Silylation of the silica surface : a review

Silylated silica surfaces have found many applications in the field of analytical chemistry (HPLC, Ion Exchange Chromatography, Size Exclusion Chromatography, GC), synthetic chemistry (heterogeneous catalysts, phase transfer catalysts), biochemistry (enzyme immobilization, affinity chromatography) and industries (composites, high-tech materials, semiconductor devices). In all cases, the knowledge of their chemical composition and surface characteristics is of great importance for the understanding and eventual improvement of their performance. This review presents a general description of the silica surface and a summary of the different modification techniques that have been developped to silylate oxide surfaces. The chlorosilylation of the silica surface (in liquid and gaseous phase) and the modification with aminosilanes are discussed in more detail, emphasizing the analysis techniques and skills that enable researchers to get a more profound insight into the reaction mechanisms and the nature and concentration of the created surface groups

Octane hydroisomerization over hexagonal mesoporous aluminosilicates synthesized from leached saponite

A thorough investigation is performed on the catalytic activity of aluminosilicate FSM's, produced via a new successful short time synthesis route using leached saponite and low concentration of CTAB. The influence of an additional Al incorporation, utilizing sodium aluminate, aluminum nitrate, and aluminum isopropoxide is also studied. The catalytic activity of the samples is tested in the hydroisomerization of octane. All the samples have a lower catalytic activity compared to the BETA zeolite, but, the combined selectivity to isomers and aromatics is better for the original acidified FSMSap H+ sample and the sample with additional incorporation of Al(NO3)(3), which indicates to a certain extent, the suitability of these materials as reforming catalysts

Modification of the silica surface with aminosilanes

The liquid-phase silylation of silica with various silanes is discussed. In each case, the reaction mechanism involved is more complex than often believed. Many reactions can occur simultaneously and the resulting surface layer depends largely on the synthesis conditions. Special attention should be given to the role of water in the synthesis. It can occur as physisorbed molecules on the substrate prior to modification, but it may be involved in the reaction mixture itself or even as humidity during the post-reaction curing step. In each case, the water molecules have an enormous impact on the modification reactions, causing a polymerization of the silane molecules and resulting in a thick but irreproducible and irregular surface layer

The creation of silicon nitride coatings by successive gas-phase reactions with trichlorosilane and ammonia : chemical surface coating : a new route towards thin ceramic layers on silica

The technique of Chemical Surface Coating consists of subsequent, consecutive reactions of different reagents with the surface of a substrate. This article presents an overview of the creation of a stable Si-N coating, by means of a reaction cycle of trichlorosilane and ammonia with the surface of silica gel. The reactions involved are much more complex than often believed. Upon trichlorosilylation of tile silica, at least 5 different surface species are created; after ammoniation, at least 6 different surface species are present on tile silica surface. These species have been characterized by FTIR, Si-29 CP MAS NMR and by XPS. At 840 degrees C, more than 60% of the nitrogen functions, present on the surface, are in the form of nitrides

Pore structure characterisation of pillared clays using a modified MP method

The data of nitrogen adsorption on pillared clays (PILC) are converted to comparison plots (t-plots) to derive their pore size distribution (PSD). As in the MP method, the surface area of a group of pores having similar pore sizes is calculated from the slopes of tangent lines at two succeeding points on a comparison plot. By the modified MP method in this work, the tangent line is extrapolated to the adsorption axis on the t-plot, and the difference between intercepts is used to obtain the volume of the group of pores. From the information of surface area and pore volume, the average width of the pore group can be calculated and hence the PSDs of PILCs are obtained by carrying out such calculation procedures from high to low t. With this method, PSDs of several pillared clays are calculated over a wide pore size range, from micropores to mesopores. It is found that the modified MP method could result in the underestimation of the width of ultramicropores due to the enhancement in adsorption energy in these pores. Nevertheless, the method can be very useful in calculating the surface area and pore volume, as well as a mean width of these pores. For super-micropores and mesopores, pore size can also be underestimated, due to deviation of the pore shape from a slit. The principles of the improved MP method, as well as problems associated with it are thoroughly discussed in this paper. In general, this modified method provides practically meaningful results which are consistent with the pore dimension obtained from powder X-ray diffraction measurements, but involves no complicated theoretical treatment or assumptions

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 degrees C. These hydrophobic MCM-48 supported VOx catalysts are thermally stable up to 500 degrees 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 degrees 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 structural stability of these materials