Zeolites structural stability

Over the past 2 decades zeolites have been exchanged with just about every imaginable cationic species in an attempt to find something that works as well as rare earth does in maintaining zeolite structural stability and catalytic activity. As yet no commercially viable substitute for rare earth has been found which provides the same zeolite stability, activity and product selectivity at a commercial price. 

MCM-22 crystallizes in thin sheets or plates. Within these sheets, there is evidence of the existence of a buried T-site (a Si or A1 atom) in the framework structure that is not accessible to a channel wall. This buried T-site appears to be unique to MCM-22 and gives the zeolite structural stability even under severe conditions that would destroy other less-stable zeolites. MCM-22 has been actively studied and over 50 US patents have been issued on its use in varied areas, including hydroprocessing, aromatic/olefin alkylation, paraffin/olefin alkylation and dispro-portionation processes, as well as specialty chemical applications. 

Selectivity (1), capacity (2) and storage ability (3) originate from the characteristics of Ag itself, while the stability (4) of Ag-FER(60.2) is contributed by the structure stability of the FER(60.2) and insensitivity of the adsorption property of Ag to its existing states. The higher Si02/Al203 ratio of FER(60.2) is the most probable reason for the zeolite-structure stability. The insensitivity of Ag is a favorable property as a stable HC trap material, and its origin will be elucidated by a future study. 

The dependence of structure collapse temperature on hydrothermal aging temperature is depicted in Figure 2 (right). Fe decreased the Y zeolite structure stability, V had an even more severe effect. Ca and Ni had less influence on the structure collapse temperature. 

Figure 2 shows that the framework Si02/Al203 ratio increased with increasing hydrothermal aging temperature, while structure collapse temperature decreased. This was attributed to the imbalance between dealumination and Si migration into the A1 defects. The structural vacancy resulting from rapid dealumination decreased the zeolite structural stability. 

Rare Earth Level. Rare earth (RE) elements serve as a bridge to stabilize aluminum atoms in the zeolite structure. They prevent the... 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

T sites were T9 and T10 sites—if thermodynamics controls the structure of Ti-containing MFI zeolite. The stability sequence of T sites was found to be T9 > T10 > T12 > T1 > T6 > T5 > T3. The exact location of Ti ions in TS-1 is still controversial. There are no similar investigations for other Ti silicates. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Overall the period since the 1980s can be described as a period of explosion in the discovery of new compositions and structures of molecular sieves. This can perhaps be seen most vividly by comparing the numbers of structure types contained in the various editions of the Atlas of Zeolite Structure Types [4]. The first edition (1978) contained 38 structure types, the second edition (1987) 64, the third edition (1992) 85 and the most recent edition (2007) 176. Thus 112 new structure types have been discovered since 1978. However, the reader should be cautioned that a significant number of the structure types included in the Atlas are not truly microporous or molecular sieve materials (i.e., they are not stable for the removal of as-synthesized guest species, typically water or organic templates) and therefore carmot reversibly adsorb molecules or carry out catalytic reactions. Unfortunately, the Atlas gives only limited information on the stability of the structures described. 

This table shows that it is diflicult, even in a model system, to present a simple view of the nature of the adsorption site because of the number of different parameters involved in the stabilization of OJ. For zeolites the problem is apparently more diflicult than for oxides, since not only do the framework ions and the exchanged cations form two distinct types of adsorption sites but the latter can migrate within the zeolite structure. It is difficult to obtain a full description of the coordination of the exchanged cations and so far there has been no systematic study on this point. 

The section on crystallization comprises zeolite synthesis, kinetics and mechanism of formation, stability relationships, recrystallization processes as well as the genesis of natural zeolites. Recent advances in this field have been surveyed, and some new perspectives have been outlined in the review by E. M. Flanigen. Most of the studies in this field are still empirical because of the complexity of the systems involved. Considerable progress has been made, however, towards a better understanding of the processes and mechanisms governing zeolite crystallization. It is not unreasonable to expect that conditions for synthesizing new zeolite structure types can eventually be predicted. 

Table VII (51). The relevant free dimensions are often similar for zeolite and nonzeolite. Urea (free diameter 5.2 A) is like Sieve A (free diameter of windows 4.3 A) in accommodating n- but not isoparaffins. Thiourea (6.1 A) and offretite (6.3 A) have channels with similar free diameters as do 0-cyclodextrin (7-8 A) and zeolite L (7.1 X 7.8 A). In thiourea the loose fit of n-paraffins in the tunnel appears to destabilize the adducts (85, 36). The same is true of disc-shaped molecules comprising only benzenoid rings. However, if suitably bulky saturated side chains are attached (cyclohexyl-benzene or fertf-butylbenzene), then adduction readily occurs. Heterocy-clics, like unsubstituted aromatics, do not readily form adducts. Thus flat molecules also exert a destabilizing effect upon the tunnels of a circular cross section. Such stability problems do not arise with the robust, permanent zeolite structures, and this constitutes an interesting distinction. Offretite, for example, readily sorbs benzene or heterocyclics with or without alkyl side chains, provided only that they are not too large to permeate the structure.

The crystalline structure of modified zeolites determine a number of properties which are specific and favorable for catalytic reactions. The complete or partial loss of crystalline structure during catalytic reactions or regeneration is in most cases accompanied by decreased catalytic activity. Thermal stability or structural stability characteristics are therefore suitable for evaluating such catalysts or supported catalysts. 

The high thermal stability of zeolites and related micro-porous solids is one of their most attractive features. Whilst it Is clear that materials with organic components cannot withstand ultra-high temperatures, quite respectable compositional stability can be achieved. Thus the [Er(TMA)] polymer mentioned above shows no weight loss in its TGA curve before 550°C. However for porous solids another key issue is that of structural stability. Many open framework coordination polymers lose their crystalline structure upon mild heating, or even evacuation, through loss of guest molecules. 

The formation and location of the active complex is obviously strongly dependent on the zeolite topology and composition indeed, among the many zeolite structures examined, only the faujasite topology with the charge density of zeolite Y was found stabilize a maximum number of active sites. 

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