933
Thermal Decomposition of CF300CF3
Kinetics of the Thermal Decomposition of Bis(trifluoromethy1) Peroxide B. Descamps and W. Forst* Molecular Dynamics Group, Department of Chemistry, Universite Laval, Quebec G 1K 7P4,Canada (Received July 2 1, 1975: Revised Manuscript Received January 23, 1976)
The pyrolysis of CF3OOCF3 (BTMP) was studied in the gas phase from 1 to 200 Torr pressure and between 236 and 272 OC in a nickel reactor by the static method. Decomposition was limited to 5% to avoid complications. The reaction proceeds according to the overall stoichiometry BTMP CF30F COF2, as determined by analysis of the products. The reaction is self-inhibited by COF2 and accelerated by the addition of inert gases Nz, C02, and C2F6. Addition of 1-400 Torr of CzF6 was studied at 232 and 260 “C. A mechanism is proposed that accounts for the initial rate. Its main features are: (1)steady-state concentration of CF30 radicals maintained by the equilibrium BTMP 2CF30; (2) the rate-determining step is the unimolecular decomposition of the CF30 radical which is pressure dependent and reversible: CF30 (+M) COF2 F (+M); this also accounts for the inhibition by COF2. The expression for initial rate is u = h z ( ( k l / h 5 ) [BTMP]]1/2,with 122 pressure dependent. The observed experimental activation energy of 49.7 kcal/mol therefore corresponds to E2 + %(E1- E5).
-
+
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%
Introduction It is now generally agreed that bis(trifluoromethy1) peroxide (CF~OOCFS,BTMP) decomposes thermally according to the overall stoichiometry CF3OOCF3
+
-+
CF30F + COFz
(1)
and that between 200 and 300 OC the reaction reaches equilibrium well before all the peroxide has dec0mposed.l This work concerns the kinetics of reaction 1 (the “forward” reaction) in a thermal system, of which several studies have appeared p r e v i o u ~ l y . ~I t- ~largely confirms the experimental results of the most recent work, that of Kennedy and Levy,4 but examines in more detail the effect of inert gases, and consequently offers a somewhat different interpretation. A preliminary account of our findings has been given some time ag0.j Experimental Section Materials. BTMP and CF30F were both purchased from Peninsular ChemResearch, Gainesville, Fla. As received, BTMP contained a small amount of COFz which was removed by pumping at -130 “C; CF30F contained as impurities CF4, COF2, and BTMP. The CF4 impurity in CF3OF was removed by pumping a t liquid nitrogen temperature (with some loss of CF30F). The COF2 impurity was difficult to remove by pumping, presumably because of formation of an azeotropic mixture6 with CF30F. The COF2 was therefore first converted to CO1 on a column of silica gel, and the CF30F was then distilled off at -196 OC, leaving BTMP and COz behind in the residue. COF2, CF4, and CzF6 were purchased from the Matheson Co. and were used without further purification. Apparatus. Experiments were done in a static system consisting of a 442-m1 cylindrical reactor fabricated from metal and placed inside a large thermostated oven with a heavy aluminum core to increase its thermal inertia. Dead space was 11 ml. Three different reactor materials were used: stainless steel 18/8, monel, and nickel 200 (a commercial grade of pure nickel), all argon-welded. Temperature was measured with a chromel-alumel thermocouple placed between the reactor and oven core. A conventional vacuum
system was used for the preparation of gas mixtures, except that parts exposed to CF30F and COFz a t room temperature were made of copper, Monel, or stainless steel; the rest was made of glass. Analysis. The analytical problem was to determine small amounts of CF30F and COF2 in the presence of a large excess of BTMP. Positive ion mass spectrometry is of little help because of the similarity of the mass ~ p e c t r aInfrared .~ spectroscopy is of limited usefulness since it permits the determination of COF2, but not of CF30F, in the presence of BTMP. Infrared was used occasionally to provide an independent check on COFz. The best method was found to be gas chromatography, with a GOW-MAC gas density detector, nitrogen carrier gas, and a silica gel c o l ~ m (Dan~~~ vidson, 40/50 mesh), 30 cm long and 0.32 cm 0.d. This column separates, in order of elution, air, CF4, CF30F, CzF6, COz, and BTMP within about 15 min at 26 “C. We have confirmed by infrared analysis that this column transforms COF2 quantitatively into COS which is eluted. No reaction products other’than COFz and CF30F were found, except that a t high conversion (-50%) a trace of CF4 appeared. The detector response was calibrated for COF2, CF30F, and BTMP, and all three calibrations were linear. Procedure. BTMP, either pure or in a suitable mixture, was introduced into the reactor by expansion from a reference volume, giving (measured) initial pressure Po. The ensuing pressure increase inside the reactor was followed manometrically by a differential pressure transducer (Pace, Model KP 15). All runs used for the determination of reaction rates were allowed to proceed to no more than 5% decomposition of BTMP to avoid complications due to the back reaction. A small initial portion of each pressure-time recording, representing the first 1%decomposition, was unusable, generally because of a pressure disturbance in the system following admission of the gases into the reactor. Only the portion of the recording between 1 and 5% decomposition was used for rate measurements. The pressure in the reactor a t 1%decomposition is called Pi, and the corresponding rate is called the “initial” rate uin; this was deemed preferable to obtaining the actual initial rate by extrapolation back to time zero. Reaction was stopped by expanding the contents of the reactor into a liquid nitrogen The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
B. Descamps and W. Forst
934
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trap; after a suitable warm-up period, an aliquot of the mixture was injected into the gas chromatograph for analysis, or, occasionally, into an infrared cell with.NaC1 windows for analysis by infrared. Preliminary Results No consistent results could be obtained in the stainless steel reactor, presumably due to surface effects, and it was therefore discarded. The monel reactor was first cleaned and polished with nitric acid and then conditioned with fluorine gaslo a t about 600 Torr for 17 days at 380 “C. Immediately following such conditioning, fairly good reproducible results on the pyrolysis of BTMP were obtained, but after a few runs reproducibility deteriorated and analysis showed that the reaction yielded an appreciable excess of COF2 relative to CF30F. Since the stoichiometry of reaction 1 requires the two products to be formed in equal amounts, the results in the monel reactor indicate substantial ( D(CF30-OCF3), so that the pyrolysis must start by 0-0 rupture:
D(H-CH20H).
H 2 9 8 K,kcal/mol
F COF2 CF4 CF30F CF3 CF300CF3 CF30
= In C1+ m In [BTMPIi, o.70[c2F6])o.33 (6)
With constant [C2F6] and variable [BTMPIi, this is the equation of a straight line of slope m. Experimental results treated in accordance with eq 6 yield15 m = 0.55 f 0.09, again over the range 232-260 “C. The total order of vin is m n = (0.33 f 0.11) (0.55 f 0.09), which has been determined previously as CY = 0.93 f 0.07; thus the partial orders m and n determined from inert gas experiments are consistent, within experimental error, with the total order determined in the absence of foreign inert gas. Initial rates give only information about the numerator of eq 2. It now remains to be determined if the denominator of eq 2 is also dependent on the concentration of inert gas. Experimental data obtained between 232 and 260 OC with X = C2F6 were used to calculate C1 and C2 from eq 2, and it turns that C1 is a constant independent of [C2F6], as we already know, but c2 increases with [C2F6], although it is independent of [BTMP]. This suggests that C2 should be written
N
TABLE 111: Standard Enthalpies of Formation
Uin
([BTMPIi,
Assuming that D(H-CH200CH3)
5 ---+
AH = -45 kcal/mol
CF3OOCF3
6 +F+ CF30F
AH = -45 kcal/mol
which are both exothermic. The disproportionation reaction CF30
+ OCF3
-
CF30F
+ COFl
AH FZ -22 kcal/mol
appears to be very slow compared with reaction 5, even with photolytically generated2*-26 CF30, although a similar reaction is one of the major reactions producing CH3OH in the pyrolysis of dimethyl peroxide.23 The reason for the dissimilarity between the perfluorinated and perhydrogenated systems seems to be again the weak 0-F bond in CF30F. Another possible chain-terminating step F+F+Fz must be eliminated because molecular fluorine is not a product of the pyrolysis. In fact addition of a small pressure of molecular fluorine is without any marked effect on
937
Thermal Decomposition of CF3OOCF3 the pyrolysis (see Results) and this is consistent with the mechanism above which involves fluorine atoms but not F2. It can be deduced from the data of Lloydz7 that the equilibrium constant for the thermal dissociation F2 + 2F is about 10-14 mol/cm3 at 250 “C, the temperature of our experiments, so that the thermal dissociation of fluorine could provide significant amounts of fluorine atoms only a t substantially higher fluorine pressures than those we have used. Rough calculations in support of this are given below in connection with eq 17. Inhibition. The mechanism proposed so far does not account for the observed inhibition by COF2. The most likely candidate for inhibition is the reverse of reaction 2 F
+ COF2
3
AH = -23 kcal/mol
CF30
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+
-
which is an expression fairly close to the empirical eq 7, except for the inert gas effect which has not yet been considered. Equation 8 is also strongly reminiscent of the wellknown rate expression for formation of HBr in the H2 + Br2 reaction,14bwith which mechanism I shares the principal characteristics (corresponding features of the H2 + Brz reaction in parentheses): the equilibrium BTMP F= 2CF30 (Br2 e 2 Br) furnishes the steady-state concentration of CF30 (Br) radicals, and the rate-determining step is reversible, CF30 == COF2 F (H2 Br + HBr H), which leads to a very efficient inhibition of the reaction by COFz (HBr). Thus the numerator of the rate expression (8) contains the rate constant for the rate-determining step, k z , multiplied by the square root of the equilibrium constant for the chain-initiating reaction, (hJk5)1/2;the denominator contains the ratio of rate constants for inhibition, k3, and chain propagation, k4. Insofar as the pressure dependence of inert gases is concerned, reactions 1,5, and 4 are unlikely to be in their pressure-dependent region at the pressures used in this work because they involve BTMP, a large molecule. Reaction 2 is the unimolecular decomposition of a small radical originally formed in the strongly endothermic reaction 1. As formed in reaction 1, the radical CF3O is almost certainly “cold”, i.e., without sufficient internal energy to decompose further, unless it acquires additional energy by collision. Examples of such behavior are n ~ m e r o u s . ~ ~ - 3 ~ Assuming a strong-collision (Lindemann) mechanism for the collisional activation of CF30, reactions 2 and 3 become
+
M
+
a
+ CF30
b C
d
+
CF30* + M
CF30* eCOFz
(9)
-
is the pseudounimolecular rate constant in the usual sense for the forward dissociation CF30 COF2 F, and the replacement of k3 by kbi, where
+
(10)
-
can be looked upon as the pseudobimolecular rate constant for the reverse reaction COF2 F COF3. Equation 8 thus becomes
+
k uni( k l/h 5)l/’ [BTMP]’/’ 1 kbi[COF2]/k4[BTMP] Comparing constants between the numerators of eq 7 and 11we find V =
a reaction that has been invoked p r e v i o u ~ l yin~the ~ ~ sys~~ tem COF2 CFsOF and which Aymonino3*has observed in the photolysis of F2 in the presence of COF2. (However recently Schumacher et al.31 have speculated that in a photochemical system the reaction might be F COFz CF20F.) Mechanism I. Reactions 1-6, but excluding reaction 6, will be called Mechanism I. This mechanism can be solved exactly in the usual steady-state approximation, yielding
+
into mechanism I results in the replacement of kz by kuni, where
+F
where the asterisk represents internal excitation energy sufficient for decomposition. Introducing this refinement
+
C 1[M]0.33= kuni(k ~ / k s ) l / ~
(12)
Comparing denominators between eq 7 and 11 we find Cz’[M] = k&d. The pseudoconstant kuni depends on pressure roughly as [Mlq, where 0 I q 5 1, so that the 0.33 order with respect to [MI = C2F6 found experimentally is entirely what one would expect for a unimolecular decomposition in the pressure-dependent region. Due to the size of the BTMP molecule, we can expect it to be an efficient energy transfer agent, so that when [MI = BTMP (i.e., in the absence of inert gas), it would very likely take only moderately high (-1 atm) pressure for kuni to become of zero order with respect to [MI, i.e., vin to become proportional to [BTMP]1/2. This is precisely what has been found by Kennedy. and Levy4 at BTMP pressures above about 200 Torr. The size of BTMP will also cause the first-order region of kuni (i.e., the 3/2-order region of uin) to be located at pressures (-< 0.1 Torr) well below the lowest BTMP pressure used by us or by Kennedy and Levy.4 Mechanism I thus appears to account quite satisfactorily for the main features of the pyrolysis, even though the chain termination via reaction 6 has not been considered. This reaction cannot be dismissed because it was foundz9to be the rateidetermining step in the inverse reaction CF30F COFz BTMP, and therefore must intervene in the forward reaction as well. The complete mechanism comprising all the reactions 1-6 cannot be solved in closed form, but we can solve the mechanism consisting of reactions 1-4 and 6, but excluding reaction 5. It is sufficient to examine only the expression for vin. Application of the steady-state hypothesis leads to
+
-
A t pressures below -100 Torr, 122 and k6 will be both pressure dependent, but since vi, depends only on the ratio k2/k6, vin will be substantially independent of [MI, contrary to experiment. Hence this mechanism cannot be the principal mechanism. This is not to suggest that reaction 6 does not occur, but merely that under our experimental conditions (low conversion) reaction 6 is relatively unimportant. At low conversion, the principal constituent of the reaction mixture is BTMP, and under such conditions most fluorine atoms would be expected to disappear via reaction 4 rather than reaction 6. Evaluation of Constants. We now return to mechanism I The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
B. Descamps and W. Forst
938
The new constants c1 and c2 are different from the old set C1 and C2 because they contain the inert gas effect. Since kuni, and therefore c1, are pressure dependent, it is convenient to extrapolate c1 to infinite pressure. Extrapolation of kuni yields k,, and therefore
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-0
-
_---
800-
400-
Ij :
400-
clm= (1.2 f O.l)lO1s ex~[-(49700 f 3 700)/RT] Torr1IZs-l
(15)
The activation energy corresponding to cl, is thus E, = 49.7 kcal/mol; recall this refers to the “clean” nickel reactor. I t may be of interest that in the monel reactor, E, -40 kcal/mol, a substantially lower value, indicating apprecia-
-
one would expect for the recombination of two radicals, eq 16 yields E , 26.6 kcal/mol. This appears to be in line with the estimate made by B e n ~ o nthat ~ ~ a unimolecular chain-propagating step cannot have an activation energy larger than -64 cal/mol deg, which works out to 35.5 kcall mol at 250 ‘C. We have found previously that AH 23 kcal/mol for reaction 2; when combined with the above value of E,, this yields E d 3 kcal/mol for the activation energy of the reverse reaction F COFz CF30, which seems entirely plausible. Extrapolation of c2 vs. l/Po gives Czm
1200
_--
ius plot for cl, yields the result
-
