Mesoporous silica surface area

Recently, aminopropyl-fxmctionalized SBA-15 materials were found to catalyze the synthesis of flavanones with good yields and high selectivity [82]. It was observed that the catalytic activity of the materials decreases with increasing loading of amine groups on SBA-15 probably due to the diminution in the mesoporous silica surface area and pore volume. 

The acylation of anisole wifh AAN was examined in the presence of silica-supporfed PW.i The supporfed catalysfs are prepared by impregnating silica (surface area 300 m x g i) or mesoporous silica MCM-41 (surface area 1250 m x g i) wifh a mefhanol solution of PW. The acylations are carried out in liquid phase in a glass reactor charged with aromatic substrate and AAN, the substrate taken in excess over the acylat-ing agent no solvent is used. 

The reduction of porous silica with magnesium vapor is a highly exothermic reaction. The process with thermal moderators is an attractive route to generating highly mesoporous silicon (surface areas >500 m /g). Without thermal moderators, large batches of macroporous silicon can be generated by combustion synthesis. Electrochemical reduction with molten calcium or lithium salts rather than thermal reduction with magnesium is also under evaluation. The use of liquid aluminum to reduce mesoporous silica deserves much further study. 

Use of open mesoporous high-surface-area silica supports such as hexagonal mesoporous silica (HMS), SBA-15, or SBA-16 [203-205] was also envisaged in order to maximize the dispersion of the active phase. Addition of Al species was also performed [206] to anchor Ni and Mo species on these supports. However, control of the location of active species inside these mesoporous frameworks still remains problematic [207], Other options include the use of multi walled carbon nanotubes but their feasibility as effective support remains to be proved [208]. 

Physico-chemical characterizations were performed on the finished ceria-doped silicas. Surface area measurements (BET) and mesopore size distribution (BJH) were carried out by means of Sorptomatic 1900 (Carlo Erba) instrument. X-ray diffraction patterns were recorded with a D 5005 X-Ray Diffractometer (SIEMENS) using Cu Ka radiation coupled with a graphite monochromator. The crystallite sizes of ceria phase were calculated from the line broadening of the most intense reflection using the Scherrer equation [13]. 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

We showed that these mesoporous silica materials, with variable pore sizes and susceptible surface areas for functionalization, can be utilized as good separation devices and immobilization for biomolecules, where the ones are sequestered and released depending on their size and charge, within the channels. Mesoporous silica with large-pore-size stmctures, are best suited for this purpose, since more molecules can be immobilized and the large porosity of the materials provide better access for the substrates to the immobilized molecules. The mechanism of bimolecular adsorption in the mesopore channels was suggested to be ionic interaction. On the first stage on the way of creation of chemical sensors on the basis of functionalized mesoporous silica materials for selective determination of herbicide in an environment was conducted research of sorption activity number of such materials in relation to 2,4-D. 

The second case study. This involves all silica micro- and mesoporous SBA-15 materials. SBA-15 materials are prepared using triblock copolymers as structure-directing templates. Typically, calcined SBA-15 displays pore sizes between 50 and 90 A and specific surface areas of 600-700 m g with pore volumes of 0.8-1.2cm g h Application of the Fenton concept to mesoporous materials looks simpler since mass transfer would be much less limited. However, it is not straightforward because hydrolysis can take place in the aqueous phase. 

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Galameau A., Cambon H., Di Renzo F., Fajula F. True microporosity and surface area of mesoporous SBA-15 silicas as a function of synthesis temperature. Langmuir 2001 ... 

Mesoporous silicas have characteristics of high specific surface areas and pores with defined dimensions and uniform distribution. These features make mesoporous systems ideal candidates as host materials to guest bio-molecules. Protein stability may be enhanced due to reduced autolysis in the case of protease enzymes, and more generally reduced protein aggregation, as a result of the separation of the molecules adsorbed on the surface. 

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