Atmospheric Oxidation of Piperazine by OH has a Low Potential To

Letter pubs.acs.org/journal/estlcu

Atmospheric Oxidation of Piperazine by OH has a Low Potential To Form Carcinogenic Compounds Lavinia Onel,* Matthew Dryden, Mark A. Blitz, and Paul W. Seakins School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K. S Supporting Information *

ABSTRACT: Piperazine [HN(CH2CH2)2NH, PZ] is widely recognized as an efficient solvent for carbon capture (CC). We present the first determination of the rate coefficient, k, and the branching ratios for the reaction of OH with PZ in the gas phase using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence giving k298 K of (2.38 ± 0.28) × 10−10 cm3 molecule−1 s−1. The reaction has a negative temperature dependence parametrized as kOH+PZ = (2.37 ± 0.03) × 10−10(T/298)−(1.76±0.08). The high rate coefficient suggests that gas phase processing in the atmosphere will compete with uptake onto aerosols. The branching ratios, abstraction from C−H versus N−H, have been determined by analysis of OH temporal profiles obtained in the presence of O2/NO. The result (rN−H = 0.09 ± 0.06) shows that the potential for forming the carcinogenic nitrosamines or nitramines from PZ oxidation is smaller than for the oxidation of the benchmark CC solvent monoethanolamine (MEA).

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INTRODUCTION Carbon capture (CC) by aqueous solutions of amine-based compounds will be an important technology for mitigating CO2 emissions from coal- and gas-fired power plants and is currently used in pilot plants worldwide.1 The total reaction of CO2 with the benchmark CC solvent monoethanolamine (MEA) involves two molecules of amine (two amine groups) for one molecule of CO2. Piperazine [1,4-diazacyclohexane, HN(CH2CH2)2NH] is a six-membered ring that has two amine groups available for the reaction with CO2. In addition, the PZ + CO2 reaction is known to occur rapidly.2−4 Therefore, PZ has attracted interest for its use in postcombustion CC technology as a promoter for CO2 absorption and an alternative solvent to MEA.4−8 However, a concern with the large scale use of amines in CC is their emission into the atmosphere, which has potential to result in the formation of toxic nitrosamines and nitramine products following OH-initiated oxidation, which would have an impact on human health and the environment.9−11 Once released into the atmosphere, amines can be lost via heterogeneous uptake or gas phase reaction with OH, which is the primary daytime oxidant in the atmosphere. Determining the overall rate coefficient for OH reaction allows assessment of the fraction of amine processed in the gas phase. Reactions of radicals with amines cannot be modeled with conventional structure−activity relationships (SARs) and hence must be determined experimentally.12 The reaction of OH with amines occurs via H abstraction at both the α position (reaction R1a) and the amine group (reaction R1b). HN(CH 2CH 2)2 NH + OH → I1 + H 2O

(R1a)

HN(CH 2CH 2)2 NH + OH → I2 + H 2O

(R1b)

© 2014 American Chemical Society

where I1 represents a carbon-centered radical and I2 a nitrogencentered radical (Scheme 1 in the Results). The reactions of I2 with NO and NO2 produce nitrosamine (R2NNO) and nitramine (R2NNO2). Both of these compounds are carcinogenic13 and were observed, but not quantified, in piperazine oxidation experiments conducted in the EUPHORE atmospheric chamber by Nielsen et al.14 Therefore, the determination of the branching ratio of reaction R1b is essential for the quantification of the yields of the carcinogenic compounds in the atmospheric oxidation of PZ. At present, there is very little information about the atmospheric oxidation of PZ, with no direct isolated studies. The EUPHORE experiments indicated that PZ has a short life with respect to reaction with OH in the atmosphere.14 However, the determination of the overall rate coefficient and the branching ratios in the OH + PZ reaction by Nielsen et al. was prevented by adsorption/desorption processes on the chamber walls and particle formation. In addition to the EUPHORE study, there is one theoretical study that reports on bond enthalpy calculations: ∼392 kJ mol−1 for the C−H bond and ∼404 kJ mol−1 for the N−H bond.15 The result suggests that both C−H and N−H bond abstractions are important in the reaction of OH with PZ. However, there are no theoretical calculations of the branching ratios of reactions R1a and R1b. In addition, previous work showed that theory is currently unable to predict the branching ratios in the reaction of OH with a series of amines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA).16 Therefore, it is essential to Received: Revised: Accepted: Published: 367

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PZ + OH Reaction in N2. The kinetic studies were performed under pseudo-first-order conditions using PZ concentrations in large excess over the initial OH concentration (>100:1 [PZ]: [OH]0). Under these conditions, the fluorescence intensity, If, which is proportional to [OH], decayed according to the singleexponential equation

perform direct measurements for the determination of the branching ratios. In addition, as a potential mechanism for the removal of waste material from carbon capture processes is incineration, kinetic studies at elevated temperatures are required. In this study, we used pulsed laser photolysis−laser-induced fluorescence (LP−LIF) to determine the rate coefficient of the OH + PZ reaction over the temperature range of 298−506 K. Previous studies of OH + alkylamine reactions showed that at relatively low pressures (99.9%, BOC) were passed along with the carrier gas nitrogen (99.998%) through calibrated mass flow controllers, mixed in a manifold, and introduced into the reaction cell. The OH precursor was tertiary butylhydroperoxide (TBHP, SigmaAldrich, 70% in H2O), and the OD precursor was deuterated acetone (acetone-d6) in the presence of oxygen.19 Both radical precursors were made up and stored as diluted mixtures in N2 in glass bulbs. Gaseous mixtures of PZ could not be prepared because of the low vapor pressure of PZ of ∼28 Pa at 298 K.20 Therefore, PZ was introduced into the reactor from a glass bubbler by blowing nitrogen gas over solid PZ to transfer the amine vapor. Radicals were generated by pulsed excimer laser photolysis at 248 nm and probed by laser-induced fluorescence using probe radiation at 282 nm. Fluorescence at 308 nm was detected by a photomultiplier tube. The time delay between photolysis and probe lasers was scanned to build up an entire temporal profile of the OH radicals. An example of the OH temporal profile is shown in the inset of Figure 1. Determination of PZ Concentrations. The concentrations of PZ were determined using in situ absorption measurements at 185 nm over a path length of 31 cm using the technique presented for MEA previously.18 Absorptions were typically 5− 25% and were converted into PZ concentrations using the absorption cross section of the amine determined in separate experiments. The determined cross section is σ185 nm(PZ) = (2.01 ± 0.23) × 10−17 cm2 molecule−1 (Supporting Information) and is 2−3 times higher than the σ185 nm values of MEA,18 TMA, and DMA21 and by a factor of ∼6 higher than the σ185 nm of MA.22

Figure 1. Pseudo-first-order coefficient vs PZ concentration for the OH(OD) + PZ reaction: (black square) OH, N2 bath gas; (red circles) OD, N2/O2 bath gas. A temperature of 298 K and a 14 Torr total pressure were used for both OH and OD experiments. Errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. kPZ+OH = (2.42 ± 0.28) × 10−10 cm3 molecule−1 s−1, and kPZ+OD = (2.14 ± 0.28) × 10−10 cm3 molecule−1 s−1. The inset shows a typical OH fluorescence decay trace recorded for the OH + PZ reaction.

PZ + OH Reaction in the Presence of O2 and of O2/NO. Kinetic measurements for the PZ + OH reaction in the presence of O2 and in the absence of NO were taken at total pressures of 7 and 14 Torr at 298 K and at 14 Torr at higher temperatures (up to ∼500 K). All the measurements in the presence of both O2 (1.5 and 3 × 1016 molecules cm−3) and NO (1−5 × 1014 molecules cm−3) were performed at room temperature and 14 Torr.

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RESULTS AND DISCUSSION PZ + OH Reaction in N2. The average rate coefficient at room temperature is (2.38 ± 0.28) × 10−10 cm3 molecule−1 s−1; hence, it is very close to the gas kinetic limit and a factor ∼3−10 higher than that found for the reaction of OH with alkylamines and ethanolamine,16,18,23,24 and more than twice the value that would be predicted from a simple SAR analysis based on the OH + DMA reaction rate coefficient.16 The very fast loss process (lifetime of ∼15 min with respect to OH for an OH concentration of 5 × 106 molecules cm−3) suggests that the PZ processing in the daytime atmosphere will be dominated by the loss in the gas phase. 368

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Figure 4. HO2 yield as a function of NO concentration for the OH + PZ/NO/O2 system: (black circle) 1.58 × 1016 molecules of O2 cm−3 and (red circles) 3.03 × 1016 molecules of O2 cm−3. A temperature of 298 K and a 14 Torr total pressure of N2 were used. The HO2 yield was calculated using numerical simulations (Supporting Information). The horizontal blue line corresponds to the average HO2 yield of 0.91.

Figure 2. Temperature dependence of the rate coefficient for the OH(OD) + PZ reaction: (blue circles) OH, N2 bath gas; (red circles) OD, N2/O2 bath gas. The errors are at the σ level. All the data can be parametrized as kOH+PZ = (2.37 ± 0.03) × 10−10(T/298)−(1.76±0.08).

Figure 2 shows the rate coefficient for the OH + PZ reaction as a function of temperature (the data are tabulated in the Supporting Information). The measurements are in very good agreement with the measurements for the OD + PZ reaction at room temperature and ∼500 K. The negative temperature dependence is more pronounced than for the OH reaction with alkylamines and ethanolamine.16,18,24 PZ + OH Reaction in the Presence of O2 and of O2/NO. In the presence of oxygen, the rate coefficient for the OH + PZ reaction does not change, showing that no OH is regenerated by the reaction of O2 with the carbon-centered radical, I1, formed by the initial OH abstraction (reaction R1a). The result is in agreement with the lack of OH recycling in the reactions of OH with primary and secondary alkylamines and ethanolamines in the presence of O2.16,25 The reaction of O2 with both RCHNHR′ and RCH2NR′ radicals produces HO2.26 However, the O2 + RCHNHR′ reaction has a rate coefficient of ∼10−11 cm3 molecule−1 s−1,27 while the O2 + RCH2NR′ reaction is much slower, with a rate of 10−18 to 10−19 cm3 molecule−1 s−1.12,28 Therefore, on the millisecond scale of our experiments, the I1 radical reacts with oxygen, generating HO2, while the I2 + O2 reaction does not produce HO2 (Scheme 1).17 At the wavelength used for the photolysis of the OH precursor (TBHP), 248 nm, the photodissociation of PZ is significant but can be accounted for (see the Supporting Information). The dominant coproduct of the photodissociation of PZ to H atoms is intermediate I2. In the presence of NO/O2, the H atoms are titrated to OH by reaction with O2 and NO2 (Supporting Information), which reacts further with PZ. An example of a OH signal generated by the H reactions in the absence of TBHP is shown in the Supporting Information. Figure 3 shows an example of the time evolution of OH recorded in the presence of TBHP, NO, and O2. Numerical analysis of the OH profiles based on Scheme 1 and presented in the Supporting Information determined that the HO2 yield, ΦHO2, of reaction R1 with O2/NO and, hence, r1a is 0.91 ± 0.14. Here the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. Figure 3 also shows the OH profile obtained if ΦHO2 would be unity and zero, compared to the experimental trace. As shown in Scheme 1, the yield of HO2 does not vary with the concentration of NO and O2, which indeed was confirmed by our experiments. Figure 4 is an example showing that ΦHO2 is independent of NO concentration.

Scheme 1. Photochemistry Generated in the LP−LIF Experimentsa

a

Photolysis light at 248 nm was used.

Figure 3. Decay trace of OH fluorescence (red circles) recorded at 298 K and a total pressure of 14 Torr of N2 for the OH + PZ reaction in the presence of 3.2 × 1014 molecules of NO cm−3 and 1.52 × 1016 molecules of O2 cm−3. The fit () results in a HO2 yield, ΦHO2, of 0.89. Two OH profiles generated using numerical simulations, ΦHO2 = 1.00 (blue line) and ΦHO2 = 0.00 (magenta line) are also shown.

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The branching ratio of reaction R1b, determined to be 0.09 ± 0.06, represents an upper limit for the carcinogenic compound production and is smaller than r1b in the reaction of OH with MA, DMA, and EA, found to lie in the range of 0.24−0.48.17,26 Very importantly, the branching ratio of reaction R1b is also smaller than the r1b of 0.36 ± 0.04 found for the OH + MEA reaction using the same technique,25 suggesting that the release of PZ into the atmosphere from a CC plant will result in a toxic compound yield smaller than the release of MEA.

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(10) Zhu, L.; Schade, G. W.; Nielsen, C. J. Real-Time Monitoring of Emissions from Monoethanolamine-Based Industrial Scale Carbon Capture Facilities. Environ. Sci. Technol. 2013, 47 (24), 14306−14314. (11) Veltman, K.; Singh, B.; Hertwich, E. G. Human and Environmental Impact Assessment of Postcombustion CO2 Capture Focusing on Emissions from Amine-Based Scrubbing Solvents to Air. Environ. Sci. Technol. 2010, 44 (4), 1496−1502. (12) Karl, M.; Dye, C.; Schmidbauer, N.; Wisthaler, A.; Mikoviny, T.; D’Anna, B.; Muller, M.; Borras, E.; Clemente, E.; Munoz, A.; Porras, R.; Rodenas, M.; Vazquez, M.; Brauers, T. Study of OH-initiated degradation of 2-aminoethanol. Atmos. Chem. Phys. 2012, 12 (4), 1881−1901. (13) Låg, M.; Lindeman, B.; Instanes, C.; Brunborg, G.; Schwarze, P. Health Effects of Amines and Derivatives Associated with CO2 Capture; The Norwegian Institute of Public Health: Oslo, 2011. (14) Nielsen, C. J.; D’Anne, B.; Bossi, R.; Bunkan, A. J. C.; Dithmer, L.; Glasius, M.; Hallquist, M.; Hansen, A. M. K.; et al. Atmospheric Degradation of Amines (ADA). Summary report from atmospheric chemistry studies of amines, nitrosamines,nitramines and amides; CLIMIT Project 208122; University of Oslo: Oslo, 2012. (15) Bråten, H. B.; Bunkan, A. J.; Bache-Andreassen, L.; Solimannejad, M.; Nielsen, C. J. Final report on a theoretical study on the atmospheric degradation of selected amines; NILU: OR 77/2008; University of Oslo: Oslo, 2008. (16) Onel, L.; Thonger, L.; Blitz, M. A.; Seakins, P. W.; Bunkan, A. J. C.; Solimannejad, M.; Nielsen, C. J. Gas-Phase Reactions of OH with Methyl Amines in the Presence or Absence of Molecular Oxygen. An Experimental and Theoretical Study. J. Phys. Chem. A 2013, 117 (41), 10736−10745. (17) Onel, L.; Blitz, M. A.; Dryden, M.; Thonger, L.; Seakins, P. W. Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine and Ethylamine. Environ. Sci. Technol. 2014, 48, DOI: 10.1021/es502398r . (18) Onel, L.; Blitz, M. A.; Seakins, P. W. Direct determination of the rate coefficient for the reaction of OH radicals with monoethanol amine (MEA) from 296 to 510 K. J. Phys. Chem. Lett. 2012, 3 (7), 853−856. (19) Carr, S. A.; Baeza-Romero, M. T.; Blitz, M. A.; Price, B. J. S.; Seakins, P. W. Ketone photolysis in the presence of oxygen: A useful source of OH for flash photolysis kinetics experiments. Int. J. Chem. Kinet. 2008, 40 (8), 504−514. (20) Verevkin, S. P. Thermochemistry of amines: Strain in sixmembered rings from experimental standard molar enthalpies of formation of morpholines and piperazines. J. Chem. Thermodyn. 1998, 30 (9), 1069−1079. (21) Burton, G. R.; Chan, W. F.; Cooper, G.; Brion, C. E.; Kumar, A.; Meath, W. J. Valence shell absolute photoabsorption oscillatorstrengths, constrained dipole oscillator strength distributions, and dipole properties for CH3NH2, (CH3)2NH, and (CH3)3N. Can. J. Chem. 1994, 72 (3), 529−546. (22) Hubin-Franskin, M. J.; Delwiche, J.; Giuliani, A.; Ska, M. P.; Motte-Tollet, F.; Walker, I. C.; Mason, N. J.; Gingell, J. M.; Jones, N. C. Electronic excitation and optical cross sections of methylamine and ethylamine in the UV-VUV spectral region. J. Chem. Phys. 2002, 116 (21), 9261−9268. (23) Carl, S. A.; Crowley, J. N. Sequential two (blue) photon absorption by NO2 in the presence of H2 as a source of OH in pulsed photolysis kinetic studies: Rate constants for reaction of OH with CH3NH2, (CH3)2NH, (CH3)3N, and C2H5NH2 at 295 K. J. Phys. Chem. A 1998, 102 (42), 8131−8141. (24) Atkinson, R.; Perry, R. A.; Pitts, J. N., Jr. Rate constants for the reactions of the hydroxyl radical with dimethylamine, trimethylamine, and ethylamine over the temperature range 298−426 K. J. Chem. Phys. 1978, 68 (4), 1850−1853. (25) Onel, L.; Thonger, L.; Blitz, M. A.; Seakins, P. W. Branching Ratios in Reactions of OH Radical with Monoethanol Amine, NMethylethanol Amine and N,N-Dimethylethanol Amine. Manuscript in preparation.

ASSOCIATED CONTENT

* Supporting Information S

Details of the determination of the cross section of PZ at 185 nm, 298−506 K data, and conditions for the OH(OD) + PZ reaction and details about the numerical analysis performed for the OH + PZ/NO/O2 system. 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]. Phone: +44 (0)113 3436594. Fax: +44 (0)113 3436565. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received support from NERC (Grant NE/1013474/ 1).

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REFERENCES

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