Metal-Substituted Mesoporous Silica Molecular Sieves

Metal-Substituted Mesoporous Silica Molecular Sieves 

In general, the surface of pure silicate mesostructures is weakly acidic. It is found that the incorporation of metal ions into the framework can introduce acidic and ion-exchange functionality and catalytically active sites. Various metal ions, such as Al +, Ti , V +, Ga +, and Fe +, have been incorporated into S BA-15 to enhance its catalytic performance. In contrast to zeolites, which have crystalline structures, the incorporation of metal ions in mesoporous silicates caimot be strictly defined as intra- or extra-framework incorporation since these ions are highly dispersed on the framework. A wide range of compositions with different coordination numbers and chemical environments can contribute to amorphous framework structures. For example, both tetrahedrally and octahedrally coordinated aluminum in S BA-15 are involved in the formation of the amorphous pore walls, and may be defined as intraframework Al. The former may exist inside the pore walls, while the latter may be located on the pore surface. 

we give an example for the synthesis of Ti-containing mesoporous silicas. This kind of catalyst has abilities for the selective oxidation of olefin and other unsaturated compounds, such as the epoxidation of 1-hexene, cyclohexene, and styrene [61,62]. The ratio of Si/Ti, structure, and hydrophobic nature of the material are the three most important factors in the catalytic activity. Therefore, a full understanding of the synthesis is necessary. 

Normally, the Ti content is limited to 0.15-3 wt%. Too high a content would lead to the destruction of the ordered mesostructure or the aggregation of titania particles. The addition of n-(trimethoxysilylpropyl)ethylenediaminetilacetic acid (EDATAS) during the hydrothermal synthesis may improve the titanium 

Sample Ti/Si ratio Conversion %) Selectivity (%) H2O2 efficiency 

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Metal-Substituted Mesoporous Silica Molecular Sieves I 287... 

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

Many transition metal-substituted molecular sieves could be used as catalysts in the liquid phase oxidation of aniline with alkylperoxides. Even though TS-1 had a good activity in this reaction it was probably not the best catalyst, the reaction being limited by the diffusion of reagents and/or products in the channels of the zeolite. This is why we preferred large pore zeolites or mesoporous silicas, which have the additional advantage with respect to TS-1 to be active with bulky oxidants like tert-butyl hydroperoxide. 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

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