T h e primary cracking of pure hydrocarbons both with and without catalysts has been studied in terms of the distribution by carbon number of the cracked fragments to allow arriving a t a mechanism of molecular disintegration. The secondary reactions of the cracked fragments have been followed by analyses of the product fractions to allow a further definition of the nature of the cracking system. On the basis of this work, cracking systems are assigned to two fundamental classes; each class is described by a set of characteristic reactions covering both the primary cracking and the secendary reactions. Correspondingly, two types of reaction mechanisms are proposed, one a free radical (thermal type) mechanism based on the Rice-Kossialroff theory of cracking, the other a carbonium ion (acid-activated type) mechanism RIOR work on the catalytic cracking of pure hydrocarbons P has led to a general characterization of the rates of cracking and product distributions of the principal classes of petroleum hydrocarbons (10-13). I n addition, a number of secondary reactions of olefins have been investigated and the effects of structural isomerism on the rates of cracking of several types of hydrocarbons were examined (9, 54). Consistent mechanisms of reaction are now proposed, based on the primary hypothesis that any hydrocarbon reacting over this type of catalyst is transformed into a carbonium ion (33, which then cracks or undergoes secondary reactions according to definite rules. This hypothesis is directly coupled with the requirement that the acidic oxide type of cracking catalyst must make available reactive positive hydrogen ions (protons) capable of producing carbonium ions on contact with the hydrocarbon feed. A similar type of approach was proposed independently by Thomas (52).The properties of carbonium ions, which are postulated to represent the reactive form of the hydrocarbon in conventional catalytic cracking, also determine the mechanism of reaction and the type of product in many other acid-catalyzed hydrocarbon reactions, such a8 the isomerization, polymerization, parafKn alkylation, and hydrogen transfer reactions of olefins, the isomerization of paraffins, and the alkylation of aromatics. Funda-derived from the work of Whitmore and others on the properties of carbonium ion systems. Cracking catalysts are available for either type of reaction mechanism; those which accelerate free radical type reactions are nonacidic, and those which accelerate carbonium ion type reactions are acidic. Commercial acid-treated clay and synthetic silica-alumina cracking catalysts belong to the latter class. Activated carbon, a highly active, nonacidic catalyst, gives a unique product distribution which is explained as a quenched free radical type of cracking. Activated pure alumina has weakly acidic properties and produces moderate catalysis of both types of reaction mechanism, the primary cracking corresponding to a free radical mechanism and the secondary reactions of product olefins following a carbonium ion mechanism. me...

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Cracking of eleven naphthenes containing 6 to 18 carbon atoms was studied over a silica-zirconia-alumina catalyst. It was found that naphthenes are quite susceptible to the action of the catalyst and that both the ring and any side chains contribute to the total cracking. The rate of cracking increases rapidly with increased molecular CRACKING of paraffins and olefins over a siiiea-zireoniaalumina catalyst under conditions similar to those employed in the commercial cracking of petroleum fractions was described in previous articles of this series (4, 5). These studies gave a picture of some of the reactions favored by cracking catalysts, with far less ambiguity than would result from observation of the cracking of petroleum fractions comprising a mixture of hydrocarbons. This method has been extended to a study of the naphthenes or alicyclic hydrocarbons, which are prominent constituents in the majority of petroleum fractions employed for cracking. An effort was made to secure a wide variety of naphthenic hydrocarbons, covering carbon numbers (C No. = n in C»Hm) from 6 to 18. These were cracked by the catalyst and procedure previously described (4)} the definitions and terminology are the same.The catalyst, obtained from Universal Oil Products Company, analyzed 86.2% silica, 9.4% zirconia, and 4.3% alumina by weight. This catalyst gives results similar to those obtained with the present commercial silica-alumina cracking catalysts. Properties and sources of hydrocarbons follow, with compounds arranged in the order of increasing molecular weight:Cyclohexane from Eastman Kodak Company was washed with concentrated sulfuric acid to remove aromatics and distilled. The melting point was 6.1 °C., boiling range 80.2-81.2 °C., di°0.7785, w2D°1.4263.Methylcyclopentane was isolated from a California petroleum by repeated fractionation. It had a boiling point of 72.0 °C., dj°0.7488, and n2D°1.4100.

Assuming the cross sectional area of the molecule to be about 20 sq. Á., then ab sin ß must be divided by four to give a value of 19.5 sq. Á. This unit cell, therefore, has eight molecules. Within a few tenths of an ángstróm, the calculated length of two fully extended molecules of hexanolamine oleate, based on analogous work on the saturated fatty acids, sodium soaps and sodium acid soaps, agrees with the value of c sin ß here reported. The value of sin ß is therefore close to unity. By analogy with the sodium salts of the fatty acids, this form may appropriately be called the alpha form.The density, calculated from the unit cell parameters and sin ß of unity, is 1.096 g./cc.-a result in agreement with known data on sodium soaps.Summary 1. Two forms of sodium oleate are distinguished by X-ray powder diagrams.2. The unit cell parameters of a pure sample of one of these forms of sodium oleate are obtained after fibrillation of the sample, produced by extrusion through a small orifice under pressure. The parameters agree with values previously found for the gamma forms of saturated sodium soaps.3. Samples of sodium oleate used in previously published physicochemical investigations in this laboratory are identified by X-ray diffraction.4. The parameters of the unit cell of hexanolamine oleate are obtained after fibrillation of the sample, as already described.