Zeolite beta mechanisms of nucleation and growth.

摘要

Zeolites are microporous aluminosilicates widely used in industrial processes as catalysts, supports, and adsorbents. 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 trivalent heteroatom substitutions (e.g., [AlO2] -) and structure directing agents (e.g., tetraethylammonium (TEA +) and Na+) effect zeolite nucleation and growth because it is can be synthesized having [SiO2] to [AlO2] - ratios from infinity to 3. Zeolite beta has been prepared in dilute (H2O/SiO2 = 80) and concentrated (H2O/SiO 2 = 20) synthesis solutions. The rates of zeolite beta nucleation and growth in dilute colloidal solutions have been characterized using dynamic light scattering and small angle X-ray scattering. By blending TEA + and Na+ in dilute solutions at low Al concentration (Si/Al = 50) it has been determined that nucleation rate decreases and 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 the role of ion pairs (e.g., [≡SiO- TEA +], [[AlO2]- TEA+], and [[AlO 2]- Na+]) in the precursor particles and the zeolite beta product was investigated in detail. Thermal gravimetric analyses have helped determine relation between synthesis condition and the ion pairs formed in the zeolite beta product. On the basis of the energies associated with these ion pairs a state diagram of zeolite beta including surface structure and phase selectivity has been developed. The relative rate of zeolite beta nucleation to growth has also been investigated by changing the ratio of TEA + and Na+. The synergy between these cations and anionic surface adsorption sites has led to synthesis of nearly monodisperse zeolite beta crystal particles having sizes from about 100 to 500 nm.