May, 3057
MERCIJRY PHOTOSENSITIZED OXIDATION OF ETHANE
assumption as to character of activity coefficients. Such adsorption inversion frequently has been reported.lD-z3 The inversion may be explained by assuming a heterogeneous surface, part of which preferentially adsorbs one component and part of which preferentially adsorbs the second component. It might also be due to a short range attraction of one component (e.g., hydrogen bonding to an oxidized surface) and a long range attraction of the second component, as has been suggested by Hansen and Fackler.12 Several systems have been investigated over the entire concentration range in which no significant inversion occurs. Of these, perhaps the most suitable test of the model is the system waterformic acid-charcoal investigated by Blackburn and I(ipling.24 Judging from the activity behavior of the aqueous-higher fatty acid systems26 the formic acid-water system should be nearly ideal. Assuming p = 0, the maximum in the surface excess mole fraction plot should occur at x = 0.15 compared to 0.17 observed; the isotherm should approach x = 1 almost linearly as is observed, the ratio of maximum adsorption to intercept of tangent at z = 1 on the ordinate axis should be 0.66 compared to 0.73 observed. The methanol-benzenesilica gel system investigated by Bartell and SchefflerZ1 shows a maximum surface excess a t mole (19) W o . Oetwald and R. de lzaguirre, Kolloid Z., 86, 289 (1025). (20) F. E. Dartell and C. K. Sloan, J. A m . Chem. Soc., 81, 1843 (1929). (21) F. E. Bartell and G. H. Sheffler, ibid., 88, 2507 (1931). (22) J. J. Kipling and D. A . Tester, J . Chem. Sac., 4123 (1952). (23) R. 5. Hansen snd R. P.Craig, THISJ O U R N A88, ~ , 211 (1954). (24) A. Blackburn and J. J. Kipling, J . Chem. Soc., 1403 (1055). ~ (25) R. 5. Hansen, F. A. Miller and 8. D. Christian, T H IJOURNAL, 68, 801 (1955).
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fraction 0.20 and a ratio of maximum adsorption to tangent intercept of 0.61 approximately. Assuming 0 = 1 (the methanol-benzene system is of course not regular but does show appreciable positive deviation from ideality'") the calculated maximum surface excess occurs at 2 = 0.22, and the maximum adsorption to tangent interce t ratio is 0.65. Results published by Hansen an8Craigzs for the systems water-acetic acid-graphon and water-ethanol-graphon do not agree a t all well with behavior predicted by the model, in that the adsorption maxima occur a t about half the expected mole fraction and the surface excess mole fraction curves approach z = 1 with approximately zero slope. Behavior predicted by the model for slightly soluble systems agrees fairly well with that observed for aqueous solutions of higher alcohols and fatty acids using carbon blacks and graphite5 as adsorbents by Hansen and Craig.2a Especially noteworthy are the generally similar behaviors of the isotherms for diff ererit aolutes when compared as functions of z/z', and the approximate ln-'/g x/x' behavior as x -F 2'. I n so far as the model predicts a surface excess proportional to ln-'/la as a -t 0, it predicts a behavior rather similar to the frequently reported Freundlich isotherm behavior and the principle dependence on activity rather than mole fraction also has been emphasized by Hansen and CraigBas It must be noted, however, that isotherms investigated at sufficiently low mole fractions are almost invariably linear and not logarithmic, and that at these concentrations the fact that the molecules are not arbitrarily small makes a continuum theory unrealistic.
THE MEltCUltY PHOTOSENSITIZED OXIDATION OF ETHANE BY J. S. W A T S O N AND ~ ~ B. DEB. DARWENT'~ Olin itfathieson Chemical Corp., New Haven, Conn. Received October 16. I066
The kinptics of the mercury photosensitixcd oxidation of ethane have been inVCRtigated at R variety of pressures, compoaitions and intensities at temperatures between 40 and 200". The rate is essentially independent of pressure, composit>ion and temperature within prescribed limits. The quantum yield is approximately unity and independent of the intensity. The reaction is not a chain process and a mechanism has bcon proposed that is consistent with the Irinetics.
I. Introduction Investigations of the mercury photosensitized reactions of ethane have been reported, in single pass flow systems, by NalbandyanZa and Gray.2b Nalbandyan's experiments were conducted a t high relative oxygen concent,rations ( [02 J/o[C2Ha = r = 9.0),and a t temperatures up to 310 . He concluded that peroxides were the initial products and that conditions could be reached such that they were the only products of the reaction. H e suggested a mechanism involving the formation of free ( 1 ) (a) Department of Applied Chemistry; National Reaearch Cormeil, Ottawa, Canada: (b) Department of Chemistry, Catholio University of America, Washington 17, D. C. ( 2 ) (a) A. B . Nalbandyan, Doklndy Aknd. Nauk U.S.S.R., 6 6 , 413 (1949); (b) J. A. Gray, J . Cbem. Soc., 3150 (1952).
radicals, by the action of excited mercury on ethane, which reacted with oxygen to give peroxy radicals. The peroxy radicals reacted with ethane t o form organic peroxides and regenerated active free radicals-thus resulting in a chain reaction. At long contact times the peroxides decomposed t o form aldehydes which then either decomposed or were oxidizied to CO and COz. The peroxides were not identified but the aldeh des were found to be formaldehyde and acetalde yde, in the ratio of about 5 : 1 and approximately independent of contact time. Gray's experiments,Zb at lower temperatures and T 0.1, confirmed Nalbandyan's conclusion that peroxides were the primary products and also showed that ethyl hydroperoxide was the only
g
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J . S. WATSONAND R. DEB.DARWENT
578
peroxidic substance produced in the initial stages of tho reaction. He did not find significant quantities of diethyl peroxide, hydrogen peroxide or water in the products and suggested the mechanism Hd'So)
+ hv -+ HgQ(),' C2Ho+ H + Hg(lSd
f J a ( W + CzHs -+ CzHs.043.
CzHa + Oa +CzHs.0.0. + CaHe +CnHsO.OH + CzHs
(0)
(a) (1) (2)
containing the chain reactions 1and 2. However, from the known values for the quenching cross-sections of ethanea and oxygenJ4it may be shown6 that, even with r as small as 0.1, more than 00% of the Hg(aPl)atoms shouId he deactivated by oxygen W8pp1) OP+On* Hg('So) (b) thus requiring fairly long chains if the quantum yield is of the order of unity, Since neither of the previous investigators' attetnpted to study the kinetics of the reaction there is very little proof of the suggested mechanism, othar than the fact that ethyl hydroperoxide is the sole initial product. Such a kinetic study forms the basis of this communication. 11. Experimental
+
+
Materials.---IIigh purity, research grade ethane and ro pnnc, from the Phillips Petroleum Company, were usex to woid possible complications due to the presence of unsaturatcrl hydrocarbons. Thme gases were further purified by tr:tp-t,o trap distillatioils in uucuo and the middle fractions slorc!ti in 2-litjer flasks. The oxy en was usually taken from n rommerrial cylinder and roughy purified by evacuatin :I s:implo condcnscd at -195'. In some expcriments hi& puri t#y oxygen, prepared hy heating mercuric oxide, waA iicled 1.0 test the effect of possible impurities in the commerviid product. The results were found to be independent of thr pwity of the oxygen. Apparatus.-A closed system, in which the ases were cirwIatr(1 by a suitable pump, was used to enabfe the rates of di~appcnranceof O2 and C2He to be measured. The apparatus included a quartz spiral reaction vessel of 15-cc. capacity, separated by a cylindrical copper shutter from an asinlly placed low pressure mercury lamp, enclosed in an electrically heated furnace. The gases were circulated over tho surface of mercury warmed to about 80",through the reaction vesscl and t)raps,maintained at -80" to remove the ethyl hydroperoxide, and through a Beckman Oxygen Analyzer, by which the concentration of oxygen was measured throughout tho experiment. The total volume of the reaction syRtein was 720 cc. The pump permitted the contact time (.) to be varied between 0.5 and 20 sec. with slightly pulsating flow. The measurements of the total )ressure and concent,rctt,ionof 0 2 were not unduly affected Ly the pulsation of the ump . Connections were provided to the storage, analyticnrand vacuum systems. Procedure.---Beforc starting an experiment the whole system was thoroughly evacuated. Ethane was added to the requisite pressure and then frozen at -195' in one of the traps which was then isolatcd from the rest of the system. Oxygen was admitted to t,he rest of the apparatus to the reuired pressure and the Beckinan analyzer checked against %e manometer. The gnscs wcre mixed by evaporat,ing the ethane into the rest of t,hc syst,em and operating the pump. The lamp was brought to its stcatly opcrating condition with the ehutter closed and one of tbe traps cooled to -80". The reaction was started by opening the shutter and allowed to proceed for a measured timc, (luring which the total pressure and the 02 pressure were observed. Analysis.-The uncondensable products (CO, 132, c y 4 ) , together with any rinreacted 0 2 wcre separated by freezing out the ethane and condengable products at, -196". The pressure of the uriconderiunhle gases was mr:asurcd nncl t,he!y -
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(:I) 1%. tIt!ti. ~ ) u r \ v c i l ~.I., C/,ertr. /'/t!/N,, i n , 1 s : j Y ( I M I I ) . (4). M . W . Rrulsn%ky,I ' h ~ x .Rma.. 36, ! ) I 9 (lW:
