(432f) Zeolite Beta Nucleation and Growth

To synthesize zeolite crystal particles with uniform morphology and controlled size the role of structure directing agents (organic and inorganic cations) in zeolite nucleation and growth needs to be understood. Zeolite beta is a model material to study how the interaction between isomorphous Al substitutions and structure directing agents effect zeolite nucleation and growth rates since it is can be synthesized having Si/Al from ∞ to 3. The broad Al concentration range over which zeolite beta can be synthesized is due to the availability of several structure directing agents to synthesize zeolite beta. I report a two-step nucleation theory to describe zeolite beta nucleation. In this theory the density and composition of the precursor particles becomes similar to zeolite beta upon heating and afterwards a slower evolution of the precursor particle structure occurs to complete nucleation. By blending TEA+ and Na+ in dilute (H2O/SiO2 = 80) zeolite beta synthesis solutions at low Al concentration (Si/Al = 50) I show that the nucleation rate decreases and the growth rate increases (colloidal stability decreases) because of competition between TEA+ and Na+ for surface adsorption sites and occlusion into the precursor particles. Using more concentrated synthesis solutions (H2O/SiO2 = 20) as an example I discuss how interactions between cations and trivalent heteroatom substitutions, [AlO2/4]-, in precursor particles can effect their density and the phase selectivity of the synthesis solution. I conclude that Na+ is important to control the density of the precursor particles in the synthesis of zeolite beta since it is necessary to nucleate zeolite beta at high Al concentrations. On the other hand, it slows down nucleation at low Al concentrations. Finally, the understanding of how interactions between TEA+, Na+, and [AlO2/4]- sites in silica effect the nucleation and growth (in the colloidal sense) rates of zeolite beta is used to synthesize nearly monodisperse zeolite beta crystals with sizes ranging from about 100 to 500 nm.