High-temperature synthesis and formation mechanism of stable, ordered MCM-41 silicas by using surfactant cetyltrimethylammonium tosylate as template

Highly ordered 2D hexagonal mesostructured silicas with thick pore walls have been directly hydrothermally synthesized at high temperatures in a range from 130 to 175 °C by using the new surfactant cetyltrimethylammonium tosylate (CTATos) as template. The mesoporous structure of the synthesized MCM-41 could be maintained after heating it to reflux in boiling water for at least 24 h. The crystallization temperature, the nature of surfactants, and the relative amount of TAAOH (tetraalkylammonium hydroxide, such as TMAOH and TEAOH) to surfactant were found to be critical parameters that affect the ordering of mesophases. On the basis of the combined characterizations of X-ray diffraction (XRD), N₂ adsorption, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), ¹³C cross-polarization magic-angle spinning (CPMAS) solid-state NMR spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM), a new mechanism was proposed to understand the formation mechanism of highly ordered MCM-41 silicas. The enlargement of pore-wall thickness is attributed to the migration and subsequent deposition of the silicate species in the inner pore channel. This process was accelerated by the ion-exchange interaction of tetraalkylammonium cations (TAA⁺) on CTA⁺ cations. Well-ordered MCM-41 silicas with thick pore walls were directly synthesized byusing the new surfactant cetyltrimethylammonium tosylate as template. A newmechanism was proposed to understand the unit-lattice expansion and pore-wall thickening.