(199b) Nanoparticle Precursors and Phase Selectivity In Hydrothermal Synthesis of Zeolite Beta
AIChE Annual Meeting
2008
2008 Annual Meeting
Materials Engineering and Sciences Division
Advances in Porous Materials: From Synthesis and Characterization to Applications
Tuesday, November 18, 2008 - 8:50am to 9:10am
The crystallization mechanism of zeolite beta in solutions composed of 1 SiO2 : 80 H2O : 0.5 TEAOH : 0.1 NaOH : Y NaAlO2 : Z B(OH)3 where Y is 0 and 0.02 and Z is 0 and 0.05 was investigated using small angle x-ray scattering, dynamic light scattering, XRD, and electron microscopy. In this system there is a critical aggregation concentration (cac) for silica above which silica monomers and oligomers aggregate into nanoparticles and there is a negative correlation between the cac and the boric acid concentration. Initially primary nanoparticles (< 3 nm) are formed above the cac. When these solutions are heated 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 beta, > 200 nm). Initially the tertiary particles have poor long range order and an oblate morphology. They evolve into zeolite beta crystals with longer range order and smooth square bipyramidal morphology by solution mediated monomer dissolution and reattachment.