Determination of the Acidity of High Surface AlF₃ by IR Spectroscopy of Adsorbed CO Probe Molecules

Abstract

The surface of highly Lewis acidic high surface AlF3, HS-AlF3, prepared in different ways was investigated by FTIR spectroscopy of adsorbed CO and compared to the previously known and well-characterized β-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 β-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 Brønsted 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.

Sol–gel prepared nanoscopic metal fluorides – a new class of tunable acid–base catalysts

“…The one sample with a high fluorine content, F : Mg molar ratio of 1.6, is designated as to Carbon monoxide, CO, is a very suitable probe molecule for the characterisation of acid surface sites. By interaction of CO with a surface acid site, the linear (CO) stretching vibration is shifted towards higher wave numbers as compared to gaseous CO [5]. The higher this shift, the higher is the acidity of the specific surface site.…”

“…When CO is interacting with OH groups, the position of the linear (CO) stretching vibration is blue-shifted to the range 2150-2170 cm -1 [5], and simultaneously the ν(OH) vibration is redshifted due to the perturbation by CO [31]. A problem with CO adsorption measurements on hydroxylated magnesium fluoride arises from remaining water on the surface.…”

“…Particular properties of these materials are the significantly increased Lewis acidity in comparison to conventionally prepared metal fluorides. For example, high surface area (HS-) AlF 3 is one of the strongest known solid Lewis acids [5][6]. Another interesting example is MgF 2 which usually is a neutral solid.…”

Aluminum Fluoride‐Supported Platinum and Palladium as Highly Efficient Catalysts of n‐Pentane Hydroisomerization

“…Notably, pure HS-AlF 3 after postfluorination exhibits exclusively extremely strong Lewis acid sites. [5] Consequently, the presence of Brønsted sites seems to be surprising. However, owing to its extremely strong Lewis acidity, these sites easily become coordinated by hydrogen donor molecules, for example, H 2 O.…”

“…[1] Those studies were motivated by recent knowledge concerning preparation of nanoscopic aluminum fluoride [2][3][4] characterized by highly specific surfaces, and, remarkably, by outstanding high surface acidity. [5,6] These interesting properties suggest potential applications of HS-AlF 3 in bifunctionally catalyzed reactions, such as hydroisomerization of n-alkanes. Strong acidity of aluminum fluoride has already been combined with hydrogenative properties of gold to create an example of bifunctional catalysts in the reaction of menthol synthesis.…”