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Framework composition of molecular sieves

Release Date:

2023-04-10

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Molecular sieves are inorganic silicoaluminate compounds that exhibit excellent thermal stability, enabling them to withstand high temperatures and facilitating regeneration for repeated use. Their framework is resistant to degradation by microorganisms and other agents. The structural framework of molecular sieves consists primarily of silicon–oxygen tetrahedra and aluminum–oxygen tetrahedra. Because aluminum has a valence of +3, one oxygen atom in each AlO4 tetrahedron remains unbalanced, imparting a net negative charge to the entire aluminum–oxygen tetrahedron. To maintain electrical neutrality, positively charged metal cations must reside near these tetrahedra to counteract the negative charge. The strong electrostatic field generated between these cations and the negatively charged molecular sieve framework profoundly influences its adsorption properties. Molecular sieves display significantly stronger adsorption for polar substances than for nonpolar ones; moreover, owing to this robust electric field, they can also adsorb compounds containing double bonds or extended π‑conjugated systems through induced polarization. Generally, the greater the charge on the cation and the smaller its ionic radius, the stronger the resulting electric field, the more pronounced the induced polarization of double bonds, and the higher the adsorption capacity for such compounds. For example, 5A molecular sieves can effectively adsorb olefins and alkynes such as ethylene and propylene.

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Framework composition of molecular sieves

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