Zeolites structural characteristics

One of the major problems in all the ammoximation processes using aqueous H202 + TS-1 with NH3 is that, under the basic conditions (pH > 10) prevailing during the reaction, some of the lattice Si ions of the zeolite structure in TS-1 are leached into solution, leading to catalyst destruction. This leaching is a common characteristic of all silicates. Innovative catalyst formulations and process modifications are needed to overcome this problem. 

Aluminum-deficient Y zeolites. The properties of aluminum-deficient Y zeolites, including ultrastable zeolites, have been reviewed in several papers (9,33-35). During the last several years, new techniques have been applied to study these materials. This led to a better understanding of their structural characteristics and of the correlations between structure and properties. We shall discuss the structure and properties of aluminum-deficient Y zeolites, with the emphasis on more recently published results. 

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

Faujasites of the X and Y type are the most frequently studied zeolite structure type for this reachon. Because the key step in the reachon is a hydride hansfer, zeolites with low Si02/Al203 ratios are favored. The other preferred characteristic is a large pore opening and hence a low diffusion barrier to product diffusion. These details and others were reviewed recently by Feller et al. [50] (Table 12.8). 

Table 2.7 Composition and Structural Characteristics of Some Fibrous Zeolites... 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

Structural Determinations. Si-0 and Al-0 vibrations at 1200-350 cm-1 give information on zeolite structure (21-25). Qualitatively, the resolution of the bands around 1150 and 1050 cm-1, the intensity and sharpness of the bands around 580 and 390 cm-1, and the presence of a shoulder at ca. 500 cm-1 are characteristic of the faujasite structure. 

Both materials used as hosts are distinguished by different geometries and pore dimensions, that is, the zeolite Y is constituted of spherical supercages of 1.3 nm of diameter tetrahedrally interconnected through 0.74 nm windows. On the other hand, the structural characteristics of MCM-41 have been expressed in terms of a honeycomb-like structure with a pore diameter of 3.5 nm and a wall thickness of ca. 1.1 nm [67], The Ni-Y zeolite was prepared by Ni(N03)2 exchange of the NaY sample, and the CBV100 was provided by the PQ Corporation Malvern, PA, USA. [67],... 

Beran S (1984) Quantum chemical study of the effect of the structural characteristics of zeolites on the properties of their bridging hydroxyl groups, J Mol Catal 26 31—36... 

There are 18 reported MeAPO structure-types (Table I). Eight of these are zeolite structure analogs (17, 20, 34, 35, 37, 43, 44, 47) which were subsequently observed In AlPO,-based materials. Seven structure-types were first Identified In the A1P0 (5, 11, 14, 16, 31), SAPO (41), or MeAPSO (46) families (14.15). The remaining structures (36, 39, 50) were first observed In the MeAPO family. The x-ray powder diffraction patterns characteristic of the structure-types 5, 11, 17, and 20 (6.16,17) and 14, 18, and 31 (7) have been previously published. The x-ray powder patterns... 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

ECR-1 is a true boundary phase between two compatible structures - mazzite and mordenite - which normally crystallize with greatly disparate Si/Al ratios (3.4 and >5 respectively). Yet ECR-1 has an Si/Al ratio identical to mazzite, even though half of its component sheets are structurally characteristic of mordenite (in the structure proposal). This compositional incompatibility is further reflected in the almost complete lack of mixtures of mazzite with mordenite in the experiments so far completed. The ongoing search for ECR-1 with mordenite overgrowths is therefore an important objective in the understanding of crystallization mechanisms in this system, with particular interest in the possible compositional variation between all three component zeolites. 

The unique nature of ZSM-5 as a catalyst and as a zeolite has been well documented (L-3 ), as shown by the wide spectrum of chemistry it can catalyze as well as the wide range of morphologies and composition in which it can be synthesized. In addition, ZSM-5 exhibits ion exchange properties that, in some cases, are very different from other zeolites. Results from the study of these ion exchange characteristics have been used effectively in developing optimal methods for conversion of ZSM-5 into catalytic forms and in supplying information concerning the zeolite structure not discernible by other techniques that could lead to more active, selective and stable catalysts. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

Catalyst Structural Characteristics. Structural features of AFS and USY materials have been characterized in this work in terms of unit cell size, presence of extraframework material, active-site distributions, and pore-size distributions. These features are similar for both sets of USY and AFS samples which indicates that structural characteristics are not related to the source of Y zeolite. 

As shown in Figure 7, calcined zeolites with similar unit cell sizes demonstrate different octane-producing behavior. Hence, structural characteristics such as extraframework aluminum and spatial distribution of acid sites are important factors affecting zeolite acidity. After steaming, the octane behaviors of AFS and USY zeolites are similar reflecting similarities in zeolite acidities. 

The zeolite-carbon adsorbents from mineral-carbon adsorbents group are novel and exhibit not quite well recognized properties with their unique, modified porous structure. The characteristic structures for zeolite, active carbon and intermediate structure exist in these materials. Such a structure results fi-om the modification of a surface of a mineral matrix by depositing carbon material. The efifectivity of enrichment of the structure of zeolite-carbon adsorbents (in relation to crystalline zeolite structure) in hydrophobic micropores (0.4 - 2 nm) and macropores (above 50 nm) is proportional to the fi action of carbon phase. Such combination of hydrophilic properties of mineral phase and hydrophobic properties of organic phase results in various sorptive properties of the material and the range of their application can be consequently extended. Additionally, the chemical resistance of these adsorbents for their exploitation in aggressive conditions takes place. 

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