Showing papers on "MCM-41 published in 1996"

Study of catalysts comprising heteropoly acid H3PW12O40 supported on MCM-41 molecular sieve and amorphous silica

Abstract: With the aim of obtaining pure Keggin-type heteropoly acid (HPA) species on siliceous surfaces, the preparation of HPA catalysts, comprising of 10 to 50 wt.% H3PW12O40 (PW) supported on an amorphous silica or a mesoporous all-silica molecular sieve MCM-41, was studied. The state of HPA on the siliceous surface was characterized by 31P NMR, XRD and TEM. Impregnating MCM-41 or amorphous SiO2 with an aqueous solution of PW gave catalysts with, in general, two HPA species: one with intact Keggin structure (A) and the other with a different structure (B), supposedly, H6P2W18O62 (P2W18) and/or H6P2W21O71 (P2W21). The relative amount of species A and B depends on HPA loading, with A dominating. At higher loadings, 30–50 wt.%, A is practically the only one present on the surface; at lower loadings, both species exist, the amount of B increasing as the HPA loading decreases. In contrast, catalysts prepared by impregnation with a methanol solution of HPA contained exclusively Keggin-type A over the whole range of PW loading. In the PWMCM-41 catalysts, as shown by TEM, the PW species are mainly located inside the MCM-41 pores. The B species was about 8 times as active as A in the liquid-phase trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol.

Tubules-Within-a-Tubule Hierarchical Order of Mesoporous Molecular Sieves in MCM-41

Abstract: The recently discovered mesoporous aluminosilicate MCM-41 consists of hexagonal arrays of nanometer-sized cylindrical pores. It is shown that this material can be synthesized by cooperative condensation of silicate and cylindrical cationic micelles. Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 micrometers in diameter. The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher order structure may take place through a liquid crystal phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. The hierarchical organization of this "tubules-within-a-tubule" particle texture is similar to that of the frustules of marine diatoms.

Synthesis of mesoporous molecular sieves: influence of aluminum source on Al incorporation in MCM-41

Abstract: Three series of mesoporous aluminosilicate molecular sieves, MCM-41, with various Si/Al ratios were synthesized using different aluminum sources (aluminum isopropoxide, pseudo boehmite and aluminum sulfate). XRD analysis, temperature programmed desorption ofn-butylamine,27Al and29Si MAS NMR, and catalytic alkylation test indicated that aluminum isopropoxide is a better source for incorporating aluminum in the framework of MCM-41 type molecular sieves with better crystallinity and acid characteristics.

Mechanical Stability of Pure Silica Mesoporous MCM-41 by Nitrogen Adsorption and Small-Angle X-ray Diffraction Measurements

Abstract: We report experimental data on the mechanical stability of the silica mesoporous material MCM-41 Using X-ray diffraction and nitrogen adsorption we show that the ordered mesoporous structure of MCM-41 can be affected considerably by mechanical compression at pressures as low as 86 MPa and essentially destroyed at 224 MPa

Synthesis and characterization of the gallosilicate mesoporous molecular sieve mcm-41

Abstract: A range of mesoporous gallosilicate molecular sieves with the MCM-41 structure have been synthesized using gallium nitrate as the source of gallium and characterized in detail by elemental analysis, XRD, 71Ga and 29Si MAS NMR, N2 adsorption measurements, and TEM. The quality of the products is very sensitive to the pH of the gel mixture. Atomic absorption and XRD show that Ga and Si in the gel are incorporated into the MCM-41 structure in nearly equal proportions. 71Ga and 29Si NMR demonstrate that in all as-made samples gallium is 4-coordinate and forms a part of the MCM-41 structure. Although upon calcination gallium is partially expelled from the structure of samples with Si/Ga ≤ 20, Ga in samples with Si/Ga ≥ 30 is unaffected. N2 adsorption measurements and TEM demonstrate the high mesoporosity of gallosilicate MCM-41-30. The pore size distribution indicates that the structure of sample MCM-41-20 partially collapses upon calcination and macropores are formed.

Disintegration of Mesoporous Structures of MCM-41 and MCM-48 in Water

Abstract: It has been found that mesoporous structures of MCM-41 and MCM-48 disintegrate readily in distilled water around 370 K, while the structures can be stable in 100%-steam of 1 atmospheric pressure at much higher temperatures around 820 K. Thus, the structure disintegration is thermodynamically more favorable in water than under the steaming condition. X-ray powder diffraction and magic angle spinning 29Si NMR spectroscopy indicate that the disintegration of the mesoporous structures in water occurs due to silicate hydrolysis.

Immobilization of the heteropoly acid (HPA) H4SiW12O40 (SiW12) on mesoporous molecular sieves (HMS and MCM-41) and their catalytic behavior

Abstract: Supported catalysts, consisting of SiW12 immobilized on hexagonal mesoporous silica (HMS) and its aluminum-substituted derivative (MCM-41) with different loadings and calcination temperatures, have been prepared and characterized by X-ray diffraction, FT-IR and NH3-temperature programmed desorption. It is shown that SiW12 retains the Keggin structure on the mesoporous molecular sieves and no HPA crystal phase is developed, even at SiW12 loadings as high as 50 wt%. In the esterification of acetic acid byn-butanol, supported catalysts exhibit a higher catalytic activity and stability and held some promise of practical application. In addition, experimental results indicate that the loaded amount of SiW12 and the calcination temperatures have a significant influence on the catalytic activity, and the existence of aluminum has also an effect on the properties of supported catalysts.

Synthesis and characterization of highly ordered MCM-41 in an alkali-free system and its catalytic activity

Abstract: MCM-41 mesoporous molecular sieve materials are synthesised using aqueous ammonia solution to adjust the pH of the reactant gel. Highly ordered MCM-41 with Si/Al ratio as low as 14 was obtained and characterised by27A1 MAS NMR, XRD, N2-adsorption, benzene sorption, and NH3-TPD measurements. The acidity of MCM-41 materials obtained in this system was conveniently generated through straightforward calcination of the as-synthesised sample. More mild acidic sites generated could be due to the avoidance of the multiple calcination procedure and/or the trace sodium species which are the poisons to Bronsted acid. The catalytic activities forn-heptane cracking and isomerization ofm-xylene were investigated, and these were in accordance with the known properties of MCM-41.

Synthesis and characterization of the mesoporous galloaluminosilicate molecular sieve MCM-41

Abstract: A range of mesoporous galloaluminosilicate molecular sieves with the MCM-41 structure and different compositions have been synthesized using gallium and aluminium nitrates as the source of Ga and Al and characterized using atomic absorption, powder X-ray diffraction (XRD) and 71Ga and 27Al magic-angle spinning (MAS) NMR. Ga, Al and Si are incorporated into the MCM-41 structure in proportions that clearly reflect the composition of the synthesis gel. NMR demonstrates that in all as-made and calcined samples gallium is four-coordinate and forms a part of the MCM-41 structure. However, 7% of Al is expelled from the structure of [120,1,3]-MCM-41 upon calcination.

Synthesis and characterisation by X-ray absorption spectroscopy of a suite of seven mesoporous catalysts containing metal ions in framework sites

Abstract: MCM-41 type mesoporous silicas have been prepared in which one or more of the following elements are accommodated in framework sites: titanium, iron, chromium, vanadium, manganese, boron and aluminium. XRD and FTIR are used as aids to characterisation, which is achieved chiefly — and to a degree that arrives at valence states, bond lengths and coordination numbers of the metal ion — by X-ray absorption spectroscopy (XAS). Ti-containing MCM-41, as well as the Fe-, V- and Cr-containing variants, yield self-consistent, XAS-based, structural data of the respective metal-ion sites. Some of these (especially those containing Ti) are exceptionally good catalysts for the selective oxidation of large organic molecules such as limonene and norbornene.

Synthesis and properties of boron containing MCM-41☆

Abstract: B-MCM-41 is a boron containing mesoporous material. It was synthesized in the system SiO2/B2O3/H2O/template using micell like species of quarternary ammonium salts, N(CH3)3R+X−, with different chain lengths of R as templates. In the as synthesized material boron is tetrahedrally coordinated, whereas in the calcined form boron partly changes the coordination to trigonal planar. B-MCM-41 possesses high porosities with a relatively narrow pore size distribution. However, the order of the silicate framework is poor and shows only local order without periodicity in the silicate framework structure.

Solid-state NMR studies of the borosilicate mesoporous molecular sieve MCM-41

Abstract: A series of boron-substituted mesoporous MCM-41 sieves has been synthesized with a wide ranging content of boron (Si/B ratio from 40 to 5) and has been studied by XRD, 11B and 29Si solid-state NMR. Boron enters the silicate framework and there are two different 4-coordinate boron sites in the as-prepared products. The interplanar d100 spacing varies from 36 to 40 A depending on the Si/B ratio. There is no significant contraction of the unit cell upon calcination, but some 4-coordinate boron is converted into the less stable 3-coordination. The 3-coordinate boron can be fully reconverted to 4-coordination by equilibrating the sample with the water vapour over a saturated aqueous solution of NH4Cl at room temperature. When the sample is immersed in water for 3 days at 50 °C, some 3-coordinate boron is removed from the framework. The 4-coordinate B is removed upon further treatment of the hydrated calcined sample with water vapour at 150 °C.

Synthesis, Characterization, and Catalytic Activity of Ti‐MCM‐41 Structures.

Abstract: Ti-containing MCM-41 mesoporous materials with regular 35 A pore diameters have been obtained by direct synthesis. The OH/SiO2 ratio and alkali metal cation content are important variables to prepare good materials. By means of IR spectroscopy, XRD, N2 and Ar adsorption, thermal analysis, DRUV and EXAFS-XANES techniques, it has been shown that samples with MCM-41 structure were prepared, Ti is in framework positions and no extraframework TiO2 was detected. The materials prepared are selective catalysts for oxidation of different types of olefins. The intrinsic activity of Ti-MCM-41 is lower than Ti-Beta and TS-1 when using H2O2 as oxidant. When using tertbutyl hydroperoxide the activity of Ti-MCM-41 is closer to that of Ti-Beta and much higher than for TS-1. Ti-MCM-41 has a clear advantage over the other Ti zeolites in the oxidation of large organic molecules of interest for production of fine chemicals.

Immobilization of Cobalt Complexes on Mesoporous MCM-41 Support Materials

Abstract: The pore surface of mesoporous MCM-41 type materials have been functionalized with silanes to yield surface bound chelate ligands that include ethylenediamine (ED), diethylenetriamine (DET) and ethylenediaminetriacetic acid salt (EDT). Additionally, The grafted DET ligand was reacted with salicylaldehyde to generate the supported tetradentate Schiff base ligand designated as DES. The MCM-41 bound ligands were used to prepare a series of covalently attached cobalt(II) complexes that potential as oxygen carriers and catalysts. These materials were characterized by FTIR and cyclic voltammetry as well as elemental analysis.

Odd-even effect in the synthesis of mesoporous silicate molecular sieves in the presence of alkyl cetyl dimethyl ammonium bromide

Abstract: New quaternary ammonium salts with the formula, (C16H33)(CnH2n+1)(CH3)2N+Br (n =1 to 12) have been synthesized and used as templates to prepare crystalline mesoporous molecular sieves The effect of the CnH2n+1 chain length on the nature of the obtained product and its d-distance has been investigated A correlation between the nature of the obtained phases and the conformation of the carbon chain of the occluded surfactant was revealed by13C CP MAS NMR

Influence of the Synthesis Parameters on Textural and Structural Properties of MCM-41 Mesoporous Materials

Abstract: MCM-41 mesoporous materials were synthesized under different hydrothermal conditions using organic templates, i.e. CTMA-Br, DTMA-Br. Reaction temperature (120 to 195 °C) and reaction time (24 to 120 hr) varied and their influence on the structure properties of MCM-41 was sought i.e. d = 36 to 41.6 A for distinct micelles diameters. Also, insertion of polyaromatics into the surfactant micelles was determinant in the final pore diameter, i.e d = 35.4 to 48.6 A.

Composite molecular sieve comprising MCM-41 with intraporous ZSM-5 structures

Abstract: The development of new improved fluid catalytic cracking (FCC) catalysts is a great challenge. An important aspect of FCC architecture is a gradual decreasing accessibility of active sites. In general catalyst particles contain macroporosity with progressively smaller pores going via mesopores towards zeolitic micropores. Recently Mobil workers developed the mesoporous molecular sieve MCM-41, a member of the M41S family. The flexibility of MCM-41 in tuning its unidimensional pores in the range of 15-100 Angstroem and its Si/Al ratio between 10 and infinity, makes this new mesoporous alumino-silicate at first sight an excellent starting material. The authors explored a new route in preparing composite materials with both mesopores and micropores. This paper reports on a tetrapropylammonium hydroxide treatment of alumino-silicate MCM-41 leading to a composite molecular sieve consisting of MCM-41 with ZSM-5 structures. These materials are active in hexane cracking at mild conditions.

Synthesis and Characterization of the Mesoporous Galloaluminosilicate Molecular Sieve MCM‐41.

Abstract: A range of mesoporous galloaluminosilicate molecular sieves with the MCM-41 structure and different compositions have been synthesized using gallium and aluminium nitrates as the source of Ga and Al and characterized using atomic absorption, powder X-ray diffraction (XRD) and 71Ga and 27Al magic-angle spinning (MAS) NMR. Ga, Al and Si are incorporated into the MCM-41 structure in proportions that clearly reflect the composition of the synthesis gel. NMR demonstrates that in all as-made and calcined samples gallium is four-coordinate and forms a part of the MCM-41 structure. However, 7% of Al is expelled from the structure of [120,1,3]-MCM-41 upon calcination.

Synthesis and catalytic properties of cobalt- and molybdenum-containing mesoporous MCM-41 molecular sieves

Abstract: Mesoporous MCM-41 type silicas containing molybdenum and cobalt have been prepared with pore sizes in the range 30–38 A and 54–59 A. Catalytic properties of these materials have been examined with respect to the oxidation of cyclooctene and aniline.