The nature of the transitory product in the gas-phase ozonolysis of ethene

~: , ; 4 ; -'x " 7 L , ~+ ~ . ~

17 November 1995

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 246 (1995) 150-156

The nature of the transitory product in the gas-phase ozonolysis of ethene Peter Neeb, Osamu Horie, Geert K. Moortgat Max-Planck-lnstitutfiir Chemie, Division of Atmospheric Chemistry, Postfach 3060, D-55020Mainz, Germany

Received 18 July 1995; in final form 25 August 1995

Abstract

One of the reactants for the formation of previously identified transitory product in the gas-phase ozonolysis of C2H 4 was shown to be HCOOH. The most probable structure of this compound is HOO-CH2-O-CHO. Its concentration increased with the addition of HCOOH but decreased with the addition of HCHO which had previously been assumed as one of the reactants. This compound slowly decomposed to formic acid anhydride and water.

decomposition , HCO + OH,

I. Introduction

CH2OO *

Despite numerous studies of the gas-phase ozonolysis of alkenes [1], the reactions of the stabilized Criegee intermediates have received very little attention. The only stabilized Criegee intermediate whose reactions have been studied experimentally is CHzOO, formed in the ozonolysis of ethene and terminal alkenes [2-6]. The initially formed excited Criegee intermediate CH2OO * partly decomposes to yield CO, CO 2, H 2, H, and OH, and partly is collisionally deactivated to form the stabilized intermediate CH2OO,

C H 2 0 O , stabilization)CH2OO.

0 3 "-I-C2H 4 ~ (primary ozonide)

CH2OO* + HCHO,

(1)

CH2OO* decomposition) CO2 + H2,

(2a)

decomposition CO2 + 2H,

(2b)

CH2OO* decomposition) CO qt_ H 2 0 ,

(2c)

cn2oo*

(2e)

The reactions of CH2OO intermediate which have been studied are mostly with carbonyl compounds (HCHO and CH3CHO) [2,3,6], CO [2], or SO 2 [5]. Notably, the reaction with HCHO (3) has been accepted to yield a transitory product, 'compound X', tentatively assigned as hydroxymethyl formate, H O C H z - O - C H O [2-4,6]. Compound X is assumed to dissociate to formic acid anhydride, (CHO)20 (hereafter abbreviated to FAN), and H2, via (4) [2], or to react with 02 to form FAN and H202, via (5) [4], CH2OO + HCHO --~ compound X ( H O C H 2 - O - C H O ) ,

(3)

H O C H 2 - O - C H O ~ (CHO)20 + H 2 ,

0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)01073-4

(2d)

A H = 77kJmo1-1 ,

(4)

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

H O C H e - O - C H O + O2(wall ) -~ (CHO)20 + H202, A H = - 5 9 k J m o l - l .

(5) More recently, the CH2OO intermediate has been considered to be involved in the formation of formic acid [7], h y d r o x y m e t h y l hydroperoxide, HOCH2OOH [7,8], and H 2 0 2 [9] via reaction with water vapour, giving CH2OO increased importance in atmospheric chemistry. There are however some experimental and thermochemical considerations which suggest that compound X is not H O C H 2 - O - C H O as previously assumed. (1) If reaction (3) takes place, addition of HCHO would increase the initial rate of compound X formation [2,4]. This is not readily apparent from the results of Su et al. [2], where they added 10.2 ppmv HCHO to the reaction mixture containing O3//C2H4 = 10.4 p p m v / 9 . 2 ppmv (Fig. 8(B) of their paper). Also, the observed increase in the yield of compound X in the matrix-isolation study of Horie and Moortgat [6], where reactant concentrations of about a factor of 100 or so higher than those used by Su et al. [2] or Niki et al. [3] were employed, was only about 30%, and could have been caused by secondary reactions. (2) Occurrence of reaction (3) assumes that the HCHO yield relative to the C2H 4 conversion, ¥ (HCHO), is lower than unity. (Unless otherwise noted, the product yields and the 0 3 consumption, AO 3, are expressed relative to the C2H 4 conversion, with ¥(product) notation.) Although the data of Su et al. [2] and of Niki et al. [3], ¥(HCHO) = 0.56 and 0.77, respectively, seem to support the occurrence of reaction (3), we found ¥(HCHO) = 1.0 in our recent preliminary study with initial concentrations of = 4.5 ppmv of C2H 4 and = 2 ppmv of 03 [7]. (3) Reaction (4) is calculated to be 77 kJ mol-1 endothermic, based on AHf° = - 5 4 0 kJ mo1-1 for compound X, estimated by the group-additivity method [10]. Even if reaction (4) may involve heterogeneous processes [3,4], it is unlikely that reaction (4) proceeds at measurable speed at room temperature. Similarly, although reaction (5) is calculated to be exothermic, the formation of H202 as a

151

major product from this reaction appears unlikely [7,9]. In view of these considerations, we have studied the effect of the addition of HCHO and HCOOH to the reaction system C 2 H 4 - O 3 in the low parts-permillion concentration ranges, and obtained evidence indicating that HCOOH is one of the reactants for compound X formation.

2. Experimental Ozonolysis was carried out in an evacuable, 570 1 spherical glass reactor in 7 3 0 _ 3 Torr (1 Torr = 133.3224 Pa) synthetic air. The reaction temperature was kept constant at 295 + 2 K by the laboratory air conditioner. Due to the large size (--- 1 m diameter) and complex geometry, no attempts were made to vary the temperature. The apparatus and procedure have been briefly described previously [7,11]. The initial reactant concentrations were in the range of 1 to 16 ppmv (1 ppmv = 2.39 x 1013 molecule cm -3 at the above temperature and pressure) for C 2 H 4 and 1 to 10 ppmv for 0 3. The majority of the experiments was performed with [C2Ha] 0 = 4 ppmv and [03] 0 = 2 ppmv. The concentrations of the added compounds were in the range of 5 - 3 0 ppmv for HCHO and 1-10 ppmv for HCOOH. Synthetic air was prepared by filling the reactor with CO-free N 2 and 02 to a pressure of 700-715 Torr. Ozone was generated either externally in a quartz-tube spiral surrounding a Hg Pen-ray lamp during the filling of 02, or internally with another Hg Pen-ray lamp mounted at the bottom plate of the reactor. To this mixture of air and 03, pre-mixed C 2 H a / N 2 (100 ppmv C2H 4) from a pressurized cylinder was directly added. Also in several cases a dilute C 2Ha//N2 mixture prepared in a transfer cylinder of 1.38 1 was flushed into the reactor by N 2 carrier, until the final pressure of = 730 Torr was reached. During the filling, two teflon stirrers were activated to ensure thorough mixing of the reactants. The addition of HCHO or HCOOH was done in a similar way, immediately before the C2H 4 injection. The reactants and the products were analyzed by long-path (43.2 m path-length) FTIR (Bruker IFS28) spectroscopy. Either 32 or 128 scans at a resolution of 0.5 cm -l were co-added. Approximate relative

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

152

errors in the concentrations are as follows: 03, C 2 H 4 , CO and HCHO + 5%, and CO 2, FAN and HCOOH __+10%. The calibration of HCOOH was performed using a pyrolytic method [12] to by-pass difficulties due to HCOOH dimer formation. Formaldehyde monomer was prepared by heating paraformaldehyde to = 120°C. Two different calibrations were made for HCHO: a pyrolytic method [13] and a standard volumetric method using paraformaldehyde as the HCHO source. Formic anhydride was prepared according to the procedure of Muramatsu et al. [14]. The reference spectrum of compound X was obtained with a computational stripping procedure, and its concentration was estimated by the band strength of the carbonyl absorption. The error limits for the concentration were estimated at + 30%. All chemicals and gases were of highest purity available commercially and used without further purification.

Reaction time/103 I

(a) 1.5

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1.0

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6

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3.1. The effect of HCHO addition

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3. R e s u l t s a n d d i s c u s s i o n

~

(b)

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1.5

Fig. 1. The 03 consumption and the product yields relative to the C2H 4 conversion. [C2H4] 0 = 4.01 ppmv, [03] 0 = 2.13 ppmv. (a) AO 3 (=[O3]0--[O3](t)), HCHO, CO and CO 2. (b) HCOOH, FAN, compound X and Total-X (FAN + compound X). Note the difference in the y-axis scale in parts (a) and (b).

the mo 3 obtained by a linear fit for the C 2 H 4 conversion ~< 1 ppmv are summarized in Table 1, together with the results of other experimental runs.

Table 1 Ozone conversion and product yields relative to the ethene consumption 4.01 2.13

[03] o (ppmv)

added gas concentration (ppmv)

03 consumption HCHO CO CO 2 HCOOH Total-X b a

Added gas.

0.94 0.92 0.29 0.20 0.04 0.18

b Total-X = compound X + FAN.

I

Ethene conversion/ppmv

In the absence of added compounds, HCHO, CO, CO 2, HCOOH, FAN and compound X were identified as the products. The product yields and the 03 consumption, AO 3, are plotted against the c 2 n 4 conversion in Fig. 1. The approximate reaction time is marked at the top of the figure. The formation of HCHO, CO and CO 2 increased linearly with the C 2 H 4 conversion (Fig. la). Their relative yields and

I f 2 H 4 ]0 ( p p m v )

16

4.09 2.50

4.04 2.30

4.09 2.09

4.09 1.93

HCHO 15

HCHO 30

HCOOH 2.0

HCOOH 10.3

1.05 a 0.57 0.18 0.36 0.16

1.10 a 0.64 0.17 0.62 0.13

0.96 0.96 0.23 0.19 - 0.67 0.54

0.98 1.01 0.24 0.23 - 0.85 0.52

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

These results are in good agreement with the results of our recent preliminary study [7]. It should be noted that the HCHO yield is nearly unity. Results for HCOOH, FAN and compound X are illustrated in Fig. lb. FFIR spectra of compound X and FAN are illustrated in Figs. 2a and 2b, respectively. The compound X spectrum was obtained through computational spectral stripping from an experimental run with the highest initial concentrations employed, [ C 2 H 4 ] 0 = 16 ppmv and [03] 0 --- 9.5 ppmv. This spectrum is identical, regarding the relative intensities of typical absorptions and their positions, to the spectrum obtained by Niki et al. [3]. The concentration of compound X was observed to increase linearly with the C2H 4 conversion at first and decrease at higher conversions a n d / o r longer reaction times. The yield profile of FAN on the other hand showed clearly secondary product nature. The sum of the yields of compound X and FAN was shown to increase linearly with the C2H + conversion, confirming that FAN is formed from compound X. The sum of compound X and FAN, defined as the Total-X = (compound X + FAN), can therefore be taken as the total yield of compound X. Approximate yield of the Total-X is also listed in Table 1. The following discussion about the effect of HCHO and HCOOH addition on the formation of compound X

0.02 I

CompoundX

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[-.

4

i

2000

i

i

+

i

i

i

i

1500

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Wave number/ cm"1 Fig. 2. The FFIR spectra. Top: compound X. The concentration is = 0.1 ppmv. The spectrum was obtained from the product spectrum of the ozonolysis run at [C2H4] 0 = 16 ppmv and [O3] 0 = 9.5 ppmv, by subtracting spectral contributions of C2H4, 0 3, HCHO, FAN and HCOOH. Bottom: formic acid anhydride (FAN). The concentration is 0.1 ppmv.

153

(a) HCOOH

0.8

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[HCHO]0/ ppmv

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Ethene Conversion/ppmv Fig. 3. The effect of the HCHO addition. [C2H4]0 = 4.0-4.1 ppmv, [03]0 = 2.3-2.5 ppmv. (a) HCOOH yield relative to the C2H4 conversion. (b) Total-X (= compound X+ FAN) relative to the C2H4 conversion.Note the differencein the y-axis scale in parts (a) and (b).

will be based on the formation of the Total-X as a single compound. The carbon balance of the results shown in Fig. 1 was calculated to be 90 + 20% based on the above calibration for compound X, assuming that compound X contains two carbon atoms. The effects of HCHO addition are illustrated in Figs. 3a and 3b for HCOOH and Total-X, respectively. The results of the linear fit for AO3, ¥(CO), ¥(CO 2) and ¥(HCOOH) are summarized in Table 1. Though not shown, the behaviour of compound X and FAN was similar to those observed in the absence of the added HCHO, shown in Fig. lb. The significant increase in HCOOH formation with the addition of HCHO (Fig. 3a) is presumably due to the HO2-HCHO reaction cycle, which is always a source of secondary HCOOH [6]. Also, the large increases in ¥(CO) with the added HCHO (Table 1) are probably due to the reaction of HCHO with OH

154

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

radical, formed as a product in the C2H 4 ozonolysis [1]. The extent of the OH radical formation in the reaction was not studied quantitatively. However, linear increases in [HCHO] and in [AO3] with respect to the C2H 4 conversion (Fig. la), and the results of AO 3 and Y(HCHO) shown in Table 1 indicate that ¥ ( O H ) may be at most of the same order of the error limits for the concentration of HCHO or 03, about 5%. A possible primary channel yielding OH radicals is reaction (2d). If formed, OH reacts mainly with c 2 n 4 at early stages of reaction, and later also with HCHO formed as a product. Another probable OH source is the reaction of 03 with HO e radicals which are generated in a subsequent reaction following (2b),

I (a) [HCOOH]0 = 2 ppmv Total-X

vvvv~

©• FANC°mpd'X

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o~ o o,?o o°°°°°°°°°a'°c~°°°° ,

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(b) [HCOOH]0 = I0 ppmv • ~ HCOOH L x Total-X i I • Compd.X o FAN 1.0 ~

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(6) 0

Formation of the secondary OH radical could be enhanced with the added HCHO, which reacts with OH to produce CO and HO2, regenerating OH on the one hand and forming eventually H C O O H on the other hand [6]. Noteworthy is that the formation of Total-X tended to decrease with the increase in [HCHO] 0, as seen from Fig. 3b. The results are contrary to the accepted mechanism in which compound X is formed in reaction (3), and also disagree with the results of our previous study [6]. A close examination of the figure reveals that, under these conditions, the profiles of the Total-X show 'delay' at low C2H 4 conversion, and this appears to become more enhanced at higher [HCHO] 0. As the C2H 4 conversion increases, the Total-X formation increases to some extent. Since the formation of HCOOH increases significantly at higher C 2H a conversions, it was anticipated that the presence of HCOOH could affect the compound X formation.

3.2. The effect of HCOOH addition When H C O O H was added, the yield of compound X increased markedly, and the associated decrease in the added H C O O H was observed. Results are shown in Fig. 4 and in Table 1. The formation of the Total-X was prompt with the added HCOOH. The extent of ozone removal, ¥(HCHO), ¥ ( C O ) or ¥ (CO 2) did not change upon addition of H C O O H as

0.5

1.0

1.5

Ethene Conversion/ppmv Fig. 4. The effect of the HCOOH addition. [ C 2 H 4 ] 0 = 4.1 ppmv, [03]0 = 1.9-2.1 ppmv. (a) [HCOOH]0 = 2.0 ppmv. (b) [HCOOH]0 = 10.3 ppmv. Note in both cases that the results for HCOOH are the decrease in the added HCOOH.

seen in Table 1. From the above results, it is evident that one of the precursors to compound X is HCOOH, and not HCHO as previously assumed. It is most probable that the other precursor is the stabilized Criegee intermediate CH2OO, by analogy to reaction (3). The formation of compound X can therefore be expressed as follows: CH2OO + H C O O H ~ compound X.

(7)

This reaction is analogous to the liquid-phase reaction of Criegee intermediates (zwitterions) with hydroxylic compounds such as CH3OH to form an alkoxy hydroperoxide, illustrated for CH2OO, reaction (8) [15]. It should be noted that the rupture of the H - O bond in CH3OH has taken place in the reaction. In the equation below, this is made conspicuous by the use of italic letters, CH2OO + C H 3 O H --~ H O O - C H 2 - O - C H 3 .

(8)

If this analogy can be extended to reaction (7), it follows that compound X has the structure of H O O -

155

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

CH2-O-CHO. Reaction (7) is then rewritten as follows: CH2OO + HCOOH ~ H O O - C H 2 - O - C H O .

(9)

Compound X may be called hydroperoxymethyl formate, and abbreviated to HPMF. The peroxidic structure of (9) seems most probable for reasons to be described below. The new structure contains an OOH group instead of an OH group of the previous assignment [3]. Previously, the two FTIR spectroscopic features of compound X around 920 and 820 cm-1 had not been given any assignment [18]. Since these two low-lying absorptions are characteristic to O - O stretching vibrations [19], the structure of HPMF is consistent with the observed spectroscopic features. Presence of two peaks at 3585 and 3405 cmobserved by Niki et al. [3] was also confirmed in this work. They attributed these peaks to an O - H stretching vibration and to an internally bonded O - H group, respectively, and suggested the presence of cis (internally bonded O - H ) and trans (free O - H ) isomers. This view is consistent with the new structure, HOO-CH2--O--CHO, where the O - H band at 3405 cm-1 is closely related to the well-established intramolecular hydrogen bonding in peroxy acids [19,20]. Decomposition of HPMF to FAN should yield H 2 0 as the co-product and is 235 kJ mo1-1 exothermic, based on the enthalpy of formation of HPMF, AH ° = - 4 7 0 kJ mo1-1 [10], H O O - C H 2 - O - C H O ~ FAN + H20, A H -- - 235 kJ mol- l,

(1o)

This reaction is thus thermochemically more favourable than reaction (4) or (5) due to the elimination of H 2 0 instead of H 2 or H 2 0 2 in the reaction. An alternative structure of the Criegee intermediate, methylenebis(oxy), OCH20 [16], may also be considered as the reactant. If the rupture of the O - H bond in HCOOH is to occur as in (9), a peroxidic product with the structure of H O - C H 2 - O O - C H O would result. The most likely decomposition of this compound should occur at the weakest O - O bond first, which would yield products other than FAN. Therefore, the dioxymethylene isomer CH2OO is considered more consistent than the methylenebis

(oxy) form with the experimental results, and also with the liquid-phase ozonolysis [15,17]. The mechanism for the formation of HPMF is still only speculative. The case of the added HCOOH is straightforward as shown in reaction (9). In the absence of the added HCOOH, it might be possible either that the CH2OO intermediate first rearranges to HCOOH, (11) [2,21], and then reacts with remaining CH2OO intermediate, as in (9), or that two CHzOO intermediates undergo an association reaction, (12), CH2OO* ~ HCOOH*

+M) HCOOH,

CH2OO + CH2OO ~ H O O - C H 2 - O - C H O .

(11) (12)

In order to explain the slight but clearly measurable decrease in the Total-X yield despite an enhanced HCOOH formation (Fig. 3a), we must conclude that HCHO reacts with CH2OO to produce yet unidentified or wrongly assigned products. As seen in Fig. 4, the depletion of the added HCOOH was larger than the sum of HPMF and FAN, even taking into account the large uncertainty in the [HPMF] determination. Although HCOOH tends to be adsorbed on the reactor wall, its effect was of minor importance under the experimental conditions. Also, the formation of the HCOOH dimer is considered negligible [12]. Previously, we observed in the C2H 4 ozonolysis with added HCHO [13], that the concentration of the HCOOH dimer was about a factor 4 larger than would be obtained in equilibrium mixtures, which might be related to the present results. However, no further speculation was invoked in the absence of experimental evidence. At present, we are unable to explain the excess depletion of the added HCOOH. The unexpected results obtained by the addition of HCOOH (Fig. 4) were also observed in the ozonolysis of other alkenes, including isobutene, isoprene, trans-2-butene, and 2,3-dimethyl-2-butene. Particularly, the formation of HPMF and its increase with added HCOOH in the ozonolysis of terminal alkenes were found to be an almost exact replica of the results observed in the C2H 4 ozonolysis. New spectral features were obtained in the residual spectra from the ozonolysis of these alkenes in the presence of the added HCOOH (Neeb et al., unpublished data. A detailed account will be given else-

156

P. Neeb et al. / Chemical Physics Letters 246 (1995) 150-156

where). D u e to the n e w findings of this study, results o f previous studies based on reaction (3), including our study [6], should be reinterpreted. It is clear, h o w e v e r , that further studies are necessary to understand the m e c h a n i s m o f the gas-phase o z o n o l y s i s o f simple alkenes.

Acknowledgements This w o r k was supported by the E u r o p e a n C o m mission and by D e u t s c h e - F o r s c h u n g s g e m e i n s c h a f t ( D F G ) through S F B - 2 3 3 , the d y n a m i c s and c h e m istry o f the hydrometeors.

References [1] R. Atkinson, S.M. Aschmann, J. Arey and B. Shorees, J. Geophys. Res. 97D (1992) 6065, and references therein. [2] F. Su, J.G. Calvert and J.H. Shaw, J. Phys. Chem. 84 (1980) 239. [3] H. Niki, P.D. Maker, C.M. Savage and L.P. Breitenbach, J. Phys. Chem. 85 (1981) 1024. [4] C.S. Kan, F. Su, J.G. Calvert and J.H. Shaw, J. Phys. Chem. 85 (1981) 2359. [5] S. Hatakeyama, H. Kobayashi, Z.-Y. Lin, H. Takagi and H. Akimoto, J. Phys. Chem. 90 (1986) 4131.

[6] O. Horie and G.K. Moortgat, Atmos. Environ. 24A (1991) 1881. [7] O. Horie, P. Neeb, S. Limbach and G.K. Moortgat, Geophys. Res. Letters 21 (1994) 1523. [8] S. G~ib, E. Hellpointner, W.V. Turner and F. Korte, Nature 316 (1985) 535. [9] K.H. Becker, J. Bechara and K.J. Brockmann, Atmos. Environ. 27A (1993) 57. [10] S.W. Benson, Thermochemical kinetics, 2nd Ed. (Wiley, New York, 1976). [11] O. Horie, P. Neeb and G.K. Moortgat, Intern. J. Chem. Kinetics 26 (1994) 1075. [12] M. Finkbeiner, P. Neeb, O. Horie and G.K. Moortgat, Fresenius J. Anal. Chem. 351 (1995) 521. [13] O. Horie and G.K. Moortgat, Fresenius J. Anal. Chem. 340 (1991) 641. [14] I. Muramatsu, M. Itori, M. Tsujii and A. Hagitani, Bull. Chem. Soc. Japan 37 (1964) 756. [15] R. Criegee, Angew. Chem. 87 (1975) 765; Angew. Chem. Intern. Ed. Engl. 14 (1975) 745. [16] P.S. Bailey, Ozonation in organic chemistry, Vol. 1 (Academic Press, New York, 1978). [17] S.A. Kafafi, R.I. Martinez and J.T. Herron, in: Modern models of bonding and delocalization, eds. J.F. Liebman and A. Greenberg (VCH Publishers, Weinheim, 1988) p. 283. [18] M.E. Jacox, J. Phys. Chem. Ref. Data 13 (1984) 945. [19] G. Socrates, Infrared characteristic group frequencies (Wiley, New York, 1980). [20] P.D. Maker, H. Niki, C.M. Savage and L.P. Breitenbach, Anal. Chem. 49 (1977) 1346. [21] S. Hatakeyama and H. Akimoto, Res. Chem. Intermed. 20 (1994) 503.