Cooperative Catalytic Performance of Lewis and Bronsted 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 chemocatalysis. 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)](+)center dot center dot center dot Cl- ion pair displays better catalytic activity than [Al(OH)(H2O)(4)](2+)center dot center dot center dot 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.