P. Van Der Voort

Silylation of micro-, meso- and non-porous oxides: a review

Changing the surface characteristics of silicon or mixed Si-Al oxides has created new prospects in both catalysis and separation technology. This review presents the surface modification of these materials by silylation over a wide range of pore sizes. The influence on the porosity, the adsorption characteristics and thermodynamic background of these modifications are summarised. The reaction mechanisms are reviewed for a wide range of silylation reagents.

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.

Plugged hexagonal templated silica: a unique micro- and mesoporous composite material with internal silica nanocapsulesElectronic supplementary information (ESI) available: Fig. S1: X-ray diffractogram of a PHTS material. Fig. S2: TEM images of SBA-15 and PHTS-2. Fig. S3: hydrothermal stabilities. See http://www.rsc.org/suppdata/cc/b2/b201424f/

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro- and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures.

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

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.

The synthesis of stable, hydrophobic MCM-48/VOx catalysts, using alkylchlorosilanes as coupling agents for the molecular designed dispersion of VO(acac)2

The use of dimethyldichlorosilane as a coupling agent for the grafting of VOx structures on the MCM-48 surface produces a material that is simultaneously hydrophobic (immiscible with water) and very active (all V centers are accessible, even for water molecules). The VOx surface species are grafted by the molecular designed dispersion of VO(acac)2 on the silylated surface, followed by a calcination in air at 450 °C. These hydrophobic MCM-48 supported VOx catalysts are thermally stable up to 500 °C. The grafted VOx surface species are very resistant toward leaching-out in aqueous media. Also, the structural and hydrothermal stability has improved enormously. The crystallinity of the materials does not decrease when the samples are subjected to a hydrothermal treatment at 150 °C and 4.7 atm pressure. A reaction mechanism is proposed and consolidated by FT-IR, Raman spectroscopy, and UV−vis diffuse reflectance. Pore size distributions, water adsorption isotherms, and X-ray diffractograms confirm the structur...

Synthesis of stable and directly usable hexagonal mesoporous silica by efficient amine extraction in acidified water

The amine template of hexagonal mesoporous silica (HMS) can be efficiently recovered and re-used by a new extraction procedure in acidified water rendering a directly usable high quality mesoporous support.

A new strategy towards ultra stable mesoporous titania with nanosized anatase walls

A new and generally applicable synthesis procedure is developed in order to synthesise micelle-templated mesoporous titania built up of nanosized anatase walls with thermal stability up to 600 degrees C.

Reaction of NH3 with trichlorosilylated silica gel: a study of the reaction mechanism as a function of temperature

Activation of the silica substrate with chlorosilanes increases the ammonia uptake by a factor of 5–10, compared with untreated silica. The reaction mechanism of ammonia with trichlorosilylated silica gel is examined in detail. Physisorbed NH4Cl causes a shift in the Si—NH2 stretching vibrations. Several side reactions occur during the chlorosilylation step, the ammoniation step and the thermal treatment of the reacted sample. Amine functions subsequently convert to silazanes and eventually nitrides. A novel mechanism for silazane formation is proposed. The different species on the silica surface are quantified as a function of the reaction temperature.

Gas-phase chlorosilylation of silica gel: effectiveness, surface coverage and stoichiometry

The five different reaction mechanisms for the gas-phase chlorosilylation of silica have been studied. A comprehensive, unified approach to the problem, based on effectiveness, surface coverage and stoichiometry of silylation is presented. It is proven that all five reactions do occur, but each mechanism has a preference for a rather limited region of pretreatment and reaction temperatures.