Silicate-Based Mesoporous Materials

We need to say that there are two types of mesoporous silicas random mesoporous structures and ordered mesoporous structures (Galameau et al. 2001). Mesoporous silicas, especially those exhibiting ordered pore systems and uniform pore diameters, have shown great potential for sensing applications in recent years (Melde et al. 2008). 

Sol-gel chemistry is frequently employed in designing random mesoporous structures of silicates (Brinker and Scherer 1990 Corma 1997). Liquid silicon alkoxide precursors (Si(OR) ) are hydrolyzed and condensed to form siloxane bridges, a process that is often described as inorganic polymerization and is represented below  

Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, 

Type of material Pore size (A) Examples Pore size range (A) 

Typical anionic surfactants are sulfonated compound with a general formula R-SOjNa and sulfated compounds of R-OSOjNa, with R being an alkyl chain consisting of 11-21 carbon atoms. 

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F. Hoffmann, M. Cornelius, J. Morell, M. Froba, Silica-based mesoporous organic-inorganic hybrid materials on the basis of silicates. Angew. Chem. Int. Ed. 45 (2006) 3216. 

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. 

Based on those propositions mentioned above, we tried to design a mesoporous material having micro crystalline wall by controlling the ratio of Q4 silicate species formed around TPA and Q2,3 silicate species interact with the micelles. To synthesize micro-mesoporous composite material through the control of Q2-3 and Q4 groups, two different templates were used and nucleation step of microporous material was introduced prior to the crystallization. And also we have attempted to monitor microenvironment of micro-mesoporous composite materials during the nucleation and crystallization steps using TG-DTA and photoluminescence with pyrene probe. 

The effects of post-synthesis alumination on purely siliceous MCM-41 material with A1(NC>3)3 on acidity have been studied by FTIR, NH3-TPD, and IPA decomposition reaction. The FTIR results of pyridine absorption show that both Lewis and Bronsted acid sites are increased by the post-modification. The amount of NH3 adsorbed on the alumina-modified MCM-41 samples increases with the loading of Al onto the surface of MCM-41. Due to the improved acidity, the alumina-modified MCM-41 materials show considerably higher catalytic activity for dehydration of isopropanol than purely siliceous MCM-41. In addition, XRD and N2 adsorption results show that all MCM-41 samples maintained their uniform hexagonal mesoporous structure well after they have been subjected to post-synthesis alumination with the loading of Al species on Si-MCM-41 varied from 0.1 wt. % up to 10 wt. % (calculated based on AI2O3). 

TTus paper is concerned with the particular class of molecular sieves having periodic mesoporous structure with pore sizes in the range of 2 to 10 nm. They are comprised of the M41S mesoporous molecular sieves and solids with related structures. In the first part, the preparation methods and characterization techniques will be reviewed and discussed. Silicate-based materials and non-silicate materials will be dealt with separately. In the second part of this review particular emphasis will be put on potential applications reported in both the patent and the open literature. Early progress in tWs field has been presented in the previous Summer School by Casci [37]. Potential catalytic applications of M41S were also reviewed rprPTitlv 1381... 

Periodic mesoporous materials may have important applications in the area of seperation of biological materials. The fabrication of composite and non composite membranes based on M41S silicates has been reported in the patent literature [262]. Another area with potential growth is the encapsulation of technologically advanced materials. Preliminary findings dealing with the following materials have been reported ... 

Kubota, Y., Nishizaki, Y., Ikeya, H., Saeki, M., Hida, T., Kawazu, S., Yoshida, M., Fujii, H., Sugi, Y. Organic-silicate hybrid catalysts based on various defined structures for Knoevenagei condensation. Microporous and Mesoporous Materials 2004, 70, 135-149. 

The majority of the hitherto reported non-siliceous mesoporous materials are hexagonally ordered or totally disordered. We recently reported a zirconium-based cubic mesostructure [15], which at that time could not be stabilized to result in a mesoporous material. Using the phosphate route developed previously for the stabilization of the hexagonal phase [13] we now also succeeded in Sorption isotherm of a phos- obtaining a mesoporous cubic... 

With respect to applications, there will certainly be more and more investigations where ordered mesoporous materials are used as catalysts or catalyst supports. However, the more skeptical note of the section on catalysis shall be repeated here In many cases, much cheaper and simpler alternatives exist, and the properties of ordered mesoporous materials are not so much superior to justify the higher effort of their synthesis. On a longer time scale, non-siliceous compositions will probably be used more frequently in catalysis. If one analyzes the catalytic processes implemented today, the majority is not based on silica as catalyst or support, and the single most important area of aluminumsilicates, acid catalysis by zeolites, seems to be less attractive for ordered mesoporous aluminumsilicates, unless a crystallization of the walls to zeolitic structures or the assembly of such materials from colloidal zeolites to enhance the acid strenght becomes possible. 

Equations (3) and (4) can conveniently be used in different methods of mesopore size evaluation, of which the BJH approach appears to be particularly suitable [55]. These relations were carefully tested on numerous MCM-41 samples and shown to be in excellent agreement with pore sizes calculated using Eq. (2), with differences usually below 0.1-0.2nm. Pore size calculations based on Eqs. (3) and (4) can conveniently be performed for both modified and unmodified ordered mesoporous silicas and can be expected to provide highly accurate results, especially for siliceous samples with cylindrical pores. Therefore, the BJH method of calculations based on these equations was used in our recent studies of modified MCM-41 materials [46,59]. It needs to be noted that Eq. (3) was derived for nitrogen capillary condensation (adsorption) data and should not be used in calculations based on desorption data. Moreover, it was shown that the desorption data are not appropriate for the pore size evaluation for mesoporous materials with pore sizes in the range of about 4-5.5 nm, due to the transition between reversible and irreversible adsorption behavior at relative pressures of about 0.4, as already described above. Because of this behavior, the relation between the capillary evaporation (desorption) pressure and the pore size would have a more complicated and less useful form than Eq. (3) derived for the capillary condensation process [55]... 

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