Structure mesoporous materials

Incorporation of titanium oxide species within the framework of mesoporous silicas has been shown to produce highly efficient photocatalytic materials. Extremely careful preparation conditions [84] leads to highly structured materials comprising anatase nanoparticles of dimension between 5 and 10 run. The channeled structure, together with the hydrophobic/hydrophilic character, are also key features controUing their enhanced photoreactivity. The photocatalytic activity of such mesoporous catalysts has been studied for the degradation of phenol in aqueous solutions [85]. It was observed that for structured mesoporous materials with low Ti content, the turnover frequency was four times greater than that for standard P25. 

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. 

Synthesis, eharacterization and catalytic activity of titanium containing mesoporous materials with TS-1 wall structure... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

Since the discovery by researchers at Mobil of a new family of crystalline mesoporous materials (1), a large effort has been expended on synthesis, characterization, and catalytic evalrration (2). MCM-41 is a one-dimerrsiorral, hexagonal structure. MCM-48 is a cubic structine with two, norrintersecting pore systems (3). MCM-50 is a layered stractme with silica sheets between the layers (4). Many scientists also looked into other mesoporous materials, of note the HMS (Hexagonal Molecular Sieve) family (5) and SBA-15 (acronym derived from Santa Barbara University) (6), bnt to date few materials have been both catalytically significant and inexpensive to synthesize. 

A joint research project between Lummus Technology and the Delft University of Technology led to the discovery of a new mesoporous material, named TUD-1 (8). TUD-1 is a three-dimensional amorphous structure of random, intercoimecting pores. The original emphasis was on the silica version, which has since been extended to about 20 chemical variants (e.g., Al, Al-Si, Ti-Si, etc.). 

Figure 41.4 shows a typical XRD (X-Ray Diffraction) pattern of TUD-1, along with a TEM image (12). Similar to other mesoporous materials, TUD-1 has a broad peak at low 20. However, it has a broad background peak, commonly called an amorphous halo, and lacks any secondary peaks that are evident for example in the hexagonal MCM-41 and cubic MCM-48 structures. The TEM shows that the pores have no apparent periodicity. In this example the pore diameter is about 5 nm. 

Some micro- and mesoporous materials exhibit anisotropic pore structures, which may yield different values for the diffusivities in the three orthogonal spatial directions. In such systems, the self-diffusion should be described by a diffusion tensor rather than by a single scalar self-diffusion coefficient. By measuring over a... 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Sol-gel processing forms the basis for various routes employed for the fabrication of a wide diversity of functional materials. To impart a structural organization at various length scales, the syntheses are performed using templates. Most consist of a self-organized ensemble of surfactants and co-polymers [1-10]. They have been successfully applied to control the geometry and dimensions of pores that are periodically arranged as in the initial structures. Mesoporous silica materials of the MCM family, which were first synthesized by a team from the Mobil oil company [11,12], are a well-known example. 

As seen in the comparison of mesoporous silica and PMO in protein adsorption behavior, the nature of the framework of mesoporous materials has unavoidable influence on the protein adsorption. Therefore, adsorption of protein on mesoporous structures composed of hydrophobic materials such as carbon is worthy of detailed investigation. In this section, systematic research on protein adsorption on mesoporous carbon materials by Vinu and coworkers is mainly introduced. 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

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. 

Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase. 

Figure 1 shows that the catalysts maintain their mesoporous structure with type IV isotherm. It can be observed a reduction in surface area, pore volume and pore diameter and slight increase in textural porosity as the concentration of aluminum increases (Table 1), due to the increase in the wall thickness in the mesoporous material as we have found previously [3],... 

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