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Mar 12, 2012 - passing nitrogen over the bubbler, at 296 K and 17 Torr, with t-BuOOH. The inset to the figure shows a typical OH decay trace and fit t...
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Direct Determination of the Rate Coefficient for the Reaction of OH Radicals with Monoethanol Amine (MEA) from 296 to 510 K L. Onel, M. A. Blitz, and P. W. Seakins* School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: Monoethanol amine (H2NCH2CH2OH, MEA) has been proposed for large-scale use in carbon capture and storage. We present the first absolute, temperature-dependent determination of the rate coefficient for the reaction of OH with MEA using laser flash photolysis for OH generation, monitoring OH removal by laser-induced fluorescence. The room-temperature rate coefficient is determined to be (7.61 ± 0.76) × 10−11 cm3 molecule−1 s−1, and the rate coefficient decreases by about 40% by 510 K. The temperature dependence of the rate coefficient is given by k1= (7.73 ± 0.24) × 10−11(T/295)−(0.79±0.11) cm3 molecule−1 s−1. The high rate coefficient shows that gas-phase processing in the atmosphere will be competitive with uptake onto aerosols. SECTION: Atmospheric, Environmental and Green Chemistry

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arbon dioxide capture from the flue gas of large combustion sources, with subsequent long-term storage, is a possible mechanism for the medium-term reduction of CO2 emissions. The reversible reaction of CO2 with amines to form carbamates, which can be separated from the flue gas and subsequently heated to release neat CO2 for processing and storage, is one possible method of capture. Large quantities of amines will be required, and inevitably, there will be some release of amines into the atmosphere at various points in the process.1 The atmospheric processing of amines has received relatively little study; however, it has been suggested that the OH-initiated oxidation of amines in the presence of nitrogen oxides can lead to the formation of toxic nitrosamines and potentially nitrous oxide, N2O, a potent greenhouse gas.2 Gasphase oxidation of the amine will be in competition with uptake and processing on particles.3,4 The sources and sinks of a variety of amines have recently been reviewed by Ge et al.5 Monoethanol amine, MEA, HOCH2CH2NH2, has been suggested as an appropriate amine for carbon capture; see, for example, Rao et al.6 and citations therein. Very recently, Karl et al.7 have performed a relative rate study at ∼305 K of the reaction of OH with MEA in the EUPHORE chamber, determining a rate coefficient of (9.2 ± 1.1) × 10−11 cm3 molecule−1 s−1 for reaction R1 using 1,3,5-trimethylbenzene as the reference compound. OH + HOCH2CH2NH2 → products

These studies are carried out with a large excess of the amine over OH (pseudo-first-order conditions). Crowley and coworkers have shown that for low volatility and polar compounds such as amines, uncertainties in the determination of the absolute amine concentration can be the largest source of error in the bimolecular rate coefficient.10 Following this work, we have been careful to use in situ absorption measurements to determine amine concentrations and have validated our methodology by reproducing the room-temperature rate coefficient for the reaction of OH with trimethylamine, which has previously been measured by Carl and Crowley10 and Atkinson and co-workers.11 OH + (CH3)3 N → products k2 = (3.58 ± 0.22) × 10−11 cm3 molecule−1 s−1 (R2)

Using the laser flash photolysis, laser-induced fluorescence apparatus described in the Experimental Methods section, we determined a value of k2 = (3.24 ± 0.14) × 10−11 cm3 molecule−1 s−1 for reaction R2, in excellent agreement with that of Carl and Crowley. Trimethylamine has a higher vapor pressure than MEA but still can be lost to the walls or photolyzed. The good agreement with the work of Carl and Crowley suggests that our precautions in handling amines, described below, are effective. Reaction R2 was also studied earlier by Atkinson et al.;11 the value of k2 obtained in that flash lamp photolysis, resonance fluorescence study, (6.09 ± 0.61) × 10−11 cm3 molecule−1 s−1, is approximately twice that of Carl and Crowley, and the latter discuss potential interferences from radical−radical reactions and uncertainties in amine concentrations as the source of the difference.

(R1)

The relative rate determinations are challenging; there is rapid uptake of MEA on the walls of the chamber, and the relative rate plots are reported to be curved over the entirety of the experiment (although the initially linear regions of the plots were used to extract rate coefficients), suggesting other possible removal processes. EUPHORE experiments are limited to ambient temperatures. In order to confirm the relative rate studies and to extend the rate coefficient data to a wider range of temperatures to give both practical and mechanistic information, we have studied reaction R1 using laser flash photolysis with laser-induced fluorescence detection of OH.8,9 © 2012 American Chemical Society

Received: February 17, 2012 Accepted: March 12, 2012 Published: March 12, 2012 853

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Carl and Crowley used two-photon photolysis of NO2 at 410−450 nm to generate O(1D), reacting the O(1D) with H2 as the source of OH. The relatively long wavelength photolysis light avoids complications from amine cophotolysis, although there are potential complications from OH regeneration via the reaction of any O(3P) with the amine. Other OH sources such as nitric acid could react with the amine prior to reaching the reaction cell. Most of our experiments have been carried out with t-butylhydroperoxide (t-BuOOH) photolysis at 248 nm as the OH precursor, with no obvious complications from either prereaction or amine cophotolysis, as demonstrated by the invariability of the returned rate coefficients with laser power. Additionally, OH was generated from the photolysis of acetone in the presence of oxygen;8 the returned value of k1 was independent of OH precursor. Figure 1 shows examples of the

Table 1. 296−510 K Data and Conditions for Reaction R1 temperature/K 296 296 376 476 510

OH precursor, pressure of N2 t-BuOOH, 25 Torr acetone/O2, 25 Torr t-BuOOH, 25 Torr t-BuOOH, 25 Torr t-BuOOH, 25 Torr

10−14 × [MEA]/ molecules cm−3

1011 × k1/cm3 molecule−1 s−1

0.5−5.5

7.62 ± 0.10a

0.5−5.5

7.59 ± 0.31

0.8−8

6.83 ± 0.02

0.6−6

5.41 ± 0.01

0.6−6

4.66 ± 0.02

a

Errors are statistical at the 2σ level from the bimolecular plots; see the text.

Figure 2. Temperature dependence of k1. An unweighted fit to the data gives k1= (7.73 ± 0.24) × 10−11(T/295)−(0.79±0.11) cm3 molecule−1 s−1.

Several theoretical studies12,13 have suggested that the reaction of OH with amines proceeds via complex formation. The complexes formed are relatively tightly bound for such systems; Solimannejad et al.12 calculated binding energies of 33−37 kJ mol−1 for OH with several simple aliphatic amines at varying levels of calculation (MP2, M05-2X, B3LYP). Tian et al.13 calculated a similar value of 35.4 kJ mol−1 for OH with methylamine at a higher level of calculation (CCSD(T)/6-311+ +G(2d,2p)//CCSD/6-31 G(d)). The negative temperature dependence observed in the temperature region of 296−510 K supports this argument. We have observed biexponential decays in the study of analogous amines (e.g., (CH3)2NCH2CH2OH), again supporting complex formation, and this behavior will be described in detail in a subsequent publication. The recent measurements on reaction R1 by Karl et al.7 were performed in the EUPHORE chamber, determining k1 relative to the reaction of OH with 1,3,5-trimethylbenzene (TMB) at 303.6 K ((9.3 ± 1.0) × 10−11 cm3 molecule−1 s−1) and 306 K ((8.1 ± 0.8) × 10−11 cm3 molecule−1 s−1). Simple averaging of the two experiments gives 8.7 × 10−11 cm3 molecule−1 s−1 at the mean temperature of 304.5 K. This result is in good agreement with the present determination. Both methods are subject to potential sources of error: wall loss, alternative removal processes, errors in the reference rate coefficient (itself measured relative to the reaction of OH with α-pinene14) for the relative method, uncertainties in amine concentration, undetected OH loss, or regeneration in these studies. However, the agreement from the two complementary studies at around room temperature suggests that the rate coefficient for the removal of OH is well-established for atmospheric modeling.

Figure 1. Bimolecular plots for the reaction of OH with MEA. The figure shows the results from three separate experiments taken on different days to illustrate the reproducibility of the results. MEA introduced by nitrogen bubbling, at 296 K and 25 Torr, with (■, black solid line) t-BuOOH OH precursor, and (▲, red dashed line) acetone/O2 OH precursor. (○, blue dotted line) MEA introduced by passing nitrogen over the bubbler, at 296 K and 17 Torr, with t-BuOOH. The inset to the figure shows a typical OH decay trace and fit to eq E1 with t-BuOOH as the OH source.

bimolecular plots obtained at room temperature using both methods of OH generation and also differing methods of delivering MEA into the reaction vessel. The different methods all give similar values for the room-temperature rate coefficient. The values of the experimental conditions for the study of reaction R1 and associated bimolecular rate coefficients from 296 to 510 K are listed in Table 1 and illustrated in Figure 2. The errors reported in Table 1 are statistical errors at the 2σ level from the bimolecular plots. We estimate that the total error (primarily from the determination of the amine concentration) is approximately 10%. Table 1 shows that the rate coefficient declines by approximately 40% over the temperature range of 296−510 K from 7.6 to 4.7 × 10−11 cm3 molecule−1 s−1. k1 can be parametrized as k1= (7.73 ± 0.24) × 10−11(T/295)−(0.79±0.11) cm3 molecule−1 s−1. The corresponding Arrhenius parametrization would be k1= 2.88 × 10−11 exp(2429 J mol−1/RT). Atkinson et al.11 observed negative temperature dependences for the reaction of OH with several aliphatic amines with rate coefficients decreasing by ∼17−22% over the temperature range of 298−425 K. 854

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us to determine the concentration of MEA in the absorption cell and hence determine the cross section from the measured absorption at 185 nm. The value determined was (8.53 ± 0.24) × 10−18 cm2. OH precursors (t-BuOOH, Sigma Aldrich (70% in water), and acetone (99.9% purity)) were purified by freeze−pump− thaw cycles, and they were made up and stored as dilute mixtures in nitrogen. The low vapor pressure of MEA made it impossible to make up mixtures of MEA, and hence, MEA was introduced into the cell from a bubbler of MEA. Two methods were used, first passing nitrogen over the bubbler flushing out the MEA head space vapor and second by bubbling nitrogen through the MEA. The first technique eliminates the potential for aerosol formation from bubble breaking, but only low concentrations could be obtained. In the second method, a wider range of MEA concentrations could be obtained. MEA was delivered through several bends in the tubing, which should minimize aerosol transfer. The reaction cell was heated by encasing the central portion of the cell, up to the windows of the cell, in a ceramic oven. Temperatures in the cell were measured using a K-type thermocouple mounted close to the observation region.

Karl et al. point out that the measured rate coefficient is significantly greater than that predicted from structure−activity relationships, and this discussion will not be repeated. The fast loss process (lifetime ≈ 40 min with respect to OH loss for [OH] = 5 × 106 molecules cm−3) suggests that the initial gasphase processing of MEA should be competitive with heterogeneous loss. Further studies on the mechanism and products of this reaction are currently in progress in our laboratory.

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EXPERIMENTAL METHODS Reactions R1 and R2 were studied by laser flash photolytic generation of OH, with the decay of OH being monitored under pseudo-first-order conditions by laser-induced fluorescence. The OH precursor, amine, and nitrogen bath gas were premixed and flowed into a stainless steel eight-way cross reactor. The total pressure in the cell (generally 25 Torr of nitrogen; some experiments at 17 Torr) was controlled by throttling the exit valve and measured using a baratron pressure gauge. OH was predominantly generated by the laser flash photolysis of t-butylhydroperoxide (t-BuOOH, (CH3)3COOH) at 248 nm using a KrF excimer laser (Lambda Physik 2101, 12.5 mJ pulse−1 cm−2). Following generation, the OH concentration was monitored by off-resonance laser-induced fluorescence. The 282 nm radiation (OH A2Σ, (ν = 1) ← X2Π, (ν = 0)) was generated by frequency doubling the 564 nm output of a pulsed Nd:YAG (Powerlite Precision II 8010) pumped dye laser (Sirah PRSC-DA-24, Rhodamine 6G). Fluorescence was monitored at 308 nm through an interference filter using a photomultiplier mounted perpendicularly to the axes of the photolysis and probe lasers. The time delay between the photolysis and probe pulses was varied to map out the OH profile, and a typical decay is shown in the inset to Figure 1. The OH signal, If, is given by eq E1 If, t = If,0e−k ′ t

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NERC, Grant Reference NE/ I013474/1.

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REFERENCES

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(E1)

where k′ is the pseudo-first-order rate coefficient, k′ = k1[amine] + kloss, and kloss represents the other first-order loss processes for OH, such as diffusional loss or reaction with the precursor. k′ was determined by a nonlinear least-squares fitting of eq E1 to the data with k′ as a variable parameter. In order to extract the bimolecular rate coefficient, k1, the experiment was repeated for a range of amine concentrations; plotting k′ versus [amine], as in Figure 1 yields k1 as the gradient. Amine concentrations were determined using in situ absorption spectroscopy. An Oriel mercury lamp was mounted at the entrance of one of the arms of the cell and the transmitted light detected at the other side of the cell through a 185 nm filter by a Perkin-Elmer photomultiplier, with the absorption occurring over a path length of 31 cm. Absorptions were typically in the region of 3−15%. In order to convert absorption into absolute amine concentration, the absorption cross section for the amine needs to be known. This has not been determined for MEA. MEA poses particular challenges due to its low vapor pressure (40 Pa at 298 K15). In order to determine the cross section, we directly mounted a short (10 cm) absorption cell onto a glass bubbler. MEA was rapidly passed through the cell. In some experiments, a slow flow of nitrogen gas blew over MEA, while in other cases, we did not apply any nitrogen flow. The vapor pressure of MEA as a function of temperature has been measured by Kapteina et al.,15 allowing 855

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(10) 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, 8131−8141. (11) Atkinson, R.; Perry, R. A.; Pitts, J. N. Rate Constants for Reactions of OH Radical with (CH3)2NH, (CH3)3N, and C2H5NH2 over the Temperature Range 298−426 K. J. Chem. Phys. 1978, 68, 1850−1853. (12) Solimannejad, M.; Nielsen, C. J.; Scheiner, S. Complexes Pairing Aliphatic Amines with Hydroxyl and Hydroperoxyl Radicals: A Computational Study. Chem. Phys. Lett. 2008, 466, 136−140. (13) Tian, W.; Wang, W. L.; Zhang, Y.; Wang, W. N. Direct Dynamics Study on the Mechanism and the Kinetics of the Reaction of CH3NH2 with OH. Int. J. Quantum Chem. 2009, 109, 1566−1575. (14) Aschmann, S. M.; Long, W. D.; Atkinson, R. TemperatureDependent Rate Constants for the Gas-Phase Reactions of Oh Radicals with 1,3,5-Trimethylbenzene, Triethyl Phosphate, and a Series of Alkylphosphonates. J. Phys. Chem. A 2006, 110, 7393−7400. (15) Kapteina, S.; Slowik, K.; Verevkin, S. P.; Heintz, A. Vapor Pressures and Vaporization Enthalpies of a Series of Ethanolamines. J. Chem. Eng. Data 2005, 50, 398−402.

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dx.doi.org/10.1021/jz300200c | J. Phys. Chem. Lett. 2012, 3, 853−856