Etienne F. Vansant

Surface modification of silica gels with aminoorganosilanes

Abstract The surface modification reaction of silica gel with aminoorganosilanes proceeds in two steps. For both the reaction step and the curing step, the chemical and physical interactions of the silane molecules with the silica surface have been modelled. From ethanol leaching tests, the reaction phase interaction, in dry conditions, may be characterized as 22% proton transfer, 10% hydrogen bonding and 68% siloxane bonding. The deposition of the aminosilane molecules is governed by the specific surface area and surface hydration. For the bifunctional N-β-aminoethyl-γ-aminopropyltrimethoxysilane, a two-step deposition is observed. The rate of siloxane bonding in the curing phase is limited by the number of alkoxy groups, and results in a turnover of the aminosilane molecules. A new application of aminosilane-modified silica gel is developed in converting the aminosilane layer to SiC. Thus, the liquid phase path of the chemical surface coating process, for the controlled synthesis of advanced ceramics, is set up.

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.

Enhanced Brönsted acidity created upon Al-grafting of porous clay heterostructures ia aluminium acetylacetonate adsorption

Montmorillonite-PCH (porous clay heterostructure) and saponite-PCH are inorganic silica-like materials with large surface areas (∽1000 m2 g−1) and pore volumes (0.7 cm3 g−1). They are obtained by introducing a surfactant template in-between the clay plates and subsequently polymerizing a silica source around them. The acidity of porous clay heterostructures has been improved. Based on the ‘ molecular designed dispersion’ method, the Al-grafting onto the PCH surface using aluminium acetylacetonate complex is carried out, involving interaction between the surface silanol groups of the PCH-support and the complex, followed by a calcination step. In this way, stable Bronsted acid sites are created, remaining even after thermal treatment as high as 300°C. Evaluation of this acidity has been performed by NH3 and acetonitrile-d3 (CD3CN) adsorptions.

The effect of water on the structure of supported vanadium oxide structures: an FT-RAMAN, in situ DRIFT and in situ UV-VIS diffuse reflectance study

A promising way to create supported metal oxide catalysts consists of the irreversible adsorption and subsequent thermolysis of metal acetylacetonate complexes. Spectroscopic techniques are often used in the evaluation and assessment of the final catalytic surface structure. In this paper, the use of FT-RAMAN, in situ UV-VIS diffuse reflectance DRS and in situ diffuse reflectance infrared spectroscopy is discussed in the evaluation of silica supported VOx structures. It is argued that water drastically influences the surface structure of these catalysts. Its effects are noticeable in the stages of pretreatment of the support, the actual synthesis and in the post-synthesis storage of the catalyst

Surface probing of synthetic faujasites by adsorption of carbon dioxide. Part 2.—Infra-red study of carbon dioxide adsorbed on × zeolites exchanged with mono- and bi-valent ions

Adsorption of carbon dioxide in X-type zeolites exchanged with alkali and alkaline earth metal cations reveals bands in the infra-red spectra due to CO2 in a “bent” configuration. The spectra are interpreted in terms of carboxylate and carbonate species. Lattice oxygens of the type O1 seem to be involved. The small number of these species fails to correlate with any structural site. When the amount Ca2+ ions in the samples is high enough to occupy sites in both the small and the supercages, new structural species are formed.

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.

Preparation of supported vanadium oxide catalysts: adsorption and thermolysis of vanadyl acetylacetonate on a silica support

Adsorption and subsequent thermolysis of vanadylacetylacetonate [VO(acac)2] on a silica support yields supported V—O structures. The initial concentrations of the complex, the drying temperature and the calcination are the factors with the highest impact on the nature of the vanadium oxide coating. Too high initial concentrations of the complex induce coalescence and clustering of the adsorbed product. During the drying or curing step, important rearrangements occur in the adsorbed layer, creating additional covalent bonds between the complex and the substrate. Calcination is accompanied by thermolysis: a conversion of the supported VO(acac)2 towards supported vanadium oxide. The evolved products were identified and a conversion mechanism is suggested. The thermolysis of adsorbed VO(acac)2 is a fast and effective way to create supported vanadium oxide catalysts with a high surface area.

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.