Mesopore channels

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. 

Scheme 1. Inclusion of size-controlled PVP-protected Pt nanoparticles in calcined mesoporous SBA-15 silica matrices. Mechanical agitation by low-power sonication affords a high dispersion of nanoparticles ranging in size from 1 to 7nm in the mesopore channels. The method is referred to as capillary inclusion (Cl). The technique is limited by the size of nanoparticles that can fit into the 6-9 nm diameter mesopores [13]. (Reprinted from Ref [13], 2005, with permission from American Chemical Society.)...

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. 

IUPAC classification, mesoporous materials are defined as porous materials with diameters in the range 2-50 nm, which is rather dose to the dimensions of functional biomolecules such as proteins. Therefore, unexplored phenomena and functions could be observed for biomolecules confined in mesopore channels due to their restricted motion and orientation. In this chapter, we briefly introduce recent developments on the immobilization of biomolecules in mesoporous media, where the use of mesoporous silica and mesoporous carbon are mainly discussed. 

Fig. 4.4 Pore-filling models for protein adsorption in a mesopore channel (a) separated single-molecularadsorption (b) separated double-molecular adsorption (c) separated triple-molecular adsorption (d) interdigitated triple-molecular adsorption where adjacent layers are interdigitated by 1/4 of the protein diameter through changing the relative orientation. Adapted from [37],...

The photoemission spectra of the GFP in buffer solution and of the hybrid material GFP/SBA-15, are reported in Figure 4. All samples were excited at 475 nm and show a well resolved photoemission band at 502 nm with a shoulder at 536 nm. The shape of the emission profile for GFP/SBA-15 follows closely that of the GFP in buffer solution, but the intensity of the signal is higher in the case of the hybrid. This result evidences that the photoemission efficiency is enhanced by the protein confinement inside the mesoporous channels. 

One difficulty with many synthetic preparations of semiconductor NCs that complicates any interpretation of NMR results is the inevitable distribution of sizes (and exact shapes or surface morphologies). Therefore attempts to make semiconductors as a sort of molecular cluster having a well-defined stoichiometry are of interest to learn potentially about size-dependent NMR parameters and other properties. One approach is to confine the semiconductor inside a template, for instance the cuboctahedral cages of the sodalite framework or other zeolite structures, which have been characterized by multinuclear NMR methods [345-347], including the mesoporous channel material MCM-41 [341, 348]. 

The effect of the modifiers appears to be more evident in the case of Ru-MCM-41 catalysts. The size-controlled mesoporous channels of MCM-41 affords a better interaction of the prostaglandin intermediates with modifiers [274,275],... 

Ichikawa et al. first reported [18-20] that Pt nanoparticles are produced by the conventional dry H2 reduction of a H2PtCl6 impregnated FSM-16 sample, while Pt nanowires are selectively synthesized in the mesoporous channels by UV-... 

Figure 15.4a shows a typical TEM image of the Pt wires, which clearly extend as dark stripes along the length of the mesoporous channels. The Pt wires (3nm in diameter) are consistent with the pore diameter of FSM-16 (2.7nm), and their length ranges extend to several hundred nanometres, reflecting the 1D channel structure of mesoporous silica templates (Figure 15.1). Moreover, the high-...

XRD patterns of Pt/FSM-16 [25] (and HMM-1 [32]) show no significant change at 26 = 1-10° before and after the incorporation of metal nanowires and nanoparticles (Figure 15.7). This indicates that the pore structures and mesoporous channels of FSM-16 (and HMM-1) remained unchanged in the synthesis of the Pt wires and Pt particles [18-20, 23, 24] by wet photo-irradiation with methanol -i- water vapor and dry H2 reduction, respectively. Furthermore, in the high 26 region, typical peaks assigned to Pt fee crystalline are observed for both samples of Pt nanowire/FSM-16 and Pt nanoparticles/FSM-16 [25]. 

Pt-Cl (light blue line), and those of the mean length (nm) of Pt nanowires elongating in the mesoporous channels of FSM-16 (yellow bars), which were measured by TEM observation as shown in Figure 15.11. 

Nanostructured germanium structures with mesoporous channels running through them can represent a different form of framework materials with exciting physical properties. When bulk germanium is penetrated by a regular array of mesopore channels, nanosized walls can be created that can exhibit electronic and optical characteristics similar to those of discrete nanodots [33]. 

Additionally, the microwave treatment during the crystallization process at high temperature may cause the metastable mesophase to collapse into the denser or amorphous phase in synthetic mixture as well as provide the favorable condition for the formation of silicalite-1. A summary of parameters obtained by nitrogen sorption is shown in Table 2. In Table 2, pore diameters of major peaks ( ) for sample II-IV are increased from 2.5 to 2.87 nm as extending the microwave irradiation. It implied that the additional space created in the mesoporous channels, as a consequence of the pore size enlargement, that is filled by extra water [16]. 

Figures 4-b and 4-d depict the pore size distribution curves of the SBA samples after these different treatments. For the sample SBA-A treated in acidic medium, the BET surface area (869 m2g" ), the mean pore diameter (6.4 nm) and the pore size distribution curve are similar to those from the pure parent silica SBA. For neutral treatment, the surface area (667 m2 g 1) decreases slightly. This can be related to the reduction of the microporous phase of the sample as shown in the pore size distribution curve. However, the mean pore diameter remains unchanged. Conversely, the structural properties of SBA-B are modified after treatment in basic solution. In this case, we observe a strong decreasing of the specific surface (454 m2 g 1) accompanied by a total loss of the microporous phase and an increasing of the mean mesoporous diameter (7.2 nm). It seems that in basic medium, a leaching phenomenon inside the mesoporous channels does occur, leading to a partial dissolution of the wall and resulting in smaller wall thickness (4.3 nm). Compared with the results on MCM-41, which show that the mesoporous structure collapses in basic solution [9,10], we can say that the stability of SBA materials in this medium is much higher.

The mechanical stability of PSM and AMM-5 samples was investigated by pressing the sample in a die (having a diameter of 16 mm) under different pressures for 15 min. The effects of compression on the surface areas and pore properties of the materials are shown in Table 1. It can be seen that the surface areas of both PSM and AMM-5 samples decrease under high pressure compression. The decrease of surface area, which is proportional to the pressure exerted on the samples, is accompanied with the decrease of pore volume, with no obvious decrease of the pore diameter for both samples. The results indicate that, under high pressure compression, some of the mesoporous channels of MCM-41 have collapsed completely and not constricted to pores of smaller diameter. 

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