Structural characteristics of 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... 

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... 

The physical meaning of any energetic correlation with either framework density or molar volume is not obvious. Some of the most important structural characteristics of zeolites are their high porosity and high internal surface area. They are nanomaterials... 

Besides zeolites, a diverse range of microporous materials with novel open-framework structures have been discovered. The framework atoms of microporous materials have expanded to include most of the elements in the periodic table.151 The framework elements are not limited to A1 and Si atoms alone, and the primary building units are not only confined to tetrahedra. This chapter will mainly describe the structural characteristics of zeolites and some zeolitic open-framework materials. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

It is appropriate here to treat structural characteristics of zeolites. A brief summary has been given in the Chapter 9, Section 9.4.1 in which a typical structure is illustrated by Fig. 9.19 for MFI type (ZSM5, siHcalite) zeolites, being almost exclusively used for defect-free membrane s)mthesis. For a more detailed discussion the reader is referred to the works of Breck [92] and of van Bekkum et al. [93]. 

It is not the place here to treat structural characteristics of zeolites. Nevertheless a very brief summary with a focus on silicalite/ZSM5 systems is necessary... 

In interpreting framework structure characteristics of zeolites from mid-infrared spectra, it is important that the over-all pattern in the spec-... 

Each modified catalyst sample preserved the crystal structure characteristic of zeolite ZSM-5. No reflexions due to separate copper oxide phases could be detected for the solid-state exchanged material. 

A. Chatterjee, T. Iwasaki, T. Ebina, H. Tsuruya, T. Kanougi, Y. Oumi, M. Kubo, A. Miyamoto et al.. Effects of structural characteristics of zeolites on the properties of their bridging and terminal hydroxyl groups. Appl. Surf. Sci. 130-132, 555-560 (1998)... 

In part because of the open crystal structure and resulting low density characteristic of zeolite minerals, analcime is the most voluminous reaction product in the simulations. 

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

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

The adaptation of zeolites to a particular purpose can be done by ion exchange and by different chemical and physical treatments. Physicochemical characteristics of zeolites often reflect the modifications introduced in the structure. Different methods are used to study the modifications and their correlations with sorption properties and catalytic activity. In this section G. T. Kerr reviews the chemistry involved in the thermal activation of NH4Y zeolites. 

Finally, a note of caution. The patterns are useful in helping to establish the structural purity of a zeolite phase, yet they may not always allow one to readily and unambiguously determine the framework type of the sample. This assignment is often not straightforward and may require more sophisticated analyses. W. J. Rohrbaugh and E. W. Wu review the factors affecting the diffraction characteristics of zeolite materials (ACS Symposium Series 411 279-302 (1989)). 

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],... 

A zeolite membrane is a membrane in which the transfer is controlled by the porous structure of the zeolite. Compared to sol-gel membranes, zeolite membranes can present some advantages for CMR applications. The most useful feature is that the pores of zeolites arc in the ultramicroporous range and have a very narrow size distribution (in this case pores are linked to the structure and not to the texture as in sol gel membranes). These characteristics of zeolite... 

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. 

An important characteristic of zeolites, determining to a large extent their efficiency as catalysts, adsorbents, and ion exchangers, is their structural resistance to various factors, including thermal, chemical, and mechanical ones. Different theoretical aproaches are now being applied to this problem. Although quantum-chemical methods are only now coming into use in this field, certain results have already been obtained and some of them are summarized below. 

Astala, Auerbach, and Monson277 used plane wave LDA calculations to examine structural characteristics of five all-silica zeolites SOD, LTA, CHA, MOR, and MFI. The first four of these materials can be examined in calculations with 36-72 atoms in a periodic unit cell, but the MFI structure is considerably larger and requires 288 atoms in a single unit cell. Astala et al. argue that LDA calculations give more accurate results for the structure and energies of silica polymorphs than GGA calculations. Demuth et al. used LDA and GGA calculations to examine the properties of Al substitutions in morde-nite both in a protonated form and in the presence of Na cations.278... 

Fig. 2 shows a comparison of the main channels of these zeolites. Chevron discovered that the open 12-membered ring structure characteristic of beta zeolite coupled with its high acidity made it an excellent catalyst for aromatic alkylation. These properties were key in the production of alkyl aromatics such as EB and cumene in extremely high yields and with product purities approaching 100%. Moreover, Chevron discovered that the combination of high activity and porous structure imparted a high degree of tolerance to many typical feed contaminants. 

As far as shape selective catalysis is concerned, the structure characteristic of greatest interest is the pore/channel system of these molecular sieves. As we know, depending on the largest channel, zeolites are characterized as small, medium or large pore if they contain apertures made by rings of 8, 10 or 12 linked tetrahedra. 

Samples dealuminated with SiCl4 were prepared following (2) and working at 250-500°C. Textural and structural characteristics of the zeolite samples are given in Table I. The high purity (99.5%) n-heptane was from Carlo Erba. The physicochemical properties of the vacuum gas-oil are given elsewhere (3). 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

The object of this study was to apply mid-infrared spectroscopy to zeolite structural problems with the ultimate hope of using infrared, a relatively rapid and readily available analytical method, as a tool to characterize the framework structure and perhaps to detect the presence of the polyhedral building units present in zeolite frameworks. The mid-infrared region of the spectrum was used (1300 to 200 cm"1) since that region contains the fundamental vibrations of the framework (Si,Al) 04 tetrahedra and should reflect the framework structure. Infrared data in similar spectral regions have been published for many mineral zeolites (30) and a few synthetic zeolites (23, 49, 50). There is an extensive literature on infrared spectra of silica, silicates, and aluminosilicates (17). However, no systematic study of the infrared characteristics of zeolite frameworks as related to their crystal structure has appeared. 

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