A density functional theory study of an SN2 reaction mechanism for hydrolysis of Al-O-Si linkages: implications for dissolution of aluminosilicates under acidic and basic conditions

Abstract

Density functional theory (DFT) calculations were performed on a molecular cluster model under acidic conditions (KAlSi 6 O 6 (OH) 16 (H 2 O)(HCl)·11H 2 O) to determine the energy of activation of Al-O-Si hydrolysis when H 2 O was bonded to the -Al(OH) 3 moiety of the model to produce [5] Al, and the bridging O (O br ) of Al-O-Si was protonated and stabilized by a Cl − . Similarly, we calculated the energy of activation of hydrolysis of the Al-O-Si linkage in a model under basic conditions (K 2 AlSi 6 O 6 (OH) 17 ·17H 2 O), where a OH − was bonded to -Al(OH) 3 to form [5] Al, and O br was stabilized by K + . This work did not evaluate the energy needed to bond an additional H 2 O/OH − to the Al in the acidic and basic models, respectively; the goal was to determine the dissolution energy of Al-O-Si hydrolysis after those additions had occurred. Stepwise dissolution calculations were performed using DFT and the resulting energies of activation of hydrolysis were compared with experimental data and computational chemistry results from orthoclase (KAlSi 3 O 8 ) and albite (NaAlSi 3 O 8 ) dissolution or hydrolysis. The calculated activation energy results agree with experimental data within computational and experimental uncertainties. The results are also useful for explaining the observed solution pH and salt-dependence of aluminosilicate dissolution rates. Rate constants were also calculated that could provide a useful guide for interpreting experimental kinetics data for aluminosilicate dissolution.