ISOMERIZATION STEPHEN
F. PERRY, STANDARD
OIL DEVELOPMENT COMPANY, LINDEN, N.
1.
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SOMERIZATION is a contributing reaction in several commercial processes which made news during the year. Both the isomerization of methylcyclopentanes to cyclohexanes (which are dehydrogenated to aromatics) and the isomerization of paraffins take place in the recently developed Platforming process (11). This process, which may be applied to both virgin and thermally cracked naphtha fractions for octane improvement and desulfurization, utilizes a supported platinum catalyst at temperatures of the order of 800”to 900’ F. and pressures of 500 to 900 pounds per square inch in the presence of recirculated, hydrogen-rich gas. The major reactions involved are stated to be dehydrogenation, hydrocracking, and isomerization. The extent of octane improvement possible by paraffin isomerization depends on the nature and amount of paraffins present in the feed stock and is limited by equilibria, which are less favorable at higher temperatures. Commercial production of benzene from petroleum, by a procem comprising the isomerization and dehydrogenation of methylcyclopentane and the dehydrogenation of cyclohexane, has been announeed (23). No process details were revealed. As mentioned in last year’s review (20),it is possible to carry out the isomerization and dehydrogenation reactions in a single step (IO). Although the isomerization equilibrium is not particularly favorable a t the relatively high temperatures required for dehydrogenation, the removal of cyclohexane by dehydrogenation allows isomerization to proceed. In the Oxo process, for the addition of carbon monoxide and hydrogen to olefins to yield aldehydes and alcohols, using cobalt carbonyl as catalyst, isomerization of the olefins by double bond migration ie a factor (19, 14). Alpha olefins undergo the Oxo reaction most readily, and it appears that with other olefins, double bond migration to the alpha position may occur to a large extent prior to the addition (18). Commercial use of the Oxo process has begun (4). Further studies have been made of isomerization accompanying alkylation (92). No additional new commercial applications of isomerization as a unit process have come to our attention during the year, but publication of the results of experimental work on a variety of isomerization reactions has continued.
PARAFFINS In the two previous annual reviews (19,go), only the aluminum halides were mentioned as catalysts for p a r a 5 isomerization. Although apparently unused commercially, boron fluoride is covered extensively in the patent literature as a p a r 5 immerization catalyst, generally promoted by hydrogen fluoride (9). Marked similarities in the operating conditions, reaction products, and the effect of inhibitors indicate that the reaction mechanism with this catalyst is the same as with the aluminum halides (13). Concentrated sulfuric acid in a weak catalyst for para& isomerization; in its presence a rhift of a methyl group along the chain has been noted with molecules having at least one tertiary carbon atom, such as Zmethylpentane, 2,Mimethylpentane and 2,2,4-trimethylpente. Production of more highly branched isomers was slight, however, and no reaction occurred with n+ctane or neohexane (16). In the presence of concentrated sulfuric acid containing deuterium as a tracer, the reactions of
hydrogen exchange, racemization, and isomerization of 3-methylheptane were observed a t relative rates of 30:2: 1(9). The hydrogen exchange reactions of the butanes have been studied as part of a comprehensive investigation of the reactions of hydrocarbons in the presence of acid-type catalysts by means of isotopic tracer methods, with most interesting result8 (I, 24). In the presence of aluminum chloridwn-alumina catalyst, no exchange of hydrogen between the paraffis and the catalyst was detected, but intermolecular exchange of the tertiary hydrogen in isobutane, and also of the secondary hydrogen in nbutane, occurred readily. Intermolecular exchanges of primary hydrogen with secondary or tertiary hydrogen occurred slowly; primary hydrogene did not exchange intermolecularly with each other. Although the activity of the catalyst for isomerization declined markedly during the experiments, ita activity for hydrogen exchange did not vary. In the preeence of concentrated sulfuric acid, it was found that the primary hydrogen of isobutane exchanged with the acid, whereas the tertiary hydrogen did not. Conversely, the tertiary hydrogen underwent intermolecular exchange but the primary hydrogen did not. With n-butane in sulfuric acid, no hydrogen exchange reactions were observed. The above experiments indicata that hydrogen exchange with the catalyst is not necessary for isomerization and that hydrogen exchange can take place without the occurrence of isomerization. An additional paper has been published giving experimentally determined equilibrium values for the isomeric hexanes and comparing them with previous data (6). For a more detailed discussion of paraffin isomerization than has been attempted in these reviews, reference should be made to a chapter by Pines in “Advances in Catalysis” (7).
NAPHTHENES Further work has been reported on the mechanism of the isomerization of methylcyclopentane to cyclohexane, in the presence of aluminum bromide-hydrogen bromide ($1). It was previously shown that under mild conditions, no reaction occurred unless there was present a trace of olefin which could initiate the proposed ionic chain mechanism. It is now shown that irradiation of the reactants with ultraviolet light will cauee the reaction to proceed in the absence of the olefin. Irradiation apparently causes the dissociation of hydrogen bromide resulting in the formation of chain initiators. Small amounte of benzene inhibited the reaction, as in previous experiments.
OLEFINS In a study of the structural isomerization of 1-hexene (17), activated alumina (H-40 grade), phosphoric acid, acid treated Doucil, and Universal Oil Products’ cracking catalyst (Type B ) were the most active catalysts tested. These produced close to equilibrium mixtures of the hexenes, together with some higher and lower boiling hydrocarbons, at temperatures within the range 500” to 900’ F. Equilibrium data are given. The use of sulfur dioxide as a catalyst for cis-trans isomerization of olefins and for double bond shifts was the subject of two , related papers (I26). The mechanism of both reactions is thought to involve the intermediate addition of sulfur dioxide to the double bond and the formation of a biradical. Dienes with a methylene group between two double bonds were isomerized to 1715
INDUSTRIAL AND ENGINEERING CHEMISTRY
1716
conjugated structurea, but when two methylene groups were present between the double bonds (1,Shexadiene) no reaction occurred.
OXYGEN-CONTAINING COMPOUNDS In a reaction similar to that described just above, vinyl acetic acid, having a methylene group between a carbon-carbon double bond and a carboxyl group, shifted its double bond readily in the presence of sulfur dioxide to form crotonic acid ($66). The isomerization of propylene oxide over chromic oxide gel catalyst a t 300”to 350”C. has produced allyl alcohol and propionaldehyde in combined yields up to 95%, with small amounts of acetone and products of siae reactions. Under best conditions allyl alcohol waa the major product (16). The aluminum chloride-catalyzed isomerization of p-cresol has been etudied (6, 8). At atmospheric pressure, the reaction rate conetant waa found proportional to the amount of aluminum chloride present in excess of 1 mole per mole of cresol. The catalyst combined with cresol in a 1:1molal ratio to form an inactive complex, accompanied by liberation of gaseous hydrogen chloride. With a 2: 1 catalyst to cresol ratio, up to 64% yield of m-creeol was obtained. The rate of formation of o-cresol was comparatively low. Under pressure of hydrogen chloride, isomerization was obtained with only 0.51 to 0.54 mole of catalyst per mole of cresol, showing that the formation of catalyst complex was reversible, but the yield of isomers was low and by-product formation high.
LITERATURE CITED (1) Beeck, O., Otvos, J. W., Stevenson, D. P., and Wagner, C. D., J. C h . Phys., 17,418 (April, 1949). (2) . . Boer, J. H., Houtman, J. P. W., and Waterman, H. J., Fats, Oib, and D e k ~ e n i s6, , 441 (1949). (3) Booth, H. S., and Martin, D. R., “Boron Fluoride and Its Derivatives,” New York, John Wiley & Sons,Inc.. 1949. (4) Clark, W., Wurld Petrolam, 20,44 (February 1949). ( 6 ) Dick, H. A., and Kata, W. E., M.S. thesis, Mass. Institute of
Technology (1949).
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Vol. 42, No. 9
Evering, B. L., and d’ouville, E. L., J . Am. Chem. Soc., 71, 440 (1949). (7) Frankenburg, W. G., Komarewsky, V. I., and Rideal, E. K., “Advances in Catalysis,” Vol. I, New York, Academic Preas. Inc., 1948. (8) French, F. E., Jr., D.Sc. thesis, Mass. Institute of Technology (1949). (9) Gordon, G. S., 111, and Burwell, R. L., J. Am. Chem. SOC.,71, 2255 (1949). (IO) Greensfelder, B. S., and Fuller, D. L., Ibid., 67,2171 (1945). (11) Haensel, Vladimir, Petroleum Refiner, 29,131 (April 1950). (12) Holm, M. M., Nagel, R. H., Reichl, E. H., and Vaughan, W. E., U. S. Dept. of Commerce, Wash., D. C., O T S R e p t . PB61383. (13) Hughes, E. C., and Darling, S.M., (preprint) presented before the Division of Petroleum Chemistry, 117th Meeting, AMERICAN CHEMICAL SOCIETY, Houston, Tex. (14) Klopfer, O., paper, Chemisches Institut der Universitat Rostock, Germany (Oct. 29, 1948); (abstract Angew. Chem., 61, 266 (June 1949). (16) Komarewsky, V. I., and Ruther, W. E., (preprint) presented before the Division of Petroleum Chemistry, 116th Meeting, AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. (16) Lundsted, L. G., Vaughn, T. H., Schwoegler, E. J., and Jacobs, E. C., presented before the Division of Organic Chemistry, 116th Meeting, AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. (17) Naragon, E. A., (preprint) presented before the Division of Petroleum Chemistry, 117th Meeting, AMERICANCHEMICAL SOCIETY, Houston, Tex. (18) Naragon, E. A., Millendorf, A. J., and Larson, L. P., Ibid., 117th Meeting, AMERICANCHEMICAL SOCIETY, Houston, Tex. (19) Perry, S. F., IND.ENQ.CHEM.,40, 1624 (1948). (20) Ibid., 41, 1887 (1949). (21) Pines, H., Aristoff, E., and Ipatieff, V. N., (preprint) presented before the Division of Petroleum Chemistry, 117th Meeting, AMERICAN CHEMICAL SOCIETY, Houston, Tex. (22) Pines, H., La Zerte, J. D., and Ipatieff, V. N., (preprint), Ibid., 116th Meeting, AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. (23) Stenerson, H., C h m . Eng. Newj, 28,1334 (April 17,1950). (24) Wagner, C. D., Beeck, O., Otvos, J. W., and Stevenson, D. P., J. Chem. Phys., 17, 419 (April 1949). (25) Waterman, H. I., van Steenis, J., and De Boer, J. H., Research, 2, 583 (December 1949). (6)
RECEIVED June 21. 1950.
NITRATION WILLARD dec. CRATER
HERCULES POWDER COMPANY, WILMINGTON, DEL.
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N CONTINUATION of the review of nitration processes and the mechanism of nitration (6,Y), an over-all picture of nitration is presented. This review covers the treatment of organic compounds with nitric acid or ita equivalent to produce both nitrates and nitro compounds.
MANUFACTURE OF NITRATES A study of the nitration of glycerol was made by Fierz-David and Fehrlin (If). They found that on nitration of glycerol with mixed acid there are formed, in addition to the nitrates, mixed mononitrate monosulfates which can be isolated in a pure st4Rte as salts of brucine. On the other hand, the mixed dinitrate monosulfates and the mixed mononitrate disulfates hydrolyze so easily that they cannot be obtained in a pure state. Considering all factors, the authors were able to show that on nitration of glycerol the yield of glycerol derivatives is quantitative. They point out that the formation of diglycerol tetranitrate or any destruction of the glycerol is not to be accepted. The manufacture of a “dense” type of nitrocellulose as carried out a t a Scottish factory is described by Picton and Kelland (2.9). The cellulose in the form of sheet wood pulp is machine cut to
the desired particle size. The wood pulp chips are nitrated batchwise using an acid to cellulose ratio of 10 to 1, which is stated to be much lower than the ratio used with cotton linters. The nitric aeid content of the mixed acid may be varied from about 40 to 70%, depending on the type of nitrocellulose to be produced. The spent acid is recovered by centrifuging the charge, care being taken to avoid prolonged wringing so that a minimum of denitration takes place. After wringing is complete, the charge is drowned in water and then stabilized. The dense nitrocellulose thus produced is said to retain essentially ita granular structure, thus giving it a marked advantage in handling over that produced from cotton linters. Watanabe ($66) published a series of articles on the preparation of nitrocellulose. His work includes composition of mixed acidsLe., sulfuric-nitric acid and acetic-nitric acid mixtures. Also, the nitration of cellulose in organic medium was studied, using mixtures of nitric acid-carbon tetrachloride and nitric acidacetic anhydride-carbon tetrachloride. The stability of nitrocellulose was also investigated. The nitration of cellulose using nitric acid vapors as the nitrating medium has been investigated by Trombe and co-workers
