The Bronsted Acidity of Protonic Zeolites

Most data agree that when the A1 content is relatively low, the amount of Bronsted sites in zeolites actually strictly depends on A1 concentrahon, according to theory. The ratio between catalytically active sites and A1 ions ranges apparently from 80 to 100% for highly siliceous extraframework-species-free zeolites. 

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis the Use of Hindered Basic Probes Catalytically active sites also exist on the external surface and at the pore mouth of zeolite crystals. These sites are considered to be responsible for unwanted non-selective catalysis. On the other hand, H-zeoUtes also catalyze reactions of molecules that do not enter the cavities because of their larger size. So, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk and surface Si/Al compositions of a zeolite could be different and different preparation procedures can be chosen to modify this ratio. 

The external surfaces of H-FER [143-145], H-MFl [146, 147] and H-MOR [147, 148] have been studied by IR spectroscopy of adsorbed hindered aliphatic nitriles, pyridine [145, 146], lutidine and aromatic hydrocarbons [149]. Terminal silanols and Lewis acid sites exist at the external surface of H-FER, H-MFl and H-MOR. Interestingly, the acidity of the external silanol OH groups of zeolites can be enhanced with respect to those of silica, appearing similar to those of SA. The very 

DPPN = 2,2-diphenylpropionitrile py = pyridine 2,6-lut = 2,6-dimethylpyridine, 2,6-lutidine 2,6,ditbpy = 2,6, di-tert-butyl-pyridine MA = methylamine DMA = dimethyl amine TMA = trimethylamine pX, mX, oX = para-, meta- and ortho-xylene. 

Zeolite catalysts are frequently applied after treatments that tend to increase their stability and also to further enhance surface acidity and shape selectivity effects. These treatments, such as steam dealumination, can cause a decrease in the framework A1 content and the release of aluminum-containing species from the framework. This can contribute to the stability of the framework, but extraframework species can also contain additional catalyticaUy active acid sites. These particles can also narrow the size of the zeolite charmels or of their mouths, so improving the shape selectivity effects. Extra-framework material (EF) can also 

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