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| Zeolites - Shape-Selective Catalysis by Zeolites |
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| Shape-selective catalysis are those reactions that depend on the pore structure of the catalyst and the size of the reactant and product molecules. In such reactions, zeolites are used as catalysts. Zeolites are microporous aluminosilicates of the general formula Mx/n [(AlO2)x (SiO2)y].z H2O where n is the charge of the metal cation, Mn+. M is usually Na+, K+ or Ca2+ and z is the number of moles of water of hydration, which is highly variable. The characteristic of zeolites is the openness of the [(Al2)O2]n framework. In this framework, some of the silicon atoms are replaced by aluminium atoms. Zeolites are found in nature and they are also synthesized for catalytic selectivity. Because of the three dimensional cage like structure, zeolites can be used as ion-exchange materials and selective adsorbents. |
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| Zeolites as ion-exchange materials have been replaced by synthetic cationic and anionic exchange resins, but their use as selective adsorbents for gases or liquids is extensive. The zeolites act as molecular sieves for the reactants. To achieve this, zeolites are heated in vacuum so that the water of hydration is lost. Once the water is removed, the zeolite structure becomes porous. The pore size in zeolites is generally in the range 260 - 740 pm. The size of the porous cavities influences the catalytic activity of the catalyst. Only those molecules can be adsorbed in the pores whose sizes are small enough. This facilitates the molecules to enter the cavities and leave easily. Hence, zeolites act as selective adsorbents. Depending on the size of the reactants and product molecules relative to the sizes of the cavities of zeolites, reactions occur in a specific manner. For e.g., alcohols are converted into gasoline using a zeolite catalyst ZSM-5 (Zeolie Sieve of Molecular Porosity-5). The formula of ZSM-5 is Hx[(AlO2)x(SiO2)96-x].16 H2O. The alcohol is dehydrated in the cavities by the protons and this results in gasoline. Zeolites are used in cracking long-chain hydrocarbons in the petrochemical industry. |
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