Pore space microporous materials

Much more attention has been given to zeolites. Zeolites are crystalline microporous materials whose structure is based on a three-dimensional tetrahedral network of AlO and SiO (the Al/Si ratio can be varied from 1 to 0). The excess negative charges carried by AIO4 units are compensated by cations (Na, H+) which ensure the high hydrophilicity of the aluminated zeolites. The crystallinity of zeolites ensures also a very precise pore size. Typically, the size of zeolite pores ranges from 3 to 8 A, and the inner diameter of the interior spaces from 5 to 13 A. 

Now the research effort goes toward experimental verification of the elevation phenomena in the simplest geometry, a slit. Our main interest is in the range of a few to several nanomenters. Some experimental studies have already reported freezing point elevation in slit pores [8-10], but the materials used were activated carbon fibers (ACFs), which have only micropores less than 2nm. In such small pores the first layer adjacent to the attractive pore wall, which is known to form a frozen phase at a temperature well above the bulk freezing point, will occupy most of the pore spaces, and the freezing behavior in the interior of the pore space is difficult to be detected. Further, there may still remain some controversy if a liquid confined in a larger nanopore would exhibit elevation unless an experimental verification is made over such sizes. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions.[1,2] Their fascinating properties, such as ion-exchange, separation, catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

The reason for the interest in zeolites lies in their unusual crystal structure at the molecular level. They are not solidly packed, like many crystalline materials, but they have continuous pores (channels) running through them, which intersect at cavities (cages) within the structure. These open spaces can occupy up to 50% of the crystal volume. The diameters of these pores and cavities fall between 0.2 and 2.0 nm, leading to zeolites being classified as microporous materials. 

It is important to appreciate the range of porous materials that have been studied, and the different challenges that they pose the NMR spectroscopist. The pore spaces will range from microporous systems (pore diameters less than 20 A), through mesoporous systems (pore diameters in the range 20-500 A), up to macroporous systems (pore diameters greater than 500 A). For instance, zeolites have pore diameters less than 10 A, while sandstones tend to have pore sizes in the range 0.1-100 pm. The precise NMR technique used to characterize pore spaces or transport within these materials will be different. Some samples will have fairly well-defined pore sizes (such as zeolites and... 

Some information can be obtained on porous media from conventional NMR spectroscopy, and this is discussed in Section 2. Relaxation time measurements have been widely used to characterize porous solids, and this technique is discussed in Section 3. Pulsed field gradient (PFG) methods may be used to probe the local structure of the pore space and to characterize transport within it, and these are discussed in Section 4. Magnetic resonance imaging (MRI) techniques can also be used to characterize the pore space and to measure transport, and applications are discussed in Section 5. The bulk of this review will be concerned with mesoporous and macroporous materials, as it is for these systems that NMR is particularly useful in characterizing the pore space. However, some applications of NMR techniques to probe the pore space and transport within microporous materials will be mentioned in Section 6. Finally, some general conclusions are given in Section 7. 

Polymers confined in the nanosized spaces of the PCP channels typically show properties that are distinctly different from those shown for the same materials in the bnlk state becanse of the formation of specific molecular assemblies and conformations [19, 20]. The inclusion of polymers within crystalline microporous hosts (pore size < 2 mn) with ordered and well-defined nanochannel sfiuctures has atfiacted considerable levels of attention because, in confiast to amorphous bulk polymer systems and polymers in solution, this approach can prevent the entanglement of polymer chains and provide extended chains in resfiicted spaces. 

Porous materials are netwoiks of solid material, which contain void spaces. These materials can be further classified depending on the size of the pores present in the material. Microporous solids are materials that contain permanent cavities with diameters of less than 2 mn. Mesoporous materials contain pores ranging from 2 nm to 50 nm and macroporous materials contain pores of greater than 50 nm [60]. The field of microporous materials contains several classes, which are well known [61], including naturally occurring zeolites, activated carbons and silica. Synthetic microporous solids have recently emerged as a potentially important class of materials. 

In Sections 15.2 through 15.6, the terminologies macropores and micropores are based on that used in porous electrode theory, with the term macropores denoting the electrolyte-filled continuous interparticle space in between carbon particles, serving as transport pathways for ion transport across the electrode, whereas the term micropores is used for all the pore space within the carbon particles (intraparticle porosity see Figure 15.5b). In Section 15.7, the formal lUPAC terminology for porous material characterization is used where macro-, meso-, and micropores are distinguished on the basis of the pore sizes in a porous material. ... 

Microporous inorganic solids, such as zeolites, clays, and layered oxide semiconductors offer several advantages as organizing media for molecular electron transport assemblies. Because these materials are microcrystalline, their internal pore spaces have well-defined size and shape. This property can be exploited to cause self-assembly, by virtue of size exclusion effects, ion exchange equilibria, and specific adsorption, of photosensitizers, electron donors, and electron acceptors at the solid/solution interface. 

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