Oligomerization and Cracking

When facile oligomerization and cracking do occur over ZSM-5 zeolite, the overall result is establishment of an olefin quasi-equilibrium (ref. 9). 

A self-consistent explanation of all the chemistry can be developed on the basis of Rri6nsted acidity of the zeolite, proton transfer and electrophilic methylation reactions, and the well known rearrangement, oligomerization and cracking reactions of carbenium ions. 

Figure 14 In situ C MAS NMR study of the reactions of methyl iodide- C on zeolite CsX. Methyl iodide (-15 ppm) was partially converted to a framework-bound methoxy (58 ppm) upon adsorption at 298 K. Ethylene (120 ppm) formed between 448 and 498 K, and oligomerized and cracked to a mixture of hydrocarbons at 523 K. denotes spinning sideband.

It has been shown in the literature that isobutane (Pc = 36.5 bar, = 408 K)/butene (Pc= 40.2 bar, Tcf= 420 K) alkylation on solid acid catalysts at supercritical temperatures suffers from increased butene oligomerization and cracking reactions at these temperatures, increasing the catalyst deactivation potential [16-18]. Lower temperatures tend to favor the alkylation reaction. Supercritical operation at 95°C can be facilitated by diluting the isopar-affin/olefin feed with suitable amounts of a low inert solvent such as CO2 (Pc = 73.8 bar. Pc = 304 K), and has been shown to give rise to steady alkylation activity on USY and beta zeolites [19]. However, the alkylate yields are very low (< 10%) on these catalysts, attributed to severe pore diffusion limitations on these catalysts. 

The distribution of product formed at 557 K differs considerably from that obtained at 462 K. First, a broader distribution of carbon number was obtained, indicating the rapid oligomerization and cracking of oligomer. Second, few of the C -products were double-branched, C7 —Cg alkenes being produced in a relatively shape selective manner. The product distribution of propene oligomerization was examined in detail also by Quann et al ... 

The distribution of C2-C6 alkene-products is close to equilibrium in the sub-atmospheric conversion of C2, C3, Cs, Ce, or Cio alkene at 544 K. This again indicates the very rapid oligomerization and cracking of oligomers. 

A hydrocarbon pool mechanism via alkylation/dealkylation of hydrocarbon scaffolds. Olefins interconversion via methylation, oligomerization, and cracking. 

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