Mesoporous aluminosilicate

Synthesis of multiwall carbon nanotubes by using mesoporous aluminosilicates... 

Multiwall carbon nanotubes (MWCNTs) have been synthesized by catalytic chemical vapor deposition (CCVD) of ethylene on several mesoporous aluminosilicates impregnated with iron. The aluminosilicates were synthesized by sol-gel method optimizing the Si/Al ratios from 6 to 80. The catalysts are characterized by nitrogen adsorption, X-ray diffraction, 27A1 NMR, thermogravimetric analysis (TGA) and infrared. The MWCNTs are characterized by TGA and transmission and scanning electron microscope. 

We can state reasonably activity in alcohol dehydration and high selectivity to ethers of large pore H-zeolites. The concentration and strength of acid sites is important. The mesoporous aluminosilicate showed very little activity despite very large pores because of low number and low strength of acid centers. 

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

VII. Zeolites and Mesoporous Aluminosilicates Modified with Alkaline Earth Metal Oxides... 

Because of their well-defined porous structures and spatial limitations, zeolites and mesoporous aluminosilicates are shape-selective catalysts. The availability of... 

A particular selectivity was observed in the isomerization of m-xylene over MCM-41 mesoporous aluminosilicate. When compared to silica-alumina, MCM-41 exhibits very similar acidity characteristics and activities, and the disproportionation to isomerization ratios are not very different. However, there exists a large difference in the relative rates of formation of the para and the ortho isomers namely, the ortho isomer is preferentially formed (ortho para > 2.5). This was attributed to the presence of the regular, noninterconnected channels of MCM-41, in which xylene molecules undergo, before desorption, successive reactions of disproportionation and transalkylation. 

T. (2004) Noncovalent anchoring of asymmetric hydrogenation catalysts on a new mesoporous aluminosilicate application and solvent effects. Chem. Eur. ]., 10, 5829. 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

Zhang, Y. W., Okubo, T. and Ogura, M. Synthesis of mesoporous aluminosilicate with zeolitic characteristics using vapor phase transport, Chem. Commun. 2005, 2719-2720. 

Mesoporous aluminosilicates have attracted much recent attention because of their potential use as versatile catalysts and catalyst supports, especially for large molecules, but their acidity is always much weaker than that of zeolites. They must be modified to enhance their acidity. Mesoporous Si-MCM-41 supported metal chloride catalysts[29] have shown high activity in the acylation of benzene by benzoyl chloride even in the presence of moisture in the reaction mixture. The time required for 54 % conversion of benzoyl chloride at 80 °C was 2.2 h in the presence of InC13/Si-MCM-41, against 18 h with H-BEA. 

A recent article reported the use of strongly acidic mesoporous aluminosilicates prepared from zeolite seeds in the acylation of anisole with octanoyl chloride.[30] The mesoporosity improving the transport of the reactants and the presence of strong acid sites lead to high conversion (>90 %) and high selectivity (100 %). 

Shih, P.-C., Wang, J.-H., Mou, C.-Y. Strongly acidic mesoporous aluminosilicates prepared from zeolite seeds acylation of anisole with octanyl chloride, Catal. Today, 2004, 93-95, 365-370. 

The method of incipient wetness has one major drawback, which is the destruction of structured support materials. Immobilisation on zeolites or MCM-41 leads to the decomposition of the carrier when a highly acidic ionic liquid is added directly. The reason for this seem to be superacidic properties of HCI in ionic liquids. Especially the MCM-41 materials, which are mesoporous aluminosilicates with a very high surface area, are interesting carrier materials. Alternatives to the method of incipient wetness have been developed and will be presented elsewhere. 

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