Aluminosilicates, zeolite structures

The motivation to replace aluminium in aluminosilicate zeolite structures by other elements arose from the need to adjust their properties to intended applications. Since the nature and strength of the bridging hydroxyl groups (Si - OH - T, T = Al, Fe, Ga, B,. ..) depend on T atom, and thus the proton - T distance and resulting acid strength of the modified material. At comparable bond angles, the proton - T distance decreases in order Fe, Ga, Al. This means that electrostatic repulsion between the proton and T increases in the same way, and that the acidity is expected to increase in the same order [48]. These changing in the acidity are easily accessible by adsorption calorimetry technique. 

Many different zeolite structures are already known, but there is permanent need for new or improved ones to satisfy novel and specific industrial and technological applications. To successfully accomplish this task, a deeper understanding of the zeolite crystallization process is certainly needed. One of the important parts in that study is the structural investigations of their amorphous aluminosilicate precursors (gels). 

In general, zeolites are crystalline aluminosilicates with microporous channels and/or cages in their structures. The first zeolitic minerals were discovered in 1756 by the Swedish mineralogist Cronstedt [3], Upon heating of the minerals, he observed the release of steam from the crystals and called this new class of minerals zeolites (Greek zeos = to boil, lithos = stone). Currently, about 160 different zeolite structure topologies are known [4] and many of them are found in natural zeolites. However, for catalytic applications only a small number of synthetic zeolites are used. Natural zeolites typically have many impurities and are therefore of limited use for catalytic applications. Synthetic zeolites can be obtained with exactly defined compositions, and desired particle sizes and shapes can be obtained by controlling the crystallization process. 

Zeolite structures typically consist of silicon and aluminum finked by tetrahedrally coordinating oxygen atoms. However, similar structures as found for these aluminosilicates can be formed by substitution of the aluminum by other elements (e.g., Ga in gallosilicates or Ti in titanosilicates). Even the substitution of both Si and A1 is possible, as for example in aluminophosphates or... 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Table 1.4 lists some of the major new structures reported in the 1990s. Interestingly, as organic SDAs tended to dominate discovery of new frameworks, there were no new aluminum-rich synthetic zeoUtes reported in either the 1980s or the 1990s. The new aluminosilicate structures were all high silica or pure sihca in composition. It awaited the 2000s for new aluminosilicate zeolite materials with low to medium Si/Al to be reported (see below). 

The first part of the book documents the history, structure, chemistry, formulation and characterizations of zeolites in Chapters 1-4. The past 60 years have seen a progression in molecular sieve materials from aluminosilicate zeolites to micro-porous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosihcate and metallophosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and, most recently, hybrid metal organic frameworks (MOFs). 

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

Since titanosilicates generally require specific synthesis conditions in comparison to silicalites and aluminosilicates, many efforts made to synthesize those of numerous zeolite structures have led to a very limited success. This has also been the case with the MWW zeolite. Although it is possible to hydrothermally synthesize MWW... 

As previously described, the hydrothermal synthesis of aluminosilicate zeolites is carried out with highly reactive aluminosilicate gels in autogenous conditions. This kind of synthesis may involve the structure-directing role of alkaline cations in solution-mediated crystallization of the amorphous gel [55],... 

Aluminosilicate zeolites because of their structure, composition, and properties offer a superior ionic strength environment [172,173], Even though these materials are electronic insulators, when hydrated, they are solid solutions of high ionic mobility, and when dehydrated exhibit fair ionic conductivity (see Section 8.2.7) [38,112,119,172], The properties of aluminosilicate zeolites that are responsible for affecting the charge-transfer reactions in electrochemical systems are [172,174] ... 

This is consistent with the observation that in zeolite structures the cation is partially coordinated to the framework and never wholly surrounded by water molecules. Moreover, this intimate association of aluminosilicate and cation species may well be the template for the crystallising zeolite. 

One approach (4) is to calculate, for a certain zeolite structure, the Madelung and polarization energies for fixed lattice positions. The heat of formation due to ionic bonding is calculated both for the zeolitic aluminosilicate with varying amount of aluminum and... 

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