The mechanism of the oligomerization reaction of silica, the initial step of silica formation, has been studied by quantum chemical techniques. The solvent effect is included by using the COSMO model. The formation of various oligomers (from dimer to tetramer) was investigated. The calculations show that the anionic pathway is kinetically preferred over the neutral route. The first step in the anionic mechanism is the formation of the SiO-Si linkage between the reactants to form a five-coordinated silicon complex, which is an essential intermediate in the condensation reaction. The rate-limiting step is water removal leading to the oligomer product. The activation energies for dimer and trimer formation ( approximately 80 kJ/mol) are significantly higher than those of the subsequent oligermerization. The activation energies for the ring closure reaction ( approximately 100 kJ/mol) are even higher. The differences in activation energies can be related to the details in intra- and intermolecular hydrogen bonding of the oligomeric complexes.

The mechanism of the initial stage of silicate oligomerization from solution is still not well understood. Here we use an off-lattice kinetic Monte Carlo (kMC) approach called continuum kMC to model silicate oligomerization in water solution. The parameters required for kMC are obtained from density functional theory (DFT) calculations. The evolution of silicate oligomers and their role in the oligomerization process are investigated. Results reveal that near-neutral pH favors linear growth, while a higher pH facilitates ring closure. The silicate oligomerization rate is the fastest at pH 8. The temperature is found to increase the growth rate and alter the pathway of oligomerization. The proposed pH and temperature-dependent mechanism should lead to strategies for the synthesis of silicate-based materials.

The silicate oligomerization reaction is key to sol−gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have been devoted to investigating the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Most of the reported computational studies of silica oligomerization employ a quantum chemical phase model supplemented with a continuum model for the solvent. We report a density-functional theory based molecular dynamics simulation of silica oligomerization in a bulk solution of explicit water molecules. Our study provides thermodynamics, kinetics, and mechanism of the reaction pathway for the anionic bond formation of siliceous oligomers. We reveal that in the water cleavage step a direct and a water mediated proton transfer pathways may occur and show that changing from a continuum to a explicit water model the rates of SiO−Si bond formation of linear and three-ring oligomers are significantly enhanced, whereas the overall thermodynamics becomes less favorable.