Mesoporous materials, formation

Effect of pH value and silicium source on mesoporous materials formation... 

To study the influence of the silica source on the mesoporous materials formation, we have performed synthesis by adding different source of silica neutral TMOS and ionic sodium silicate to the micellar solution. Syntheses were made either under acidic (pH = 2) or basic (pH = 10) conditions for TMOS. Table 1 contains essential information of the obtained products. 

Polyoxyethylene alkyl ether and in particular decaoxyethylene oleyl ether CigHj5 (CH2CH20)io can be used as templating agent for silica mesoporous materials formation. The synthesis can be performed under both acidic or basic conditions with tetramethoxysilane (TMOS) as silica source while no mesoporous compound was obtained with sodium silicate. 

This coated micelle mechanism remained a central concept in the mechanism of mesoporous material formation for several years. It was used by Yang et to explain the formation of spectacular rope-like,... 

Patarin, J., Lebeau, B. and Zana, R. (2002) Recent advances in the formation mechanisms of organized mesoporous materials. Current Opinion in Colloid and Interface Science,... 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

Microporous nanoparticles with ordered zeolitic structure such as Ti-Beta are used for incorporation into walls or deposition into pores of mesoporous materials to form the micro/mesoporous composite materials [1-3], Microporous particles need to be small enough to be successfully incorporated in the composite structure. This means that the zeolite synthesis has to be stopped as soon as the particles exhibit ordered zeolitic structure. To study the growth of Ti-Beta particles we used 29Si solid-state and liquid-state NMR spectroscopy combined with x-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). With these techniques we monitored zeolite formation from the initial precursor gel to the final Ti-Beta product. 

With the combined methods of 29Si NMR spectroscopy, X-ray diffraction, HRTEM and SAED we were able to characterize the Ti-Beta particle growth. 29Si NMR spectroscopy gave us an opportunity to see the formation of nanoparticles even before they were detectable with other techniques such as XRD. The above mentioned techniques enabled us to obtain sufficient knowledge to prepare Ti-Beta nanoparticles which were than successfully incorporated in novel micro/mesoporous materials [1],... 

For n-decane isomerization, when a good balance between the metal phase and the acidic phase of the catalysts is reached, the isomerization and cracking yield curves of the catalysts are a unique function of the conversion, meaning that these yields do not depends on the porosity nor the acidity of large pore materials. Formation of the most bulky isomers, such as 4-propylheptane and 3-ethyl-3-methylheptane was favored in mesoporous solids (figure 1). Criteria based on the formation of these particular isomers are linked with mesoporosity and could be useful to discriminate between zeolites catalysts with and without mesopores. 

Yamamoto, Ichikawa, and coworkers—mesoporous material and zeolite supported Au and Pt catalysts—formate intermediates observed with Au/zeolites. 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

Fig. 5. Schematic of the proposed formation of mesoporous materials from silicate-bearing hectorites.

A further point of interest is that, in addition to ferrierite, high selectivity for isobutylene is also obtained on alumina catalysts modified with halides323 325 344 345 and over mesoporous materials with low A1 content.325 346 Selective formation of isobutylene on these samples is attributed to the low concentration of active sites. This results in distant location of activated 1-butene molecules, thereby suppressing the bimolecular mechanism.339 346 347... 

SEM micrographs of the mesoporous materials prepared from C 6TMAB-HN03-TE0S-H20 composition, with and without the addition of PEO-6000 polymer, are shown in Fig. 1. In the absence of this polymer additive, mesoporous silica with gyroidal particulate morphology (Fig. 1 A) was formed even under the shear flow conditions. On the other hand, the addition of this polymer promotes the formation of mesoporous silica materials with millimeter-sized, rope-... 

The synthesis and characterization of the structural defects within aluminosilicate mesoporous materials were provided. We further discussed the fascinating adsorption-desorption hysteresis behaviors and the influencing factors in the formation of the structural defects. However, mesoporous MCM-41 can act as catalyst support for many catalytic reactions, especially involve bulk oiganic molecules, due to its large surface area and pore size. The ability to synthetically control the connectivity of the mesoporous materials may have important applications in catalysis. 

For a molar ratio of decane/TMB equal to 0.66 (x = 1 and y = 15), the effect of the crystallization time and temperature on the formation of mesoporous materials has been studied. 

From Figure 6 it is clear that whatever is crystallization temperature, 80°C or 100°C, the crystallization time should not excess 4 days. After this delay, the amorphisation of the material is completely reached, the value of the specific surface area drops sharply and no homogeneous pore size distribution is obtained. Lower crystallization temperatures, for example 60°C, should be studied. It should be noted that for a given molar ratio of decane/TMB the variation of crystallization temperature and time can lead to the formation of both MCM-41 and MCM-48 We would like to show here only the effect of crystallization temperature and time on the formation of mesoporous materials. We neglect at the moment which kind of mesoporous materials is formed at a given crystallization temperature and time This will be discussed in the following section. 

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