Preparation of Metal Zeolites

Various techniques are available for the introduction of metals into zeolites. Ion exchange and impregnation, e.g., by the incipient wetness or imbiberaent techniques, are often used. The former method introduces preferentially the cation into the zeolite, whereas the latter method also incorporates an equivalent number of anions. In both cases, the introduction of ions has to be followed by calcination and reduction steps. For zeolite-based catalysts the ion-exchange method is often preferred (97,9S). 

Ion exchange of Na or forms of zeolites normally starts with a dilute aqueous solution of ionic precursors. The ions are solvated or coordinated 

The reduction of these precursors will produce multiatom particles and water vapor. This reduction process has a much smaller activation energy compared to the reduction of ion precursors, resulting in isolated atoms and protons. 

A second strategy to prevent escape of transition metal ions into small zeolite cages is blocking of these cages with less reducible and catalytically inactive cations, such as Mn , Ca , or Sr. Considerable enhancement of the reduction of Ni and Co has been achieved in this way (79,108,133). 

Removal of coordinating ligands by careful calcination prior to reduction, is therefore extremely important for metal/zeolite catalysts, because it controls the cation locations and thus the metal particle growth mechanism during subsequent reduction. It has been demonstrated that the ultimate metal dispersion depends on the temperature of the calcination (50,69,71,79,107). An optimum calcination temperature can be defined for obtaining maximum dispersion of metals in zeolites. 

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