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Zeolites

TermDefinition
What is a zeolite? Inorganic crystalline solid with high surface area, attributed to small pores (1-100A in diameter) present throughout structure. A mineral with a honeycomb-type structure and a unique "open" microporous molecular structure.
General structure of a zeolite They are an aluminosilicate framework structure with corner sharing SiO4 and AlO4 tetrahedra alternating throughout, in a lattice-like configuration.
Pore characteristics of a zeolite Sharply defined and shape selective. Act like molecular sieves.
Rules that dictate the build of zeolites Two aluminium ions cannot be adjacent to one another. Si/Al ratio of 1 to infinity.
Some properties of low Si/Al ratios Alkaline when in water, hydrothermally unstable, hydrophilic.
Some applications for natural zeolites Pollution control. Mining. Crops/Pastures. Horticulture. Stockfeeds. Aquaculture. Turf. Vermaculture.
Building of zeolites Structure of fused 4 and 6 membered rings.
How to control zeolite structure? Pore size and shape can be controlled by growing zeolites around a template.
Steps of zeolite synthesis To autoclave: sodium silicate, sodium aluminate, template, H2SO4 at 100-150C. Gels. Crystals: Filtering, washing, drying, calcination. Ion-exchange, drying, calcination. Zeolite.
Techniques for zeolite characterisation XRD, FTIR, Raman, surface area, XPS.
Shape-selective catalysis by zeolites Certain zeolites can be used to catalyse reactions and be specific to what product(s) is formed.
Reactant-selective Only one type of reactant molecule can be allowed to pass through the catalyst pores, due to size and shape.
Product-selective Only one type of product molecule can be allowed to pass through the catalyst pores, due to size and shape. Therefore, only a certain product is formed. "Observed" product
Applications of shape-selective catalysts Undesirable impurities can be continuously converted to easily removable compounds or harmless substances, the selectivity of a desired product can be increased and coke formation can be reduced.
What is Zeolite Y? ZY is a synthetic zeolite with structural type FAU, with a high Si/Al ratio. Ultra-stable Y.
Applications of Zeolite Y? FCC - Fluid CAtalytic Cracking is the process to "crack" heavy gas oils from larger molecules into small molecules, by breaking carbon bonds. Alkylation - Introduction of an alkyl group onto a molecule.
Green chemistry and zeolites Solid acid properties have been utilised extensively. Catalysts for: Friedels-craft acylation, salt-free esterfication of amino acids, green caprolactam process. Used in methanol to gasoline production.
Zeolite A Largest volume zeolite produced. Water softener in laundry detergents. Replaced harmful tri-phosphates.
 

 



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