August, 1983
PYROLYSIS OF
ACETYLENE, VINYLACETYLENE,
AND
DIACETYLENE
3 579
COMPARATIVE STUDIES OF PYROLYSIS OF ACETYLENE, VINYLACETYLENE, ,4ND DIACETYLENE BY K. C. Hou
AND
ROBBIN C. AXDERSON
Department of Chemistry, The University of Texas, A wtin, Texas Received .Youember 29, 1962 Since vinylacetylene and diacetylene are both straight-chain unsaturated compounds which might be significant intermediates in the pyrolysis of acetylene, a comparative study of the thermal reactions of these three compounds was made. Reaction patterns were determined in a flow system, using temperatures in the range of 500-800", and mixtures of 25 mole ";/o hydrocarbon in helium. Products were analyzed chromatographically. A reactor was also designed to discharge directly into a niass spectrometer, so that tests might be made for free radicals and the product gases also analyzed in the mass spectrometer. Good results were obtained in detecting methyl radicals formed in reaction if di-t-butyl peroxide and lead tetramethyl, but no evidence was found for any free radical fragments from the acetylenic compounds a t teniperatures u p to 700". Iliacetylene was barely detectable in the low temperature reaction of acetylene, and no evidence was found to indicate it is a likely intermediate in the acetylene reactions under such conditions. Vinylacetylene is readily detected and shows appropriate reactivity, but little tendency to decompose to acetylene. It seems likely that it, like benzone, is a reactive side product rather than a key intermediate.
Certain results in recent years1,2 indicate that the pyrolysis of acetylene to form carbon involves straightchain unsat.uratcs as intermediates rather than aromatics. Since the initial stages of thermal reaction are second order, vinylacetylene is of interest as a possible intermediate in the first stages of reaction. However, hydrogen appears as an early product so diacetylene is another leading possibility; and there is also a question whether these might be secondary products result,ing from a free-radical mechaiiism involving C2H or CzH3. C h e r t ~ n for , ~ example, observed diacetylene bands in the spectra of polymerization products produced by a high-frequency discharge in acetylene, and Munson4 found vinylacetylene being formed and then reacting in the pyrolysis of acetylene at 500-850". Accordingly, a comparative study of the reactivit,y of these hydrocarbons was undertaken, with particular attention to mass spectrometric observations which might give some indications of the presence of free radicals. The temperature range (500-800°) and the concentrations (about 25 mole yo hydrocarbon in helium) used were such as to span the range in which thermal reaction of acetylene changes from polymerization to decomposition. Since this was started, I h n e y and Slysh5 have reported finding diacetylene prominent in a fioy system where carbon was formed from acetylene, and Greene, et u Z . , ~ have found evidence for this also in shock-tube experiments on acetylene, but Skinner' and Bradley and Kistiakowsky* found evidence for vinylacetylene as intermediate in similar shock-tube experiments. Experimental Two types of experiments were used to compare reactions of the hydrocarbons. (1) G. Porter, "Fourth International Combustion Symposium," Williams and Wilkins Co., 1954. 11. 218. ( 2 ) F. G . Stehling, J. D. Frai;ee, and R. C. Anderson, "Eighth Symposium (International) on Combustion," Williams and WiIkins Cb., 1962, p. 774. For review of earlier work, see E. W. R. Steacie, "Atomic and Free Radical Reactions," 2nd Ed., Reinhold Publ. Corp., New York, N. Y.,1954; also 0. J. Minkoff, rrf. 14. (3) R. Cherton, Bull. soc. roy. sci. Liege, IO, 604 (1941). (4) M. S. B. Munson, Dissertation for the Ph.D. degree, Univ. of Texas, 1959. (5) C. R. Kinney and R. 8. Slyah, "Proc. Fourth Conf. on Carbon," Pergamon Press, 1960, p. 301; J . Phys. Chem., 65, 1044 (1961). (6) E.F. Greene, R. L.Taylor, and W. L. Patterson, ibid., 62, 238 (LQ.58). Cy. also Combust. Flame, 6 , 55 (1961). (7) G. B. Skinner, paper presented a t American Chemical Society National Meeting, Rt. Louis, Mo., March 21-30, 1961. (8) J. N. Bradley a n d G. E. Kistiakowsky, J . Chem. Phys., 85, 264 (1961).
Flow Reactor.-In one series, a flow reactor was used which consisted of a \-ycor tube, 2 cm. in diameter and 32 rm. long, wrapped with coils of Nichrome wire and covered with asbestos to give temperature control. Temperature readings were made with a therrnocouple in a well a t the middle of the tube. These could be held within i 4 O up to 1000". Flow rates of the gases were checked with capillary-type flowmeters and were reproducible within 2%. Diacetylene was introduced by vaporizing into a stream of helium a t controlled temperatures. The total gas flow rate in the reactor was adjusted to 600 cc./min., giving a contact time of 9-10 sec. The products of reaction were determined with a gas chrornatograph. In one set of experiments liquid polymers were collected and analyzed both chromatographically and by mass spectrorneter. These showed the usual very complex mixture of product including benzene, toluene, xylene, styrene, indene, naphthalene, biphenyl, and others. However, since the major interest here was in the earlier stages of reaction, usually the gaseous products only were checked. Product gases were collected in n sampler tube on leaving the reactor and samples for analysis were taken from this by syringe. Three colunins were used: with 40-60 mesh charcoal for measuring hydrogen and methane, with silica gel for light hydrocarbons, and with Reoplex 400 (Geigy Chem. Corp., Ardsley, ?rT. Y.) for aromatic compounds (essentially benzene in these experiments). In the second series of experiments, the mass spertromcter was used in an attempt to detect possible free-radical intermediates. Reactor in Mass Spectrometer.-For the second series of experiments, a Consolidated Model 21-620 mass spectronieter was modified so as to permit detection of possible free radirals formed in reaction. A small reactor was installed in the sample path so that its exit was only about 2 mm. from the electron beam in the cycloid tube. This reactor was made of alumina tubing 2 mm. in diameter and 2 cm. long. It was heated by means of a resistance wire coil wrapped around the tubing. Johns-Manville refractory cement was used to give thermal insulation. The entire assembly was baked out at 1000" in an electric furnace before installing for a series of tests. By changing the voltage across the heating coil, any desired temperature up to 800" could be obtained. The temperature in a run was determined from the power supplied to the heating system. This was calibrated by measurements made with a chromei-alumel thermocouple inserted into the reactor. The electron beam energy of the C.E.C. 21-620 mass spectronieter is 70 v. under normal operating conditions. This is high enough so that any of the reagent and product molecules might be split in reactions occurring on electron impact. A diode clamping circuit was therefore installed to limit the maximum energy of the electron beam. This permitted decreasing the electron energy to as low as 6 v. Thus electron energies great enough to ionize free radicals already formed, but not so great as to give dissociation reactions, could be used, Measurement of the bias voltage, proportional to electron energy, was made with a Tektronix-type oscilloscope. Materials.-To avoid explosion, mixtures of 25% hydrocarbon
K.
1580
c. Hou 9 N D ROBBINc. ~ D E R S O N
in helium were used. The helium was E. S.Bureau of Mines Grade A. The acetylene was commercial acetylene, purified by bubbling through saturated sodium bisulfite solution and then drying. The vinylacetylene was obtained commercially and then redistilled, scrubbed with sodium bisulfite and sodium hydroxide, and dried. Mass spectrometric analysis indicated about 95oj, purity, with ethylene as the other constituent. The diacetylene was synthesized from 2-butyne-1,4-diolg by a two-step process involving replacement of OH by halogenlo and dehydrohalogenation .I1 For the methyl radical tests, di-t-butyl peroxide of minimum purity 99% was supplied by the Lucidol Divis., Wallace and Tiernan, Inc. Tetramethyllead was recrystallized from toluene solution supplied by the Ethyl Corporation.
Results Data showing the reactivity of the three reagent hydrocarbons in the flow reactor are given in Table I. The general order of decreasing reactivity is diacetylene > vinylacetylene > acetylene.
Vol. 67 TABLE I1 PATTERN OF PRODVCTS
____500'
Mole % hydrogen found----600° 700'
1.2 0.5 0.3
0.0 0.0
0.0
_-__ 500'
1 .o
0.4
500°
0.8 0.12 0.0 5000
19 .,5 0.1
Mole
2.8 1.5 1.8
800C
8.4 3.0 5.6
rinylacetylene found--600° 700°
8000
2.0 0.2 8.5
0.3 0.0 0.0
1. 0 0.0 0.8
Mole % benzene found---800° 7000
1.2 0.2 0.0
2.4 0.5 0.15
\lole % acetylene found-----. BOO' 700'
13.0 0.3 0.02
6.8 0.3 0.1
8000
2.0 0.2 0.1 8000
1.5 0.1 0.04
TABLE I REACTIONS O F ACETYLENE, DIACETYLENE, AND VINYLACETYLEKE hydrogen increases products such as yc of reagent pyrolyzing methane and ethylene and decreases carbon formation. Temp., "C. 400 600 700 800 500 It is notable that diacetylene was not detected in the Acetylene 0 22 48 73 92 pyrolysis of vinylacetylene, and only traces are obDiacetylene 4 68 96 100 100 served in the acetylene reaction. Benzene also is Vinylacetylene 0 66 97 100 28
COMPARISON
O F THE
Material balance for carbon
yo of carbon recovered in the exhaust. gases Temp., "C. 400 500 600 700 800 Acetylene 130 97.6 8 9 . 2 75.2 41.0 Diacetylene 97.5 35.8 9.4 5.5 4.8 Vinylacet,ylene 100 99 . O 35.0 9.5 6.4 Material balance of hydrogen
% of hydrogen recovered in various products in the exhaust Temp., "C. Acet*ylene Diacetylene Vinylacetylene
400 100 100 100
gases 500 600 700 800 99.6 9 8 . 4 96.0 9 4 . 0 42.6 22.6 27.6 34.8 91 . O 37.5 21.5 27.4
The gaseous products from pyrolysis of acetylene followed familiar patterns-hydrogen, methane, ethylene, acetylene, vinylacetylene, and benzene being the detectable ones. With diacetylene, the amount of gaseous products is less, benzene in particular being definitely decreased. With vinylacetylene, products were similar to those with acetylene, but with less benzene again and with traces of propylene and butadiene appearing. The same order of reactivity was indicated by the appearance of the exhaust from the reactor. Diacetylene gives a black smoke, indicating appreciable carbon formation, a t temperatures for which acetylene and vinylacetylene still give just a yellow or brown mist of polymeric products. Some comparative data on products of reaction are given in Table 11. Results for methane are similar to those for hydrogen and for ethylene to those for benzene. (9) P. Pomerantz, A. Fookson, T. W. Nears, S. Rothbwg, and F. L. Howard, J . Res. Natl. Bur. Std., 62, 51 (1954). (10) A. W. Johnson, J . Chem. Soc., 1009 (1946). (11) J. B. Armitage, H. B. H. Whitmg, and M. C. Whiting, ibzd., 44 (1951).
formed much less extensively by the C4 hydrocarbons than by acetylene. The preliminary tests with di-t-butyl peroxide and lead tetramethyl in the mass spectrometer gave clear evidence of CH3 radicals, using an electron voltage of 10 e.v.-above the ionization potential of the methyl radical but not above the appearance potential for formation of CH3+from the parent molecule by electron impact. Typical results for lead tetramethyl, which seem to be in agreement d h those of Hipple and Stevenson12are given in Fig. 1. Similar results were obtained with di-t-butyl peroxide, in general agreement with those of Lossing and Ti~ki1er.l~Comparison of the CH4+ and CH3+ peaks indicates that the sensitivity of the mass spectrometer in detecting the free methyl radicals is essentially the same in the present instrument as in theirs. Thus the unit had sufficient sensitivity to detect free radicals formed from these hydrocarbons, but repeated tests, especially a t mass 25, on all three compoundsusing 15 v. and gas pressures of 60 p-simply gave negative results. Some tests were also made with the electron beam a t 7 0 v. t o see whether any unusual pattern of fragments might be observed in the reactor. Comparative values for mass 24 (CJ) mass 25 (CZH), and mass 27 (C2H3) relative to the parent peak were checked carefully. The only possibly significant increase occurring in the reactor ~ 7 a sfound a t mass 27; but it was also found that ethylene could be detected early in the reaction of acetylene and diacetylene as well as of vinylacetylene. Tests in a freshly cleaned reactor gave somewhat more ethylene; so this seems to involve a surface effect rather than the main gas-phase process. ,411 interesting sidelight is that, in the higher mass ranges in some tests, the amount of mass 52 (CdH4) appeared slightly greater than mass 50 (CbH,), but (12) J. A. Hipple and D. P. Stevenson, P h g s . Reo., 63, 121 (1943). (13) F. P. Lossing and A. W. Tickner, J. Chem. Phys., 20, 907 (1952).
August, 1963
PYROLYSIS OF ACETYLENE, YINYLACETYLEKE, AND DIACETYLESE
tests for either were quite faint under the conditions used. Discussion It is notable in these results that, although diacetylene and vinylacetylene are more reactive than acetylene, all show patterns indicating that reaction starts under the mild conditions used here by steps which must involve addition rather than decomposition. No evidence is found for any free radical such as C2H or Cz&. The results also give no indication of the formation of diacetylene as a first stage in reaction of acetylene under mild conditions. The mass spectrometer tests did not indicate any mass 50 product before mass 52, etc., and in the gas chromatographic studies it may be seen that the production of hydrogen from acetylene is not commensurate with what should appear if the acetylene were reacting to forim diacetylene. At higher temperatures, and with different rates of activation such as can exist in the shock tubes, this pattern could be different. It would be interesting to see what results the shock tubes might give a t comparable temperatures. The general reactivity of vinylacetylene, both in pyrolysis as shown above and in interaction with acetylene,2 is not inconsistent with a possible role as intermediate in acetylene reactions. However, if, as indicated by the results of Bradley and Kistiakowsky, there is a dimer (the "A" polymer) which establishes rapid equilibrium with acetylene, this could apparently not be vinylacetylene. The latter shows very little tendency to split to acetylene-even under conditions, such as the 600" tests, where vinylacetylene is clearly reactive and acetyleiie itself is still observable in readily detectable amounts. Vinylacetylene itself should apparently be considered, like benzene, as a reactive side product but not an essential intermediate. A dirner which is formed without rearrangement of hydrogen and which exists as an activated molecule or free radical is probably the actu,al intermediate in reaction. It is also interesting to note that the mass spectrometer tests here give no support to the suggestion of Minkoff14that excited states which form on the surface
200
400
600
138 1
800
Temp., "C.
Fig. 1.-Temperature
us. ion current.
Xass 15 from Pb(CH,)4
at 10 e.v.
and diffuse into the gaseous acetylene are es entia1 in reaction. At the very lorn pressures uszd in the rea:tor in the mass spectrometer, there is definite evidence of surface activation and reaction, but with a9parently a different pattern of products from that of the ordinary reaction and with no evidence of any active groups appearing in gas phase. Acknowledgment.-This research was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of the fund, and also to the Office of Scientific Research, U. 5.Air Research and Development Command, for support of the project in part. (14) G. J. Minkoff, Can. J. Chem., 36, 131 (1958).
