Mesoporous metal leaching

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],... 

Metal-modified aluminophosphates show great potential in the oxidation reactions under mild conditions. However, the low thermal stability and the metal leaching from the framework reduce their wider applications. Some of the metal-modified mesoporous catalyst and their catalytic applications are listed in Table 15. 

Transition metals and their complexes can be immobilized in the mesopores or incorporated in the structure to make silica-supported metal catalysts. For instance, titanium catalysts for selective oxidation can be formed by modifying the mesoporous structure with either Ti grafted on the surface (Tif MCM-41) or Ti substituted into the framework (Ti->MCM-41). The grafted version makes the better catalyst for the epoxidation of alkenes using peroxides, and has good resistance to leaching of the metal. 

Thus, mesoporous structures, because of their pores, make it possible to incorporate large catalytically active transition metal complexes. Covalently grafting these complexes in the hydrophobic patches provides better dispersion of the catalyst, as well as resistance to leaching. Improvement in catalytic performance is to be expected when materials with even larger pore diameters are studied. It will also be necessary to passivate the silanol groups responsible for promoting adsorption on the catalytic surface. 

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

Table 2 summarizes the variation in porosity at various stages of treatment of oil fly ash. Acid treatment of oil fly ash removes the most of the inorganic matter from the surface and inside of fly ash particles and produces micro and mesopores. Heating of ash with acid leached out most of the metals in the form of phosphates, sulfates and nitrates. Activation of ash with acid produces more mesopores than raw ash (see Fig. 2). 

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