N-heptane cracking

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Marques, J.P., Gener, L, Lopes, J.M., Ramoa Ribeiro, F., and Guisnet, M. (2005) n-Heptane cracking on dealuminated HBEA zeolites. 

Figure 6. Comparison between H-Beta zeolites (open circles and dashed lines) and HY zeolites (continuous lines) for n-heptane cracking at different Si/Al ratios.

H-Beta are more active for n-heptane cracking than HY-zeolites with the same Si/Al ratio, while for gas-oil cracking the opposite occurs. Nevertheless we have not looked here to mesoporosity of the zeolites which may be critical for explaining the activity in gasoil cracking. Steaming causes a smaller decreases in activity of H-Beta than on HY zeolite. H-Beta gives a lower ratio than... 

Hopkins (161) found that a steady decrease in n-heptane cracking activity occurred over La- and Ca-exchanged Y zeolites as the catalyst calcination temperature was increased from 350° to 650°C. The lanthanum form was about twice as active as the calcium form. Reduction in activity with increasing activation temperature was attributed to removal of acidic framework hydroxyl sites as dehydration becomes more extensive. The greater activity of La—Y with respect to the calcium form was thought to result from the greater hydrolysis tendency of lanthanum ion, which would require more extensive dehydration to result in the same concentration of acidic OH groups as found on Ca—Y. 

Results obtained by Hopkins (161) substantiated Benesi s ammonia poisoning observations. The activities for n-hexane and n-heptane cracking... 

Results from Figure 2, show that sample U1F-25 is about 30% less active than sample USY-1 for n-heptane cracking. This result clearly shows that the sites associated to framework A1 are not the only ones responsible for the cracking activity, and also that the differences in gasoil activity given in Figure la are related with the presence of the EFAL. 

Cruz et al. (36) also found that AFS preferentially dealumi-nates the surface of zeolite Y crystals to a depth of about 100 A and produces an extra-framework-Al-free zeolite. They found that the activity for n-heptane cracking is the same as for a steamed dealuminated sample, while the gas oil cracking was lower, being controlled by the outer shell. 

Olefins resulting from n-heptane cracking (step 1) are transformed through various reactions (oligomerization, cyclization, hydrogen transfer etc...) into soluble coke molecules sterically blocked in the cavities or at channel intersections (step 2), The same reactions transform soluble coke molecules into non soluble molecules (step 3) that overflow onto the outer surface of the zeolite crystallites. Non soluble coke molecules could also overflow in the mesopores created during zeolite... 

Figure 3 A Isomerization of n-hexane over PtHMOR catalysts31 and of dichlorobenzene over HMOR catalyst 2. B n-heptane cracking over HFAU samples41 dealuminated by steaming (i.e. with a large amount of extraframework Al species (a) followed by acid treatment, i.e. with a low amount of these species (b). Initial activity Ao vs.Xai. ...

Figure 2. Formation of coke during n-heptane cracking at 450 C over a HUSY zeolite. Amounts of soluble coke families with formulae CnH2n-26 (A), CnH2n-32 (B),CnH2n-36 (C) and of insoluble coke (I) as functions of total coke content...

Figure 4. n-Heptane cracking at 450 C over various protonic zeolites. Deactivating effect of coke molecules. Residual activity Ar versus the number of coke molecules nK (10 0 molecules g"l -Reproduced with permission from ref 9. 

Figure 5. n-Heptane cracking at 450 C over various protonic zeolites. Relative decrease in activity (1-AR) as a function of the ratio of the number of coke molecules to the number of strong acid sites (nK/nA2). Experimental values USHY (A) HZSM-5 ( ) Straight lines with slope = 1, 2, 3 HMOR ( ) HERI ( ) slope = 10,20. Reproduced with permission from ref.9. 

Figure 3 Coke formation during n-heptane cracking at 350°C over dealuminated HY zeolites.

Distribution of the main families of coke components (wt %) during n-heptane cracking at 450°C over HZSM5 as a fimction of total coke content. 

Alkenes resulting from n-heptane cracking (Step 1) are transformed into soluble coke molecules which are trapped in cages and pores. Soluble coke molecules are transformed into... 

Figure 5 Change in the concentration of soluble ( ) and insoluble (O) coke molecules (n, lO V ) as a fimction of the percentage of coke formed during n-heptane cracking at 450°C on HY (a) and on HZSM5 (b).

Figure 7 Formation and growth of coke molecules in the pores of HOFF during n-heptane cracking at 450°C (a) naphthalene, (b) methylphenanthrene and (c) methylchrysene.

---