Metal Species and Oxide Clusters Encapsulated in Zeolites

2 Metal Species and Oxide Clusters Encapsulated in Zeolites Basicity and General Aspects 

Alkaline or alkaline earth metal oxides are well known basic catalysts and much of work has been devoted to their characterization and to the study of their catalytic activity [64,65]. Taking into account the strong basic character of these oxides it is possible to enhance the basicity of zeolites by over-exchanging them with alkali [66-69] or alkaline earth metals [70-72] and producing after thermal de- 

Faujasite zeolites with a three dimensional strueture and 12-A supercavities are the most studied supports. Davis et al. [67,68] prepared eatalysts by impregnation of CsNaX and CsNaY with eesium acetate then thermal decomposition. They studied for first time the effect of loading cesium into zeolite on the acid-base properties, by studying the decomposition of propanol. Since then, special emphasis has been devoted to the study of cesium oxide loaded into faujasite zeolites yielding basicities in the range of superbasicity [66-69,73-76]. 

The size of the alkali metal cation has an impact on the basic properties of the final catalysts this is reflected in the order of basic strength found for several metal oxides encapsulated in X-zeolite Na O/NaX K,0/KX = Rb O/RbX CSj O/CsX. This order indicates that stronger basic sites are formed when the radius of the monovalent cation increases the sites are even stronger than those measured for the corresponding bulk alkaline oxides [67], A similar order of basicity was observed for a series of alkaline earth oxides occluded inside the cages of faujasite [70]. 

The chemical composition of the zeolitic host also affects the basic properties. When the basicity of Cs-over-exchanged zeolites X and Y, with Si/Al ratios of 1.2 and 2.5, respectively, were measured by stepwise thermal desorption of CO, it was found that different cesium oxide species are present CS2O inside the CsNaX zeolite and (Cs20)2 inside the CsNaY zeolite [73,74]. The basicity of the occluded CS2O species was, moreover, greater than that of intrazeolitic clusters (Cs20)2, as deduced from catalytic activity and TPD results. 

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