TRANSFORMATION OF LAMELLAR SILICATE INTO THE MESOPOROUS MOLECULAR-SIEVE MCM-41

Mesoporous silicates synthesized in strongly basic media from sodium s ilicate 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), Si-29 magic-angle-spinning (MAS) NMR and transmission electron microsc opy (TEM). The gel mixture (pH > 14) is transparent. Upon acidificatio n, a layered phase with interplanar spacing d = 35 Angstrom rapidly fo rms at pH = 13 and gradually transforms into the lamellar phase with d = 31 Angstrom. The Si-29 MAS NMR spectrum of the layered phase is bro ad and dominated by the resonance from Si(OSi)(3)O-M+ (Q(3)) structura l units (M stands for the surfactant cation). The spectrum of the lame llar phase is well resolved with a more intense line from the Si(OSi)( 4) (Q(4)) structural units, indicating a more regular atomic arrangeme nt. The hexagonal phase is obtained at pH = 11.5 upon acidification of the gel, or when the pH of the intermediate product containing the la mellar phase is adjusted to 7-9 and the hydrothermal treatment continu ed. The hexagonal phase gives a broad Si-29 NMR spectrum dominated by the Q(4) resonance, indicating a less ordered local atomic arrangement in comparison with the lamellar phase. We suggest that the transforma tion is driven by the increasing reactivity of the silicate species wi th increasing acidity of the gel, which facilitates further aggregatio n of uncondensed silicate between the layers.