Silico Lewis acidity

The specific surface areas vary between 0.6 and 11.7 m /g. The main impurities of the AI2O3 samples are CaO, Si02, Na20 and MgO. The eventual combination of these elements on the alumina surface is not known. The values of 7, determined at 100°C are in the 90-100 mj/m range and there is no evident correlation between 7 whatever the impurities contents. However, it is known, particularly in catalysis, that the association of AI2O3 and Si02 leads to silico-aluminates of increased Lewis acidity. To check this, we evaluated an acid base parameter, which is simply the ratio of the retention volumes of a basic probe, i.e. diethyl ether, and of an acidic probe, i.e. dichloroethane ... 

Silico)aluminophosphate-molecular sieves are interesting materials as catalysts for the condensation of phenoi with carbonyl compounds, presumably because of the low acidity and the spacious constraints, decreasing catalyst deactivation. Especially ALPO-5, having a very low acid strength, gives good yields in the reaction of phenol with formaldehyde towards dihydroxydiphenylmethanes. In case of the phenol/isobutanai reaction, mainly monosubstituted products are obtained shape selectivity seems to retard a second condensation step. Ortho-substitution prevails, possibly caused by a contribution of Lewis-acid catalysis. 

One of the driving forces behind recent developments in the synthesis and characterization of new acids is the requirement to replace mineral acids by reagents and catalysts that will lead to reduction in waste, in other words to an improved atom economy. Thus, HCl used in the synthesis of diamino diphenyl methane, an intermediate in polyurethane production, can be replaced by a silico-aluminate catalyst in which the acid sites of the zeolite are accessible through the external surface. Less conventionally, simple aromatic hydrocarbons can be nitrated using 60-70% nitric acid in the presence of the strong Lewis acids, lanthanide-perfluoroalkane sulfonic acid salts, such as the ytterbium(lll) compounds (VI) and... 

Fripiat, J. J., A. Leonard, and J. B. Uytterhoeven, 1965. Structure and properties of amorphous silico aluminas. II. Lewis and Bronsted acid sites. J. Phys. Chem. 69 3274-3279. 

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