Mesoporous material pore dimension

Materials with controlled pore sizes and functionality, particularly in three dimensions would have many uses (53-59). Numerous totally inorganic microporous and mesoporous materials have been subject of thousands of papers, and applications of the former (e.g., zeolites) have a sizable impact on the global economy at present (myriad uses from production of gasoline to a host of chemicals) (60-66). However, the use... 

The field of mesoporous materials has developed rapidly since the first reports on these materials in 1992, as these last examples show. The trend is to utilize inexpensive, multifunctional micelle- or aggregate-forming surfactants or templates which may adopt many different liquid crystal-like configurations in aqueous solution. Formation of a silicate structure with well-defined pore dimensions and connectivity may then be accomplished by the appropriate choice of the synthetic conditions. Additional microporous and macroporosity may be incorporated by using macroporous host materials, as in the case of Stucky of the work by and coworkers, who created mesophases with unprecedented architecture.[47]... 

A nanomaterial can be loosely defined to be any material containing heterogeneity at the nanoscale in one or more dimensions. In the broadest sense, then, the following are nanomaterials phase-separated glasses or crystals with domains in the nanoregime, zeolites and mesoporous materials with pores of nanometer dimensions, clays with nanometer sized alternations of aluminosilicate layers and interlayer hydrated cations, and nanoscale leach layers at the mineral-water interface. 

The mesoporous materials discussed here comprise silicates and aluminosilicates that are formed from synthesis conditions comparable to zeolites. The main difference with the latter is the use of supra-molecular assemblies of surfactant molecules, e.g. alkyltrimethylammonium ions, as the structure directing agents. Due to the large dimensions of these spherical or cylindrical micelles, the framework of the silicates is not so well crystallized as in the case of zeolites. This causes lower framework stability as well as weaker Bronsted acidity. Upon calcination, large pores of uniform diameter (say 4-10 nm, depending on the surfactants used) become accessible. 

Zeolites, as previously described, are crystalline microporous solids with well-defined structures made up of interlocking microporous Si04 and A104. Microporous means that the pores have dimensions of less than 20 A, on the order of the size of many petroleum-related molecules, and their crystalline nature means that they have a narrow pore distribution (mesoporous materials have pore sizes between 20 A and 500 A, macroporous materials have pores larger than 500 A). This combination of features not only restricts the size of molecules that can enter the pores, but also the dimensions of the transition state and of the molecules that can successfully leave. For these reasons, zeolites have been termed shape selective. ... 

In order to avoid this limitation, the interest in synthesising mesoporous materials is nowadays increasing in catalysis as the high pore dimension (10-150 A) allows the access to bulky organic molecules. In 1991, the Mobil Oil Company synthesised a mesoporous alumino-silicate through a basic synthesis using alumino-silica gel and alkyl-... 

Porous silicas are usually mesoporous materials and they can be made with a variety of pore dimensions. In particular, silica glasses can be made with well-defined pore diameters, typically in the range 30-250 A, using sol-gel methods. Such a system provides a good model for testing the models of relaxation behaviour of fluids in porous solids. It is normally found that the two-site fast-exchange model for relaxation described above for macroporous systems is still valid. For instance, H and relaxation times have been measured during both adsorption and desorption of water in a porous silica. Despite hysteresis in the observed adsorption isotherms, it was found that the relaxation times depended solely on water content.For deuterated water in some porous silicas, multicomponent relaxation behaviour for T2 and Tip has been observed, and this has been attributed to the fractal nature of the pore structure. 

The host structures can be classified by the dimensionality of their pores or cavities hosts with cavities extending and connecting in three dimensions (3-D), such as some zeolites and mesopore materials (see Figure 8.1 and Table 8.1) cavities which extend in two directions (2-D), as is the case with all lamelar materials, such as clays materials with cavities in a single direction (1-D), for example, carbon nanotubes and finally there are materials with small cavities or with closed cavities (0-D), such as fullerenes and cyclodextrins. 

The azobenzene-modified mesoporous silica materials are ideal platforms to study photocontrolled transport behavior because of their easy fabrication, easy integration into nanodevices, and well-controlled pore connection, pore dimension, and surface chemistries. To date, all the related work reported in the literature has dealt with numerous nanochannels. It will be a milestone if we can study the photocontrolled transport behavior of a few nanochannels or even a single nanochannel with azobenzene-modified pore surfaces. However, significant engineering problems exist in the fabrication of such a single-channel device. 

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