Cooperative Catalytic Performance of Lewis and Brønsted Acids from AlCl₃ Salt in Aqueous Solution toward Glucose-to-Fructose Isomerization

The mechanism of glucose-to-fructose isomerization, as one of the key intermediate steps in biomass valorization, remains an intriguing topic in potential chemo-catalysis. In the present work, the catalytic mechanism of glucose-to-fructose isomerization in AlCl₃ aqueous solution has been theoretically investigated at the PBE0/6-311++G(d,p), aug-cc-pvtz level. The catalytic activities of possible active species from the hydrolysis of AlCl₃ in aqueous solution, that is, Lewis acids ([Al(OH)(H₂O)₄]²⁺ and/or [Al(OH)₂(H₂O)₂]⁺) and Bronsted acid (H₃O⁺) together with the counterpart anion Cl–, have been evaluated. The glucose-to-fructose isomerization includes aldose ring-opening, aldose-to-ketose tautomerization, and ketose ring-closure. Toward the global glucose-to-fructose isomerization, the Lewis acid behaves dominantly in the aldose–ketose tautomerization and the Bronsted acid acts predominantly toward both aldose ring-opening and ketose ring-closure. Furthermore, [Al(OH)₂(H₂O)₂]⁺···Cl– ion pair displays better catalytic activity than [Al(OH)(H₂O)₄]²⁺···2Cl– ion pair. Alternatively, the individual [Al(OH)(H₂O)₄]²⁺ shows better catalytic activity than [Al(OH)₂(H₂O)₂]⁺. The counterpart cation Cl– has a more stable effect on the corresponding intermediates than transition states, which indirectly affects the catalytic activity of Lewis acid. For the individual Lewis acids ([Al(OH)(H₂O)₄]²⁺ and [Al(OH)₂(H₂O)₂]⁺), the basic −OH ligand facilitates the cleavage of the O–H bond and the acid −H₂O ligand boosts the formation of the O–H bond, both of which cooperatively play a catalytic role. The individual [Al(OH)(H₂O)₄]²⁺ displays better catalytic performance than [Al(OH)₂(H₂O)₂]⁺, which stems from its higher Bronsted basicity of the −OH ligand, higher Bronsted acidity of the −H₂O ligand, and the lower highest occupied molecular orbital–lowest unoccupied molecular orbital gap. These findings provide a deep insight into the catalytically active species from Lewis acid metal salt in aqueous solution toward glucose chemistry.