M41S materials

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]. 

Fig. XVII-27. Nitrogen adsorption at 77 K for a series of M41S materials. Average pore diameters squares, 25 A triangles, 40 A circles, 45 A. Adsorption solid symbols desorption open symbols. The isotherms are normalized to the volume adsorbed at Pj = 0.9. (From Ref. 187. Reprinted with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

In the above work quats are occluded in zeolite lattices in a molecular way. In the recently disclosed (ref. 8) superwide pore M41S-materials quats arranged in the... 

In 1992, Mobil reported a novel family of molecular sieves known as the M41S materials that established an entirely new area in nanoporous solids (168,169). The M41S materials expanded the range of pore sizes into the mesoporous domain (20-300 A). The impact of this discovery on molecular sieve research has been profound. In 1998, an international conference solely devoted to mesoporous molecular sieves was founded (170). The journal Microporous Materials even re-... 

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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],... 

For the characterisation of M41S materials, often only powder X-ray and nitrogen adsorption studies are used. However, the use of transmission electron microscopy in combination with these techniques yields valuable information that is indispensable in order to understand the structure of mesoporous materials. 

Figure 3.3 Idealized models of mesostmctured M41S materials (a) MCM-41 (2D hexagonal, space group pbnun ). (b) MCM-48 (cubic, space group Ia- id), and (c) MCM-50 (lamellar, space group pi).

Two other major types of M41S materials are MCM-48 (with 3-D pores) and MCM-50 (with a pillared layer structure). 

The XRD pattern of the sample of MCM-41 showed that the material was high quality MCM-41. This sample was characterized in the same way as PS. Figures 2a and lb show the N2 adsorption isotherm at 77 K and the CO2 adsorption isotherm at 273 K at subatmospheric pressures, respectively, for MCM-41. Figure 2b also presents the complete CO2 adsorption isotherm obtained at high pressures. The shape of the N2 adsorption isotherm is typical for M41S materials and the pore volume obtained from the isotherm is 0.76 cm g . The micropore volume obtained by applying the DR equation to the CO2 adsorption isotherm (Figure lb) for MCM-41 is 0.14 cm g" , which is the same as the one for PS. 

This well accepted method [28] has been used extensively in the characterization of M41S materials [11-12,14]. From the application of this method to MCM-41, it has been concluded that this material contains no significant amounts of microporosity. This is the main evidence presented so far in order to conclude that MCM-41 is exclusively mesoporous. As it happens with any good method its limitations need to be considered in order to avoid misinformation. In the case of the a method the choice of the reference isotherm is crucial. All the reference silicas should be nonporous in order to allow a reliable analysis of MCM-41. Unfortunately, we observed that most of them have a steep rise in their N2 adsorption isotherms at 77 K at low relative pressures and BET surface areas varying from 40 [29] to 400 mVg [30], For this reason, our sample of MCM-41 was heat-treated so as to sinter the silica particles and thus obtain a nonporous silica (BET surface area 1.5 mVg) and as similar as possible to our MCM-41. The N2 adsorption isotherms for a reference silica [29] and our sintered MCM-41 are shown in Figure 7. 

Beck et aL 5348687 20.09.94 M41S Materials Having Nonlinear Optical Properties... 

In addition to the importance of the M41S materials for size- and shape-selective applications, these materials have been also regarded as a suitable mesoporous model adsorbent for testing theoretical predictions of pore condensation. Pore condensation represents a first order phase transition from a gas-like state to a liquid-like state of a pore fluid in presence of a bulk fluid reservoir, which occurs at a pressure p less than the saturation pressure po at gas-liquid coexistence of the bulk fluid [6,7]. In this sense pore condensation can be regarded as a shifted gas-liquid bulk phase transition due to confinement of a fluid to a pore. Recent work has shown that in fact the complete phase diagram of the confined fluid is shifted to lower temperature and higher mean density as compared with the bulk coexistence curve [e.g., 8,9]. 

The application of solvent extraction to removal and recovery of templates or SDAs from zeolite channels was initiated by Whitehurst[7] in the 1990s for the extraction of surfactant from mesoporous M41S materials. This method or its improved analogs have become one of the most important techniques to recover surfactants from mesoporous molecular sieves. However, it is still difficult to use this technique to remove and to recover the SDAs from microporous molecular sieves because, first, the size of SDA molecules is similar to that of the channel openings and the molecules are not able to diffuse out from the channels, and, secondly, there are usually strong interactions between the microporous frameworks and the SDA molecules that prevent the SDA molecules from being extracted solely by solvents. Modification of the conventional solvent-extraction technique, such as addition of chemical agents which can adjust the... 

Long-chain quaternary ammonium surfactants CwTMAX (n = 10-18, X = Br-, Cl-, or OH-) are the most used templates for M41S materials including MCM-41 and MCM-48. 

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