Growth mechanisms of silicalite-1

In-situ seeded dynamic light scattering growth experiments for silicalite-1 were performed for various solution pH's and electrolyte concentrations. The pH was controlled using ratios of TPAOH (Pr₄NOH) and TPABr (Pr ₄NBr) while maintaining a constant TPA⁺ concentration. The electrolyte concentration was varied through the addition of NaCl, CsCl, and CaCl₂. The growth rate was found to decrease linearly with increasing pH and was unaffected by the presence of electrolytes, but at high enough salt concentrations the silicalite-1 particles experience a critical coagulation concentration. The growth rates were simulated using a transport model that incorporates a DLVO interparticle potential to account for the energy barrier for particle aggregation. The model is based on the assumption that the subcolloidal particles present in the solution directly add to the crystal surface. It was shown that the model can predict the pH growth rates; however, the model overpredicts the screening effects of electrolytes in solution. Preliminary computations indicate that Stern-layer stabilization, due to the large size of adsorbing TPA, may be involved in the stability of the seeds and subcolloidal particles. This is the first time the predictive capability of the silicalite-1 growth model has been tested for a wide range of reaction conditions.