Kinetics of the thermal reactions of ethylene. Part 11. Ethylene-ethane mixtures

Transcription

1 Kinetics of the thermal reactions of ethylene. Part 11. Ethylene-ethane mixtures M. L. BOYD' AND M. H. BACK Chemistry Department, University of Ottawa, Ottawa, Canada Received November 21, 1967 Mixtures of ethane and ethylene have been pyrolyzed in the temperature range "C and at pressures from cm. The products were similar to those obtained from the pyrolysis of ethylene by itself, described in Part I, with a marked increase in the yields of the saturated products. The initial rates of product formation and the dependence of these rates on the concentration of ethane suggest that the initiation step is the same as that proposed in the pyrolysis of ethylene alone, viz. 2CzH4 -t CzH, + CzHs and that the reaction C2H4 + CzH6 -> 2CzHs is not an important source of radicals. A simplified mechanism is outlined to account for the main effects of ethane on the free radical chain polymerization. Canadian Journal of Chemistry, 46, 2427 (1968) Introduction The results of the study of the pyrolysis of ethylene reported in Part I (1) were interpreted in terms of a free radical chain polymerization. The initiation step postulated was the bimolecular reaction of two ethylene molecules to form a vinyl and an ethyl radical. It seemed of interest to look for the occurrence of an analogous reaction between ethane and ethylene, and for this purpose the pyrolysis of mixtures of ethane and ethylene was studied in the temperature range 563 to 600 "C. The pyrolysis of mixtures of ethane and ethylene at 600 "C has been studied by Silcocks (2). In several respects, such as the products formed and their relative importance, the present results are in agreement with the earlier work. In the present work the rates of formation of the products were measured at an earlier stage of the reaction and this has allowed a clearer definition of the induction period and the changes observed in it on addition of ethane. Differences in interpretation can for the most part be attributed to this distinction. Experimental The apparatus, procedure, and analytical methods were identical with those described in Part I. The ethane and ethylene used were Phillips Research Grade, degassed at -196 "C. The trap-to-trap distillation in vaciro prior to each experiment, as described previously, was also 'Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada. made here. The reaction vessel used in all erperiments was the quartz cylinder of ml capacity with a surface/ volun~e ratio of 1.0cm-'. Homogeneous gaseous mixtures of ethane and ethylene were obtained by condensing both gases in the central cold finger of the 1 l flask described in Part I. On warming, convection currents were generated and complete mixing occurred in a short time. Results Mixtures containing 15 cm ethylene and varying amounts of ethane (1545 cm) were pyrolyzed at 563, 584, and 600 "C. Yields of the following ~roducts were measured as a function of time: n-butane, methane, hydrogen, propylene, butene, butadiene, propane, and benzene. Figures 1-5 show representative yield-time plots for the main products, from which initial rates were obtained. Figure 6 shows the initial rates of the main products as a function of ethane pressure. The &der of these rates with respect to ethane was determined by least squares analysis of log rate against log pressure curves. These orders are given in Table I. In brief, the main effects of the addition of ethane upon the pyrolysis of ethylene may be summarized as follows: (1) The yields of n- butane, methane, and hydrogen, each considerably increased, were linear with time in the initial stages. The rates of each were first order with respect to ethane. (2) The initial rates of propylene and butene were scarcely affected, but the induction periods observed in both cases were markedly reduced, usually to the point when their existence was doubtful. (3) The initial rate

2 2428 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, cm C2H6 eo- BUTANE 600 c METHANE ' O ' / 30cm C2Hs 16 0 TIME (51 FIG. 1. Yield of butane as a function of time. of formation of butadiene was inversely proportional to the concentration of ethane. The yield plots for butane and methane suggested a small induction period, but since it was less than 20 s its existence is questionable. The rate of production of propylene as a function of temperature is shown in Fig. 7 along with the rates obtained in the absence of ethane. A similar plot was obtained for the rate of formation of butene. Ethane clearly has a negligible effect on the rates of production of propylene and butene. Secondary Products The shape of the yield-time plots for propane and benzene were unchanged upon addition of ethane and in both cases the yields were not appreciably altered. The propane yield increased W I I I I I I I TIME (51 FIG. 2. Yield of methane as a function of time. from zero time whereas benzene appeared to rise from the end of the induction period. Discussion The Initiation Step One of the objects in the study of the mixtures of ethane and ethylene was a search for the occurrence of the bimolecular initiation step 1 C2H4 + C2H6 -f C2H5 + C2H5 analogous to that suggested in the pyrolysis of ethylene alone, 12 I C2H4 + C2H4 -t CzH3 + CzH5. A third reaction which it is necessary to consider is the dissociation of ethane. [3 1 C2H, -t 2 CH,. TABLE I Activation energies and order with respect to ethane of the initial rates Temperature (" c) Butane Methane Hydrogen Propylene Butene Butadiene Activation energy

3 PROPYLENE BOYD AND BACK: KINETICS OF THE THERMAL REACTIONS OF ETHYLENE. PART I[ Dc I BUTENE 0 A C2 ti4 only v TlME (51 FIG. 3. Yield of propylene as a function of time: (0) 15 cm C2HG; (A) 30 crn C2HG; (0) 45 crn C2HG. A summary of the relevant rate parameters is given in Table 11. The frequency factors and rate constants for reactions [I] and [2] have been calculated on the assumption that the rate constant for the reverse disproportionation of radicals is in both cases equal to 3 x 1012 cc mole-' s-' at 298 OK (3). For comparison, the general expressions for the rate constants have been obtained using data at 298 OK (4). The rate constant for reaction [3] has been discussed by Lin and Laidler (5). We use the following average value suggested by them, IOIG e-88000/rt -1 k,,j - ' 3.d S. The last colun~n of Table I1 lists the rates for the three reactions for equal pressures of ethane and ethylene. Reactions [I] and [3] would therefore be expected to be the main initiation processes. Nevertheless the evidence from the measurement of the initial rates of formation of the main products is against the occurrence of these steps. Initiation by reaction [I ] and termination by mutual combination of radicals would give a radical concentration proportional to [C2H4]'/2 [C2H6]1/2. Initiation by reaction [3] and termination by mutual combination of radicals would give a radical concentration proportional to TlME (51 FIG. 4. Yield of butene as a function of time: (A) 15 crn C2HG; (0) 30 crn C2HG; (0) 45 cm C2H6. [C2H6]'/2. If the products were then formed through a series of radical addition and abstraction reactions similar to those outlined in Part I, the order with respect to ethane of the rates of formation of these products should be either 0.5 or 1.5. None of the rates measured showed this dependence. The orders with respect to ethane of the rates of methane and butane were very close to one and were constant over the range of pressure and temperature studied. The rates of formation of propylene and butene were essentially independent of ethane concentration. Since the activation energy for reaction [3] is 22 kcal/mole greater than that for reaction [2], initiation by reaction [3] should raise the activation energy for the rates of formation of the products by about 1 I kcal/mole, assuming the activation energy for the propagation steps remains the same. This was not observed for any of the products. The activation energy for butene and propylene was unchanged while that for butane decreased by about 20 kcal/mole. We conclude, therefore, that in the mixtures of ethane and ethylene, ethane is not i~lvolved in the initiation process. The question remains of why reaction [2] appears to be the predominant initiation reaction. The lneasurenlents and the data pertaining only

4 TABLE II Thermodynamic data and reaction rate parameters 5 Rate at z 857 "K $ log A K(857 OK) (CZH4 = 15 cm) S0(298 OK)* AHr(298 OK) AH(298 OK) AS(298 OK)* (cc mole-' s-' (cc mole-' s-' (mole cc-' s-' n (cal/deg mole) Ref. (kcal/mole) Ref. Reaction (kcal/mole) (cal/deg mole) or s-') or S-') x 10lz) I CzH CzHs CzHs x 3.03 : CzH < 'Standard state: 1 atm of ideal gas. s p m e

5 BOYD AND BACK: KINETICS OF THE THERMAL REACTIONS OF ETHYLENE. PART I Y I I I TIME (s) FIG. 5. Yield of butadiene as a function of time. to reactions [l] and [3] are the most reliable and the simplest explanation lies in the possibility of errors in the thermodynamic data used to calculate k,. The most uncertain quantities are those of the vinyl radical. An error of 5 kcal/mole in the heat of formation of the vinyl radical would raise k, to the same value as k, at 857 OK. This would, however, require a C-H bond dissociation energy in ethylene 5 kcal/mole lower than the current value of 103 kcal/mole (9, 10). Less uncertainty is probably involved in the entropy of the vinyl radical but a combination of errors in both quantities might change k, considerably. Another uncertainty is the reverse radical disproportion rate constants. k-, could be greater than k-, by a factor of about 5 or 10 (11, 12). It thus appears probable that k, has been underestimated. PRESSURE OF ETHANE FIG. 6. Rate of product formation as a function of ethane pressure. Left-hand scale applies to all products except methane. Right-hand scale applies to methane. by abstraction from ethane which may now compete successfully with addition to ethylene. It is suggested, in fact, that the four main products, butane, methane, butene, and propylene, arise during the polymerization by abstraction of hydrogen from ethane by the corresponding radical. In the case of the unsaturated radicals, C,H, and C,H,, this abstraction process must be faster than the addition to ethylene, since the rate of formation of butene and propylene is essentially independent of ethane concentration. The rates of formation of butene and propylene will then be practically unchanged from the rates observed in pure ethylene. In the latter system the rates of formation of butene and propylene were essentially the rate of propagation of the free radical polymerization, The Mechanism The proposed mechanism may be briefly summarized in general terms. The presence of ethane provides a more readily abstractable a 0,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, hydrogen atom than is available from ethylene. IOOO/ T (OK-[) The polymer will tend to be more saturated and FIG. 7. Arrhenius plot of the rate of propylene probably of a shorter length. More may formation. Pressure of ethylene = 15 cm. Pressure of be diverted into lower molecular weight products ethane: (0) 0 cm; (G) 15 cm; (v) 30 cm; (0) 45 cm. cm

6 2432 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, 1968 since after the induction period the rate of [lo1 C4H9 -i- C2H4 -> R decomposition of the polymer was equal to its ~111 C4H9 -I- C2H, -> C4Hlo + C2H5 rate of formation. In the mixtures of ethane and ethylene the radical abstraction reaction will be [I2] R->CH, + R comparable to the rate of propagation of the [I31 CH, -t C2H4 -> C3H7 free radical polymerization. However, since the [141 CH, -t C2H6 -. CH, + C2H5 abstraction occurs during the polymerization process no induction period will be observed. [I51 R 4- CzH6 -> RH + C2H5 In the case of methane and butane it must be [I61 R + C2H4 -> R' -t C2H5 assumed that the abstraction from ethane by the saturated radical is slow compared with the [t 1 2CzHs -> C4H10 addition to ethylene in order to give the first order dependence on ethane. The n-butyl radical is probably formed largely by addition of ethyl radicals to ethylene, since the activation energy for butane formation is close to that for butene [C2H" (k2/kt)1~2[~z~4~ and propylene. The methyl radical, on the other Making the assumptions already discussed, the radical concentrations may be expressed as follows, hand, probably arises from decomposition of [GHal = - [CzHj] radicals formed during the polymerization, since the activation energy for methane formation is k,[c2h3iic2h41 large. The rate of methane formation is greatly = kg[c?h~l increased in the mixtures and it may be that the saturated polymer radical tends to lose a methyl k 6 k 7 k 4 [C4Hgl = - [ C~HS] kio group more readily than the unsaturated polymer formed in pure ethylene. Since the rates of methane and propylene are not equal, decom- ki2ks[czh~] position of the n-butyl radical is not an important [CH31 = ki3(kii[c2h61 + kir[c~h,l + k12) ' source of methyl radicals. and the steady state expressions for the rates of The characteristics of the rate of formation of formation of products are then given as butadiene were rather different from the other products and it is suggested that the mechanism R,,,, = k7(k2/kt)1/2[~2~412 for formation of butadiene in the mixtures is the same as in pure ethylene. The negative order kiik6 Rc,,l, = -( K~/K,)"~[C~H~][C~H~] with respect to ethane is accounted for by kg reaction of its precursor radical with ethane to ki4ki2kj form products other than butadiene. RCH, = (kz/kt)1/2[c2~41[c2~61. A summary of the most probable reactions is presented as a guide but is not intended to be The activation energy for butene will thus be comprehensive. For simplicity the termination close to that observed in pure ethylene. The step is again taken as the of ethyl activation energy for butane will be similar to radicals. The general term R denotes any radical that for butene while the activation energy for higher than C, and R' represents an unsaturated methane may be much larger because of reaction polymer molecule. [12], the decomposition of the polymer radical. The weaknesses of the assumptions necessary [2 I C2H4 + C2H4 -> CzH3-1 CZHS to account for the orders of the rates with respect [4 1 C2H, + C2H4 -> C4H7 to ethane are apparent. The results, however, are [5 I C2H3 + C2H6 -+ CzH4 + C2H5 not easy to tie together in a consistent mecha- [6 I CzHs + CzH4 + C4H9 nism. The disappearance or the induction period is the fact which suggests the products are formed during the free radical polymerization. 17 I C2H5 + C2H4 + CZH, + C2H3 I C4H7 4- C2H4 -> R Otherwise a change in the induction period must [91 C4H7 + C2H, -> C4H, + C2H5 arise by a change in lc,, the conlposite rate

7 BOYD AND BACK: KINETICS OF THE THERMAL REACT~ONS OF ETHYLENE. PART constant for decomposition of the polymer, or Acknowledgments by participation of ethane in the decomposition The authors to thank the National process. This may be seen from the equation ~~~~~~~h council of canada and the D ~ ~ ~ given in Part I for the concentration of polymer of university ~ g of the ~ province i ~ of ~ as a function of time. Ontario for financial support. M. L. B. thanks Dr. J. Convey, Director of the Mines Branch, k [I] = -I [C?H 1 [1- e-bjch411 1 for the opportunity to participate in this work. kd Although it was suggested in Part I that decomposition of the polymer did involve reaction with ethylene, reaction with ethane is much less likely. An increase in lc,, due possibly to the larger degree of saturation of the polymer formed in the presence of ethane, is an alternative explanation. This in fact may be a more reasonable mechanism for the formation of propylene, since the induction period for this product did not disappear entirely as it did for butene. There is, however, a further difficulty in suggesting a change in lc,. The rates of butene and propylene were essentially unchanged in the mixtures, but a decrease in k, would lower the steady state concentration of polymer I and hence lower the rate of formation of product which arises from its decomposition. 1. M. L. BOYD, T-M. WU, and M. H. BACK. Can. J. Chem. This issue. 2. C. G. SILCOCKS. Proc. Roy. Soc. London, Ser. A, 233, 465 (1956). 3. A. SHEW and K. 0. KUTSCNKE. J. Chern. Phys. 26, 1020 (1957). 4. Selected values of physical and thermodynamic properties of hydrocarbons. American Petroleum Institute Research Project. Carnegie Press, Carnegie Inst. of Technology, Pittsburgh M. C. LIN and K. J. LAIDLER. Trans. Faraday Soc. 64, 79 (1968). 6. S. W. BENSON. J. Chern. Educ. 42, 502 (1965). 7. J. H. PURNELL and C. P. QUINN. J. Chem. Soc (1964). 8. S. W. BENSON and G. R. HAUGEN. Private cornmunication. 9. A. G. HARRISON and F. P. LOSSING. J. Am. Chern. SOC. 82, 519 (1960). 10. A. F. TROTMAN-DICKENSON and G. J. 0. VERBEKE. J. Chem. Soc (1961). 11. A. G. SHERWOOD and H. E. GUNNING. J. Phys. Chem. 69, 2323 (1965). 12. N. A. WEIR. J. Chern. Soc (1965).