Study of pure-silica Zeolite Nucleation and Growth from Solution
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Zeolites are microporous crystalline materials, which are widely used in catalysis, adsorption, and ion-exchange processes. However, in most cases, the synthesis of novel zeolites as functional materials still relies on trial-and-error methods, which are time consuming and expensive. Therefore, the motivation for this thesis work is to understand the zeolite synthesis mechanismand further develop knowledge for manipulating zeolite properties and ultimately the rational design of porous materials. This work focused on formation of silicalite-1 (pure-silica ZSM-5) from basic aqueous solutions containing tetraorthosilicate (TEOS) as silica source, and tetrapropylammonium (TPA) cations as the organic structure-directing agent. The presence of silica precursor particles with size of 2-5 nm in these mixtures prior to and during hydrothermal treatments have been observed through dynamic light scattering (DLS), small-angle X-ray (SAXS) and transmission electron microscopy (TEM). However, to quantify composition and the molecular structure transformation of these silica precursor particles during zeolite synthesis is still a technical challenge. Another important yet unresolved question is how organocations interact with these nanoparticles and direct zeolite nuclei. Unlike many studies performed analyzing the inorganic phase (silica) present in synthesis mixtures, this study quantified the organocation-silica particle interaction and its ultimate effect on zeolite growth mainly through probing the behavior of the organocations. Pulsed-field gradient (PFG) NMR was used to capture the mobility change of organocations, and was complemented with scattering measurements (DLS, SAXS) on the silica nanoparticles. On the basis of the measurement results, the thermodynamic and kinetic properties of the organic-inorganic interaction were derived. Upon aging at room temperature, this interaction manifested as binding of TPA onto the silica particles due to electrostatic interactions, and such binding behavior can be well described by the Langmuir adsorption model. Upon hydrothermal treatment, a fraction of TPA adsorbed at room temperature dissociates from the growing silica nanoparticles and the corresponding desorption profiles were fitted well by the pseudo-second order kinetic model. The addition of tetramethylammonium (TMA) as "competitors" promoted TPA desorption kinetics and hindered silica nanoparticle growth due to stronger association of TMA with particles than that of TPA. Finally, the TPA adsorption strength increased via addition of monovalent salts with increasing ionic size whereas that of TMA shows an opposite trend. This suggests one potential route for tuning the organic-silica precursor particle interactions and thus possibly affecting some kinetics steps in the synthesis.