EOTES
August, 1962 from wELich nitrogen probably is freer to escape than from the more strictly stoichiometric crystal lattices of BN, AlX, and Mg3N2. ,4 possible mechanism by which metal nitrides could undergo decomposition to the elements with escape of nitrogen molecules and sometimes metal atoms to the gas phase might involve these steps: (I) escape of metal and nitrogen atoms from crystal ledges or imperfections to adsorbed positions on the surface; ( 2 ) equilibration of atoms in the adsorbed layer to yield Sz molecules adsorbed and metal crystal nuclei if the metal is suAiciently non-volatile (like boron); ( 3 ) escape of adsorbed r\12 and adsorbed metal atoms to the gas phase. Acknowledgments.-The authors wish to acknowledge the ffinancial support of this research by the Wisconsin Alumni Research Foundation, the National Aeronautics and Space Administration and its subcontractor, the Gallery Chemical Company, and the Atomic Energy Commission.
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CRACKING 0%HYDROCSRBONS OVER A PROMOTED ALUMINA. CATALYST BY J. H. SIXFELTAND J. C. ROHRER
1559
Results When n-heptane or methylcyclopentane is passed over the catalyst used in this work, the observed reactions are essentially limited to cracking. The reaction products are mixtures of paraffins and olefins, the former resulting in large part from hydrogenation of the latter when the reactions are carried out in the presence of hydrogen. In the case of n-heptane, cracking refers to the production of olefins and paraffins of lower carbon number than the reactant. I n the case of methylcyclopentane, cracking refers t o the production of hexenes and hexanes as well as lower carbon number paraffins and olefins. At the low conversion levels (1 to 12%) shdied in the present work, the average distribution of the products by carbon number is shown below C1 CZ C3
C? C6
n-Heptane
Methylcyolopentane
17 23 27 23 10
12 17 10 22 2 37
CS
With respect t o other reaction products, trace amounts of isoheptanes, toluene, methylcycloAlumina possesses some catalytic activity for hexane, and dimethylcyclopentanes were observed in the products from n-heptane. Trace quantities cracking reactions of saturated hydrocarbons. The presence of halogens (C1 or F) has been shown of cyclohexane and benzene were observed in the to promote the cracking activity of alumina,1,2 products from methylcyclopentane. A complete resolution of the paraffins and olepresumably because of a n increase in the number or strength of acid sites on the surface. To obtain fins was not obtained by the analytical procedure further information regarding the nature of crack- used in this work. However, in the case of ning reactions over halogen-promoted alumina, heptane some data were obtained at 527' on the data have been obtained on the kinetics of cracking relative amounts of propane and propylene in the of n-heptane and methylcyclopentane over an reaction products. Of particular interest is the alumina c:l;talyst containing 1.2 wt. % chlorine. effect of hydrogen pressure on the ratio of propane For the most part, the reactions were carried out in to propylene, as shown below the presence of hydrogen, with one of the objectives Propane/propylene of the study being the determination of the effects 0.5 of hydrogen pressure on reaction rates and product 5.0 distribution. Increasing the hydrogen pressure tenfold thus inExperimental Procedure.--The n-heptane and methylcyclopentane were creases the ratio of propane/propylene tenfold. contacted with the catalyst in the presence of hydrogen, us- However, the ratios are about 70-fold lower than ing a flow reactor technique described elsewhere.415 The the equilibrium ratios' at these conditions. Similar reaction products were analyzed by a procedure, also described el sew lie re,^ consisting of a combination of chromato- results have been observed previously when cracking methylcyclopentane over a pure alumina. graphic columns coupled directly to the reactor outlet. Materials.--Phillips pure grade n-heptane and methylRates of cracking of n-heptane and methylcyclocyclopentane (> 99 mole % pure) were used throughout. pentane are shown in Table I as a function of temBoth these hydrocarbons and the hydrogen Ryere dried t o less than 5 p.p.m. water using procedures described pre- perature and hydrogen partial pressure. The rev i ~ u s l y . ~ The J alumina catalyst used in this study con- action rates were measured a t low conversion levels tained I .2 wt. % chlorine. The catalyst was calcined in air (1 to 12%) and were evaluated using the relation Esro Research and Engineering Co., Linden, N . J . Received March 1, lB68
for 4 hr. a t 5 9 3 O , and had a surface area of 210 m.2/g. Prior to calcination, X-ray diffraction data obtained in this Laboratory showed the alumina to be @-alumina trihydrate, a form of alumina which has been described elsewhere.6
(1) C. G. Myers and G. W. Munns, Jr., Ina!. Eng. Cham., 60, 1727 (1958). ( 2 ) K. W. Mcllenrg, R. ,J. Bertolacini. H. M. Brennan, J. L. Wilson. and H. S. Seelig, Paper No. 117, Section 11, "Proceedings of the Second International Congress on Catalysis," Paris, France, 1960. (3) J. H. Sinfelt,and J . C . Rohrer, J . Phgs. Chem., 66, 2272 (1961). ( 4 ) J. H. Sinfelt, H. Hurwitz. and J. C. .Rohrer, ibid., 64, 892
(1960). (6) J. E. SinEalt and J, 12, RQhrdir.)ibid,, 611,078 (lQt3l).
F
r=-Ax
w
where F represents the feed rate of the hydrocarbon reactant in g. moles per hour, W is the weight of catalyst in grams, and Ax is the fraction o€ the hy(0) H. C. Stumpf, A. 6 Ruasell, J W.Sewsnmr n?d C 11 Tucker, Ind. Ens. Chem., 42, 1398 (1950) (7) "Selected Values of Physical and Thermodynamic Prnp)CrtiPS of Hydrocarbons and Related Componnds," 4P1 Research P I ~ ~ P 44, PP Carnagis Press, Tno,, New Pork, S , Y . , 1953
NOTES
1560
drocarbon reactant which is converted to products of the cracking reaction. The rates of cracking are higher in the presence of hydrogen and increase with increasiiig hycirogtm pressure. The data suggest that the effect of the partial pressure of the hydrocarbon reactant i3 small. Throughout the raiige of conditions studied, the rate of cracking of n-heptane is lower than that of methylcyclopeiitaiie. Furthermore, the data on the effect of temperature on the reaction rates indicate an apparent activation energy of 43 kcal./mole for n-heptane as compared to 30 kcal.jmole for methylcyclopentane.
Discussion Cracking reactions of hydrocarboris over acidic catalysts are generally interpreted in terms of carbonium ion mechanisms.8 According to carbonium ion theory, the rate of cracking is related to the ease of formation of carbonium ions. Hydrocarbons containing a tertiary hydrogen atom form ions more readily than those containing only primary and secondary hydrogen atoms. This offers an explanation for the higher rate and the lower apparent activation energy of cracking of methylcyclopeiitane as compared to n-heptane, since the former contains a tertiary hydrogen atom. TABLE I SUMMARY OF CRACKING RATESOF YL-HEPTAXE (n-C,) A N D METHYLCYCLOPENTANE (hICP) 471
471
Temp., O C . 471 471 471
Ob
2 0
6 . 0 20.0
1 0 2.5
1.0 4.2
1.0 7.5
Ob
2.0
6 0
c
Cracking of n-C7 Hzpressure, atm. n-C7 pressure, atm. Rate, T, X 10ZQ Cracking of MCP Hz pressure, atm. MCP pressure, atm. Rate, r, X loaa a
1.0 12
499
527
17.0
17 0
17.0
1.0 3 . 7 9 . 9 10
3.7 28
3.7 74
17 0
17 0
17.0
10 1 . 0 1.0 3 . 5 19 26 26 28
3 5 64
20.0
Gram moles converted per hr. per g. of catalyst.
at 2.0 atm. substituted for I&.
3 5 118
* Na
Vol. 66
for the cracking of hydrocarbons over silicaalumina. The effect of hydrogen pressure on the parafinto-olefin ratio in the reaction products is further evidence for the hydrogenation activity of alumina catalysts, which has been noted before.losll The promotional effect of hydrogen pressure on the rate of cracking at the lower hydrogen pressures conceivably could be explained in several wayss : Increasing hydrogen pressure might serve to increase surface acidity (concentration of protons), to increase the rate of desorption of products via hydrogenation, or possibly to free the surface of carbonaceous residues which lower catalyst activity. (IO) V. C. I?. Holm and R. W. Blue, Ind. Eng. Chem., 43, 501 (1951). (11) S. 'N. Weller a n d S. G. Hindin, J . Pkys. Cliem., 60, 1501 (1956).
OXIDATION-REDUCTION AFALYSIS OF CRYOGENICALLP STABLE PRODUCTS O F DISSOCIA4TEDWATER IrL4FOR BY HENRYM. GLADNEY AND DAVIDGARVIN~ Prick Chemical Laboratory, Princeton University, Princeton, New Jerliev Received March 1 , 1962
Deposit,s having similar characterist,ics may be obt'ained by trapping a.t 77°K. the products from discharged water vaporz--4 or hydrogen peroxide& and from the reactions of hydrogen atoms wit'h either o ~ y g e n ~or . ~o. ~ o n e . ~ ,The ~ , ~deposits, in part crystalline and in part glassy, recryst'allize with evolution of oxygen upon tvarming,1-3.6 and, when melted, form concentrated solutions of hydrogen peroxide. The deposit from discharged water vapor is weakly paramagnetic.10911Similar oxygen evolution and recrystallization was observed when mixtures of hydrogen peroxide vapor and oxygen were frozen a t 4"K.I2 This method, however, failed to produce gas-evolving solids a t 77°K. In view of these results, we have undertaken chemical analyses of the frozen deposits to determine whether or not there is present a species stable only a t low temperature and what relation it has to the oxygen evolution.
I n comparing the rates and activation energies for the cracking of n-heptane and methylcyclopentane, it appears that a compensation effect Experimental exists. Thus, in the Arrhenius relation, k = ko Degassed water was vaporized, metered through a capilexp( - E / K T ) , the preexponential factor IC0 for lary (0.31 g./hr.) into a low pressure (0.1 t o 0.4 mm.) flow methyIcyclopentane would appear to be about 2000 (1) National Bureau of Standards, Washington 25, D. C. times lower than for n-heptane. This conclusion (a) W. H. Rodebush, C. W. J. Wende. and R. W. Campbell, is derived from the data on the temperature de- J . (?) Am. Chem. Soc., 89, 1924 (1937); (b) R. A. Jones and C. A. Winkpendence of the rate of cracking a t 17 atm. hydro- ler, Can. J . Chem., 29, 1010 (1951). gen pressure. Here the rates of cracking have been (3) M. A. Hogg and J. E. Spice, J . Chem. Soc., 3971, (1QL57). (4) P. Giguere, Discussions Faraday SOC.,14, 101 (1953). treated like rate constants, since the rates appear J. S. Batzold, C. Luner, and C. A. Winkler, Can. J . Chem., 31, to be nearly zero order with respect to both hy- 262(5) (1953). drogen and hydrocarbon pressures at this level ( 6 ) J. D. hfcKinley, Jr., and D. Garvin, J . Am. Chem. SOC.,77, of hydrogen pressure. Thus, while the activation 6802 (1955). (7) IC. H. Geib and P. Harteok, Be?., 86, 1551 (1932). energy for cracking of methylcyclopentaiie is 13 (8) N. I. Kobosev, I. T. Bkorokodov, L. I. Nekrasov, and E. I. kcal./mole lower than for cracking of n-heptane, Makaroira, Zh. Fiz. Khim., 31, 1843 (1957). the lower preexponential factor for the former (9) P. Giguere and D. Chin, ,J. Ciiem. Ph$/s.,31, 1685 (1959). largely compensates for this, so that the differences (la) Et. Livingston, J. Ghormlay, and H. Zeldes, ibid.,24, 483 (1956). (11) A. I. Gorbanev, S. D. Kaitsmazov, 8. M. Prokhodov, and A. J3. in rates are not many-fold. A compensation effect Zh. Fis. Khim., 31, 515 (1Pii7). also has been noted by Franklin and Nicholson9 Tsentsiper, (12) J. W. Edwards. Chapter 8 in "Formation and Trapping of Free (8) H. H. Voge, "Catalysis," Vol. 6. Reinhold Publ. Corp., New York, N. Y.. 1958, p. 445. (9) J. I,. Franklin a n d D. E. Nicholson, J . Phys. Chem., 61, 814 (1957).
Radicals," A. M. Bass and H. P. Broida, Editors, dcademio Press, New York, N. Y., 1960, pp. 288-292; J. W. Edwards and J. S. Hashman, Am. Chem. SOC. Abstracts of 132d Natl. kreeting. Sept., 1957, IJ. 42-8.
