Synthesis of Embryonic Zeolites with Controlled Physicochemical Properties
Résumé
Colloidal zeolite precursors, with a sharp particle size distribution (ca. 3-5 nm) and identical chemical composition, the so-called embryonic zeolites (EZs), are prepared by a strict control of aluminosilicate precursors polymerization in zeoliteyielding systems. The organic structure directing agent (OSDA) acts as a sacrificial template and is eliminated by high temperature combustion after the synthesis. Physicochemical properties of the EZs, such as pore size and volume and specific surface area are determined by the size of the OSDA and the synthesis conditions employed; i.e. the larger the OSDA, the higher the microporous volume and the specific surface area of the derived EZs. The EZs belong to the family of extra-large microporous (1-2.5 nm) materials. Upon calcination EZs retain their structure/porosity, and most of their aluminum remains in tetrahedral coordination to provide BrØnsted acidity. Pyridine adsorption shows a lower acidic strength for embryonic zeolites with respect to their crystalline counterparts (zeolites). An appropriate combination of extra-large micropores (1-2.5 nm) and Brønsted acid sites (~25µmol.g-1) leads to improved catalytic performances in the dealkylation of TiPBz, a proxy for bulky molecules reacting only on the external surface of crystalline zeolites. By varying the size of the OSDA for synthesizing the EZ, materials with controlled porosity, acidity, accessibility, and catalytic activity are prepared and their properties extend those of existing crystalline zeolites.
Domaines
Matériaux
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