Silicate Glass and Mineral Dissolution: Calculated Reaction Paths and Activation Energies for Hydrolysis of a Q3 Si by H3O+ Using Ab Initio Methods

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Abstract
Molecular orbital energy minimizations were performed with the B3LYP/6-31G(d) method on a [((OH)3SiO)3SiOH−(H3O+)·4(H2O)] cluster to follow the reaction path for hydrolysis of an Si−O−Si linkage via proton catalysis in a partially solvated system. The Q3 molecule was chosen (rather than Q2 or Q 1) to estimate the maximum activation energy for a fully relaxed cluster representing the surface of an Al-depleted acid-etched alkali feldspar. Water molecules were included in the cluster to investigate the influence of explicit solvation on proton-transfer reactions and on the energy associated with hydroxylating the bridging oxygen atom (Obr). Single-point energy calculations were performed with the B3LYP/6-311+G(d,p) method. Proton transfer from the hydronium cation to an Obr requires sufficient energy to suggest that the Si−(OH)−Si species will occur only in trace quantities on a silica surface. Protonation of the Obr lengthens the Si−Obr bond and allows for the formation of a pentacoordinate Si intermediate ([5]Si). The energy required to form this species is the dominant component of the activation energy barrier to hydrolysis. After formation of the pentacoordinate intermediate, hydrolysis occurs via breaking the [5]Si−(OH)−Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the [5]Si−(OH) bond, proton transfer from the Si−(OH2)+ back to form H3O+, and a reversion of [5]Si to tetrahedral coordination was predicted. The activation energy for Q3Si hydrolysis calculated here was found to be less than that reported for Q3Si using a constrained cluster in the literature but significantly greater than the measured activation energies for the hydrolysis of Si−Obr bonds in silicate minerals. These results suggest that the rate-limiting step in silicate dissolution is not the hydrolysis of Q3Si−Obr bonds but rather the breakage of Q 2 or Q1Si−Obr bonds.