Zeolites with different structure types

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

Investigations are being performed to gain further understanding of the photocatalysis of a-methylstyrene by TiOg added zeolite for example in using other types of zeolites (with different structure and acidity). 

The Al MAS NMR spectra of zeolites are in general, much simpler than their Si NMR counterparts since, according to the Lowenstein s rule (which forbids Al-O-Al pairings), only a single tetrahedral A1 environment, namely, Al(OSi), exists in the zeolite framework. The AlO units in hydrated zeolites show only small deviations from the tetrahedral symmetry and thus only weak quadrupole interactions (quadrupole constant of about 1-2 MHz). Consequently, a single narrow line is usually observed in the Al MAS NMR spectra of hydrated zeolites (Fig. 11) with a typical chemical shift of abont 60ppm. The isotropic Al chemical shifts of tetrahedral framework aluminum in hydrated zeolites of different structure type cover a rather small range from about 55 to 68 ppm [18]. 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Reference cites the literature from which the crystal data, atomic coordinates, and displacement factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if authors and users would inform us of any errors or omissions. A listing of the references for isotypic species can be found in the Atlas of Zeolite Framework Types (Baerlocher, McCusker and Olson (2007)). A list of references to structure analyses of zeolites with different cations, up to 1982, is given in the Compilation of Extra Framework Sites in Zeolites, Mortier (1982). 

Determination of the heats of adsorption of substances of different structure demonstrates the dependence of the interaction energy with a zeolite on the type of the exchange cation as well as on the structure of the adsorbed molecule. It is possible to evaluate the heat of adsorption of a complicated molecule by means of the increments of the functional groups forming this molecule. These measurements are still needed for molecules with branched chains, with 7r-bonds, and with conjugated bonds on zeolites of different structure and composition. 

In this study protonated large pore zeolites of different structures (HY, HBeta and HMordenite) and framework Si-to-Al ratios were used in liquid phase in a batch reactor. The zeolites were calcined at 500°C and the hydrolysis was conducted at 75°C. The procedure was optimised in terms of solvent, activation, type and amount of catalyst for the hydrolysis of nitroacetanilides, currently carried out with 10 % sulphuric acid [14], and then extended to other substituted amides. The reaction, followed by GC with nitrobenzene as internal standard, was clean and no by-products or degradation were detected. 

At present, 201 different structural types have been recognized by the International Zeolite Association (IZA), differing in the size, shape, and connectivity of their channels and typicaUy incorporating 8-30 rings [5]. The size of zeoHte chaimel entrances sometimes exceeds 1 nm, but it should be stressed that for industrial applications, the maximum channel size in use is still only 0.74 nm (12-ring). One structural motif can also be used to describe materials with different chemical compositions most elements in the periodic table can be introduced into zeoHte... 

Figure 8.4 shows the relationship between the standard SCR conversion and NO oxidation reaction conversion using Fe/zeolites prepared by different Fe loading methods and Fe loading amounts. The data which is used from Ref. [13] by Delahay et al. demonstrates a positive correlation between them, implying that the active sites for NO reduction are identical with the NO oxidation sites. Figure 8.5 shows the relationship between standard SCR conversion and NO oxidation reaction conversion when several Fe/zeolites with different pore structures and Si/Al2 ratios were used [16]. As can be seen from the hgure, the SCR conversion in this case correlates well with the NO oxidation conversion. Thus, the activity relationship is preserved even when different types of zeolites are used. 

Dimensions of alkane molecules sorbed by zeolites are comparable to the dimensions of zeolite pores. So the diffusion and intramolecular dynamics of alkanes confined in the narrow pores of zeolite is defined by the size and geometry of the zeolite pores as well as by dimensions and the structure of the adsorbed alkane. This means that alkanes with different structures and dimensions can exhibit a unique NMR line shape in the zeolite structure of the same type. 

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

Vedrine and coworkers studied vibrational bands for different types of zeoUtes with different particle sizes [95]. They concluded that during the synthesis of ZSM-type zeolite that the presence of vibrational bands at 550 and 450 cm indicate that a ZSM-type zeolite may have formed. Absence of the 550cm" band indicates that such a structure has not formed. The 550cm" band is characteristic of five-member pentasil rings which are a structural unit of ZSM-type zeohtes. 

T-O-T stretch measured by framework IR, as discussed in Section 4.5.3.2. The comparison of areas as described above does provide quantitative information about the relative changes in acidity between the samples since the area is direction proportional to the concentration (Beer-Lambert law, discussed in Section 4.5.2.) It most cases, this relative, but quantitative comparison between samples is sufficient to provide information about how various treatments or modifications have altered acid site distributions. Since extinction coefficients can change with zeolite type (Table 4.5), these comparisons are best for samples of the same zeolite type. Therefore, caution should be used when comparing data from samples with different zeolite structures. 

The adsorption of NO and CO has been used to characterize the properhes of Co in Co-exchanged zeoHtes [148-151]. NO is a selective probe for Co and CO is selective for Co species. Datka and coworkers used the combination of CO and NO adsorption IR to quantitahvely determine the concentration of Co as an oxide and the Co present as in exchange positions, oxide-like clusters, and cobalt oxide in a series of Co-exchanged ZSM-5 and ferrierite (PER) zeolites [151]. They established conditions under which the CO and NO would react selectively with the various types of sites and estabhshed absorphon coefficients for the quanhtative calculations. Differences in the distributions of the various forms of Co species were found to be dependent on both the structure and framework Si/Al of the zeolite. 

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