M41S mechanism

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],... 

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. 

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. 

Mesoporous molecular sieves materials5-8 designated M41S (which include the MCM-41 class of materials) have made a further major impact on the area of synthesis of porous materials. A variety of open framework structures that are mesoporous have recently been reviewed by Thomas.9 Activated charcoal, MCM-41, mesoporous tungsten oxide, and substituted MCM-41 materials are mentioned. This article primarily emphasizes potential applications of such materials and possible mechanisms of reaction. The mesoporous sysems are compared briefly to microporous materials such as zeolites, ALPOs, MeALPOs and SAPOs. 

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... 

The discovery of a novel family of mesoporous molecular sieves (M41-S) has opened the way to new mesostmctured materials [1]. The synthesis involves a templating mechanism in which not single molecules, but supramolecular arrangements such as the liquid crystalline phases of surfactant molecules, serve as structure-directing agents. 

Different mechanisms for the interactions between the sihcate precursors and the organic template, as well as the liquid crystal templating mechanisms, have been discussed (Ting et al., 1999 Tanev and Pinnavaia, 1995). Two other major types of M41S materials are MCM-48 (with 3-D pores) and MCM-50 (with a pillared layer or lamellar structure). 

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 key feature in the preparation of mesoporous molecular sieves is that the templates used are surfactant aggregates instead of the traditional single organic molecule or metal ion. The mechanisms responsible for the formation of mesoporous molecular sieves, such as the M41S family of materials, from their precursors have for the past decade attracted much attention and speculation. A number of mechanisms have been suggested, each apparently... 

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