Acid-base pairs, alumina surfaces

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Basic sites and acid-base pairs on alumina surfaces... 

The published data concerning the structural, surface, and catalytic properties of aluminas are reviewed, and these properties are related to the preparation procedures. The experimental and computational investigations of the structural characteristics of the polymorphs most useful for applications in catalysis, which are y- q-, 8- and 0-AI2O3, are critically analyzed. The thermodynamics of the various polymorphs and the kinetics of the phase transitions are considered. The available information on Bronsted sites (i.e., hydroxyl groups), Lewis acid sites, and acid-base pairs on... 

There are not many industrial appHcations of alumina itself as a catalyst, but some of them are very important. It is remarkable that most applications are assumed to require the Lewis acidity or the acid—base pairs of alumina, but are performed in the presence of water at moderate temperatures, implying that the surface is still largely hydroxylated. 

This reaction (Equation 3.3), which occurs at 250—300 °C with almost total yield (187), might find renewed interest in the future to convert bioethanol produced by fermentation into bioethylene (375,376) in the frame of a new industrial organic chemistry based on renewables. Ethanol dehydration has also been used recently as a test reaction for the investigation of the surface properties of aluminas (187,377—379). At low conversions, ethanol can be converted into diethylether with high selectivitiy. IR spectra show that ethanol adsorbs in the form of ethoxy groups, which are formed either by dissociation on Lewis acid—base pairs or by substitution of hydroxyl groups (187). 

Alumina with neutral, basic and acidic (both Lewis type) surfaces was introduced into PE0ME-LiC104 electrolytes of various concentrations and examined with a variety of techniques. At higher salt concentrations when aggregation of ions occurs, fillers led not only to a decrease of the viscosity, but also to an increase in the fraction of free ions. The best results were obtained in the case of acidic fillers (Fig. 2.7). This results from interaction of acidic surface states with anions (Lewis bases) present in electrolyte as free ions, ion pairs, triplets, etc. The latter especially affect the viscosity and cation transference number. With the use of acidic filler, the fraction of free ions was increased as confirmed by Fuoss-Kraus calculations, as well as by Fourier transform infrared (FT-IR) investigations (Marcinek et al. 2005). The increase in the fraction of free ions means an increase of the number... 

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