Organic zeolites stability

MOFs can be considered as organic zeolite analogs, as their pore architectures are often reminiscent of those of zeolites a comparison of the physical properties of a series of MOFs and of zeolite NaY has been provided in Table 4.1. Although such coordinative bonds are obviously weaker than the strong covalent Si-O and Al-O bonds in zeolites, the stability of MOF lattices is remarkable, especially when their mainly organic composition is taken into account. Thermal decomposition generally does not start at temperatures below 300 °C [3, 21], and, in some cases. 

Inclusion Compounds Selectivity, Thermal Stability, and Kinetics, p. 696 Mesoporous Materials, p. 845 Nomenclature in Crystal Engineering, p. 967 Organic Zeolites, p. 996 Phthalocyanines, p. 1069... 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Since the first attempts of Barrer (1) and Kerr (2) to synthesize counterparts of some natural zeolite materials in presence of te-tramethylammonium hydroxyde, a wide number of new zeolites have been prepared from various organic cation-containing mixtures (3). Soon, zeolites with high Si/Al ratio appeared attractive because of their potential thermal, hydrothermal and acid stabilities. In that respect, a variety of organics have proved successful in pro-ducing Si-rich zeolites (3). 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

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. 

Spray drying requires formulahon of a relatively stable, usually aqueous dispersion of zeolite and colloidal binder particles, which are then fed to the spray drier [88, 89], Organic dispersing agents may be added to the spray drier feed slurry to stabilize the dispersion while the slurry is fed to the drier. 

A second area that will be important in the future is the continued development of MOFs and ZlFs [152]. Much as the discovery of AlP04-based materials revolutionized the catalyhc use of zeolites when only aluminosilicates were known, MOFs and ZlFs have the potential to revolutionize low temperature processes such as oxidations and organic reachons [153]. Newly discovered materials along these same lines are covalent organic frameworks, the so-called COFs [154]. These materials have similar channels to those known for MOFs and ZlFs but tend to have higher thermal stability. 

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