THE PYROLYSIS OF TETRAFLUOROETHYLENE by George A. Drennan and Richard A. Matula Fluid Dynamics Laboratory Department of Mechanical Engineering College of Engineering The University of Michigan Ann Arbor, Michigan This research has been sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant AF-AFOSR-1144-67, and administered through the Office of Research Administration, The University of Michigan. Reproduction in whole or in part is permitted for any purpose of the United States Government.

THE PYROLYSIS OF TETRAFLUOROETHYLENE George A. Drennan and Richard A. Matula ABSTRACT The pyrolysis of tetrafluoroethylene was studied in the temperature and pressure range 300 to 455~C and 25 to 760 torr. The rate of reaction was determined to be second order with respect to the tetrafluoroethylene, and three independent numerical values of the second order rate constant were determined by simultaneously measuring the tetrafluoroethylene concentration, the octafluorocyclobutane concentration and the total pressure as a function of reaction time. The temperature dependence of the reaction rate constants are given by I s-/,- *- t(? kin /^C'-2 eyp 5~v 7<t2 -2. 13 t,,20 ) RT i A- i,.' /c' -yc\ cc/mole-sec-1 cc/mole-sec1 cc/mole-sec 1

INTRODUCT ION The vapor phase dimerization of tetrafluoroethylene (C2F4) in the temperature and pressure ranges 290-470~C and 100 to 700 torr have been studied by Lacher, Tompkin and Park(l). Their experiments were conducted in a one-liter Pyrex vessel, and the kinetic results were based on total pressure measurements as a function of reaction time. The rate of consumption of C2F4 was found to be second order with respect to C2F4 and the second order Arrhenius rate constant (k2) was reported to have an activation energy of 26,299 kcal/mole and a frequency factor of 16.5 x 101 cc/mole-sec.- Atkinson and co-workers (23) studied the decomposition of C2F4 at temperatures from 300 to 8000C. At temperatures below 600 C, they reported that the second order dimerization of C2F4 to octafluorocyclobutane (c-C4FQ) and the first order back reaction were much faster than any of the other reactions which were taking place. In the temperature range 600 to 800~C hexafluoroethane (C2F6) and octafluorobutenes were also formed, and at temperatures above S00 C C2 F and tars were the primary products. The dimerization of C2F4 in the temperature and pressure ranges 300-5500C and 200 to 550 torr was studied in a static Pyrex reactor. The kinetic data were based on the measurement of total pressure as a function of reaction time, and the second order Arrhenius rate constant (<2) for the dimerization of C2F4 in this temperature range was reported to have an activation energy of 25.4 kcal/mole and a frequency factor of 10.3 x 10 cc/molc-sec

-2 (4) Butler( studied the first order thermal decomposition of c-C4F8 to C2F4 in the temperature range 360 to 560~C. These experiments were conducted in a one liter Pyrex flask, and initial reactant pressures between 0.003 and 600 torr were considered. Based on the measured equilibrium composition in this system and the numerical value of the first order rate constant for the decompsoition of c-C4F8, Butler calculated that the second order rate constant (k2) for the rate of decrease of C2F4 had an activation energy of 24.0 kcal/mole and a frequency factor of 20 x 10104 cc/mole-sec1 Butler also found that during the course of c-C4F8 decomposition that a slow parallel decomposition forming perfluoropropene (C3F6) occurred with a first order rate constant (k3). The Arrhenius 17.2 -1 parameters for k3 were determined to be 1017 sec and (3) 87.2 kcal/mole. Atkinson and Atkinson(3) also showed that the formation of C3F6 from c-C4F8 is a first order reaction. Their experiments were conducted in a nickel pyrolysis tube and covered the temperature range 550 to 650~C. Based on these experiments, the first order rate constant (k3) was reported to have an activation energy of 79.0 kcal/mole and a frequency 16 -1 factor of 3.9 x 101 sec 1 In order to evaluate the second order rate constant (k2) for the dimerization of C2F4 based on the experimental determination of total pressure as a function of reaction time, the previous investigators'2 were forced to make certain assumptions. The present investigation was undertaken in order

-3 to determine three independent numerical values of the rate constant (k2). The three independent values of k2 were determined by simultaneously measuring the C2F4 concentration, the c-C4F8 concentration, and the total pressure as a function of reaction time. The concentrations of C2F4 and c-C4F8 were determined with the aid of GSC chromatography. The numerical values of k2 based on total pressure are compared directly to the values reported in References (1) and (2), and the values of k2 based on the concentration measurements were used to check the assumptions which were made in order to determine k2 from the total pressure measurements. The present series of experiments were conducted in the temperature and pressure ranges 300-455~C and 25-760 torr. EXPERIMENTAL Apparatus. The experiments were conducted in a cylindrical, Vycor reactor which was enclosed in a horizontal wire-wound furnace. The vessel was approximately 250 mm long and had a volume of 455 ml. Prior to instillation in the furnace the reactor was cleaned with a 5% HF-H20 solution. Power was supplied to the furnace from a commercially available temperature controller which was capable of controlling the temperature to within + 0.5~C over a period of several hours. In order to insure that temperature gradients along the furnace cavity

-4 were negligible, a manually controlled guard heater was installed at each end of the furnace cavity. The temperature of the reactor vessel was monitored by four chromel-alumel thermocouples which were placed in contact with the reactor wall and equally spaced along the longitudinal axis of the vessel. The sampling tube which extended to the geometric center of the vessel and the pump tube which was sealed flush with the reactor wall were both made of 6mm Vycor tubing. Both the sampling and pump tubes were terminated outside of the furnace by 2 mm greaseless vacuum stopcocks. An Aerograph model 202-B dual column, hot wire, thermal conductivity gas chromatograph was used to identify and quantitatively determine the gaseous products as a function of reaction time. A Beckman IR-10 infrared spectrophotometer with a spectral range 300-4000 cmr was used as a back up instrument for the identification of any species which escaped detection by the gas chromatograph. A 4 foot column of 50/80 mesh Poropak (Waters Associate, Inc.) Type N maintained at 100~C was used to separate the C2F4 pyrolysis products. The column was packed in 1/4 inch O.D. type 316 stainless steel tubing, and the helium carrier gas flow rate was maintained at 75 ml/min. Prior to final installation in the chromatograph, the column was activated by heating it to 200~C while purging with helium (75 ml/min) for two hours. The concentrations of the various products were determined by comparing the electrical output of the chromatograph from the unknown sample to the output from a calibration mixture of known component concentrations.

-5 The C2F4 used in this study was purchased from Columbia Organic Chemicals, Inc., Columbia, South Carolina, and it was stored in a steel cylinder as a liquified gas under its own vapor pressure of approximately 20 atm. at 20~C. The manufacturer stabilized the liquid phase, by adding 1% by weight of alpha-pinene to the liquid. The supplier specified that the minimum purity of the gas phase was 99%. Subsequent gas chromatographic analysis of the C2F4 indicated that the major gas phase impurity was c-C4F8 and that traces of CO2, CF4 and C2F6 were also present. The mole fraction of the c-C4F8 impurity was determined to be approximately 9 x 10. In order to determine if the rate of C2F4 pyrolysis was effected by residual inhibitor which may have been present in the gaseous C2F4 supplied from the cylinder, a number of preliminary experiments were conducted in which both purified C2F4 and C2F4 taken directly from the cylinder were pyrolyzed. Purified C2F4 was obtained by withdrawing a sample of C2F4 from the cylinder and collecting that fraction of the sample which was volatile at -126~C and condensible at -196~C. Heicklen and Knight(5) report that this purification technique yields C2F4 with less than 0.1% of any impurity. In all cases the experimental results were identical for both purified and cylinder C2F4. Therefore in all experiments the C2F4 was taken directly from the cylinder and used without further purification. The c-C4F8 and C3F6 used in these experiments were purchased

-6 from the Matheson Company, East Rutherford, New Jersey and Air Products and Chemicals, Inc., Allentown, Pennsylvania, respectively. Both of the gases had impurities of less than 1% and were used directly without further purification. Procedure. Reactants and calibration mixtures were introduced into the reactor and gas chromatograph through a glass manifold equipped with greaseless vacuum stopcocks. A gas sampling valve was used in conjunction with a 2 ml sample volume to inject samples into the gas chromatograph. All pressure measurements were made with a Wallace and Tiernan Type 145 Precision Dial Manometer which has a range of 0 to 30 in. of Hg. vacuum and a least count of 0.05 in. of Hg. A mechanical vacuum pump, vented through a standard laboratory fume hood, was capable of evacuating the system to a pressure -3 of approximately 10 torr. The actual experimental data have been obtained by applying standard experimental techniques. The reactants were introduced into the reactor, which was maintained at a controlled temperature, and the time dependence of the total pressure, and the concentration of both reactants and products was determined by withdrawing a sample from the reactor at various reaction times and analyzing the sample with the aid of the gas chromatograph and the IR spectrophotometer. These data which were obtained with temperature, initial pressure and reactant composition as independent variables were used for the evaluation of the necessary rate equations and the appropriate Arrhenius parameters.

-7 RESULTS Data Analysis. The results of previous investigators have indicated that the rate of C2F4 pyrolysis, in the temperature range 300-550~C, can be represented by [CAX CC FY] o L' (L1) Butler(4) has shown that if the temperature is less than 500~C the second term on the right hand side of Eq. (1) is insignificant with respect to the first term. Therefore the rate expression for the pyrolysis of C2F4 at temperatures below 500~C can be represented by the equation'dt+ -CFJ (2) The integrated form of Eq. (2) yields the rate constant (k2) as a function of parameters which were experimentally determined #; - T- - _C/; _-./~-(en/ (3) where t is the reaction time in seconds and FC2F4t and C2F4 are the measured concentrations of C2F4 (moles/cc) at time t and t = 0 respectively. Assuming that the back reaction is insignificant and that the only important products are C2F4 and c-C4F8, Eq. (1) can be rewritten in terms of the c-C4F8 concentration.

-8 gdtCyF) ((2F t 4-Th4) -4) 2.Av/r (4) The numerical value of the second order rate constant for C2F4 pyrolysis (kI) can be determined by integrating Equation (4). e (t~c.C4J * l -cv97F_) __/^ -/ (5):f8 -S... The numerical value of k2 as calculated from Eq. (5) should be equal to the value of k2 calculated from Eq. (3). However, since the two k2 s are based on independent experimental measurements, the "prime" nomenclature is used to differentiate between the two experimental values of the rate constant k2. Since the stoichiometry of the reaction has been assumed and the only products are C2F4 and c-C4F8, the numerical value of k2 can also be determined by measuring the total pressure of the products as a function of time. The numerical value of k2 based on total pressure measurements is given the symbol k" ^-fc Si) ea/ (6) I" t [t t ao c; d )] /C (6) where R is the universal gas constant, T is the reaction temperature, Pt is the total system pressure at time t and P0 is the initial pressure.

-9 C4 Pvrolvsis. The order of the C2F4 pyrolysis reaction was determined at 365~C by applying the half-life method. During the course of these experiments, the initial C2F4 pressure was varied from 740 to 175 torr, and the C2F4 half-life was approximately 2 x 103 sec when the initial C2F4 pressure was 175 torr. A least mean square fit to the data indicated that the order of reaction was 1.98, and hence for all practical purposes the rate equation for the pyrolysis of C2F4 is given by Eq. (2). Once the reaction was established to be second order, the pyrolysis of C2F4 was studied over the temperature and initial pressure ranges 300 to 450~C and 50 to 200 torr. The three independent rate constants given by Eqs. (3), (5) and (6) were calculated from the experimentally determined time dependence of the C2F4 concentration, c-C4F8 concentration and total pressure. The numerical values of these rate constants based on a series of experiments with a reaction temperature and initial C2F4 pressure of 452~C and 50.8 torr are listed in Table 1. The average values of the three rate constants as a function of temperature are given in Table 2. The numerical value of each of the rate constants listed in Table 2 are based on 5 to 10 data points. The Arrhenius expressions for the temperature dependence of the three rate constants obtained by a least-mean-squares fit of the experimental data are given by, ^c (_-S5p^3 tOp )'cQ/*/eC ec-t (7) RT

-10 K>-J^'~":' exp(-253& tD),cc/o/-Sec' (8) \'2 Rr,8,, D ^',,. > ~~(tep(-^SSIL5^ cc/^ tc cOCo e r r/,/'/(9) c-CFP Pvrolvsis. The thermal decomposition of c-C4F8 was studied in the temperature and pressure ranges 452 to 552~C and 100 to 200 torr. These experiments were conducted in order to confirm the results of previous investigators concerning the relative rates of the second order production of c-C4F8 from C2F4 and the first order decomposition of c-C4F8. The results of these experiments confirmed the validity of the assumption that the term representing the first order back reaction in Eq. (1) is insignificant with respect to the contribution of the second order term for temperatures less than 460~C. (3,4) It has been previously shown 3'4) that both C2F4 and C3F6 are formed by parallel, unimolecular reactions during the course of c-C4F8 pyrolysis. These unimolecular reactions are given by C-C —....2 (10) c~-CC -- Cry eCF (11)

-11 (4) Butler(4) has shown that both kl and k3 can be determined if k2 is known and the concentrations of c-C4F8, C2F4 and C3F6 are measured during the course of c-C4F8 pyrolysis. The numerical values of k1 and k3 based on our limited c-C4F8 pyrolysis data at both 452 and 552~C are in reasonable agreement with the (4) results given by Butler DISCUSSION The pyrolysis of C2F4 has been studied in the temperature and pressure ranges 290 to 470~C and 100 to 700 torr. This reaction was shown to be second order with respect to the concentration of C2F4 and the numerical value of the second order rate constant based on three independent experimental measurements has been determined. These results were obtained by simultaneously measuring the C2F4 concentration, the c-C4F8 concentration and the total pressure as a function of reaction time. The temperature dependence of the second order rate constant (k") based solely on total pressure measurements, has (1*2) been previously reported in the literature (12) In the temperature range of interest, the values of k2 reported in References (1) and (2) vary by approximately 26%. The numerical values of k2 and k2 evaluated in the present study are within approximately 5 to 12% of each other and they generally fall

-12 between the results of the previous investigators. However, the second order rate constant (k2) based on the measured c-C4F8 concentration, is approximately 20% lower than k2. A number of possible reasons for the variance between k' and k2 and k" were considered. An error analysis was made in order to determine if the variance in the rate constants was due to experimental errors. This analysis revealed that the expected deviation in the rate constants based on the estimated experimental errors were not large enough to account for the measured deviations. The experimental determination of the second order rate constants k2 k2 and k" were based on two assumptions: (1) C2F4 and c-C4F8 were the only important products; (2) the first order rate of decomposition of c-C4F8 was negligible with respect to the second order rate of formation of c-C4F8. Careful gas chromatographic analysis of the reaction products indicated that no significant side reactions were occurring when the reaction temperature was less than or equal to 452 C. As discussed earlier, the validity of the second assumption was confirmed by studying the thermal decomposition of c-C4F8 at 452~C. The possible effects of any heterogeneous effects were also considered. The previous studies') were conducted in Pyrex reactor vessels in which the surface to volume ratios were varied by a factor of 100 with no significant effect on the rate of reaction. The present investigation was conducted in a Vycor reactor with yet another surface to volume ratio. Since heterogeneous reactions are strongly influenced by both the reactor

-13 material and surface to volume ratio it was concluded that the consistency of the results obtained by similar methods over a broad range of experimental conditions preclude any significant surface effects. ACKNOWLEDGEMENT This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant Number AF-AFOSR-1144-67*

REFERENCES 1. J. Lacher, G. Tompkin and J. Park, J. Am. Chem. Soc., 74, 1693 (1952). 2. B. Atkinson and A. Trenwith, J. Chem. Soc. (London) 2082 (1953). 3. B. Atkinson and V. Atkinson, J. Chem. Soc. (London) 2086 (1957). 4. J.N. Butler, J. Am. Chem. Soc., 84, 1393 (1962). 5. J. Heicklen and V. Knight, U.S. Air Force Report No. SSD-TR-66-105 (June, 1966).

Table 1 SECOND ORDER RATE CONSTANT FOR TETRAFLUOROETHYLENE PYROLYSIS AT 725~K t (seconds) (cc/mole-sec- ) 300 300 600 600 900 900 1200 1800 2240 2278 2379 2355 2295 2260 2250 2381 1947 1904 1791 1865 1871 1886 2520 2520 2380 2380 2250 2250 2502 2561 Average 2305 1877 2420

Table 2 TEMPERATURE DEPENDENCE OF SECOND ORDER RATE CONSTANT FOR TETRAFLUOROETHYLENE PYROLYSIS T k2 k2 ki ~K (cc/mole-sec ) -— ~~~~~~~~~~~~~~~~~.J.. 588 638 663 688 725 36.0 194 411 828 2305 30.5 156 328 620 1877 34.2 183 361 937 2420

Unclassified _ ]my %_ -D.. l nI I I DOCUMENT CONTROL DATA R & D (Security classilicalion of title, body of abstract and indexing annotation must be entered vWhen the overall report Is claaesfled) 1 I.. 1. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICAtION The University of Michigan Department of Mechanical Engineering 2b. GROUP Ann Arbor, Michigan 3. REPORT TITLE The Pyrolysis of Tetrafluoroethylene 4. DESCRIPTIVE NOTES (Type of report and inclusive dates) 5. AU THOR(S) (Piret name, middle initial, last name) George A. Drennan and Richard A. Matula 6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b. NO. OF REFS January, 1968 17 5 d. 8a. CON TRACT OR GRANT NO. 9d. ORIGINATOR'S REPORT NUMBER(S) AF-AFOSR-1144-67 | Fluid Dynamics Laboratory c. 9b. OTHER REPORT NO(S) (Any other numbers that may be aa ldned this report) 10. DISTRIBUTION STATEMENT 11. SUPPLEMENTARY NOTES 12. SPQ^SfI fiILT. -T. Sci nt fi ITeUNhT Other T5 orce 6fl "ce of Scientific ~Tech~., Other I Research (SREP) 1400 Wilson Boulevard I —-Arlington, uirgcinai 2?209 - 13. ABSTRACT The pyrolysis of tetrafluoroethylene was studied in the temperature and pressure range 300 to 4550C and 25 to 760 torr. The rate of reaction was determined to be second order with respect to the tetrafluoroethylene, and three independent numerical values of the second order rate constant were determined by simultaneously measuring the tetrafluoroethylene concentratio the octafluorocyclobutane concentration and the total pressure as a function of reaction time. The temperature dependence of the reaction rate constants are given by vv //,/t10 (.2K)4f( y *X I cc / _ _.^?/^ A/6e2 /jiy _ _-9/ ^-/o e/p { ) f~r -) c~w^"-Sc. N11hL Milk~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I I D NOV6 1 4 73 Unclassified Security Classification

cigClaasifiaed Security Classification 14 T LINK A LN I K S LINK C KEY WORDS - ROLE WT ROLE WT ROL E WT Pyrolysis Tetrafluoroethyl ene Octa fluorocyclobutane Thermal Decomposition of Fluorocarbons Chemical Kinetics Rate Expressions Rate Constants i I I I I iA~~~~~~~~~~~~~~~~~~~JnI I I I I Unclassified *.,.'* (.,-. i (',.,; ",.1. -