Hydrogen Production in Aromatic and Aliphatic Ionic Liquids

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Article

Hydrogen Production in Aromatic and Aliphatic Ionic LiquidsSurajdevprakash 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

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Hydrogen Production in Aromatic and Aliphatic Ionic Liquids

Surajdevprakash B. Dhiman a George S. Goff

b, Wolfgang Runde

c and Jay A. LaVerne

ad*

aRadiation Laboratory, University of Notre Dame, Notre Dame, IN 46556

bChemistry Division, Los Alamos National Laboratory, Los Alamos NM, 87545

cScience Programs Office, Los Alamos National Laboratory, Los Alamos NM, 87545

dDepartment of Physics, University of Notre Dame, Notre Dame, Indiana 46556

Corresponding author: [email protected]

Abstract

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

butylammonium 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 γ-

rays 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

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

methylimidazolium, 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 (ESI-

MS). 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

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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, 1-

methylimidazole, 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.

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2. Experimental Section

2.1 Materials

Samples of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ,

[emim][Tf2N], and N-trimethyl-N-butylammonium 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-methyl-

butylamine, 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 γ-

rays 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

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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

4He

2+ 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

.

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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

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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 γ-

radiolysis 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

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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

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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 N-

methyl-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 γ-

rays 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

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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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Vibrational and NMR Spectroscopic Study. Phys. Chem. Chem. Phys. 2010, 12,

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

Compound

G(H2)

γ−rays

He-ion

(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

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Figure Legends

Figure 1. Structure of the ionic liquids (A) 1-ethyl-3-methylimidazolium

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.

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21

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.

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Figure 1. Structure of the ionic liquids (A) 1-ethyl-3-methylimidazolium

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

bis(trifluoromethylsulfonyl)amide, [N1114][Tf2N].

N

N

CH2

CH3

+

NS CF3

O

O

F3C S

O

O

-

H3C

H2C

CH2

H2C

N

CH3

CH3

H3C+

N

S CF3

O

O

F3C S

O

O

-

CH3

(A)

(B)

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Figure 2. Production of H2 as a function of irradiation dose in the gamma and helium ion

radiolysis of [emim][Tf2N].

0 10 20 30 40 500

2

4

6

8

10

12

G = 0.29 molecules/100 eV

G = 0.098 molecules/100 eV

[emim][Tf2N]

5 MeV 4He

γ-rays

H2 Production (molecules/1017)

Dose (kGy)

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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 10 100 10000.01

0.1

1

N

NH

N

N

CH 3

N

N

CH2CH3

CH3

+

imidazole

methyl imidazole

emim

4He

1H

γ-ray

G(H

2) (molecules/100 eV)

Track Average LET (eV/nm)

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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.

1 10 100 10000.1

1

10

C4H9

N

CH3

CH3

H3C+

N

CH3

C4H9

H3C

HN

CH3

C4H9

HN

C 4H 9

dimethylbutylamine

4He1

Hγ-ray N-methyl-1-butylamine

N-butylaniline

N1114

G(H

2) (molecules/100 eV)

Track Average LET (eV/nm)

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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.

4000 3000 2000 1000

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

[emim][Tf2N]

difference in γ irradiation

Transimittance (%)

Wavenumber (cm-1)

unirradiated

γ irradiated

5 MeV He ion

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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.

4000 3000 2000 1000-0.2

0.0

0.2

0.4

0.6

0.8

1.0

difference in γ irradiation

[N1114][Tf

2N]

Transimittance (%)

Wavenumber (cm-1)

unirradiated

γ -irradiated

5 MeV He ion

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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.

200 220 240 260 280 300 320 340 360 380 4000

1

2

3

4

[emim][Tf2N]

Absorbance

Wavelength (nm)

unirradiated

270 kGy

500 kGy

750 kGy

1000 kGy

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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.

0

1

2

3

4

[N1114][Tf

2N]

unirradiated

270 kGy

500 kGy

750 kGy

1000 kGy

Absorbance

Wavelength (nm)

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For Table of Contents Only

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Hydrogen Production in Aromatic and Aliphatic Ionic Liquids