Structure and colloidal stability of nanosized zeolite beta precursors

We report the mechanism of zeolite beta nucleation and growth at 120 °C in Al-containing solutions. Two solutions with molar compositions 1 SiO ₂/80 H₂O/0.25 TEA₂O/4 CH₃CH ₂OH/(0.05 + Y) Na₂O/K Al₂O₃, where Fis 0 and 0.01, were studied using attenuated total reflectance-Fourier transform spectroscopy, small angle scattering, and cryogenic-transmission electron microscopy. First, at room temperature, supersaturated silica self-assembles into primary particles (<3 nm). Upon heating the primary particles, the least stable ones aggregate into secondary particles. Some of the secondary particles are stable as spheroidal monomer particles, but others are unstable and aggregate into small clusters. After 4 days of heating, secondary particles are mostly composed of amorphous silica, but their density is similar to that of zeolite beta. All-silica secondary particles are stable in solution for extended periods of time (>100 days), and no all-silica zeolite beta product was obtained. On the other hand, after 6 days of heating solutions containing Al, we observe that Al-containing secondary particles aggregate into tertiary particles that have the structure of zeolite beta. We conclude that as silica reorganizes from amorphous into zeolite beta, Al-containing secondary particles become less stable (in the colloidal sense) and aggregate with, tertiary particles.