Synthesis of hierarchical MFI zeolite microspheres with stacking nanocrystals

MFI nanozeolite microspheres with intercrystal mesopores were synthesized by a steam-assisted crystallization-aggregation method, starting with close packed amorphous SiO2 or Al-containing SiO2 colloidal nanoparticles. The obtained samples with different Si/Al ratios were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM)/transmission electron microscopy (TEM) observations, nitrogen adsorption analyses and Al-27 magic angle spinning nuclear magnetic resonance (NMR). The SEM images clearly showed that the mean size of nanozeolite microspheres was ca. 0.5-3 mu m, agglomerated from zeolite nanocrystals and varied with different Si/Al ratios. N-2 sorption analyses as well as TEM micrographs indicated the dual-porosity of samples, one is from intracrystal micropores, and the other is from intercrystal mesopores. Al-27 MAS-NMR suggested nearly all of the Al atoms were located at tetrahedral-coordinated sites in the framework. Furthermore, nanozeolite microspheres with various Si/Al ratios, even Si/Al equal to five, could be prepared by this facile route and less structure-directing agent was needed compared to conventional approach. This simple method could be easily extended to prepare other hierarchically structured zeolites. (C) 2008 Elsevier Inc. All rights reserved.