Calcination behavior of different surfactant-templated mesostructured silica materials

The removal of the template by calcination from mesostructured M41S and SBA-type silica materials was studied by combining high temperature X-ray diffraction, thermogravimetry-differential thermal analysis and mass spectrometry, allowing detailed in situ investigations during the thermal treatment. A comparison was made between materials with different mesoscopic structures, resulting from different synthesis routes and chemical treatment. The in situ XRD studies showed a strong increase in scattering contrast observed for the low angle reflections occurring when the template is removed from the inside of the pores, irrespective of the type of mesostructure. In agreement with the XRD investigations, the TG-DTA/MS experiments proved that the removal of the surfactant is a stepwise mechanism. Marked differences in the scattering contrast variations and chemical reactions were observed depending on the synthesis conditions and the type of surfactant, which highlight the role of the silica-surfactant interfaces. MCM-41 and MCM-48 materials synthesized in the presence of alkyltrimethylammonium surfactant under alkaline conditions showed a template removal mechanism based on an Hofmann degradation at low temperatures, followed by oxidation and combustion reactions above 250 degreesC. On the other hand, acidic conditions employed for the synthesis of SBA-3 type materials seems to favor reactions of oxidations after the evaporation of water and hydrochloric acid at low temperature. in that case, large contraction of the hexagonal unit cell was usually observed. Most of the block-copolymer template is removed from SBA-15 at lower temperatures, in a single oxidation step. The SBA-15 framework possibly catalyzes the oxidation of the block copolymer template species. In addition, the presence of framework porosity or pore connectivities seems to be responsible for the strong scattering contrast variations observed below 250 degreesC. Residual carbonaceous species and water are removed from the structure upon heating from 300 degreesC up to 550 degreesC. During this subsequent process a large contraction of the hexagonal unit cell is observed, possibly due to further framework condensation. In addition, a brief survey of the previous investigations reported in the literature related to the decomposition of structure-directing agents is given. (C) 2003 Elsevier Inc. All rights reserved.