RADIOLYSISOF PENTANE ADSORBEDON MINERAL SOLIDS
Jan., 1958
33
TABLE I1 FORMATION O F ISOTOPIC METHANES IN THE RADIOLYSIS O F ETHANE: ETHANEds The ethane mixture used in all these runs consisted of 52 mole % ethane, 48 mole % ethane-&. Run no.
Press. (mm.) CIH~CPDO
CD4
Relative concn. methaneso CDIH CDeH; CHaD
CD4/ CDaH
CH4
CHd CHID
20 404.5 98.1 44.4 26.5 24.4 100 2.21 4.10 21 249.7 100.4 44.8 13.1 31.4 100 2.24 3.18 22 304.4 96.8 43.8 15.6 29.8 100 2.21 3.36 20ab 404.5 88.0 33.7 10.0 23.3 100 2.61 4.29 22ab 304.4 92.7 35.6 18.9 24.0 100 2.60 4.17 27c 235.9 (15.3) (5.3) .. 2.89 a The sensitivity corrections were not used in calculating these concentrations. The measured sensitivities for CH, and CD4 were 24.6 and 23.9 divisions/micron, respectively. 6% ethylene added in these runs. 27% ethylene addod.
..
and StevensonI2 have reported the value of the reaction cross-section for the highly efficient reaction C*Hz+
+ CzHs = CsH6' + CH4
The C2Ha+ion is the second most predominant ion in the mass spectrum of ethane, and it may be this reaction which is involved in the molecular detachment formation of methane. No analogous ion-molecule reaction which might account for molecular detachment of hydrogen from ethane has been reported, and we have failed t o observe such a reaction. Molecular detachment of hydrogen may, however, occur in the primary ionization. Such a molecular detachment is inherent in one of the primary processes listed by Williams and Essexl* (12) D. 0. Schissler and D. P. Stevenson, J . Chcm. Phys., 24, 926 (1956). (13) N. T. Williams and H. Essex, ibid., 17, 995 (1949).
..
C I H ~=
..
+ HZ+ e-
c2&+
and it is t o be noted that the C Z H ~ion + is th.e most predominant ion in the mass spectrum of ethane. Hipple,14in a consideration of the energetics of the electron impact dissociation of ethane, has pointed out, earlier, that the formation of the fragment ions should involve direct detachment of molecular hydrogen CzHa = C2H3+ HZ H e' C2H6
+ + + + 2112 + e -
= C2H2+
This type of reaction may be the most important mode of formation of molecular hydrogen. Acknowledgment.-The author is indebted to Mr. P. C. Noble for performing the mass spectrometric analyses, and to Mr. F. Haber for operating the resonant transformer. (14) J. A. Hipple, Phya. Rev., 63, 530 (1938).
RADIOLYSIS OF PENTANE ADSORBED ON MINERAL SOLIDS'" BY JAMES M. CAFFREY, JR.,'~ AND AUGUSTINE 0. ALLEN Chemistry Department, Brookhaven National Laboratory, Upion, Long Island, New Yo,,k Received June 9.4,1067
When distilled onto various solids of large surface area and then irradiated with y-rays, n-pentane yields product distributions which differ widely.for the different solids, and are all different from the results of radiolysis of the bulk: liquid. The total amount of decomposition a pears to be greatly increased by the presence of silica gel, EO that a specific transfer of energy from the silica to the adsorbe! pentane must be postulated. The results appear to reopen the question of the role of radioactivity in the genesis of petroleum.
Introduction chemically inert solid. Third was the old question of the effect of The radiolysis of organic compounds in the adsorbed state does not appear t o have been radiation on the constitution of petroleum and studied. Three questions prompted us to investi- petroleum gases. The radioactive element congate the subject. First was the simple question tent of many petroleum source bed rocks is high whether the presence of a solid surface would alter enough so that, over geological periods of time, the nature of the products of radiolysis t o an im- chemical action of the radiations on the contained portant extent. Such an effect would widen the organic material might be expected. To discover possibilities for application of high energy radiation what importance this effect might have in explaining the constitution of petroleum, a considlerable in chemical syntheses. Second was the question whether energy given program of radiation chemistry was carried out at up t o one phase by radiation could be transferred the Massachusetts Institute of Technology under t o another phase; that is, might the radiolysis of the auspices of the American Petroleum Institute the organic compound be sensitized, or perhaps (Project 43c). Studies were made of the radioprotected against, by intimate admixture with a activity of various petroleum source sediments and the effect of radiation on various organic com(I) (a) Research performed under the auspices of the U. S. -4tomic pounds. The conclusions were that the theory of Energy Commission. (b) Guest Scientist, from the Texaco Reradioactive effects in petroleum source beds must searoh Center, Beacon, N. Y.
34
JAMES M. CAFFREY,JR., AND AUGUSTINE0. ALLEN
V d . 62
e
% n-C5 H,, ON SOLID ADSORBENT
+
Fig. 1.-Hydrogen yield ( G H ~molecules per 100 ev. absorbed by n-C5Hln adsorbent) versus the weight per cent. of npentane in the solid adsorbent-pentane mixture: o, 1OOp SiOl; A, 62p Si02; o, 0 . 0 2 SioZ; ~ 0 , less than 44p Fez03; e , 1p attapulgite; e, 5p montmorillonite; 0 , silica gel.
be ruled out on three counts.2 First, petroleum gases contain little or no hydrogen, but the irradiation of organic compounds in the laboratory always produces hydrogen as the major gas constituent. Second, some petroleum contains little or no olefinic compounds, but these compounds are important products of hydrocarbon radiolysis. Third, and most important, the radioactivity of some petroleum source rocks is so small that if the energy of the radiations is divided between the mineral and organic phases in effective proportion to their stopping powers a conversion of only about 1% of the organic matter could be expected over geological periods of time. The conclusions were based entirely on results of radiolysis of organic compounds in bulk but the organic compounds in source sediments may be largely adsorbed as monomolecular films on mineral particles. To elucidate these questions we have chosen to study the radiolysis of a hydrocarbon adsorbed on a variety of mineral powders of large surface area. Normal pentane was chosen because it is readily distributed over a solid powder by distillation and (2) W. L. Whitehead, in "Nuclear Geology," H. Fad, editor, John Wiley and Sons, lno., New York, N . Y . , 1954, Chap. 7. (3) (a) H. A. Dewhurst, THISJOURNAL,61, 1466 (1957); 62, 15 (1958); (b) A. E. de Vries (Brookhsven National Laboratory), priv a t e communication.
because its radiolysis in bulk has been thoroughly studied. 3
Experimental The n-pentane was Phillips pure grade, shaken 48 hours with concentrated sulfuric acid, washed with potassium hydroxide solution and fractionally distilled through a 20plate bubble cap column, then filtered through a silica gel column. Vapor phase chromatography showed that the material contained about 0.3% isopentane. Reported yields of isopentane in the irradiations are corrected for this blank. The various solid powders used are described in Table I. The solids were preconditioned by evacuation a t 300-450" followed by pre-irradiation in vacuo by 106 r. of y-rays. A fixed volume of 10 cc. of solid powder was weighed (in air) into an irradiation cell which had a vacuum break-off seal. After further evacuation of the adsorbent a t 400' for one hour, the pentane (thoroughly degassed) was distilled onto the powder. The irradiation cell was then sealed under vacuum. The sample was irradiated with Co60 -prays at a dose rate of 269,000 r. per hour for total doBes of either -15 X lo6or -36 X 1 0 6 r. (based on FeSOl dosimetry). After irradiation the samples were opened into a vacuum system. The tube was heated to 100" and all the volatile material was distilled into a liquid air trap. The gaseous fraction, consisting of hydrogen, methane and a trace of ethylene, was pumped through the trap into a McLeod gage where the gas was measured, and then to a Saunders-Taylor combustion analysis apparatus where the percentages of hydrogen and methane were determined. The material retained in the liquid nitrogen trap was analyzed by vapor phase chromatography on a 2-m. long 6-mm. i.d. column containing tri-m-cresyl phosphate supported on crushed firebrick. The column was unable to separate ethylene from ethane, but did separate propane from propylene and iso- and n-
RADIOLYSIS OF PENTANE ADSORBED ON MINERAL SOLIDS
Jan,, 1958
35
TABLE I SOLIDS USED Trade name
Approx. chemical constitution
Particle diameter,b
Specific surfaae
B
(m.l/g.)
Supplier
d SiOz 62
