Bronsted thermal analysis

Pyridine adsorption at room temperature on activated cloverite evidences the presence of Lewis and Bronsted acid sites. Pyridinium species hardly persists after evacuation at 423 K, showing that the Bronsted acidity is not very strong. Spectra analysis confirms that pyridinium species occur from the interaction with P-OH groups since the 3673 cm- and 944 cm- bands are regenerated by heating at the expense of the pyridinium species. The 3700 cm- v(OH) band hardly reappears by thermal evacuation suggesting that an irreversible reaction occurs during this treatment. Note that pyridine adsorption on phosphated alumina leads to protonation (8) due to interaction with free POH hydroxyls. 

Reactive amines other than ammonia have also been employed to distinguish Bronsted acid sites in various zeolites using a combination of TPD and thermo-gravimetric analysis (TGA) techniques [135]. The method is based on the fact that surface Bronsted sites may induce thermal decomposition of aliphatic amines to alkenes and ammonia over a narrow temperature range. The number of amine molecules reacted is equated to the number of strong Bronsted acid sites. By choosing amines of appropriate sizes it is possible to discriminate between acid sites located in pores of different diameters. 

In general, when results obtained from titration methods are compared with the catalytic activity, the correlation is not necessarily good. One of the reasons is that only a small fraction of the total acid sites measured by titration are active for a given catalytic reaction. The low temperature at which amine adsorption is measured with the use of color indicators favors adsorption on aU sites, including the weakest ones. Thermal desorption methods may discriminate between sites of different strength but are unable to distinguish between Lewis and Bronsted sites. When coupled with vibrational spectroscopy data, the TPD technique may, indeed, be adequate for analysis of surface acidity [136]. However, it is almost... 

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