The coordination of cyclic β-D-glucose (CDG) to both [Al(OH)(aq)](2+) and [Al(OH)2(aq)](1+) ions has been theoretically investigated, using quantum chemical calculations at the PBE0/6-311++G(d,p), aug-cc-pvtz level under polarizable continuum model IEF-PCM, and molecular dynamics simulations. [Al(OH)(aq)](2+) ion prefers to form both six- and five-coordination complexes, and [Al(OH)2(aq)](+) ion to form four-coordination complex. The two kinds of oxygen atoms (on hydroxyl and ring) of CDG can coordinate to both [Al(OH)(aq)](2+) and [Al(OH)2(aq)](+) ions through single-O-ligand and double-O-ligand coordination, wherein there exists some negative charge transfer from the lone pair electron on 2p orbital of the coordinated oxygen atom to the empty 3s orbital of aluminum atom. The charge transfer from both the polarization and H-bond effects stabilizes the coordinated complex. When the CDG coordinates to both [Al(OH)(H2O)4](2+) and [Al(OH)2(H2O)2](1+) ions, the exchange of water with CDG would take place. The six-coordination complex [(ηO4,O6(2)-CDG)Al(OH)(H2O)3](2+) and the five-coordination complex [(ηO4,O6(2)-CDG)Al(OH)2(H2O)](1+) are predicted to be the thermodynamically most preferable, in which the polarization effect plays a crucial role. The molecular dynamics simulations testify the exchange of water with CDG, and then support a five-coordination complex [(ηO4,O6(2)-CDG)Al(OH)2(H2O)](1+) as the predominant form of the CDG coordination to [Al(OH)2(aq)](1+) ion.

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 Brønsted 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 Brønsted 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 Brønsted basicity of the −OH ligand, higher Brønsted 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.