J . Phys. Chem. 1994, 98, 5679-5685
5679
FTIR Product Study of the C1-Initiated Oxidation of CH3Cl: Evidence for HCI Elimination from the Chloromethoxy Radical E. W. Kaiser' and T. J. Wallington Ford Motor Company, Research Laboratory, Drop 3083/SRL, Dearborn, Michigan 481 21 - 2053 Received: December 27, 1993; In Final Form: February 7, 1994"
The C1-atom-initiated oxidation of CH3CI has been investigated a t 296 K by UV irradiation of mixtures of CH3C1, Cl2, and 0 2 in Nz. The observed products, measured using Fourier transform infrared (FTIR) spectroscopy, are HC(O)CI, CO, HCI, and CH2C100H. The ratio HC(O)CI/CO ( R ) is directly related to the 0 2 pressure (1-147 Torr). The HCl yield is inversely related to the 0 2 pressure. At 1 Torr 0 2 , the C O yield is >60%, while the HC1 yield is 155%. We interpret these data as evidence for a competition between the decomposition and 0 2 reactions of the CHzClO radical: CH2ClO H C O HCI (ks);CH2CIO + 0 2 H C ( 0 ) C l HOz(k6). The first reaction involves a three-center molecular elimination of HCl, observed previously for 1-chloroethoxy radicals. The ratio R is inversely related to the total pressure (12-700 Torr), indicating that cm3/molecule a t the first reaction is in the falloff regime. The ratio ks/kg was determined to be 5.0 X 700 Torr total pressure. We have also measured the rate constant ratio for reaction of H C O with Cl2 (kll) and 0 2 (kg) to be killkg = 1.15 (f0.12). This ratio agrees with that derived from literature values of these rate constants, verifying that the H C O radical is present during the oxidation process.
-
+
Introduction
CH,CHCIO
+
CH3C0,
-
CH,CO
C H 3 C ( 0 ) 0 2 RO,
+ HCl
CH,CO,
CH,
+ R O + 0,
+ CO,
c1, + hv
-
2c1
0,
CH,CI CH,CIO, CH,CIO,
+ CH,ClO, + CH,CIO, CH,ClO
CH,CI
CH,ClO,
CH,ClO
HC(0)CI
0,
HCl
CH20
CH,CIOH
(3)
+ 0, CH,ClOH + 0,
CH,CIO
HC(0)CI
(2)
HO,
HC1
(4) (5) (6) (7)
(A)
(B) (C)
Because three-center elimination of HCI from chloroalkoxy radicals might be expected to have a substantial activation energy barrier, the occurrence of reaction A is somewhat surprising in light of the fact that the excitation of the CH3CHCIO radical is expected to be relatively low. For this reason, we believe that it is important to study oxidation reactions of other chlorinated hydrocarbons to determine whether three-center HCI elimination from alkoxy radicals is typical. The chloromethoxy radical is the simplest chlorinated alkoxy radical, and a reaction similar to A is certainly possible. Therefore, a study of the oxidation of methyl chlorideat ambient temperature is important. The C1-atom-initiatedoxidation of CH3CI has been studied previously by Sanhueza and Heicklen3 and Niki et aL4 In both studies HC(0)Cl was observed as the major carboncontaining product in a yield of 90-95% with CH2C100H tentatively identified as a minor product in a yield of up to 10%. CO was not reported as a product in either study. The results obtained by Sanhueza and Heicklen3and by Niki et al.4areentirely consistent with the typical C1-initiated oxidation mechanism of alkanes (reactions 1-7):
0
- + + - + - + + + - +
C1+ CH,C1
In product studies using either FTIR' or time-resolved diode laser2 spectroscopic analysis of the C1-atom-initiated oxidation of ethyl chloride at 296 K, high yields of C02 and higher yields of HC1 than can be explained by H atom abstraction from C2H5C1 were observed. To rationalize these observations, a unimolecular decomposition channel was proposed for 1-chloroethoxy radicals, in which intramolecular elimination of HCI occurs, followed by addition of 0 2 to CH3CO with accompanying CO2 formation:
-
+
(1)
Abstract published in Advance ACS Abstracts, May 1, 1994.
0022-365419412098-5679$04.50/0
In the experiments performed by Sanhueza and Heicklen3and by Niki et relatively high concentrations of 02were employed (up to [O,] = 358 Torr). In the present work a study of the C1-initiated oxidation of CH3Cl has been performed using lower 0 2 concentrations of 1-147 Torr. In contrast to the results presented by Sanhueza and Heicklen3 and Niki et al.,4 CO was observed as a significant product. In addition, yields of HCI were observed which were much greater than can be explained by the reaction of C1 with CH3CI. Within the reaction scheme above (reactions 1-7), no reaction path exists for formation of CO as a primary product, and the HCI formed should be equal to the CH3CI consumed plus that from decomposition of CH2CIOH. Our results require reactions 8 and 9 to explain the observed data: CH2C10 (+M) HCO
-
+ 0,
+ HCI (+M) C O + HO,
HCO
-+
(8)
(9)
Experimental Section The Fourier transform infrared (FTIR) spectroscopic technique has been described in detail previously5 and is only discussed briefly here. The FTIR spectrometer (Mattson, Sirius 100) was interfaced to a 140-L Pyrex reactor, which was surrounded by 12 fluorescent blacklamps (GE F15T8-BL). TheCH+2loxidation was initiated by C1 atoms generated in the UV irradiation of mixtures of C12, CH3Cl,O2, and N2 at 296 K. Reactant loss and product formation were monitored by FTIR absorption spectroscopy, using an optical path length of 25 m and a spectral 0 1994 American Chemical Society
Kaiser and Wallington
5680 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
resolution of 0.25 cm-I. Infrared spectra were derived from 32 co-added interferograms. The products from the oxidation were quantified by fitting the observed spectra after irradiation to reference spectra of the pure compounds. With the exception of CH2C100H, reference spectra were acquired by expanding known volumes of a reference material into the reactor. IR spectral features characteristic of an alkyl hydroperoxide wereobserved in thespectra after irradiation. While no standard spectrum is available for CHzClOOH at this time, comparison of the integrated intensity of the observed band a t 795-840 cm-* with the analogous band from CHzFOOH, whose intensity has been calibrated,6 was used to estimate the yield of this species. HCl, CO, HC(O)CI, and CH3Cl were identified and quantified using their characteristicfeatures over the following wavelength ranges: 2580-31 30; 2050-2250; 1750-1825; 13001550 cm-I. Ultrapure oxygen, nitrogen, synthetic air, and chlorine (>99%) were supplied by Matheson Gas Products and used without further purification. Methyl chloride (Matheson Gas Products, >99%) was purified by fractional distillation prior to use. The initial concentrations of the gases used in the FTIR experiments were 75-1 1000mTorr of CHsCI, 29-1900 mTorr of C12,and 1-147Torrof02withN2added tomakeuptotal pressures from 12 to 700 Torr at room temperture (296 K). The accuracy of the HC(0)Cl standard was checked by observing the C O generation during the dark decay of HC(0)Cl in the reactor, which occurred with a first-order rate constant of 0.0055 min-I. C O was the only carbon-containing product observed from the decomposition of HC(0)Cl. The CO concentration after the HC(0)Cl loss was complete equaled the initial HC(0)Cl concentration determined from the reference standard to within the f 5 % experimental uncertainty. HC1 standards were prepared by diluting mixtures containing 4.6 f 0.46% HC1 in N2 to partial pressures of 10-40 mTorr of HC1 at a total pressureof 1 atm of air in the reactor. The accuracy of this calibration was verified by performing a C1-initiated oxidation of C2H6 in the reactor. Previous experiments' have shown that AHCl/AC& = 0.99 0.04 for this reaction. During the present experiments, a C1-initiated oxidation of C2H6 was also performed, and the measured ratio was determined to be AHCI/AC2H6 = 0.95 f 0.05. This result suggests that the HC1 concentrations determined using this standard may be slightly underestimated although they are correct to within the *5% experimental error.
Results and Discussion Figure 1 presents typical IR spectra beforeand after irradiation for two mixtures with different initial 0 2 pressures. Bands characteristic of HC(O)CI, CO, and HC1 are evident after irradiation. H202was also an observed product with a yield of 10-20% of the CH3Cl loss independent of 0 2 . This is consistent with the=1O%H2O2yieldobserved by Nikiet ale4Quantification of the H202 yield is complicated by its rapid decomposition in our reaction chamber to give HzO and was not pursued further. The initial conditions, product yields, and CH3Cl consumption for all experiments are tabulated in Table 1 . All of the data in Table 1 are raw, uncorrected partial pressures. CH2C100H was quantified as described above. In all cases, the yield of CH2ClOOH was 13 f 2% with no systematic trends apparent as initial concentrations of 02, CH3C1, and C12 were varied. This average yield for CH2C100H was used in the calculations of carbon balance and yields of other products under all experimental conditions. The yields of HCI, CO, and HC(0)Cl calculated from the entries in Table 1 are presented in Figure 2 as afunction of the 0 2 pressure in the reactor, with the yield of product P defined as the ratio [AP]/[ACH3Cl]. In most cases, the consumption of CH3CI was too small (
