Mesoporous molecular sieves, formation

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

Nb-containing MCM-41 sieves represent Lewis acidity proven by FTIR study conducted after pyridine adsorption [3,4], Hydrogen forms of niobium-containing MCM-41 materials exhibit lower Bransted acidity than that in hydrogen aluminosilicate mesoporous molecular sieves (see the band at 1549 cm 1 in Figure 6 [3]). The dehydroxylation of H-NbMCM-41 samples causes the formation of the following lattice species ... 

Vartuli, J. C., Schmitt, K. D., Kresge, C. T., Roth, W. J., Leonowicz, M. E., McCullen, S. B. Hellring, S. D., Beck, J. S., Schlenker, J. L., Olson, D. H. and Sheppard, E. W. Effect of surfactant silica molar ratios on the formation of mesoporous molecular-sieves-inorganic mimicry of surfactant liquid-crystal phases and mechanistic implications,... 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

A catalyst based on the mesoporous molecular sieve MCM-41 aluminosilicate has been used for the oligomerization of propene and butenes[51-53] at low temperatures (between 353 and 473 K). The catalyst shows higher activity and selectivity for the formation of trimers and tetramers as compared with HZSM-5 and HZSM-23. 

As a typical example of CEDFT calculations, we present in Fig. 1 the capillary condensation isotherm of N2 in a cylindrical pore mimicking the pore channel in MCM-41 mesoporous molecular sieves. The isotherm is presented in co-ordinates adsorption N versus chemical potential p Calculations were performed at 77 K for the internal diameter of 3.3 nm up to the saturation conditions, point H. We used Tarazona s representation of the Helmholtz free energy [6] with the parameters for fluid-fluid and solid-fluid interaction potentials, which were employed in our previous papers [7]. We distinguish three regions on the isotherm. The adsorption branch OC corresponds to consecutive formation of adsorption layers. Note that the sharp transitions between the consecutive layers are not observed in experiments. They are caused by a well-known shortcoming of the model employed, which ignores intrinsic to real... 

In the present work, AlMCM-41 materials with different nsi/n,y-ratios were used as support for copper and zinc species, which were introduced during the synthesis of the mesoporous molecular sieve. Characterization of the metal-containing materials was primarily achieved by carbon monoxide adsorption and temperature programmed reduction In particular the aim of this work was to distinguish between isolated copper species grafted on the wall of MCM-41 and/or or the formation of copper oxide clusters located in the channels... 

Bein and coworkers investigated the formation of poly(methyl methacrylate) (PMMA) through polymerization of methyl methacrylate (MMA) in channels of microporous crystals including Na-Y, mordenite, zeolite / , and ZSM-5, and mesoporous molecular sieves such as MCM-41 and MCM-48.[83] MMA in zeolites can also polymerize as acrylonitrile does, and with an increase in the host channel size the polymerization degree is enhanced. Electron microscopic observation indicates that the polymerization reaction proceeds mainly inside the zeolite channels, because almost no polymers are observable on the external surface of the zeolite particles. This is further confirmed by the fact that the polymer/zeolite composite lacks the characteristic glass transition temperature for a bulk polymer. 

The textiuai characteristics of the samples as a function of heating temperature and duration are listed in Table 1. From these data, it appears that the increase of heating temperature fevours the formation of large pore mesoporous molecular sieves. Too long heating at higher temperature, 100°C for exan le, will however lead to the destruction of the conqx>imds. Their X-ray diffraction pattern will not show any... 

This technique has recently been developed for the synthetic formation of mesoporous molecular sieves (Section 6), which contain cavities with diameters between 2.0 and 20 nm, ten times larger than those in the microporous zeolites. 

These mesoporous molecular sieves are prepared using a liquid crystal templating mechanism in which micelles, which are assemblies of cationic alkyl trimethylammonium surfactants [CH3(CH2) N+(CH3)3] X , act as a template for the formation of the silicaceous material (Figure 6.2). In the silicate-rich aqueous solution, the hydrophobic tails of the surfactant cluster together, leaving the positively charged heads to form the outside of the rod-like liquid crystal micelles. The silicate anions are attracted to, and surround the micelles, aggregating into an open-framework amorphous solid, which precipitates. The solid is filtered off, and heated in air at up to 700 °C (calcination), which removes the surfactant and leaves the... 

In blank tests, silicious MCM-41 or bulk M0O3 did not exhibit photocatalytic reactivities in the presence of a mixture of NO and CO under UV-irradiation. The photocatalytic decomposition reactions of NO in the absence and presence of CO were performed on Mo-MCM-41 mesoporous molecular sieves. UV-irradiation of the Mo-MCM-41 in the presence of NO led to the evolution of N2 as well as N2O and N02- Moreover, it was found that the photocatalytic decomposition reactions of NO was dramatically enhanced by the coexistence of CO, leading to the formation of N2 and CO2. UV-irradiation of Mo-MCM-41 in the presence of a mixture of NO and CO leads to the formation of N2 and CO2 with a good linearity against the UV-irradiation time, while the turnover number (TON), (defined as the value of the number of photo-formed N2 molecules divided by the total number of Mo species in the catalyst) exceeded unity after irradiation for 2 h, as shown in Fig 5. These results clearly indicate that the reaction proceeds photocatalytically [6]. 

Basic quantitative relationships and trends governing the formation of the M41S mesoporous molecular sieves MCM-41 and MCM-48 are presented. The syntheses with hydroxide/surfactant molar ratio close to unity afforded high quality MCM-41 and proved particularly suitable for quantifying the effects of synthesis mixture composition and synthesis conditions upon the nature and properties of the mesoporous products. Increasing temperature and duration of crystallization resulted in unit cell expansion and a more robust silica framework. Syntheses with silica to surfactant molar ratio equal to 5 1 and higher gave MCM-41 while MCM-48 was observed at lower (3.5 1) ratios. 

The influence of alkali/silica molar ratio in the synthesis of mesoporous molecular sieve materials was studied in a simple synthesis system containing tetraethylorthosilicate, water, alkali and the cetyltrimethylammonium bromide at room temperature. The resulting silicate materials were characterized by XRD, Si MAS NMR, TEM and N2 adsorption-desorption isotherms. The results suggest that the formation of different surfactant-silicate aggregation array morphology not only relies on an interaction between silicate ions and the surfactant, but also on a proportion of the silicate species to surfactant. 

The synthesis and kinetics of formation of SBA-1 mesoporous molecular sieve was greatly affected by the addition of trimethylbenzene (TMB) isomers. The synthesis of SBA-1 was performed at 0°C using tetraethyl orthosilicate (TEOS) as the silicon source and cetyltriethylammonium bromide (CTEABr) as the template under acidic conditions. XRD shows that a well ordered hexagonal p6mm SBA-3 mesophase rapidly forms and then transforms to cubic Pm3n SBA-1 mesophase with proper TMB/CTEABr molar ratios. The presence of 1,2,3-TMB was the most favored to form p6mm hexagonal mesophase, and 1,2,4-TMB showed an intermediate behavior. 

In addition, nanocomposite materials of silica-based mesoporous molecular sieve, MCM-41 with conducting COPANI ofpolyfmethyl aniline) inside the channels (COPANI/S-MCM-41) have been also prepared and their nanocomposite formations have been confirmed through N -adsorption isotherm, SEM and TEM measurement (see Figure 14.18a-d). However, the suspension of COPANI/S-MCM-41 shows decreased ER properties than those of PANI/MCM-41 or PANI alone, because the COPANI/S-MCM-41 has lower conductivity (see Figure 14.18e) [100]. 

Zhou, W. and Klinowski, J. 1998. The mechanism of channel formation in the mesoporous molecular sieve MCM-41. Chem. Phys. Lett. 292 207-212. 

This article describes the preparation, properties (including characterization techniques), and potential applications of mesoporous materials. The article begins by examining the chemistry of surfactant/inorganic precursor solutions and discusses its application to the synthesis of mesoporous materials. The most common methods of characterization and the properties of the materials are described and finally potential applications are mentioned. This article is therefore intended to provide a general overview of the synthesis, formation mechanisms, characterization, properties, and applications of mesoporous molecular sieves. 

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