Nathan D. Hould, Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, 150 Academy St., Newark, DE 19716 and Raul F. Lobo, Center for Catalytic Science and Technology, Department of Chemical Engineering, Univesity of Delaware, 150 Academy St., Newark, DE 19716.
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.