Acid-base properties pure oxides

The present chapter reviews the results of an investigation devoted to the study of the photodecarboxylation of ethanoic acid adsorbed over various solids. A series of pure insulator and semiconductor oxides covering a wide range of acid-base properties and a series of mixed insulator-insulator and insulator-semiconductor oxides of various composition were used for performing reactivity studies in a continuous photoreactor working in gas-solid regime. The nature of the species adsorbed over the various solids was monitored by IR spectra. 

In the tert-butylation of phenol it is found that hydrotalcites are more active than the pure oxides, MgO and y-Al203. The formation of the 0-alkylated products on these basic catalysts indicate the presence of weak acidic sites. It was also quite interesting to observe the formation of the alkenyl phenols along with the normal products whose mass differed by 2 units from the normal alkyl phenols. The distribution of the products varied with reaction temperature and the acid-base properties of the cat ysts. 

Purely diffuse-layer models have become quite popular since the database for hydrous ferric oxide has been published by Dzombak and Morel [76]. The model calculations by these authors have shown that it is possible to describe a wide range of experimental sorption data within a relatively simple model framework. However, the description of acid-base properties of hydrous ferric oxide with this model is not convincing. Substantial failures with respect to true predictions can therefore be expected whenever dynamic systems involving the transport of protons are considered and variations of pH are possible (Lutzenkirchen et al., in preparation). Nevertheless, for conditions in which this is not the case (i.e., buffered systems), the database is very useful but should not be used in the context of mechanistic discussions. 

Thus, in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

A practical method of modification of polysaccharides by clean oxidation using H2O2 as oxidant and cheap iron phthalocyanine as catalyst has been developed. Since no acids, bases or buffers and no chlorinated compounds were used, a pure product can be recovered without additional treatment. Importantly, this flexible method provides materials with a wide range of DScho and DScooh just by an appropriate choice of the reaction conditions. Oxidized polysaccharides thus obtained possess various, tailormade hydrophihc/hydrophobic properties which have been tested successfully in cosmetic and other apphcations. 

In Fig. 2 we have represented both the r Acetone values and the total site density (nj) as a function of catalyst composition. Qualitatively, the variation of r Acetone with increasing A1 content is similar to that followed by nj thereby suggesting that acetone conversion depends on both acid and base sites. Pure MgO was the most active catalyst whereas AI2O3 showed the lowest activity. This is because Al-0 pairs are much less active than Mg-0 pairs for promoting the proton abstraction and carbanion stabilization steps involved in aldol condensation reactions. We have showed [1] that the acetone aldolization rate is controlled on basic catalysts by the number of metal cation-oxygen anion surface pairs. Mg-rich Mg AlOx oxides are less active than MgO because they exhibit a lower base site density and also poor acidic properties. In contrast, Al-rich Mg AlOx oxides are more active than AI2O3 due to a proper combination of acid and base sites. 

As already noted, the most relevant aspects of the chemical reactivity of the higher binary rare earth oxides, i.e, ceria, praseodymia and terbia, deal with their redox properties. Typical acid-base processes, like hydration and carbonation phenomena, very important for the sesquioxides, have much less significance in the case of the higher oxides. Except for praseodymia [188,189], the information available about the intemction of the higher binary oxides, particularly ceria, with CO2 and H2O, under the usual experimental conditions, suggests the occurrence of purely surface processes [190]. Accordingly, this section of the chapter will be mainly devoted to the redox chemistry of the higher rare earth oxides. 

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