Self-assembly of silica nanoparticles and their role in the mechanism of silicalite-1 crystallization.

摘要

Developing new, nanostructured materials for emerging applications in areas ranging from energy to pharmaceuticals largely involves self-assembly, and often requires syntheses of materials at small length scales with exact precision in physical properties (e.g., size, morphology). Rational design of materials, such as zeolites, requires knowledge of their mechanism(s) of formation; and thus, the overarching motivation for this thesis was to develop a molecular-level understanding of the processes involved in zeolite crystallization. To this end, we have focused on the all-silica zeolite, silicalite-1, which is synthesized in basic aqueous solutions of silica and an organic structure-directing agent, the tetrapropylammonium (TPA+) ion. Molecular precursors result in the spontaneous formation of stable, silica nanoparticles (1--6 nm) that serve as potential building units in both nucleation and growth of silicalite-1.; In this thesis, we employ a hierarchical approach involving analyses along multiple length scales, using combined experiments and modeling to: (i) elucidate the driving force(s) leading to nanoparticle self-assembly, (ii) experimentally assess the energetic contributions to their colloidal stability in solution, (iii) quantify nanoparticle composition and molecular structure, (iv) identify changes in nanoparticle properties with heat treatment (i.e., initial stages of nucleation), (v) probe relevant time scales involved in viable growth and dissolution pathways, (vi) develop combined kinetic and silicate speciation models to predict both growth and dissolution, and (vii) evaluate hypotheses in the literature regarding nanoparticle structure and its role in the mechanism of silicalite-1 crystallization.; Nanoparticle self-assembly is investigated through a combination of pH, conductivity, small-angle scattering, and microcalorimetric measurements. We developed a chemical equilibrium model, based on silica condensation and silanol dissociation, capable of predicting established phase behavior of silica nanoparticles along with their critical aggregation concentration (CAC). This model calculates higher surface charge for nanoparticles compared to those for zeolite, suggesting that electrostatics are largely responsible for the colloidal stability of precursors. In addition, the model offers explanations for thermodynamic phenomena associated with nanoparticle self-assembly and silicalite-1 crystallization (e.g., the observed exothermic-endothermic transition near synthesis completion).; Silica nanoparticles possess a core-shell structure comprised of a hydrated silica core surrounded by a shell of adsorbed cation (e.g., TPA+). We analyze cation effects by substituting TPA+ with other tetraalkylammonium ions and alkali metals. The self-assembly (i.e., CAC) and growth of nanoparticles is nearly independent of the cation, while organic ions imparts a steric stabilization that increases the nanoparticle resistance to coagulate. We also use a combination of small-angle X-ray and neutron scattering (SAXS and SANS) and microcalorimetric measurements to elucidate the composition and molecular structure of silica nanoparticles. Analyses are performed on both as synthesized and heat-treated particles. The latter are shown to evolve by an Ostwald ripening process, whereby a fraction of the nanoparticles grows at the expense of others that dissolve over time. During this evolution, the overall composition (e.g., scattering length density, Si/TPA+ molar ratio) changes from a material that is initially amorphous-like to one that is closer to that of silicalite-1.; Nanoparticle and silicalite-1 dissolution rates are measured at varying reaction conditions, and are compared to those of other silicates, showing two distinct features: (i) dissolution rates and enthalpies of reaction can be used to compare, or identify, the molecular structure of silicates, and (ii) nanoparticles are initially disordered, but exhibit an internal reori