Catalytic Cracking and the Catalysts

Weisz and Miale compared the activity for hexane cracking of a number of zeolites (Table 4.25) with a highly active silica — alumina (10% alumina). The zeolites are at least 10 times as active as amorphous silica —alumina. The catalytic process, however, cannot utilize the activity from a pure zeolite catalyst. The catalyst must be modified to decrease the acid-strength to avoid excessive formation of coke and low molecular weight gases, at the expense of gasoline. Moreover, the catalyst must be able to withstand the thermal and hydrothermal conditions experienced in regeneration. It must also withstand breakup in the mechanical circulation systems. A detailed description of the preparation of industrial catalysts is found in the literature. 

Structure of zeolite Cations in base exchanging solution a 

Currently, only Y-type zeolites are of any commercial importance as cracking catalysts. In many cases, rare earth ions are incorporated into Y-type zeolites. So-called ultrastable forms of Y-zeolites are also used. These may be prepared by extracting some of the aluminum from the zeolite framework. The ultrastable Y-zeolites can retain their crystal form at temperatures as high as 1200 K. 

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