(588c) Modeling the Formation of Nanoparticles During Early Stages of Zeolite Growth

A significant amount of experimentation has been carried out to characterize the growth kinetics of zeolites, but the mechanism is still unclear. Since the key events in zeolite nucleation are not easily accessible by experimental methods, molecular modeling may be able to shed some light on how zeolites form. In the past decade, silica nanoparticles have been found to play an important role in the clear-solution synthesis of zeolites. Jorge et al. developed a simple lattice model to describe the formation of silica nanoparticles in the early stages of the clear-solution templated synthesis of silicalite-1, and obtained qualitative agreement with published experimental observations for some of the properties of the nanoparticles, including the core-shell structure [J. Am. Chem. Soc. 127, 14388(2005)]. Their model was based on simple cubic (SC) lattice, which does not model the four-coordinate bonding character of silica solids. In this work we are exploring two approaches to improving the lattice model by incorporating four-coordinate interactions. It is anticipated that this effort will lead to a more realistic structure for the nanoparticles and provide a basis for modeling the growth at later stages.

A four-coordinated tetrahedral network can be generated by imposing a second neighbor repulsion on a body-centered cubic (BCC) lattice . In this way we successfully generated nanoparticles with four-coordinated tetrahedral network. The nanoparticles possess a core-shell structure with silica in the core and template at the shell. More importantly, some template particles were able to penetrate the precursor nanoparticles, indicating that the nanoparticles generated by this model have some porosity. In addition to the SC and BCC models where a unit of silica is represented by a single site we have developed an atomistic Silicon-Oxygen lattice model. The Silica-Oxygen model represents silica as a rigid tetrahedron as one silicon atom at the center and four oxygen atoms (or hydroxyls) at the corners. We have developed a set of rules for resolving the configurations of these tetrahedra after condensation or hydrolysis reactions. This atomistic lattice model gives a quite realistic treatment of the structure of polymerizing silica.