Gas-Phase Ozonolysis of Selected Olefins: The Yield of Stabilized

Sep 20, 2012 - The gas-phase reaction of ozone with olefins represents an important path for the conversion .... Criegee intermediate and highly oxyge...
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Gas-Phase Ozonolysis of Selected Olefins: The Yield of Stabilized Criegee Intermediate and the Reactivity toward SO2 Torsten Berndt,*,† Tuija Jokinen,‡ Roy L. Mauldin, III,‡,§ Tuukka Petaj̈ a,̈ ‡ Hartmut Herrmann,† Heikki Junninen,‡ Pauli Paasonen,‡ Douglas R. Worsnop,‡,∥,⊥ and Mikko Sipila‡̈ †

Leibniz-Institute for Tropospheric Research, Leipzig, Germany Department of Physics, University of Helsinki, Helsinki, Finland § University of Colorado at Boulder, Boulder, Colorado, United States ∥ University of Eastern Finland, Kuopio, Finland ⊥ Aerodyne Research, Inc., Billerica, Massachusetts, United States ‡

S Supporting Information *

ABSTRACT: The gas-phase reaction of ozone with olefins represents an important path for the conversion of unsaturated hydrocarbons in the atmosphere. The current interest is focused on the formation of stabilized Criegee intermediates (sCI) and possible further reactions of sCI. We report results from the ozonolysis of 2,3-dimethyl2-butene (TME), trans-2-butene and 1-methyl-cyclohexene (MCH) carried out in an atmospheric pressure flow tube at 293 ± 0.5 K and RH = 50% using chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry to detect H2SO4 produced from SO2 oxidation by sCI. The yields of sCI were found to be in good agreement with recently observed data: 0.62 ± 0.28 (TME), 0.53 ± 0.24 (trans-2-butene) and 0.16 ± 0.07 (MCH). The rate coefficients for sCI + SO2 from our experiment, (0.9−7.7) × 10−13 cm3 molecule−1 s−1, are within the range of recommendations from indirect determinations as given so far in the literature. Our study helps to assess the importance of sCI in atmospheric chemistry, especially for the oxidation of SO2 to H2SO4. SECTION: Kinetics and Dynamics in the case of cyclic olefins (cyclohexene: no stabilization; αpinene: ∼15%). The knowledge regarding rate coefficients for bimolecular reactions of sCI with atmospheric trace gases is very sparse.6 For instance, estimates of 7 × 10−14 cm3 molecule−1 s−111 or 7 × 10−12 cm3 molecule−1 s−112 for the reaction of CH2OO + SO2 have been used in atmospheric modeling and to set relative rate coefficients of other sCI reactions on an absolute scale.6 Very recently, Welz et al.13 reported a rate coefficient for CH2OO + SO2 of (3.9 ± 0.7) × 10−11 cm3 molecule−1 s−1 at 298 K and 4 Torr pressure being out of range of the reactivity assumed so far. We conducted gas-phase ozonolysis experiments for TME, trans-2-butene and 1-methyl-cyclohexene (MCH) in the atmospheric pressure flow tube IfT-LFT (Institute for Tropospheric Research − Laminar Flow Tube)14 at 293 ± 0.5 K using synthetic air as the pressure gas (RH: 50%); see Supporting Information for more detailed explanations. Propane or n-butane served as an OH radical scavenger. The resulting propyl and butyl radicals from the H-abstraction by OH radicals react in a fast step with O2 forming the corresponding peroxy radicals. The fate of the peroxy radicals

M

echanistic investigations regarding the reaction of ozone with unsaturated compounds have been the subject of investigations for more than 100 years.1−3 While earlier studies focused on the liquid phase process, the importance of ozonolysis for removal of gaseous olefins in the atmosphere was realized more recently.4,5 The gas-phase reaction of ozone with olefins produces an energy-rich primary ozonide, which decomposes very rapidly, forming a carbonylic substance and a biradical, the so-called Criegee intermediate (CI).3,6 This still energy-rich CI can undergo unimolecular reactions or can be collisionally stabilized by the pressure gas.7−9 The unimolecular pathways lead to the formation of an excited vinyl hydroperoxide (VHP) that rapidly decomposes producing OH and to the formation of dioxirane. Quite recently, the importance of stabilized VHP for product formation has been taken into consideration.9 For the stabilized Criegee intermediate (sCI), bimolecular reactions with other species (in competition to the unimolecular steps) are possible depending on the reactivity and availability of suitable substances in the atmosphere. The fraction of stabilization of different CI under atmospheric conditions is currently not well characterized because the determination has mostly been done indirectly so far.6 Recently published results by Donahue’s group9,10 show efficient stabilization of CI from noncyclic olefins (2,3-dimethyl-2butene (TME): ∼65%; trans-5-decene: 100%) and clearly less © 2012 American Chemical Society

Received: August 9, 2012 Accepted: September 20, 2012 Published: September 20, 2012 2892

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is governed by the self-reaction forming finally carbonylic substances or by the reaction with HO2 forming hydroperoxides. Results from qutantum chemical calculations point to a very slow reaction of peroxy radicals with SO2 making this path negligible.15 At the flow-tube outlet SO2 was injected to the gas stream by nozzles converting sCI to SO3, sCI + SO2 → SO3 + carbonyl. The possible SO2-catalyzed isomerization step in the case of sCI with H-atoms bound to the COO carbon, sCI + SO2 → SO2 + RCOOH, is believed to be not fast enough to compete with the SO3 formation.15 The presence of high water vapor content ensures rapid H2SO4 formation from SO3 within 0.1 ms.16 H2SO4 concentrations were measured with a chemical ionization atmospheric pressure interface time-of-flight (CIAPi-TOF) mass spectrometer17 0.5−1.4 s downstream from the point of the SO2 injection. In Figure 1 the usability of this

Figure 2. H2SO4 concentrations as a function of SO2 for different residence times after SO2 injection. SO2 was added at the outlet of the flow tube Ift-LFT. (a) TME; (b) trans-2-butene and MCH. Data for MCH with a residence time of 0.8 and 1.1 s were omitted for clarity. Initial olefin concentrations were (unit: molecules cm−3) TME = 9.6 × 1011; trans-2-butene = 5.35 × 1012; MCH = 4.7 × 1012.

Figure 1. Observed H2SO4 concentration for different gas additions injected at the flow-tube outlet (CI-inlet with 0.5 s residence time from SO2 injection until detection with mass spectrometric methods). Initial flow tube concentrations (unit: molecules cm−3): O3 = 1.06 × 1012, propane = 5.3 × 1016, TME = 9.5 × 1011. Concentration of the additions: SO2 = 7.5 × 1012, HCOOH = 2.6 × 1014.

approach for sCI detection via SO2 conversion to H2SO4 is demonstrated. Immediately after switching on/off the SO2 flow, the resulting increase/drop of the signal is visible without any delay pointing to a H2SO4 formation not affected by the walls. Furthermore, adding HCOOH together with SO2 to the reaction gas, H2SO4 formation is clearly suppressed as expected from the rate of the competing reactions sCI + SO2 versus sCI + HCOOH.18 Ongoing kinetic investigations regarding the reaction of sCI with a series of compounds in our laboratory confirm the very high reactivity of sCI toward small carboxylic acids. Figure 2a,b shows the measured H2SO4 concentration as a function of SO2 concentration for the three investigated olefins. The black dots depict the runs with the CI-inlet without additional tubing (SO2 residence time of 0.5 s) and the open circles with a different length additional tubing between SO2 injector and the CI-inlet resulting in residence times of up to 1.4 s (see the more detailed description in the Supporting Information). An increase of the H2SO4 signal was observed with increasing SO2 concentrations, resulting in nearly constant H2SO4 levels for [SO2] ≥ 1015 molecules cm−3. Under these conditions, all sCI were converted to H2SO4, indicating also that the rate coefficient for sCI + SO2 definitely has to be higher than 10−15 cm3 molecule−1 s−1.

The sCI concentration is measured at the flow-tube outlet (at the point of SO2 injection), and it is defined by the steady state sCI concentration according to [sCI]ss − tube = Y ·k1· [O3]· [olefin]/k loss

(I)

where Y stands for the yield of sCI, k1 stands for the rate coefficient for O3 + olefin, and kloss is the pseudo-first order rate coefficient for the total sCI loss including thermal decomposition and possible reaction with water vapor (sCI wall loss is negligible under these conditions). The reactant conversion was small for the chosen conditions of ozonolysis in the flow tube (O3 conversion: 5.4−6.8%; olefin conversion: 1.2−6.4%). Using SO2 in large excess, [SO2] ≥ 1015 molecules cm−3, sCI is immediately transformed to H2SO4, i.e., [sCI]ss‑tube = [H2SO4]ss‑tube. After SO2 injection, however, the olefin ozonolysis is still ongoing forming continuously more sCI and subsequently H2SO4. Thus, increasing the residence time between the SO2 injection and H2SO4 detection yields increasing levels of H2SO4 (see Figure 2a,b). The total sCI and subsequently H2SO4 concentration after SO2 injection ([SO2] ≥ 1015 molecules cm−3) is given by the fraction formed in the CI-inlet (0.5s) and the fraction arising from the flow tube ([H2SO4]ss‑tube) considering also further H2SO4 production in 2893

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the additional tubing between flow-tube outlet and the CI-inlet including the wall loss in this section:

overall uncertainty of the sCI yields. The rate coefficients for kloss are not affected by the precision of the H2SO4 calibration. The observed sCI yield for (CH3)2COO of 0.62 ± 0.28 from TME ozonolysis agrees well with recently reported values from another study (∼0.65)9,10 also using end-product analysis of a typical sCI reaction product. Lower stabilization yields have been derived from an earlier experiment (∼0.30)19 or from quantum chemical calculations (≤0.38).20 In the case of the CH3CHOO sCI yield from trans-2-butene, our value of 0.53 ± 0.24 is somewhat higher than the former experimental data, 0.4221 and ∼0.45,22 but in line with the expected trend starting from (CH3)2COO, that with lowering of the carbon number stabilization becomes less efficient.10 For both CI arising from MCH, the observed sCI yield is relatively small, 0.16 ± 0.07, reflecting the findings from other groups that CI of cyclic olefins are generally less stabilized under atmospheric conditions than CI from linear olefins.10,23 For the structurally similar, but larger molecule α-pinene, a sCI yield of ∼0.15 was reported in the literature.10 The kloss data observed stands for the thermal decomposition channel of sCI forming OH8 and the possible reaction of sCI with water vapor, i.e., kloss = kdec + k(sCI + H2O)·[H2O], [H2O] = 2.9 × 1017 molecules cm−3 in our experiment. Estimated rate coefficients for the reaction with water vapor are highly uncertain, spanning a range of a few 10−18−10−16 cm3 molecule−1 s−1.6 It should be noted that the estimates of k(sCI + H2O) mostly arose from ethylene ozonolysis, i.e., for CH2OO, which only exits as the anti-species. For the sCI from TME ozonolysis, only the syn-species is possible. From trans-2butene and MCH ozonolysis, syn- and anti-conformers can be formed, and ab initio quantum chemical calculations favored the syn-species.24 Results from computational studies show rate coefficients for sCI + H2O on the order of 10−19−10−21 cm3 molecule−1 s−1 in the case of sCI with a syn-structure.25−27 For anti-CH3CHOO, rate coefficients of the reaction with water vapor on the order of 10−15−10−16 cm3 molecule−1 s−1 have been obtained.25−27 Our experimental kloss data are about 3 s−1, i.e., k(sCI + H2O) ≤ 10−17 cm3 molecule−1 s−1. Assuming that k(sCI + H2O) is in the order of 10−18 cm3 molecule−1 s−1 or less, our kloss data can be compared with literature values for kdec. From TME ozonolysis, kdec = 6.4 ± 0.9 s−1 (experiment) and 3.5 s−1 (theory) are reported for stabilized (CH3)2COO at 100 Torr8 being in reasonable agreement with our kloss = 3.0 ± 0.4 s−1. Another theoretical study suggested kdec = 250 s−1.20 In the case of CH3CHOO from trans-2-butene ozonolysis, kdec = 76 s−128 and kdec = 2.5 s−121 have been found from indirect experimental determinations. The latter value is close to our kloss = 2.9 ± 0.9 s−1. It is to be noted here that kdec can be strongly temperaturedependent. Therefore, reaction temperatures significantly different from our 293 ± 0.5 K can cause clearly different rate coefficients for kdec.

[H 2SO4 ] = Y ·k1·[O3]·[olefin]·[0.5s + 1/k wall − (1/k wall − 1/kloss) ·exp( −k wall·(t − 0.5s))]; t ≥ 0.5s

(II)

The rate coefficient kwall stands for the diffusion controlled H2SO4 wall loss in the additional tubing. H2SO4 wall loss was negligible in the case of the CI-inlet (0.5s) due to the given geometry; see Supporting Information for a more detailed explanation of eq II. The measured H2SO4 concentrations for [SO2] ≥ 1015 molecules cm−3 are given in Figure 3 as a function of the different SO2 residence times along with the best fit curves according to eq II.

Figure 3. Mean values of the measured H2SO4 concentrations in the presence of [SO2] ≥ 1015 molecules cm−3 for the different residence times after SO2 injection. The error bars represent two standard deviations of the mean values. The full curves represent results from a least-squares analysis according to eq II. The dashed lines are obtained from the TME system, setting kloss = 1.5 or 6 s−1 as a fixed parameter in eq II.

The free parameters Y (sCI yield) and kloss in eq II were determined from a least-squares analysis (see Table 1). The needed O3 and olefin concentrations were measured just before the port of the SO2 injection. This approach is sensitive for the determination of kloss and Y. Setting kloss = 1.5 or 6 s−1 (half or twice as much the best-fit value) for TME ozonolysis as a fixed parameter in eq II, the sCI yields of 0.52 and 0.69, respectively, are obtained, and the measurements are not adequately reproduced by the fitting curve (cf. Figure 3). It is to be noted that the measured sCI yields are linearly dependent on the H2SO4 calibration, i.e., a maximum error of ±45% due to the H2SO4 calibration uncertainty has to be taken into account dominating the

Table 1. sCI Yield, the Rate Coefficient for the Total sCI Loss, and the Rate Coefficient for the Reaction sCI + SO2 olefin

ozonolysis reaction rate (molecules cm−3 s−1)

sCI yieldb

klossa (s−1)

k(sCI + SO2)a (cm3 molecule−1 s−1)

TME trans-2-butene MCH

8.60 × 108 9.61 × 108 7.65 × 108

0.62 ± 0.28 0.53 ± 0.24 0.16 ± 0.07

3.0 ± 0.4 2.9 ± 0.9 2.4 ± 0.4

(7.7 ± 1.4)·10−13 (1.4 ± 0.4)·10−13 (9.3 ± 2.6)·10−14

a

Given error limits represent two standard deviations of statistical errors. bResulting error limits (root-mean-square error) include two standard deviations of statistical errors (2.2−7.5%) and an uncertainty of ±45% arising from the H2SO4 calibration 2894

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sensitivity study (retaining kloss = 1.5 or 6 s−1; cf. Figure 3) shows that kloss values of ∼3.0 s−1 are needed in a relatively narrow range to describe the time-dependent measurements. Of course, we cannot totally rule out any hidden, systematic errors. However, it is highly unlikely that our kloss values are affected by errors of a factor of ∼102−103, too high or too low in order to hit the range of the data for k(sCI + SO2) given in the literature.13,15,29 It is noteworthy that our k(sCI + SO2) coefficients are within the range of estimates of 7 × 10−14 cm3 molecule−1 s−111 or 7 × 10−12 cm3 molecule−1 s−112 (from CH2OO + SO2) as used in atmospheric modeling. Finally, the atmospheric process of H2SO4 formation from sCI + SO2 appears to be efficient enough to produce substantial fractions of H2SO4 beside the OH + SO2 path,30 even with rate coefficients for sCI + SO2 of a few 10−13 cm3 molecule−1 s−1. Besides the k(sCI + SO2) itself, the formation yields of sCI and their thermal stability are important parameters. Olefins with exocyclic double bonds and/or large molecules with a high carbon-number appear to be the most promising candidates due to their high stabilization yields. A comparison of results regarding the H2SO4 formation from limonene (exocyclic and endocyclic double bond) and α-pinene (only endocyclic) under identical conditions supports this tendency.30 Much more work is needed to evaluate sCI's role in atmospheric chemistry, both in terms of oxidation capacity and source strengths. In the atmosphere, a vast number of organic compounds, each producing specific sCI, are emitted by the biosphere.31

In the next set of experiments, we varied the SO 2 concentration injected downstream from the flow-tube outlet over 5 orders of magnitude maintaining the reaction conditions from the experiments before (CI-inlet, 0.5 s). Figure 4 shows the measured H2SO4 concentrations depending on SO2 concentrations for the three olefins along with the best-fit curves.

Figure 4. Measured H2SO4 concentrations as a function of SO2 concentrations injected at the outlet of the flow tube (CI-inlet, t = 0.5 s). The full curves represent the best-fit results from a least-squares analysis determining k(sCI + SO2) using the sCI yield Y and kloss from the experiments before, dashed lines were observed assuming half or twice as much of k(sCI + SO2).

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ASSOCIATED CONTENT

S Supporting Information *

Least-squares analysis was used for the determination of the rate coefficient for sCI + SO2 → ... → H2SO4 minimizing the difference between measured [H2SO4] and [H2SO4] from integration of the resulting differential equations describing the reaction sequence: O3 + olefin → Y·sCI + ... with k1, sCI → loss with kloss, and sCI + SO2 → ... → H2SO4 with k(sCI + SO2) (see more information in the Supporting Information). The values for k(sCI + SO2) are given in Table 1. They are not affected by the precision of the H2SO4 calibration. (CH3)2COO from TME ozonolysis is roughly 5−8 fold more reactive toward SO2 than the sCI from ozonolysis of trans-2-butene and MCH. Due to this very limited data set, it is impossible to draw any conclusions regarding a structure−reactivity relation. Generally in all three cases, the measurement points for low SO2 concentrations are somewhat above the best-fit curve. The reason is not clear at the moment. It could be speculated that the reaction scheme considered here for H2SO4 formation is too simple and, consequently, the SO2 oxidation starting from olefin ozonolysis is more complex as assumed at the moment. Resulting curves assuming double or half of the value for k(sCI + SO2) are also depicted in Figure 4 giving an impression regarding the sensitivity of this method. Our rate coefficients of the reaction sCI + SO2, (0.9−7.7) × 10−13 cm3 molecule−1 s−1, are clearly higher than the upper limit for k(sCI + SO2) ≤ 4 × 10−15 cm3 molecule−1 s−1 deduced from 2-methyl-2-butene ozonolysis under atmospheric conditions.29 On the other hand, our k(sCI + SO2) values are about 2−3 orders of magnitude lower than a recently published absolute rate coefficient for CH2OO measured at 298 K and 4 Torr total pressure,13 and lower than results from quantum chemistry for CH 2 OO and (CH 3 ) 2 COO. 15 The rate coefficients k(sCI + SO2) from our study have been determined relative to the rate coefficient of the overall sCI loss (kloss). A

Experimental setup and reaction conditions; methods of chemical analysis; mathematical approach. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank K. Pielok, R. Gräfe and A. Rohmer for technical assistance. This work was partially funded by Academy of Finland (1251427, 1139656, Finnish centre of excellence 1141135), EU Pegasos project and European Research Council (ATMNUCLE).

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