Calcination of the MCM-41 mesophase: mechanism of surfactant thermal degradation and evolution of the porosity

Abstract

Sample controlled thermal analysis (SCTA) has been used to carefully and reproducibly eliminate the organic surfactant template from pure silica mesoporous MCM-41. The reproducibility allowed a number of intermediate species to be isolated permitting complementary measurements. To understand the mechanisms by which the surfactant is removed, evolved gas analysis as well as ¹H and ¹³C MAS NMR were used. The liberation of the porosity and evolution of the surface hydrophobicity were followed by gas adsorption, XRD and immersion microcalorimetry.

It would seem that two types of cetyltrimethylammonium surfactant exist in the confined state, of which the large majority decomposes at a lower temperature (up to 200 °C) within the silica organic host than in the pure state. A small quantity of organic fragments are still observed within the pore structure up to 500 °C. Gas adsorption shows a pore blocking effect occurs during the surfactant removal.