I
R. M. LINCOLN, R. L. ROGERS, H. BURWASSER, and V. J. KEENAN Research and Development Department, The Atlantic Refining Co., Philadelphia 1, Pa.
Irradiation of Petroleum Hydrocarbons Comparative Effects of Particle Size
T o
study the mechanism of hydrocarbon radiolysis, previous work ( 6 ) has been extended to compare the effect of light 2s. heavy particles on cyclohexane, methylcyclohexane, heptane. and 2,4-dimeth>-lpentane. These hydrocarbons \+-ere chosen to explore possible differences due to the presence of methyl groups.
Experimental For heavy particle radiation, a boroncontaining compound was dissolved in the hydrocarbon. Exposure of the solution to radiation within a n atomic reactor effected the boron-10 ( n , a ) lithium-7 reaction. One-million volt electrons from a resonant transformer (6) ivere used for the light particle radiations. Irradiations in the atomic reactor Ivere performed a t Brookhaven National Laboratories. Upton, S. Y . The hydrocarbons were pumped from a 15-liter reservoir through 100 feet of 3,’g-inch aluminum tubing (1 .j-liter capacity) coiled within fuel element hole C-12-2 North, then through a water-cooled coil back to the reservoir. T h e flow rate was kept constant at 218 cc. per min. Tubing uithin the pile was insulated to minimize heat exchange with the pile. Thermocouples connected to a recorder gave a continuous record of charge temperature just before entering and after leaving the pile. With the reactor operating at 24 megaIvatts, these 3” C . a t the temperatures Tvere 25’ entrance and 50’ 4 3” C. a t the exit. T h e circulating pump \vas a 10-gallonper-hour Lapp Pulsafeeder equipped with a 5-inch Ful-Floiv filter on the inlet. T h e contents of the reservoir were sampled periodically ivithout replacement. Irradiations \vith million volt electrons were carried out Tvith General Electric CO.’S 1000 k.v.p. resonant transformer electron beam generator operating a t 1 ma. Hydrocarbon was exposed to the beam in the irradiation cell apparatus ( 6 ) . Reaction temperature was maintained at 3.5’ C . by cooling with ice water. Samples of about 1070 of the charge \cere removed periodically and replaced with fresh reactant. A Consolidated Engineering (Chicago. Ill.) mass spectrometer (Model 21-103) of high resolution and containing a heated inlet system (7) was used for
part of the analysis ( 3 ) . .4lthough the high molecular weight irradiation products formed are a mixture of new compounds for which no calibration was available, the analytical error is estimated as not greater than ztO.2070 relative. Further analysis of the liquid samples were performed using gasliquid partition chromatography. Hydrocarbon samples u p to 16 carbon atoms were analyzed and identified with a chromatograph for high temperature (Consolidated Electrodynamic Corp.. Model S o . 26-201). The column contained Dow Corning silicone vacuum grease as stationary phase. supported on firebrick. T h e hydrocarbons used were pure grade (Phillips Petroleum Co.) Trimethyl borate (purchased from Matheson, Coleman. and Bell) was selected as a soluble boron compound of suitable volatility.
Results and Discussion Subsequent to these experiments, oxygen dissolved in hydrocarbon liquids has a pronounced inhibitory effect on the normal course of the hydrocarbon radiolysis ( 4 ) . No significant amounts of oxygenated compounds other than small amounts of carbon monoxide, carbon dioxide. and water in the reacted hydrocarbon could be detected with mass spectrometry or chromatography. As no effort was made in this rvork either to deaerate or to maintain air saturation in the hydrocarbon liquid, no G values are reported in this work. However. because the degree of aeration for the in-pile experiments and for electron gun irradiations was about the same, the data are significant for internal comparisons. Values of the neutron flux for the reactor location in lvhich the experiments were performed \\ere obtained from Brookhaven ( 2 ) . After making suitable
Table I.
corrections for size and absorption of the reactor container, the value of the thermal neutron flux was 1.O x 1OI2n per sq. cni. per sec. No attempt was made for more exact dosimetry. This neutron flux value was obtained for all in-pile experiments. T h e total dose did not exceed e.v. per gram and extent of reaction did not exceed loyo. Maximum in-pile tirne for each experiment was 5 days. T o investigate the possibility that trimethyl borate (TMB) may have sensitized the reaction in the pile by means other than fission recoil, a n O.57hf solution of T M B in n-heptane was irradiated kvith I-million volt electrons from the resonant transformer. Comparison ivith boron-free irradiation showed that T M B had no effect on electron-induced radiolysis of heptane. Therefore, in a neutron-gamma flux: the T M B kvould effect the radiolysis in no way other than being a source of recoil particles resulting from boron fission.
In-Pile Experiments Cyclohexane. .4ddition of small amounts of trimethyl borate produced a sizable increase in the rate of formation of the txvo major products, cyclohexene and dicyclohexyl. T h e plot of amounrs of products as a function of reaction time indicates a linear relationship. This permits averages of the amounts produced per unit time giving rates to a precision no worse than 20(T0 for an!single experiment (Table I). Addition of boron and the resultant increase of ionization track density produce a greater relative increase in rate of formation of cyclohexene than of dimeric materials. Also unsaturation is increased in the dimeric material as boron concentration is raised. These results are similar to “track effects” obtained in the radiolysis of aqueous sys-
Addition of Small Amounts of Trimethyl Borate Increases the Rates of Product Formation Moles/Hour
TMB, Moles/Liter
Cyclohexene
Dicyclohexyl
0.00 0.11 0.27 0.46 0.54
0.5
0.2 0.3
2.4 3.3 6.8
...
0.5 0.6
...
X lo2 Cyclohexylcyclohexene
hlethylcyclohexene
Total Dimer
0.1
0.2
0.03
0.2 0.2 0.5
...
... ...
...
...
...
7
VOL. 5 1 , NO. 4
...
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APRIL 1959
547
tems (7). Data on the other systems support the suggestion that the olefinic materials formed represent products from a molecular or direct action process. Dorfman ( 5 ) , Stevenson ( 8 ) , and Dewhurst ( 4 ) indicate that hydrogen is produced molecularly. T h e distinction between a molecular process and track effect should be made clear. A molecular process may or may not be unimolecular. I n aqueous systems ( 7 ) , the evidence strongly suggests that the molecular products are formed from bimolecular reactions. Considerable support comes from the fact that G values are dose or intensity dependent. I n h y d r o c a r b o n liquids, the evidence, from Dewhurst’s work ( 4 ,would indicate that olefin is formed primarily by a molecular process. The authors’ work in addition implies a track effect and thereby suggests that disproportionation plays an important role in olefin production. The temperature independence in Dewhurst’s work may account for the existence of “hot” radical reactions leading to olefin production. This track effect may be due to the dissolved oxygen acting as a radical scavenger. Methylcyclohexane. Lack of suitable calibrations for mass spectrometric analysis made complete identification of the dimers impossible. From some available calibrations, however, 1,2dicyclohexylethane was found to be one of the isomers. Smaller amounts of other materials of mass 174 could not be identified via mass spectrometry. With addition of trimethyl borate, the rate of methylcyclohexane dimer formation show a much greater increase than that for the cyclohexane dimer. This may be attributed to the presence of a methyl group on the ring, which may promote dimer formation through interaction involving the methyl groups of neighboring molecules. The appearance of unknown trimeric and tetrameric materials along with increasing amounts of unsaturation in the dimer (up to 50% at 10% conversion) made qualitative analysis of dimer isomers impossible. Chromatographic analysis established the presence of a product (about 370 by weight of the radiolytic products) having 7 to 14 carbon atoms. Isolation and subsequent mass spectrometric analysis
Table II.
of this product showed it to be ethylcyclohexane. The unexpectedly high yield of this product, whose formation is explained by radical recombination, suggests that it is formed in regions of high radical or ion concentration. This compound was present under all conditions of irradiation. n-Heptane a n d 2,bDimethylpentane. Results of these experiments follow the general pattern set by the two previous hydrocarbons. That is, inclusion of trimethyl borate enhanced decomposition of the starting material by a factor of 10 or more. Complete quantitative analysis via mass spectrometry was impossible, due to lack of calibration data for the many possible c14 branched olefins. In addition, a breakdown of the highly branched dimer olefins corresponds to that of the monomer olefin making quantitative identification of monomer olefin with dimeric olefin very difficult. Chromatographic analysis was used to effect a better product separation and fractions were analyzed with a mass spectrometer. T h e method was not sufficiently refined to collect reliable rate data, but allowed an estimate of the ratio C7olefin to total C14 hydrocarbon. Ratios are shown below in Table 11, column 2. Chromatography shows a greater proportion of products intermediate between c14 and C7 from 2,4-dimethylpentane than from n-heptane. For 2,4-dimethylpentane the peak areas indicated three to four times as much total intermediate ((28, C9, CIO) produced as C W G values are not reported. As the olefin-dimer ratio seems to be a critical characteristic and is a ratio of G values (in the presence ofair), the data have been summarized (Table 11). For greater precision, the ratios from each analysis were averaged rather than averaging the rates for each product and taking their ratio. Comparison with 1 Million Volt Electrons
Hydrocarbon products, similar to those from the in-pile experiments of these irradiations, were produced with the same pseudo-zero order kinetics. T h e olefin-dimer ratio increases in
Comparison of Olefin-Dimer Ratios Shows an Increase from Electron to Recoil Particle Radiation 1 Million Volt
Electrons
Cyclohexane
0.4
1.9
W ( n ,a)Li7
TMB, Mole
3.5 0.2 4.7 0.1 5.6 & 0 . 4 1.8 0.2 9 3a 7.1 1.1
0.11 0.27 0.46 0.54
* * * *
1.1 f 0 . 3 Methylcyclohexane 2.0 0.2 %-Heptane 0.51 7.0 1.1 2,4-Dimethylpentane The scatter of the analytical a Average of 3 runs made with varying amounts of TMB. data, however, was too great to warrant reporting separate ratios as in cyclohexane.
* *
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INDUSTRIAL AND ENGINEERING CHEMISTRY
going from electron to heavy recoil particle radiation. This is again suggestive of a “track effect” occurring in the radiolytic decompositions of hydrocarbons. T h e exception of 2,4-dimethylpentane may be due to the dimeric material being no longer a major product and olefin-dimer ratio no longer critical. The apparent identity in olefin-dimer ratios between the two types of radiation is therefore fortuitous. Conclusion
In hydrocarbon radiolysis, a track effect probably occurs. If densely ionizing radiation favors molecular processes occurring in the track, then the monomeric olefin is produced. However, other possible processes, as free radical disproportionation external to the track, may be important in the olefin production. Dimer yields do not increase as rapidly as olefins, indicating that the dimer comes from a nonmolecular process. At the reaction temperatures used in this work, hydrogen abstraction would be effected most easily by hot radicals. These radicals would be the light species resulting from random scission of the starting material-i.e., H, CHa, CzHs. The net result is the appearance of thermalized radicals corresponding to the reactant molecule minus one hydrogen. These would explain the major product in the bulk reaction being a compound of twice the carbon number of the starting material. Acknowledgment
‘The authors wish to thank Eleanor Donall for the mass spectrometric interpretations and William J. Rooney for much of the experimental work. Literature Cited (1) Allen, A. O., Holroyd, R. A., J. Am. Chem. SOC. 77, 5852 (1955). (2) Brookhaven National Laboratories,
Upton, N. Y., “Research Reactor Facilities-Irradiation Services and Radioisotopes” (August 1956). (3) Brown, R. A., Melpolder, F. W., Young, W. S., Petrol. Processing 7, 204 (1952). (4) Dewhurst, H. A,, 132nd Meeting, ACS, New York, September 8-1 3,1957. (5) Dorfman, L. M., J. Phys. Chem. 60, 836 (1956). (6) Keenan, V. J., Lincoln, R. M., Rogers, R. L., Burwasser, H., J . Am. Chem. SOC. 79, 5125 (1957). (7) O’Neal, M. J., Jr., Wier, T. P., Jr., Anal. Chem. 23,830 (1951). (8) Stevenson, D. P., Schissler, D. O., J . Chem. Phys. 23,1353 (1955). RECEIVED for review April 7, 1958 ACCEPTEDNovember 12, 1958 Division of Industrial and Engineering Chemistry, Symposium on Uses of Radiation in Industrial Chemical Reactions, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
