Hydrogen Production in Aromatic and Aliphatic Ionic Liquids - The

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Hydrogen Production in Aromatic and Aliphatic Ionic Liquids Surajdevprakash B. Dhiman, George S Goff, Wolfgang Runde, and Jay A LaVerne J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp402502d • Publication Date (Web): 15 May 2013 Downloaded from http://pubs.acs.org on May 22, 2013

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The Journal of Physical Chemistry

Hydrogen Production in Aromatic and Aliphatic Ionic Liquids Surajdevprakash B. Dhiman a George S. Goffb, Wolfgang Rundec and Jay A. LaVernead* a

Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556 Chemistry Division, Los Alamos National Laboratory, Los Alamos NM, 87545 c Science Programs Office, Los Alamos National Laboratory, Los Alamos NM, 87545 d Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556 Corresponding author: [email protected] b

Abstract The radiolytic production of molecular hydrogen in the ionic liquids N-trimethyl-Nbutylammonium

bis(trifluoromethanesulfonyl)imide

[N1114][Tf2N]

and

1-ethyl-3-

methylimidazolium bis(trifluoromethylsulfonyl)imide [emim][Tf2N] has been examined with γ-rays, 2-10 MeV protons and 5-20 MeV helium ions to determine the functional dependence of the yield on particle track structure. Molecular hydrogen is the dominant gaseous radiolysis product from these ionic liquids, and the yields with γ-rays are 0.73 and 0.098 molecules per 100 eV of energy absorbed for [N1114][Tf2N] and [emim][Tf2N], respectively. These low yields are consistent with the relative insensitivity of most aromatic compounds to radiation. However, the molecular hydrogen yields increase considerably on going from γrays to protons to helium ions with [emim][Tf2N] while they remain essentially constant for [N1114][Tf2N]. FTIR and UV-Vis spectroscopic studies show slight degradation of the ionic liquids with radiation.

Keywords: radiolysis, H2 production, track effects

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1. Introduction Ionic liquids (ILs) have received increased attention in the last few years because of their unique properties compared to those of common organic solvents

1-9

. The interest for

ILs stems from their potential as green solvents because of their negligible vapor pressure, low flammability and thermal stability that make them attractive alternatives for volatile organic solvents 6,7. These properties can be tuned by possible variation of anions, cations and functional groups of these ILs 10. Because of their characteristics as environmentally friendly solvents, ILs have been used in range of chemical processes and also as constituents in electrochemical applications

11-13

. Ionic liquids are being extensively explored for various

catalytic reactions and separation processes

14-17

. Studies have demonstrated that ILs can be

used for solvent extraction of metal species from aqueous media, which is encouraging for the potential use of ILs for the reprocessing of used nuclear fuel

17-21

. The wide

electrochemical window of ILs will potentially enable direct electrodeposition processes, which could potentially reduce the number of steps for nuclear waste treatment or reprocessing of used nuclear fuel. Ionic liquids have shown promise as alternatives to high temperature molten salts for the direct deposition of electropositive metals such as lanthanides and uranium metal. Alternatively, Giridhar et al. demonstrated the feasibility for extracting uranium (U(VI)) from nitric acid solutions using tri-n-butylphosphate (TBP) with IL diluents, followed by electrodeposition of UO2 from the IL phase 22-25. However, for ILs to find application in separations technologies for advanced nuclear fuel cycles, they must be chemically stable in the high radiation fields of the fission products and their radioactive decay products. The radiation chemistry of several ILs suggested for use in nuclear separations processes have been probed by a variety of research groups 26-35. A preliminary assessment of the radiation effects of α, β and γ on imidazolium cation based hydrophilic ILs such as 2 ACS Paragon Plus Environment

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[bmim][NO3], [emim][Cl], and [hmim][Cl] where [bmim], [emim] and [hmim] are 1-butyl-3methylimidazolium,

1-ethyl-3-methylimidazolium

and

1-hexyl-3-methylimidazolium,

respectively, show that less than 1 % of the samples undergo radiolysis even when exposed to a dose of 400 kGy, which suggests a very high radiation stability similar to that of benzene 26. This work presents the only published results for radiolysis of ILs with alpha particles. Other hydrophobic ILs containing 1-butyl-3-methylimidazolium cation (bmim) and inorganic anions such as hexaﬂuorophosphate (PF6−) and bis(triﬂuoromethylsulfonyl)imide [Tf2N]− exhibit high stability up to 1200 kGy

27

. The degradation products were identified using

spectrometric techniques such as NMR and electrospray ionization mass spectrometry (ESIMS). Further detailed investigations of the γ-radiolysis of [bmim][PF6] 31 and [bmim][BF4] 32 were carried out by spectrophotometric ATR-IR and differential scanning calorimetry. Changes in the physical properties of these ILs such as density, viscosity, conductivity, surface tension and refraction index after γ-radiolysis have been reported. Pulse radiolysis techniques have been used to examine primary effects of radiation in ILs

36-39

. Solvated

electrons react slowly in tetraalkylammonium-based ILs, whereas electrons are very rapidly captured by the alkylimidazolium cation followed by the formation of imidazoyl radicals. Other more recent work has focused on EPR techniques to evaluate primary radical species. 40-42

A variety of observations in organic materials show that radiation is much more likely to break C-H bonds than C-C bonds. The H atom formed in this process can then undergo a hydrogen abstraction reaction to form molecular hydrogen, H2. The molecular radicals remaining will undergo abstraction, combination, and disproportionation reactions. Molecular dissociation can also occur. These various processes can lead to a wide variety of products that are often difficult to quantify, but the sum of these products is generally equal to the H2 formation. Therefore, measurement of the H2 yield is generally a good indication of total 3 ACS Paragon Plus Environment


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molecular decomposition and radiation stability. A similar predictability with ILs is expected especially for those ILs that have a major organic component in the cation. However, only a few studies have examined the production of H2 in the γ-radiolysis of ILs 26,30,33. If ILs are to be employed as solvents or electrolytes in the reprocessing of used fuel then formation of gaseous products such as H2 during their exposure to radiation can lead to serious safety and maintenance problems. Final product yields in a given medium depend on the characteristics of the irradiating particles, such as the type, energy and linear energy transfer (LET= stopping power, -dE/dx) because of the variation in track structure. The transuranic elements decay by emitting alpha particle and the radiation chemical effects in ILs can be considerably different than with more conventional γ-radiolysis, resulting in unexpected high yields of H2 that can have dangerous consequences in actinide separation processes. Basic information on the radiation chemistry occurring in particle tracks has been obtained in the decomposition of various organic liquids 43-45

. Unfortunately, no systematic radiation chemistry studies have examined the production

of H2 or other volatile products as a function of the type of irradiating particles in the radiolysis of ILs. The present work examines the H2 yields from two ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,

[emim][Tf2N],

and

N-trimethyl-N-butylammonium

bis(trifluoromethylsulfonyl)imide, [N1114][Tf2N]. Schematic representations of these two compounds are given in Figure 1. Irradiations were performed with γ-rays, 2-15 MeV protons and 5-20 MeV helium ions in order to probe track structure effects. These results from these ILs are compared with that obtained with structural surrogates such as imidazole, 1methylimidazole, N-methyl-butylamine, N,N-dimethyl-butylamine and N-butylaniline. FTIR and UV-Vis spectroscopic techniques are used to examine the structural changes occurring in radiolysis. 4 ACS Paragon Plus Environment

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2. Experimental Section 2.1 Materials Samples [emim][Tf2N],

of and

1-ethyl-3-methylimidazolium N-trimethyl-N-butylammonium

bis(trifluoromethylsulfonyl)imide

,

bis(trifluoromethanesulfonyl)imide,

[N1114][Tf2N], were obtained from the laboratory of Prof. J. F. Brennecke and not further purified. Samples of [emim][Tf2N] were also obtained from Fluka and no differences between the suppliers could be detected. The water content was determined to be 500 ppm in both ILs except where otherwise specified, and was obtained by the coulometric Karl Fischer method using a Metrohn 831 KF Coulometer. Imidazole, 1-methylimidazole, N-methylbutylamine, N,N-dimethyl-butylamine and N-butylaniline were obtained from Alfa Aesar in the highest purity possible and used as received. Radiolysis of all compounds was performed under inert atmospheres of ultrahigh purity argon or helium (99.9999%). 2.2 Irradiations Samples for H2 analysis and for spectroscopic analysis following radiolysis with γrays were placed in Pyrex tubes (diameter ~ 1 cm, length ~ 10 cm), evacuated and flame sealed. Sample weights were typically between 0.5 to 1 g and the γ-radiolysis of the samples was carried out using a self-contained Shepherd 109-68

60

Co source at the Radiation

Laboratory of the University of Notre Dame. The dose rate was ~220 Gy/min as determined using the Fricke dosimeter 46. Energy deposition in each medium is proportional its electron density relative to that of the Fricke dosimeter. Each sample was weighted before radiolysis to avoid uncertainties in density, especially with the ILs. The electron fraction per molecule was determined from the molecular formula. The heavy ion irradiations were performed in purged and vigorously stirred cells using the 10 MV FN Tandem Van de Graaff accelerator at the Nuclear Structure Laboratory 5 ACS Paragon Plus Environment


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located in the University of Notre Dame Nuclear Science Laboratory. The ions used in these experiments were: 2, 5, 10 and 15 MeV 1H+; 5, 10, 15 and 20 MeV 4He2+ ions. The window assembly and the radiation procedure are the same as previously reported 47,48. Energy loss of the heavy ions in passing through all the windows was determined from a standard stopping power compilation 49. The samples were irradiated with completely stripped ions at a charge beam current of about 1.5 nA. Total energy deposition was obtained from the product of the integrated beam current and the ion energy incident to the sample. The sample cell was made of quartz with a thin mica window of about 5 mg/cm2 attached to the front for the beam entrance. The cell had inlet and outlet ports designed for purging the sample before and after irradiation. 2.3 Gas analysis Gas chromatographic analysis of H2 was performed using a SRI 8610 apparatus equipped with a thermal conductivity detector. The chromatographic column was 6.4 mm diameter 13X molecular sieve 3 m long maintained at 40 ºC. Argon gas of ultra-high purity was used as carrier gases in the analysis of H2 gas. Following γ-irradiation each sample tube was broken in the sampling port of the gas chromatograph. An inline technique was employed to determine the H2 production by heavy ion irradiation where the inlet and outlet of the sample cell are connected to the gas chromatograph by a four-way valve. Calibration was performed by injection of pure H2 gas under normal conditions. The error in gas measurement is estimated to be about 5%, which includes gas measurement and manipulation. 2.4 Spectroscopic analysis Infrared transmittance spectra of the ILs were recorded using a Bruker Vertex 70 FTIR spectrometer with a resolution 4 cm-1 and 512 scans in the range of 400 - 4000 cm-1. 6 ACS Paragon Plus Environment

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Irradiated samples were mixed with KBr powder, so that the weight fraction of the ILs was about 10 %. These mixtures were used to make pellets of 2 mm thickness and were then analyzed using the FTIR spectrometer. UV-Vis absorption spectra of the irradiated samples were recorded using a diode array spectrophotometer (Hewlett-Packard HP8453). Samples were diluted with acetonitrile before measurements unless otherwise stated. 3. Results and Discussion H2 production The production of H2 in the γ-ray and heavy ion radiolysis of all the compounds was examined as a function of dose up to about 50 kGy. Plots of the volume of H2 produced as a function of the absorbed dose were linear and passed through the origin for all compounds examined here, suggesting that initial H2 formation is being examined with little complication due to secondary effects that may occur at higher doses. Figure 2 shows the typical linear relationship between H2 production and irradiation dose for [emim][Tf2N] irradiated with both γ-rays and with 5 MeV He ions. Radiation chemical yields, G values, are proportional to the slope of the data as shown in Figure 2. All yields reported here are expressed in the traditional G value unit of molecules/100 eV (1 molecule/ 100 eV = 1.036 x 10-7 mol/J). The yields for H2 in the γ-radiolysis and 5 MeV He ion radiolysis of [emim][Tf2N] as obtained from the data of Figure 2 are 0.098 and 0.29 molecules/100 eV, respectively. Radiation chemical yields for γ-radiolysis and the 5 MeV He ion radiolysis of the compounds examined here are given in Table 1. Radiation chemical yields are found to be dependent on the LET if the mechanism involves a second or higher order process

44,45

. Increasing the density of energy deposition

increases the concentration of reactive species, which increases the reaction rates of second order processes with respect to first order processes. An examination of the yield of H2 with 7 ACS Paragon Plus Environment


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LET not only gives an idea of the underlying mechanism, but the information also is useful for a crude prediction of H2 yields for other heavy ions. The yields of H2 in [emim][Tf2N] and [N1114][Tf2N] are given as a function of LET in Figures 3 and 4, respectively. The first noticeable difference is the much lower yield of 0.098 molecules/100 eV in the γ-radiolysis of [emim][Tf2N] as compared to 0.73 molecules/100 eV for [N1114][Tf2N]. The addition of an aromatic entity to the compound results in a much lower yield of H2 than observed with an aliphatic compound. An earlier work also suggested that ILs with aromatic entities will have slightly lower H2 yields 33. These results confirm that the expected trend in H2 yields for ILs is similar to that found with organic compounds structurally similar to the IL cation

44

. For

instance, the H2 yield with benzene is 0.038 molecules/100 eV while that for cyclohexane is 5.6 molecules/100 eV. Aromatic compounds have long been considered to be radiation resistant because of these observations. Such a result seems to be common with γ-radiolysis, but not with alpha particle radiolysis. The relatively low yield of H2 for [emim][Tf2N] increases considerably with increasing LET of the incident particle from γ-rays to H ions to He ions. He ions of 5 MeV result in an H2 yield of 0.29 for [emim][Tf2N] which is about three times that found with γradiolysis and approaching the γ-radiolysis yield of 0.73 for [N1114][Tf2N]. Clearly the type of particle can have a large influence on the radiolytic outcome. This trend of increasing H2 yield with increasing LET has been observed with a variety of aromatic compounds

44,45

.

Intra-track chemistry of highly excited states is thought to lead to this increase in aromatic organic compounds 50, perhaps similar processes may be occurring in ILs. Interestingly, the yield of H2 with [N1114][Tf2N] shows very little dependence on particle LET; the H2 yield with γ-rays is very nearly the same as with 5 MeV He ions. These results are consistent with the radiolysis of cyclohexane and other aliphatic hydrocarbons. The production of H2 from aliphatic compounds is mainly due to H atom abstraction and combination reactions that tend 8 ACS Paragon Plus Environment

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to cancel out with increasing LET. Although the LET dependence of H2 from [N1114][Tf2N] is somewhat similar to common aliphatic compounds, the absolute yield is low. For instance, the H2 yield is 5.6 for cyclohexane and only 0.73 for [N1114][Tf2N]. Clearly the molecular structure can have an effect on the total H2 yield, at least in aliphatic compounds. However, the near common dependence of H2 production with LET in a wide variety of aromatic compounds may indicate that the response for one compound may be used to estimate the radiolytic yield from another. This use of surrogates would save considerable time and effort in the case of the radiolysis of ILs. An argument could be made that the radiolytic decomposition of these ILs is not necessarily small with γ-radiolysis and that the observed results for H2 are just due to an increase in the combination reactions of H atoms. H atoms can be expected to add to the aromatic ring and in benzene and in pyridine these addition reactions are known to lead to polymer formation.

51,52

The increase in H2 yields with increasing LET in the ILs are

definitely due to a second order process, which one might assume to be due to the recombination of H atoms. In such a scenario, the actual decomposition of the parent compound would be nearly independent of LET and only the product distribution changes. All products have not been identified in the ILs, but studies with benzene and pyridine clearly show that the sums of all product yields are low in the γ-radiolysis of these compounds. Low total decomposition yields are common in the γ-radiolysis of aromatic compounds, and similar results are expected for the ILs. 53 Furthermore, H2 yields in the radiolysis of benzene have been shown to be independent of the concentration of an added H atom scavenger, which indicates that H atoms are not the source of H2 for these compounds.

51

The source of

H2 in several aromatic liquids has been identified as being due to second order reactions of excited states in the high LET track,

44,50

and a similar process has been assumed for the ILs



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examined in this work. This assumption would imply that the absolute decomposition of ILs is small with γ-rays, but increases with increasing LET. An attempt was made to find suitable surrogates for the production of H2 in ILs. These surrogates may give insight into the possible underlying mechanism leading to H2 production without designing complicated and expensive materials. Figure 3 shows the results for the H2 yields in imidazole (a solid) and methyl imidazole (a liquid) with γ-rays, H ions and He ions. Although the yields with γ-rays for imidazole are a little low, the results show that these two compounds have about the same general trend as [emim][Tf2N] over the entire range of LET. The aromatic entity is dominant for much of the formation of H2 in these compounds, especially with increasing LET. The fact that the aromatic organic component is part of an IL has little effect on the increasing yield of H2 with increasing LET. These findings show that the physical state of the compound is not very important, which should be of considerable interest to the engineering of separation systems. However, there are details that suggest other factors may be contributing to the absolute yields of H2. Radiation chemical yields of H2 with these aromatic compounds seem to be slightly greater with the addition of side chains to the ring. These chains may be giving the compound more “aliphatic” nature and thereby increasing the H2 yield. Further studies involving a systematic variation of the number and length of side chains should help elucidate the processes involved. Energy deposition in the ILs can be considered to be distributed between the cation and the anion. A different method for comparison of the absolute production of H2 from the γ-radiolysis of the surrogates with that of the IL would consider energy deposition to the cation of the IL alone. Energy is distributed within a molecule according to the fraction of electrons within each component. A simple summation of the electrons in each IL gives values of 0.30 for [emim] in [emim][Tf2N] and 0.32 for [N1114] in [N1114][Tf2N]. The 10 ACS Paragon Plus Environment

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resulting H2 yields are 0.33 and 2.28 molecules/100 eV for the γ-radiolysis of [emim][Tf2N] and [N1114][Tf2N], respectively. One can readily see from Table 1 or in Figures 3 and 4 that this simple conversion results in better agreement for H2 production from the ILs with the surrogates. The assumption that the energy deposition between the cation and anion of the ILs can be considered separately may be checked by variation of the anions. Increasing the aliphatic nature of the cation in ILs seems to increase the yield of H2 33, and those yields seem to be nearly independent of LET. However, the absolute yields with the aliphatic compounds are not very predictable. The large aliphatic side chain in [N1114][Tf2N] would suggest a much larger yield of H2 than is observed. A few simple amine analogs of [N1114][Tf2N] like N-methyl-butylamine and N,N-dimethylamine have much higher yields of H2. Substitution of a phenyl ring for a methyl group on going from Nmethyl-butylamine to N-butylaniline has a dramatic lowering of the radiolytic yield of H2 with γ-rays. Increasing the LET leads to an increase in H2 yields with N-butylaniline, just as predicted from other studies with aromatic compounds. The results suggest that the more aliphatic ILs will have little or no LET dependence for H2 production, but predicting the absolute H2 yields is still not obtainable. Water will often be associated with ILs in many practical applications. Many ILs are also hydrophilic and readily absorb water. One can reasonably expect that water will affect the production of H2 when present as a large fraction of ILs. The measurements of H2 reported above were performed with 500 ppm water, which corresponds to about 1 mole% for [emim][Tf2N]. A different set of experiments were performed with 3000 ppm water in [emim][Tf2N] and no significant difference in the yields of H2 was observed. Other work reported a similar independence of H2 yields with low water content 30.



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Spectroscopy Optical spectroscopy has been used to examine irradiated [emim][Tf2N] and [N1114][Tf2N] for major changes in the parent molecule with radiolysis. Typical FTIR spectra of [emim][Tf2N] and [N1114][Tf2N] are shown in Figures 5 and 6 respectively. Both FTIR spectra are strongly dominated by contributions from the bis(trifluoromethanesulfonyl)imide [Tf2N] anion in the regions between 400 to 800 cm-1 and from 1000 to 1400 cm-1

54,55

. Very

little change can be observed in the FTIR spectra and most small changes occur in a very busy part of the spectrum, which make analysis difficult. A difference spectrum for [emim][Tf2N] as shown in Figure 5 was obtained by subtracting the spectrum of that for γrays irradiated to 100 kGy from the spectrum of the unirradiated compound. One can see a slight increase in absorbance bands from 2800 to 3300 cm-1 corresponding to the ring HCCH asymmetric stretching (3161 cm-1), ring NC(H)NCH stretching (3125 cm-1), CH3(N)HCH asymmetric stretching (2993 cm-1), CH2HCH asymmetric stretching (2968 cm-1) and terminal CH3HCH asymmetric stretching (2951 cm-1). Several other bands are observed in the range of 1400–1600 cm-1 corresponding to ring in-plane symmetric/asymmetric stretching, CH3(N) and CH2(N) CN stretching (1574 cm-1), ring in-plane asymmetric stretching and CH3(N) stretching (1471 cm-1), CCH HCH asymmetric bending, CH3(N) HCH symmetric bending and

terminal

CH3

HCH

asymmetric

bending

(1460

cm-1)

and

ring

in-plane

symmetric/asymmetric stretching, CH3(N) CN stretching and CH3(N)HCH symmetric bending (1431 cm-1). No one specific product can be identified unambiguously, but the results suggest that general radiolytic degradation is relatively minor. In the case of [N1114][Tf2N] (Figure 6), the changes are even smaller than for [emim][Tf2N]. The technique for observing the FTIR spectra involved the making of KBr pellets and small variations in concentration could also account for the observed absorbances.


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With increasing radiation dose, the color of [emim][Tf2N] changed from colorless to yellow and then to dark orange; however, [N1114][Tf2N] remained colorless. In order to analyze the colored products, acetonitrile was used as solvent for recording the UV-Vis spectra with dilution factor of 100 and 5 for [emim][Tf2N] and [N1114][Tf2N], respectively. Acetonitrile exhibits negligible absorption above 200 nm. Figure 7 shows the absorption spectra of [emim][Tf2N] prior to and after γ-irradiation. The absorption for the irradiated [emim][Tf2N] increases considerably with dose in the wavelength region from 240 nm to 400 nm compared to unirradiated [emim][Tf2N]. A small shoulder is observable at around 287 nm. This new peak indicates the formation of new radiolysis products that have a stronger absorption than the IL. Similar absorption peaks at around 290 nm for other ILs containing the 1,3-dialkylimidazolium cation associated with different inorganic anions was reported by Yuan et al

35

. A shift of the peak from 290 nm to 297 nm was observed on increasing the

alkyl chain length from butyl to hexyl. The presence of an ethyl group in [emim][Tf2N] results in a peak at 287 nm, which is consistent with that expected from the relative lengths of the side chains. In the case of [N1114][Tf2N] (Figure 8), a distinct absorption peak at 320 nm appeared with no identification being made. Interestingly, another UV-Vis study with [emim][Tf2N], [bmim][Tf2N], and [N1114][Tf2N] found the same bathochromic shift, but no noticeable peaks in this wavelength region.

56

That work used open containers for the

irradiations in order to avoid gas buildup. The presence of oxygen probably quenched normal radical reactions and modified product outcome from that found here. Conclusions The production of H2 in the ILs [emim][Tf2N] and [N1114][Tf2N] and a few possible surrogates

such

as

imidazole,

1-methylimidazole,

N-methylbutylamine,

N,N-

dimethylbutylamine and N-butylaniline with γ-rays, H ions, and He ions were investigated. The presence of an aromatic entity seems to lower the yield of H2 for all compounds 13 ACS Paragon Plus Environment


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examined, regardless of components of the compound or its physical state. A strong LET dependence of H2 is observed for all of the aromatic compounds, which suggests a common mechanism for H2 production. Such a result may aid in predicting the radiolytic outcome for other ILs. All of the aliphatic compounds seem to have little dependence of H2 yields on LET, even though the absolute yields were not always obvious. Optical analysis of the ILs following radiation suggests little radiolytic decay, which is consistent with the relatively low H2 yields. Acknowledgments The authors acknowledge the Laboratory Directed Research and Development Program at Los Alamos National Laboratory for financial support during this project. The authors thank Prof. Michael Wiescher for making available the facilities of the Notre Dame Nuclear Structure Laboratory, which is supported by the U.S. National Science Foundation. Ionic liquid samples were supplied by the laboratory of Prof. J. F. Brennecke of the University of Notre Dame. The work was performed using the facilities of the Notre Dame Radiation Laboratory, which is supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533. This contribution is NDRL-4963 from the Notre Dame Radiation Laboratory.


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References

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(20) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Griffin, S. T.; Rogers, R. D. Traditional Extractants in Nontraditional Solvents: Groups 1 and 2 Extraction by Crown Ethers in Room-temperature Ionic Liquids. Ind. Eng. Chem. Res. 2000, 39, 3596-3604. (21) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Task-specific Ionic Liquids for the Extraction of Metal Ions from Aqueous Solutions. 2001, 135-136. (22) Giridhar, P.; Venkatesan, K. A.; Srinivasan, T. G.; Rao, P. R. V. Extraction of Uranium(VI) from Nitric Acid Medium by 1.1M Tri-n-butylphosphate in Ionic Liquid Diluent. J. Radio. Nucl. Chem. 2005, 265, 31-38. (23) Giridhar, P.; Venkatesan, K. A.; Srinivasan, T. G.; Rao, P. R. V. Electrochemical Behavior of Uranium(VI) in 1-Butyl-3-methylimidazolium Chloride and Thermal Characterization of Uranium Oxide Deposit. Electrochim. Acta 2007, 52, 3006-3012. (24) Giridhar, P.; Venkatesan, K. A.; Subramaniam, S.; Srinivasan, T. G.; Rao, P. R. V. Extraction of Uranium (VI) by 1.1 M Tri-n-butylphosphate/Ionic Liquid and the Feasibility of Recovery by Direct Electrodeposition from Organic Phase. J. Alloys Comp. 2008, 448, 104-108. (25) Giridhar, P.; Venkatesan, K. A.; Subramaniam, S.; Srinivasan, T. G.; Rao, R. R. V. Electrochemical Behavior of Uranium(VI) in 1-Butyl-3-methylimidazolium Chloride and in 0.05 M Aliquat-336/Chloroform. Radiochim. Acta 2006, 94, 415-420. (26) Allen, D.; Baston, G.; Bradley, A. E.; Gorman, T.; Haile, A.; Hamblett, I.; Hatter, J. E.; Healey, M. J. F.; Hodgson, B.; Lewin, R.; et al. An Investigation of the Radiation Stability of Ionic Liquids. Green Chem. 2002, 4, 152-158. (27) Berthon, L.; Nikitenko, S. I.; Bisel, I.; Berthon, C.; Faucon, M.; Saucerotte, B.; Zorz, N.; Moisy, P. Influence of Gamma Irradiation on Hydrophobic Room-Temperature Ionic Liquids [BuMeIm]PF6 and [BuMeIm](CF3SO2)2N. Dalton Trans. 2006, 2526-2534. (28) Bosse, E.; Berthon, L.; Zorz, N.; Monget, J.; Berthon, C.; Bisel, I.; Legand, S.; Moisy, P. Stability of [MeBu3N][Tf2N] Under Gamma Irradiation. Dalton Trans. 2008, 7, 924931. (29) Harmon, C. D.; Smith, W. H.; Costa, D. A. Criticality Calculations for Plutonium Metal at Room Temperature in Ionic Liquid Solutions. Radiat. Phys. Chem. 2001, 60, 157159. (30) Le Rouzo, G.; Lamouroux, C.; Dauvois, V.; Dannoux, A.; Legand, S.; Durand, D.; Moisy, P.; Moutiers, G. Anion Effect on Radiochemical Stability of RoomTemperature Ionic Liquids under Gamma Radiolysis. Dalton Trans. 2009, 6175-6184. (31) Qi, M.; Wu, G.; Chen, S.; Liu, Y. Gamma Radiolysis of Ionic Liquid 1-Butyl-3methylimidazolium Haxafluorophosphate. Radiat. Res. 2007, 167, 508-514. (32) Qi, M.; Wu, G.; Li, Q.; Luo, Y. γ-Radiation Effect on Ionic Liquid [Bmin][BF4]. Radiat. Phys. Chem. 2008, 77, 877-883. (33) Tarabek, P.; Liu, S.; Haygarth, K.; Bartels, D. M. Hydrogen Gas Yields in Irradiated Room-Temperature Ionic Liquids. Radiat. Phys. Chem. 2009, 78, 168-172. (34) Yuan, L. Y.; Peng, J.; Xu, L.; Zhai, M. L.; Li, J. Q.; Wei, G. S. Influence of GammaRadiation on the Ionic Liquid C(4)mim PF(6) During Extraction of Strontium Ions. Dalton Trans. 2008, 6358-6360. (35) Yuan, L. Y.; Peng, J.; Xu, L.; Zhai, M. L.; Li, J. Q.; Wei, G. S. Radiation-induced Darkening of Ionic Liquid C(4)mim NTf2 and its Decoloration. Radiat. Phys. Chem. 2009, 78, 1133-1136. (36) Behar, D.; Gonzalez, C.; Neta, P. Reaction Kinetics in Ionic Liquids: Pulse Radiolysis Studies of 1-Butyl-3-methylimidazolium Salts. J. Phys. Chem. A 2001, 105, 76077614.


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(37) Behar, D.; Neta, P.; Schultheisz, C. Reaction Kinetics in Ionic Liquids as Studied by Pulse Radiolysis: Redox Reactions in the Solvents Methyltributylammonium Bis(trifluoroniethylsulfonyl)imide and N-butylpyridinium Tetrafluoroborate. J. Phys. Chem. A 2002, 106, 3139-3147. (38) Grodkowski, J.; Neta, P.; Wishart, J. F. Pulse Radiolysis Study of the Reactions of Hydrogen Atoms in the Ionic Liquid Methyltributylammonium Bis(trifluoromethyl)sulfonyl Imide. J. Phys. Chem. A 2003, 107, 9794-9799. (39) Wishart, J. F.; Neta, P. Spectrum and Reactivity of the Solvated Eelctron in the Ionic Liquid Methyltributylammonium Bis(trifluoromethylsulfonyl)imide. J. Phys. Chem. B 2003, 107, 7261-7267. (40) Shkrob, I. A.; Chemerisov, S. D.; Wishart, J. F. The Initial Stages of Radiation Damage in Ionic Liquids and Ionic Liquid-Based Extraction Systems. J. Phys. Chem. B 2007, 111, 11786-11793. (41) Shkrob, I. A.; Marin, T. W.; Chemerisov, S. D.; Hatcher, J. L.; Wishart, J. F. Radiation Induced Redox Reactions and Fragmentation of Constituent Ions in Ionic Liquids. 2. Imidazolium Cations. J. Phys. Chem. B 2011, 115, 3889-3902. (42) Shkrob, I. A.; Marin, T. W.; Chemerisov, S. D.; Wishart, J. F. Radiation Induced Redox Reactions and Fragmentation of Constituent Ions in Ionic Liquids. 1. Anions. J. Phys. Chem. B 2011, 115, 3872-3888. (43) Burns, W. G. Decomposition of Aromatic Substances by Different Kinds of Radiation. Trans. Faraday Soc. 1962, 58, 961-970. (44) La Verne, J. A.; Baidak, A. Track Effects in the Radiolysis of Aromatic Compounds. Radiat. Phys. Chem. 2012, 81, 1287-1290. (45) LaVerne, J. A.; Chang, Z.; Araos, M. S. Heavy Ion Radiolysis of Organic Materials. Radiat. Phys. Chem. 2001, 60, 253-257. (46) LaVerne, J. A.; Schuler, R. H. Radiation Chemical Studies with Heavy-Ions - Oxidation of Ferrous Ion in the Fricke Dosimeter. J. Phys. Chem. 1987, 91, 5770-5776. (47) LaVerne, J. A.; Schuler, R. H. Track Effects in Radiation-Chemistry - Core Processes in Heavy- Particle Tracks as Manifest by the H2 Yield in Benzene Radiolysis. J. Phys. Chem. 1984, 88, 1200-1205. (48) LaVerne, J. A.; Schuler, R. H. Track Effects in Radiation-Chemistry - Production of HO2 in the Radiolysis of Water by High-LET Ni58 Ions. J. Phys. Chem. 1987, 91, 6560-6563. (49) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range of Ions in Solids; Pergamon: New York, 1985. (50) Baidak, A.; Badali, M.; LaVerne, J. A. Role of the Low-energy Excited States in the Radiolysis of Aromatic Liquids. J. Phys. Chem. A 2011, 115, 7418-7427. (51) Enomoto, K.; LaVerne, J. A.; Pimblott, S. M. Products of the Triplet Excited State Produced in the Radiolysis of Liquid Benzene. J. Phys. Chem. A 2006, 110, 41244130. (52) Enomoto, K.; LaVerne, J. A.; Araos, M. S. Heavy Ion Radiolysis of Liquid Pyridine. J. Phys. Chem. A 2007, 111, 9-15. (53) Roder, M. Aromatic Hydrocarbons. In Radiation Chemistry of Hydrocarbons; Elsevier: Amsterdam, 1981. (54) Kiefer, J.; Fries, J.; Leipertz, A. Experimental Vibratnional Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide and 1-Ethyl-3-methylimidazolium Ethylsulfate. Appl. Spect. 2007, 61, 1306-1311. (55) Noack, K.; Schulz, P. S.; Paape, N.; Kiefer, J.; Wasserscheid, P.; Leipertz, A. The Role of the C2 Position in Interionic Interactions of Imidazolium Based Ionic Liquids: A 17 ACS Paragon Plus Environment


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Vibrational and NMR Spectroscopic Study. Phys. Chem. Chem. Phys. 2010, 12, 14153-14161. (56) Bridges, N. J.; Visser, A. E.; Williamson, M. J.; Mickalonis, J. I.; Adams, T. M. Effects of Gamma Radiation on Electrochemical Properties of Ionic Liquids. Radiochim. Acta 2010, 98, 243-247.


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Table 1. Yields of H2 for γ-rays and 5 MeV He ions.

G(H2) Compound

He-ion γ−rays (5 MeV)

Imidazole

0.048

0.16

1-methylimidazole

0.14

0.51

N-methylbutylamine

5.79

4.93

N,N-dimethylbutylamine

3.83

3.99

N-Butylaniline

0.49

1.20

[N1114][Tf2N]

0.73

0.85

[emim][Tf2N]

0.098

0.29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

Figure Legends

Figure

1.

Structure

of

the

ionic

liquids

(A)


bis(trifluoromethylsulfonyl)imide, [emim][Tf2N], and (B) N-trimethyl-N-butylammonium bis(trifluoromethylsulfonyl)amide, [N1114][Tf2N].

Figure 2. Production of H2 as a function of irradiation dose in the gamma and helium ion radiolysis of [emim][Tf2N].

Figure 3. Yields of H2 in [emim][Tf2N], 1-methylimidazol and imidazole as a function of track average LET for () 1H and () 4He ions. The dashed lines show the low LET limiting yield obtained with γ-rays.

Figure 4. Yields of H2 in [N1114][Tf2N], N-methylbutylamine, N,N-dimethylbutylamine and N-butylaniline as a function of track average LET for () 1H and () 4He ions. The dashed lines show the low LET limiting yield obtained with γ-rays.

Figure 5. FTIR spectra of [emim][Tf2N] irradiated with γ-rays (100 kGy) and with 5 MeV He ions (20 kGy). The difference spectrum was obtained by subtracting the original FTIR spectrum of the IL from the γ-irradiated spectrum.

Figure 6. FTIR spectra of [N1114][Tf2N] irradiated with γ-rays (100 kGy) and with 5 MeV He ions (20 kGy). The difference spectrum was obtained by subtracting the original FTIR spectrum of the IL from the γ-irradiated spectrum.


Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. UV-visible absorption spectra of [emim][Tf2N] irradiated with γ-rays to doses of 270, 500, 750, and 1000 kGy and diluted by a factor of 100 after irradiation with acetonitrile.

Figure 8. UV-visible absorption spectra of [N1114][Tf2N] irradiated with γ-rays to doses of 270, 500, 750, and 1000 kGy and diluted by a factor of 5 after irradiation with acetonitrile.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure

1.

Structure

of

the

ionic

liquids

(A)

Page 22 of 30


bis(trifluoromethylsulfonyl)imide, [emim][Tf2N], and (B) N-trimethyl-N-butylammonium bis(trifluoromethylsulfonyl)amide, [N1114][Tf2N].

CH3

(A) N

+ O

-

O

N F3 C

N CH2

S

S

O

O

CF3

CH3

(B) H2 C H3C

CH3

H2 C C H2

N

+

CH3

H3C

O

-

O

N F3 C

S

S

O

O

CF3


Page 23 of 30

Figure 2. Production of H2 as a function of irradiation dose in the gamma and helium ion radiolysis of [emim][Tf2N].

12

[emim][Tf2N] 17

H2 Production (molecules/10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

10

5 MeV He G = 0.29 molecules/100 eV

8

6

4

G = 0.098 molecules/100 eV

γ-rays

2

0

0

10

20

30

40

50

Dose (kGy)



Figure 3. Yields of H2 in [emim][Tf2N], 1-methylimidazol and imidazole as a function of track average LET for () 1H and () 4He ions. The dashed lines show the low LET limiting yield obtained with γ-rays.

1

4

He N

G(H2) (molecules/100 eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

1

γ-ray

H

N CH 3

methyl imidazole CH3

0.1

0.01

N

+

emim N CH2CH3

N

imidazole N H

1

10

100

1000

Track Average LET (eV/nm)


Page 25 of 30

Figure 4. Yields of H2 in [N1114][Tf2N], N-methylbutylamine, N,N-dimethylbutylamine and N-butylaniline as a function of track average LET for () 1H and () 4He ions. The dashed lines show the low LET limiting yield obtained with γ-rays.

10

G(H2) (molecules/100 eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

γ-ray

1

H

He

CH3

N-methyl-1-butylamine

HN

C4H9 CH3 N

H3C

dimethylbutylamine

C4H9

CH 3

1

0.1

N1114

C4H9 N

+

CH3

H3C

N-butylaniline HN

C 4H 9

1

10

100

1000

Track Average LET (eV/nm)



Figure 5. FTIR spectra of [emim][Tf2N] irradiated with γ-rays (100 kGy) and with 5 MeV He ions (20 kGy). The difference spectrum is the gamma irradiated spectrum minus that for the unirradiated.

1.0 0.8

Transimittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

unirradiated γ irradiated

0.6

5 MeV He ion 0.4

[emim][Tf2N] 0.2

difference in γ irradiation 0.0 -0.2 4000

3000

2000

1000 -1

Wavenumber (cm )


Page 27 of 30

Figure 6. FTIR spectra of [N1114][Tf2N] irradiated with γ-rays (100 kGy) and with 5 MeV He ions (20 kGy). The difference spectrum is the gamma irradiated spectrum minus that for the unirradiated.

1.0

0.8

Transimittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unirradiated

0.6

γ -irradiated

5 MeV He ion 0.4

[N1114][Tf2N] 0.2

0.0

difference in γ irradiation -0.2 4000

3000

2000

1000 -1

Wavenumber (cm )



Figure 7. UV-Visible spectra of [emim][Tf2N] irradiated with γ-rays to doses of 270, 500, 750, and 1000 kGy and diluted by a factor of 100 after irradiation with acetonitrile.

4

[emim][Tf2N] 3

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

unirradiated 270 kGy 500 kGy 750 kGy 1000 kGy

2

1

0

200 220 240 260 280 300 320 340 360 380 400

Wavelength (nm)


Page 29 of 30

Figure 8. UV-Visible spectra of [N1114][Tf2N] irradiated with γ-rays to doses of 270, 500, 750, and 1000 kGy and diluted by a factor of 5 after irradiation with acetonitrile.

4

[N1114][Tf2N] unirradiated 270 kGy 500 kGy 750 kGy 1000 kGy

3

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

0

Wavelength (nm)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

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Hydrogen Production in Aromatic and Aliphatic Ionic Liquids - The

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