Developing nanostructured materials for emerging applications in areas ranging from energy to pharmaceuticals involves the rational design of synthetic pathways, largely through self-assembly, that ideally can be tuned to attain desired material properties, such as particle size and morphology. The ability to a priori control these properties is desirable and advantageous on many levels, but requires rather detailed knowledge regarding the interfacial interactions, kinetics, thermodynamics, and mechanistic aspects governing growth. To this end, our research focuses on improving the existing, but not well-developed, molecular-level understanding of microporous silicate (i.e., zeolite) crystallization. Microporous silicates have received much attention over the past decade due to increased interest in their implementation in expanding technologies (e.g., optical electronics ^1a, selective separations ^1b-d) – all of which entail adapting the economies of scale approach to industrial zeolite production to selectively tailor materials at much smaller length scales. We employ a hierarchical approach using combined experiments and modeling to study the all-silica zeolite, silicalite-1, which has served as a prototype for mechanistic studies of microporous growth.

Investigations largely focus on silica nanoparticles (2-6 nm) that are precursors, and possible building units, in the synthesis of silicalite-1. We have shown that silica nanoparticles evolve by Ostwald ripening during the induction period, leading to a compositional shift from an initially amorphous-like particle to one more akin to silicalite-1 ^2a. Similarly, both the kinetic rates and enthalpies of dissolution reveal that the molecular structure exhibits an internal reorientation from an initially disordered to a more zeolite-like arrangement ^2b. The results of these studies, aside from providing relevant time scales involved in silicalite-1 crystallization, offer insight into the early stages of nucleation, suggesting that solution-mediated processes may be the dominant mechanism.

The kinetics and thermodynamics of nanoparticle formation were examined through a combination of small-angle scattering and microcalorimetry measurements, identifying particle self-assembly at a specific silica concentration and pH ³. We developed a model coupling silica speciation and electrostatic double layer theory to predict the self-assembly, phase behavior, and surface charge (i.e., colloidal stability) of silica nanoparticles. This model provides an explanation for thermodynamic phenomena associated with silicalite-1 crystallization ⁴, and it has served as a basis for establishing a predictive silicalite-1 growth model, capable of quantitatively capturing growth rates over a wide range of reaction conditions. We will present these collective findings within the context of mechanisms proposed in the literature, highlighting advancements made toward understanding silica nanoparticle self-assembly, composition, structure, and its roles in both nucleation and crystallization of silicalite-1. In a broader scope, these techniques offer a general methodology that can be applied to other microporous, mesoporous ^4a, and/or biomineral silica-based materials to develop predictive growth models, thereby serving as a framework toward rational syntheses of novel nanomaterials.

1. (a) Tosheva and Vltchev, Chem. Mater. 2005, 17, 2494, (b) Lai et al., Science 2003, 300, 456, (c) Lee et al., Science 2003, 301, 818, (d) Lai et al., Adv. Func. Mater. 2004, 14, 716.

2. (a) Rimer et al., J. Phys. Chem. B 2005, 109, 12762, (b) Rimer et al., Chem. Mater. 2007, 19, 4189.

3. (a) Rimer et al., Langmuir 2005, 21, 8960, (b) Fedeyko et al., J. Phys. Chem. B 2004, 108, 12271, (c) Rimer et al., J. Phys. Chem. C, 2008, submitted.

4. (a) Rimer et al., Chem. Eur. J. 2006, 12, 2926, (b) Yang et al., Chem. Mater. 2002, 14, 2803.