Surface Modification of Al30 Keggin-Type Polyaluminum Molecular Clusters

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al13 and Al30, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al30 clusters, whose surface has been modified with chelated metals (Al3+ and Zn2+) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Al32IDA [(Al­(IDA)­H2O)2­(Al30O8(OH)60­(H2O)22)]­(2,6-NDS)4­(SO4)2­Cl4­(H2O)40, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, P1̅ with a = 13.952(2) Å, b = 16.319(3) Å, c = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. Zn2Al32 [(Zn­(NTA)­H2O)2­(Al­(NTA)­(OH)2)2­(Al30(OH)60(O)8­(H2O)20]­(2,6-NDS)5­(H2O)64, (NTA = nitrilotriacetic acid), also crystallizes in P1̅ with unit cell parameter refined as a = 16.733(7) Å, b = 18.034(10) Å, c = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al30 clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process.