Synthesis and Applications of Colloidal Zeolites and Transition Metal Functionalized Ordered Mesoporous Carbons

Abstract: Porous materials have attracted a great deal of attention of the materials science community, owing to their physical and chemical properties including high surface areas, narrow pore size distribution, tunable pore diameters and variety of chemical compositions. These properties make them suitable for various applications such as adsorption, sensing, catalysis, energy, carbon capture and drug delivery. Zeolites and carbons are among the most commonly and extensively investigated porous materials. Zeolites are crystalline microporous aluminosilicate structures with multidimensional channel systems. During the last decades, they have been broadly used and studied in the fields of chemical synthesis and applications such as ion exchange, gas separation and catalysis. Due to the high demand for these materials in the chemical industry and their unique characteristics, there is still a need for understanding their formation in order to develop new synthetic methods and tailor their properties for specific target-oriented applications. In this thesis, colloidal zeolite particles were prepared, studied and further utilized in two applications. One of the first goals was to understand the formation of colloidal zeolites and more specifically the premature termination of their growth during synthesis. A pH-dependent equilibrium between the condensation and dissolution reactions is presented in this thesis to explain this behavior. By a controlled decrease in pH of the synthesis mixtures, it was possible to shift the equilibrium towards condensation and to favor the increase in particle size and reach full conversion. We showed that it is possible to cycle between zeolite growth and dissolution by cyclic acid and base adjustments of the pH. In addition, we gained new insights about the duration of the nucleation stage. Likewise, the chemical solution equilibria growth mechanism was studied in the synthesis of colloidal ZSM-5 particles, and besides observing an increase in the particle size and conversion yield, shifting the dynamic pH-dependent equilibrium led to an improved incorporation of aluminum into the zeolite framework. As the second goal in this thesis, we used the colloidal zeolite particles for the preparation of two new materials: a hybrid zeolite-cellulose foam for CO2 capture and hierarchical micro-/mesoporous zeolite microspheres. The hybrid foams were produced by mixing colloidal silicalite-1 particles and cellulose nanofibrils/gelatin and applying a freeze-casting. The foams exhibited ultra-high loading of silicalite-1 particles and a linear relationship between the silicalite-1 and the CO2 adsorption capacity with a selectivity towards CO2-over-N2. In addition, a novel mesoporogen-free evaporation-driven colloidal assembly was developed to prepare mesoporous silicalite-1 microspheres. The synthesized materials exhibited interconnected porosity, high pore volume, and the method can be used with other colloidal particles to tailor the pore size, pore volume and surface area. Moreover, the method does not require long preparation times or high temperature which has a positive effect on production at industrial scale. Porous carbon materials have received growing interest due to their unique properties, including porosity, high surface areas, low density and electrical conductivity. Here, special attention has been casted on the development of N-doped mesoporous carbon for energy conversion in fuel cell technologies. Numerous efforts have been devoted to developing inexpensive platinum-free catalysts, active towards oxygen reduction reaction (ORR). In this thesis, we synthesized Fe-N-doped ordered mesoporous carbon (OMC) materials using different types of iron salt and studied the effect of the counter anion on their structural and catalytic properties. Although the change of the anion led to a significant increase in the Fe content in the resultant Fe-N-OMC, it was found by rotating disc electrode (RDE) and fuel cell measurements, that such increase does not provide an noticeable improvement in the rate of ORR, suggesting that the additional fraction of iron consists of less active Fe species.