Increasing the ethanol concentration in the tetraethoxysilane (TEOS)-hexadecyltrimethylammmonium bromide (C 16 TMABr)-ammonia-water system at room temperature permits one to obtain a succession of different mesophases in the order MCM-41 f MCM-48 f lamellar phase f radial hexagonally ordered phase. First, the original hexagonal (MCM-41) phase is replaced by a cubic phase (MCM-48) and later, upon ethanol addition, by a lamellar phase. Such phase succession is the result of the cosurfactant behavior of the ethanol. At lower alcohol concentration, scanning electron microscopy (SEM) shows only undefined or barely spherical structures, indicating that the ethanol has only a limited effect on the external morphology. When the alcohol concentration is further increased, it will mainly act as a cosolvent producing spherical particles. A TEM study reveals the radial arrangement of the pores within the spherical particles. A hexagonal closed pore packing can be considered on a local scale around the center of the spherical particle. This hexagonal pore arrangement is the result of a combination of a very slow equilibrium toward the hydrolysis of the TEOS, its good homogenization in the synthesis solution due to the solvating effect of the alcohol, and the interference of the alcohol on the cooperative process of the micelle formation. To complete the study, parallels have been drawn with other alcohols such as methanol and propanol.
The molecular designed dispersion method is used to prepare supported transition metal oxide catalysts
(Mo, Cu, V). The corresponding transition metal acetylacetonate complexes were reacted with silica or
alumina, and subsequently thermally converted into the supported metal oxide. The deposition of these
acetylacetonate complexes depends on the geometry and stability of the complex, the support properties
and the synthesis procedure. By thorough control of the reaction parameters, this novel synthesis method
enables the creation of uniform and highly dispersed supported transition metal oxides. Therefore, the
molecular designed dispersion method plays an important role in the development of a new generation
of catalysts. This paper is forwarded to the special issue of the Langmuir journal, devoted to the ISSHAC-3
symposium.
Chromium acetyl acetonate [Cr(acac) 3 ] complexes have been grafted onto the surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMCM-41) and Al-containing silica MCM-41 with an Si:Al ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N 2 adsorption, thermogravimetrical analysis , diffuse reflectance spectroscopy in the UV-Vis-NIR region (DRS), electron spin resonance (ESR) and Fourier transform infrared spectroscopy. Hydrogen bonding between surface hydroxyls and the acetylacetonate (acac) ligands is the only type of interaction between [Cr-(acac) 3 ] complexes and SiMCM-41, while the deposition of [Cr(acac) 3 ] onto the surface of AlMCM-41 takes place through either a ligand exchange reaction or a hydrogen-bonding mechanism. In the as-synthesized materials, Cr 3 is present as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from O h symmetry. After calcination, Cr 3 is almost completely oxidized to Cr 6 , which is anchored onto the surface as dichromate, some chro-mate and traces of small amorphous Cr 2 O 3 clusters and square pyramidal Cr 5 ions. These materials are active in the gas-phase and slurry-phase polymer-ization of ethylene at 100 8C. The poly-merization activity is dependent on the Cr loading, precalcination temperature and the support characteristics; a 1 wt % [Cr(acac) 3 ]-AlMCM-41 catalyst pre-treated at high temperatures was found to be the most active material with a polymerization rate of 14 000 g poly-ethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr 6/5 and the oxidation and coordination environment of Cr n species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nano-fibres of polyethylene with a length of several microns and a diameter of 50 to 100 nanometers. These nanofibres (partially) cover the outer surface of the MCM-41 material. The catalyst particles also gradually break up during ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index between 0.56 and 1.38 g per 10 min; this indicates the very high molecular weight of the polymer .
A new synthesis pathway for MCM-48 is developed using fumed silica as the silicate source and a Gemini
surfactant (GEM 16-12-16) as the surface directing agent. Different synthesis parameters have been optimized
in order to obtain a highly ordered mesostructure. The determining factor in the stabilization of the final
structure is proven to be a double post-synthesis hydrothermal treatment. This treatment, performed successively
in a minimum of pure water at 403 K, reduces the pH of the gel, allowing a better condensation of the silicate
structure. Other parameters of importance are the OH-/Si ratio, the gelation time, and the synthesis temperature,
influencing the solubility of the fumed silica, the condensation and the structuring of the silicate framework,
and the pore diameter, respectively. The synthesis temperature has also a positive effect on the particle
dimension, producing uniform grains of 4−5 μm. The optimized parameters (3-day gelation time followed
by two consecutive post-synthesis hydrothermal treatments) give a highly reproducible synthesis, with a high
silica yield (70−75%). The synthesized samples have been characterized using XRD, N2 sorption isotherms,
SEM, and infrared (DRIFT) measurements.
Hexagonal MCM-41 can be transformed into cubic MCM-48 and finally into spherical
particles by the addition of alcohol during the synthesis of a mesoporous silica material.
X-ray diffraction suggests that the structure of these spherical particles is of the MCM-41
type. Transmission electron microscopy however reveals that the structure of the
mesoporous silica spherical particles consists of a core in the form of a truncated
octahedron with an MCM-48 cubic structure and radial pores grown on the surfaces of the
truncated octahedron. Spherical MCM particles therefore consist of a mixture of cubic and
hexagonally arranged pores.
Silica-supported molybdenum oxide catalysts have been prepared by liquid and gas phase deposition, followed by calcination of the deposited molybdenyl acetylacetonato complex. Fourier-transform infrared spectroscopy indicated a hydrogen bonding anchorage mechanism for the liquid phase deposition and a two step reaction mechanism for the gas phase deposition. After calcination of the absorbed molybdenum complexes, the supported molybdenum oxides were characterized by combining Fourier-transform infrared and Raman spectroscopy. X-ray di †raction was used to probe the possible clustering towards crystallites. An evaluation of the molecular designed dispersion method has been made by comparing the deposited molybdena structures obtained by the designed dispersion of with catalysts prepared by the conventional impregnation MoO 2 (acac) 2 method using ammonium heptamolybdate. It is concluded that the molecular designed dispersion method results in a better grafting (more SiÈOÈMo bonds) and thus a stronger metal oxideÈsupport interaction than the conventional impregnation methods.
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