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43464-AC10
Structural Evolution of Organic-Silica Hybrid Nanoparticles: Understanding the Early Stages of Zeolite Crystallization
This PRF grant has supported one graduate student (Jeffrey Rimer) who finished his Ph. D. dissertation in December 2006. He worked mainly on two projects related to his doctoral thesis. In the project on the self-assembly of germanium dioxide nanoparticles, he worked closely with an undergraduate student (Daniel Roth) who worked in my group doing undergraduate research.
Self-Assembly of Germanium Dioxide Nanoparticles.
The self-assembly of germania nanoparticles in basic aqueous solutions occurs at a critical aggregation concentration (CAC) corresponding to a 1:1 GeO2-to-OH- molar ratio. A combination of pH, conductivity, and small-angle X-ray scattering (SAXS) measurements were used to monitor the effect of incremental additions of germanium (IV) ethoxide to basic solutions of sodium hydroxide or tetraalkylammonium cations. Plots of pH versus total germania concentration at varying alkalinity were used to generate a phase diagram with three distinct regions containing monomers (region 1), small nanoparticles (region II) and a gel phase (region III). The diagram was analyzed with a thermodynamic model based on the chemical equilibria of germania speciation and dissociation. The model uses the GeO-H dissociation constant (pK = 7.1) as the single fitting parameter and quantitatively captures trends in the CAC and pH. SAXS patterns reveal that the germania nanoparticles have either a cubic or a spherical geometry of dimension ~1 nm that is independent of solution pH and identity of the cation. Based on these and previous literature reports, we propose the germania nanoparticle structure is that of the cubic octamer (double 4-member ring, Ge8O12(OH)8), which is common among condensed GeO2 materials and building units in [Ge, Si]-zeolites. Comparisons between germania and silica solutions show distinct differences in their phase behavior and nanoparticle structure. The results in combination with previous studies of siliceous solutions, provide a framework for ongoing studies of combined germania-silica phase behavior, which are necessary to understand the effect of germanium on the structure of templated zeolites. The results of this research have been reported in reference [1].
Dissolution of Silicalite-1 Crystals in Basic Aqueous Solutions.[2]
The dissolution kinetics of silicalite-1 was studied using dynamic light scattering to monitor the temporal changes in particle radius over a range of temperature and pH. Silicalite-1 dissolution increases with solution alkalinity and level off around pH 12. Over the course of dissolution, the pH decreases due to the dissociation of dissolved silica, leading to a simultaneous reduction in the ionic conductivity. Time-dependent changes in solution conductivity follow trends similar to the particle radius, and have been used to show that the addition of electrolytes increases the rate of silicalite-1 dissolution. Comparisons of rates and activation energies with previously reported values for quartz and amorphous silica show significant differences between amorphous and crystalline silicates, while the rates of dissolution decrease from amorphous silica to silicalite-1 to quartz. Electron microscopy was used to examine the dissolution of silicalite-1 crystals (< 200 nm), whereby alkaline treatment results in the formation of smoother surfaces with pits. We are using these data to develop models of zeolite growth and dissolution that can describe the effects of pH and temperature for growth and dissolution simultaneously. These data will help identify the process parameters that can be used to control crystal size (and morphology).
Finally, we also investigated two competing mechanisms of zeolite growth: nanoparticle addition vs. monomer addition mechanisms. Growth rates of silicalite-1 in combination with thermodynamic aspects of the aqueous chemistry of silica are used to evaluate two zeolite growth models. Analysis of a nanoparticle addition growth model indicates that, in its current state, the model does not adequately predict the effect of ionic strength and pH on growth rates. Initial studies of a monomer addition model based on solution chemistry indicate the model is capable of quantitatively predicting growth rates as a function of temperature, pH, silica concentration, and ionic strength. The detailed results of these investigations can be found in reference [3].
References:
1. Rimer, J.D.; Roth, D.; Vlachos, D.G.; Lobo, R.F.; “Self-assembly and Phase Behavior of Germanium Oxide Nanoparticle in Basic Aqueous Solutions” Langmuir 2007 23, 2784.
2. Rimer, J. D.; Trofymluk, O.; Navrotsky, A.; Lobo, R.F.; Vlachos, D.G.; “Kinetic and Thermodynamic Studies of Silica Nanoparticle Dissolution”, Chem. Mater. 2007 19 4189.
3. Rimer, J. D.; Vlachos, D.G.; Lobo, R.F.; “Kinetics of Silicalite-1 Crystallization”, to be published in the Proceedings of the 15th International Zeolite Conference, Beijing, Aug. 2007.
