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Gas Phase Reactions of OH with Methyl Amines in the Presence or Absence of Molecular Oxygen. An Experimental and Theoretical Study Lavinia Onel, Lucy Thonger, Mark A. Blitz, Paul William Seakins, Arne J. C. Bunkan, Mohammad Solimannejad, and Claus Jørgen Nielsen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp406522z • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on October 1, 2013
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Gas Phase Reactions of OH with Methyl Amines in the Presence or Absence of Molecular Oxygen. An Experimental and Theoretical Study
L. Onel, L. Thonger, M.A. Blitz and P.W. Seakins* School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK A.J.C. Bunkan, M. Solimannejada and C.J. Nielsen* CTCC, Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern, 0315 Oslo, Norway
Abstract The rate coefficients for the reaction of OH with the alkyl amines: methylamine (MA), dimethylamine (DMA), trimethylamine (TMA) and ethylamine (EA) have been determined using the technique of pulsed laser photolysis with detection of OH by laser induced fluorescence as a function of temperature from 298 K to ~600 K. The rate coefficients (1011 × k/ cm3 molecule-1 s-1) at 298 K in nitrogen bath gas (typically 5 – 25 Torr) are: kOH+MA = 1.97 ± 0.11, kOH+DMA = 6.27 ± 0.63, kOH+TMA = 5.78 ± 0.48, kOH+EA = 2.50 ± 0.13. The reactions all show a negative temperature dependence which can be characterised as: T k OH+MA = (1.889 ± 0.053) × 10 −11 298K
T k OH+TMA = (5.73 ± 0.15) ×10−11 298K
− ( 0.56± 0.10 )
− ( 0.71±0.10)
, k OH+ DMA = (6.39 ± 0.35) ×10 −11 T 298K
, k OH+EA = (2.54 ± 0.08) ×10−11 T 298K
− ( 0.75± 0.18 )
,
− ( 0.68±0.10)
.
OH and OD reactions have very similar kinetics. Potential energy surfaces (PES) for the reactions have been characterized at the MP2/aug-cc-pVTZ level and improved single point energies of stationary points obtained in CCSD(T) and CCSD(T*)-F12a calculations. The PES for all reactions are characterised by the formation of pre and post reaction complexes and submerged barriers. The calculated rate coefficients are in good agreement with experiment; the overall rate coefficients are relatively insensitive to variations of the barrier heights within typical chemical accuracy, but the branching ratios vary significantly. The rate coefficients for the reactions of OH/OD with MA, DMA and EA do not vary with added oxygen, but for TMA a significant reduction in the rate coefficient is observed consistent with OH recycling from a chemically activated peroxy radical. OH regeneration is pressure dependent and is not significant at 298 K and atmospheric pressure, but the efficiency of recycling increases strongly with temperature. The PES for OH recycling have been calculated. There is evidence that the primary process in TMA photolysis at 248 nm is the loss of H atoms. Keywords: Kinetics, Amine Oxidation, Atmospheric Lifetimes, OH recycling. a - Permanent address: Arak Univ, Department of Chemistry, Arak 3815688349, Iran. E-mail:
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Introduction Methyl amines (CH3NH2, methylamine - MA, (CH3)2NH, dimethylamine - DMA, (CH3)3N, trimethylamine - TMA) are released into the atmosphere from a variety of sources including animal husbandry, food processing, marine sources and biomass burning as detailed in a review of amine chemistry by Ge et al.1 who also review previous measurements of atmospheric concentrations. Annual emission rates of methyl amines in 1995 were estimated to be in the order of 300 Gg N per annum by Schade and Crutzen.2 The proposed use of monoethanol amine (MEA, HOCH2CH2NH2) in carbon capture3 will significantly increase amine release including methylamine emissions as these can be formed during degradation of the MEA during repeated capture/desorption cycles.4,5 The atmospheric fate of amines includes heterogeneous uptake6 or gas phase oxidation, primarily via reaction with the OH radical.5 There have been limited measurements of the kinetics of OH reactions with simple alkyl amines7-10, predominantly at or near room temperature, or more complex amines such as MEA11-13 and morpholine,14 but data suggest that gas phase oxidation should be competitive with heterogeneous uptake, with a lifetime with respect to removal by OH of a few hours. The atmospheric chemistry of amines has been reviewed recently by Nielsen et al.5 who emphasise the need for additional kinetic and mechanistic data. Particular concern exists for products such as nitrosamines (R2N-NO) and nitramines (R2N-NO2), which are known carcinogens15 and are formed as minor products following abstraction of H from the N-H site. The main pathway following abstraction of an N-H bond from a primary amine is imine formation5, e.g. OH + CH3NH2 → CH3NH + H2O
(R1a)
CH3NH + O2 → CH2=NH + HO2
(R2)
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The rate of reaction of oxygen with an R2N radical formed from the abstraction of N-H from a secondary amine is very slow under atmospheric conditions and hence there is a greater potential for nitrosamine and nitramine formation. Abstraction of a hydrogen atom from the alkyl group is thought to proceed via conventional oxidation pathways to form an amide, e.g. OH + CH3NH2 → CH2NH2 + H2O
(R1b)
CH2NH2 + O2 → OOCH2NH2
(R3a)
OOCH2NH2 + NO → OCH2NH2 + NO2
(R4)
OCH2NH2 + O2 → (O)HCNH2 + HO2
(R5)
The atmospheric fate of amides has recently been reviewed by Barnes et al.16 Reaction 3a is in competition with 3b leading to imine formation: CH2NH2 + O2 → CH2=NH + HO2
(R3b)
Few reports of direct measurements exist on the branching ratio for the two OH abstraction channels in alkyl amines, but it is thought that abstraction from the alkyl group dominates.5 For example in their recent study on MEA oxidation, Karl et al.12 simulated product data with 80% of the OH + MEA reaction proceeding via abstraction of the α hydrogen to the NH2 group and only 15% from the NH2 group while Nielsen et al. reported 84% and 8%, respectively.17 N-H bond strengths decrease from primary to secondary amines18 and hence abstraction of the nitrogen bonded hydrogen may play a greater role in secondary amine chemistry. Studies of the temperature dependence of the reaction of OH with amines provide information on the mechanism of the reactions but also, as amines are components of fuel 3 ACS Paragon Plus Environment
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nitrogen, they provide practical information for combustion modellers. Additionally, as a potential mechanism for removal of waste material from carbon capture processes is incineration, more information is required on higher temperature kinetics. In this paper we use laser flash photolysis coupled to laser induced fluorescence detection of OH to investigate the kinetics of the reactions of OH with MA, DMA and TMA as a function of temperature (298 - ~600 K) and oxygen. OH + CH3NH2 → products
(R1)
OH + (CH3)2NH → products
(R6)
OH + (CH3)3N → products
(R7)
Additionally the reaction of OH with ethylamine (R8) was studied to compare with previous measurements. OH + C2H5NH2 → products
(R8)
In the presence of oxygen, the OH + TMA reaction regenerates OH radicals therefore we report on the yield of OH as a function of temperature and pressure. Whilst the OH yield at atmospheric conditions is insignificant, such processes provide mechanistic insights into amine oxidation systems. The experimental work is supported by theoretical calculations of the potential energy surfaces for OH abstractions and for recycling of OH in the presence of oxygen.
Experimental Kinetic studies of OH radical reactions with MA, DMA, TMA and EA took place in a slow flow, stainless steel reaction cell. The apparatus was similar to that described in previous 4 ACS Paragon Plus Environment
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publications.11,19 The reaction cell (volume ~ 1000 cm3) is a 10-way cross with four axes in the horizontal plane, two of which are used for the photolysis and OH probe lasers, with OH fluorescence being observed in the mutually perpendicular axis above the reaction cell. The OH precursor (if used), amine and oxygen (if used, 99.999%) were introduced along with the carrier gas nitrogen (99.998%) or helium (CP Grade 99.999%), through calibrated mass flow controllers and a mixing manifold before flowing into the reaction cell. The total pressure (measured using a baratron pressure gauge 0-100 or 0-1000 Torr) in the cell (typically 5-25 Torr for kinetic measurements for MA, DMA and EA and extending to 160 Torr for TMA) was controlled via a needle valve between the reaction cell and rotary pump. The total flow rate (typically ~130 sccm) was such that a fresh portion of gas was irradiated with each photolysis pulse. Tests showed that the returned rate coefficients were independent of total flow rate as long as fresh gas was irradiated with each pulse. The cell could be heated by means of cartridge heaters inserted into the central body of the cell. Temperatures were measured by thermocouples located just above and below the reaction zone. The OH precursors (tertiary butylhydroperoxide, Sigma Aldrich, 70% in H2O or acetone/O2) and amines (all Sigma Aldrich, MA >98%, DMA 99%, TMA 99%, EA 99%) were subject freezepump-thaw cycles and made up as dilute mixtures in nitrogen or helium, stored in darkened five litre bulbs. For TMA the stated impurities are NH3 (
