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J. Phys. Chem. 1992, 96, 295-297
Decomposition of 1,3,5-Trioxane at 700-800 K S. Hochgreb* and F. L.Dryer Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08540 (Received: July 16, 1991)
The unimolecular decomposition of 1,3,5-trioxaneinto formaldehyde is studied experimentally in the temperature range 700-800 K in an atmospheric pressure flow reactor. The measured rate constant for the decomposition of trioxane in this temperature exp(-(47.5 2.4) kcal/mol/RT), is compared to previous evaluations at different pressures and range, k = 10'5,28*0.06 temperatures, using the RRKM method to verify the falloff behavior of the decomposition process. Two different sets of parameters were used in the calculations,one purely theoretical, and one estimated from a previous set of shock tube experiments. The comparison between model and experiment is shown to be satisfactory, and the present results are estimated to be close to the high-pressure limit. The overall results show that the rate of decomposition of trioxane is much faster than reported rates of decomposition and oxidation of formaldehyde under a wide range of conditions, so that trioxane can be effectively used as a source of monomeric formaldehyde for kinetic studies.
*
Introduction A variety of reasons have led to an increased interest in the study of trioxane chemistry in recent years. The primary interest in this study, also shared by other kineticists, was the use of trioxane as a source of pure formaldehyde in kinetic studies, a method which had not been used thus far in flow reactors. Recent investigations related to research on monopropellant chemistry, including CHzO and nitrogenated species, have also made use of this trimer as a source of CH20 in shock tubes.]** As a cyclic heteroatom compound, trioxane is also an interesting model for reactions of other similar molecules, such as the energetic materials RDX and HMX.3*4 Finally, the concerted nature of the decomposition of the ring into three formaldehyde molecules has attracted attention to this compound from the point of view of verifying theoretical model^.^^^ The decomposition of trioxane proceeds by the concerted rupture of three C-O bonds in the ring to form CHzO:
Measurements of this rate constant had been published prior to this study, both in the range 545-620 K7v8based on measurements of pressure rise in pure trioxane mixtures. A very recent shock tube study by Irdam and Kiefer5 spanning the range 9OQ-3000 K and pressures from 150 to 300 Torr was published after the present experiments had been concluded. Finally, Aldridge and co-workers4 have measured a rate constant for trioxane in a static reactor over the temperature range of 523-603 K at 800 Torr. The purpose of the present trioxane decomposition experiments was twofold (a) to determine the rate of decompition of trioxane a t the temperatures used in the formaldehyde pyrolysis and oxidation experiments in the range 945-1 100 K, to ensure that all trioxane is converted into formaldehyde prior to appreciable formaldehyde reaction; and (b) to verify that the only product of trioxane decomposition is formaldehyde. The results in the present paper cover the range 700-800 K at 1 atm and should be considered complementary to the studies ( I ) Choudhury, T. K.; Lin, M. C. Combust. Sci. Technol. 1990,64, 19. (2) Lin, C. Y.;Wang, H. T.; Lin, M. C.; Melius, C. F. Int. J. Chem. Kinet. 1990, 22, 455. (3) Zabarnick, S.;Fleming, J. W.; Lin, M. C . Int. J . Chem. Kinet. 1988, 20, 117. (4) Aldridge, H. K.; Lin, X.;Lin, M. C.; Melius, C. F. Unimolecular Decomposition of 1,3,5-Trioxane. Comparison of Theory and Experiment. Proceedings of the Eastern States Section of the Combustion Institute, Orlando, FL, Combustion Institute: Pittsburgh, PA, 1990. (5) Irdam, E. A.; Kiefer, J. H. Chem. Phys. Lett. 1990, 166, 491. (6) Melius, C. F. ONR Workshop on Energetic Material Initiation Fundamentals; CPIA Pub. 516 CPIA: Laurel, MD, 1988; Vol. 1, p 226. (7) Burnett, R. Le G.;Bell, R. P. Trans. Faraday SOC.1938, 34, 420. (8) Hogg, W.; McKinnon, D. M.; Trotman-Dickenson, A. F.; Verbeke, G. J. 0.J . Chem. SOC.1961, 1403.
TABLE I: Rate Constants for Unimolecular Decomposition of Trioxane E, log A' kcal/mol T, K P,Torr diluent ref 14.80 14.95 15.86 15.78 f 0.19 15.28 f 0.06
47.4 47.4 50.0 50.9 f 0.5 47.5 f 2.4
545-620 545-620 900-1270 523-603 700-800
0.1-1 5 0.1-100 150-350 800 760
trioxane 7' trioxane 8' Kr Ar N2
5 4
this workb ' A in s-I. bLeast-squaresanalysis of the data. ' A and E values are for the highest pressures. in the literature mentioned above. This work also determined the products of the decomposition of trioxane (by gas chromatography) to be exclusively formaldehyde. This result has been confmed by Aldridge et a1.4 using Fourier transform infrared (FTIR) analysis. Calculations based on the RRKM method are also presented here, based on structural and energetic parameters suggested by Irdam and Kiefer5 and Meliuse6 The results are discussed in comparison to all available experimental data.
Experimental Section The experiments were performed in the atmospheric pressure turbulent flow reactor facility (APFR) at Princeton, which has been described in detail in previous papers?J0 A diluted mixture of nitrogen containing small concentrations of reactants flows through a 1-m-long tube at a controlled initial temperature. High Reynolds numbers and small longitudinal temperature and species gradients render diffusion effects unimportant under the present experimental conditions. Samples of the reactant mixture are extracted along the reactor by an axial probe and stored for later analysis. The temperature is measured by a silica coated, Pt 30% Rh/Pt 6% thermocouple located at the tip of the sampling probe. Trioxane (melting point 62 OC, boiling point 114 "C) was melted and vaporized continuously into the reactor with the use of a heated cylinder piston assembly. The very large activation energy of the decomposition of the trimer limited the range of temperatures that could be studied in the flow reactor-typical reaction times (for 80% of reaction) varied from 9 ms at 800 K to over 450 ms at 700 K. Formaldehyde and trioxane concentrations were determined by gas chromatography. Calibrated mixtures of trioxane in nitrogen were made using the partial pressure of trioxane at ambient temperature (about 7 Torr). Formaldehyde calibration factors used were obtained from a series of complete oxidation experi(9) Yetter, R. A.; Dryer, F. L.; Rabitz, H. A Comprehensive Mechanism for Carbon Dioxide, Hydrogen and Oxygen Kinetics. Comb. Sci. Technol., in press. (IO) Norton, T. S.;Dryer, F. L. Int. J . Chem. Kinet. 1990, 22, 219.
0022-365419212096-295$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
296
Hochgreb and Dryer
TABLE 11: Parameters for RRKM Calculations ~~~
parameter frequencies, cm-' molecule
transition state
moments of inerita, amu molecule transition state energy barrier, kcal/mol
A2
I K"
MELb
296, 296, 524, 524, 945 945, 1070, 1070, 1178, 1178 1305, 1305, 1410, 1410, 1481 1481, 2850, 2850, 3025, 3025 1122, 1242, 1383, 466, 752 978, 1235, 1495, 2850, 3025 100, 100, 200,200,945 945, 1400, 1400, 1178, 1178 1200, 1200, 1200, 1200, 1481 1481, 2850, 2850, 3025, 3025 1000, 1242, 1100, 200, 700 1200, 1495, 2850, 3025
263, 263, 417, 518, 518 755, 958, 958, 971, 1048 1075, 1075, 1208, 1208, 1229 1236, 1310, 1308, 1381, 1438 1438, 1498, 1498, 1518, 2820 2820, 2841, 2997, 2997, 3002 134, 134, 247, 248,273 434, 434, 528, 894, 914 914, 1108, 1214, 1214, 1222 1248, 1248, 1379, 1415, 1415 1561, 1597, 1597, 2879, 2879 2884, 3001, 3001, 3004
96.4, 96.4, 173.0 125.3, 120.5, 249.2 47.0
93.4, 93.4, 169.0 114.8, 114.8, 210.1 49.1
Collision Parametersc a,
A
elk, K 500 93.3 190 71.1
6.093 3.542 3.610 3.798
C3H603
Ar Kr N2
500-700"
(cm-')
(AE)down
a Frequencies in IK for the molecule are from Pickett and Strauss,14moments of inertia from Melia et al.I5 Transition state properties were chosen5 to generate high-pressure rate parameters most consistent with the data. bAll parameters in MEL have been calculated by BAC-MP4.6 CFrom Tsang and Hampsont6 and Irdam and Kiefer.5 dSee text.
.-
1
-1 1
1-
10
-
----_ k ikl (MEL)
-
8-
-2
6-
700 K 706 0 769 770
-3
-4
-
1-
.........
kin( (IK)
-t
wrhige et a1 (800torr) Burnett and Bell (15 torr) ~oop et a1 (100 torr)
mi6 work (780torr)
A 800 -5
-8
] \' 11. I
100
-6
0
50
I 100
.
I 150
.
I 200
.
0.5
time (ms)
Figure 1. Experimental results for the decomposition of trioxane in the range 700-800 K. Inset shows the calculated values for the decomposition rates k = -d In (C3H,03)/dt and the Arrhenius fit in this temexp(-(47.5 & 2.4) kcal/mol/RT). The perature range, k = 10'5~28*o~" experiment for T = 700 K was excluded from the fit in view of the short observation time.
ments." Typical exponential decay was observed for all temperatures (Figure 1) at trioxane concentrations ranging from 0.17 to 0.29%. The calculated rate constant is depicted in the inset, k (s-l) = 1015.28*0." exp(-(47.5 f 2.4) kcal/mol/RT).
Results and Discussion The rate of unimolecular decomposition of trioxane should increase with total concentration up to a high-pressure limit. Hogg et a1.8 determined that trioxane decomposition is strongly dependent on pressure below 100 Torr with trioxane as bath gas. Irdam and Kiefer5showed through experiment and calculations that the falloff behavior extends to pressures up to 360 Torr at temperatures above 900 K. Finally, calculations of Aldridge et al.4 based on the theoretical model of Melius6 estimated that pressure dependence starts below 500 Torr in the temperature range 523-603 K. The experimental and theoretical evidence indicates that, under our present conditions at atmospheric pressure and moderate temperatures, the exponential decay constant ( 1 1) Hochgreb, S.; Dryer, F. L. Symp. (Int.) Combust. [Proc.] 23 1990,
171.
1 .o
1.5
2.0
2.5
250
1ooo/T Figure 2. Experimental rate constants for the decomposition of trioxane (symbols) and calculated high-pressure limits for the MEL (short dashes, log A, = 15.83, E, = 52.43) and IK (large dashes, log A, = 15.83, E, = 49.7) sets of parameters. Experimental data are shown only for the
highest pressure in each set.
measured is close to the high-pressure rate constant k,. Table I summarizes the results of all rate constants measured by a variety of experimental methods under a wide range of conditions. In order to verify the range of falloff behavior of the decomposition of trioxane, and to confirm the assumption that the present experiments are close to the high-pressure limit, detailed rate calculations were made using the RRKM method. The UNIMOL program, recently developed by Gilbert, Jordan, and Smith,12 which calculates specific rate constants as a function of energy and solves the master equation for unimolecular decomposition, (12) Gilbert, R. G.; Jordan, M. J. T.; Smith, S . C. UNIMOL-A Program for the Calculation of Rate Coefficients for Unimolecular and Recombination Reactions. Dept. of Theoretical Chemistry: Sydney University, Australia, 1990. (13) O!ef, I.; Tardy, D. C. Chem. Reu. 1990, 90, 1407. (14) Plckett, P. J.; Straws, H. L. J . Chem. Phys. 1970, 53, 376. (15) Melia, T. P.; Bailey, D.; Tyson, A. J . Appl. Chem. 1967, 17, 15. (16) Tsang, W.; Hampson, R. F. J. Phys. Chem. ReJ Data 1986,15,1087. J" 7) The average energy transferred per collision is defined as ( MLhn, = ;(E'- E)P(E',E) dE'/StP(E',E) dE'for E'< E, and (ME),,, = S,(E - E)P(E',E) dE'. In the program, the exponential model for the transition probabilities was used, P(E,E') exp(E'- E)/cr, so that a = (M)&.(see Gilbert et a1.I2).
-
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 291
Decomposition of 1,3,5-Trioxane
1.01
______----
e
I O torr
t
0.8-
0
-
0
0.4-
0.1
5 ,"
.
8
9
10
11
12
10000/T
Figure 3. Experimental (symbols) and calculated values for the decomposition rate of trioxane at 900-1200 K and 150 Torr (open symbols, dashed lines) and 350 Torr (filled symbols, solid lines), using the IK and MEL sets of parameters.
was used for this purpose. Two sets of parameters for the structure and energetics of trioxane, from Irdam and Kiefer5 (IK) and Melius6 (MEL) (Table 11), were used in the calculations. The high-pressure rate constants obtained with both sets of parameters are shown in Figure 2 along with reported experimental results (notice that the rates obtained by Aldridge et al. at 800 Torr are lower than those reported by Hogg et al. for pressures under 100 Torr). The high-pressure rates obtained using the MEL parameters are lower than the experimental values, which are in better agreement with the high-pressure rate values obtained using the IK parameters. However, one must keep in mind that the values in MEL are purely theoretical, whereas the parameters in IK have been adjusted by them5 in order to obtain good agreement with the experimental data. Comparisons of the falloff characteristics for both sets of parameters are shown for the experimental conditions of Irdam and Kiefer and of Hogg et al. Good agreement between model and experiment is achieved for the IK set using ( AEdown) = 500 cm-' (
