Catalytic Dehydrogenation of Naphthenes by Sulfur Dioxide JOSEPH D. DANFORTH AND MARY JANE BENDER] Department of Chemistry, Grinnell College, Grinnell, Iowa
I
iX R E C E S T years the reforming
of to give a maximum yield of high octane fuels has been extensively investigated. One of the important reactions encountered in these reforming processes has been the dehydrogenation of naphthenes to aromatics. The recent demand for benzene and its homologs has given further impetus to the study of processes for the conversion of petroleum naphthas to aromatic hydrocarbons. Activated carbon has been used as a reforming catalyst ( I , 2) but has not been adopted commercially for this purpoae. One of the important problems encountered was the introduction of sufficient process heat to maintain the highly endothermic reaction ( 2 ) . The present approach to the problem of reforming was based on the use of a hydrogen acceptor or mild oxidizing agent which Tyould remove the naphthenic hydrogens by oxidation, thereby providing ample quantities of process heat. It was essential that the acceptor molecule be reduced to a substance which could be oxidized and recycled to the process. Substantially no conversion of sulfur dioxide to hydrogen sulfide was observed when the catalyst chamber was packed with glass chips and cyclohexane-sulfur dioxide mixtures were charged a t 500' C. A silica-alumina cracking catalyst gave some conversion of sulfur dioxide to hydrogen sulfide, but this catalyst rapidly became inactive. Activated carbon was found to be an efficient catalyst for this reaction, and this paper describes some of the results obtained when cyclohexane, methylcyclohexane, ethylcyclohexane, decalin, and a mixture of methylcyclohexane and hexane were treated with activated carbon in the presence of sulfur dioxide. EXPERIMENTAL
The graphite block furnace used in this work has been described (5). During a run the temperature was measured a t the top, middle, and bottom of the catalyst bed a t 10-ml. intervals of liquid charge, and the temperature of the run was recorded as the average of these measurements. The hydrocarbon was charged from a copper bellows pump. All runs were made a t atmospheric pressure. Sulfur dioxide was charged from its cylinder through a needle valve. The rate of flow was measured by a rotameter (Fischer and Porter Tube No.-08-15 5-21654-3) and controlled by the needle valve. A modified Piros Glover fractionating column packed with 1/12-inch glass helices and equipped for automatic control of reflux ratios was used in all fractionations. PROCEDURE. In making a run the liquid hydrocarbon was pumped downward through the reaction tube containing 75 ml. of catalyst. Shortly after the flow of hydrocarbon was started, the sulfur dioxide flow was begun. The liquid product was collected in an iced receiver and the gaseous products, after passage through an appropriate solvent for hydrogen sulfide removal, were collected in a gas bottle. Shortly before completion of the run the charging of sulfur dioxide was discontinued to avoid carry-over of sulfur dioxide to the reaction flask when the chamber was flushed with nitrogen. Since most of the runs were carried out on an activated carbon catalyst, it was not possible to determine coke formation by the usual combustion procedure. 1
Present address, Monsanto Chemical Co., Nitro, W. Va.
Coke was estimated in certain runs by the gain in weight of the charcoal catalyst after completely flushing with nitrogen. Hydrogen sulfide was estimated quantitatively in the runs on cyclohexane by bubbling the gaseous products through a solution of iodine in potassium iodide and titrating the unreacted iodine with sodium thiosulfate. In other runs the hydrogen sulfide was absorbed in sodium hydroxide and was not quantitativelv determined. The quantity of hydrogen sulfide dissolved in the water and hydrocarbon products was neglected. The gaseous products after removal of hydrogen sulfide, were collected in a gas-bottle and analyzed for olefins and oxygen (air). Residual gas was considered as hydrogen and paraffins. The liquid product containing a water layer was weighed and transferred t o a separatory funnel to recover and weigh the water and remaining hydrocarbon. The dissolved hydrogen sulfidc was removed from the hydrocarbon layer by washing with dilute caustic. The hydrocarbon layer was dried over anhydrous calcium chloride and was fractionated, wing decalin as a boiling base. When the boiling points of the products did not permit efficient separation by fractionation, the per cent ole6ns in the liquid product was determined by bromine number and thr amount of aromatics estimated by adsorption in fuming sulfuric acid. When decalin was charged, the naphthalene was recovered by crystallization a t ice temperature and washed and recrystallized from hexane. In the calculations of hydrocarbon recoT-ery, the weight per cent of hydrogen recovered in the water and hydrogen sulfide has been recorded in cases where water and hydrogen sulfide were determined. MATERIALS.Commercial grade sulfur dioxide of 99.9% purity, cyclohexane (melting point 4" to ' 6 C.), methylcyclohexane (practical), and ethylcyclohexane (boiling point 130' to 134" C ) were purchased from the Matheson Co. Decalin was Du Pont commercial grade. Catalysts were activated alumina from the Harshaw Co. (Puralox) and the SoconyVacuum Co. synthetic bead cracking catalyst contained 10% A120a on Sios. Columbia activated carbon was designated as Grade S, 4//14 mesh. RESULTS OF CONVERSION
When sulfur dioxide was charged with cyclohexane over glass chips at 500' C., the sulfur dioxide was present in the products of the reaction, and only insignificant and undetermined deposits of sulfur were present in the unreacted cyclohexane. Replacement of the glass chips by the silica-alumina cracking catalyst resulted in the initial conversion of sulfur dioxide to hydrogen sulfide. However, this catalyst rapidly lost activity, and unreduced sulfur dioxide came through into the reaction chamber where it reacted with the hydrogen sulfide present in the receiving flask to form voluminous deposits of sulfur. Thus, evidence of catalyst failure was marked by the sudden appearance of sulfur deposit8 in the receiver. Because of the interaction of sulfur dioxide and hydrogen sulfide, all runs were made under conditions adjusted to avoid an excess of sulfur dioxide in the charge. I n general, the moles of cyclic paraffin were three to five times the moles of sulfur dioxide, and, under these conditions, complete conversion of sulfur dioxide to water and hydrogen sulfide was obtained on activated carbon.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 8
The results of the dehydrogenations of methylcyclohexane ir. the presence of carbon dioxide and sulfur dioxide are shown in Table 111. Distillation of the liquid products showed approxi(Conversion of methylcyclohexane with 75 mi. of catalyst) mately 25% toluene in the run with sulfur dioxide; when carbon Activated Activated Cracking Carbon Alumina Catalyst dioxide was used, toluene was not present in amounts capable of 500 490 498 Temnerature, C. detection by the method of analysis employed. 1.55 1 39 1.32 LHSV" 115 121 52 Duration of run, min. In the viork of Sanford and Friedman (2), an activated carbon Charged, moles 1.57 1.57 0.i8 catalyst promoted with 3.64% sodium carbonate gave about 15% Methylcyclohexane 0.45 0.48 0.24 Sulfur dioxide conversion of methyl cyclohexane to toluene at 508" C., using 0 92 0 95 Water recovered, mole Hydrocarbon, wt. % of charge steam as a diluent. That the unpromoted activated carbon 1 5 0 . 5 6 Gas 90.5C 88.0d ..b used in the present work had little dehydrogenation activity in Liquid Toluene inliquid (esLd. from diatn. c:urve), the absence of sulfur dioxide was shown by a test performed by 25 10 0 % the authors in which substant'ially no hydrogen gas or benzene was recovered when 28 grams of cyclohexane was passed over 38 ml. of activated carbon in a 1-hour process period at 508" C. C Product perfectly clear. d Traces of sulfur in product. The maximum conversion to benzene which could have been obtained by the most optimistic interpretation of t'he data was TABLE 11. EFFECT OF TEMPERATURE ON COKVERSIOK O F CYCLOless than 2.0ojo. When this conversion is compared with the HEXANE TO BESZENE BY ACTIVATEDCARBOSCATALYST conversions obtained in the presence of carbon dioxide, it seem Temperature, C. reasonable t,o assume that substantially no conversion of naph499 460 421 thenes to aromatics occurs on the present activated carbon; car1.33 1.33 1.40 LHSV, 120 120 114 Duration of run, min. bon dioxide funct'ions as an inert diluent and provides a basis Charged, moles for comparison with the runs using sulfur dioxide; and the pre1.m 1.61 1.61 Cyclohexane Sulfur dioxide 0.47 0.47 0.47 sence of sulfur dioxide and a catalyst are essential for significant Recovered, moles 1 .@2 Water 1 04 0.98 conversions of naphthenes to aromatics under the test conditions 0 32 Hydrogen sulfide 0.44 0.41 used. ' of charge Hydrocarbon, wt. %
TABLE I. COMPARISON OF CATALYSTS
O
O
92.0
90.5
91.0 0.3 1.7 7.0 1.6 19 0
In Table I are summarized data relevant to the activities of a cracking catalyst, act,ivated alumina, and activated carbon for the conversion of methylcyclohexane to toluene at 500" C. using sulfur dioxide as a hydrogen acceptor. The liquid product from activated carbon was clean, showing no sulfur deposit after 200 ml. of methylcyclohexane JTas charged for a period of 2 hours. The liquid product from the activated alumina catalyst contained significant quantities of toluene, but was dark in color and a reddish brown liquid presumed t o contain sulfur compounds 1%-asobt'ained by boiling off the toluene and unreact,ed methylcyclohexane. -4lthough hydrogen sulfide was formed initially in the presence of the cracking catalyst, the catalyst soon lost activity and the liquid product contained much sulfur which made separat'ion and complete analysis impossible. The amounts of toluene in the liquid products from activated carbon, activated alumina, and a cracking catalyst, estimated to the nearest 5% were 25, 10, and 0 weight %, respectively. In these runs, as well aE in subsequent runs, gas formation represented a relatively Emall proportion of the charge. In order to demonstrate the range of temperatures in which sulfur dioxide could ef5ciently dehydrogenate cyclohexane in the presence of activated carbon, tests were conducted at 499", 460°, and 421' C. with other conditions closely similar. In this series of tests the hydrogen sulfide xas determined by absorption in iodine, and aromatics in the hydrocarbon product were estimated by absorpt'ion in fuming sulfuric acid with an appropriate correction for the olefins present. The data, summarized in Table 11, indicate substantially complet'e reduction of sulfur dioxide at 460" and 499" C. and less complete, though not entirely unsatisfactory reduct'ion at 421" C. It appears that a rather wide range of temperature can be used satisfactorily in this reaction. In none of the liquid products was there any visible formation of sulfur. The product at 421" C., however, was somewhat off-color and presumably contained minor quantities of sulfur compounds. The liquid product contained approximately 20 weight yobenzene at, all three temperatures.
111. DEHYDROGENATION O F IIETHYLCYCLOIIEXASE O N ACTIVATEDCARBOS G S I X G SULFCR DIOXIDE AKD C.4RBON
TABLE
DIOXIDE Temperature, C. LHSV Duration of run, min Charned Gai, inole Methylcyclohexane, grams Water recovered, mole Hydrocarbon, wt. YC of charge Liquid Gas Hydrogen (in ISzO and H&),wt. yc Coke and unaccounted, wt. yo Toluene in liquid (estd. from distn. curve), %
TABLE
Sulfur Dioxide 490 1 39 115
Carbon Dioxide 485 1.62 99
0.45 154 0.92
0.35 I54 0.00
00 5 1.5 1.8
06 4
1.9 0.0 1.7 0
6 2
25
IT'. EFFECTO F SULFUR DIOXIDE O S DEHYDROGEXATIOS or DECALIN ON A % CARBON ~ ~
Decalin charged, grams Temperature, C. Duration of run, niin. T HclV
%'ate; recovered, mole Hydrocarbon, wt. % of charge Liquid Gas Crude naphthalene in liquid, 70
Sulfur Dioxide Charged, Mole 0 24 0 088 88 489 493 56 52 1 42 1 52 0 54 0 92 7 2.3 32 5
97 5 3 0 4 5
In another comparison, decalin was dehydrogenated a t 490" C. xvith and without sulfur dioxide. The data, summarized in Table IV, show that the presence of sulfur dioxide resulted in R liquid product, containing 32,5y0 naphthalene, while only 4.5% naphthalene mas present in the liquid formed in the absence of sulfur dioxide. Ethylcyclohexane (78 grams, 100 ml. j was charged to 75 ml. of activated carbon for a process period of 57 minutes at 492" C. A total of 0.26 mole of sulfur dioxide mas charged throughout the run. Hydrocarbon liquid, gas, and coke were 90.5, 3.8, and 3.0 w-eight % of the charge, respectively. The gas contained only 1.0% olefins, as determined by absorption in bromine water, and had a molecular weight of only 9.8. Water recovered was 0.59 mole. The liquid product contained 2.7 weight % olefins, determined by bromine number, and 27.4 weight yo aromatics,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
measured by adsorption in fuming sulfuric acid. The aromatics boiled in the 130' to 140" C. range, but a sharp separation of ethylcyclohexane and ethylbenzene by distillation was not obtained. SELECTIVE DEHYDROGENATION
I t has been demonstrated that the pure hydrocarbons, cyclohexane, methylcyclohexane, ethylcyclohexane, and decalin can be converted to the corresponding aromatic by sulfur dioxide and an activated carbon catalyst. Since an actual feed stock from petroleum sources presumably would contain paraffin hydrocarbons as well as the naphthenes, it was of interest to determine the extent to which a naphthene could be selectively dehydrogenated in the presence of a paraffin hydrocarbon. Two hundred ml. (149.4 grams) of a blend containing 22.2 weight % n-hexane and 77.8 weight % ' methylcyclohexane was charged to 75 ml. of activated carbon for a process period of 106 minutes a t 493' C., while 0.4 mole of sulfur dioxide was charged. Water recovery was 0.95 mole and hydrocarbon liquid, gas, and coke were 88.8, 2.3, and 2.3 weight %, respectively. Weight loss of hydrogen to water and hydrogen sulfide was 2.0 weight % of the charge, leaving 4.6% unaccounted. By fractionation of the liquid product, it was estimated that a loss-free recovery of 21.4, 56.1, and 22.5 weight % hexane, methylcyclohexane, and toluene was obtained, illthough high accuracy is not claimed for this type of estimate, calculations from the data show that 29.8 grams of hexane was recovered from 33.2 grams charged. On a loss-free basis, 31.3 grams of toluene was recovered, which is about 85% of
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that which could theoretically be formed from 0.40 mole of sulfur dioxide and represents a 28.5% conversion of methylcyclohexane. These data indicate clearly that selective dehydrogenation of methylcyclohexane occurs in the presence of hexane. The extent to which such selectivity could be attained on naphtha feed stocks is a subject for additional work. CONCLUSION
It has been shown by this work that sulfur dioxide in the presence of activated carbon dehydrogenates naphthenes containing six-membered carbon rings to form the corresponding aromatic hydrocarbon. It appears that some selectivity in the dehydrogenation of naphthenes in the presence of paraffins can be achieved. ACKNOWLEDGMENT
The authors wish to express their appreciation t o the Office of Naval Research for financial support of this research. LITERATURE CITED
(1) hlarisic, M. M. (to The Pure Oil Co.), U. S. Patent 2,428,715 (Oct. 7 , 1947). (2) Sanford, R. A., and Friedman, B. S., presented before the Division of Petroleum Chemistry, at the 123rd Meeting, AM. CHEM.SOC., Los Angeles, Calif., March 1953. (3) Stright, P., and Danforth, J. D., J. Phys. Chem., 57, 448 (1953). RECEIVED for review July 24, 1953. ACCEPTED April 12, 1954. Presented before t h e Division of Petroleum Chemistry a t t h e 124th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Chicago, 111.
Properties of Irradiated Polyethylene EFFECT OF INITIAL MOLECULAR WEIGHT ELLIOTT J. LAWTON, J. S. BALWIT, AND A. M. BUECHE Research Laboratory, General Electric Co., Schenectady, N . Y .
C
ROSS-linking of polyethylene when irradiated with high energy electrons has been reported (8). Charlesby and Little (2, IO) have also reported cross-linking of polyethylene when subjected to the radiation from an atomic pile. The irradiated polyethylene appears to behave as would be generally expected for a cross-linked material. It is no longer soluble in solvents for the unirradiated material. It shows improved form stability a t elevated temperatures, the tensile strength and elongation change upon irradiation, and it changes from a plastic to an elastic material. The experiments described here were undertaken to determine the effect of initial molecular weight on the amount of radiation required for cross-linking and to determine the effect of irradiation on some of the physical properties. EXPERIMENTAL
The high-energy electron source used was the General Electric Research Laboratory 800-kv. (peak) resonant transformer unit (3, 7 ) . Irradiation dose was accumulated a t the rate of 0.14 X lo6 roentgens per pecond. Ionization dose u-as measured with a specially constructed thin-walled ionization chamber. The samples were irradiated in a nitrogen atmosphere to exclude any effects due t o ozone when irradiated in air. The thickness of the samples usually was about 40 mils, which is about one third of the total beam penetration and in the range for fairly uniform dose distribution throughout. The samples were irradiated so as always to minimize the small temperature rise that accompanies the irradiation. The temperature rise was estimated never to exceed about 40" C.
The polyethylene used was obtained from the Bakelite Co. and from E. I. du Pont de Nemours & Co. The particular materials used had the following designations and reported molecular weights: Designation D Y G T blend D Y L T blend D Y K F blend DYNH blend D Y N K blend DXH-35 blend Alathon-8
Molecular Wt. 7,000 12,000 19,000 21 , 0 0 0 24,000 35,000 19,000
Source Bakelite Co. Bakelite Co. Bakelite Co. Bakelite Co. Bakelite Co. Bakelite Co. Du P o n t Co. (contains antioxidant)
The powders were pressed into sheet form a t a temperature ranging from 120' to 150" C., depending on the molecular weight. No attempt was made to relieve the stresses in the sheets. Swelling measurements were made in toluene a t a temperature of 90' i 5" C. Samples '/4 inch wide and 1 inch long were placed in test tubes containing toluene and maintained at 90" C. for several hours. The minimum irradiation dose for swelling was taken as that value a t which the material would not dissolve but would just hold together in the form of a swollen strip. An Instron tensile test machine was used in making the stressstrain measurements. This machine makes an automatic record of load ws. elongation during each test period. During each test the specimen was stretched a t a constant rate of 0.5 inch per minute. The temperature of the sample for the high temperature tests was regulated by a small electrically heated oven that surrounded both sample and machine clamping jaws. The temperature was measured by means of a fine-wire thermocouple (wire 0.005 inch in diameter) located very close to the center of the test section but not touching it. For the low temperature tests,
