Heterogeneous versus Homogeneous Nucleation and Growth of Zeolite A

Abstract

The effectiveness of the addition of nanoparticles of various materials to act as substrates for zeolite A nucleation and growth was investigated in an attempt to obtain submicron zeolite A crystals. It was found that the important parameters influencing nucleation and growth rates of zeolite A crystals over the substrates were (1) the surface energies of the substrates, zeolite A and the synthesis medium, and (2) the size of the substrate. Nucleation rates of zeolite A over substrates was found to be significantly lower as compared to nucleation rates for homogeneously nucleated zeolite A, even though the energy barrier for nucleation was found to be significantly lower in the former case. This could be attributed to the steric factors associated with the substrate. Indeed, we proved mathematically this to be the case. It was found that the nucleation becomes progressively easier on substrates as the contact angle between the zeolite A embryo and the substrate goes from π to 0. It was interesting to note that synthesis using two Titanias (Hombikat and Hombifine) as substrates gave very promising results in obtaining submicron zeolite A crystals and crystals in the size range of about 0.3 μm on the surface on these substrates were obtained. No zeolite A crystals were found to undergo heterogeneous nucleation and thus grow on other substrates like Fe2O3, TiO2 (Ishihara), Ce2O3, SnO2, ZnO, TiO2 (Nanotek), CuO, TiO2 (Kemira), TiO2 (Degussa). For most of them, the contact angle between the substrate and the zeolite A embryo does not fall between 0 and π/2, which is the range required for heterogeneous nucleation and growth. It was found that the Zeta potential of only TiO2 Hombikat and TiO2 Hombifine goes to zero at pH ≈ 12.6. This leads to an increase in their mean particle sizes thus increasing the ratio of the size of the substrate to the critical zeolite A embryo size, a factor that was found to be crucial for heterogeneous nucleation. X-ray Diffraction (XRD), Laser scattering particle size analyzer, Scanning Electron Microscopy (SEM), EDAX, and Energy dispersive XRF have been used to characterize the zeolites synthesized.