Zeolites typical properties

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. 

The properties of the nanocrystallites are determined, not only by the confinements of the host material, but also by the properties of the system, which can include a range of factors including the internal/external surface properties e.g. of the zeolite or the lability of a micelle. The particle size is controlled by the system chosen e.g. in zeolites the nanocrystallite diameter is limited by the pore size of the zeolite (typically smaller than 2 nm). 

Charge-balancing surface cations in natural zeolites (typically alkaline and alkali-earth cations) can be replaced quantitatively, and essentially irreversibly, by long-chain organic cations, e.g., hexadecyltrimethylammonium (HDTMA ), ethylhcxadecyldimethylammonium, and cetylpyridinium, having also surfactant properties [65]. This involves only the "external"... 

Typical properties of commercially available molecular-sieve zeolite adsorbents are presented in Table... 

Typical properties and the composition of zeolites used in FCC catalysts are listed in Table 5.8, with the molecular diameters of relevant hydrocarbons given in Table 5.6. 

Ion Exchange. Certain solid substances have the property of exchanging one ion for another if placed in a solution containing the ions. Typical substances with this property are the zeolites and certain synthetic resins. 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

The XRD patterns demonstrated that the MCM-22 zeolites were well crystallized and pillars have been created in the MCM-36 sample, respectively. Thus, the last material exhibited a typical intense peak at 29 2°, corresponding to a Aspacing of 4 nm. The textural properties of solids (Table 1) indicated that the pillaring in MCM-36 resulted in increases in BET specific surface area and external surface area compared with the MCM-22 zeolite. 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

When developing a liquid phase adsorptive separation process, a laboratory pulse test is typically used as a tool to search for a suitable adsorbent and desorbent combination for a particular separation. The properties of the suitable adsorbent, such as type of zeolite, exchange cation and adsorbent water content, are a critical part of the study. The desorbent, temperature and liquid flow circulation are also critical parameters that can be obtained from the pulse test. The pulse test is not only a critical tool for developing the equilibrium-selective adsorption process it is also an essential tool for other separation process developments such as rate-selective adsorption, shape-selective adsorption, ion exchange and reactive adsorption. 

Table 10.1 Summary of common zeolite structures used in membrane applications along with important properties and typical synthesis conditions. 

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