Zeolite Structural Features

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Zeolite structures sometimes remain unsolved for a long time, because of either their complexity, the minute size of the crystallites or the presence of defects or impurities. One extreme example of stacking disorder is provided by zeolite beta [1,2], Different stacking sequences give rise to two polymorphs (A and B) in zeolite beta that always coexist in very small domains in the same crystal. Not only do the small domains make the peaks in the powder X-ray diffraction pattern broad and thereby exacerbate the reflection overlap problem, but the presence of stacking faults also gives rise to other features in the diffraction pattern that further complicate structure solution. 

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. 

If one examines the evolution of new zeolite structures over the past decade the most interesting discoveries have been made with high silica compositions. Many of these phases can be prepared in essentially all silica forms. Purists would prefer to classify such molecular sieves as organosilicates or porosils (1), in part because the physical properties differ from more classical low Si/Al ratio zeolites. In particular, the high silica zeolites tend to be more thermally stable and chemically robust. Additionally, the higher the Si/Al ratio the more hydrophobic the zeolite. These features are desirable for catalysts that may be used in catalytic processes such as cracking (3). 

Using the more advanced quantum chemical computational methods it is now possible to determine the fundamental electronic properties of zeolite structural units. The quantum chemical basis of Loewenstein s "aluminum avoidance" rule is explored, and the topological features of energy expectation value functionals within an abstract "nuclear charge space" model yield quick estimates for energy relations for zeolite structural units. 

The exceptional catalytic properties and structural features of zeolites are a powerful stimulus for both experimental and theoretical research. With the advent of the computer age and with the spectacular development of advanced quantum chemical computational methods in the last decade, one may expect that molecular quantum theory will find more and more practical and even industrial applications. The most rapid progress is expected to occur along the borderline of traditional experimental and theoretical chemistry, where experimental and computational (theoretical) methods can be combined in an efficient manner to solve a variety... 

Alkylation over the MWW Zeolite. The MWW (or MCM-22) zeolite developed by Mobil as catalyst for ethylbenzene and cumene production deserves particular attention. Indeed, this zeolite presents unique structural features (Figure 12.5). Its structure is constituted of three independent pore systems " large supercages (inner diameter of 7.1 A dehned by a 12-member-ring [12-MR], height 18.2 A) each connected to six others through 10-MR apertures... 

The important structural feature of zeolites, which can be exploited for various uses, is the network of linked cavities forming a system of channels throughout the structure. These cavities are of molecular dimensions and can adsorb species small enough to gain access to them. A controlling factor in whether molecules can be adsorbed in the cavities is the size of the window or port into the channel, thus the... 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

Not every parameter used in the various force fields will be described in great detail, as this is not a review concerned with the simulation of aspects of zeolite structure. Instead, we aim to present the essential features of the various parameters that are used and to group force fields into certain families that essentially originate from one of a handful of key references. 

The spectra of alkaline earth ion-exchanged samples, with the exception of the barium form (211), have hydroxyl absorption bands at 3645 and 3540 cm-1, similar to those found in H—Y zeolite. The barium form behaves like the alkali-exchanged zeolites. The similarity of the spectra of the alkaline earth forms with that of the hydrogen form suggests that the acidic hydroxyls are associated with the same structural features (151). Band frequencies in the region of 3600 to 3560 cm-1 vary with the cations and are thought to result from hydroxyl groups associated with the divalent cations (211). They are weakly acidic or inaccessible to adsorbate molecules since the band intensity is not affected by adsorption of pyridine (209). 

The NMR data on clean dealuminated samples of zeolite Y indicate that with good sample preparation and the advanced techniques for structural studies, it will be possible to elucidate further the relationship between structural features and catalytic properties. ... 

A zeolitic structure can be described in various crystallographic terms. For many systems it is now possible to specify the following structural features the SBUs, the framework density, the coordination sequences, the unit cell dimensions and composition, the direction of the channels and the aperture (window) dimensions (Atlas of Zeolite Structure Types, 1992 Thomas et al., 1997). The framework density, FD, is defined as the number of T atoms per 1000 A1 (i.e. per 1 nm3) of the structure. 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

The structural features of dealuminated zeolite samples were characterized using X-ray powder diffraction, porosimetry and solid-state NMR measurements. Hexadecane cracking was used as a probe reaction to investigate catalytic properties of pure zeolites. 

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. 

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