Electron Irradiation of Hydrocarbons I. Radiolysis of Liquid N-Pentane

Bull. SOC. Chim. Belg., 73, pp. 689-702, 7 fig. (1964)

ELECTRON IRRADIATION OF HYDROCARBONS

1. RADIOLYSIS OF LIQUID n-PENTANE

P. CLAES (*) and S. RZAD (Louvain)

R ~ M E

Ce travail a pour objet l’ktude de l’action d’electrons d’une tnergie de l’ordre de 30 KeV sur le pentane n* liquide. Nous avons rnis au point une methode d’analyse des differents gaz forrnes au cours de la radiolyse. Nous nous sornmes attaches surtout A Btudier l’influence de I’intensitt d’irradiation et de la dose totale dd te au systeme. L’Btude de ces effets nous a permis de rnettre en tvidence les reactions qui prennent une part prepondbante aux effets chimiques observes.

I. INTRODUCTTON

Lately, a great many articles concerning radiation chemistry of hydrocarbons have been published, Detailed results on the radiolysis of n-pentane were published by Wagner (1) and by de Vries and Allen (2).

The present work aims to study the action of 30 KeV electrons on liquid pentane. The influence of dose rate and total dose absorbed by the irradiated system on the yields of gaseous products was investigated. The analysis of light hydrocarbons formed during the radiolysis is difficult since most of these compounds are soluble in the irradiated liquid. The partition between gaseous and liquid phases is characterized by an equilibrium obeying Henry’s law. We worked out an analytical method allowing the quantitative determination of these hydrocarbons.

EXPERIMENTAL

Materials Fluka purissirnum grade n-pentane was used. It was 99,98% pure. The

only detectable impurity is isopentane, its amount was not affected by irradiation as measured by gas-liquid chromatography.

(*) Charge de Recherches du Fonds National de la Recherche Scientifique. (1) C.D. WAGNER, J. Phys. Chern., 64,231 (1960). (2) A. E. DE VRW and A. 0. ALLEN, J. Phys. Chem., 63,879 (1959).

690 P. CLAES AND S. RZAD

Irradiations

The apparatus described previously (3) has been improved. Indeed, preliminary irradiations on liquid n-pentane gave poor reproductibility. The sample introduction described in the preceding paper (3) implied sealing of some parts of the pyrex connection between the irradiation cell and the manometer. This manipulation might produce a pyrolysis of a part of the sample. On the other hand, the volume of the introduced liquid was not known with enough accuracy. Figure 1 shows the new connection we used in this research. (m) is a magnetic break-seal wich allows communication of the apparatus with

,I

Fig. 1 - Connexion between the irradiation cell and the manometer,

the system of products recuperation. ( d ) is a Dewar causing reflux of the pentane to the irradiation cell. The teflon cock (c) allows the isolation of an homogeneous part of the gaseous phase. The sample, introduced through the groundglass joint (j), was degased in the trap ( t ) . After irradiation, the sample was condensed in trap ( t ) . The reproductibility obtained by this method was much better. The runs were carried out without interruption on 6ml of n- pentane.

Dosimetry

The initial hydrogen yield, G(H2)o = 5,55, from cyclohexane was used as a chemical dosimeter. This G(H2)o is an average of the values reported by several authors (4-13).

- ~~ - ~~

(3) P. CLAES, P. HUYSKENS, J.L. GA~LLIEZ et S. RZAD, J . Chim. Phys.,

(4) R.H. SCHULER and A.O. ALLEN, J . Am. Chenz. SOC., 77, 507 (1955). (5, J.P. MANION and M. BURTON, J. Phys. Chem., 56, 560 (1952).

61, 271 (1964).

ELECTRON IRRADIATION OF HYDROCARBONS 69 1

The so estimated doses ranged from 2,9. lo5 rads to 1,04. lo7 rads. The dose rate is a function of the heating of the filament. Physical measurements allow us to determinate the relative dose rates corresponding to different heating current values.

Analysis

Analyses were carried out with a Carlo Erba “Fractovap ” chromatograph Model B, Cat no 5390. A two meter column packed with 3% di-2-ethyl-hexyl- sebacate on silicagel was used for the analysis of the gas phase. The carrier gas was helium. The katharometer was calibrated by passing known quantities of the various hydrocarbons under the conditions of temperature and flow rate used in the irradiation product analyses.

11. ANALYTICAL METHOD

This work is concerned only with gaseous products.

1. Hydrogen

After irradiation the liquid was condensed in the liquid nitrogen trap (t) (fig. 1). At this temperature the vapour pressure of the organic compounds is negligible. However we observed that hydrogen was absorbed in the solid hydrocarbon to an extent rigorously proportional to the pressure. We measured the pressures of known amounts of hydrogen in the presence of n-pentane or cyclohexane maintained at liquid nitrogen temperature. These pressures are independent of the nature of the hydrocarbon. Figure 2 shows a graph giving the experimental pressure P, measured when the hydrocarbon is maintained at liquid nitrogen temperature versus the hydrogen pressure P existing in the apparatus when the hydrocarbon is at room temperature. It is seen

~~~~~

(6) M. BURTON, J. CHANG, S. LIPSKY and M.P. REDDY, Rad. Res., 8 ,

( 7 ) W. S. GUENTNER, T. J. HARDWICK and R. P. NEJACK, J. Chem. Phys.,

(*) G. R. FREEMAN, J. Chem. Phys., 33, 71 (1960). (9 P.J. DYNE and W.M. JENKINSON, Can. J. Chem., 39, 2163 (1961). (lo) P. J. DYNE and J.A. STONE, Can. J. Chem., 39, 2381 (1961). (I1) L. J. FORRESTAL and W. H. HAMILL, J. Am. Chem. Soc., 83, 1535

(12) J.F. MERKLIN and S . LIPSKY, Biological effects of’ ionizing radiations

(l3) R.H. SCHULER, J. Phys. Chem., 61, 1472 (1957).

203 (1958).

30, 601 (1959).

(1961).

at the molecular level, International Atomic Agency, Vienna, 73 (1962).


Fig. 2 - Experimental pressure Pe measured when the hydrocarbon is maintained at liquid nitrogen temperature versus the hydrogen pressure P existing in the apparatus when the hydrocarbon is at room temperature.

that the accuracy of the method is very good. Using the experimental graph we were able to determine from measured pressures the numbers of hydrogen molecules formed during the radiolysis.

2. Organic Gases

The quantitative analysis of organic gases is more difficult. Indeed, the gaseous organic phase is a mixture of light hydrocarbons partly soluble in the irradiated liquid. In order to obtain the total amount of a produced gas from its partial pressure in the vapour phase, the partition coefficient of this gas between the two phases needs to be known. The coefficient of each gaseous product was measured in the same conditions as during the analysis.

A gas, partly soluble in a liquid, partitions itself according to Henry’s law :

- K CL CU _ -

where CZ is the concentration of the gas in the liquid phase, Cg its concentration in the gas phase and K a constant for a given set of gas, liquid and temperature.

ELECTRON IRRADIATION OF HYDROCARBONS 693

After transformation, we get :

ntot = n, [I +K ;] where ntot = nl + ng is the total number of moles of a given gas, nr and ng are the numbers of moles respectively in the liquid and gas phases. VZ and VB are the volumes of these phases.

The percentages of the organic gases in the gas phase were obtained by means of a chromatogram of an homogeneous part of this phase. The total pressure measured at 15OC (temperature at which the partition coefficients were determined) reduced by the hydrogen pressure yields the total organic pressure. Knowing the total pressure and the chromatographic percentages, we can obtain the partial pressures of the different organic products in the gas phase. Using formula (2) and the partition coefficients, we are able to calculate the amounts of gaseous hydrocarbons formed during the irradiation.

EXPERIMENTAL PART

Figure 3 shows the apparatus used for determination of coefficients K.

I manometer

Fig. 3 - Apparatus used for the determination of partition coefficients.


Ethylene Propane Propylene Butane Isobutane

This apparatus is constitued by: 1 . a pyrex tube ( t ) containing a cell filled with n-pentane, which we can

break by means of a magnetic hammer; this tube may be removed from the apparatus;

2. a volume thermostated at 1 5 o C ( 0 ) ;

3. a manometric system, in which a part is composed of gauged volumes(g). The bulb (6) containing the gas to be experienced may be fixed onto the

vacuum line by means of a ground joint. The total volume (of which sizes were quite the same as those of the irradiation apparatus) of the apparatus was previously gauged.

The amount of introduced gas was measured manometrically. After breaking of the cell, the liquid, distilled into the volume ( v ) , is thermostated at 15oC. The difference between the sum of the vapour pressure of the liquid and the initial pressure of the gas, and the pressure measured at equilibrium provides the quantity of dissolved gas. The results are given in table I:

4,66 33,lO 25,OO

151,ll 91,96

TABLE I

Liquid = n-pentane

T = 15OC

Owing to their high solubilities, most of the butane and the isobutane is found in the liquid phase.

I

1

Fig. 4 - Chromatographic cell.


"'tot. 10-19

In order to get an homogeneous part of the gaseous phase, the irradiated pentane was thermostated at l5OC in ( t ) (Fig. 1). Manometric measurements at equilibrium furnished the amount of gases formed during the radiolysis and present in the gas phase. Afterwards an homogeneous part of this phase was isolated by closing the stopcock (c) (Fig. l), and entirely pumped through (m) by means of an Antropoff pump into the chromatographic cell (Fig. 4).

This latter was fixed to the chromatograph with rubber tubing pipes. The carrier gas circulated first from (a) to (b) through (3) and pushed out the air contained in this part of the cell. Once the chromatograph stabilization was performed, the volume (v ) was heated in order to evaporate the pentane condensed on the walls. The closing of (3) and the simultaneous opening of (1) and (2) allowed the carrier gas to pass through ( v ) and the sample to be rapidly introduced onto the column.

xorg. 10-19

111. RESULTS AND DISCUSSION

(1)

Results of radiolysis of liquid n-pentane are summarized in table 11.

3.15

TABLE I1

- 1.744 4.658

10.305 10.071 13.662 18.867

- 0.340 1.281 1.823 1.633 2.243 3.490

Dose rate ( 10-20eV/h.)

2.96 5.91 5.91 5.91 5.91 5.91

17.73 5.91 5.91

( 5 ) (6) (7) (8) (9)

N H ~ . 1049

37.81 113.42 113.42 170.13 226.85

0.329 0.671 1.388 1.404 3.377 8.482 8.438

11.419 15.377

Columns four, five and six of table I1 show successively the number of hydrogen molecules N H ~ the total number of molecules present in the gas phase Ntot and the number of organic products molecules present in the gas phase Nbrg = Ntot - after each radiolysis. In figure ( 5 ) we plotted NH,, Ntot and Nbrg versus the absorbed dose.

The hydrogen curve of Figure (5 ) shows the number of molecules is not a linear function of the dose absorbed by the system, since the rate of hydrogen production decreases as the dose raises. In figure (6)


15

10

5

Fig. 5 - Graph giving Nbt (m), N H ~ (0) and ,N'& ( A) versus the absorbed dose.

the hydrogen yield is plotted as a function of energy absorption. By

extrapolation at zero dose we found a G(H2)o value of 4,98

The G(H2) values reported in the litterature ranged from 3,5 to 5,04 mo1/100 eV (1) (2) (14-17).

Our results seem to be in fairly good agreement with these yields corresponding to different absorbed doses. However Hardwick obtained a G(H2)o value of 6,35 mo1/100 eV (18) which is higher than ours. This difference might fairly well be due to the dosimetry used in both works.

molecules 100eV

~ _ _ _ _ ~~~

(14) J.W. SUTHERLAND and A.O. ALLEN, J. Am. Chem. SOC., 83, 1040

(16) H.A. DEWHURST, J. Phys. Chem., 61, 1466 (1957). (16) H.A. DE'WHURST, J. Am. Chem. Soc., 80, 5607 (1958). (17) R.H. SCHULER and R.R. KUNTZ, J. Phys. Chem., 67, 1004 (1963). ('8) T.J. HARDWICK, J . Phy.s. Chem., 66, 1611 (1962).

(1961).


It will be noted that the 5,04 yield found by Schuler and Kuntz(17) is measured at very low absorbed dose (lJ.105 rads) and has to be very close to the G(H& value. The agreement between Schuler’s value and ours is very good. The continuous decrease in hydrogen gas yield as a

20 40 Dose (rads 0-’I Fig. 6 - Graph giving G(Hs) against the absorbed dose.

function of energy absorption was observed for various hydrocarbons by several authors (7) (8) (10) (19-22). It seems now evident that this decrease is due to a scavenging of hydrogen atoms by olefins formed during the radiolysis.

(19) T.D. NEVITT and L.P. REMSBERG, J. Phys. Chem., 64, 969 (1960). P O ) T.J. HARDWICK, J. Phys. Chem., 64, 1623 (1960). (21) R.A. HOLROYD, J. Phys. Chem., 65, 1352 (1961). (22 ) G.R. FREEMAN, J. Chem. Phys., 36, 1534 (1962).


The importance of this reaction gradually increases as the olefin concentration rises. This competes with other possible processes of hydrogen atoms disappearence.

H'+H'-+ Hz (2)

H ' + R - + R H (3) H' + C5H12 -+ HZ + C5Hll' (4)

H' + C5H10 -+ HZ + C5HO' (5 )

The experimental points relative to the organic gases are rather dispersed (fig. 5). A dose effect, if any, is certainly very weak. In order to elucidate this point, we plotted against the absorbed dose the heights of the chromatographic peaks. This graph (figure 7) shows more clearly a lack of dose effect for these products.

50

50 100 150 200 Dose (rt Fig. 7 - Graph giving the heights of the chromatographic peaks versus

the absorbed dose.

In table 111, the percentages of various organic gases present in the gas phase after each irradiation are given for various doses and dose rates.

These percentages do not change with dose. The yields of these different gases were calculated from the experimentally obtained straight


C2H6

line in figure 5. They are shown in table IV together with results reported by other authors.

C2H4 1 C3H6 1 C3H6

TABLE 111

Dose rate ( 10-20eV/h)

Dose (rads.

CH4

18.91 113.42 113.42 170.13 226.85

5.91 5.91

17.73 5.91 5.91

17.0 17.2 17.1 17.4 16.3

27.7 33.2 10.7 26.6 33.2 11.2 26.1 I 34.8 1 10.7

11.3 11.6 12.1

Gas

TABLE IV

de Vries-Allen (a)

0.22 0.27 0.36 0.33 0.29

'utherland-Allen (l4:

0.22

) 0.62

0.59 0.10

This work

0.12 0.26 0.28 0.25 0.19

The agreement between the three series of values is less in this case as in the case of hydrogen. It will be noted that the chromatograms of the gaseous products show the presence of traces of acetylene and isobutane. Butane and isobutane do not appear in table IV since they remain dissolved in the liquid.

Independence of the rate of organic gases production from dose indicates the alkyl radical addition on olefins (6) is a negligible process during radiolysis.

If no true, a dose effect like for hydrogen would appear. Two runs (1) and (7) of which results are presented in table I1 were

carried out at different dose rates. The experimental points corresponding to these two irradiations do not diverge from the curve drawn


Dose I Dose rate 'I? 1 (rads. I (10-20eV/h)

I

-.

6 113.42 i 5.91 17.73 7

113.42 I

through the other points (fig. 5) . In table V are given the gaseous products yields of runs (6) and (7) which were performed at the same dose but different dose rates.

TABLE V

Hz CH4 CaHa CZH4

3.19 0.13 0.28 0.31 3.17 0.12 0.26 0.28

C3Ha C3He

0.27 0.20 0.25 1 0.20

The difference between the various G values are not significative. It appears the dose rate does not influence the yields of different products formed during radiolysis.

For a large range, Futrell(23) and Hardwick (20) do not observe any dose rate effects on the various yields in the n-hexane radiolysis and Freeman (22) does not find any dose rate influence on the production of hydrogen and methane during the radiolysis of methylcyclohexane.

However, it will be noted that, for a determined range of dose rates, Holroyd (21) observes a variation of methane and ethane yields in the radiolysis of liquid neopentane. Methane yield diminishes from a stable value to a value which is no more influenced by further variations of dose rate. Inversely ethane yield increases. On the other hand, Schuler and Kuntz (17) point out a similar G(CH4) variation for the same range during the radiolysis of 2, 2, Ctrimethylpentane. These authors ascribe this effect to the transition from a region where hydrogen abstraction from the solvent by organic radicals (7) is efficient to a limit beyond which this reaction vanishes because the rate of radical recombination predominates (8).

(7)

(8)

C H j + R H + C H 4 + R

CH; + CHs -+ CzHs

This effect is certainly more apparent for branched chain hydrocarbons, which show large organic gas yields (le), than for linear ones. Holroyd, Schuler and Kuntz are dealing with dose rate ranges much

~~ ________ - ~- -~ -

(23) J.H. FUTRELL, J . Am. Chem. Soc., 81, 5921 (1959).

ELECTRON IRRADIATION OF HYDROCARBONS 70 1

wider than we are. On the other hand, because of the short range of 30 KeV electrons and the particular mode of irradiation characteristic of the method (9, the density of the energy absorbed by our system is very high. The dose rates investigated in this research are probably higher than the upper limit of the transition region revealed by Holroyd, Schuler and Kuntz.

Anyhow, several assumptions may be made to explain the lack of dose rate effect.

1. If active species arising from different tracks cannot interact because of the diffusion rate weakness, the whole effect will result from the sum of reactions occuring in space volumes independent of each other. In this case, no dose rate effect will be observed.

2. If tracks overlap, and if the active species react only with each other and never with solvent or h a 1 products, neither dose nor dose rate effects will be observed. Indeed, since radicals have to recombine, the reaction products will be identical wether the recombinations occur between radicals arising from the same track or not.

3. If reactions of radicals with solvent or reaction products are much more important than recombination, a majority of alkyl radicals will appear after a very short time. These radicals may result either from hydrogen abstraction from the solvent or reaction products molecules, or from addition of initial radicals on produced olefins. In this instance a dose effect would be seen.

These three assumptions are limit cases. The discussion presented below is very summary, a more rigorous one will be reported in a later paper.

It is well known that even very low concentrations of iodine or other scavengers trap most of radicals produced by irradiation. The first assumption seems therefore very improbable.

Since a dose effect occurs for hydrogen evolving, the third possi- bility appears to be the good one. The instantaneous hydrogen production would result from the competition of reactions (l), (4) and (9, reactions (2) and (3) occuring to a negligible extent.

For neither dose nor dose rate effects are apparent in the organic gases production, only the second assumption is possible. Organic gases production would be essentially due to radicals recombination, i.e. addition (9) or disproportionation (10).

702 P. CLAES A N D S. RZAD

A and B are respectively an alkane and an olefin. In contradistinction with hydrogen, these two reactions would predominate for organic gases production.

The authors are much indebted to Professor P. HUYSKENS for many stimulating discussions.

This work received financial assistance from the F.N.R.S. (Fonds National de la Recherche Scientifique) and from the F.R.S.F.C. (Fonds de la Recherche Scientifique Fondamentale et Collective).

Department of Physical Chemistry III (Dir. Prof. P. HUYSKENS)

39, rue des Moutons UNIVERSITY OF LOUVAIN

Communique a la Societd Chimique de Belgiqiie le 16 avril 1964

Documents

Electron Irradiation of Hydrocarbons I. Radiolysis of Liquid N-Pentane