Mesoporous molecular sieves, structural properties

In this current work, cerium-modified MCM-41 mesoporous molecular sieve was synthesized using heptahydrated cerium chloride, colloidal fumed-silica, sodium hydroxide, cethyltrimethylammonium bromide and water. The incorporation of cerium to MCM-41 improved the quality, stability and acid properties of the resulting ordered mesoporous material. Its surface and structural properties were extensively studied by nitrogen adsorption and high-resolution thermogravimetry. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Table 1 shows the structural properties of the titanium-incorporated mesoporous molecular sieves. The surface areas of all samples were over 750 m2/g, being typical of M41S group materials. After grafting, surface area and total pore volume decreased [8],... 

Review of literature concerning the photochemistry of inorganic compounds shows us that a substantial progress was achieved during the past two decades in understanding the photophysical and photochemical properties of nanometer semiconductor particles [1-4] and structurally organized semiconductor materials, including the nanostructured semiconductor films, mesoporous molecular sieves [5 - 10] etc., and, also in the elaboration of physical and chemical techniques for their synthesis and examination of their photocatalytic activity in various chemical and electrochemical redox-processes. 

The acidic properties of mesoporous molecular sieves rely on the presence of active sites in their framework. In the case of MCM-41 active sites are generated by the introduction of heteroatoms into the structure. In particular, Bronsted acid sites are introduced by isomorphous substitution of A1 for Si which is achieved by hydrothermal synthesis in which charged quaternary ammonium micelles are used as the template for charged alumino-silicate inorganic precursors. 

In recent years, increasing attention has been focused on studies concerning the production of new zeolite materials such as micropwrous and mesoporous molecular sieves due to their very unique and interesting physicochemical properties such as a pore structure of a molecular scale, the capacity for ion-exchange, a strong surface acidity and a unique... 

The mono- and bimetallic substituted MCM-41 catalysts with cobalt, vanadium or lanthanum have been prepared by direct synthesis or impregnation and characterized by various techniques, such as XRD, N2 adsorption-desorption, SEM, TEM and TGA. The catalytic activity in the selective oxidation of styrene with H2O2 has been evaluated. Adsorption of benzene and ammonia on the mono- and bimetallic mesoporous molecular sieves with cobalt and vanadium has been studied by IR spectroscopy and the results have been correlated with their catalytic properties and the characteristics of the structure. Co-incorporated catalysts prepared by direct synthesis show to be very active and selective. 

Other specific areas of micellar catalysis in which industry has expressed interest are in micellar phase-transfer catalysis and in the synthesis of mesoporous molecular sieves [92]. In the first example of the latter application, investigators at Mobil were able to control pore size and properties by synthesizing the desired mesoporous material in the presence of appropriately sized, structured, and charged micelles [96]. The burst of research activity in this area that occurred in the next few years after this discovery has been reviewed by Huo et al. [97]. 

A great deal of research effort aimed at enlargement of the zeolite and zeotype structures resulted in numerous papers on the synthesis, properties, and characterization of delaminated and pillared zeolitic materials and mesoporous molecular sieves with amor-... 

It is the aim of the present paper to review the synthesis of a selected number of microporous materials which have more recently attracted the attention of zeolite scientists. The following selection criteria were appUed (i) the review is restricted to aluminosiUcates, hence, isomorphously substituted zeolites, SAPOs, MeAPOs and the Hke are not considered, (ii) crystalline microporous aluminosilicates are exclusively dealt with, the synthesis of ordered mesoporous molecular sieves (i.e. MCM-41) is not discussed and, (iii) especially those zeolites have been selected which, based on their particular structural or physicochemical properties, have a certain potential for appHcations in catalysis or adsorption. Some readers might miss such important and interesting zeolites like Beta, EMC-2, ZSM-12, etc. However, although there is still intensive research going on in the synthesis of these zeoHtes, their main synthesis principles have been summarized earUer [5,6] and will therefore not be discussed here. 

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

M., and Jaroniec, M. (1997) Characterization of large-pore MCM-41 molecular sieves obtained via hydrothermal restructuring. Chem. Mater., 9 (11), 2499-2506 (d) Kruk, M., Jaroniec, M., and Sayari, A. (1999) Influence of hydrothermal restructuring conditions on structural properties of mesoporous molecular sieves. Microporous Mesoporous Mater., 27 (2-3), 217-229 (e) Kruk, M., Jaroniec, M., and Sayari, A. (1999) A unified interpretation of high-temperature pore size expansion processes in MCM-41 mesoporous silicas. J. Phys. Chem. B,... 

Parida KM, Dash SS, Singha S (2008) Structural properties and catalytic activity of Mn-MCM-41 mesoporous molecular sieves for single-step amination of benzene to aniline. Appl Catal A Gen 351 59-67... 

Lanthanum has also been used to modify the mesoporous molecular sieves to improve their catlytic propertis. MCM-41 is the most popular mesoporous molecular sieve of the M41S series. The structure is the hexagonal cylindrical pore, containing a diameter of 15-100... 

The mesoporous molecular sieve SBA-15 possesses the same chemical composition and structure as that of nano-Si02,so it can be used to enhance the electrochemical properties of the CSPE, in which the LP ions can enter and move through the mesoporous channels much more easily than they can through microporous inorganic materials. 

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