Pascal Van Der Voort

Influence of water in the reaction of γ-aminopropyltriethoxysilane with silica gel. A Fourier-transform infrared and cross-polarisation magic-angle-spinning nuclear magnetic resonance study

Silica gel has been modified with γ-aminopropyltriethoxysilane under varying conditions, controlling the influence of water in the different modification stages. Diffuse reflectance infrared Fourier-transform (DRIFT) spectra revealed the influence of surface water in the reaction stage and of air humidity in the curing stage. These results were confirmed and refined by 29Si and 13C cross-polarisation magic-angle-spinning nuclear magnetic resonance (CPMASNMR) spectroscopy. Combining the results of both techniques, four modification structures present on the silica surface are proposed, depending on the conditions used.

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.

Estimation of the distribution of surface hydroxyl groups on silica gel, using chemical modification with trichlorosilane

An estimate of the relative distribution of the free and bridged surface hydroxyl groups is made, using a surface modification with trichlorosilane according to the following main reaction Si—OH + HSiCl3→Si—O—SiHCl2+ HCl. The amount of chemisorbed chloride groups on the substrates is compared with data on the total amount of surface hydroxyl groups and with hydroxyl group information derived from infrared spectra. The interpretation of these data gives an indication of the reactivity, distribution and concentration of the different types of hydroxyl groups on the surface of silica gel.Trichlorosilane reacts mainly with bridged hydroxyl groups on silica gel, pretreated at low temperatures and exclusively with free hydroxyls at high pretreatment temperatures.Extrapolation of the reactivity of trichlorosilane towards the surface hydroxyl groups demonstrates that bridged hydroxyl groups are present up to pretretment temperatures of 973 K.

The Uses of Polynuclear Metal Complexes to Develop Designed Dispersions of Supported Metal Oxides: Part I. Synthesis and Characterization

One way to design a catalyst begins with a consideration of thereaction mechanism to the desired product so that only the chemistryrequired of that mechanism will be present on the surface. The reactionmechanism will suggest the structure(s) to be developed on the surface whichshould be stabilized against changes during operation. We believe that thisideal may be approached by decorating surfaces or porous powders with amonolayer of metal complexes having the desired structures. These complexesmay be partially decomposed to develop a high dispersion of the supportedmetal oxide.

Stabilized MCM-48/VOx catalysts: synthesis, characterization and catalytic activity

Abstract The high synthesis cost, the poor hydrothermal stability of the pore structure and the extensive leaching of the active metal sites in liquid phase heterogeneous catalysis are three of the most important drawbacks for the industrial use of the newly developed mesoporous crystalline materials (MCM-41, MCM-48, HMS, FSM, …). In this study, we present an integrated approach that tackles these three problems simultaneously. High quality MCM-48 is prepared by a method of surfactant extraction and recuperation. The surface of the material is stabilized using a bifunctional silane, rendering the surface essentially hydrophobic, but leaving sufficient secondary anchoring sites for a further activation with VO(acac) 2 and final calcination. The final catalyst is thoroughly characterized and its catalytic and mechanical behavior is evaluated. It is shown that these materials can withstand severe hydrothermal conditions and that the leaching of the catalytic active species in liquid media is very strongly reduced. The obtained catalysts are active and selective and can be regenerated without significant loss of activity.

Siloxane bridges as reactive sites on silica gel. Fourier transform infrared–photoacoustic spectroscopic analysis of the chemisorption of diborane

The reaction of diborane with the silica gel surface has been studied with Fourier transform infrared photoacoustic spectroscopy (FTIR–PAS). After a dissociative adsorption of diborane on the siloxane bridges, borane reacts with the hydroxyl groups of the silica gel surface, evolving hydrogen gas. Besides this general reaction (a), a new reaction (b) is observed, which is competitive with the direct boranation of the hydroxyl groups. The siloxane bridges act as reactive chemisorption sites, resulting in the formation of silane groups. [graphic omitted]

Modelling of the hydroxyl group population using an energetic analysis of the temperature-programmed desorption of pyridine from silica gel

The temperature-programmed desorption (TPD) of pyridine from silica gel has been analysed with the variable heating, constant coverage procedure proposed by Richards and Rees (R. E. Richards and L. V. C. Rees, Zeolites, 1986, 6, 17). This results in curves of activation energies of desorption as a function of coverage. A weak heterogeneity of the silica gel surface is observed in contrast with other pyridine sorption studies. Two adsorption sites, the free and bridged hydroxyl groups, contribute to the surface heterogeneity. The relative amounts of the two sites on the surface are deduced from the calculated activation energy vs. coverage profiles. An attempt is made to extrapolate the absolute concentrations of free and bridged hydroxyls from the relative site coverages.

A 3D-TEM study of the shape of mesopores in SBA-15 and modified SBA-15 materials

The mesopores in SBA-15 are curved on a mesoscopic length scale and even more so in the case of modified SBA-15, which implies that using the particle size of these types of materials in diffusion studies may strongly underestimate the path length relevant for intraparticle diffusion.

Characterization and quantification of the NH3 modification of a BCl3-treated silica gel surface

The reaction of NH3 with a well characterized silica surface containing SiOBCl2 surface species has been studied using FTIR photoacoustic spectroscopy and quantitative boron, chloride and ammonia analysis. The influence of reaction time and temperature on the reaction was studied. Characterization of the reactions occurring and the surface species formed was carried out. Both surface siloxane groups and SiOBCl2 species interact with NH3, resulting in the formation of NH4Cl, chemisorbed amine groups and SiOH groups. Borazine-like surface species are formed at higher temperatures. A quantitative description of the grafted groups has been obtained as a function of reaction temperature.

Total characterization of the BCl3-modified silica surface by means of quantitative surface analysis

The reaction of boron trichloride (BCl3) with a well characterized silica surface has been studied using Fourier-transform IR photoacoustic spectroscopy (FTIR–PAS) and quantitative boron and chloride analyses. Isolated and vicinal silanol groups, in addition to surface siloxane bridges, react with BCl3, resulting in the formation of chemisorbed monodentate or didentate surface groups. The parameters influencing the relative occurrence of these surface species have been investigated. A quantitative description of the grafted groups and reacted silanols has been obtained as a function of the pretreatment temperature of the silica, the reaction temperature and the reaction time.