O. Collart

Synthesis, Spectroscopy and Catalysis of [Cr(acac)3] Complexes Grafted onto MCM‐41 Materials: Formation of Polyethylene Nanofibres within Mesoporous Crystalline Aluminosilicates

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, N2 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, Cr3+ 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 Oh symmetry. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface as dichromate, some chromate and traces of small amorphous Cr2O3 clusters and square pyramidal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 °C. The polymerization activity is dependent on the Cr loading, precalcination temperature and the support characteristics; a 1 wt % [Cr(acac)3]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a polymerization rate of 14 000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr6+/5+ and the oxidation and coordination environment of Crn+ species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nanofibres 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.

Structure of nanoscale mesoporous silica spheres

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.

Spectroscopic characterization of an MoOx layer on the surface of silica. An evaluation of the molecular designed dispersion method

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 diffraction 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 MoO2(acac)2 with catalysts prepared by the conventional impregnation method using ammonium heptamolybdate. It is concluded that the molecular designed dispersion method results in a better grafting (more Si–O–Mo bonds) and thus a stronger metal oxide–support interaction than the conventional impregnation methods.

Reproducible synthesis of high quality MCM-48 by extraction and recuperation of the gemini surfactant

High quality MCM-48 can be repeatedly and reproducibly prepared, as a novel method for the extraction, recuperation and re-use of the gemini surfactant has been optimized. Gemini surfactants have the general formula [CnH2n+1N+(CH3)2(CH2)sN+(CH3)2CmH2m+1]·2Br, and have been shown to produce MCM-48 in a very convenient way. In each cycle, more than 95 wt% of the surfactant is extracted under mild conditions. The extracted surfactant does not degenerate during this treatment and can be re-used many times. The MCM-48 produced by this soft extraction, has a narrower mesoporous pore size distribution and a larger pore radius than materials prepared by normal temperature-programmed calcination. The surface of the extracted MCM-48 still contains sufficient silanol groups, although some silanol groups have been replaced by methoxy groups. The number of silanols on the surface of the MCM-48 has been titrated by surface reaction with hexamethyldisilazane, followed by quantification of the liberated NH3.

The Adsorption of VO(acac)2 on a Mesoporous Silica Support by Liquid Phase and Gas Phase Modification to Prepare Supported Vanadium Oxide Catalysts

Supported vanadium oxide catalysts are prepared by adsorption and subsequent calcination of the vanadyl acetylacetonate complex on silica by liquid phase and gas phase modification. The influence of the pretreatment temperature and the effect of the solvent in the liquid phase are discussed. Two types of gas phase deposition processes are used: flow-type reactions and vacuum deposition. The bonding mechanism, the influence of pretreatment temperature of the support and the influence of the reaction temperature are investigated by FTIR, XRD, TGA and chemical analysis. After calcination the obtained vanadium oxide layer is characterized by XRD and UV-Vis diffuse reflectance spectroscopy. The gas phase modification enables the creation of well dispersed supported VOx catalysts. Loadings up to 1.4 mmol g-1 (7 wt% V) without the formation of a crystalline fraction can be achieved. The selective oxidation of methanol to formaldehyde is used as a probe reaction to assess the catalytic activity and selectivity of the catalysts. It is shown that not the concentration of vanadium species, but their surface configuration is the determining factor in catalytic reactions.

Mechanical Strength of Micelle-Templated Silicas (MTS)

Abstract The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed.

A1-modified porous clay heterostructures with combined micro and mesoporosity

In this contribution, the synthesis and characterization of a new type of Al-grafted porous clay heterostructure (PCH) is discussed. The support PCH consists of small saponite clay plates, bound by a templated silica matrix. This 3-dimensional solid has a unique pore structure, with a high BET surface area (999 m2/g), microporosity (0.307 cc/g) and mesoporosity (0.634 cc/g). Based on the ‘Molecular Designed Dispersion’ method, a grafting of the PCH surface with aluminium oxide species is performed. The reaction involves the interaction between the aluminium acetylacetonate Al(acac)3 complex and the silanol surface groups of the PCH, followed by a temperature treatment. The acidic properties of the Al-modified PCH are evaluated by the adsorption of acetonitrile-d3 (CD3CN), revealing a strong Bronsted acidity.

29-O-04-A controlled dispersion of Al3+ onto a silica mesoporous material A comparative study with Al3+ incorporation

Publisher Summary This chapter describes a controlled dispersion of Al 3+ onto a silica mesoporous material. A pure silica MCM-48 is activated by a controlled dispersion of Al onto the substrate surface performed by the anchoring of the Al(acac) 3 complex onto the surface silanols. A calcinations step removes the organic ligands. The characteristics of the final structure are evaluated by comparison with Al incorporated MCM-48. The type and concentration of Lewis/BrOnsted acid sites are also investigated by the adsorption of ammonia (NH 3 ) and acetonitrile-d3 (CD 3 CN).