Nanoparticle precursors and phase selectivity in hydrothermal synthesis of zeolite β

The crystallization mechanism of zeolite in relatively dilute solutions composed of 1SiO(2)/80H(2)O/0.25TEA(2)O/4EtOH/(0.05+Y)Na2O/YAl2O3/WB2O3, 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 degrees 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 P, >200 nm). The tertiary particles initially have poor long-range order and an oblate morphology. They evolve into zeolite beta crystals with longer-range order, a smoother surface, and a square bipyramidal morphology by solution-mediated silica reorganization.