the dehydration-etherification of fructose to 5-ethoxymethylfurfural (EMF) has been theoretically investigated at G4 level in ethanol solution. Initially, fructose prefers to dehydration other than etherification with ethanol in the presence of Brønsted acid. The proton H+ should be solvated on ethanol other than fructose, resulting in [C2H5OH2]+ as the catalytically active species. Both protonation and [C2H5OH2]+ exhibit good catalytic performance, but ethanol does not. Furthermore, [C2H5OH2]+ displays better catalytic performance than protonation, which reflects the promotion catalytic role of C2H5OH on the proton H+. The turnover frequency analysis shows that the SN2 nucleophilic substitution for the etherification of 5-hydroxymethylfurfural (HMF) to EMF is the rate-controlling step in the whole reaction catalyzed by [C2H5OH2]+. The catalytic performance of [C2H5OH2]+ stems from the positive charge of OH2 group, which assists both the H2O release of fructose to HMF and the etherification of HMF to EMF. For the dehydration of fructose, the catalytic superiority of [C2H5OH2]+ to protonation comes from the shift of the intramolecular H-shift catalyzed by protonation to the intermolecular H-shift catalyzed by [C2H5OH2]+, which lowers the activation energy barrier for the H2O release. The present study is useful for understanding the roles of ethanol and Brønsted acid in acid-catalyzed dehydration-etherification of carbohydrates to biofuel in ethanol solution.

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