Effect of Dealumination on Acidic Properties

The Si/Al ratio of zeolites can be modified by removing aluminum from the framework. The dealumination can be achieved in various ways treatment with acids, steam or EDTA. The dealumination process can be expressed by the following.  

The aluminum defects (III) in the above scheme created by dealumination may be eliminated by silicon species from the zeolite structure of amorphous silica contained in the material. The dealumination can be achieved also by the reaction with silicon tetrachloride. In this case, no vacancies are formed since aluminum in the structure is directly substituted by silicon. Since the thermal stability of zeolites increases with increasing Si/Al ratio, zeolites become thermally more stable after dealumination. In the case of Y-zeolites, the stabilized zeolites are called ultra-stable zeolites. 

Since the number of acidic OH groups depends on the number of aluminum in the framework, dealumination might be expected to reduce the catalytic activity of zeolites. In some cases, however, the activity is enhanced by dealumination. As described above, the acid strength of zeolites depends on the Si/Al ratio. Thus, if the effect of increase in the acid strength surpasses the effect of the decrease in the acidic centers, dealumination results in enhancement of catalytic activity. 

Lago et also studied the effect of steaming on the catalytic activity of H — ZSM-5 for hexane cracking. The catalytic activity of H — ZSM-5 for hexane cracking was 

Strictly proportional to its aluminum content, when carefully prepared in the absence of water vapor. While severe steaming reduces the catalytic activity, mild steaming produces catalysts of hi er activity. They inferred that paired aluminum atoms in the framework are required for enhanced acidic centers, which have 45 — 75 times higher specific activity than normal acid sites. 

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