Silica micro porosity

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

The wall thickness can be adjusted from 0.2 to 2 pm it is proportional to the product of the ice crystal diameter dice and the silica concentration in the starter solution. The overall porosity of the micro-honeycomb is about 94% that of the walls around 40% (corresponding to a random packing of monosized spheres). Meso- and micro-porosity can be adjusted independently through hydrothermal treatment the initial high BET surface area (up to 900m g ) decreases and the small pores (radii < 1 nm) grow (up to 45 nm) so that mesopore volume is created. Microhoneycombs are suitable for many applications, since the macrochannels ensure alow pressure drop, and diffusion parameters in the porous walls can be engineered. 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

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. ... 

After drying, the materials usually exhibit large surface areas, typically between 100 and 900 m g due to a high micro pore content and large total porosities. Compared to fumed silica, the obtained material is cheaper but less pure. Silica gel of higher purity can be obtained by hydrolysis and condensation using metal-organic precursors as the silica precursor [111]... 

In the present paper silica-aluminas with controlled porosity in the region of micro- (ERS-8 (3), SA), (micro)/meso- (MSA (2)) and meso- (MCM-41 (1), HMS (4)) pores have been synthesized by sol-gel method using different gelling agents. The textural properties of these samples have been determined by physisorption of N2 at 77.4 K and Ar at 87.3 K and evaluated by different models, comparing their effectiveness. 

As in the manufacture of silica, porosity, pore size, and surface area of polymer packings can be adjusted over a wide range, and micro-, meso-, and macro- as well as non-porous beads are synthesized reproducibly. 

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. 

In view of possible commercial applications of rice husk silicas, e.g. as catalyst supports, their surface areas and porosities are important properties. Nitrogen sorption measurements of the three differently treated rice husk silicas show that these are porous materials with moderately large surface areas. The surface areas are, in detail 73 m /g for the calcined material, 75 mVg in the case of the material oxidized by Fenton s reagent and 51 m /g for the rice husks treated with Caro s acid. As the shapes of the ad- and desorption isotherms reveal (Fig. 3), pores ranging from micro- up to macropores are present. 

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