Quantitative mechanistic modeling of silica solubility and precipitation during the initial period of zeolite synthesis

The mechanistic details of the structural changes occurring during the initial stages of silica polymerization to form gels or zeolites remain largely unknown due mainly to the complexity of sol-gel synthesis processes. Previous simulation studies have applied simple lattice models to qualitatively replicate spontaneous silica nanoparticle formation using fitted interaction energy parameters to replicate the behavior of real systems. This study moves for the first time to the use of quantum chemical-based interaction (dimerization) energies, determined through density functional theory computations, in a coarse-grained Monte Carlo simulation of the initial stages of gel/cluster formation in sodium silicate systems across a range of concentrations. The use of accurate dimerization energies as model inputs enables semiquantitatively accurate results to be obtained, as determined by comparisons with ₂₉Si nuclear magnetic resonance data. The most concentrated system simulated (9.36 m silica concentration) undergoes Ostwald ripening, whereby a single large cluster forms, indicative of colloidal silica formation. Furthermore, as the extent of the reaction progresses, this initial nonequilibrated cluster (known as the primary amorphous phase in the zeolite synthesis literature) is progressively transformed to the secondary amorphous phase via structural rearrangements, without significant changes in the overall size of the cluster. These results are in good agreement with previous experimental studies on the nature of the amorphous phase(s) present during silicate gelation and prior to zeolite crystallization. Hence, this investigation demonstrates for the first time the successful application of multiscale simulation methodology to the coarse-grained Monte Carlo approach of a sol-gel process, revealing important quantitative mechanistic information regarding complex silicate reaction processes.