Karl Possemiers

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.

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.

Modelling of the reaction-phase interaction of γ-aminopropyltriethoxysilane with silica

The coating formed in the reaction phase of the modification of silica with γ-aminopropyltriethoxysilane was modelled using the freeze sampling method with a classical N-determination. The formation of a monolayer coating in the reaction phase is confirmed. Within the selected region of silane : silica ratio, the monolayer capacity value was determined to be 1.02 mmol g–1 on dehydrated silica gel and 0.63 mmol g–1 on the dehydroxylated substrate. The obtained monolayer structure is described and related to the specific surface area of the substrate used. APTS molecules are distributed over the silica surface with each molecule covering 64 A2.

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.

Synthesis of a polysilazane layer on silica gel by chemical surface coating

Thermally pretreated silica gel has been modified with SiCl4 and NH3. Repeating this treatment gives rise to a polysilazane layer, chemically bonded to the silica gel surface. Using FTIR, N and Cl determinations and gravimetric analysis, a total characterization of the polysilazane layer was obtained, together with quantification of the different reactions occurring. The parameters of the reaction cycles have been optimized in such a way, that a high density of silazanes (Si—NH—Si) is reached at the silica surface. The influence of the modification process upon the porosity of the silica gel was also studied.

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.

SYNTHESIS OF A COVALENTLY BOUND POLYMERIC BORAZINE STRUCTURE ON A MESOPOROUS SILICA GEL

The successive modification of a well-characterized porous silica gel with BCl 3 and NH 3 has been studied using FTIR spectroscopy with photoacoustic detection and quantitative boron chloride and ammonia analysis of the surfaces formed. A borazine-like, highly cross-linked polymeric boron–nitrogen network, chemically bound to the silica surface, is created after four BCl 3 –NH 3 reaction cycles at low reaction temperatures. Combining IR and elemental analysis results yielded a quantitative depiction of the surface structure and the reaction mechanisms.

Hydroxyl-type specificity of diborane chemisorbed on the surface of silica gel

The reaction of diborane with the surface of silica gel has been investigated by volumetric monitoring of the hydrogen evolved. The chemisorption of diborane features two phenomena. Diborane reveals hydroxyl-type specificity for the free hydroxyl groups on the surface. Diborane reacts quantitatively with all free hydroxyl groups, but, above this free hydroxyl number, chemisorption at bridged hydroxyl groups also occurs. The absolute magnitude of the reacted bridged hydroxyl groups in excess of the amount of free hydroxyl groups is determined by the relative coverage of the two hydroxyl types.

Synthesis of Si-N and B-N Preceramic Coatings on Silica Gel by Chemical Surface Coating: A Porosity Study

Silica gel has been modified by successive gas reactions with (SiCl4-NH3) and (BCl3-NH3) in order to obtain Si-N and B-N preceramic polymers, which are chemically bound to the substrate surface. The effect of the polymer synsthesis on the porosity characteristics of the substrate is determined as a function of the number of applied modification cycles, using N2 adsorption-desorption isotherms recorded at 77 K. The elemental analysis data in combination with the adsorption isotherms yield a more detailed picture of the density and the homogeneity of the coatings. Using the pore size distributions, the contribution of pore blocking and pore narrowing can be calculated as a function of the amount of reaction cycles. A new calculation method for the pore blocking and pore narrowing, which is not based on any pore shape model, is also presented and compared with the former calculation method.