Determination of the acidity of high surface AlF3 by IR spectroscopy of adsorbed CO probe molecules

The surface of highly Lewis acidic high surface AlF3, HS-AlF3, prepared in different ways was investigated by FrIR spectroscopy of adsorbed CO and compared to the previously known and well-characterized beta-AlF3. The majority of adsorbed CO shows large blue shifts up to more than 80 cm(-1) of the band of the CO stretching frequency, corresponding to very strong Lewis acidic adsorption sites. In contrast to that behavior, most of the CO adsorbed on beta-AlF3 shows a smaller blue shift of around 40 cm(-1). These results can explain the very high acidity of HS-AlF3 and the differences in catalytic activity between both samples. The influence of water is also investigated. Adsorption of water at room temperature blocks the Lewis acid sites and generates Bronsted acidity. Heating the sample under a certain partial pressure of water followed by degassing results in a significant loss of acidity. Both phenomena were confirmed by IR spectroscopy.