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A brief reinvestigation of gas phase ethane radiolysis
Ratiat. Phys. Chem. VoL 16, pp. 201-205
Perpmon Press Ltd., I~0. Printedin Great Britain
A BRIEF REINVESTIGATION OF GAS PHASE ETHANE RADIOLYSIS MICHAEL D. SCANLON a n d ROBERT J. HANRAHAN
Department of Chemistry, University of Florida, Gainesville, FI 32611, U.S.A.
(Received 18 May 1979; in revised form 20 August 1979) Abstract--The 6eCo 7-radiolysis of gaseous ethane was studied at 50 torr pressure, both pure and with 10v~added oxygen. Radiolysis yields (G) in the pure and O2-scavenged system are as follows: H2, 7.24/3.12; CI-L, 0.53/0.24; C3Hs, 0.49/0.14; n-C4Hi0, 2.13/0.27; C2H4, 0.12/1.52; C2H2, 0.05/0.29. Small yields (G ~ 0.03) were also found for i-C4HIo, C3I-I~, i-CsHn, n-CsHi2, I-C4H,, trans-2-C4H,, cis-2-C4I-Is, C6H,4 and C~H~o. A simple mechanism involving fragmentation of excited C2H~ into C2H5+ H', 2CH3', C2I-L,+H2 or C2He+2H2 followed by radical--radical and radical--substrate reactions accounts for scavengable yields of major products as well as much of the nonscavengable product distribution. Several ion-molecule reactions, investigated previously in other laboratories, are required to complete the description of the radiolysis dynamics. INTRODUCTION IN THE STUDY of the radiation chemistry of organic gases, a thorough understanding of the C2I-I~ system is of special importance because it is the simplest compound containing both C-C and C - H bonds. There have been numerous published reports on the radiolysis of gaseous C21-I6 both with and without radical or charge scavengers, °-a') including effects of density, c~"3. m electric fields, °'~ and isotopic labeling. ¢7"~e~ There have also been numerous papers dealing with relevant work on VUV photolysis ~'5-23~and ion-molecule reactions in ethane. ~u-33~ Nevertheless, deliniation of the radiolytic behavior of ethane is not complete. There is a considerable spread in yields for the pure compound and for the scavenged system as measured in various laboratories, and in general the material balance is not good. Many of the published investigations omitted measurement of minor products, or of the H2 yield in the presence of scavenger. We recently had occasion to re-measure product yields in ethane, both pure and with 10°~ added oxygen, in order to establish an "end-point" in connection with studies of the radiolysis of C2Fe-C2H6 mixtures, cu) For the reasons mentioned above, it appears worthwhile to report the results and to compare them briefly with previous work. EXPERIMENTAL Matheson Company c.p. grade ethane was purified by preparative gas chromatngraphy using a 5 m silica gel column, and transferred into 104ml nickel radiolysis vessels using standard vacuum line techniques. Analysis of all products from both the radiolysis and the ethylene
dosimeter was done using a special "Duplex" gas chromatograph combining both a thermal conductivity instrument and a flame ionization instrument with separate columns, coupled via a common input section built around a Toepler pump. ~35~Irradiations were performed using a S~2o 3,-irradiator which has been described previously. ~ The dose rate in 50 torr ethylene (G ~ 1.2° ' ) was 3.15 x 10t9eV g h. The corresponding dose rate in 50 tort ethane was calculated as 3.35 × 10*9eV g h. after correcting for a small difference in secondary electron stopping power. Additional details concerning sample preparation and analysis are given elsewhere. ~ RESULTS Ethane was irradiated at room temperature over the absorbed dose range of 1.26x 10~°-1.34x 10zl eVg -t. Radiolyses were carried out at 20 and 50 ton'. There seemed to be little, if any difference in product G-values at these two pressures. In all, 17 organic products were identified. The major products are hydrogen, methane, ethylene, propane, acetylene and n-butane. Figure 1 shows yields of the major products H2, n-C,l-lto and C2H4 as a function of dose. These yields, as well as those of the minor products CI-I,, C3H. and C2H2 are essentially linear with absorbed dose, both pure and with 10°~ added oxygen. Trace products ( G < 0 . 0 2 ) include propylene, /-pentane, n-pentane, l-butene, cis-2-butene, trans-2-butene, hexane and pentene. G-values of all products in the unscavenged system are listed in Table !. Both major and minor products were identified by their retention times and their mass spectral cracking patterns. In the case of pentene and hexane, however, only the empirical formulas could be established. Due to similarities in both G. C. retch-
201
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M. D. SCANLON et al.
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FIG. I. Product formation in ethane radiolysis as a function of dose. Left scale: //2 yield O pure; • 10% added oxygen. Right scale: n-butane yield [] pure, • 10% added oxygen; ethylene yield A pure, • 10% added oxygen. tion times and mass spectral cracking patterns of the several isomers of each compound, it could not be determined with certainty which isomers were produced. Table 2 summarizes the effect of 02 on the system. When Ch was added to ethane, it was found that among the major products the hydrogen yield was reduced by approximately 50%, the nbutane yield by 90~, the propane yield by 70%, and the methane yield by 50%. On the other hand, the major unsaturated products were greatly enhanced. For example, the ethylene yield increased by a factor of 10, and the acetylene yield increased by a factor of 6. Among the minor products, added oxygen essentially eliminated the saturated products such as /-butane, i-pentane, n-pentan¢ and hexane. All the unsaturated product yields, except for the pentenes, were increased. DISCUSSION Table 1 compares our data on the unscavenged system with the results of several other ethane radiolysis investigations. °'s) One notices quite a variation in results. Peterson and co-workers, °) Heckel and Niessner, c2) and Holland and Stone °~ did their experiments at very low doses, while
Yang and Manno, ") yon Bunau °' and this laboratory performed their investigations at higher doses. It is seen that at higher doses the yields of hydrogen and especially the unsaturated compounds are lower than for the low dose work, while the yields of higher molecular weight compounds are higher. This probably illustrates the influence which the build-up of radiolysis products (especially the olefins) can have on the observed product yields. In all of the above studies, the material balance is not very good with a hydrogen to carbon ratio of 3.9:1 for Yang and Manno, (') 3.6:1 for yon Bunau, °) 3.52: I for Peterson e t al., "~ 3.44: 1 for Heckel and Niessner, ~2~3.65: I for Holland and Stone °~ and 3.71 : 1 for this work. Table 2 shows the effect of scavenger on the radiolysis of ethane reported by various investigators. " ' 2 " ' ' ' ' ' ' ~ While there seem to be discrepancies among the several reports, probably due to differences in experimental conditions such as dose, pressure, etc. certain general trends emerge. By comparing Table 1 with 2, it is noticed that when scavenger is added, the hydrogen yield is reduced by more than half, the lower molecular weight saturated compounds such as methane, propane and butane are drastically reduced, the higher molecular weight saturates are eliminated, while the unsaturated compounds are increased. (In the case of the higher dose studies, they are increased quite substantially.) The wealth of detailed information concerning ethane radiolysis may obscure the fact that a simple mechanism based on fragmentation of excited ethane is adequate to explain most of the chief features of this system. (1)
C2Hs
, (C2Hs + + e-)
(2)
C2H~
' C2Hs" + H"
, C2H[
(3)
• C2H, + H2 (or 2H')
(4)
~ C2H, + 2H2
(5)
• 2CHf
(6)
, CH, + CH,.
Reactions (2)-(6) are supported by the extensive studies of the vacuum UV photolysis of ethane. ('s23) Ethylene formation involves the unusual intermediate CH~-CH* which rapidly rearranges. "s~ It can be coilosionally deactivated or decompose further to give acetylene. A portion of the C=Hs" radical yield is excite~l; it has been assumed to
A brief reinvestigation of gas phase ethane radiolysis
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decompose to ethylene plus hydrogen atom, in competition with deactivation. ~"~ The above scheme accounts for nonscavengable yields of H2, CtL, C,I-L and C2H~, as observed. Prior to build-up of olefins, the radical fragments react by combination/disproportionation or abstraction from substrate. (7) (8) (9) (10)
H'+H'+M H" + C2I-I~ H'+CH3"+M CH3" + C2H5"
(I I) (12) (13)
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) C~I-Im ~C~H6 + C2I-L.
Reactions (7)-(13) account for scavengable yields of H~, CH4, C2I-L, C3I-Isand n..C~-ll0. Within the dose range used in the present work, however, reactions of radicals (particularly H atoms) with C2H2 and C2I-I4 are important, so that in fact the acetylene and ethylene yields increase with added scavenger. Addition of CH3 and C2H~ radicals to ethylene produces C3 and C, radicals, respectively, which can undergo combination/disproportionation reactions leading to Cs, C6 and C~ products. Although the radical/excited molecule reaction scheme described above accounts for the major aspects of ethane radiolysis, it is known that numerous ionic processes also occur, and contribute substantially to certain product yields. In particular, ionic fragmentation of C2I-I6 at low pressures forms the primary ions C2I-I~+ (26~), C2Hs÷ (22~), C~I'L+ (100~), C2H3+ (33%), C2H2+ (23%), CH, ÷ (5%) and smaller amounts of several other ions. c~ Corresponding neutral products are primarily H2, H" and C H f . It has been established that all of the C2 ions attack substrate giving the series of unstable condensation products (C2I-I,)2+, C ~ I ~ , C.I'I~'o, C.I'I9÷ and CA'k÷. °'='='32~ The species C3H9+, Csl'Is ÷, C3H~, C3Hs ÷ and C3I'Is ÷ are formed via fragmentation, t ~ Field et al.°°~ showed that the yield of (C21-16)2÷ was rather low at pressures up to 5 ton" in the mass spectrometer, but it would still account for a small part of the yield of several products. Ausloos and co-workers" have shown that H ÷ transfer from the C41-I~', ion to molecules o f moderate basicity leads to formation
of butane; similarly, proton transfer from C3H9÷ would account for part of the C3Hs nonscavengable yield. Ion-molecule reactions are also known to produce portions of the yields of H2, CI-L, and C2I-L, as well as H" and CH3". t29"3°'32~ There is no difficulty in giving a reasonable account for all of the miscellaneous minor products in ethane radiolysis, on the basis of ion-molecule reactions and other minor processes such as carbene insertion."" The main remaining difficulty in ethane radiolysis does not concern such nuances, however, but rather focuses on the material balance. In all investigations reported to date, there is a substantial excess of H2 over apparent sources of H2 (i.e. olefinic or acetylenic products and products of higher molecular weight than ethane). Typically, the observed organic products fail to explain the H2 yield to the extent of about 2 G units. It is true that irradiations at very low dose reveal higher yields of C2I-L, and C2H2, tending to alleviate the discrepancy. However, the hydrogen yield observed at low dose also increases over the high dose value to the extent of 1 or 2 G units, so that the situation is almost comparably bad, at low dose or high. Furthermore, during the dose regime where the ethylene yield is falling off, following yield-dose graphs which are concave downward, it would be expected that yields of at least some higher molecular weight products should increase, showing yielddose graphs which are concave upward. Such behavior has not been observed. At present, an assumption of polymerization on the walls appears to be the only explanation of the discrepancy in product yields. Acknowledgement--This work was supported by the
U.S. Department of Energy under Contract No. Ey~76S-05-3106. This is Document No. ORO-3106-67. REFERENCES I. C. M. Wovmrz~, P. A. McCUSKER and D. B. P~N, J. phys. Chem. 1965, 69, 1045. 2. (a) E. HECKEL and C. R. NlUSNn, Private communication. (b) C. R. N l ~ s ~ a , M. S. Thesis, Radiolytic Processes in Gaseous M h ' l u ~ of ~hame and Perpuoromalume. E~t Carolina Uniwrsity, 1972. 3. P. T. HOLLANDand J. A. STONE,Can. 1. Chem. 1974,
$2,221. 4. K. YANGand P. J. MANNO,J. Am. Chem. Soc. 1959, $1,3507. 5. G. VONBONAU,Bet. Bunsenges. Physik Chem. 1965, 69, 16. 6. L. H. GEVANTMANand R. R. WILLIAMS,JR., J. phys. Chem. 1952, $6, 569. 7. L. M. DOVJ~IAN,J. phys. Chem. 1958, 62, 29. 8. R. A. BACK,J. phys. Chem. 1960, 64, 124.
A brief reinvestigation of gas phase ethane radiolysis 9. K. YANG and P. L. GANT, J. phys. Chem. 1961, 65, 1891. 10. L. J. STIEFand P. AUSLOOS,J. chem. Phys. 1962, 36, 2904. 11. H. H. CARMICHAEL,R. GORDON,JR. and P. AUSLOOS, J. chem. Phys. 1965, 42, 343. 12. C. M. WODETZKI, P. A. McCUsKER and D. B. P~r,':~ZSON,J. phys. Chem. 1965, 69, 1057. 13. J. GAWLOWSKIand J.A. HERMAN, Can. J. Chem. 1974, $2, 3631. 14. P. AUSLOOS,R. E. REeS~T and L. WAYNESIECK,J. chem. Phys. 1971, 34, 2612. 15. H. OKABEand J. R. McNEsBY, J. chem. Phys. 1961, 34,668. 16. R. F. HAMPSON,JR., J. R. McNEsay, H. A~MOTO and J. TANAKA,J. chem. Phys. 1964, 40, 1099. 17. H. AKIMOTO,K. Oel, and J. TANAKA,J. chem. Phys. 1965, 42, 3864. 18. R. F. HAMPSON,JR., and J. R. McNESBY, J. chem. Phys. 1965, 42, 2200. 19. J. A. PIROGand J. R. McNESsY, J. chem. Phys. 1965, 42, 2490. 20. A. H. LAUGERand J. R. McNEssY, J. chem. Phys. 1965, 42, 3329. 21. W. M. JACKSON,J. FARTSand N. J. Buccos, J. chem. Phys. 1966, 45, 4145. 22. B. C. ROQUITrE,J. phys. Chem. 1970, 74, 1204. 23. S. G. IAAS, G. J. COLLINS, R. E. REBB~T and P. AUSLOOS,J. chem. Phys. 1970, $2, 1841. 24. R. R. HUTCmNS and R. R. KUNTZ,J. phys. Chem. 1971, 75, 2903.
205
25. M, S. B. MUNSON,J. L. FRANKLIN,and F. H. FIELD. J. phys. Chem. 1964, a ~,3098. 26. G. A. W. DERWtSH,A. GALLi, A. GIARDIN1-GUIDON! and G. G. VOLIP,J. chem. Phys. 1964, 40, 5. 27. S. K. SEARLF.S, L. W. SIECK and P. AUSLOOS, J. chem. Phys. 1970, $3, 849. 28. A. S. BLAIR, E. J. HESLIN and A. G. HARRISON,J. Am. Chem. Soc. 1972, 94, 2935. 29. R. C. DUNBAR,J. StiEN and G. A. OLAH, J. chem. Phys. 1972, .f,6, 3794. 30. S. L. BENNETT,S. G. LIAS and F. H. FIELD, J. phys. Chem. 1972, 76, 3919. 31. E. HECKEL and R. J. HANRAHAN, J. chem. Phys. 1975, 62, 1072. 32. S. WEXLERand L. G. PoBo, J. Am. Chem. Soc. 1971, 93, 1327. 33. T. M c A I A ~ , J. chem. Phys. 1972, S6, 5192. 34. M. D. SCANLDNand R. J. HANRAHAN,The Radiation Chemistry of Mixtures of Ethane and Hexafluor~ ethane in the Gas Phase. Radint. Phys. Chem. To he published. 35. E. HECKELand R. J. HANP.AHAN,J. Chmmatog. Sci. 1969, 7, 418. 36. R. J. HANRAHAN, Int. J. Appi. Radiat. lso. 1962, 13, 254. 37. G. G. M~SELS, J. chem. Phys. 1964, 41, 5 I. 38. American Petroleum Institute Project 44, Catalog of Mass Spectral Data. Thermodynamics Research Center, Texas A & M University, Serial No. 2.
Perpmon Press Ltd., I~0. Printedin Great Britain
A BRIEF REINVESTIGATION OF GAS PHASE ETHANE RADIOLYSIS MICHAEL D. SCANLON a n d ROBERT J. HANRAHAN
Department of Chemistry, University of Florida, Gainesville, FI 32611, U.S.A.
(Received 18 May 1979; in revised form 20 August 1979) Abstract--The 6eCo 7-radiolysis of gaseous ethane was studied at 50 torr pressure, both pure and with 10v~added oxygen. Radiolysis yields (G) in the pure and O2-scavenged system are as follows: H2, 7.24/3.12; CI-L, 0.53/0.24; C3Hs, 0.49/0.14; n-C4Hi0, 2.13/0.27; C2H4, 0.12/1.52; C2H2, 0.05/0.29. Small yields (G ~ 0.03) were also found for i-C4HIo, C3I-I~, i-CsHn, n-CsHi2, I-C4H,, trans-2-C4H,, cis-2-C4I-Is, C6H,4 and C~H~o. A simple mechanism involving fragmentation of excited C2H~ into C2H5+ H', 2CH3', C2I-L,+H2 or C2He+2H2 followed by radical--radical and radical--substrate reactions accounts for scavengable yields of major products as well as much of the nonscavengable product distribution. Several ion-molecule reactions, investigated previously in other laboratories, are required to complete the description of the radiolysis dynamics. INTRODUCTION IN THE STUDY of the radiation chemistry of organic gases, a thorough understanding of the C2I-I~ system is of special importance because it is the simplest compound containing both C-C and C - H bonds. There have been numerous published reports on the radiolysis of gaseous C21-I6 both with and without radical or charge scavengers, °-a') including effects of density, c~"3. m electric fields, °'~ and isotopic labeling. ¢7"~e~ There have also been numerous papers dealing with relevant work on VUV photolysis ~'5-23~and ion-molecule reactions in ethane. ~u-33~ Nevertheless, deliniation of the radiolytic behavior of ethane is not complete. There is a considerable spread in yields for the pure compound and for the scavenged system as measured in various laboratories, and in general the material balance is not good. Many of the published investigations omitted measurement of minor products, or of the H2 yield in the presence of scavenger. We recently had occasion to re-measure product yields in ethane, both pure and with 10°~ added oxygen, in order to establish an "end-point" in connection with studies of the radiolysis of C2Fe-C2H6 mixtures, cu) For the reasons mentioned above, it appears worthwhile to report the results and to compare them briefly with previous work. EXPERIMENTAL Matheson Company c.p. grade ethane was purified by preparative gas chromatngraphy using a 5 m silica gel column, and transferred into 104ml nickel radiolysis vessels using standard vacuum line techniques. Analysis of all products from both the radiolysis and the ethylene
dosimeter was done using a special "Duplex" gas chromatograph combining both a thermal conductivity instrument and a flame ionization instrument with separate columns, coupled via a common input section built around a Toepler pump. ~35~Irradiations were performed using a S~2o 3,-irradiator which has been described previously. ~ The dose rate in 50 torr ethylene (G ~ 1.2° ' ) was 3.15 x 10t9eV g h. The corresponding dose rate in 50 tort ethane was calculated as 3.35 × 10*9eV g h. after correcting for a small difference in secondary electron stopping power. Additional details concerning sample preparation and analysis are given elsewhere. ~ RESULTS Ethane was irradiated at room temperature over the absorbed dose range of 1.26x 10~°-1.34x 10zl eVg -t. Radiolyses were carried out at 20 and 50 ton'. There seemed to be little, if any difference in product G-values at these two pressures. In all, 17 organic products were identified. The major products are hydrogen, methane, ethylene, propane, acetylene and n-butane. Figure 1 shows yields of the major products H2, n-C,l-lto and C2H4 as a function of dose. These yields, as well as those of the minor products CI-I,, C3H. and C2H2 are essentially linear with absorbed dose, both pure and with 10°~ added oxygen. Trace products ( G < 0 . 0 2 ) include propylene, /-pentane, n-pentane, l-butene, cis-2-butene, trans-2-butene, hexane and pentene. G-values of all products in the unscavenged system are listed in Table !. Both major and minor products were identified by their retention times and their mass spectral cracking patterns. In the case of pentene and hexane, however, only the empirical formulas could be established. Due to similarities in both G. C. retch-
201
202
M. D. SCANLON et al.
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FIG. I. Product formation in ethane radiolysis as a function of dose. Left scale: //2 yield O pure; • 10% added oxygen. Right scale: n-butane yield [] pure, • 10% added oxygen; ethylene yield A pure, • 10% added oxygen. tion times and mass spectral cracking patterns of the several isomers of each compound, it could not be determined with certainty which isomers were produced. Table 2 summarizes the effect of 02 on the system. When Ch was added to ethane, it was found that among the major products the hydrogen yield was reduced by approximately 50%, the nbutane yield by 90~, the propane yield by 70%, and the methane yield by 50%. On the other hand, the major unsaturated products were greatly enhanced. For example, the ethylene yield increased by a factor of 10, and the acetylene yield increased by a factor of 6. Among the minor products, added oxygen essentially eliminated the saturated products such as /-butane, i-pentane, n-pentan¢ and hexane. All the unsaturated product yields, except for the pentenes, were increased. DISCUSSION Table 1 compares our data on the unscavenged system with the results of several other ethane radiolysis investigations. °'s) One notices quite a variation in results. Peterson and co-workers, °) Heckel and Niessner, c2) and Holland and Stone °~ did their experiments at very low doses, while
Yang and Manno, ") yon Bunau °' and this laboratory performed their investigations at higher doses. It is seen that at higher doses the yields of hydrogen and especially the unsaturated compounds are lower than for the low dose work, while the yields of higher molecular weight compounds are higher. This probably illustrates the influence which the build-up of radiolysis products (especially the olefins) can have on the observed product yields. In all of the above studies, the material balance is not very good with a hydrogen to carbon ratio of 3.9:1 for Yang and Manno, (') 3.6:1 for yon Bunau, °) 3.52: I for Peterson e t al., "~ 3.44: 1 for Heckel and Niessner, ~2~3.65: I for Holland and Stone °~ and 3.71 : 1 for this work. Table 2 shows the effect of scavenger on the radiolysis of ethane reported by various investigators. " ' 2 " ' ' ' ' ' ' ~ While there seem to be discrepancies among the several reports, probably due to differences in experimental conditions such as dose, pressure, etc. certain general trends emerge. By comparing Table 1 with 2, it is noticed that when scavenger is added, the hydrogen yield is reduced by more than half, the lower molecular weight saturated compounds such as methane, propane and butane are drastically reduced, the higher molecular weight saturates are eliminated, while the unsaturated compounds are increased. (In the case of the higher dose studies, they are increased quite substantially.) The wealth of detailed information concerning ethane radiolysis may obscure the fact that a simple mechanism based on fragmentation of excited ethane is adequate to explain most of the chief features of this system. (1)
C2Hs
, (C2Hs + + e-)
(2)
C2H~
' C2Hs" + H"
, C2H[
(3)
• C2H, + H2 (or 2H')
(4)
~ C2H, + 2H2
(5)
• 2CHf
(6)
, CH, + CH,.
Reactions (2)-(6) are supported by the extensive studies of the vacuum UV photolysis of ethane. ('s23) Ethylene formation involves the unusual intermediate CH~-CH* which rapidly rearranges. "s~ It can be coilosionally deactivated or decompose further to give acetylene. A portion of the C=Hs" radical yield is excite~l; it has been assumed to
A brief reinvestigation of gas phase ethane radiolysis
203
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decompose to ethylene plus hydrogen atom, in competition with deactivation. ~"~ The above scheme accounts for nonscavengable yields of H2, CtL, C,I-L and C2H~, as observed. Prior to build-up of olefins, the radical fragments react by combination/disproportionation or abstraction from substrate. (7) (8) (9) (10)
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C~H5 + C2Hs
) C~I-Im ~C~H6 + C2I-L.
Reactions (7)-(13) account for scavengable yields of H~, CH4, C2I-L, C3I-Isand n..C~-ll0. Within the dose range used in the present work, however, reactions of radicals (particularly H atoms) with C2H2 and C2I-I4 are important, so that in fact the acetylene and ethylene yields increase with added scavenger. Addition of CH3 and C2H~ radicals to ethylene produces C3 and C, radicals, respectively, which can undergo combination/disproportionation reactions leading to Cs, C6 and C~ products. Although the radical/excited molecule reaction scheme described above accounts for the major aspects of ethane radiolysis, it is known that numerous ionic processes also occur, and contribute substantially to certain product yields. In particular, ionic fragmentation of C2I-I6 at low pressures forms the primary ions C2I-I~+ (26~), C2Hs÷ (22~), C~I'L+ (100~), C2H3+ (33%), C2H2+ (23%), CH, ÷ (5%) and smaller amounts of several other ions. c~ Corresponding neutral products are primarily H2, H" and C H f . It has been established that all of the C2 ions attack substrate giving the series of unstable condensation products (C2I-I,)2+, C ~ I ~ , C.I'I~'o, C.I'I9÷ and CA'k÷. °'='='32~ The species C3H9+, Csl'Is ÷, C3H~, C3Hs ÷ and C3I'Is ÷ are formed via fragmentation, t ~ Field et al.°°~ showed that the yield of (C21-16)2÷ was rather low at pressures up to 5 ton" in the mass spectrometer, but it would still account for a small part of the yield of several products. Ausloos and co-workers" have shown that H ÷ transfer from the C41-I~', ion to molecules o f moderate basicity leads to formation
of butane; similarly, proton transfer from C3H9÷ would account for part of the C3Hs nonscavengable yield. Ion-molecule reactions are also known to produce portions of the yields of H2, CI-L, and C2I-L, as well as H" and CH3". t29"3°'32~ There is no difficulty in giving a reasonable account for all of the miscellaneous minor products in ethane radiolysis, on the basis of ion-molecule reactions and other minor processes such as carbene insertion."" The main remaining difficulty in ethane radiolysis does not concern such nuances, however, but rather focuses on the material balance. In all investigations reported to date, there is a substantial excess of H2 over apparent sources of H2 (i.e. olefinic or acetylenic products and products of higher molecular weight than ethane). Typically, the observed organic products fail to explain the H2 yield to the extent of about 2 G units. It is true that irradiations at very low dose reveal higher yields of C2I-L, and C2H2, tending to alleviate the discrepancy. However, the hydrogen yield observed at low dose also increases over the high dose value to the extent of 1 or 2 G units, so that the situation is almost comparably bad, at low dose or high. Furthermore, during the dose regime where the ethylene yield is falling off, following yield-dose graphs which are concave downward, it would be expected that yields of at least some higher molecular weight products should increase, showing yielddose graphs which are concave upward. Such behavior has not been observed. At present, an assumption of polymerization on the walls appears to be the only explanation of the discrepancy in product yields. Acknowledgement--This work was supported by the
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