Microporous solids, properties

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

Coal is microporous, with certain partial molecular sieve properties. (A microporous solid herein refers to that which contains pores with diameters of a few tens of A. or less.) Micropores can be considered as entities capable of sorbing foreign molecules, and it is known that additivity of their sorption potential fields enhances the sorption owing to dispersion interactions. As the pores become progressively narrower, the vapor adsorption isotherm (Figure 1) in the initial region up to point B becomes progressively steeper (toward the... 

Our current views, with some elaboration, are summarized below. We do not consider it meaningful with coals, or with microporous solids in general, to deduce specific surface values from sorption data, nor even to apply the concept of specific surface to these materials. We believe that sorption uptake (moles per unit weight or volume of a given adsorbent) under defined conditions is the correct parameter that should be used to describe the sorptive properties of such materials. Thus, whenever the sorption uptake by an active carbon, for a particular sorbate, is required in a practical application such as solvent recovery, purification, or gas sorption, what should be determined in the laboratory is the uptake under the conditions for which the value is to be used it is not possible to predetermine unequivocally the value for one... 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Aluminosilicates form an extensive family of compounds that include layered compounds (such as clays, talc, and micas), 3-D compounds, (e.g. feldspars, such as granite), and microporous solids known as molecular sieves. The structural diversity of these materials is contributed to by aluminum s ability to occupy both tetrahedral and octahedral holes as it also does in y-Al203. Thus, aluminum substitution for silicon in silicate minerals may lead to replacement of silicon in tetrahedral sites or the aluminum can occupy an octahedral environment external to the silicate lattice. Replacement of Si with Al requires the presence of an additional cation such as H+, Na+, or 0.5 Ca + to balance the charge. These additional cations have a profound effect on the properties of the aluminosilicates. This accounts for the many types of layered and 3-D structures (see Silicon Inorganic Chemistry). 

Silicoaluminophosphates (SAPO s) (1) are molecular sieves which contain tetrahedra of oxygen surrounded silicon, aluminum, and phosphorus. These microporous solids not only exhibit properties characteristic of zeolites but also show unusual physiochemical traits ascribable to their unique chemical compositions (1,2). 

Porosity and pore structure are properties that control diffitsive transport, selective reaction, and sorption-based separations of gases in adsorbents and catalysts [1,2]. Sorption porosimetry may be used to characterize the porosity of both mesoporous and microporous solids. The pore size distribution F H) is obtained from the experimental isotherm /[/ )... 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

The BET model is used to determine the specific surface area of solids with the properties mentioned above. Thus, solids for which specific surface areas can be determined using this model are limited to meso- and macroporous solids. In the case of microporous solids, the adsorption phenomenon cannot be described using the above hypotheses. However, in the absence of a universally accepted model, the BET equation is usually used to calculate the specific surface of a microporous solid. 

Zeosils are microporous solids with tetrahedral frameworks, which are similar to those of aluminosilicate zeolites, but which are built from pure SiOz [1, 2]. With their neutral frameworks, zeosils do not show the typical properties of zeolites such as ion exchange, hydrophilicity, and catalytic activity instead, these materials are hydrophobic and non-reactive. Zeosils find their main qrplications as highly selective adsorbents for sorbing nonpolar molecules from wet gas streams or aqueous solutions. 

In the past four decades, we have witnessed the significant development of various methods to describe microporous solids because of their important contribution to improving of adsorption capacity and separation. Various models of different complexity have been developed [5]. Some models have been simple with simple geometry, such as slit or cylinder, while some are more structured such as the disk model of Segarra and Glandt [6]. Recently, there has been great interest in using the reverse Monte Carlo (MC) simulation to reconstruct the carbon structure, which produces the desired properties, such as the surfece area and pore volume [7, 8]. Much effort has been spent on studies of characterization of porous media [9-15]. In this chapter we will briefly review the classical approaches that still bear some impact on pore characterization, and concentrate on the advanced tools of density functional theory (DFT) and MC, which currently have wide applications in many systems. 

Microporous solids Zeolites are solids with aluminosilicate frameworks having pores and channels. When these are occupied by hydrated ions the compounds are used as ion exchangers when the pores are empty they have useful catalytic properties. 

The use of microporous solid catalysts such as zeolites and related molecular sieves has an additional benefit in organic synthesis. The highly precise organization and discrimination between molecules by molecular sieves endows them with shape-selective properties [12] reminiscent of enzyme catalysis. The scope of molecular sieve catalysis has been considerably extended by the discovery of ordered mesoporous materials of the M41S type by Mobil scientists [13,14]. Furthermore, the incorporation of transition metal ions and complexes into molecular sieves extends their catalytic scope to redox reactions and a variety of other transition metal-catalyzed processes [15,16]. 

The molecular-sieve zeolites are distiact from other three major npore size. Although other microporous solids are used as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the ssolecular-sieve effect, that is. sepurations caused by difference in the molecular size of the materials to be separated. The most impurtanr molecular-sieve effects are shown by dehydrated crystalline zsoliles. Zeolites selectively adsorb or reject molecules based on differences in molecular size, shepe. and other properties such as polarity. Daring the ndsorption of various molecules, the micropores fill and empty reversibly. Adsorption in zeolites is a matter of pore filling, and the usual surface-area concepts are not applicable. 

To explain the total suppression of adsorption capacity from the moment of disappearance of half of the zeolitic phase and the appearance of a closed macropososity, it seems necessary to invoke the hypotheses of Wolf (6, 7) the solid-solid transformation begins at the periphery of the particle and progresses toward the center. It is sufficient to cause the shell of product B to break in order to bring about the residual zeolitic phase and thus find the properties of the microporous solid. After thermal treatment at 670 °C, calculation indicates the presence of 37 volume % of solid B. If a cubic particle with an edge equal to 1 micron is considered, the thickness attained by the layer of solid B impervious to water is close to 0.1 micron. 

Zeolites are well known as microporous solids and acid catalysts [18]. These two charateristic properties seem to be operating in the course of photooxidation (by adsorption of organic substrates or/and by the acidity influence on the reaction mechanism). 

The wide range of structural units exhibited by the natural and synthetic phases suggests that many new open frameworks with ion-exchange and sorption properties can be obtained. Exploratory synthesis of novel microporous solids is currently an active research area. 

The diffusion coefficient (or diffiisivity) and viscosity represent transport properties which affect rates of mass transfer. In general, these properties are at least an order of magnitude higher and lower, respectively, compared with liquid solvents. This means that the diffusion of a species through an SCF medium will occur at a faster rate than that obtained in a liquid solvent, which implies that a solid will dissolve more rapidly in an SCF. In addition, an SCF will be more efficient at penetrating a microporous solid structure. However, this does not necessarily mean that mass transfer limitations will always be absent in an SCF process. For example, in the extraction of a solute from a liquid to an SCF phase, the resistance to diffusion in the liquid phase will probably control the overall rate of mass transfer. Stirring will therefore continue to be an important factor in such systems. 

Since many properties of crystalline oxides, e.g., acidity, hydrothermal stability, etc., are the essential features exploited in commercial applications of these oxides, it is not unexpected that the ordered, mesoporous materials have not yet found much commerical use. The large void volumes, pore sizes and surface areas of the ordered, mesoporous materials provide advantages over microporous solids in certain areas of application but issues such as stability remain. Thus, if crystalline, extra-large pore solids could be prepared in the pore size and void volume ranges of the mesoporous materials, they would be immediately commercialized. The question remains as to why crystalline materials of this size range have not been synthesized. Navrotsky et al. have shown that pure silica, ordered, mesoporous silicas are energetically very close to pure silica, crystalline... 

The synthesis, structural characterization and luminescence spectroscopy studies of AV-5 and AV-9 (Aveiro microporous solids no. 5 and 9), the first examples of microporous framework cerium(III) and europium(III) silicates (Na4K2X2Sii6O3gT0H2O, X = Eu, Ce) are reported. Both materials display interesting photoluminescence properties and present potential for applications in optoelectronics. This work illustrates the possibility of combining in a given framework silicate microporosity and optical activity. 

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