Silica mesoporous materials

Porous materials are classified into several kinds depending on the pore size. According to the International Union of Pure and Applied Chemistry (lUPAC) notation, microporous materials have pore diameters of less than 2 mn and mesoporous materials have pore diameters between 2 and 50 nm. Macroporous materials have pore diameters of greater than 50 nm. Hydrothermal synthesis has been the technique of choice to prepare microporous phases. Ordered porous materials, including ordered mesoporous materials and the metal organic frameworks (MOFs), have also been synthesized generally under hydrothermal conditions [1-5]. In this section, we briefly present the synthesis of mesoporous silica materials and MOFs. 

Traditionally the synthesis of ordered mesoporous silica materials, with pores in the 2-50 nm range, is templated by surfactants, hi this soft-templating approach, quaternary ammonium salts and non-ionic polyether-based surfactants are the most used templates, leading to pore distributions of 2-6 and 4-30 nm respectively [6,7]. Under specific reaction conditions, a mixture of the silica precursor and the surfactant yields a meso-structured solid, which, after removal of the template, becomes an open mesoporous structure. Many synthetic protocols have been developed by varying the surfactant, silicon source or synthesis conditions, but most preparations occur at low or high pH. Extensive reviews on the synthesis of ordered mesoporous materials are available [8-10]. 

Essentials of Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

Kim et al. reported ordered large-pore silica, MSU-H (see Fig. 14.8.3), synthesized at near neutral end pH, using P123 and sodium silicate solution at tanperatures 

FIGURE 14.8.2 Phase transition in mesoporous solids (a-d) lamellar-hexagonal (e-f) hexagonal-cubic. The circular objects around the surfactant assemblies are the metal-oxo species (From Ref. 19b, J. Mater. Chem. 8 (1998) 1631. 1998 Royal Society of Chemistry). 

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MESOPOROUS SILICA MATERIALS AS SENSITIVE COMPONENTS FOR CHEMO- AND BIOSENSORS Telbiz G.M.. Gerda V.I., Ilyin V.G., Starodub N.F. ... 

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. 

The viabiUty of using site-isolated Ta(V) centers for cyclohexene epoxi-dation was explored by grafting ( PrO)2Ta[OSi(O Bu)3]3 onto a mesoporous silica material [83]. After calcinations, the material formed is less active and selective in the oxidation of cyclohexene than the surface-supported Ti(IV) catalysts using organic peroxides however, the site-isolated Ta(V) catalysts are more active under aqueous conditions. 

Solid palladium scavengers, PVPy, QTU were pmchased from commercial somces. The mesoporous silica material, S102-SH, was prepared via reaction of SBA-15 (110 A pore diameter) with 3-mercaptopropyltrimethoxysilane (16). Specifically, a toluene suspension of SBA-15 and 3-mercaptopropyltrimethoxysilane was heated at reflux for two days under Ar. Water was then added to promote the cross-linking and the mixture was heated at reflux for an additional day. The sohds were filtered and washed with copious amounts of toluene, hexanes, and methanol to remove unreacted silanes. The solids were finally Soxhlet extracted with dichloromethane at reflux temperature for 3 days, dried, and stored in a nitrogen dry box. The final solid contained 7.5 wt% sulfur (2.3 mmole S/g solid). 

Fig. 2.29 Reaction mechanism on acetalization of cyclohexanone within unhydrolyzed mesoporous silica materials. Adapted from [107], W. Otani et al., Chem. Eur.J. 2007, 73, 1731-1736.

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. 

A non-surfactant templating route to mesoporous silica materials. Advanced Materials, 10, 313—316. 

Fig. 4.3 Adsorption isotherms of lysozyme at pH 10.5 on mesoporous silica materials (a) Cn2-MCM-41 (b) Cie-MCM-41 (MCM-41) (c) SBA-15. Reprinted with permission from [119],...

In addition to popular mesoporous silica materials, mesoporous silica supports with various morphologies have also been used for protein immobilization. Tang and coworkers synthesized lotus-leaf-like silica flakes with a three-dimensionally connected nanoporous structure and controllable thickness, which were used for immobilization of ribonuclease A [126]. The synthesized silica flakes have a thickness of200 nm, and a diameter of 3 mm, showing a much higher initial adsorbing rate of... 

Fig. 4.23 Preparation of mesoporous silica materials with chirally twisted rod-like structures by using surfactant with a chiral amino acid moiety as a structure-directing reagent.

Surface Modifications of Ordered Mesoporous Silica Materials. .122... 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

Synthetic encapsulation of 2 or 5 nm Au NPs into the pores of mesoporous silica materials MCM-41 and MCM-48 has been performed by growing the porous structure in the presence of the metal particles in an aqueous solution [176]. 

SEM micrographs of the mesoporous materials prepared from C 6TMAB-HN03-TE0S-H20 composition, with and without the addition of PEO-6000 polymer, are shown in Fig. 1. In the absence of this polymer additive, mesoporous silica with gyroidal particulate morphology (Fig. 1 A) was formed even under the shear flow conditions. On the other hand, the addition of this polymer promotes the formation of mesoporous silica materials with millimeter-sized, rope-... 

As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the N2 adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p0 of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6,7]. 

Such ammonia hydrothermal treatment is also applicable to mesoporous silica materials synthesized under different conditions, e.g., different acid source or temperature. This and other interesting issues will be reported later. 

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