Synergistic Formation of Carboxyl and Methyl Radicals in CO2 +

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Synergistic Formation of Carboxyl and Methyl Radicals in CO+Methane Mixed Gas Hydrates 2

Motoi Oshima, Kazuma Kitamura, Atsushi Tani, Takeshi Sugahara, and Kazunari Ohgaki J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp506264k • Publication Date (Web): 16 Jul 2014 Downloaded from http://pubs.acs.org on July 17, 2014

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The Journal of Physical Chemistry

Synergistic Formation of Carboxyl and Methyl Radicals in CO2+Methane Mixed Gas Hydrates Motoi Oshima*,†,§, Kazuma Kitamura‡, Atsushi Tani*,†, Takeshi Sugahara‡, and Kazunari Ohgaki‡ †

Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1

Machikaneyama, Toyonaka, Osaka 560-0043, Japan ‡

Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University,

1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

ACS Paragon Plus Environment


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ABSTRACT

The formation mechanisms of gamma-ray-induced carboxyl (HOCO) and methyl radicals in CO2+methane mixed gas hydrates, which are inclusion compounds of H2O, CO2, and methane, were investigated. The HOCO and methyl radicals were observed in CO2+methane mixed gas hydrates by electron spin resonance (ESR) at 120 K after irradiation at 77 K. The amounts of the HOCO and methyl radicals induced in the mixed hydrates are much higher than those in pure CO2 and methane hydrates. Both radicals are synergistically formed in the mixed hydrates by efficient reactions between the guest molecules (CO2 and methane) and the active species (electron, proton, and hydroxyl radical) induced from H2O.

Keywords: Carboxyl radical, Methyl radical, Clathrate hydrates, Electron spin resonance (ESR), gamma-ray-irradiation


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1. INTRODUCTION The carboxyl (HOCO) radical is an intermediate in the reaction of hydroxyl (OH) radical with carbon monoxide (CO), “CO + OH·→CO2 + H·”, and acts as a hydrogen donor to the reaction partners.1 The methyl (CH3) radical is well known as a precursor of other hydrocarbons and alcohols (for example, ethane and methanol) and as an oxidant to the reaction partners.2 The HOCO and methyl radicals are important active species for oxidation, reduction, and molecular evolution in combustion chemistry, atmospheric chemistry, and planetary science.1–4 The HOCO radical is not only formed by the reaction of “CO + OH·”. Once a water molecule is exposed to gamma-radiation, it dissociates to active species by reactions 1 and 2.5 If water and CO2 coexist, the electron and proton induced from H2O can further react with CO2 leading to the formation of the HOCO radical by reactions 2–4.6,7 H2O + gamma-ray

→ OH· + H·

(1)

H2O + gamma-ray

→ H2O+ + e–

CO2 + e–

→ CO2–·

(3)

CO2–· + H+

→ HOCO·

(4)

→ OH· + H+ + e–

(2)

In the formation of methyl radical in the mixture of methane and water, the following three reactions are mainly considered: direct dissociation of methane by radiolysis (reactions 5 and 6), and the hydrogen abstraction reaction between the methane molecule and the OH radical (reaction 7) induced by radiolysis of the water molecule (reactions 1 and 2).2,8,9 CH4 + gamma-ray

→ CH3· + H·


(5)


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CH4 + gamma-ray

→ CH4+ + e–

CH4 + OH·

→ CH3· + H2O

→ CH3· + H+ + e–

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(6)

(7)

Comparing the formation processes of HOCO and methyl radicals, it can be seen that the HOCO radical is seemingly formed by the addition of a hydrogen atom to CO2, whereas the methyl radical is formed by the abstraction of a hydrogen atom from methane. If both CO2 and methane coexist in a matrix, both HOCO and methyl radicals will be formed synergistically because hydrogen released from methane may finally transfer to CO2. Since each radical is very reactive and has a short lifetime, the formation efficiencies of these radicals in the mixed matrices (for example, matrices including H2O, CO, CO2 and methane) have not been quantitatively estimated. In the present study, the clathrate hydrates were used as reaction fields for investigating the formation processes of the HOCO and methyl radicals. Clathrate hydrates (or gas hydrates) are crystalline compounds of water molecules (hydrate cages) and so-called “guest molecules”. The guest molecules generally interact with the hydrate cages through weak van der Waals forces.10 In the case of gamma-ray-irradiated hydrocarbon hydrates, alkyl radicals induced from the hydrocarbon molecules in the hydrate cages seem to be stable below the three-phase (i.e., hydrate, ice, and gas phases) equilibrium temperature of each hydrocarbon hydrate at atmospheric pressure.8,11–15 In addition, the carboxyl radical7,16 and the hydrogen atom7,8,11–15,17–19 in the hydrate cages are more stable than those in the other matrices. It means that each hydrate cage is a particular holder for a free radical. The crystal structure of the CO2, methane, and CO2+methane mixed gas hydrates is called “structure I”, which consists of two small cages (512, dodecahedron) and six large cages (51262, tetrakaidecahedron) in the crystal lattice. In the mixed hydrates, the CO2 molecules are entrapped


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into the large cages prior to the small cages comparing to methane, because the CO2 molecule is larger than the methane molecule.20

2. EXPERIMENTAL METHODS Hereafter, xCO2 and xCH4 represent the mole fractions of CO2 and methane respectively, on a water free basis in the gas hydrates. The CO2+methane mixed gas hydrates (xCO2 = 0.19, 0.40, 0.79, 0.97: the maximum uncertainty of the mole fraction is 0.01), pure CO2 hydrate (xCO2 = 1), and pure methane hydrate (xCO2 = 0) were prepared from ultrapure water or distilled water (8.0 cm3) with CO2 and methane in a high-pressure vessel. The CO2+methane mixed gas hydrate (xCO2 = 0.70) and pure CO2 hydrate were prepared from heavy water (Cambridge Isotope Laboratories, Inc., 99.9%, 8.0 cm3). In the preparation of the mixed gas hydrates, CO2 and methane (CO2, Neriki Gas, 99.99% grade; methane, Neriki Gas, 99.9995% grade) were mixed in advance and the mixture was added to the vessel until the vessel pressure increased to 4.8– 7.3 MPa at 276.0 ± 0.5 K. The gas mixture was not added to the vessel after the pressure dropped down due to the hydrate formation. In the pure methane hydrate, methane was supplied to the vessel until the vessel pressure increased to 7.0–7.5 MPa at 276.0 ± 0.5 K. The procedure for the pure CO2 hydrate preparation has been described previously.7 In this procedure, approximately 2.0–2.6 × 10–2 moles of gases were consumed to produce the hydrates. After the high-pressure vessel was transferred to a low temperature chamber, the hydrates were taken from the vessel at 243 ± 3 K and atmospheric pressure. Small pieces of the hydrate with a diameter of 1–2 mm were collected using metal sieves and placed in a plastic vial, which was then stored at ~77 K. The hydrate formations of the samples were confirmed by Raman spectroscopy. The xCO2 in the



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mixed hydrates was analyzed by the use of TCD-gas chromatography (SHIMADZU, GC-7AG). The total amount of gas molecules in the samples (nCO2+CH4) was estimated from the ratio of the amount of gases consumed in the sample preparations and the sample volume in the ESR sample tube. The amounts of CO2 and CH4 (nCO2 and nCH4) in the samples were estimated from the nCO2+CH4 and the xCO2 analyzed from TCD-gas chromatography. The synthetic hydrate samples in the vial were irradiated by gamma-ray using a 60Co source at a dose rate of 0.2–0.3 kGy per hour for 6 hours (total absorbed dose was 1.2–1.8 kGy) in liquid nitrogen (77 K). The irradiated samples were measured at atmospheric pressure by using a commercial X-band ESR spectrometer (JEOL, JES-FA200). The measurement temperature was controlled using a nitrogen gas flow unit system (JEOL, ES-DVT4) or a dewar vessel of liquid nitrogen (77 K). The microwave power was 0.2 mW in the measurements, and the 100 kHz field modulation width was 0.1 mT. The amounts of HOCO and methyl radicals were estimated by the observations at 120 K. The hydrate samples prepared with D2O (xCO2 = 1 and 0.70), and the hydrate prepared with H2O (xCO2 = 1) were observed at 77 K. The total amount of both radicals (nHOCO+CH3) was estimated by integration of the whole spectra. The amount of methyl radicals (nCH3) was calculated using integrated value of the smaller signal at lower magnetic field because the two central intense signals apparently overlapped with the signal of the HOCO radical. This integrated value was converted to the whole value of the signal for nCH3 by taking the amount of methyl radical in pure methane hydrate (xCO2 = 0) into account. The amount of the HOCO radical (nHOCO) was estimated by subtracting nCH3 from nHOCO+CH3.


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3. RESULTS AND DISCUSSION Figure 1 shows the ESR spectra of the gamma-ray-irradiated pure methane hydrate (xCO2 = 0), CO2+methane mixed gas hydrates (xCO2 = 0.19, 0.40, 0.79, 0.97), and pure CO2 hydrate (xCO2 = 1) at 120 K. Each spectral intensity in the vertical axis is normalized by the sample volume and the radiation dose. The HOCO radical appears at the center of the spectra (denoted with open diamond in Figure 1) and the methyl radical shows quartet signals with signal intensity ratio of 1:3:3:1 (denoted with open triangles). The signals of both HOCO and methyl radicals are observed in the irradiated mixed hydrates, whereas only the individual signals of these radicals are observed in the pure CO2 and the pure methane hydrates, respectively. The peak heights, i.e., the ESR signal intensities of the HOCO and methyl radicals apparently change with xCO2. The intensities in the mixed hydrates are much higher than those in the pure CO2 and methane hydrates, even though the mixed hydrates contain smaller amounts of CO2 and methane than the pure hydrates. Although the signal of the hydrogen atom is observed in all the compositions, it promptly disappears at 120 K. The OH radical is apparently observed only in the pure CO2 hydrate (xCO2 = 1) and also quickly disappears at 120 K. The amount of the OH radical is about 10% of the HOCO radical. The results are concordant with the previous study.7 Although a small amount of OH radical is induced in all the compositions during irradiation, it will quickly react with methane in the mixed hydrate as well as the pure methane hydrate at 77 K. Therefore, we are focusing on only two radical species i.e., HOCO and methyl radicals in the following quantitative discussion.



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Figure 1. ESR spectra of the irradiated hydrate samples at 120 K for different xCO2: (a) 0 (pure methane hydrate), (b) 0.19, (c) 0.40, (d) 0.79, (e) 0.97, and (f) 1 (pure CO2 hydrate). Open diamond, open triangles, and closed triangle stand for the signals of the HOCO radical, methyl radical, and Mn2+ (marker), respectively. The ESR intensities were normalized by each sample volume and irradiation dose. The g-factor of HOCO and methyl radicals is gave ≈ 2.001 and g = 2.0029, respectively.


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Figure 2 shows the molar ratios of both the radicals to all the guest molecules (nHOCO/nCO2+CH4 and nCH3/nCO2+CH4). The nHOCO/nCO2+CH4 ratio in the pure CO2 hydrate (xCO2 = 1) is 3 × 10–6 (3 ppm). It gradually increases with a decrease in xCO2 and is 45 ppm at xCO2 = 0.19. In contrast, the nCH3/nCO2+CH4 ratio increases drastically from 10 ppm in pure methane hydrate (xCO2 = 0, xCH4 = 1) to 47 ppm at xCO2 = 0.19 (xCH4 = 0.81). With a further increase in xCO2, the nCH3/nCO2+CH4 ratio at xCO2 = 0.40 (xCH4 = 0.60) is almost the same as that at xCO2 = 0.19 (xCH4 = 0.81), and decreases to 18 ppm at xCO2 = 0.97 (xCH4 = 0.03). The yields of radical formation, i.e., the molar ratios of the HOCO radical to CO2 (nHOCO/nCO2) and the methyl radical to methane (nCH3/nCH4), were calculated in the mixed hydrate as well as the pure hydrates, and are summarized in Table 1. The mixed hydrates show higher yields of both HOCO and methyl radicals in comparison with those in pure CO2 and methane hydrates, respectively. These results show that CO2 effectively converts to the HOCO radical in the mixed hydrates in spite of the low amount of CO2 molecules enclathrated in the hydrates. The conversion from methane molecule to methyl radical also shows a trend similar to the HOCO radical.



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Figure 2. Molar ratios of the HOCO radical to the CO2+methane mixture (nHOCO/nCO2+CH4) and the methyl radical to the CO2+methane mixture (nCH3/nCO2+CH4) in the irradiated hydrate samples (xCO2 = 0, 0.19, 0.40, 0.79, 0.97, 1). The amounts of the radicals are normalized by irradiation dose. The estimated uncertainties of molar ratio (radical to guests) are smaller than the size of the symbols.


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Table 1. Yields of HOCO and Methyl Radicals (nHOCO/nCO2 and nCH3/nCH4, respectively) in the Irradiated Hydrate Samples (xCO2 = 0, 0.19, 0.40, 0.79, 0.97, 1) Molar ratio (10–6)a xCO2

a

nHOCO/nCO2

nCH3/nCH4

0

–

10

0.19

234

58

0.40

97

80

0.79

21

161

0.97

8

606

1

3

–

The estimated uncertainties of both molar ratios are within 5%.

Two reaction models can be considered for the effective radical formation in the mixed hydrates. These radicals would be formed in Model A by transferring hydrogen from methane to CO2 (not via H2O), and/or in Model B by reaction through active species induced from H2O. The following reactions can be considered in Model A. CH4 + gamma-ray

→ CH4+ + e–

→ CH3· + H+ + e–

CO2 + e–

→ CO2–·

(3)

CO2–· + H+

→ HOCO·

(4)


(6)


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In the first step (reaction 6), a methane cation (CH4+) and an electron are induced by radiolysis of methane.20 CH4+ is unstable even at 77 K, and quickly decomposes to a methyl radical and a proton. In this case, the HOCO radical would be formed by the additional reactions of CO2 with the secondary electron (reaction 3) and proton (reaction 4) induced from methane. If the reactions in Model A dominantly occur in the mixed hydrates, the amounts of the induced electron and proton in the mixed hydrates would be larger than those in pure CO2 hydrate because the electron and proton are provided not only by H2O (reaction 2) but also by methane (reaction 6) to CO2 in the mixed hydrates. In addition, the inverse reaction for methane formation would occur less often in the mixed hydrates than in the pure methane hydrate, because the electron and proton, which can combine to form a hydrogen atom, are consumed by the reaction with CO2. In this case, the methyl radical remains more in the mixed hydrate, which agrees with the present experimental results. In Model B, the following reactions can be considered. H2O + gamma-ray

→ H2O+ + e–

→ OH· + H+ + e–

CH4 + OH·

→ CH3· + H2O

CO2 + e–

→ CO2–·

(3)

CO2–· + H+

→ HOCO·

(4)

(2) (7)

Active species such as the OH radical, electron, and proton are induced by the radiolysis of H2O (reaction 2),5 and they react with methane (reaction 7) and CO2 (reactions 3 and 4). If the reactions of Model B are dominant in the mixed hydrates, the active species induced from H2O are more efficiently provided to CO2 and methane without the inverse reaction to form H2O. The


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reactions of Model B would also inhibit the re-formations of CO2 and methane such as “HOCO· + OH·” and “CH3· + H·”. The present experimental results are also supported by this model. To investigate whether Model A or B results in efficient radical formation, the induced radicals in the mixed hydrates prepared with D2O were observed at 77 K. It has been reported that the deuterated carboxyl (DOCO) radical is induced in gamma-ray-irradiated CO2 hydrate prepared with D2O.7 The identity of the radical formed in mixed hydrates provides a means to distinguish the two models. Specifically, methane is the source of hydrogen for the HOCO radical in Model A, while D2O is the source of deuterium for DOCO in Model B. It has been reported that the spectral shape of the DOCO radical differs to that of the HOCO radical because these radicals have hyperfine structures of hydrogen (deuterium).7,21 The ESR spectra of the mixed hydrate (D2O) as well as the pure CO2 hydrates with H2O and D2O are shown in Figure 3 for comparison. The results clearly show that the central signal in the mixed hydrate (D2O) is much closer to the signal of the DOCO radical in the pure CO2 hydrate (D2O). It means that the deuterium atom is dominantly supplied from D2O for the DOCO radical formation. Therefore, we conclude that the formation processes of these radicals according to Model B are dominant in the mixed hydrates.



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Figure 3. ESR spectra in the irradiated hydrate samples at 77 K: (a) CO2+methane mixed gas hydrate prepared with D2O (xCO2 = 0.70), (b) pure CO2 hydrate prepared with D2O, and (c) pure CO2 hydrate prepared with H2O. The dotted line shows the typical peak on the signal of the HOCO radical, and the broken lines show the typical peaks on that of the DOCO radical. Open triangles and closed triangle stand for the signals of the methyl radical and Mn2+ (marker), respectively. The spectra of (b) and (c) are reproduced from the previous study.7

The compositional effect on the nHOCO/nCO2+CH4 ratio drastically differs from that of the nCH3/nCO2+CH4 ratio as shown in Figure 2; the maximum values of the nHOCO/nCO2+CH4 ratio and the nCH3/nCO2+CH4 ratio are observed around xCO2 = 0.19. In the Model B, the HOCO radical is formed through a two-step process (reactions 3 and 4), whereas the methyl radical is formed through a one-step reaction (reaction 5). Therefore, the compositions with the highest yields may shift from xCO2 = 0.50 (xCH4 = 0.50) to a smaller composition of xCO2. Additionally, compared to methane, CO2 molecules are entrapped into the large cages in the mixed hydrates, and the cage


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occupancy changes with the composition.19 The HOCO radical would only occupy the large cages of structure I owing to its larger size,7 whereas the methyl radical can occupy either cage.8,22 The cage occupancies may also affect the yield of these radicals in the mixed hydrates. The similar phenomenon would occur in the mixed hydrates including CO2 and the other molecules with hydrogen atoms (e.g., alkane molecules), and may also occur in the other icy matrices including H2O together with different molecules: one (e.g., hydrocarbons and alcohols) can provide hydrogen atom to the OH radical, while another (e.g., CO2) can capture the electron and proton. Clathrate hydrates may be more effective matrices as formation fields of these radicals because each molecule is structurally arranged at appropriate sites.

4. CONCLUSIONS The formation processes of the HOCO and methyl radicals in the gamma-ray-irradiated CO2+methane mixed gas hydrates were investigated by ESR measurements. The amounts of the induced HOCO and methyl radicals in the mixed hydrates are much larger than those in the pure hydrates. The conversion ratio from CO2 to HOCO radical increases with a decrease in the xCO2, and that from methane to methyl radical shows a similar trend. Since the DOCO radical is predominant in mixed hydrates prepared with D2O, it can be deduced that the water molecule, not methane, is the source of hydrogen for the HOCO radical. These results reveal that the electron, proton, and OH radical induced from H2O efficiently react with CO2 and methane in the mixed hydrates, and the inverse reaction to form H2O and the re-formations of CO2 and methane are inhibited; the HOCO and methyl radicals would be synergistically formed.



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AUTHOR INFORMATION Corresponding Authors *Motoi Oshima, Phone +81-11-857-8407, Email: [email protected] *Atsushi Tani, Phone: +81-6-6850-5540, E-mail: [email protected]

Present Addresses § Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohiraku, Sapporo, Hokkaido 062-8517 Japan

Funding Sources This study was supported by Grant-in-Aid for JSPS Fellows No. A2412090 and Scientific Research (A) No. 21246117. We also acknowledge the scientific supports from the “Gas-Hydrate Analyzing System (GHAS)” of the Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors thank Prof. S. Sasaki, Mr. Yoshito Katsuta, all members in our laboratory at Osaka University, and Dr. T. Uchida at Hokkaido University for their help and advice. We appreciate Dr. T. Yamamoto for his support for 60Co gamma-ray irradiation at the Institute of Scientific and Industrial Research of Osaka University.

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(7) Oshima, M.; Tani, A.; Sugahara, T.; Kitano, K.; Ohgaki, K. Reactions of HOCO Radicals through Hydrogen-Atom Hopping Utilizing Clathrate Hydrates as an Observational Matrix. Phys. Chem. Chem. Phys. 2014, 16, 3792–3797. (8) Takeya, K.; Tani, A.; Yada, T.; Ikeya, M.; Ohgaki, K. Electron Spin Resonance Study on γRay-Induced Methyl Radicals in Methane Hydrates. Jpn. J. Appl. Phys. 2004, 43, 353–357. (9) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; 3rd ed.; Taylor & FrancisCRC Press: Boca Raton, FL, 2008. (10) Takeya, K.; Sugahara, T.; Ohgaki, K.; Tani, A. Electron Spin Resonance Study on γ-RayInduced Radical Species in Ethylene Hydrate. Radiat. Meas. 2007, 42, 1301-1306. (11) Takeya, K.; Nango, K.; Sugahara, T.; Ohgaki, K.; Tani, A.; Ito, H.; Okada, M.; Kasai, T. Electron Spin Resonance Study on γ-Ray-Induced Ethyl Radical in Ethane Hydrate. Jpn. J. Appl. Phys. 2007, 46, 3066–3070. (12) Tani, A.; Ishikawa, K.; Takeya, K. Thermal Stability of Methyl Radical in γ -RayIrradiated Methane Hydrate under Different Pressure from 0.003 to 1 MPa, Radiat. Meas. 2006, 41, 1040–1044. (13) Ohgaki, K.; Nakatsuji, K.; Takeya, K.; Tani, A.; Sugahara, T. Hydrogen Transfer from Guest Molecule to Radical in Adjacent Hydrate-Cages. Phys. Chem. Chem. Phys. 2008, 10, 80– 82. (14) Takeya, K.; Tani, A.; Sugahara, T.; Ohgaki, K. Thermal Stability of Radicals Induced in Xenon Hydrate. Phys. Chem. Ice 2010, Hokkaido University Press, Sapporo, 2011, 267–271.


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Synergistic Formation of Carboxyl and Methyl Radicals in CO2 +

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