Porous ordered mesoporous materials

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

The synthesis of ordered mesoporous materials in the past decade greatly expanded the range of ordered porous materials and opened up many new opportunities in the design and applications of materials. Of particular importance is the synthetic methodology that is used for the preparation of these materials. Prior to this, the synthesis of porous materials generally involved the use of individual molecules or hydrated clusters of simple ions. In the synthesis of ordered mesoporous materials, however, it is the assembly of surfactant molecules that directs the condensation of inorganic precursors. 

Since the discovery of ordered mesoporous materials, researchers have explored many possible applications that can take advantage of the unique compositional or structural features of mesoporous materials. In addition to apphcations in traditional areas such as catalysis, separation, and ion exchange, new applications that might involve mesoporous materials include stationary phases in HPLC, bio and macromolecular separations, low dielectric constant materials, enzyme immobilization, optical host materials, templates for fabrication of porous carbons, and reactions in confined enviromnents. 

The discovery of ordered mesoporous material not only expands the molecular sieves from micropore materials to mesopore materials, but also fills a gap in the porous material family. Some illustrative porous material examples are given in Figure 8.1. 

The attractive parts of the ordered mesoporous materials are that they possess some exclusive outstanding properties which other porous materials do not have. These properties include ... 

The expansion and diversification of the ordered mesoporous materials can be attributed to bringing together two advanced disciplines, i.e. microporous solids and amphiphilic compounds. The latter turned out particularly rich in phenomena that could be exploited for innovation and synthesis of novel and/or designer products. The following developments can be considered as exploiting the principles of surfactant science in the preparation of porous solids. 

Type 1 isotherms exhibit prominent adsorption at low relative pressures p/po (the relative pressure p/po is defined as the equilibrium v or pressure divided by the saturation vapor pressure) and then level off. Type 1 isotherm is usually considered to be indicative of adsorption in micropores (e.g., adsorption of benzene on microporous active carbon) or monolayer adsorption due to the stror adsorbent-adsorbate interactions (which may be the case for chemisorption, which involves chemical bonding between adsorbate and the adsorbent surface, e.g., adsorption of hydrogen on iron). In the case of nonpolar gases commonly used for charactmzation of porous solids (nitrogen, argon) [10, 12, 13, 17, 56], chemisorption is unlikely and therefore e I reflects usually adsorption on microporous solids. However, type I isotherms may also be observed for mesoporous materials with pore size close to the micropore range. In particular, in the case of adsorption of N2 at 77 K or Ar at both 77 K and 87 K in cylindrical pores, a type I isotherm would have to level off below the relative pressure of about 0.1 for the material to be exclusively microporous, as inferred fi-om tile results of recent studies of siliceous and carbonaceous ordered mesoporous materials (OMM) [57-59]. Consequently, when a type 1 isotherm does not level off below the relative... 

TEM analyses are often conducted to study the crystalline organization of micro-porous zeolites and to visualize the pore structure of ordered mesoporous materials (46-48). Moreover, high resolution TEM allows the examination of the nature and location of guests such as nanoparticles, clusters, coatings, etc. (46,47,49,50). 

Silica is a material with a low dielectric constant of k—4.0. In order to increase the packing density between multilevel interconnects, new. ultralow-k materials are needed. Because k decreases with decreasing density, crystalline and noncrystalline porous silicas with DK as low as A <2.0 were introduced to replace the current wire insulators in microelectronics for multilayer deviees. Because of their low density, ordered mesoporous materials are used for practical purposes. ... 

From a materials point of view an incredible development happened in the twentieth century with the preparation of porous metal oxides by the decomposition of metal salts and layered oxides [30, 31], the invention of aerogels [32], and sol-gel processing [33]. In addition, the preparation of zeolites by hydrothermal processing became important for the synthesis of very well-ordered, uniform pore structures in the micropore range. In 1990, the concept of biphasic micellar systems as a template for well-ordered mesoporous materials was successfully introduced... 

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. 

Because of the attractive physicochemical properties and potential applications in catalysis, biotechnology, adsorption, and separation, fabrication of hierarchically porous (macro/mesoporous) materials, especially for the three-dimensional ordered macro/ mesoporous (3DOM) materials, has been a focus in the research on materials science and engineering in recent years [99,199,200], By using close-packed arrays of monodisperse spheres, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and silica as template, metals [201,202], metal oxides [203-208], metal chalcogenides [209], silica [204,210,211], carbon [212,213], polymers [214,215], and hydroxyapatite [216] with 3DOM structures have been generated. 

Invented less than a decade ago,1 EISA has rapidly developed into a universal technique for fabrication of organized porous and patterned nanocomposite materials, ranging from metal oxides and chalcogenides to carbons, polymers, and metals.2,3 In addition to generating ordered mesoporous films, this technique may also be used to incorporate functional molecules and... 

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