Thermal decomposition of carboxylate ionic liquids: Trends and mechanisms

PCCP

View Article Online

View Journal | View Issue

PAPER

Thermal decomposition of carboxylate ionic liquids:

trends and mechanisms†

Cite this: Phys. Chem. Chem. Phys., 2013,

1

5, 20480

a

b

a

b

Matthew T. Clough, Karolin Geyer,* Patricia A. Hunt,* J u¨ rgen Mertes and

a

Tom Welton*

The thermal stability of a series of dialkylimidazolium carboxylate ionic liquids has been investigated using

a broad range of experimental and computational techniques. Ionic liquids incorporating fluoroalkyl

carboxylate anions were found to have profoundly diﬀering thermal stabilities and decomposition

mechanisms compared with their non-fluorinated analogues. 1-Ethyl-3-methylimidazolium acetate was

N

observed to largely decompose via an S 2 nucleophilic substitution reaction when under inert gas

conditions, predominantly at the imidazolium methyl substituent. The Arrhenius equations for thermal

2

decomposition of 1-ethyl-3-methylimidazolium acetate, and the C -methylated analogue 1-ethyl-2,3-

Received 28th August 2013,

Accepted 21st October 2013

dimethylimidazolium acetate, were determined from isothermal Thermogravimetric Analysis experiments.

The low thermal stability of 1-ethyl-3-methylimidazolium acetate has important implications for biomass

experiments employing this ionic liquid. For these two ionic liquids, ion pair and transition state structures

DOI: 10.1039/c3cp53648c

N

were optimised using Density Functional Theory. The activation barriers for the S 2 nucleophilic

www.rsc.org/pccp

substitution mechanisms are in good agreement with the experimentally determined values.

In addition, such processes are only economic if the ionic liquid

can be recycled and used multiple times. Hence, a thorough

Introduction

Ionic liquids are versatile materials, with applications as understanding of the long-term thermal decomposition behaviour

1

2

electrolytes, engineering fluids, pre-treatment agents for of [C C im][OAc], and related compounds, including knowledge

2

1

3

wood grinding/biomass extraction and, fundamentally, as of the upper operational temperature limit and the precise

solvents and catalysts for synthetic chemical reactions. They mechanisms that are occurring, is crucial.

are liquid at, or near, room temperature. Fully composed of

cationic and anionic species, the unsymmetrical and charge- to study the thermal stability of ionic liquids. TGA experiments

diﬀuse nature of the ions contributes to their low melting can be broadly divided into two categories: (i) ‘temperature-

points. More recently, it has been suggested that there may ramped’ or ‘scanning’ TGA experiments measure weight % as

be an entropic factor that also contributes to the low melting a function of sample temperature, with a constant heating rate

4

10

5

Thermogravimetric Analysis (TGA) has been used extensively

1

6

À1

points of ionic liquids. Dialkylimidazolium ionic liquids are (typically 10 1C min or 20 1C min ); (ii) ‘isothermal’ TGA

generally preferred for their low melting points and ease of experiments record weight % as a function of time, whilst the

7

handling and preparation.

sample is maintained at a constant temperature.

The ionic liquid 1-ethyl-3-methylimidazolium acetate, [C

OAc], has been popular in biomass pre-treatment studies. These ison of thermal stability between compounds in a series. The

2

C

1

im]-

Temperature-ramped TGA experiments allow rapid compar-

8

[

processes are carried out at elevated temperatures (120–170 1C) T_o_n_s_e_tvalue is determined from extrapolation of the steepest

9

for extended periods of time (up to 24 h). Degradation of the ionic point of the thermograph, and has been used to quantify the

1

1d,12

liquid will reduce the efficacy of the process, and the build up thermal stabilities of ionic liquids.

of volatile or corrosive decomposition products poses a hazard, value is known to vary with experimental conditions, including

However, the T_o_n_s_e_t

11c,13

particularly in large-scale industry.

heating rate, sample quantity and pan material.

Nevertheless,

temperature-ramped TGA is a suitable technique for assessing the

relative thermal stabilities of a series of compounds, when care is

taken to keep experimental conditions uniform. It is well established

that the Tonset value provides a large overestimate of ionic liquid

thermal stability. The ‘Tx/z’ parameter was proposed, where x

represents a given decomposition extent (e.g. 1%) in z length of

a

Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.

E-mail: p.hunt@imperial.ac.uk, t.welton@imperial.ac.uk

b

BASF SE, Ludwigshafen, Germany. E-mail: karolin.geyer@basf.com

1

4

†

Electronic supplementary information (ESI) available. See DOI: 10.1039/

c3cp53648c

2

0480 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

different synthetic procedures. A metathesis reaction involving

[C C im]Br, 1, and silver(I) acetate in aqueous solution yielded 3a.

2 1

Compound 3b was prepared by the neutralisation of an aqueous

solution of [C C im][OH] with acetic acid. The aqueous [C C im]-

2

1

2 1

[

OH] was generated from a metathesis of [C C im][HSO ] with

2 1 4

barium hydroxide in aqueous solution, which precipitates insoluble

18

barium sulfate. Ionic liquid 3c was a purchased commercial

19

2 1

sample of [C C im][OAc].

Thermogravimetric Analysis (TGA) was carried out on a

PerkinElmer ‘Pyris 1’ TGA apparatus. Temperature-ramped

TGA experiments were performed on 4–8 mg of the ionic liquid,

À1

using a heating rate of 10 1C min , and platinum pans. Unless

otherwise stated, nitrogen was used as the inert carrier gas.

Isothermal TGA was performed on 7–8 mg of the ionic liquid.

Thermogravimetric Analysis–Mass Spectrometry (TGA-MS)

experiments were performed on 20–60 mg of the ionic liquid,

À1

using ceramic pans. A heating rate of 10 1C min and a CP

À1

Grade helium flow of 20 ml min were employed. The ‘Pyris 1’

TGA was connected to a ‘Hiden Analytical HPR 20’ Mass

Fig. 1 Ionic liquids investigated in this study, incorporating the 1-ethyl-3- Spectrometer with a ceramic heated capillary. Electron Ionisation

+

methylimidazolium [C

2

C

1

im] cation (1–12) and 1-ethyl-2,3-dimethylimidazolium

(

EI) mass spectrometry was employed, with an ionisation energy

of 70 eV.

Diﬀerential Scanning Calorimetry (DSC) experiments were

+

[

C

2

C

1

C

1

im] cation (13 and 14).

performed on 1–9 mg of the ionic liquid, using aluminium

pans. In order to allow volatile degradants to escape, and to

avoid the risk of the pans bursting under pressure, a small

incision was made in the top of both the sample and the reference

pan. The pan was heated initially to 100 1C for 30 minutes, in order

to remove water. The sample pan was then re-weighed and the DSC

program was initiated. DSC traces were collected in the range

time (e.g. 10 hours). Hence, the T0.01/10 parameter represents

the temperature at which exactly 1% weight loss occurs in

10 hours. MacFarlane and co-workers determined the T₀_.₀₁_/₁₀

values for several imidazolium, pyrrolidinium and phosphonium

ionic liquids, and found them to be lower by typically 110 1C

1

5

compared to the Tonset temperatures.

To date, the thermal decomposition of [C

2

C

1

im][OAc] has been

partially characterised. In this study, we investigate the thermal

decomposition of [C im][OAc] and related compounds (Fig. 1)

À1

16

30–410 1C, with a heating rate of 2.5 1C min . After the first heating

À1

step, the sample was cooled to 30 1C at a rate of 20 1C min . The

2 1

C

sample was then maintained at 30 1C for 15 minutes, followed by a

second heating step using an identical program to the first heating

step. The second heating step would indicate the phase behaviour

of any residual decomposition products present in the sample pan

following the first heating stage.

using a variety of experimental and computational techniques. The

emphasis is on determining the long-term thermal stability of the

ionic liquids, and understanding the thermodynamics, kinetics,

and chemical mechanisms of degradation.

Full TGA, TGA-MS and DSC experimental conditions are

described in the ESI.†

Experimental

The ionic liquids investigated in this study are shown in Fig. 1.

With the exception of 3c, which was purchased, all compounds

Results and discussion

were prepared in our laboratory from commercial starting The thermal stabilities of ionic liquids 1–14 were compared using

materials, taking care to ensure purity at each step. The synthesis temperature-ramped TGA (Fig. 2). The temperature-ramped TGA

and characterisation of ionic liquids 1–14 is described in experiments were performed in triplicate for each ionic liquid, in

the ESI.† Compounds incorporating the bromide, 1, and order to assess the reproducibility of the Tonset measurement. For all

bis(trifluoromethanesulfonyl)imide, 2, anions have been pre- ionic liquids in this investigation, the Tonset values were reproducible

viously studied, and are included in the study for comparative to within Æ2 1C. The middle value for each ionic liquid is tabulated

1

4b,17

purposes.

2 1

Each of the carboxylate ionic liquids 4–12 can be in Table 1. The three samples of [C C im][OAc], 3a–c, give identical

regarded as an analogue of [C

2

C

1

im][OAc], 3, involving systematic onset values within the experimental error (Table 1). This implies

T

functionalisation of the acetate anion by fluorination (4 and 5), that the commercial sample, 3c, generated on large scale, is as

alkyl chain extension (6–9), including branching (10), a strained stable at high temperatures as the samples prepared in the

cyclopropane ring (11), and by incorporating a sulfur atom into the laboratory either via silver(I) or barium metathesis reactions, 3a

carboxylate group to give the thioacetate anion (12). Three separate and 3b respectively.

samples of 1-ethyl-3-methylimidazolium acetate, [C

2

C

1

im][OAc],

Trifluorination of the acetate anion in compound 4 leads

denoted 3a–c, represent the same compound prepared by three to lower thermal stability than the non-fluorinated compound

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20481

View Article Online

PCCP

Paper

about no substantial change in the Tonset value of the ionic liquid.

Therefore, temperature-ramped TGA experiments indicate that there

+

is no substantial difference in the thermal stability of [C C im]

2

1

carboxylate ionic liquids with differing length/branched alkyl chains,

and that the decomposition mechanism involves the reactive

carboxylate functional group.

2 1

The sulfur analogue, [C C im][SAc], 12, is substantially less

thermally stable than the acetate equivalent, 3. Thermal decom-

position of 12 occurs through a nucleophilic substitution

mechanism (Fig. 7, below), because of the high nucleophilicity

+

and low basicity of the thioacetate anion (pK thioacetic acid =

3.33). The negative charge on the thioacetate anion is known

Fig. 2 Temperature-ramped TGA thermographs for selected [C

2

C

1

im] and

a

+

À1

21

[

C

2

C

1

C

1

im] ionic liquids, 1, 2, 3a, 4 and 14, with heating rate 10 1C min

.

to be centred almost entirely on the sulfur atom, due to the

2

large diﬀerence in the energies of the thiol and thione forms.

Table 1

T

onset values for ionic liquids 1–14, measured under a nitrogen atmo- It was observed that over a period of six months, the yellow

À1

sphere and with a heating rate of 10 1C min

thioacetate ionic liquid, 12, gradually darkened when stored at

4

1C under nitrogen (Fig. 3).

Ionic liquid

T_o_n_s_e_tÆ 2 (1C)

301

436

The colour change suggests poor long-term stability. More-

1

2

[C

2

C

1

im]Br

im][NTf ]

2

1

over, the H NMR spectrum of the darkened liquid showed a

decrease in the integration of the thioacetate methyl peak,

2

1

3

a

b

c

[C

2

C

1

im][OAc] Ag route

im][OAc] Ba route

im][OAc] Commercial

214

216

accompanied by the appearance of a new small singlet at

1

1.75 ppm. H NMR and LSIMS mass spectrometry experiments

revealed that partial hydrolysis of thioacetate to acetate had

occurred. The convoluted synthesis and oﬀensive odour of 12

indicate that it will not be a suitable candidate for many ionic

liquid applications.

The identity of the flow gas could potentially aﬀect the

thermal stabilities of ionic liquids 1–14. Therefore, subsequent

temperature-ramped TGA experiments were performed for the

4

5

[C

2

C

1

im][CF

im][CHF

3

CO

2

]

172

263

2

1

2

CO

2

]

6

7

8

9

1

[C

2

[C

2

[C

2

[C

2

[C

2

[C

2

C

1

C

1

C

1

C

1

C

1

C

1

im][CH

im][C(CH

im][c-(C

3

(CH

2

)

2

4

6

CO

14CO

2

]

2

]

213

212

213

216

212

219

2

)

2

]

0

1

3

)

3

CO

)CO

3

H

5

2

]

archetypal ionic liquid [C

2 1

C im][OAc], 3c, under compressed air

and CO flow gases, in addition to the usual inert nitrogen.

2

1

2

[C

2

C

1

im][SAc]

188

1

3

4

[C

2

C

1

C

1

im]Br

im][OAc]

299

221

2

1

3a–c (Fig. 2). Pibiri and co-workers examined the thermal

stabilities of a series of protic perfluoroalkyl-carboxylate ionic

2

0

liquids using TGA, and found these to be low.

By contrast, ionic liquid 5, which incorporates the difluoroacetate

anion, is found to be significantly more stable than both the

non-fluorinated and trifluorinated compounds, 3 and 4, with a

2 1 2 2

Tonset of 263 1C. Therefore, [C C im][CHF CO ], 5, was found to

be an analogue of the acetate ionic liquid, 3, with improved

thermal stability.

Extension of the carboxylate alkyl chain, in compounds 6–9, has

a minimal effect on the Tonset thermal decomposition temperature

of the ionic liquid (Fig. 2). Surprisingly, even the 16-carbon palmitate

2 1 3 2 2

compound, [C C im][CH (CH )14CO ], 9, gave a thermograph very

similar to the acetate ionic liquid, 3a. Branching of the carboxylate

alkyl chain, illustrated by compounds 10 and 11, also has a very

small effect on the T_o_n_s_e_tvalue. There is a small increase in the

basicity of the carboxylate anion with the addition of electron-

a

donating alkyl substituents, indicated by the pK values of the

Fig. 3 (a) Observed decomposition of [C

2

C

1

im][SAc], 12, after six months, when

conjugate carboxylic acids (acetic acid 4.76; butyric acid 4.83;

cyclopropanecarboxylic acid 4.83; hexanoic acid 4.85; octanoic acid

1

stored under nitrogen at 4 1C. (b) Aliphatic region of the H NMR spectrum of

2 1

[C C im][SAc], 12, after six months, demonstrating the disappearance of the

21

4

.89; trimethylacetic/pivalic acid 5.03). Nevertheless, this brings thioacetate singlet at 2.12 ppm, and the appearance of a peak at 1.75 ppm.

2

0482 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

2

A TGA thermograph for 3c under a CO carrier gas is displayed

in the ESI† (Fig. E2). Prior to each scan, the ionic liquid was

heated to 80 1C for 10 hours in the TGA apparatus under a

gentle flow of each gas, to ensure equilibration of the dissolved

gases in the ionic liquid. The presence of oxygen gas in

compressed air introduces the possibility of oxidation or com-

bustion occurring. Moreover, employing CO

may indicate the presence of an N-heterocyclic carbene (NHC)

species forming in the ionic liquid, since CO could act as a

2

as the carrier gas

2

3

carbene trap. Interestingly, no change in the thermal stability

of 3c was observed under the compressed air environment and

a T_o_n_s_e_tvalue of 216 1C was recorded, suggesting that neither

oxidation or combustion are thermal decomposition mechanisms.

However, for the experiment employing a CO

gain of mass was observed for the first two hours of the 10 hour

0 1C equilibration period, followed by gradual loss of mass for the

2

carrier gas, an initial

8

remainder of the equilibration step. The sample then gave a Tonset

value of 225 1C in the subsequent temperature-ramped step,

slightly higher than for 3c in the nitrogen experiment. This result

+

À

suggests the formation of the [C C im -CO ] complex previously

2

1

2

3

observed by Rogers and co-workers. Therefore, it is crucial to

consider the nature of the gaseous environment when employing

ionic liquid 3c.

The Tonset temperature, as determined by a step-tangent

method, is well known to overestimate the thermal stability

1

4,15

of ionic liquid compounds.

Therefore, the T0.01/10 value was

determined for [C C im][OAc], 3a, and [C C C im][OAc], 14, from Fig. 4 (a) Isothermal TGA analysis of [C

2

C

1

im][OAc], 3a. (b) The t0.99 (time taken

2

1

2 1 1

for 1% decomposition to occur) values were plotted against temperature (T) in

Kelvin, giving an exponential fit. The fitted curve was extrapolated backwards to

find T0.01/10 corresponding to t0.99 = 600 min/10 hours. T0.01/10 was determined

as 102 1C.

isothermal TGA experiments using the method of MacFarlane and

14b

co-workers. The procedure is described in the ESI.† Isothermal

TGA graphs for 3a are shown below (Fig. 4); graphs for 14 are

displayed in the ESI† (Fig. E4).

For 3a and 14, the T0.01/10 parameter was measured as 102 1C

and 99 1C, respectively. For 3a, this value is significantly lower proportion of the ionic liquid that has decomposed as a

than the operating temperature of many biomass processes function of time, da/dt (1). Plotting ln(da/dt) against 1/T gives

9

employing this ionic liquid (typically upwards of 120 1C). In a linear fit with a gradient equal to ÀE /R and a y-axis intercept

a

addition, biomass procedures are often carried out for periods equal to ln(A).

substantially longer than 10 hours. Therefore, these procedures

are likely to lead to substantial degradation of this acetate ionic

liquid.

da/dt = A exp(ÀE

a

/RT)

(1)

The plot of ln(da/dt) against 1000/T for 3a is shown in Fig. 5. A

The T0.01/10 value is a reasonable approximation of long-term similar plot for ionic liquid 14 is displayed in the ESI† (Fig. E4).

À1

thermal stability, although the ability to accurately predict the

The activation energy, E

a

, was found to be 115.8 kJ mol for

À1

rate of decomposition at any given temperature is necessary. 3a and 99.6 kJ mol for 14. Although the T0.01/10 values for

The Arrhenius equation, (1), representing the ionic liquid ionic liquids 3a and 14 are very similar (102 and 99 1C,

decomposition, can be determined from the isothermal TGA respectively), the activation energies diﬀer by approximately

2

4

À1

data. Hence, the activation energy, E , and the pre-exponential 16 kJ mol . However, the pre-exponential factors diﬀer by two

a

9

7

factor, A, can be calculated. This allows rapid and accurate orders of magnitude (2.98 Â 10 for 3a, 2.19 Â 10 for 14). The

prediction of the decomposition rate at any temperature. The pre-exponential factor, A, often referred to as the ‘frequency

Arrhenius equations representing decomposition of [C

OAc], 3a, and [C im][OAc], 14, have been determined collisions occurring, whether or not the collision leads to a

using this isothermal method. reaction. The diﬀering values of A suggest significantly fewer

2 1

C im]- factor’, is intrinsically linked to the frequency of molecular

[

2 1 1

C C

Where ionic liquids thermally decompose via the reaction of collisions are occurring for 14 than for 3a, although the lower E_a

one cation with one anion, thereby generating volatile degra- value for 14 indicates that a greater proportion of collisions will

dants, the concentration of cations and anions in the bulk ionic result in a decomposition reaction. The diﬀering values of A may

liquid does not change. Therefore in these circumstances, be a result of the reduced mobility of the 1-ethyl-2,3-dimethyl-

decomposition is pseudo zeroth order. The rate of a decom- imidazolium cation, caused by the additional steric bulk and

2

position of this type can be expressed in terms of the molar van der Waals interactions of the C -methyl substituent.

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20483

View Article Online

PCCP

Paper

An understanding of the safe operating temperatures of an

ionic liquid is crucial. In addition, knowledge of the thermal

decomposition products is important; the production of volatile or

corrosive degradants represents a safety hazard, and may lead to a

reduction in the eﬃcacy of a process. Certain ionic liquids are

known to thermally decompose by nucleophilic or basic attack of the

11a,15,17b,26

anion on the cation,

into reactive species, which subsequently react with the cation.

or by initial degradation of the anion

17a,c,27

The formation of NHC species from dialkylimidazolium ionic

liquids is recognised as another potential decomposition

2

8

route.

Ionic liquids incorporating the tetrafluoroborate,

À

[

BF ] , and hexafluorophosphate, [PF ] , anions are susceptible

4

6

to hydrolysis, liberating highly corrosive hydrogen fluoride,

2

9

HF. Carboxylate anions are both moderately nucleophilic and

basic, and several chemically-plausible decomposition mechanisms

can therefore be proposed for the 1-ethyl-3-methylimidazolium

carboxylate ionic liquids (A–E, Fig. 6).

2 1

Fig. 5 Thermal decomposition kinetics for [C C im][OAc], 3a, determined from

À1

9

isothermal TGA experiments. E

a

= 115.8 kJ mol , A = 2.98 Â 10 .

Ionic liquid 14 has a measured melting point of 90–92 1C,

Thermogravimetric Analysis (TGA) coupled with Mass Spectro-

whereas 3a is a liquid at room temperature. Therefore, at metry (MS) allows the stream of inert gas to be analysed, and

temperatures high enough for 14 to be in the liquid state, chemical products from the ionic liquid thermal decomposition

decomposition will already be occurring at a rate of approxi- can be detected. The detection of peaks at m/z values corresponding

mately 1% in 24 hours. This will restrict the suitability of ionic to parent ions or fragment ions of certain decomposition products

liquid 14 for many applications.

is indicative of a particular mechanism or mechanisms occurring.

Dialkylimidazolium and trialkylimidazolium ionic liquids

When describing thermal decomposition in an open system,

such as in the TGA apparatus, the contribution to mass loss incorporating bis(trifluoromethanesulfonyl)imide (2), acetate

from evaporation of the ionic liquid must also be considered. (3a, 14), trifluoroacetate (4), octanoate (8) and thioacetate (12)

Despite once being heralded as completely non-volatile, ionic anions have been investigated in this TGA-MS study. In order

2

5

liquids are now known to exert a vapour pressure. Successful

distillation of high thermal stability [NTf ]-containing ionic

liquids has been demonstrated, by employing ultra-high vacuum

2

5b,d

(

UHV) conditions.

im][NTf ], evaporation dominated thermal decomposition

at low heating rates in thermogravimetric experiments. The

Heym and colleagues observed that for

[C

C

4 1

2

5e

enthalpy of vaporisation at 298 K, Dvap

H

298, for this ionic liquid

À1

has been measured as 133–140 kJ mol , depending on the

2

5a

chosen experimental method.

For the dialkylimidazolium and trialkylimidazolium carboxylate

ionic liquids investigated here, it is unclear whether thermal

decomposition is occurring exclusively, or whether vaporisation

of the intact ionic liquid is also involved. However, Shimizu and

co-workers have calculated the DvapH value for [C

4 1

C im][OAc]

(

using the relationship DvapH = Ucohesive + RT), and found this

À1

25j

value to be 100 kJ mol higher than for [C C im][NTf ].

4

1

2

Furthermore, Chandran et al. obtained a D_v_a_pH value of

À1

25g

187.6 kJ mol

4 1

for [C C im][OAc] from MD simulations.

The Tonset temperatures for all alkyl and fluoroalkyl carboxylate

ionic liquids here are at least 170 1C lower than for

[C

2

C

1

im][NTf

2

], 2 (Table 1).

Therefore, there is likely to be a contribution from ionic

2

5e

liquid vaporisation for ionic liquid 2. However, on the basis

of low thermal decomposition temperatures and high vaporisa-

tion enthalpies, the contribution to mass loss from vaporisation

is likely to be negligible for all alkyl and fluoroalkyl carboxylate

ionic liquids, 3–14, in this study. Instead, all mass loss is

attributed to the evaporation of volatile thermal decomposition

products.

+

Fig. 6 Postulated decomposition mechanisms for [C

2

C

1

im] ionic liquids 3a, 4, 8

2 nucleophilic

and 12: (A) N-heterocyclic carbene (NHC) formation; (B) + (C) S

N

substitution; (D) E2 Hofmann elimination; (E) decarboxylation of the anion.

2

0484 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

for the mass spectral peaks to be of suﬃcient intensity, a larger

quantity (20–60 mg) of ionic liquid was used than in the regular

temperature-ramped TGA experiments. High purity helium was

used as the carrier gas. Electron Ionisation (EI) mass spectra

were recorded at intervals throughout the experiment, in the

range m/z 10–300. A 70 eV ionisation energy was employed. The

main peaks of interest were of m/z 40 and above.

Fragmentation of small-molecule decomposition products is

possible under the harsh conditions of electron ionisation mass

spectrometry (EIMS). Therefore, the parent ion corresponding to a

given thermal decomposition product may not always be observed,

even if the decomposition product is present in the inert flow gas.

Data from the ‘National Institute of Standards and Technology’

(

NIST) Chemistry WebBook was used to identify expected

3

0

fragmentation of small-molecule decomposition products.

Fig. 8 TGA-MS investigation of [C

2

C

1

im][OAc], 3a. The solid black line repre-

For each ionic liquid, a single mass spectrum was selected

sents the TGA thermograph; coloured dashed lines represent the MS intensities

from the TGA-MS data, recorded at a time point during the main for selected m/z values; the assigned decomposition products are shown.

thermal decomposition period of the ionic liquid. Major peaks

were then assigned as parent peaks, or fragments, of decomposi-

tion products. Example MS histograms for [C C im][OAc], 3a, as a function of temperature, and overlaid with the TGA thermo-

2

1

[

C C im][CF CO ], 4, and [C C im][SAc], 12, are shown in Fig. 7. graph. An example TGA-MS graph of this type for 3a is shown in

2 1 3 2 2 1

Subsequently, the intensity of particular m/z peaks could be plotted Fig. 8. Additional TGA-MS spectra are shown in the ESI† (Fig. E5

and E6).

The thermal decomposition of [C

2 1 2

C im][NTf ], 2, has been

previously studied in detail, and a decomposition mechanism was

17c

suggested. Chen and colleagues observed a peak at m/z 64, which

+

ꢀ

was assigned as [SO ] . In addition, their experiments showed

2

smaller peaks corresponding to a number of other degradants,

including 1-methylimidazole, 1-ethylimidazole, ethene, CH

and CHF . They therefore proposed a mechanism for the decom-

position of 2, involving an anion-centred decomposition to gen-

3 2

NH

3

ꢀ

ꢀ ꢀ

erate reactive radical intermediates (CF

3

, NH

2

, F ), which then

+

react with the [C C im] cation. The results of our TGA-MS studies

2 1

on compound 2 (shown in the ESI†) are consistent with their

findings; peaks at m/z 64 and m/z 69 were seen during the main

+

ꢀ

+

decomposition period, which were assigned as [SO₂] and [CF ] ,

3

respectively. Therefore, the presence of these peaks suggests degra-

À

dation of the [NTf

liquid 2.

In the TGA-MS experiments for the archetypal carboxylate ionic

liquid [C im][OAc], 3a, the onset of thermal decomposition is

2

]

anion during the decomposition of ionic

2 1

C

accompanied by the detection of a large peak at m/z 74 in the mass

spectrometer, which is assigned to the parent ion of methyl acetate,

+

ꢀ

CH CO CH ] (Fig. 7a and 8). This strongly suggests an S 2

3 2 3 N

[

+

2 1

mechanism occurring at the methyl substituent of the [C C im]

cation, because it is unlikely that any other mechanism could

produce substantial amounts of this degradant (Fig. 6). The peak at

+

m/z 59 was assigned to [CH

3

CO

2

] , a known fragment of methyl

acetate, but not of acetic acid or ethyl acetate in EI mass spectro-

30

metry. This is further evidence for the S

N

2 mechanism occurring

at the methyl substituent.

+

ꢀ

A small peak corresponding to ethyl acetate, [CH CO CH CH ]

3

2

3

Fig. 7 TGA-MS data for ionic liquids [C

2 1 2 1 3 2

C im][OAc], 3a, (a), [C C im][CF CO ], 4,

(

m/z 88), was also observed, indicative of nucleophilic attack at

(

b), and [C im][SAc], 12, (c). The displayed MS histograms were recorded

2 1

C

the more sterically crowded ethyl substituent (Fig. 8). These

mechanisms are consistent with the nucleophilic character of

the acetate anion. An acetic acid parent peak (m/z 60) is detected

during the main thermal decomposition period of the ionic liquid. Peaks were

assigned to parents or fragments of expected decomposition products. Spectra

are shown in the region 40–150 m/z, for clarity.

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20485

View Article Online

PCCP

Paper

2

as a minor decomposition product (Fig. 8). This suggests that

2 1 1

For the C -methyl substituted ionic liquid, [C C C im][OAc],

either an E2 Hofmann elimination mechanism or NHC for- 14, the NHC decomposition route (mechanism A, Fig. 6) is not

2

mation is occurring to a small extent. No fragment at m/z 60 is possible. Instead, a proton may be abstracted from the C

expected in the EI mass spectrum of either methyl acetate or methyl group, generating acetic acid and a neutral species,

3

0

ethyl acetate, therefore our observed peak at m/z 60 cannot be 1-ethyl-3-methyl-2-methylene-2,3-dihydroimidazole (m/z 124).

regarded as a fragment of either ester decomposition product. However, no detectable peak was observed at m/z 124 during the

Interestingly, no significant peaks were observed for the decomposition of ionic liquid 14, suggesting that this mechanism

molecular ion of 1-methylimidazole (m/z 82), 1-ethylimidazole does not occur to any measurable extent. Instead, the major

(

2

m/z 96), or for the N-heterocyclic carbene 1-ethyl-3-methylimidazol- observed decomposition product in the TGA-MS experiment for

+

ꢀ

-ylidene (m/z 110) that would form in the event of NHC ionic liquid 14 was methyl acetate, [CH CO CH ] , at m/z 74.

3 2 3

decomposition (mechanism A, Fig. 6). The neutral heterocycle Therefore, it is likely that the decomposition of both acetate ionic

-ethylimidazole should be present as a product of the S_N2 liquids, 3a and 14, occurs predominantly via an S 2 nucleophilic

1

N

thermal decomposition mechanism occurring at the methyl attack at the imidazolium methyl substituent.

substituent of the 1-ethyl-3-methylimidazolium cation of 3a. The TGA-MS results therefore indicate the occurrence of S

N

2

Attempts to confirm 1-ethylimidazole as a decomposition nucleophilic substitution decomposition mechanisms (3a, 8, 12

1

product of 3a, by high-temperature H NMR experiments, are and 14), or anion-centred decomposition, generating reactive

+

described below.

intermediates which subsequently attack the [C

2

C

1

im] cation

2 1 3 2

The low Tonset value for [C C im][CF CO ], 4, can be under- (2 and 4). For each of the ionic liquids investigated in this

stood in terms of an anion-centred decarboxylation mecha- TGA-MS study, there was no substantive evidence of the anion

À

ꢀ

nism, enabled by the stability of the CF₃anion or CF₃radical behaving as a base.

that would be formed. The TGA-MS data for this compound

The decomposition of [C C im][OAc], 3a, was explained in

2 1

N

supports this hypothesis, as indicated by the presence of peaks terms of an S 2 nucleophilic substitution mechanism at the

+

ꢀ

+

at m/z 44, [CO

at m/z 69 could be attributed to multiple plausible decomposition large observed peak for methyl acetate in the TGA-MS experiment.

products of [C im][CF CO ], 4, including CHF , trifluoroacetic However, the other major expected decomposition product,

acid, methyl trifluoroacetate and ethyl trifluoroacetate. However, 1-ethylimidazole, was not observed.

2 3 2 2 1

] , m/z 69, [CF ] , and m/z 51, [CHF ] . The peak methyl substituent of the [C C im] cation, on the basis of the

2

C

1

3

2

3

0

+

the fragment at m/z 51, [CHF ] , strongly suggests the presence of

In an attempt to monitor the formation of 1-ethylimidazole

2

CHF as a major decomposition product of ionic liquid 4, since it is and other decomposition products, a neat sample of 3a was

3

not a known fragment of the potential acid or ester products heated to 120 1C for 12 hours in a sealed, thick-walled NMR

30

1

generated by S

4 is observed throughout the duration of the TGA-MS experiment high-temperature H NMR experiment was performed unlocked, in

CO is a minor contaminant of the helium carrier gas), there is a the absence of any NMR solvent. Therefore, to determine the

N

2 or E2 mechanisms. Although the peak at m/z tube. A H NMR spectrum was recorded every 30 minutes. The

1

4

(

2

visible spike in intensity corresponding to the onset of mass loss. chemical shifts of the ionic liquid and decomposition product

The TGA-MS data strongly suggests an anion-centred decarboxyla- peaks, two separate control NMR spectra were obtained, whereby

tion mechanism for the thermal decomposition of ionic liquid 4.

pure [C C im][OAc] had been deliberately spiked with a small

2 1

The TGA-MS results for the octanoate ionic liquid, 8, were quantity of an expected decomposition product, 1-methylimidazole

compared with those for the acetate ionic liquid, 3a, in order to and 1-ethylimidazole respectively. A DMSO-d capillary was used as

6

understand the eﬀect of alkyl chain extension on the prevailing a reference in each reference sample.

mechanisms of thermal decomposition. For 8, a peak was

During the 12-hour experiment, no additional peaks were

observed at m/z 158, which has been assigned as the parent observed. It is possible that the broadened ionic liquid peaks

+

ꢀ

peak of methyl octanoate, [CH

an S 2 nucleophilic attack of the octanoate anion at the methyl tion products were partially in the gas phase. However, upon

substituent of the [C C im] cation (mechanism B, Fig. 6). cooling 3a to room temperature, new peaks not present in the

3 2 6 2 3

(CH ) CO CH ] . This suggests engulfed the decomposition product peaks, or that the decomposi-

N

+

2

1

Lower intensity peaks were observed that can be assigned to ionic liquid prior to the high-temperature experiment were

1

fragments of octanoic acid and ethyl octanoate. Unlike the TGA- observed in the H NMR spectrum (Fig. 9). Several peaks in the

MS experiment for 3a, the parent peaks of 1-methylimidazole region 7.4–6.1 ppm were observed at approximately 1–2% of the

(m/z 82) and 1-ethylimidazole (m/z 96) were observed in the integration of the ionic liquid aromatic peaks. The chemical shifts

spectrum of [C im][CH (CH CO ], 8. The presence of these of these aromatic peaks matched the neutral heterocyclic molecules

2

C

1

3

2

)

6

2

two neutral imidazole products is further evidence for S

decomposition mechanisms.

N

2-type 1-methylimidazole and 1-ethylimidazole (Fig. 9a). In addition, a

triplet peak was observed at 0.69 ppm, which has been assigned to

For the ionic liquid [C C im][SAc], 12, a large peak at m/z 90 was the terminal –CH₃unit of the ethyl chain of 1-ethylimidazole

2

1

observed, corresponding to the parent ion of methyl thioacetate, (Fig. 9b). A considerably smaller peak was observed for the aliphatic

+

ꢀ

CH COSCH ] . This is strong evidence of an S 2 mechanism methyl group of 1-methylimidazole. This lends evidence towards an

3 3 N

[

+

occurring at the methyl substituent of the [C

analogous to the decomposition of 3a. This is unsurprising, substituents of the 1-ethyl-3-methylimidazolium cation, but predo-

given the known high nucleophilicity of the thioacetate anion. minantly at the less hindered methyl substituent.

2 1

C im] cation,

N

S 2 nucleophilic substitution reaction occurring at both alkyl

2

0486 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

3^À

that the CF anion, generated by the decarboxylation mechanism,

then removes a b-proton from the ethyl substituent on the

+

[

C C im] cation in an E2-type mechanism, generating the observed

2

1

1-methylimidazole.

Smaller peaks were observed at the same chemical shifts

1

in the H NMR of 5, following the 18 hour heating period at

00 1C, indicating that a very small quantity of 1-methylimidazole

1

had also been generated from the thermal decomposition of this

ionic liquid. 1-Methylimidazole was not employed at any stage of

the synthesis of compounds 4 and 5, therefore it cannot be

regarded as an impurity.

The thermal decomposition of ionic liquids 3b, 4, 11 and 14

was subsequently investigated using Diﬀerential Scanning

Calorimetry (DSC), and DHdecomposition values were measured.

In order to correlate the DSC peaks to specific decomposition

processes observed in the TGA, a temperature-ramped TGA

experiment was performed for each of the four ionic liquids

À1

at the same slow heating rate of 2.5 1C min . These TGA

thermographs were subsequently superimposed on the DSC

traces. DSC procedures are described in the experimental

section of this contribution and in the ESI.† An example graph

2 1

for [C C im][OAc], 3b, is displayed in Fig. 10. Graphs for ionic

liquids 4, 11 and 14 are displayed in the ESI† (Fig. E7).

DHdecomposition data for the four ionic liquids is shown in

Table 2.

The DSC profile for the first heating step of 3b displayed one

broad endothermic peak, between approximately 200–250 1C.

This endothermic peak occurs in the same temperature range

1

2 1

Fig. 9 Expanded regions of the H NMR spectrum of [C C im][OAc] at room

temperature, after the 12 h experiment at 120 1C. (a) New aromatic peaks were

observed in the region 7.4–6.1 ppm, which were assigned to 1-alkylimidazole

decomposition products. (b) A triplet peak was observed at 0.69 ppm; the

chemical shift and splitting pattern indicate 1-ethylimidazole.

as the TGA weight loss, and has been assigned as decomposi-

À1

Subsequently, thermal decomposition of the fluoroalkyl carboxyl- tion of the ionic liquid, with DHdecomposition = +51.9 kJ mol

ate ionic liquids [C im][CF CO ], 4, and [C im][CHF CO ], 5, (Fig. 10).

2

C

1

3

2

C

1

2

1

19

was investigated using H and F NMR. Small samples (ca. 20 mg)

The DSC profile representing the decomposition of [C

2 1

C im]-

of 4 and 5 were placed in 25 ml sealed glass pressure vessels, and [CF

3

CO ], 4, demonstrated a two-step process, corresponding to

2

maintained at 100 1C for 18 hours, before being allowed to cool to the two visible decomposition stages in the temperature-ramped

1

room temperature. The residues were dissolved in DMSO-d , and H TGA thermograph. Both steps were endothermic in nature and

6

1

9

À1

and F NMR spectra were recorded.

of similar magnitude, +15.7 and +10.5 kJ mol , respectively.

1

9

The F NMR experiments did not reveal any fluorine-

containing thermal decomposition products for either ionic

liquid 4 or 5. Two expected decomposition products from the

3 2

anion decarboxylation of 4 are CHF and CO , which are both

gaseous products (Fig. 6). Therefore, if formed, these thermal

decomposition products would have escaped when the pressure

tubes were opened. The high T_o_n_s_e_tvalue for [C C im][CHF CO ],

2

1

2

5, may explain why no fluorine-containing products were

observed for this ionic liquid after the 18 hour 100 1C heating

period.

1

By contrast, small peaks were observed in the H NMR

spectrum for 4 that were not present in the pure ionic liquid

before commencing the experiment. These peaks, at 7.18, 6.87

4

5

and 3.64 ppm, were subsequently assigned to the C , C and

CH protons on the neutral heterocycle 1-methylimidazole, by

–

3

comparing the spectrum of 4 after the heating period against a

1

6

reference H NMR spectrum of 1-methylimidazole in DMSO-d .

1

19

These H and F NMR results are compatible with the previous

observation that [C im][CF CO ], 4, decomposes via an

Fig. 10 DSC trace for the decomposition of [C

2 1

C im][OAc], 3b, shown in the

2

C

1

3

2

range 100–300 1C (blue). A temperature-ramped TGA thermograph is super-

anion-centred decarboxylation (Fig. 7b). Therefore, it is likely imposed (black).

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20487

View Article Online

PCCP

Paper

Table 2 DHdecomposition values for four ionic liquids, measured using Diﬀerential

Scanning Calorimetry (DSC). Each ionic liquid exhibited broad endothermic

decomposition

À1

Ionic liquid

DH_d_e_c_o_m_p_o_s_i_t_i_o_n(kJ mol

)

3

4

1

b

[C

2

[C

2

[C

2

[C

2

C

1

C

1

C

1

C

1

im][OAc]

im][CF CO

im][c-(C

im][OAc]

51.9

15.7, 10.5

64.7

3

2

]

1

4

3 5 2

H )CO ]

C

1

18.0

Ionic liquid 11, containing the strained cyclopropane ring,

exhibits endothermic decomposition in a similar fashion to the

acetate ionic liquid 3b. The measured DHdecomposition value of

À1

+

64.7 kJ mol for 11 is comparable to the value for 3b. No

additional peaks were observed in the DSC decomposition profile

for 11, suggesting that opening of the cyclopropane ring does not

occur. A similar shallow, broad endothermic decomposition DSC

2

trace was obtained for the C -methyl ionic liquid, [C C C im][OAc],

2

1

À1

1

4, and DHdecomposition was measured as +18.0 kJ mol

.

Fig. 11 (a) Diagram representing the location of primary cation–anion inter-

action for [C

ion pair, showing primary H-bond interactions. (c) Diagram representing a dimer

conformer of [C im][OAc].

2 1

C im][OAc]. (b) Structure of the lowest energy ‘Front Methyl + Ethyl’

The four ionic liquids investigated by DSC (3a, 4, 11 and 14)

appeared to thermally decompose entirely to volatile products;

the aluminium DSC pans were opened after the experiments, and

there was no observable decomposition residue. Accordingly,

the second heating scan of these DSC experiments displayed no

noticeable peaks. However, one very small endothermic peak was

observed in the cooling stage for [C C im][CF CO ], 4, at 247 1C.

2 1

C

2

1

3

2

This suggests that a small quantity of decomposition residue was

present after the first heating stage for this ionic liquid.

In summary, the DSC experiments indicated that for the four

investigated imidazolium ionic liquids incorporating alkyl (3a, 11,

14) and fluoroalkyl (4) carboxylate anions, thermal decomposition

is an endothermic event with positive values of DHdecomposition. In

addition, there was an absence of any decomposition residue for

the three non-fluorinated ionic liquids investigated, although a

small amount of residue may have been formed from the decom-

2 1 3 2

position of [C C im][CF CO ], 4.

Fig. 12 (a) Diagram representing the location of primary cation–anion inter-

action for [C im][OAc]. (b) Structure of the lowest energy ‘Top Methyl 1’ ion

pair, showing primary H-bond interactions.

Computational results

2 1 1

C C

The liquid structure of 1-ethyl-3-methylimidazolium acetate,

[C

2

C

1

im][OAc], and the interaction of this ionic liquid with

water and CO

, have been previously explored using experi- lowest energy ion pair of [C

mental and computational techniques.

In this investigation, DFT was used in order to understand

the ion pair structuring and, subsequently, the decomposition located in the plane of the imidazolium ring, in agreement with

C

im][OAc] and [C

are listed in the ESI† (Fig. E8 and E9, respectively).

The results suggest that the acetate anion is preferentially

C C im][OAc]

2

1

2 1 1

3

1

3

1a,d

mechanisms of the two acetate ionic liquids, [C

im][OAc]. All calculations were performed at the B3LYP- pair conformers of [C

D/6-311++G(d,p) level of theory. Full details of the computa- determined for the prototypical ionic liquid 1-butyl-3-methyl-

2

C

1

im][OAc] and previous studies on this ionic liquid.

Anion sites for our ion

[C

2

C

1

C

1

2 1

C

im][OAc] resemble those previously

3

2

tional methods are described in the ESI.†

4 1

imidazolium chloride, [C C im]Cl. Whereas the chloride

Fig. 11 and 12 represent the low energy ion pair conformers anion is monoatomic, acetate can bind strongly through

for [C C im][OAc] and [C C C im][OAc] obtained in this study, the two oxygen atoms of the carboxylate group. As a result,

2

1

2 1 1

and the naming convention of the ion pairs used in the the low energy ‘Front’ conformers are more centralised, with the

2

following discussion. Tables 3 and 4 summarise the DG and acetate binding both to the C proton and a-/b-protons on the

DE (BSSE and ZPE corrected) energies of these ion pair con- methyl and ethyl side chains. In the front and back ion pair

formers. The DH, TDS DZPE and BSSE energies are listed in the conformers, both oxygen atoms of the acetate anion remain

ESI† (Tables E2 and E3). In addition, the NBO charges for the essentially in the plane of the imidazolium ring, whereas in the

2

0488 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

Table 3 Ion pair energies for [C

2

C

1

im][OAc]. DE values are zero-point and BSSE ‘Front Methyl + Ethyl’ and ‘Top’ conformers are displayed in the

corrected

ESI† (Fig. E10). The structure of the front ion pair was largely

À1

unchanged upon rotation of the ethyl chain, with the acetate

Ion pair [C

2

C

1

im][OAc]

DE (kJ mol

)

DG (kJ mol )

2

anion remaining bound to the C proton and a-proton of the

From methyl + ethyl

Front ethyl

Front methyl

Top

Side ethyl (down)

Side methyl

Side ethyl (up)

Back methyl

Back ethyl

0.0

4.9

0.0

3.2

3.5

11.0

42.0

43.8

46.9

57.2

57.3

À1

methyl side chain. Small rotational energy barriers of o10 kJ mol

were observed for ethyl rotation of the front conformer. For the top

ion pair conformer, rotation of the ethyl chain carried the acetate

anion towards the front of the imidazolium ring, giving structures

analogous to the ‘Front Methyl’ and ‘Front Methyl + Ethyl’ minima.

In a recent molecular dynamics study, Brehm and co-workers

investigated the preferred location of the acetate anion, relative

9.4

41.4

46.0

44.9

59.6

55.7

+

to the [C C im] cation, by examining the angle between the ring

2

1

normal vector and the connecting line from the centre of the

ring to the position of the acetate carboxylate carbon atom.

2 1 1

Table 4 Ion pair energies for [C C C im][OAc]. DE values are zero-point and

3

1i

BSSE corrected

Their results indicated that in the neat ionic liquid, the acetate is

largely associated with the plane of the ring (angle approximately

901). However, their results also showed some probability of the

acetate being located out of the ring plane by up to approxi-

mately 301. To this end, we have investigated a sample of dimer

structures for [C C im][OAc] (stable geometries involving two

cations and two anions). Structures were optimised as for the ion

pairs, at the B3LYP-D/6-311++G(d,p) level of theory. Although our

calculations have not yet yielded an exhaustive list of dimer

structures, the results do indicate the possibility of the acetate

À1

Ion pair [C

2

C

1

C

1

im][OAc]

DE (kJ mol

)

DG (kJ mol )

Top methyl 1

Top methyl 2

Bottom

Top ethyl

Side ethyl

Side methyl

Back ethyl

Back methyl

0.0

À1.8

À0.6

À1.0

29.9

33.2

46.4

48.7

0.0

0.3

1.0

5.8

24.9

34.1

41.9

43.1

2 1

top and side conformers the acetate lies out of the plane of the ring. anions being located out of the plane of the imidazolium rings.

The two distinct ‘Side Ethyl’ conformers are denoted ‘up’ (acetate An example optimised dimer structure is displayed in Fig. 11c,

and ethyl chain both lie above the plane of the imidazolium ring) and in the ESI† (Fig. E11).

and ‘down’ (acetate and ethyl chain on opposite sides of the plane

of the ring).

For the ionic liquid 1-ethyl-2,3-dimethylimidazolium acetate,

2

[C C C im][OAc], the C proton has been replaced by a methyl

2

1 1

The lowest energy conformer (Fig. 11) located in this investiga- substituent, and thus there are no equivalent stable ‘Front’ ion

tion is the ‘Front Methyl + Ethyl’ structure, similar to the structure pair conformers. Instead, the low energy conformers are ‘Top’

28d

reported by Holl o´ czki and co-workers.

In this conformer, the and ‘Bottom’ structures, whereby the acetate anion sits above or

ethyl chain rotates forward in order to bind to the acetate anion below the plane of the imidazolium ring. Four distinct ‘Top’ and

through a b-proton. Two other ‘Front’ conformers, ‘Front Methyl’ ‘Bottom’ structures were located, with relative energies within a

À1

and ‘Front Ethyl’ (Table 3), are approximately 3.5 kJ mol higher in 6 kJ mol range. The lowest energy ion pair structure obtained,

energy, and lack this interaction with the ethyl b-proton. A ‘Top’ ion shown in Fig. 12b, involves the H-bond interaction of the acetate

2

pair structure was obtained, whereby the acetate anion lies above anion with protons on the C methyl substituent and a-protons

the plane of the imidazolium ring, with an energy approximately on both the methyl and ethyl side chains. The ethyl side chain is

À1

1

0 kJ mol greater than the ‘Front Methyl + Ethyl’ structure. rotated almost into the plane of the ring to facilitate this interaction.

Attempts to optimise the top conformer at the B3LYP/

Plausible thermal decomposition mechanisms of [C

2 1

C im][OAc]

6

-311++G(d,p) level of theory were unsuccessful, indicating that and [C C C

2 1 1

im][OAc] have been explored using DFT. The

the inclusion of dispersion is necessary for locating this ion pair. decomposition routes under investigation are displayed in

Bowron and co-workers observed a high probability of the Fig. 13. Mechanisms A–D are analogous to those shown for

acetate anion lying in the plane of the imidazolium ring the TGA-MS results (Fig. 6). For each of the two ionic liquids,

for [C

2 1

N

C im][OAc], which resulted in a decrease in anion possible decomposition mechanisms include the S 2 attack of

3

1d

density above and below the ring.

As a result, cation–cation the acetate anion at either the N-methyl (mechanisms B1 and

p-stacking was possible. Interestingly, in our investigation no B2) or N-ethyl (mechanisms C1 and C2) substituents on the

front ion pair conformers were located where the acetate lies imidazolium cation, or the E2 Hofmann elimination mecha-

perpendicular to the plane of the imidazolium ring.

nism (mechanisms D1 and D2) occurring at the ethyl substi-

2

Conformational analysis of the ethyl side chain was per- tuent. The decomposition mechanisms at the C position

formed for the low energy ‘Front Methyl + Ethyl’ and ‘Top’ ion (mechanisms A1 and A2) are different for the two acetate ionic

2

pair conformers of [C C im][OAc]. In each case, the C –N–C –C

liquids; for [C C im][OAc], the formation of an NHC is possible,

2 1

2

1

a

b

2

dihedral angle was rotated in increments of 301, and all other whereby the C proton is removed by the basic acetate anion

coordinates were optimised. The change in energy and structure (mechanism A1). For [C im][OAc], the carbene-forming

2 1 1

C C

of the ion pair was recorded as a function of the dihedral angle. mechanism is not possible. Instead the basic acetate anion

2

Graphs representing the conformational analysis around the may remove a proton from the C methyl substituent,

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20489

View Article Online

PCCP

Paper

Table 5 Transition state and product energies for the S

N

2 and E2 mechanisms of

[C

2

C

1

im][OAc] and [C C C im][OAc], relative to the lowest energy ion pair

2 1 1

conformer of each ionic liquid. DE values are zero-point corrected

À1

Transition state (kJ mol

)

Product (kJ mol )

Mechanism DE

DG

DH

TDS

DE

DG

DH

TDS

Bl

C1

D1

139.1 134.6 139.5

135.6 135.4 136.2

156.4 156.1 156.8

4.9

0.8

4.0 À35.3

0.6 35.9

0.2 À39.6 À3.0 36.7

0.7 61.3 À25.8 63.3 89.1

B2

C2

D2

119.8 118.5 119.1

0.5

2.3 À41.1 À1.0 40.1

116.7 119.3 114.8 À4.6 À0.1 À41.7 À5.2 36.5

138.6 137.7 138.6

0.9 61.0 À27.8 61.1 88.9

mechanism (Table 5). In addition, the calculated DHactivation ener-

À1

gies for the S

N

2 mechanisms are 15–24 kJ mol higher than the

experimentally determined E

a

values from the isothermal TGA

À1

a 2 1

experiments (E = 115.8 kJ mol for [C C im][OAc], 99.6 kJ mol

for [C im][OAc]). The TGA-MS data (Fig. 7 and 8) suggests that

2 1 1

C C

S_N2 attack occurs predominantly at the methyl substituent of the

cation, however the calculations find little difference in the activa-

tion barriers for attack at the N-methyl or N-ethyl substituents.

Fig. 13 Decomposition routes investigated for the two acetate ionic liquids in These results indicate that there is no inherent large energetic

2 1 2 1 1

this computational study: [C C im][OAc], A1–D1, and [C C C im][OAc], A2–D2.

difference between these reactions and that steric effects are most

likely responsible for the more facile reactivity at the N-methyl

substituent.

2 1 2 1 1

The DSC investigation of [C C im][OAc] and [C C C im][OAc]

indicated that thermal decomposition was an endothermic event

À1

(

DH_d_e_c_o_m_p_o_s_i_t_i_o_n= +51.7 and +18.0 kJ mol , respectively, Fig. 10).

By contrast, our gas phase calculations suggested that the

2 mechanisms of these two

DHproduct-reactant values for the S

N

ionic liquids are close to zero. However, the DSC experiments

involve both the thermal decomposition of the ionic liquid and

an associated phase change of the decomposition products from

the ionic liquid to gaseous molecules. Therefore the diﬀerence in

DH_p_r_o_d_u_c_t_-_r_e_a_c_t_a_n_tvalues from the gas phase calculations and DSC

experiments give an indication as to the magnitude of D_v_a_p

associated with the vaporisation of neutral ester and imidazole

products from the S 2 thermal decomposition mechanisms. It is

unclear whether thermal decomposition occurs from within the

liquid phase, followed by subsequent vaporisation of the neutral

species, or whether decomposition is occurring primarily at the

H

2 1 N

Fig. 14 Transition states for [C C im][OAc], representing the S 2 nucleophilic

N

substitution (B1 and C1) and E2 Hofmann elimination (D1) mechanisms, atomic

distances are in angstroms.

generating acetic acid and the neutral species, 1-ethyl-3-methyl- surface of the ionic liquid.

2

-methylene-2,3-dihydroimidazole (mechanism A2).

Decomposition reaction profiles have been studied starting from dialkylimidazolium ionic liquids is now widely recognised.

The potential for N-heterocyclic carbene (NHC) formation in

2

8

the lowest energy ion pair of [C

2

C

1

im][OAc] and [C

2 1 1

C C im][OAc], This phenomenon has been exploited in catalytic applica-

3

2

(‘Top Methyl + Ethyl’ and ‘Top Methyl 1’, respectively). The tions. In the case of [C im][OAc], removal of the C proton

N

2 and E2 decomposition mechanisms (B1–D1, B2– by the basic acetate anion would generate acetic acid and the

2

C

1

respective S

D2) are analogous for the two acetate ionic liquids, and are NHC species, 1-ethyl-3-methylimidazol-2-ylidene (mechanism

therefore discussed first. The transition state structures for A1, Fig. 13). A stable minimum exists involving the interaction

mechanisms B1, C1 and D1 are shown in Fig. 14, and the of acetic acid and the NHC, which is referred to here as the

relevant DE, DG, DH and TDS energies are displayed in Table 5. ‘Carbene-Acid Pair’ (CAP). Holl o´ czki and colleagues performed

2

The decomposition mechanisms at the C position (A1 and A2) DFT calculations on the ion pair and CAP structure of

are less trivial, and are discussed separately shortly.

[C

2

C

1

im][OAc] using a range of functionals and basis sets,

À1

For both acetate ionic liquids, the DHactivation energies for and found the energy diﬀerence to be less than 20 kJ mol

2

8d

2

the two S

N

2 nucleophilic substitution mechanisms are approxi- in each case. Their calculations indicated that the C proton

À1

mately 20 kJ mol lower than for the E2 Hofmann elimination is very labile in the gas phase. Moreover, mass spectrometry

2

0490 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

Fig. 15 Gas phase ‘Front Methyl + Ethyl’ ion pair, transition state and carbene-

acid pair (CAP) structures for mechanism A1.

experiments indicated the presence of acetic acid (m/z 60) and

the isolated neutral carbene species (m/z 110) as decomposition

products of [C C im][OAc] in the gas phase.

2 1

In our investigation, structures relating to the NHC decom-

position route were optimised, and are shown in Fig. 15. The

2

C –O(acetate) distances, displayed in blue, are not exactly equal

2

to the sum of the C –H and O–H distances because of the slightly

non-linear bond angle. Bond distances are in angstroms.

The SCF energies showed the TS to be lower in energy than

À1

the CAP product; 0.0, 12.6 and 13.7 kJ mol for the ion pair, TS

and CAP respectively. When ZPE corrections were taken into

À1

account this eﬀect was enhanced; 0.0, 1.7 and 11.5 kJ mol for

the ion pair, TS and CAP respectively. These energies suggest

that the transition state is lower in energy than the CAP, which

is implausible.

2

Fig. 16 3D energy surface for the proton transfer between the imidazolium C

carbon atom and the acetate oxygen atom of [C C im][OAc] (mechanism A1).

2 1

However, the optimised TS structure has a single strong The white line represents the lowest energy pathway from the ion pair through

À1

the transition state to the CAP (critical points shown as green circles).

negative frequency of À728 cm . Moreover, the intrinsic

reaction coordinate (IRC) protocol was employed, and demon-

strated that this TS structure does link the ion pair and CAP.

In order to understand why the TS falls at a lower energy essentially ‘vanishes’ before a CAP minimum has fully

than the CAP product, the structures in Fig. 15 were investi- developed.

gated further. The gas phase calculations (Fig. 15) indicate a

The energy, E

n

, of the transition state vibrational mode

2

À1

contraction of the C –O(acetate) distance in the TS (2.59 Å) (À728 cm

or 8.7 kJ mol ) is comparable to the energy

relative to the ion pair (2.78 Å) and CAP (2.71 Å). Thus, the diﬀerences between the ion pair, transition state and CAP

2

À1

C -carbon–oxygen distance was fixed at a range of values (o14 kJ mol ). To compare the energies of these structures

between 2.4 and 2.8 angstroms (Å), and the hydrogen atom on a more equal footing the vibrational energy associated with

moved in increments of 0.05 Å, computing the potential energy the reaction coordinate has been removed from the zero point

surface (PES) shown in Fig. 16.

corrected energies for the reactant and product. (Because the

2

This PES (Fig. 16) reveals that at short C –O distances, ( y) in TS vibration is imaginary it has already been excluded from

the range 2.4–2.6 Å, a very broad flat single energy well exists the zero point corrected energy of the TS.) This corresponds to

À1

spanning all of the ion-pair, TS and CAP structures (rear blue the C–H vibration in the ion pair (2507 cm or 30.0 kJ mol

)

2

line in Fig. 16). As the C –O distance is increased towards 2.8 Å, and the O–H vibration in the CAP (2139 cm or 25.6 kJ mol ).

À1

the ion pair is stabilised and a barrier develops between the The resulting effective energies (DEeff) are 0.0 kJ mol for the

ion-pair and CAP. The barrier increases as C –O increases in ion-pair, 31.7 kJ mol for the TS and 15.9 kJ mol for the CAP

the range 2.6–2.8 Å. (ESI,† Table E5). The relative magnitudes of these effective

A scan performed with a fixed C –O distance, ‘y’, of 2.8 Å energies provide a more reasonable reaction profile for mechanism

shows the presence of a true TS (26.5 kJ mol ), front blue line A1, with a transition state of higher energy than the ion pair

2

À1

2

À1

in Fig. 16. The CAP is approximately 9 kJ mol higher in energy and CAP.

2

than the ion pair. Thus, a more reasonable reaction profile has

been obtained.

Altogether, these results show that a TS exists at the C –O

distances of the ion-pair and CAP (B2.76 Å), but that the two

2

The white line on Fig. 16 shows the likely reaction path, ions must contract significantly (reduce C –O distance, ‘y’) to

starting from the ion pair minimum through the lowest energy follow the lowest energy path for the reaction.

2

TS (which occurs in the region of the PES where the barrier is

just developing) and finishing in the CAP product. Thus, the TS [C

Despite the known labile nature of the C proton of

2

C

1

im][OAc] in the gas phase, no strong evidence for an

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20491

View Article Online

PCCP

Paper

NHC decomposition pathway under inert conditions was pair. Introducing a weakly polar solvent environment (e

observed in our experimental investigation. Acetic acid and lowers the ion pair energy by 38 kJ mol relative to the gas

compounds relating to the NHC species were observed as only phase calculation, compared to approximately 17 kJ mol for

r

= 2.28)

À1

very minor peaks in the TGA-MS experiments; instead, the the CAP. For the more polar solvent environment (e = 33.0), the

r

À1

major observed decomposition products were indicative of energy of the ion pair is lowered by B73 kJ mol relative to the

S

N

2 nucleophilic substitution mechanisms.

To further explore NHC formation in the liquid phase, lowered by only 32 kJ mol

gas phase calculation, and in contrast the energy of the CAP is

À1

.

mechanism A1 was investigated using DFT and a Conductor-type

Polarisable Continuum Model (CPCM). The CPCM calculations extremely shallow in the region of the CAP, with a very small

The potential energy surface within the polar medium is

34

approximate the liquid phase environment and provide a rudimen- energy barrier for the reverse reaction to re-form the ion pair

À1

tary approximation of the bulk polar liquid and hence stabilisation (for example, DE_a_c_t_i_v_a_t_i_o_n= o2 kJ mol for the CH OH solvent,

3

due to a polar environment. Solvents benzene (C H , e = 2.28), Fig. 17). Thus, the PES has a ‘shelf-like’ feature that develops into a

6

r

dichloromethane (CH Cl , e = 8.93) and methanol (CH OH, e = small barrier in less polar environments; such a low barrier will be

2

r

3

r

3

3.0) were used to demonstrate the eﬀects of a medium increas- diﬃcult to observe experimentally, and will result in a highly

21,34

ingly capable of stabilising an ionic solute.

dielectric constant for the molecular solvents CH

cover the range anticipated to be typical for ionic liquids.

No stable structures were obtained using the CPCM environ- or compressed air) conditions.

The values of the reversible mechanism. This feature may explain why the CAP can

2

3

OH and CH

2

Cl

2

be trapped with CO

2

,

but is not observed as a primary decom-

35

position route at high temperatures under inert (nitrogen, helium

ment for the TS or CAP structures. This is consistent with

In addition, these results indicate that it may be possible to

recent ab initio MD simulations of the ionic liquid encourage NHC formation by diluting the ionic liquid with a

2

8h

[

C C im][OAc].

Subsequently, scans were performed with a neutral non-polar solvent, reducing the ‘charge-screening

2

1

2

fixed C –O distance, ‘y’, of 2.76 Å in the gas phase and using eﬀect’. Alternatively, forcing the C –O distance to contract,

CPCM conditions, Fig. 17. The fixed coordinate prevents con- perhaps under high pressure, may facilitate carbene formation.

traction of the ‘y’ distance, and therefore these scans do not

2 1 1

For [C C C im][OAc], the abstraction of a proton from the

2

perfectly represent the proton transfer mechanism. However, C -methyl group by the acetate anion generates acetic acid and

frequency analysis of the maximum structure in the gas phase 1-ethyl-3-methyl-2-methylene-2,3-dihydroimidazole (mechanism

scan (Fig. 17) revealed the expected (single imaginary) vibra- A2, Fig. 13). This mechanism was also initially characterised by

tional mode corresponding to mechanism A1.

gas phase DFT calculations, and subsequently using CPCM.

The CPCM scans of Fig. 17 suggest that the energy of the ion Reaction profiles and a transition state structure for decomposi-

pair is stabilised to a far greater extent than the neutral CAP tion mechanism A2 are displayed in the ESI† (Fig. E13).

and TS in a polarised medium. These results are supported by

The calculations suggest that even in the gas phase, the ion pair

the ‘charge-screening’ eﬀects observed in ab initio MD simula- is of lower energy than the neutral products. Again, the CPCM

2

8h

tions of the ionic liquid [C

2

À1

C

1

im][OAc].

2 2 3

In the gas phase calculations using CH Cl and CH OH solvents bring about a large

scan, the CAP is B8 kJ mol higher in energy than the ion stabilisation of the ion pair, and a significantly smaller stabilisation

of the transition state and product energies. Similar to [C C im]-

2

1

[

OAc], the CPCM calculations for [C C C im][OAc] indicate that the

2 1 1

ion pair is thermodynamically more stable than the neutral pro-

ducts when in a polarised medium. Moreover, the very small energy

À1

barrier for the reverse reaction (DGactivation r 3 kJ mol , CPCM

3

calculation using a CH OH solvent) indicates that the reaction is

likely to be highly reversible.

Therefore, for the two decomposition mechanisms occur-

ring at the front of the imidazolium ring (A1 and A2, Fig. 13),

the CPCM calculations indicate stabilisation of the ion pair

relative to the neutral products in an ionic environment. In

addition, for each mechanism there is a minimal energy barrier

À1

(

o3 kJ mol ) for the reverse reaction. These results suggest

that mechanisms A1 and A2, if occurring in the bulk liquid, are

likely to be highly reversible. Our TGA-MS experimental data

corroborates with this observation, because the expected neutral

decomposition products generated by these two mechanisms

were not observed.

Moreover, the measured activation energies, E , for thermal

decomposition of both acetate ionic liquids were in reasonable

2

a

Fig. 17 Change in DE during the proton shift between the C carbon atom and

the acetate oxygen atom (mechanism A1), in the gas phase, and with CPCM

N

agreement with the calculated DHactivation values for the S 2 thermal

decomposition mechanisms. Ester products corresponding to the

solvent environments of C

6

H

6

(e

r

= 2.28), CH

2

Cl

2

(e

r

= 8.93) and CH

3

OH (e

r

= 33.0).

2

The C –O distance, ‘y’, is fixed at 2.76 Å.

2

0492 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

N

S 2 mechanisms were detected as major decomposition products neutral 1-alkylimidazole products. However, the ionic liquids

in the TGA-MS experiments, and neutral imidazole products were incorporating the fluorinated anions were observed to decom-

1

clearly observed in the high-temperature H NMR experiment.

Decomposition mechanism A1 of [C C im][OAc] is likely to

pose by initial decarboxylation of the anion.

Ion pair structures and plausible decomposition mechanisms

2

1

occur in the gas phase, as observed by the mass spectrometry of [C C im][OAc] and [C C C im][OAc] were explored using

2

1

2 1 1

2

8d

experiment of Holl o´ czki and colleagues. In addition, studies Density Functional Theory methods. The activation barriers for

2

3

indicate that the NHC is highly accessible in the liquid phase.

the S

N

2 nucleophilic decomposition mechanisms were found to

À1

However, our combined experimental and computational investiga- be 20 kJ mol lower than the E2 Hofmann elimination reac-

tion suggests that thermal decomposition of [C

liquid phase does not primarily occur via the NHC route, when agreement with the experimentally determined E

2

C

1

im][OAc] from the tions, and the calculated DHactivation energies were in reasonable

values from

a

under inert conditions (nitrogen, helium or compressed air gases). the isothermal TGA experiments. The CPCM calculations for

decomposition mechanisms at the front of the imidazolium

rings suggest that each ionic liquid is thermodynamically more

stable than the neutral products that form, and that these

Conclusions

+

A broad series of [C

liquids has been synthesised, and the thermal stabilities were species may be highly accessible in [C

compared using temperature-ramped TGA. The difluoroacetate dialkylimidazolium carboxylate ionic liquids, our experimental

ionic liquid, [C im][CHF CO ], was found to be the most and computational results suggest that NHC formation is not a

2

C

1

im] and [C

2

C

1

C

1

im] carboxylate ionic mechanisms are likely to be highly reversible. Whilst the NHC

2 1

C im][OAc] and other

2

C

1

2

stable imidazolium carboxylate ionic liquid of the series, with a significant decomposition mechanism at elevated temperatures

T_o_n_s_e_tvalue approximately 50 1C higher than the non-fluori- and under inert conditions.

nated ionic liquid, [C C im][OAc]. The high thermal stability of

2

1

[

C

2

C

1

im][CHF

2 2

CO ] suggests that it may be a suitable replacement

Acknowledgements

for the trifluoroacetate analogue in applications operating at

3

6

high temperatures. Extension and branching of the anion The authors gratefully acknowledge the help of Dr Richard

alkyl chain, from two to 16 carbon atoms, had a minimal eﬀect L. Thompson and W. Douglas Carswell from the University of

on the thermal stability of the ionic liquid, as determined from Durham with the TGA-MS experiments, and Prof. John M.

temperature-ramped TGA.

Seddon and Dr Arwen I. I. Tyler for assistance with the DSC

The Arrhenius parameters for decomposition of [C C im][OAc] experiments. We are thankful to Claire Ashworth, Dr Jason

2

1

and [C C C im][OAc] have been experimentally determined, and P. Hallett, Dr Kevin R. J. Lovelock and Dr Klemens Massonne

2

1 1

allow rapid prediction of the rate of decomposition of the ionic for valuable conversations. The authors are thankful to BASF SE

liquid at any temperature, in an open system. The safe operating for the sponsorship of Matthew T. Clough.

temperature limit for [C C im][OAc] was found to be substantially

2 1

lower than previously thought, and significantly lower than the

operating temperature of many literature biomass procedures. In

light of this result, certain biomass experiments that employ

References

1 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and

B. Scrosati, Nat. Mater., 2009, 8, 621.

[

C C im][OAc] should be modified. If the basic acetate anion is

2 1

essential for the eﬃcacy of the process, then the procedure must be

carried out at a reduced temperature. Alternatively, low-melting

inorganic mixtures of lithium, potassium and caesium acetates

have been recently reported, which have improved thermal stabi-

lity. Where the acetate anion is not essential, a more stable ionic

liquid incorporating a diﬀerent anion may be employed.

Continuing to operate biomass processing procedures using

2 H. Zhao, Chem. Eng. Commun., 2006, 193, 1660.

3 A. Brandt, M. J. Ray, T. Q. To, D. J. Leak, R. J. Murphy and

T. Welton, Green Chem., 2011, 13, 2489.

4 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508.

5 T. Welton, Coord. Chem. Rev., 2004, 248, 2459.

6 (a) I. Krossing, J. M. Slattery, C. Daguenet, P. J. Dyson,

A. Oleinikova and H. Weing ¨a rtner, J. Am. Chem. Soc., 2006,

128, 13427; (b) P. A. Hunt, J. Phys. Chem. B, 2007, 111, 4844.

7 J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem.

Commun., 1992, 965.

8 (a) A. Brandt, J. P. Hallett, D. J. Leak, R. J. Murphy and

T. Welton, Green Chem., 2010, 12, 672; (b) A. Brandt,

J. Gr ¨a svik, J. P. Hallett and T. Welton, Green Chem., 2013,

15, 550.

9 (a) W. Li, N. Sun, B. Stoner, X. Jiang, X. Lu and R. D. Rogers,

Green Chem., 2011, 13, 2038; (b) D. B. Fu and G. Mazza,

Bioresour. Technol., 2011, 102, 8003.

37

[

C C im][OAc] at temperatures of 120 1C and above for

2 1

extended periods of time will result in the build-up of volatile

neutral products, posing a hazard and preventing eﬀective

reuse/recycle of the ionic liquid. Our results demonstrating the

lower than previously believed maximum operating tempera-

tures for this ionic liquid have important implications for these

experiments and procedures will need to be modified in light

of these.

It has been demonstrated that ionic liquids incorporating

the acetate, octanoate and thioacetate anions decompose primarily

N

via S 2-type nucleophilic substitution mechanisms, largely at 10 (a) C. Maton, N. De Vos and C. V. Stevens, Chem. Soc. Rev.,

the imidazolium N-methyl substituent. A high-temperature

H NMR experiment showed the formation of the expected

2013, 42, 5963; (b) S. Sowmiah, V. Srinivasadesikan,

M.-C. Tseng and Y.-H. Chu, Molecules, 2009, 14, 3780.

1

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20493

View Article Online

PCCP

1

Paper

1 (a) B. K. M. Chan, N.-H. Chang and M. R. Grimmett, Aust. J.

Chem., 1977, 30, 2005; (b) M. R. R. Prasad, K. Krishnan,

K. N. Ninan and V. N. Krishnamurthy, Thermochim. Acta,

M. A. Gilea, J. N. Canongia Lopes, L. P. N. Rebelo, J. W. Magee,

K. R. Seddon and J. A. Widegren, Nature, 2006, 439, 831;

(c) A. Deyko, K. R. J. Lovelock, J.-A. Corfield, A. W. Taylor,

P. N. Gooden, I. J. Villar-Garcia, P. Licence, R. G. Jones,

V. G. Krasovskiy, E. A. Chernikova and L. M. Kustov, Phys.

Chem. Chem. Phys., 2009, 11, 8544; (d) A. W. Taylor, K. R. J.

Lovelock, A. Deyko, P. Licence and R. G. Jones, Phys. Chem.

Chem. Phys., 2010, 12, 1772; (e) F. Heym, B. J. M. Etzold,

C. Kern and A. Jess, Phys. Chem. Chem. Phys., 2010, 12, 12089;

( f ) K. R. J. Lovelock, A. Deyko, P. Licence and R. G. Jones, Phys.

Chem. Chem. Phys., 2010, 12, 8893; (g) A. Chandran, K. Prakash

and S. Senapati, Chem. Phys., 2010, 374, 46; (h) A. Deyko,

K. R. J. Lovelock, P. Licence and R. G. Jones, Phys. Chem. Chem.

1

997, 297, 207; (c) H. L. Ngo, K. LeCompte, L. Hargens and

A. B. McEwen, Thermochim. Acta, 2000, 357–358, 97;

d) C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K.

Aki and J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, 954;

e) D. M. Fox, J. W. Gilman, H. C. De Long and P. C. Trulove,

(

J. Chem. Thermodyn., 2005, 37, 900; ( f ) A. Seeberger,

A.-K. Andresen and A. Jess, Phys. Chem. Chem. Phys., 2009,

1

7

1, 9375; (g) L. A. Daily and K. M. Miller, J. Org. Chem., 2013,

8, 4196.

12 (a) J. M. Crosthwaite, M. J. Muldoon, J. K. Dixon,

J. L. Anderson and J. F. Brennecke, J. Chem. Thermodyn.,

Phys., 2011, 13, 16841; (i) S. P. Verevkin, D. H. Zaitsau,

V. N. Eme `I yanenko and A. Heintz, J. Phys. Chem. B, 2011,

2

005, 37, 559; (b) I. H. J. Arellano, J. G. Guarino,

F. U. Paredes and S. D. Arco, J. Therm. Anal. Calorim.,

011, 103, 725.

115, 12889; ( j) K. Shimizu, M. Tariq, M. F. Costa Gomes,

L. P. N. Rebelo and J. N. Canongia Lopes, J. Phys. Chem. B,

2010, 114, 5831; (k) S. P. Verevkin, D. H. Zaitsau,

V. N. Emel’yanenko, A. V. Yermalayeu, C. Schick, H. Liu,

E. J. Maginn, S. Bulut, I. Krossing and R. Kalb, J. Phys. Chem.

B, 2013, 117, 6473.

2

1

3 M. Kosmulski, J. Gustafsson and J. B. Rosenholm, Thermochim.

Acta, 2004, 412, 47.

4 (a) D. M. Fox, W. H. Awad, J. W. Gilman, P. H. Maupin,

H. C. De Long and P. C. Trulove, Green Chem., 2003, 5, 724;

1

(b) K. J. Baranyai, G. B. Deacon, D. R. MacFarlane, 26 (a) A. G. Glenn and P. B. Jones, Tetrahedron Lett., 2004,

J. M. Pringle and J. L. Scott, Aust. J. Chem., 2004, 57, 145.

5 T. J. Wooster, K. M. Johanson, K. J. Fraser, D. R. MacFarlane

and J. L. Scott, Green Chem., 2006, 8, 691.

45, 6967; (b) M. C. Kroon, W. Buijs, C. J. Peters and

G.-J. Witkamp, Thermochim. Acta, 2007, 465, 40;

(c) H. Ohtani, S. Ishimura and M. Kumai, Anal. Sci., 2008,

24, 1335; (d) N. Meine, F. Benedito and R. Rinaldi, Green

Chem., 2010, 12, 1711.

1

6 (a) F. Wendler, L.-N. Todi and F. Meister, Thermochim. Acta,

2012, 528, 76; (b) F. Wendler, L.-N. Todi, F. Meister, Thermal

investigations of polysaccharide blend solutions in imidazolium 27 J. Huang and A. F. Hollenkamp, J. Phys. Chem. C, 2010,

ionic liquids, 241st ACS National Meeting, March 27–31, 114, 21840.

Anaheim, USA, 2011; (c) A. W. T. King, A. Parviainen, 28 (a) J. Dupont and J. Spencer, Angew. Chem., Int. Ed., 2004,

P. Karhunen, J. Matikainen, L. K. J. Hauru, H. Sixta and

I. Kilpel ¨a inen, RSC Adv., 2012, 2, 8020.

7 (a) M. C. Kroon, W. Buijs, C. J. Peters and G.-J. Witkamp,

Thermochim. Acta, 2007, 465, 40; (b) S. D. Chambreau,

J. A. Boatz, G. L. Vaghjiani, C. Koh, O. Kostko, A. Golan

and S. R. Leone, J. Phys. Chem. A, 2012, 116, 5867;

43, 5296; (b) T. L. Amyes, S. T. Diver, J. P. Richard,

F. M. Rivas and K. Toth, J. Am. Chem. Soc., 2004,

126, 4366; (c) S. T. Handy and M. Okello, J. Org. Chem.,

2005, 70, 1915; (d) O. Holl o´ czki, D. Gerhard, K. Massonne,

L. Szarvas, B. N ´e meth, T. Veszpr ´e mi and L. Nyul ´a szi, New J.

Chem., 2010, 34, 3004; (e) O. Holl o´ czki and L. Nyul ´a szi, Org.

Biomol. Chem., 2011, 9, 2634; ( f ) Z. Kelemen, O. Holl o´ czki,

J. Nagy and L. Nyul ´a szi, Org. Biomol. Chem., 2011, 9, 5362;

(g) H. Rodr ´ı guez, G. Gurau, J. D. Holbrey and R. D. Rogers,

Chem. Commun., 2011, 47, 3222; (h) O. Holl o´ czki,

D. S. Firaha, J. Friedrich, M. Brehm, R. Cybik, M. Wild,

A. Stark and B. Kirchner, J. Phys. Chem. B, 2013, 117, 5898.

29 (a) R. P. Swatloski, J. D. Holbrey and R. D. Rogers, Green

Chem., 2003, 5, 361; (b) M. G. Freire, C. M. S. S. Neves,

I. M. Marrucho, J. A. P. Coutinho and A. M. Fernandes,

J. Phys. Chem. A, 2010, 114, 3744; (c) S. Steudte, J. Neumann,

U. Bottin-Weber, M. Diedenhofen, J. Arning, P. Stepnowski

and S. Stolte, Green Chem., 2012, 14, 2474.

1

(c) Y. Chen, Y. Cao, Y. Shi, Z. Xue and T. Mu, Ind. Eng.

Chem. Res., 2012, 51, 7418.

1

8 J. L. Ferguson, J. D. Holbrey, S. Ng, N. V. Plechkova,

K. R. Seddon, A. A. Tomaszowska and D. F. Wassell, Pure

Appl. Chem., 2012, 84, 723.

1

2

9 1-Ethyl-3-methylimidazolium acetate, BASF Basionicst

‘BC 01’, CAS-No. 143314-17-4.

0 I. Pibiri, A. Pace, S. Buscemi, V. Causin, F. Rastrelli and

G. Saielli, Phys. Chem. Chem. Phys., 2012, 14, 14306.

1 CRC Handbook of Chemistry and Physics, Internet Version,

ed. D. R. Lide, Taylor and Francis, Boca Raton, FL, 87th edn,

2

007, http:/www.hbcpnetbase.com.

2 R. Fausto, L. A. E. Batista De Carvalho and J. J. C. Teixeira- 30 National Institute of Standards and Technology (NIST)

Dias, THEOCHEM, 1990, 207, 67. Chemistry WebBook, http://webbook.nist.gov/chemistry/.

3 G. Gurau, H. Rodr ´ı guez, S. P. Kelley, P. Janiczek, R. S. Kalb 31 (a) N. R. Dhumal, H. J. Kim and J. Kiefer, J. Phys. Chem. A,

2

and R. D. Rogers, Angew. Chem., Int. Ed., 2011, 50, 12024.

4 J. H. Flynn and L. A. Wall, Polym. Lett., 1966, 4, 323.

5 (a) J. M. S. S. Esperança, J. N. Canongia Lopes, M. Tariq,

L. M. N. B. F. Santos, J. W. Magee and L. P. N. Rebelo, J. Chem.

Eng. Data, 2010, 55, 3; (b) M. J. Earle, J. M. S. S. Esperança,

2009, 113, 10397; (b) J. Kiefer, K. Obert, J. Fries, A. Bosmann,

P. Wasserscheid and A. Leipertz, Appl. Spectrosc., 2009,

63, 1041; (c) M. B. Shiflett and A. Yokozeki, J. Chem. Eng.

Data, 2009, 54, 108; (d) D. T. Bowron, C. D’Agostino,

L. F. Gladden, C. Hardacre, J. D. Holbrey, M. C. Lagunas,

2

0494 Phys. Chem. Chem. Phys., 2013, 15, 20480--20495

This journal is c the Owner Societies 2013

View Article Online

Paper

PCCP

J. McGregor, M. D. Mantle, C. L. Mullan and T. G. A. Youngs,

J. Phys. Chem. B, 2010, 114, 7760; (e) W. Shi, K. Damodaran,

A, 2006, 110, 2269; (c) P. A. Hunt, I. R. Gould and

B. Kirchner, Aust. J. Chem., 2007, 60, 9.

H. B. Nulwala and D. R. Luebke, Phys. Chem. Chem. Phys., 33 M. E. M. Berger, D. Assenbaum, N. Taccardi, E. Spiecker and

012, 14, 15897; ( f ) S. Stevanovic, A. Podgor ˇs ek, A. A. H.

P. Wasserscheid, Green Chem., 2011, 13, 1411.

P ´a dua and M. F. Costa Gomes, J. Phys. Chem. B, 2012, 34 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005,

16, 14416; (g) M. Brehm, H. Weber, A. S. Pensado, 105, 2999.

A. Stark and B. Kirchner, Phys. Chem. Chem. Phys., 2012, 35 (a) C. Wakai, A. Oleinikova, M. Ott and H. Weing ¨a rtner,

2

1

4, 5030; (h) S. Zhang, Y. Zhang, X. Ma, L. Lu, Y. He and

J. Phys. Chem. B, 2005, 109, 17028; (b) H. Weing ¨a rtner,

Angew. Chem., Int. Ed., 2008, 47, 654.

Y. Deng, J. Phys. Chem. B, 2013, 117, 2764; (i) M. Brehm,

H. Weber, A. S. Pensado, A. Stark and B. Kirchner, Z. Phys. 36 C. Shi, Q. Ge, F. Zhou, M. Cai, X. Wang, X. Fang and X. Pan,

Chem., 2013, 227, 177. Sol. Energy, 2009, 83, 108.

2 (a) P. A. Hunt, B. Kirchner and T. Welton, Chem.–Eur. J., 37 S. Bajus, A. Deyko, A. B o¨ smann, F. Maier, H.-P. Steinr u¨ ck

006, 12, 6762; (b) P. A. Hunt and I. R. Gould, J. Phys. Chem. and P. Wasserscheid, Dalton Trans., 2012, 41, 14433.

3

2

This journal is c the Owner Societies 2013

Phys. Chem. Chem. Phys., 2013, 15, 20480--20495 20495

Article Doi

Thermal decomposition of carboxylate ionic liquids: Trends and mechanisms

DOI: 10.1039/c3cp53648c

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c3cp53648c

Full text of DOI:10.1039/c3cp53648c

Products guided by the article

R&D Labs maybe for 143314-17-4

Relevant to this article

Hot Product