M41S phase

In 1992, researchers of the Mobil Oil Company introduced a new concept in the synthesis of mesoporous materials. They used supramolecular arrays of surfactant molecules as templating agents in order to obtain mesostructured silicates or alumosilicates which retain after calcination an ordered arrangement of pores with diameters between 2 and 10 nm and a narrow pore size distribution comparable to that of zeolites. These materials called M41S phases give access to the regime of the mesopores which is very interesting for different kinds of new size selective applications, e.g., molecular sieves, catalysis and nanocomposites [1]. 

Figure 36.4. Schematic depiction of different configurations at the organic-inorganic interface of mesostructures a) standard M41S phase b), c) with phenylsiloxy residues attached to the silica walls d), e) with hydrolyzed and deprotonated phenylsiloxy anions incorporated into the micellar structures (the phenylsiloxy anions are not necessarily fully deprotonated). Configurations b) and d) lead to a decrease, configurations c) and e) to an increase in curvature at the interface. For further explanation, see text.

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

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. 

The development of new porous materials that could be used as adsorbents, catalysts, catalyst supports, molecular sieves, etc. [1], are very well discussed by several authors [2-9], describing interesting properties and characteristics of materials such as MCM-41, MCM-48, M41S, FSM16, lamellar phases, intercalation products, special CMS (carbon molecular sieves), fullerenes, carbon nanotubes, etc. being some of them silica based materials, and carbon based the others. 

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],... 

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 overall performance of a catalyst is known to depend not only on the inherent catalytic activity of the active phase but also on the textural properties of the solid. The ability to control the specific surface area and the pore size distribution during the synthesis of amorphous silica-aluminas has been described for both surfactant micelle templated syntheses (M41-S (1), FSM-16 (2), HMS (3), SBA (4), MSU (5), KIT-1 (6)) and cluster templated sol-gel syntheses (MSA (7), ERS-8 (8)). 

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