Mechanism of paraffin cracking

Krannila, H., Haag, W.O., and Gates, B.C. (1992) Monomolecular and bimolecular mechanisms of paraffin cracking n-butane cracking catalyzed by HZSM-5./. Catal, 135, 115-124. 

Haag, W.O., Dessau, R.M., and Lago, R.M. (1991) Kinetics and mechanism of paraffin cracking with zeolite catalysts. Stud. Surf Sd. Catal., vol. 60, Elsevier, Amsterdam, pp. 255-265. 

Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts... 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

T he expansion of the petrochemical industry and the accompanying increase in the demand for ethylene, propylene, and butadiene has resulted in renewed interest and research into the pyrolytic reactions of hydrocarbons. Much of this activity has involved paraffin pyrolysis for two reasons saturates make up most of any steam cracker feed and since the pioneering work of Rice 40 years ago, the basic features of paraffin cracking mechanisms have been known (1). The emergence of gas chromatography as a major analytical tool in the past 15 years has made it possible to confirm the basic utility of Rice s hypotheses (see, for example, Ref. 2). 

The resulting equation was found empirically by E. B. Burk (3) and M. D. Tilicheev (22). The above-studied mechanism of the cracking of n-paraffins explains the absence of dependence of the cracking rate on reaction extent. This differs from the case of the low molecular weight paraffins experimentally determined by Kasanskaya (6) and Panchenkov and Baranov (19) for n-octane and n-hexadecane. 

It should be remembered that the above reaction is also the first step in the mechanism for the alkylation of paraffins by olefins as proposed by Schmerling (9). As a matter of fact, the mechanism proposed by Schmerling for the alkylation reaction applies equally well, in reverse, for the mechanism of catalytic cracking of paraffins. 

The mechanism of paraffin hydrocracking over bifunctional catalysts is, essentially, the carbenium ion chemistry of acid cracking coupled with metal-centered dehydrogenation/hydrogenation reactions. The presence of excess hydrogen and the hydrogenation component of the catalyst result in hydrogenated products and inhibition of some of the secondary reactions and coke formation. 

The first three carbon depositing reactions involve only methane and carbon monoxide, and are of particular interest in plants with methane feeds. These reactions can also occur in reformers with higher molecular weight feedstocks, since the reactants exist a short distance into the reformer tube, even when no methane exists in the feed. As mentioned in Reference 23, The mechanism of thermal cracking (pyrolysis) of higher hydrocarbons is more complicated. Reference 13 highlights the relative risk of carbon laydown from several paraffinic, aromatic, and olefinic compounds. 

Theory. Porous adsorbent catalysts of the silica alumina type are widely used. The fact that one or more hydrous metallic oxides are present in all successful catalysts suggests that water in some way is important in this type of catalysis. Nevertheless, completely satisfactory explanations of the mechanism of catalytic cracking have not been presented. Greensfelder and associates have studied the cracking of dozens of paraffinic, naphthenic, and aromatic hydrocarbons. They find that... 

The Parak process for the recycling of polyolefins and the production of raw materials for the production of paraffin waxes, is described. The process is claimed to provide a link between mechanical and feedstock recycling, employing elements of feedstock recycling, e.g. melting and cracking. The main product obtained is paraffin wax, which can be used for coatings for cardboard and paper, and corrosion protection. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Haag, W.G. and Dessau, R.M. (1984) Duality of mechanism in add catalyzed paraffin cracking, in Proceedings of the Eighth International Congress on Catalysis, vol. 2, Verlag Chemie, Weinheim, p. 305. 

Sie, S.T. (1993) Acid-catalyzed cracking of paraffinic hydrocarbons 2. Evidence for protonated cyclopropane mechanism from catalytic cracking experiments. Ind. Eng. Chem. Res., 32, 397. 

A mechanism that postulates prevention of paraffin formation during gas oil cracking with the dual zeolite catalyst can explain the above data. Such a prevention could take place by more than one route. ZSM-5 present in the catalyst could prevent certain secondary reactions that lead to the formation of gasoline range... 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

Sundaram, K.M. and G.F. Fioment, Modeling of Thermal Cracking kinetics. 3. Radical Mechanisms for the Pyrolysis of Simple Paraffins, Olefins, and Their Mixtures., Ind. and Eng. Chem. Fund., 17,174—182,1978. 

Mechanism of Hydropyrolysis of -Paraffins. The hydrogen carbon ratio in the total hydropyrolysis product from 1 is higher (H C > 2.2) than that in 1 itself (H C = 2.12) in all experiments performed (Tables II and III). This clearly indicates participation of hydrogen in the process. To account for the observed differences between hydropyrolysis and conventional thermal cracking (Table I), and to rationalize the variations in product composition as a function of reaction conditions,... 

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