Catalvtic Cvclization of d
J
Aliphatic Hydrocarbons J
to Aromatics ARISTID V. GROSSE, JACQUE C. NIORRELL, AND WILLIAM J. MATTOX Universal Oil Products Company, Riverside, Ill.
are Ipatieff (14), Davidson (z), Hague and Wheeler ( I l ) , Hurd (IS), and Frolich with his co-workers (8). However, a simple consideration of the classical van’t Hoff-Le Bel space models of paraffin or unsaturated hydrocarbons with six or more carbon atoms in a row shows that they have a natural tendency to form six carbon atom rings-this is in conformity with von Baeyer’s strain theory. It is necessary to split off only two hydrogen atoms from two end carbon atoms of the open ring to form a cyclohexane ring, which by further dehyFRONT VIEW O F SEMICOMhlERCIAL-PLAXT DEHYDROGENATION drogenation is converted into an aromatic hydrocarbon. PROCESS The conversion of n-heptane into toluene is illustrated in Figure 1; two practically equivalent mechanisms are indicated. I n recent years we have developed efficient and practical oxide catalysts, containing, in order of their effectiveness, HE problem of converting aliphatic hydrocarbons into metals of the VI, V, and IV groups of the periodic system, aromatics is an old one. The importance of the successfor the dehydrogenation of straight-chain hydrocarbons to ful solution of this problem lies in the value of aromatics the corresponding olefins ( 5 ) . Working with the same and as high antiknock motor fuels, solvents, raw material for similar types of catalysts along the idea mentioned above, explosives (TXT), and a practically endless variety of organic we have discovered the new reaction of cyclization. The chemicals useful for dye intermediates, pharmaceuticals, announcement of this discovery by patent applications (9) medicinals, synthetic resins, etc. I t is extremely important and publications was delayed as a result of prolonged disfrom the viewpoint that a practically unlimited supply of cussion of the subject with the United States War Department raw material is available in the form of petroleum and coal in connection with national defense. per se, whereas the present source of raw materials for aroSubsequent to our discovery, Moldavsky and his co-workers matics is coal tar of which there is only a limited supply. (16) published their results. They apparently did not attach Various solutions of the problem have been attempted by any practical significance to their work since the life of their purely thermal cracking ever since Faraday’s memorable catalysts was short. discovery (7) of benzene in 1825. The new reaction permits the more or less quantitatiye Faraday’s work was followed by investigations by a numconversion of aliphatic hydrocarbons containing six or more ber of technologists among whom Letny (I6),Rudnew (do), carbon atoms in a chain into the corresponding aromatic hyDvorkovitch ( S ) , Xkiforoff ( l 7 ) ,Pamfilou ( I @ , and Zelinsky drocarbons. For instance, n e have been able to conyert (22) should be mentioned. During the World War extensive n-heptane into toluene with yields of about 75 per cent of t!ie ir vestigations were carried out in this country by Rittman theoretical (i. e., 0.75 mole of toluene per mole of n-heptane) (19) and Egloff with Twomey and Moore ( 6 ) . in one pass, or over 90 per cent on recycling. In these investigations (4) it has been usually assumed, with The new reaction will be generally referred to ab cyclizathe work of Berthelot (1) and Haber ( I O ) as a basis, that the tion. Strictly speaking, the conversion of paraffins into aromatics are formed over the acetylene or ethylene route. aromatics is, in the terminology used in our laboratories, This assumption of the formation of aromatics by a build-up process from smaller aliphatic fragments has been further “dehydrocyclodehydrogenation”, since the cyclization of paraffins to hydroaromatics is preceded by dehydrogenation. supported by evidence of a number of workers, among whom
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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1. THEDEHYDROCYCLIZATION REACTION
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hydrogenating metal as will finally leave only the oxides of the desired metal on the particles of the support without any interfering constituents. Such compounds are usually water-soluble metal nitrates or ammonium salts of the metallic acid. Chromic acid m a y be used in the case of chromium. One convenient laboratory procedure for the preparation of a simple cyclization catalyst containing 8 per cent ch r om iu m sesq u ioxide on alumina is as follows :
For greater convenience we will use the shorter term “dehydrocyclization”. From the same viewpoint “cyclodehydrogenation” applies to the conversion of olefins to aromatics. The well-known dehydrogenation of cyclohexanes to the corresponding aromatics (Zelinsky) is often referred to as aromatization. Broadly speaking, cyclization in the sense used here is also covered by this term. However, “true cyclization” applies only to the isomerization of monoolefins into cycloparaffins (or diolefins into cycloolefins or bicycloparaffins).
Ten to twelve mesh granules of activated alumina are impregnated with an aqueous solution of the required amount of chromium trioxide (CrO3) or chromic nitrate [Cr(NO&, as.]. Grade A activated alumina of the Aluminum Corporation of America, sold as a drying agent, is recommended. The dried particles are preferably reduced in situ in an atmosphere of hydrogen before use. The carbon or carbonaceous deposit on the catalyst should not be allowed to accumulate in excess of 5-10 per cent of its weight. The regeneration is accomplished by burning off the carbon in a stream of air or oxygen-containing gases, its rate being adjusted so as not to allow the catalyst temperature to exceed 900” C.; the preferred upper limit may, however, be lower.
Catalysts
Greater activities of catalysts and larger space-time yields may be attained with mixed catalysts containing two, three, or even more dehydrogenating metal oxides on the supports. The relation between the activity and the nature of different
Some of the pure oxides which were not supported on carriers (for instance, chromium sesquioxide, CrzOa,and molybdenum sesquioxide, Mo203)!+-ere found to produce partial cyclization a t 450” to 500” C. At higher temperatures they lose their activity rapidly as a result of crystallization and are not suitable for practical purposes. The introduction of suitable carriers allows the preparation of active catalysts with a useful life of over 1000 hours. The catalysts used here consist of minor molar proportions of oxides of the transition metals of the VI (e. g., chromium or molybdenum), V (e. g., vanadium), and IV (e. g., titanium and cerium) groups of the periodic system supported on carriers of relatively low catalytic activity, such as specially prepared alumina or magnesia. Other substances giving no unfavorable reaction with the metal oxides and providing a stable and large surface may be substituted as carriers. The catalysts may be prepared in a variety of ways. Generally the dehydrogenating metal compounds may be deposited upon the carriers from aqueous or other solutions or may be mechanically admixed Tyith the carriers, either in the wet or dry condition. The principle is always to use such a procedure and such compounds of the deSIDE VIEW
O F SEMICOMMERCIAL-PLANT DEHYDROGENATION PROCESS
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mutual loosening or stabilization of lattices, and changes in activation properties. The factors just mentioned are sufficient to show the wide variety of possible catalysts. Catalyst stability over a long period of time, its ruggedness in regeneration] and its carbonforming tendency, as well as its price, may play a decisive role in the final decision as to its practicability. A logical development of highly efficient catalysts is materially aided by the application of x-ray crystal structure and Hahn's radioactive surface-characterizing methods (12).
C
/NCR€AS/NG CONTACT T/ME
OF WHEPTASE INTO TOLUFIGURE 2. DEHYDROCYCLIZATION ENE WITH METALOXIDECATALYSTS
metals and their amounts is complicated. This is illustrated in Figure 2 for the specific case of dehydrocyclization of h e p tane into toluene a t 500" C. by mixtures of chromium] molybdenum, and vanadium oxides on alumina. The sum of the dehydrogenating metal oxide molecules per 100 molecules of alumina was kept constant in all mixtures. All oxides were introduced by means of the corresponding ammonium metalates. This Darticular examde shows that a t all contact times a vanadium-molybdenum catalyst is less active than the corresponding simple chromium catalyst. However, the chromium-molybdenum and, even more so, the vanadiumchromium catalysts show improved results over the simple catalyst a t longer contact times, corresponding to higher once-through yields. The triple molybdenum-chromiumvanadium oxide catalyst shows higher activity than the single or any double dehydrogenating metal oxide catalysts. It should be particularly stressed that the activity of our catalysts, like that of many others, depends to a great extent on the method of preparation. This is evident from the fact that their activity is not determined primarily by the weight ratio of the dehydrogenating to the supporting oxide but by other factors. Among these factors are the surface concentration of the dehydrogenating metal oxide, the surface availability, the condition of the surface and the interrelation, both physical and chemical, between the dehydrogenating metal oxide and the supporting oxide. The surface availability is defined as the surface area in square centimeters per gram of catalyst accessible to the gases or vapors. By physical interrelation is meant the crystallographic, lattice, and diffusional relations between t h e components; chemical interrelation includes compound formation, FIGURE 3.
Cyclization of Pure Hydrocarbons A large number of pure aliphatic hydrocarbons were cyclicized with a variety of catalysts under varying conditions. Among the hydrocarbons used were paraffins] mono- and diolefins] and acetylenes. Specific examples are n-hexane and n-hexenes into benzene, n-heptane and n-heptenes into toluene, and n-octane and n-octenes into ortho-, meta-, and para-xylenes and ethylbenzene. The same catalysts have been successfully used for the dehydrogenation of naphthenic hydrocarbons. Space permits only a brief outline here of the results obtained. Complete description of the individual cases will be published later. Factors affecting the composition of the products and their yields are the composition of the feed, chemical composition and physical structure of the catalyst] catalyst activity and life, contact time or space velocity, and temperature and pressure under which the reaction is carried out. I n this
A P P A R l T U S FOR CYCLIZ 4TION AT
ATMOSPHERIC PRESSURE
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hydrogen and 2.9 per cent methane and ethane. The liquid product consisted of only a trace (0.5 per cent) of hydrocarbons boiling below 50" C. (due to carboncarbon link splitting) and analyzed 12.1 weight per cent toluene, 11.5 weight per cent heptenes (bromine number of product 16.4), and 75.5 weight per cent unreacted n-heptane (n': 1.3881). Based on these data, the calculated recycle yield of toluene was about 89 weight per cent or about 97 weight per cent of the theoretical. The once-through yields of toluene, gas (mainly hydrogen), carbon, heptenes, and unreacted heptane obtained with the same catalyst a t 550' C. and a t difFIGURE4. DEHYDROCYCLIZATION OF n-HEPTANE WITH 8 WEIGHT PERCENTCHROMIC ferent space velocities are given in FigOXIDE-OK-ALUMINA CATALYST AT 550" C. ure 4. The calculated recycle yields are given in Table I. article only the cyclization of n-heptane into toluene will be A maximum once-through yield of 72 per cent of theory described. was obtained at a liquid space velocity of about 0.39 per hour. With automatic regeneration of the catalyst after each cycle APPARATUS AND PROCEDURE. The catalyst was placed in a it is possible t o maintain a useful life of the catalyst of over quartz tube which was heated in a vertical, automatically con1000 hours with substantially unchanged once-through and trolled, electric, metal block furnace (Figure 3). The liquid was fed from the top by means of a Tropsch-Mattox adjustable feed over-all yields of toluene. A slow decrease in the activity of pump ( H ) ; it was preheated by running down a quartz spiral the catalyst may be fully counteracted by slight gradual inand then distributed evenly by running down a length of quartz crease in the catalyst temperature. chips above the catalyst. The cyclization process and the catalysts used are covered The catalyzate was brought out rapidly from the catalyst space and chilled in ice or solid carbon dioxide; the usual train of by patents and pending applications in the United States and cooled receivers, manometers, and gasholders was used to collect in foreign countries. the products. For long runs an electrical cycle controller was used which Literature Cited automatically regenerated the catalyst with air. ANALYTICAL METHODS.The liquid products were first sta(1) Berthelot, M., Ann. chim. phys., 9, 445 (1886); 12, 52 (1887); bilized and separated into fractions of 10" to 50" C. boiling range 16, 143 (1889) ; "Carbures d'hydrogkne" (collected works), by Podbielniak distillation. In such fractions the per cent ole1851-1901. fins was determined from the bromine number. The olefins in (2) Davidson, J. ISD. ENG.CHEM.,10,901 (1918). the bulk of the fraction were then converted to the bromides by (3) Dvorkovitch, J. Soc. Chem. I n d . , 12, 5,403 (1893). adding bromine, as calculated from the bromine number, at 0' C., (4) Egloff, Gustav, M e t . & Chem. Eng., 15, No. 12, 3 (1916). and the hydrocarbons were distilled off in a vacuum from the (5) Egloff, Gustav, Proc. Am. PetroleumInst., 111, 16, 137-8 (1935). bromides. In the olefin-free distillate the aromatics mere ab(6) Egloff, G., and Twomey. T . J . , J . P h y s . Chem., 20, 121, 515. 597 sorbed with sulfuric acid until the residue was stable to the (1916); M e t . & Chem. Eng., 15, No. 1, 15, No. 5, 245 (1916); nitrating test. If identification of the aromatic was desired, the Egloff, Gustav, Twomey, T. J., and Moore, R. J., Ibid., 15, olefin-free distillate was nitrated directly and the nitrohydroNo. 7, 387, No. 9. 523 (1916). carbons were isolated. In the aromatic-free residue the paraffin (7) Faraday, M., Trans. Roy. Soc. (London), 1825,440. content could readily be determined by either of the following (8) Frolich, P. K., Simard, R., and White, A., IND.ENG.CHEM.,22, methods or a combination of them: carbon-hydrogen analysis, 240 (1930) : Schneider, V., and Frolich. P. K., I b i d . , 23, 1405 index of refraction, aniline point. (1931). The gases were analyzed by the conventional Podbielniak dis(9) Grosse, .1. V., and Morrell, J. C., U. S. Patents 2,124,566-7, tillation or Gockel method. 2,124,583-6 (July, 1938), applied Seyt.-Oct., 1936. J . Gasbeleucht., 39, 377, 395, 435, 452, 799, 813, 830 (10) Haber, F., (1896). TABLE I. CALCUL~TED RECYCLE YIELDS (11) Hague and Wheeler, J. Chem. Soc., 1929, 378. Liquid space velocity, vol./vol./hr. 0.034 Toluene recycle yields. weight yo of theoretical 66 (see equation. Figure 1)
10.195
0.385
0.78
5.00
10.00
73
79
80
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RESULTS.The following example is chosen to illustrate in detail the results obtained under one particular set of conditions: Pure n-heptane (d:' 0.6837, nz: 1.3879) was dehydrocycliized a t 500" C. over a 6 atomic per cent chromic oxideon-alumina (8-10 mesh granule) catalyst a t a liquid space velocity of 3.60 volumes per volume of catalyst per hour and at atmospheric pressure for one hour. The recovery of the different products, in weight per cent of the charge, was as follows: Liquid products (d'O 0 6922 n Gas u n ~ o n d e n s a b l e ~ a-t 78' C. Carbon
g
1 3943)
1
981 , 0 17
The gas, 74.2 volumes (at normal temperature and pressure) per volume of heptane charged, contained 97.1 per cent
(12) Hahn, O., "Applied Radiochemistry", Cornell Univ. Press, 1936. (13) Hurd, C. D., and Spence, L. U., J . Am. Chem. Soc., 51, 3561 (1929). (14) Ipatieff, V . N.. Ber., 44, 2978, 2987 (1911); 46, 1748 (1913). (15) Letny. Dinglers polytech. J.,177, 58 (1865). (16) Moldavsky, B. L., and Kamusher, H . D., Compt. rend. acad. sci. C. R. S. S.. [N. S.]1, 355-9 (1936) ; Moldavsky, B. L., Karnusher, H. D., and Kokylskaya, M. V., J . Gen. Chem. (U. S . S. R.), 7, 169-78, 1835-9 (1937); Moldavsky, B. L., Bezprosvannaya, F., Kamusher, H. D.. and Kokylskaya, M. V., Ibid., 7, 1840-7 (1937). (17) Nikiforoff, Chem.-Ztg., 20, 8 (1896). (18) Pamfilow, Ibid., 21, 1069 (1897). (19) Rittman, W. F., J. IND.ENG.CHEX.,7, 945 (1915); Rittman, W. F., Byron, C.,and Egloff, Gustav, I b i d . , 7, 1019 (1915); Rittman. W.F.. Dutton, C . B.. and Dean, E. W , , 'c. S. Bur. Mines, Bull. 114 (1916); Rittman, IT. F.. and Egloff, Gustav, Met. & Chem. Eng.,14, l(1916). (20) Rudnew, Dinglers polytech. J.. 239, 72 (1581). (21) Tropsch, H., andMattox. W.J., IND. EKG.CHEV.,26,1338 (1934). (22) Zelinsky, N., Chem.-Ztg., 26, 68 (1902). PRESENTED before t h e Division of Organic Chemistry a t t h e 96th Meeting of t h e American Chemical Society, Milwaukee, Wis.
