The Chemistry of Catalytic Cracking

The field of catalytic cracking is reviewed. Cracking uses the largest tonnage of commercial catalysts. It produces the largest volume of products. It has been instrumental in creating the most varied, richest, and most influential of our industries. It is therefore not surprising that a great deal is known about cracking catalysts and catalytic cracking. What is surprising is that a coherent picture of the chemistry of this process is only now beginning to emerge. In brief, the picture is as follows. Catalytic cracking occurs on acid catalysts containing strong Bronsted and Lewis sites. It is accompanied by a vast variety of other processes such as skeletal isomerization, cyclization, aromatization, disproportionation, and dehydrogenation. Each of these processes occurs on sites of appropriate strength and nature and not on other sites which may be present. The weak sites catalyze reactions such as cis-trans isomerization, stronger sites catalyze double-bond isomerization, followed in turn by skeletal isomerization, cracking, and coke formation on sites of increasing strength. Further complications are introduced by the size, tortuosity, and surface configuration of the pores present in crystalline cracking catalysts. Appreciation of the significance of these effects seems to be new to the field of heterogeneous catalysis. Itmore » now seems certain that they have enormous potential to affect selectivity and to introduce specificity to the list of important properties of heterogeneous commercial catalysts. 365 references, 25 figures, 22 tables.« less