Mesoporous metal-substituted

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],... 

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. 

Other Metal-substituted Silica Mesoporous Materials... 

Tanev et al. have reported the synthesis of mesoporous materials via a route which involves self-assembly between neutral primary amines and neutral inorganic framework precursors.12 The regularity of the pore structure in these materials has been illustrated by lattice images which show a honeycomb like structure. The system of channels of these molecular sieves produces solids with very high internal surface area and pore volume. This fact combined with the possibility of generating active sites within the channels produces a very unique type of acid catalyst. In the case of transition metal substituted M41S, the principal interest lies in their potential as oxidation catalysts, especially Ti and V substituted MCM and HMS type materials, and more recently synthesised large pore materials.13... 

Acid catalysis (e.g., metal-substituted material, zeolite/ mesoporous composites, sulfonic acid-functionalized)... 

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

Heterogeneous catalysis is of fundamental importance in the chemical industry and in other technologically relevant applications. This chapter mainly illustrates the synthesis for the mesoporous solids that can be used as catalysts and catalysts carriers, such as siKca, organic-group-functionalized silica, metal-substituted sihca, phosphate, carbon, nonshiceous oxides, nonoxides, and so... 

A remarkable discovery of mesoporous silicates with pore openings from 2 to 50 nm in 1991 was quickly followed by a successful preparation of mesoporous aluminophosphates with hexagonal pore arrangement and amorphous nature of the framework (12-15). Up to date, a variety of silicon- and metal-substituted mesoporous aluminophosphate materials with different incorporated metal... 

Hsu, B.-Y. and Cheng, S. (1998) Pinacol rearrangement over metal-substituted aluminophosphate molecular sieves. Microporous Mesoporous Mater., 21 (4-6), 505-515. 

In comparison, 40 is much more acidic than zeolites and metal-substituted mesoporous acidic catalysts. The KIT-5-(100)-20TA demonstrates an advanced and higher activity and superiority of performance in the synthesis of coumarins via Pechmann reaction (10MI2843). 

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. 

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 development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

Complexation of metal ions and subsequent incorporation of the resulting metal complex into the oxide matrix during surfactant-templated synthesis prevents aggregation and leads to a homogeneous distribution of metal centers in the mesostructure. Copper- and vanadium-substituted mesoporous silicas were prepared in this way [107,108], Such materials have great potential in the field of catalysis. 

Similarly the alkali metal-exchanged titanium beta was shown to be an effective catalyst for olefin epoxidations with TBHP [47]. Titanium has also been incorporated into mesoporous molecular sieves, such as MCM-41, by framework substitution [48] or by grafting Ti(IV) species to the internal surface by reaction... 

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