Nanoparticle Precursors and Phase Selectivity in Hydrothermal Synthesis of Zeolite β

The crystallization mechanism of zeolite β in relatively dilute solutions composed of 1SiO 2 /80H 2 O/0.25TEA 2 O/4EtOH/(0.05 + Y )Na 2 O/ Y Al 2 O 3 / W B 2 O 3, where Y is 0 and 0.01 and W is 0 and 0.025, was investigated using small-angle X-ray scattering, dynamic light scattering, X-ray diffraction, and electron microscopy. In this system there is a critical aggregation concentration (cac) for silica above which silica monomers and oligomers aggregate into primary nanoparticles (<3 nm), and there is an inverse correlation between the cac and the boric acid concentration. When these solutions are heated (120 °C) some of the primary particles transform into larger secondary particles (6−50 nm). In solutions without aluminum the secondary particles remain stable for extended periods of time. In solutions with aluminum the secondary particles aggregate into tertiary particles (zeolite β, >200 nm). The tertiary particles initially have poor long-range order and an oblate morphology. They evolve into zeolite β crystals with longer-range order, a smoother surface, and a square bipyramidal morphology by solution-mediated silica reorganization.