Role of Water in Silica Oligomerization

Abstract

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.

Effect of Counter Ions on the Silica Oligomerization Reaction

“…During the water removal step the Li + cation is coordinated to the side of the silica dimer not involved in the water removal. This is consistent with the observation that the calculated barrier of the water removal step does not depend on the presence of cation Li + , the barrier is found comparable to that of the case without cation [45,47] (around 40 kJ mol À1 ).…”

“…For a representative example of a combined plot of the measured constraint forces and associated free energy profile see refs. [45] and [47].…”

“…The product of this step is a 5-fold coordinated silicon which is generally observed in other theoretical and experimental studies. The Li + cation remains coordinated to the reacting, negatively charged, silica oxygen up to the transition state with a distance similar as that in the reactant state ( , respectively [45,47] Hence, the presence of a Li + cation gives rise to an enhanced stabilization of the reactant state relative to the transition and intermediate state. This can be rationalized by the notion that 1) the negative charge is more localized on the reactant state (silicate monomer) than on the intermediate silicate dimer state yielding a more stable cation-anion interaction in the reactant state and 2) that the positive charge of the cation reduces the strength of the bond between active oxygen and silicon atom, that is, the attraction of [SiO dÀ ···Si d + ] will be less favourable for SiOÀSi bonding with a Li + cation present.…”

“…Free energy profiles of the silica condensation reaction have been recently presented by Mora-Fonz et al [20,28] and by Trinh et al [27,45,47] These studies revealed that the formation of the small ring fragment is thermodynamically favorable in high pH media. Especially, it is important to include explicitly the water molecules [45,47] and the counter ions in modelling the silica oligomerization. [28] At different solution pH, the silica condensation reaction has a different mechanism and activation barrier.…”

Acid dissociation mechanisms of Si(OH)4 and Al(H2O)63+ in aqueous solution

“…Furthermore, these species can form larger polynuclear clusters through condensation reactions, which eventually lead to the crystal growths, e.g. Al/Si containing (hydr)oxides, zeolites and clay minerals (Casey and Rustad, 2007;Casey et al, 2009;Trinh et al, 2009). In all of these processes, the dissociations of the coordinated hydroxyls and waters are key reaction steps and therefore, it is obvious that the relevant acidity constants and dissociation mechanisms are central for understanding their geochemical properties (Casey et al, 2009).…”