Transformation of lamellar silicate into the mesoporous molecular sieve MCM-41

Abstract

Mesoporous silicates synthesized in strongly basic media from sodium silicate in the presence of cetyltrimethylammonium chloride, a cationic surfactant. The formation of a lamellar phase and its transformation into hexagonal MCM-41 were studied by powder X-ray diffraction (XRD), ²⁹Si magic-angle-spinning (MAS) NMR and transmission electron microscopy (TEM). The gel mixture (pH>14) is transparent. Upon acidification, a layered phase with interplanar spacing d=35 Å rapidly forms at pH=13 and gradually transforms into the lamellar phase with d=31 Å. The ²⁹Si MAS NMR spectrum of the layered phase is broad and dominated by the resonance from Si(OSi)₃O–M⁺ (Q³) structural units (M stands for the surfactant cation). The spectrum of the lamellar phase is well resolved with a more intense line from the Si(OSi)₄ (Q⁴) structural units, indicating a more regular atomic arrangement. The hexagonal phase is obtained at pH=11.5 upon acidification of the gel, or when the pH of the intermediate product containing the lamellar phase is adjusted to 7–9 and the hydrothermal treatment continued. The hexagonal phase gives a broad ²⁹Si NMR spectrum dominated by the Q⁴ resonance, indicating a less ordered local atomic arrangement in comparison with the lamellar phase. We suggest that the transformation is driven by the increasing reactivity of the silicate species with increasing acidity of the gel, which facilitates further aggregation of uncondensed silicate between the layers.