Mobil Lube Dewaxing by Hydrocracking

Furthermore, the products from n-decane cracking were exclusively unbranched. In contrast, cracking the same feed over amorphous silica-alumina or the calcium form of the zeolite gave branched products (Table 10.1). This implied that not only was there selectivity for the shape of molecules that could access the reactive site, but also for those that departed. The eventual finding for 

FIGURE 10.1 Key illustrations of zeolite shape selectivity in chemical reactions. 

Catalytic Percent Decane lso- n-Butane lso- n-Pentane Products, 

Source P. B. Weise and V. J. Frilette, Intracrystalline and Molecular-Shape-Selective Catalysis by Zeolite Salts, Journal of Physical Chemistry 64 382 (1960). With permission. 

Zeolites, as previously described, are crystalline microporous solids with well-defined structures made up of interlocking microporous Si04 and A104. Microporous means that the pores have dimensions of less than 20 A, on the order of the size of many petroleum-related molecules, and their crystalline nature means that they have a narrow pore distribution (mesoporous materials have pore sizes between 20 A and 500 A, macroporous materials have pores larger than 500 A). This combination of features not only restricts the size of molecules that can enter the pores, but also the dimensions of the transition state and of the molecules that can successfully leave. For these reasons, zeolites have been termed shape selective.  

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