Oxidation heteroatom substituted zeolites

Heteroatoms (B, Al, Fe, Ga, and Ti) may be incorporated into the framework of high-silica and all-silica materials in the presence of fluoride as well, giving rise to active acid catalysts. Usually, transition metal ions will hydrolyse to form hydroxide or oxide precipitates in a high-pH solution. Therefore, there is a limitation to the content of transition metals in heteroatom-substituted zeolites. However, this limitation can be significantly increased by using fluoride during the synthesis because fluoride can coordinate to the transition metal atoms to form stable complex, which will help transition metal atoms incorporate into the framework of zeolites. 

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

Elucidation of the structure of a solid catalyst is paramount to any understanding of its activity. Without such information, inferences about its activity would be speculation. Often it is instructive to determine the structure of a catalyst after a treatment such as oxidation, reduction, or exposure to a reactant, or with the catalyst in a particular state it may be helpful to compare a fresh catalyst with a spent or a regenerated catalyst. XAFS spectroscopy used in this manner is "static the structure of the catalyst is determined in a specific well-defined state determined by the treatment and gas environment during the measurement. Two such examples are discussed here the determination of the location of the isomor-phous substitution of a heteroatom (tin) into a zeolite framework (zeolite beta), and the structure of dispersed rhenium oxide supported on y-Al203. 

Even though the metal-substituted, mesoporous solids allow the oxidation of molecules that is not possible with zeolites, there are several issues that still need to be addressed. First, the activity of the metal-loaded catalysts decreases with increased metal loading, e.g. for Ti-MCM-41, the peak activity for alkene epoxidation is attained at 2 wt. % [44aj. Second, metal leaching can occur and care needs to be exercised in concluding that oxidation is taking place at the framework site rather than via metal ions leached into solution [184, 185]. Leaching has been shown to occur for V-substituted mesoporous materials in the oxidation of alkanes [184], X-ray absorption spectroscopy indicates that the inclination of the heteroatoms to remain in the MCM-41 framework after calcination follow the order Ti > Fe > V > Cr [56],... 

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