Porous materials pore structure characterization

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

Pores, and especially mesopores (with sizes between 2 and 50 nm) and micropores (with sizes less than 2 nm), play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. In addition to pore structural characterization described above, the description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures [29]. 

A number of physical parameters are necessary to standardize chromatographic properties of aluminas in thin-layer chromatography. Because these aluminas are porous materials, the parameters characterize the pore structure and specific surface area. The actual values of pore diameters, specific surface areas, and pore volumes of aluminas most frequently used in thin-layer chromatography are listed in Table 3. 

The geometric structure of porous materials can be characterized by a poro-gram the integral pore size distribution function (PSDF), which describes the distribution of pore volume versus pore radii V and r or the differential PSDF (dV/dr), r. 

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

There are two conventional definitions in describing the fractality of porous material - the pore fractal dimension which represents the pore distribution irregularity56,59,62 and the surface fractal dimension which characterizes the pore surface irregularity.56,58,65 Since the geometry and structure of the pore surfaces are closely related to the electro-active surface area which plays a key role in the increases of capacity and rate capability in practical viewpoint, the microstructures of the pores have been quantitatively characterized by many researchers based upon the fractal theory. 

Subsequently, the characterization of the pore structures of the porous materials using gas adsorption method was discussed in detail. The types and characteristics of the adsorption isotherms and the hysteresis loops were introduced. In addition, the BET (Braunauer, Emmett, and Teller) theory92 for the determination of the surface area and various theoretical models for characterization of the pore structures according to the pore size range were summarized based upon the adsorption theory. 

The available transport models are not reliable enough for porous material with a complex pore structure and broad pore size distribution. As a result the values of the model par ameters may depend on the operating conditions. Many authors believe that the value of the effective diffusivity D, as determined in a Wicke-Kallenbach steady-state experiment, need not be equal to the value which characterizes the diffusive flux under reaction conditions. It is generally assumed that transient experiments provide more relevant data. One of the arguments is that dead-end pores, which do not influence steady state transport but which contribute under reaction conditions, are accounted for in dynamic experiments. Experimental data confirming or rejecting this opinion are scarce and contradictory [2]. Nevertheless, transient experiments provide important supplementary information and they are definitely required for bidisperse porous material where diffusion in micro- and macropores is described separately with different effective diffusivities. 

The characterization of porous materials exhibiting a composite pore structure encompassing micro-meso-and perhaps macro-pore sizes, is of particular significance for the development of separation and reaction processes. Among the characterization methods for materials exhibiting ultramicropore structures, DpDubinin-Radushkevich (DR)[2], Dubinin-Astakov (DA) [3], Dubinin-Stoeckli (DS) [4], as well as the Horvath-Kawazoe (HK), [5] methods are routinely used for the evaluation of the micropore capacity and the pore size distriburion (PSD). 

The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. 

Thermoporometry. Thermoporometry is the calorimetric study of the liquid-solid transformation of a capillary condensate that saturates a porous material such as a membrane. The basic principle involved is the freezing (or melting) point depression as a result of the strong curvature of the liquid-solid interface present in small pores. The thermodynamic basis of this phenomenon has been described by Brun et al. [1973] who introduced thermoporometry as a new pore structure analysis technique. It is capable of characterizing the pore size and shape. Unlike many other methods, this technique gives the actual size of the cavities instead of the size of the openings [Eyraud. 1984]. 

In the context of microporous and mesoporous materials, lUPAC has provided a variety of recommendations for nomenclature and characterization of porous materals, that can be found in the literature. Microporosity should not be based on structural data but on adsorption data. Sorption by materials that show Type 1 isotherms is an indication of a microporous material. Pore size distributions less than 20 A are related to microporous materials like zeolites. Materials having pores between 20 A and 500 A are refered to as mesoporous materials. Materials that have pores larger than 500 A are refered to as macroporous. 

Solid-state NMR spectroscopy is nowadays a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties (like diffusion). The objective of this paper is to highlight recent solid-state NMR results of zeolitic materials, based on new techniques, methods and pulse sequences. The intention is not to review recent NMR results, since a large number of such papers is easily available and one of the latest was presented during the 10th IZC Summer School on Zeolites in Wildbad Kreuth, Germany, two years ago (1). 

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