Synthesis and Properties of Colloidal Zeolite
Several zeolite phases, zeolite N-Y, N-A, ZSM-2, (hydroxy)sodalite and TPA-silicalite- 1, have been synthesized in the form of stable suspensions of discrete colloidal crystals wherein the average particle size is less than 150 nanometers and in certain cases, less than 50 nanometers, with a narrow particle size distribution, PSD. Clear homogeneous tetraalkylammonium (alumino)silicate solutions containing controlled amounts of alkali metal cations are hydrothermally treated at 100 °C without stirring and under reflux. The zeolite sols display the typical colloidal characteristics of Tyndall light scattering, susceptibility to elevated electrolyte contents and a low rate of sedimentation.
The absence of a solid amorphous aluminosilicate in the synthesis solution throughout the course of the crystallization permits the use of analysis techniques common within the field of colloidal science, i.e. dynamic light scattering and ultracentrifugation. Growth rates, particle sizes and the corresponding PSD, could be determined with a high degree of precision - the two latter parameters were confirmed by electron microscopy and XRD peak broadening.
The colloidal zeolite crystals are XRD crystalline and possess a high degree of stability under prolonged hydrothermal treatment. The colloidal nature of the zeolite is illustrated by the fact that the particles in aqueous suspension are coagulated by excess amounts of electrolyte and are not compatible with organic solvents. The ion-exchange of colloidal zeolite may be achieved however by the use of ion-exchange resins and the sols may be transformed to organosols. The topics of crystallinity, structural order and solubility are adressed.
The method of direct analysis has been employed to monitor the course of crystallization as well as the process of particle size tailoring. Furthermore, the mechanism of zeolite crystal growth has been investigated by applying experimental data to the classical equations describing crystal growth.