Mechanism of Glucose–Fructose Isomerization over Aluminum-Based Catalysts in Methanol Media

The catalytic isomerization of glucose to fructose has been deemed a vital step in biorefinery, while the isomerization mechanism in alcoholic media still remains ambiguous. Hereby, density functional theory (DFT) calculations were carried out to investigate the isomerization mechanism of glucose over aluminum-based catalysts in methanol media. Al³⁺ was apt to coordinate with methanol and cyclic β-d-glucose (CDG) to form various complexes. It was found that [Al­(CH₃O)₂(CH₃OH)₂]⁺ was the most stable one in +1 charge complexes based on the DFT calculations and ESI-MS experiments. Furthermore, the four-coordination complex [(η²_O4,O6-CDG)­Al­(CH₃O)₂]⁺ was predicted to be the most preferable. Ionic species formed between Al³⁺ and the solvent can further assemble with glucose to catalyze the isomerization. The isomerization proceeds mainly by three steps, including ring-opening, hydride shift, and ring-closing with the migration of H from the C2–H to the O1–H bond (hydride shift) as the rate-determining step. The coordination of Al³⁺ with methanol showed a significant catalytic effect by shortening the proton transfer distance, which resulted in a markedly reduced overall reaction barrier for isomerization. The calculations provided a better insight into the glucose transformation in the methanol media with Al-based catalysts.