7
Vapor-Phase
γ-Radiolysis
o f Benzene,
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T o l u e n e , Ethylbenzene, and the X y l e n e s K. E. WILZBACH and LOUIS KAPLAN Argonne National Laboratory, Argonne, Ill. 60439 Benzene, toluene, ethylbenzene, and the three xylenes have been irradiated in the vapor phase with gamma rays. Products and yields have been compared with those in liquid-phase radiolysis. G values for disappearance in the vapor phase range from 6 to 10, more than five times greater than in the liquid phase. The principal product in each case is "polymer." All of the identified products are also found in the liquid phase, but relative yields are markedly different. The high yields of acetylene and some other products in the vapor phase suggest that ionic processes are more important here than in the liquid phase. nphe stability of aromatic hydrocarbons to radiation has been cited so frequently in the literature that it has come to be accepted (5, 6) as a characteristic of aromaticity, a consequence of electron delocalization. These conclusions are based on the results of irradiations in the liquid phase. That they may not apply to the isolated aromatic molecule, however, is suggested by the few reported studies of the radiolysis of aromatic hydrocarbons in the vapor phase. Such studies have thus far been limited to two compounds—benzene (8, 10, 11) and isopropylbenzene (cumene) (9), and differences in the character of the radiation or the conditions of irradiation preclude a simple assessment of the effect of phase on these systems. To provide further information on this point, we have investigated the γ-radiolysis of benzene, toluene, ethylbenzene, and the xylenes in the vapor phase and have determined yields of the gaseous products, "poly mer," and some products of intermediate volatility. These results are compared with those of parallel irradiations of liquid toluene and o-xylene and with published (2, 12) data for the other hydrocarbons in the liquid phase. 134 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
7.
WTJLZBACH
AND
135
Vapor-Phase y-Radiolysis
KAPLAN
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Experimental Chemicals. The o-xylene was an American Petroleum Institute stand ard sample, with stated impurities of 0.005 ±: 0.004 mole % . Other hydrocarbons were purified by gas chromatography. Purity was checked by gas chromatography on three columns of different selectivity: except for 0.02% p-xylene in the m-xylene, no more than 0.002% of any impurity was detected. Samples were stored in vacuo. Irradiation. Samples were irradiated in the Argonne high level gamma irradiation facility. γ-Rays, from spent reactor fuel elements, ranged in energy from 0.22 to 2.5 Mev., with an average of about 0.75 Mev. The dose rates for various samples varied from 1 Χ 10 to 4 Χ 10 rads per minute. The temperature of the samples was about 30 °C. Dosimetry. The intensity of the flux at the sample site was measured periodically with a ferrous sulfate dosimeter, using GFe(iii) = 15.5. Energy absorbed in liquid samples was based on this dosimetry and was corrected for the electron density of the samples. To determine energy absorbed i n the vapor samples, nitrous oxide was irradiated, at compa rable electron densities, i n the vessel used for the hydrocarbons; the G value for nitrogen production was taken to be 11.0 (7). Procedure. For liquid-phase studies, weighed samples (ca. 0.5 ml.) were irradiated in sealed, evacuated glass tubes (ca. 0.7 m l . ) . Vapor samples were irradiated in a 700-ml. glass cylinder. Small weighed sam ples (12-77 mg.) of hydrocarbon were introduced into the vessel i n sealed tliin-walTed capillary tubes prior to evacuation and sealing-off. Irradiated samples were processed on the vacuum line to yield a noncondensable gas fraction ( H + C H ) , a fraction containing the C to C hydrocarbons, and a liquid fraction containing the C to C i components. The weight of "polymer" was taken to be the difference between the initial weight of hydrocarbon and the sum of the measured weight of the liquid fraction and the calculated weights of the gaseous fractions. The residual polymeric material was not investigated further. Analysis of Products. The three fractions collected from each sample were analyzed by gas chromatography. The noncondensable fraction, containing hydrogen and methane, was analyzed on silica gel at room temperature. The fraction containing C - C hydrocarbons was analyzed at 75 °C. on silica gel treated with didecyl phthalate. Aliquots of the liquid fraction were analyzed on three columns of different selectivity: Bentone-34-didecyl phthalate; silicone SE-30; and m-polyphenyl ether (five-ring). Products were identified, and their yields were determined by comparison of retention volumes and peak areas with values for known amounts of authentic samples. The analytical procedure d i d not permit reliable analyses for C to C hydrocarbons. C and C products are formed from all of the alkylbenzenes, especially from ethylbenzene. A few unidentified peaks were observed i n the analyses of the liquid fractions but generally in low yield. 4
2
4
4
2
6
2
4
3
3
4
0
5
4
Results Our results on the γ-radiolysis of benzene, toluene, ethylbenzene, and the xylenes i n the vapor phase, and of toluene and o-xylene in the
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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136
RADIATION CHEMISTRY—Π
liquid phase, are shown i n Tables I and II. The results listed are those of specific experiments; duplicate runs gave product yields which agreed within 5-10% with those tabulated. The yields of products were closely proportional to dose over the limited range investigated, 20-100 Mrads. No systematic study of effects of dose rate and of pressure on yields was carried out; increasing the pressure of toluene from 7 to 22 torr produced no significant change. Included in the tables are the results of Burns and Jones (2) on the C o radiolysis of hquid benzene and of Verdin (12) on the C o radiolysis of ethylbenzene and m- and p-xylene in the liquid phase. To the extent that they overlap, our results on liquid o-xylene agree reasonably well with those of Verdin (12) as do our results on liquid toluene with those of Weiss and Collins (14). Comparison of the results of vapor- and liquid-phase radiolysis shows that the yields of all products from each hydrocarbon are markedly greater in the vapor phase. As in the liquid phase, "polymer" is the pre dominant product, accounting for 83-96% of the hydrocarbon consumed. The 100-e.v. yields (G values) of "polymer" increase with alkyl substitu tion from 6 for benzene to 8.7 for ethylbenzene; the yield i n each case is about five to six times that observed in the liquid phase. 6 0
e o
The gaseous products (hydrogen, methane, and the C hydrocar bons) are formed also with high G values i n the vapor-phase radiolyses. Yields of H increase markedly with alkyl substitution, from 0.15 for benzene to about 0.8 for ethylbenzene and p-xylene, paralleling the trend in the liquid phase but about four times greater in each case. Methane and ethane yields also increase with alkyl substitution; from the alkylbenzenes the yields of methane are 10-20 times greater and those of ethane are more than 100 times greater in the vapor than i n the hquid phase. Yields of ethylene and acetylene are also much higher in the vapor phase. Acetylene yields are decreased slightly by alkyl substitution; ethylene yields are increased markedly by ethyl substitution but only moderately by methyl substitution. 2
2
The liquid products include compounds i n which alkyl groups have been replaced by hydrogen and vice versa. In vapor-phase radiolysis, G values for replacement of a single alkyl group (e.g., benzene from toluene and ethylbenzene; toluene from xylenes and ethylbenzene) are ca. 0.3, about 10 to 20 times those i n the liquid phase. Replacement of two alkyl groups is still an important process i n the radiolysis of the xylene vapors although almost negligible in liquid o-xylene. Replace ment of hydrogen by methyl occurs both i n the ring and in side chains. In the vapor phase, the replacement of an aromatic hydrogen is affected by its position; G values range from 0.007 to 0.06, with the order of preference being meta > ortho > para. In the liquid phase, the corre sponding process is essentially statistical, with a G value of 0.001 per
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
7.
WBLZBACH AND K A P L A N
Vapor-Phase
137
γ-Rodiolysis
hydrogen. I n the vapor, side-chain hydrogens are substituted about as readily as ring hydrogens, with the alpha position favored over the beta (as judged from ethylbenzene).
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Table I.
y—Radiolysis of Benzene, Toluene, and Ethylbenzene in Vapor and Liquid Phase Vapor Phase Ben zene
Conditions Pressure, torr Dose, e.v. gram" X 10" Rate, e.v. gram" sec." Χ 10"
Tolu ene
Liquid Phase
Ethyl benzene
a
Benzene
b
Tolu ene
26.4
7.5
7.6
8.8
9.1
1.7
8.7
6.1
2.3
2.8
2.2 c
liq.
Ethyl benzene
c
liq.
liq.
1
21
1
1
1β
Products Hydrogen Methane Ethane Ethylene Acetylene Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Propylbenzene i-Propylbenzene o-Ethyltoluene m-Ethyltoluene p-Ethyltoluene "Polymer"
^product
0.15 0.018 0.004 0.08 0.73 0.02 0.007
6.0
0.47 0.15 0.06 0.10 0.64 0.20 0.07 0.02 0.06 0.007 0.003 0.002 0.003 0.002 6.3
0.76 0.51 0.92 0.40 0.56 0.38 0.34
0.039
0.019
0.001 0.002 0.002 0.001
0.02 0.01 0.034 0.11 0.035 0.10 0.012 8.7
^product
0.11 0.017 0.0001 0.0004 0.007 0.018
0.94
0.158 0.0259 0.0057 0.0071 0.0021 0.016 0.018
< 0.002
1.16
Products found also include propane, butanes, butylbenzenes, and diethylbenzenes. Data of Bums and Tones (2). «Data of Verdin (10).
a
6
In radiolysis of xylenes, the products include benzocyclobutene and the isomeric xylenes. The former is formed from o-xylene i n the vapor phase with a G-value of 0.08, 40 times that i n the liquid phase. Its yield from the other xylenes is much smaller. The G values for isomerization of the xylenes in the vapor phase are about 0.3. In each case both possible isomers are formed with significant yields. In the liquid phase, the isomerization of o-xylene is much more selective; m-xylene is produced with a G of 0.01; the para isomer could not be detected.
Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
138
RADIATION CHEMISTRY
Table II.
γ—Radiolysis of o-, m-, and J-Xylene in Vapor and Liquid Phase Xylene Vapor
Conditions
0-
Pressure, torr Dose, e.v. gram" X
Π
m-
p-
Xylene Liquid 0-
m-
—
4.7
3.0
2.9
8.2
8.4
8.0
8.2
1.9
1.8
1.7
5.7
—
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1
XO-21
Rate, e.v. gram" sec." Χ 10" 1
1
1β
Products Hydrogen Methane Ethane Ethylene Acetylene Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Benzocyclobutene o-Ethyltoluene m-Ethyltoluene p-Ethyltoluene 1,2,3-Trimethylbenzene 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene "Polymer" "Verdin
^product
^product
0.65 0.26 0.39 0.16 0.53 0.13 0.36 0.028 0.18 0.07 0.08 0.13 0.004 0.001
0.65 0.32 0.41 0.14 0.56 0.10 0.21 0.012 0.10 0.11 0.01 0.002 0.10 0.002
0.84 0.33 0.41 0.15 0.59 0.15 0.38 0.011 0.08 0.26 0.004 0.001 0.004 0.11
0.18 0.025 0.0002 0.0004 0.0028 0.001 0.031
