Alkanes cracking of, 164

The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. These are clarified heavy distillates from the catalytic cracking of gas oils. About 95% of U.S. feedstock use is decant oil. Another source of feedstock is ethylene process tars obtained as the heavy byproducts from the production of ethylene by steam cracking of alkanes, naphthas, and gas oils. There is a wide use of these feedstocks in European production. European and Asian operations also use significant quantities of coal tars, creosote oils, and anthracene oils, the distillates from the high temperature coking of coal. European feedstock sources are 50% decant oils and 50% ethylene tars and creosote oils. 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

Protonated cyclopropane ring closure and ring opening steps are discussed in Section 13.8.1 within the context of alkene skeletal siomerization. Carbonium ion decomposition is further discussed in Section 13.8.4.2 within the context of mono-molecular cracking of alkanes. 

Bhan, A., Gounder, R., Macht, J., and Iglesia, E. (2008) Entropy considerations in monomolecular cracking of alkanes on acidic zeolites. J. Catal., 253, 221-224. 

Less information is available about the cracking of alkanes. Three sources [212,232,233] confirm that, in the series of straight-chain alkanes, the rate increases with the molecular weight. The data could be correlated by the Taft equation when the molecule was divided into the reaction centre and such substituents as R—CH2—CH3 [125]. The reactivity of hexane isomers was studied at 550°C on an A1203—Si02— Zr02... 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

On the other hand, a remarkable difference between catalysis by Y and 13 zeolites has been found for the Claisen-Sohmidt condensation of acetophenone and benzaldehyde (Table 5). When the cross aldolic reaction is carried out in the presence of HY, together with the expected trans and ois chalcones 5, the 3,3-diphenylpropiophenone 6 is also formed, this product being not detected on 13 zeolites. A likely explanation for the absence of 6 using zeolite beta is that the crystalline structure of this zeolite exerte a spatial constraint making difficult the formation of a big size molecule like 6, especially in the smaller channel. Similar effects due steno limitations on 6 catalysis have been found for the formation of multi-branched products during the cracking of alkanes (ref 8). 

As a model for cracking of alkanes, the reaction of 2-methylpentane (16, 2MeP) over SbF5-intercalated graphite has been studied in a flow system, with the hydrocarbon being diluted in a hydrogen stream.104,105 A careful study of the product... 

Finally, it may be possible to use the rate of a chemical reaction to determine the actual number of active acidic sites. Furthermore, it may be possible to obtain an estimate of the range of strengths of acid sites by using different reactant molecules. For example, it is known that dehydration of alcohols requires only relatively weak acid sites, whereas cracking of alkanes requires very strong acid sites. 

A simplified reaction network for the cracking of alkanes on zeolite catalysts is presented in Fig. 2.7. 

The cracking of alkanes initially produces unsaturated, low molecular weight byproducts which can polymerise and, through coke formation, cause a rapid loss of catalyst activity. This problem can be minimised by using a zeolite with low cokeforming tendencies, e.g. ZSM-S, or by incorporation of a hydrogenation function. 

The narrowness of the zeolite pores also influences their catalytic nature. A faujasite in the K" form, even though it is not acidic, catalyzes the cracking of alkanes. The product distribution in cracking is consistent with free-radical intermediates and the induced homolytic rupture of C—H or C—C bonds. The activity arises from the strong electric flelds of the ions in the confining pores. 

H C Beimaert, J R Alleman, G B Marin. A fundamental kinetic model for the catalytic cracking of alkanes on a USY-zeolite in the presence of coke formation. Ind. Eng. Chem. Res. 40, 1337-1347,2001. 

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