Role of Water in Silica Oligomerization

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.