Mesoporous silica materials porosity

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. 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

Various mesoporous silica materials were synthesized using a non-ionic surfactant as template in the presence of different platinum salts in the synthetic gel. The salts were found to influence the structure and porosity of the solids obtained. Thus, our results show that platinum salts promote the hydrolysis of tetraethylorthosilicate (TEOS) and the presence of (NH4)2PtCl4 or H2PtCl6 leads to the formation of materials with smaller pore sizes and less condensed than those obtained in the absence of platinum salts or in the presence of (NH3)4PtCl2. 

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

The synthesis of ordered micro- and mesoporous silica materials often relies on the use of organic templates that are saciified in order to evacuate the pores. Inspired by the synthesis of amorphous microporous sihca materials by Maier et al.[l] applicable in molecttlar shape selective catalysis [2] and controlled release [3, 4] in this work we continued the exploration of silica synthesis imder strongly acidic conditions. We observed the outcome of the synthesis to be strongly dependent of the Si-soiuce, the type of mineral acid and the synthesis temperature. In this paper the synthesis of silica materials with a porosity ranging from micro- to mesoporous is demonstrated. This approaeh of synthesis of silica with tunable nanopores not involving sacriftcial templates will be convenient for many applications. 

Textural mesoporosity is a feature that is quite frequently found in materials consisting of particles with sizes on the nanometer scale. For such materials, the voids in between the particles form a quasi-pore system. The dimensions of the voids are in the nanometer range. However, the particles themselves are typically dense bodies without an intrinsic porosity. This type of material is quite frequently found in catalysis, e.g., oxidic catalyst supports, but will not be dealt with in the present chapter. Here, we will learn that some materials possess a structural porosity with pore sizes in the mesopore range (2 to 50 nm). The pore sizes of these materials are tunable and the pore size distribution of a given material is typically uniform and very narrow. The dimensions of the pores and the easy control of their pore sizes make these materials very promising candidates for catalytic applications. The present chapter will describe these rather novel classes of mesoporous silica and carbon materials, and discuss their structural and catalytic properties. 

Among the inorganic templates, zeolite produces more regulated pores as compared to the silica template. If nano-channels in zeolite are completely filled with carbonaceous precursor and then the carbon materials are extracted from the zeolite framework, one can obtain the porous carbon of which structure reflects the porosity of the original zeolite template. The ordered mesoporous silica templates, e.g., MCM-4 838,39,47 and SBA-1547 have been employed to prepare the ordered porous carbons by the procedures involving the pore filling of the silica template with carbonaceous precursor followed by carbonization and silica dissolution. The resulting pore sizes of the ordered mesoporous carbons are smaller than about 10 nm. 

In the present section we comment further on the chemical modifications of these materials when the R group is chosen for the preparation of micro-and mesoporous silicas. From a general point of view, the control of the porosity of silica via organic molecular templating is an attractive topic connected to molecular recognition, catalysis, chemical sensing and selective adsorption, etc. Many attempts have been made to control the pore size distribution in sol-gel derived silica30,196. 

The strategy of this method is to utilize the inherent porosity of bulky substrates in the construction of hierarchical stractures by incorporating additional pore systems. Diatoms are unicellular algae whose walls are composed of silica with an internal pore diameter at submicron to micron scales. Zeolitization of diatoms, in which zeolite nanoparticles are dispersed on the surface of diatoms followed by a hydrothermal conversation of a portion of the diatom silicas into zeolites, resulted in the formation of a micro/mesoporous composite material. Similarly, wood has also been used as a substrate to prepare meso/macroporous composites and meso/macroporous zeolites. After the synthesis, wood is removed by calcination. ... 

This paper describes a novel process for the preparation of spherical mesoporous silica spheres in the submicrometer and micrometer size range. Tetra-n-alkoxysilanes are hydrolysed and condensed in the presence of n-alkylamine as nonionic template and ammonia as catalyst. The porosity and the morphology parameters can be independently adjusted in wide ranges. These materials are promising adsorbents in separations techniques and valuable catalyst supports. 

Two different synthesis paths for making mesoporous silica possessing both framework-eonfined and textural mesoporosity has been presented. In the first synthesis path, adjustment of the reaction pH resulted in a gel-structure consisting of smaller particles and larger fraction of textural mesopores compared to ordinary MCM-41 materials. The lowest reaction pH resulted in the smallest particle size and highest amount of textural mesopores. In the second synthesis path, TEOS was allowed to prehydrolyze for different periods of time before surfactant addition. Longer prehydrolysis times resulted in a higher fraction of textural porosity and thicker pore walls. 

We will not discuss here models for pores in carbons, as this topic is treated in Chapter 5, and elsewhere in specialist [15] or general reviews [106, 107]. For similar reasons, we will not discuss porosity control [44, 108] in detail. However, porous carbons prepared by the template technique, especially the ordered ones, deserve special attention. Ordered mesoporous carbons have been known to scientists since 1989 when two Korean groups independendy reported their synthesis using mesoporous silicas as templates [109, 110]. Further achievements have been described in more recent reports [111, 112]. One might have expected that the nanotexture of these materials would merely reflect the nature of the precursor used, namely phenol-formaldehyde [109] or sucrose [110] in the two first ordered mesoporous carbon syntheses (as is well known, these two precursors would have yielded randomly oriented, isotropic carbon had they been pyrolyzed/activated under more conventional conditions). However, the mesopore walls in some ordered mesoporous carbons exhibited a graphite-like, polyaromatic character [113, 114], as described in Chapter 18. This information was obtained by nitrogen adsorption at low relative pressures, as in classical... 

In this study, we will focus our attention mostly on 7-AI2O3, Si02, mesoporous silica and active carbon as support materials. Their respective surface area and porosity are reported in Table 18.2. 

Methods for preparation of silica samples exhibiting or not porosity are known since a long time [10]. Before the discovery of mesoporous silica [11] zeohtes were the most important materials with micropores forming a regular array of channels with uniform size [12-14]. Mesoporous materials were mainly represented by amorphous silica [1], pillared clays, sihcates [15,16] and certain forms of alumina. A common characteristic of those materials is the irregular spatial distribution of pores and a wide spectrum of pore sizes [17]. In 1992 Beck et al. announced the synthesis and characterization of new mesoporous silica-based materials [11,18]. These mesoporous molecular sieves received the general... 

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