Decomposition of Al promoted by salicylic acid under acidic condition: Mechanism study by differential mass spectrometry method and DFT calculation.

Decomposition of the polycation AlO(OH)(HO) (Al) promoted by ligand is a vital subject to advance our understanding of natural and artificial occurrence and evolution of aluminum ions, especially in the case of acidic condition that dissolved Al species can be released from the Al-bearing substances. However, the microscopic pathway of synchronous proton-promoted and ligand-promoted decomposition process for Al is still in the status of ambiguity. Herein, we applied differential mass spectrometry method and DFT calculation to study the initial detailed process of Al decomposition under the presence of proton and salicylic acid (HSal). Mass results showed that the mononuclear Al-HSal complexes dominated the resulting Al species, whereas the monodentate complex AlHSal was not observed in the spectra. The difference of decomposition levels between the ligand/Al ratio 0.2 and 0.5 cases revealed that proton and ligand performed synergistic effect in initial Al decomposition process, and the proton transfer determined the ring closure efficiency. The ring closure reaction is the prerequisite for the collapse of Al structure and detachment of the mononuclear complex. DFT calculations reveal that hydrogen bond plays an important role in inducing the formation of chelated complex accompanying proton transfer. Attachment of protons at the bridging OH can elongate and weaken the critical bond between targeted Al and µ-O resulting from delocalization of electron pairs in the oxygen atom. These results demonstrate the detailed mechanism of Al composition promoted by ligand and proton, and provide significant understanding for further application and control of Al.