M41S hexagonal phase

The original members of the M41S family consisted of MCM-41 (hexagonal phase) (Figure 2.23), MCM-48 (cubic Ia3d phase), and MCM-50 (a stabilized lamellar phase) [110,111,117], The structure of MCM-41 has a hexagonal stacking of uniform diameter porous tubes, whose size can vary from about 15 A to more than 100 A [111,117],... 

MCM-41 is the hexagonal phase of silica-based mesoporous materials collectively designated as M41S [1]. 

The structure of M41S-type materials is built up of pores with amorphous walls that are formed around micelles of templating material (surfactants). One of the extreme structures of M41S-type materials (MCM-41) is a hexagonal ordering of the pores, an other extreme is a worm-hole disordered type of arrangement of the pores. A lamellar layered structure is another form in which these type of materials often (partially) appear, but this phase collapses to amorphous material upon removal of the surfactant (eg by calcination). A cubic ordering of the pores is also encountered. This form has been named MCM-48 and will not be discussed in the current paper. 

Mesoporous materials of the M41S family with their regular arrays of uniform pore openings and high surface areas have attracted much attention since their first synthesis in 1992 (61), because their properties were expected to open new applications as catalysts and/or adsorbents. These materials are formed by condensation of an amorphous silicate phase in the presence of surfactant molecules (usually ammonium salts with long alkyl chains). However, the chemistry of the steps of the synthesis process is still not fully clear. Ideas put forward so far include (a) condensation of a silicate phase on the surface of a liquid crystalline phase preformed by the surfactant molecules (62) (b) assembly of layers of silicate species in solution followed by puckering of those layers to form hexagonal channels (63) and (c) formation of randomly disordered rod-like micelles with the silicate species... 

The second mechanism was very general - it only stated what caused M41S formation and required further studies to elaborate mechanistic details. This has been addressed in a series of papers by Stucky et al. [10,11,23,43]. First, however, we would Hke to analyze two specific proposals concerning attempted elucidation of the self-assembly mechanism. Both pertained to the hexagonal (MCM-41) phase and relied on the identification of putative precursors to MCM-41. Stucky et al. [10] observed the presence of a layered siHca/surfactant phase upon mixing of the reactants. With time the hexagonal MCM-41 was produced, which led to the proposal of solid-state transformation, as depicted in Fig. 10. 

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