Zeolites organic analogs

Aoyama Y. Functional Organic Zeolite Analogs Top. Curr. Chem. 1998 198 131-161... 

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. 

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). 

Recently, the first stable organic analog of a zeolite was prepared [58] which showed catalytic properties in the base-catalyzed Knoevenagel condensation. In... 

De Vos, Sels, and Jacobs illustrate strategies of immobilizing molecular oxidation catalysts on supports. The catalysts include complexes of numerous metals (e.g., V, Cr, Mn, Fe, Co, and Mo), and the supports include oxides, zeolites, organic polymers, and activated carbons. Retention of the catalyt-ically active metal species on the support requires stable bonding of the metal to the support at every step in the catalytic cycle, even as the metal assumes different oxidation states. Examples show that catalysts that are stably anchored and do not leach sometimes outperform their soluble analogs in terms of lifetimes, activities, and selectivities. 

The order for the nucleation rste (see Figure 6 TEA < orga-nics-free < TPA), and the observation that when X - 5 NaOH + 3 wt % seed ZSM-5, ZSM-5 is formed about as fast as with X - 5 TPAOH (see Table II), do not support a precursor role for D5R silicates in all these synthesis reactions. This is because, on the basis of the D5R concentrations in analogous silicate solutions, the order TEA = TPA > organics-free is expected (cf. Table III) (4). If particular zeolite precursors are responsible for the formation of ZSM-5 then, clearly, TEA has a very retarding effect on their mutual condensation rate. 

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

CAS-1 is the first microporous calcosilicate zeolite-like crystal material and can be easily synthesized with or without organics. The reversible cation-exchangeability and the selectively adsorptive properties of CAS-1 are analogous to those for zeolites and molecular sieves. CAS-1 can withstand high temperature calcination indicating that it has a good thermal stability that is common for zeolites. 

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