THE POLYMERIZATION AND HYDROGENATION OF ACETYLENE' H. AUSTIN TAYLOR AND ANDREW VAN HOOK Nichols Chemistry Laboratory, New York Universitu, New York City Received November 94, 1994
The literature on the polymerization and hydrogenation of acetylene is voluminous (2), but practically all of it deals with the mechanism (3) and products ( 6 ) of reaction. The only work dealing in any detail with the kinetics of the reaction is due t o Pease (8) and Schlaffer and Bmnner (12). Pease concludes that the rate-determining process is the primary reaction involving two molecules of acetylene. No quantitative data on the velocity of the hydrogenation are available. It was deemed desirable therefore to study both reactions more thoroughly. The majority of previous work has been carried out by the dynamic method and suffers consequently, for such a complex reaction, from definite uncertainties of the time of contact and the analysis of the products. Since Pease, by the dynamic method, has shown that the only reaction of importance in the early stages of the thermal treatment of acetylene is the polymerization, and since the static method is admirably fitted for a study under such conditions, particularly if a back extrapolation to zero time is advisable, the static method as used previously by Taylor and his coworkers (14) was adopted. POLYMERIZATION
Acetylene was generated by dropping water on Baker's calcium carbide, the gas evolution being moderated by suspension of the carbide in alcohol. The gas was purified by passage through acid copper sulfate and dried over calcium chloride. Analysis by precipitation as copper acetylide gave, as a mean of several determinations, its purity as 98.7 per cent. As shown later, the only objectionable impurity in the acetylene is oxygen, and absorption in acetylene-saturated alkaline pyrogallol showed considerably less than 0.1 per cent of oxygen in the acetylene used. It is repeatedly mentioned in the literature that acetylene decomposes more readily and smoothly in the presence of carbon formed in its pyrolysis. This became quickly appreciated during the work because of the di61 Abstract from a thesis presented by Andrew van Hook in partial fulfillment of the requirements for the degree of Doctor of Philosophy a t New York University.
811
812
H. AUSTIN TAYLOR AND ANDREW VAN HOOK
culty of obtaining duplicate results if the reaction system was thoroughly cleaned after each experiment. It was found, however, that after one run had been completed, the reaction vessel was apparently seasoned in some manner, and subsequent experiments at a given temperature and pressure were almost ideally reproducible. A change in either of these factors, however, necessitated a new seasoning. After each run the apparatus was thoroughly evacuated for at least one hour with a mercury vapor pump backed by an oil pump. To illustrate this reproducibility, data are presented in table 1 of two runs, fortunately at exactly the same initial presTABLE 1 Reproducibility of results after seasoning of the reaclion vessel Temperature, 535°C.; initial pressure, 399 mm. of CzH2 TIME
I
(1
PRESSURE DECREASE
1
P R E S S U R E DECREASE
1
I1
minutes
mm.
7nm.
minutes
mm.
mm.
0.5 1 .o 1.5 2 3 4
42 79 108 130,5 160 179.5
42 80 110 131 161 181
6 8 10 20 30 120
204 219.5 230 253 261 272
205 220 230 252.5 260 272
TABLE 2 Constancy of percentage pressure decrease with acetylene alone TEMPERATURE
I
BND-POINT
"C I
495 515 535 Mean. . . . . . , . . . . . . . . , . . . . . . . . . . ,
0.671 (Average of 26 observations) 0.687 (Average of 10 observations) 0.669 (Average of 6 observations) 0.675
sure, at 535"C., showing the rate of pressure decrease with time of 399 mm. of acetylene. With acetylene alone the total percentage pressure decrease on completion of reaction was constant, independent of temperature and initial pressure. These facts are shown in table 2 as the ratios of the total pressure decrease to the initial pressure. The end-point of the reactions occurring at temperatures from 495°C. to 535°C. may therefore be taken as approximately two-thirds. It is worth while to mention in this connection some observations made on a new reaction vessel which had not been seasoned. The first run is slower,
POLYMERIZATION AND HYDROGENATION OF ACETYLENE
813
as measured by the actual rate of pressure decrease, by an amount approximately equivalent to a 5°C. change in temperature. The end-point too is much lower, about 0.36 as against 0.66, and would seem to indicate the preference of the decomposition reaction on the clean surface and its progressive decrease with seasoning. In order to test specifically the order of the polymerization reaction a comparison of the fractional lives for various initial pressures was made. As a result of the distillation of the liquid products formed during polymerization Pease (8) decided that the major reaction involved four molecules of acetylene, giving a tetrapolymer. This is not inconsistent with the end-points herein quoted, since admittedly these are composite of both TABLE 3 Times necessary for one-tenth, one-.fourth, and one-half of the acetylene to react Temperature, 495°C. INITIAL PRESSURE
h/4
P
Pt
k
-
mm.
ainutec
748 729 558.5 380 365 103.5 100 92.5 79 70.5 42 29.5 24
0.50 0.51 0.68 1.15 1.18 7.0 7.3 8.0 10.6 11.4 24.9 38 57
minutes
374 372 380 436 430 725 729 738 835 800 1042 1150 1370
0.0037 0.0038 0.0037 0.0032 0.0033 0.0020 0.0019 0.0019 0,0017 0.0018 0,0014 0.0012 0.0010
1.25 1.27 1.71 3.0 3.1 19.5 20.6 22.0 29.0 30.5 72.5 136 215
(112
Pt
k
-minutes
935 925 955 1140 1160 2020 2060 2150 2280 2145 3020 4000 5150
0.0045 0.0046 0.0044 0.0037 0.0036 0.0021 0.0021 0.0020 0.0019 0,0020 0.0014 0.0011 0.0008
3.35 3.67 4.69 8.3 8.8 59.0 67.2 70.0 89.0 91.0 212 410 500
2510 2675 2620 3160 3210 6100 6700 6460 7010 6400 8860 12050 12000
0.0050 0.0047 0.0048 0.0049 0.0040 0.0021 0.0019 0,0020 0.0018 0.0020 0.0014 0.0011 0.0011
polymerization and decomposition, despit the inherent suspicion of a trimer that the value of two-thirds would suggest. Assuming then with Pease the formation of the tetramer, the fractional life will be the time necessary for the pressure to fall by an amount P / f - P / 4 f , where P is the initial pressure of acetylene and l/f the chosen fraction. In table 3 are listed the times necessary for &, t, and of the acetylene to react. The products of initial pressure and time are included as a test of the bimolecularity of the reaction. The velocity constants quoted are calculated from these times by the usual expression for a bimolecular reaction k = l / t . z/u(u - z), the units used being seconds and atmospheres. The figures indicate that in the early stages the reaction is bimolecular but deviates as the reaction proceeds, and furthermore, that the deviation
814
H. AUSTIN TAYLOR AND ANDREW VAN HOOK
is greater the smaller the initial acetylene pressure. Similar data at the other temperatures studied show that as the temperature increases the effect of the disturbing factor, which is undoubtedly decomposition, also increases. The values of the velocity constant obtained above from the one-tenth life were verified by a back extrapolation to zero time. Thus, by plotting the observed values of the pressure change in unit time against the time and extrapolating to zero time, the limiting rate of pressure change is found at the very start of reaction, and dividing this by the square of the initial pressure gives the velocity constant of the reaction a t its inception. These values were slightly less than those found from the onetenth life and are used in the subsequent calculation. Since higher pressures favor polymerization the velocity constant calculated above is found to approach a limiting value at pressures which are only slightly above those actually studied. It is justifiable then to estiTABLE 4 Values of the velocity constant TEMPERATURE
VELOCITY CONSTANT
WORKER
'C.
420 495 500 515 525 535 550
0.005-0.008 0.0039 0 .OOll-O .0015 0.0078 0.0030-0.0045 0.0145 0,004941.011
Schlaffer and Brunner Taylor and van Hook Pease Taylor and van Hook Pease Taylor and van Hook Pease
mate this limiting value by plotting the velocity constants against the reciprocal of the pressure and extrapolating to infinite pressure. By this means the value of the velocity constant for the earliest reaction and under the ideal conditions of infinite pressure is obtained. In table 4 the values so found at the temperatures studied are given. For comparison there are included in the table also the values available in the literature already cited. In general the values found are almost double those reported by Pease. This is undoubtedly due to the approximate nature of the calculation in Pease's work, using a dynamic method, and to the devices used above to extrapolate to zero time and infinite pressure. Indeed, as Pease found, the value of k is usually the greater for those runs in which the least hydrogen and methane are found, that is, in which the pyrolysis was least. With the above values of the velocity constant an estimate of the energy of activation of polymerization can be obtained, using the simple Arrhenius equation, For the temperature interval 495OC. to 515OC. the value
POLYMERIZATION AND HYDROGENATION OF ACETYLENE
815
calculated is 41,700calories; for the interval 515°C. to 535"C.,it is 39,300 calories, giving as a mean 40,500calories. A comparison between the observed velocity constants and those calculated on the basis of the activated collision theory of bimolecular reaction is now possible. The total numbers of collisions occurring per cubic centimeter per second is given by 2n2u22 / r k T / m , where n is the number of molecules per cubic centimeter, u the diameter, and m the mass of the molecules. The diameter of the acetylene molecule is not given in the literature, though some scant data are available on the viscosity. Vogel (17)gives 93.5 micropoises as the value at 0°C. The temperature variation is not known, though for ethylene, which has an almost identical viscosity at O'C., the variation with the temperature is given by the Sutherland constant of 226. Assuming the same value for acetylene, the moleccm. The ular diameter a t high temperatures evaluates to 3.64 X number of effective collisions will be times the total number of collisions, and since two molecules take part in collision the rate of disTABLE 5 Ralio of observed to calculated velocitu constants RATIO
RE.4CTION
C2Hz CzHa C2H4 C4He CsHs
+ CzHz............................... + H Q . ................................ + CzH4.. .............................
+ C4Hs.. ............................. + C6Hs ...............................
REFERENCE
1:3 1:220 1:370 1:10,300 1:530
appearance of acetylene will be twice this product. At 788'K. and a concentration of 1 mole per liter this product is equal to 1.33 in liters per mole per second. The value of k determined by experiment when expressed in these same units is 0.50. The ratio of the observed to the calculated velocity constants, 1:3, may be compared with similar ratios found in other association reactions. Vaughan (16) has recalculated many of the available data and summarized the results on a comparable basis. In table 5 are listed the known examples. The smallness of the ratio for acetylene is in harmony with the simplicity of the molecule, and though too much reliance may not legitimately be placed on the actual figure (the limits are probably 1 and 1 :10)the position of the reaction in the series is significant. HYDROGENATION
In the reactions with hydrogen measurements were made over the same temperature range, using different mixtures of acetylene and hydrogen.
816
H. AUSTIN TAYLOR AND ANDREW VAN HOOK
Thus mixtures were made up which were approximately 1:1, 1:8, 1:16, and 1 :24 acetylene to hydrogen, and each mixture was investigated over the whole temperature range and at partial pressures of the acetylene from about 20 to 70 mm. The observed end-point, that is, the ratio of the pressure decrease to the partial pressure of acetylene, varied with both temperature and composition, as is illustrated in table 6. To discuss these figures as well as the subsequent treatment necessitates a consideration of the possible reactions which might occur. In the first place polymerization of acetylene will be proceeding, accompanied by the decomposition mentioned earlier. Since under the experimental conditions the partial pressures of acetylene used are rather small, a larger proportion of decomposition might be suspected as compared with that in the previous work with acetylene alone. In this connection Bone and Coward (1) and Pring and Fairlee (11) claim that decomposition is favored relative to polymerization by the presence of hydrogen. In the second place, hyTABLE 6 Variation in observed end-point with temperature and composition TEMPERATURE
END-POINT
TEMPERATURE
0.81 1.24 1.60 1 .9a 1.32
515 515 535 535 535
~~~~~
"C.
495 495 495 495 515 U l U g G l l ~ U l V l lV I
MIXTURE
c~H~:H~
END-POINT
"C.
1:l 1:s 1:16 1:24
1:s
a b G u y r G l l G U V G u l l y l G u ~~ ~ ~ u ULIIUI. u l ~ 4
1: 16 1:24
1:s
1:16 1:24 IIIC
1.93 2.28 1.51 2.15 2.51 I ~ ~ ~ V I V I IU L
~
b
versible, but the equilibrium constant at the temperatures in this study is of the order of 10' (5) and the reverse reaction may thus be neglected. Hydrogenation of ethylene to ethane might be expected, as well as the possible hydrogenation of the acetylene polymers. It would seem necessary to invoke all of these to account for the observed end-points. However, a study of the individual rates of pressure change showed there was a very gradual approach to the end-point, a pressure decrease approximately equal to the initial acetylene pressure occurring relatively rapidly, followed by a considerably slower decrease for many hours and even days before any semblance of an end-point was eventually reached. It is probable that the major reactions occurring in this early stage involve acetylene polymerization, decomposition, and hydrogenation almost exclusively, and since little is known of the decomposition it can only be grouped with the hydrogenation and allowance made for the known polymerization. The relative occurrence of these two tendencies may be miicrh 1v est im n.t,ed
-
POLYMERIZATION AND HYDROGENATION OF ACETYLENE
817
Assuming, in anticipation of later results, that the hydrogenation is bimolecular, the rate of disappearance of acetylene will be given by
and the observed pressure change by
With mixtures high in hydrogen content it is apparent that the second term on the right will contribute predominatingly to the rate. Calculations, using as low a ratio as 1:10 for acetylene to hydrogen and arbitrary values of kl/k2, show the relative insignificance of the polymerization. The value of Icl/kz actually found is about 5, and it is legitimate therefore to neglect the polymerization at higher hydrogen ratios in order to approximate the value for k 2 for a more exact evaluation at the lower ratios. The method is virtually one of successive approximations. Thus, assuming only hydrogenation to ethylene to occur early in the reaction, for acetylene to hydrogen ratios of 1 :32 or 1 :24, the one-tenth or one-quarter lives will be the times necessary for the pressure to decrease by one-tenth or onequarter of the initial pressure. Calculating an approximate velocity constant from these values gives a value of 5.0 for the 1:24 mixture and 5.5 for the 1 :32 mixture for the ratio of the rates of polymerization and hydrogenation for the same acetylene pressure at temperatures from 495°C. to 535°C. Using this latter value and assuming the major reactions t o be 4CzH2 C2H2 +H2
--f
+
(C2H2)4 Ci"
the times taken for one-tenth of the acetylene to be hydrogenated for the various mixtures used may be evaluated. Thus for the 1:32 mixture the one-tenth life will be the time for the pressure to decrease an amount equal to 0.110 of the initial pressure, and similarly for other acetylene to hydrogen ratios. The times so found at one temperature as typical of the other temperatures are given in table 7, together with the velocity constants calculated from them by the usual bimolecular equation. There can be no doubt that the previous assumption of bimolecularity is completely justified. Since, however, account has only been taken of the acetylene polymerization which is occurring simultaneously with the hydrogenation, and no account was taken in the above of the acetylene decomposition, more reliable estimates of the velocity constants of the hydrogenation would be obtaiued from data taken as early in the reaction as possible. To this end a procedure similar to that used for the polymerization was adopted, involving an extrapolation to zero time of the observed
8 18
l
H. AUSTIN TAYLOR AND ANDREW VAN HOOK
N
I
~
~
$ tl/10~
~
I
TABLE 7 Velocity constants Temperature, 515°C. ~
I
b
1
t1/4
k
I
I
11/2
Ii
C2Hz:Hz = 1:24
mm.
27 28.2
27.2 37.9 39.3
!:y
,
1
1 1 0.00047 0.00043
:4":
~
0.00040 0.00038
~
;t
~
I 1 I :; 1
0.00044 0,00039
CzH2:Hz = 1:16 5.5 5.1 * 4.4
0.00049 0.00038 0.00043
19.5
0.00043 0.00036 0.00038
53
0.00039 0.00038 0.00038
0.00040 0.00039 0.00044 0.00042
93 73
0.00037 0.00037
45.5
0.00037
CzHz:Hz = 1 : s 29.7 41.9 60.4 665.
12.0 9.5 6.2 5.8
0.00040 0.00040 0.00042 0.00041
35.5 28.0 17.5 16.5
TABLE 8 TEMPERATURE
khydrogenation
E
OC.
0.00019 0.00028 0.00039 0.00055 0,00076
495 505 515 525 535
43540 40410 43010 41470
rate of pressure change and calculation from this value of the velocity constant. Thus
Since h / k z is approximately 5.5 IC2
=
- dP/dt 5.5 p&Hz
+
2PCzHz PHe
In this way the probability that the values of ICz are vitiated by the occurrence of acetylene decomposition is reduced as far as would seem possible. The constants so obtained are presented in summary in table 8 for the five temperatures studied. The energies of activation for the succeeding temperature intervals are also included and give an average value of 42,000
POLYMERIZATION AND HYDROGENATION OF ACETYLEXE
819
calories. This value may be compared with that calculated by Sherman and Eyring (13) on the basis of quantum mechanics, namely 46,400 calories. This discrepancy is not unexpected in that the calculated value relates to 0°K. and depends on the choice of 10 per cent for the coulombic energy. Furthermore the value found is very close to that obtained by Pease for ethylene hydrogenation, 43,200 calories. The comparison between the observed velocity constant at a particular temperature and that calculated on the basis of collision theory may again be made as in the previous case. Taking the total number of collisions occurring per cubic centimeter per second between unlike molecules as 2N1N2ui2 1/2?rkT(ml+m2)/mlm2, where AT1 and N2 are the numbers of molecules per cubic centimeter of acetylene and hydrogen respectively, of masses ml and m2,u12 is the mean of their two diameters, so with 3.6 X cm. as the diameter of acetylene and 1 X lo-* cm. for hydrogen, using 42,000 calories as the energy of activation, it is calculated for the temperature 788'K. that the velocity constant should be 0.724 liter mole-1 sec.-l The observed value in these same units is 0.252, whence the ratio is again found to be approximately 1:3, also in harmony with the values for other reactions previously quoted in view of the simplicity of the molecules partaking in the reaction. SUMMARY
The homogeneous thermal polymerization of acetylene and its hydrogenation have been investigated in the temperature range 495°C. to 535°C. The principal processes are interpreted as bimolecular with energies of activation of 40,500 and 42,000 calories, respectively. The efficiency of the association is high, in that the ratio of the observed rates to those calculated on the basis of activated collisions is 1:3 in both cases. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)
BONEAND COWARD: J. Chem. SOC.93, 1197 (1908). AND SCHAAD: J. Phys. Chem. 36,1457 (1932). EQLOFF,LOWRY, HURD:Ind. Eng. Chem. 26, 50 (1934). KASSEL:J. Am.Chem. SOC.63,2143 (1931). KASSEL:J. Am.Chem. SOC.66,1359 (1933). AND TRICOT: Chimie & industrie 13,361,537 (1925). KOVACHE MATHEWS:J. Am. Chem. SOC.22,106 (1900). PEASE: J. Am.Chem. SOC. 61,3470 (1929). PEASE:J. Am. Chem. SOC. 63, 613 (1931). PEASE:J. Am. Chem. SOC.64, 1876 (1932). PRINGAND FAIRLEE: J. Am. Chem. SOC.62,1158 (1930). SCHLAFFER ANn BRUNNER:Helv. Chim. Acta 13, 1125 (1930). AND EYRING: J. Am. Chem. SOC.64,2661 (1932). SHERMAN TAYLOR: J. Phys. Chem. 34,2761 (1930). J. Am.Chem. SOC.64,5873 (1932). VAUQHAN: J. Am.Chem. SOC.66,4115 (1933). VAUGHAN: VOQEL:Ann. Physik 43, 1235 (1914).
