Density functional theory study and kinetic analysis of the formation mechanism of Al30O8(OH)56(H2O)2618+ (Al30) in aqueous solution

Abstract

Abstract The formation mechanism of Al 30 O 8 (OH) 56 (H 2 O) 26 18+ (Al 30 ) has been investigated by the density functional theory based on the supermolecule model and kinetic analysis on the 27 Al nuclear magnetic resonance (NMR) experimental results in monitoring Al 30 synthesis process. The theoretical chemistry calculations on the four possible schemes show that δ-Na–Al 13 is the reasonable intermediate followed by the substitution of Na with Al to form δ-Al 14 , and Na + plays an important role in stabilizing the intermediate (δ-Na–Al 13 ) in the transformation. The kinetic analysis on the 27 Al NMR experimental data indicates that e-Al 13 decomposes and isomerizes in the formation of Al 30 , while Al monomers facilitate the decomposition of e-Al 13 and so the isomerization of e-isomers to δ-isomers effectively. The favorable formation mechanism of Al 30 includes three steps: (1) e-Al 13 decomposes and rearranges into the isomer δ-Al 13 ; (2) Na + reacts with δ-Al 13 to stabilize the intermediate δ-Na–Al 13 , followed by Al monomers replacing Na to form δ-Al 14 ; (3) δ-Al 14 reacts with the Al monomers in the solution to finally form Al 30 . Both Al monomers and Na + are important in the transformation. Al monomers are the basic building units and helpful to the isomerization while Na + can well stabilize the isomer δ-Al 13 to yield intermediate δ-Na–Al 13 . The results also show that other isomers of e-Al 13 (β-Al 13 and α-Al 13 ) form in the formation of Al 30 , and their calculated 27 Al NMR tetrahedral resonance shifts are consistent with the experimental 27 Al NMR tetrahedral signals in the preparation process of Al 30 .