M41S formation

This review of the proposed M 41S formation mechanism will address the issues of how M 41S materials formed, factors controlling this formation and principles of phase selection/transformation. The concepts concerning M41S formation have been formulated based on the studies involving silica (or aluminosilicate)-cationic surfactant systems. Their extension to other compositions will be discussed at the end of this section. As M41S materials are always prepared in the presence of amphiphiles (surfactants), it is safe to conclude that surfactant molecules play a vital role in their formation. Furthermore, the M41S materials... 

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. 

The adsorption isotherms are often Langmuirian in type (under conditions such that multilayer formation is not likely), and in the case of zeolites, both n and b vary with the cation present. At higher pressures, capillary condensation typically occurs, as discussed in the next section. Some N2 isotherms for M41S materials are shown in Fig. XVII-27 they are Langmuirian below P/P of about 0.2. In the case of a microporous carbon (prepared by carbonizing olive pits), the isotherms for He at 4.2 K and for N2 at 77 K were similar and Langmuirlike up to P/P near unity, but were fit to a modified Dubninin-Radushkevich (DR) equation (see Eq. XVII-75) to estimate micropore sizes around 40 A [186]. 

Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2],... 

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... 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

Baturin G. H., Merkulova K. I., and Chalov P. I. (1972) Radiometric evidence for recent formation of phosphatic nodules in marine shelf sediments. Mar. Geol. 13, M37-M41. 

Other workers in the past, however, Baltzer (B7, B8), Bonis and Kalk (B26, K3), Bucher (B43, B44), Kapp (K7), MitcheU (M41), and Udaondo and Zunino (U3), considered that mucin plays no significant role in the buffering of HCl in the stomach, and that the frequently described decrease in mucus content in peptic ulcer may be instrumental in ulcer formation by a mechanism other than decrease of its buffering capacity. 

Figure 4. Mobil group proposed formation pathways of M41S [49].

The original reaction mechanism for the growth of M41S and MCM-41 materials was proposed by Mobil researchers. 1 This proposed mechansim involved formation of rod-like structures of micelles and concomitant formation of a hexagonal array of rods, after which an inorganic species would encapsulate the rods and surround the surfactant species. Calcination of these composite materials led to the... 

The initial templates for the M41S family materials are quaternary ammonium surfactants with positive charge and long tail (hydrocarbon chain). The synthesis was believed to proceed via the formation of a lyotropic liquid crystal, which is an ordered assembly of micelles (details will be discussed in the mechanism section). The... 

---