Stability of Ionic Liquids in Application Conditions

1974

Current Organic Chemistry, 2011, 15, 1974-1991

Stability of Ionic Liquids in Application Conditions Ewa Maria Siedlecka1,*, Magorzata Czerwicka1, Stefan Stolte2 and Piotr Stepnowski1 1

Faculty of Chemistry, University of Gdask, ul. Sobieskiego 18, PL 80-952 Gdask

2

Department Sustainable Chemistry, Centre for Environmental Research and Sustainable Technology, University of Bremen, Leobener Str. UFT, 28359 Bremen, Germany Abstract: Ionic liquids have a large variety of applications in nearly all areas of the chemical industries. They are used for example as lubricants, plasticizers, solvents and catalysts for synthesis, matrices for mass spectroscopy, solvents for extraction and in the manufacture of nano-materials, and as electrolytes. There are numerous studies in the corresponding literature concerning the unique properties, preparation methods, and different applications of ionic liquids. Nowadays, researchers try to answer questions concerning the stability of ionic liquids within practical applications. In this review, a general description of the stability and - degradation products of ionic liquids formed under different physicochemical conditions such as high temperature, irradiation, varying electrical current density, oxidizing agents are discussed. The main focus of this paper is on how the structure of ionic liquid cations and anions as well as impurities (e.g. water) influence the stability of ionic liquids in application and in utilization conditions. Additionally, it is also very important to consider the stability of ionic liquids with respect to environmental issues and for the hazard assessment of those compounds. These aspects, as well the possibility to use strongly oxidizing conditions (advanced oxidation processes) to remove ionic liquid structures from wastewaters are discussed within this review.

Keywords: Ionic liquids, stability, intermediates of degradation, pathway of degradation. INTRODUCTION Many papers dealing with ionic liquids (ILs) assert that they are a new class of chemical compounds but the truth is that they have been intensively studied within the last decade and they are already known since a long time. The first room temperature ionic liquid which turned up in literature was ethylammonium nitrate in 1914 [1-2]. Although there have been some experiments considering industrial applications of ILs, they were negated for many decades. The golden age of ionic liquids started at the end of the last century. Starting from the nineties until now the number of publications dedicated to ILs has increased [3]. The most popular definition of ionic liquids describes them as salts with a melting point below 100oC, typically consisting of usually bulky and asymmetric organic cations and inorganic or organic anions [1-2]. Estimates of the theoretically possible number of these chemicals range in the millions, but nowadays only several hundred are in practical use. First, they were treated as alternative solvents in chemical synthesis, now the range of applications has broadened. Beside organic chemistry (organic and catalytic reactions) ILs are also utilized in biotechnology, electrochemistry, extraction processes, and analytical chemistry (separation techniques) [1-4]. The most popular examples of ionic liquid applications are presented in Fig. (1). The wide applicability of ILs is based on their exceptional physico-chemical properties i.e. like low vapor pressure, low melting point, non-flammability, thermal stability, electrochemical stability, a wide electrochemical window, and a good conductivity. However, the increasing usage of ILs during the last few years comes along with an increasing probability of finding them in industrial wastewaters and therefore in the environment. Thus, there is the need to investigate the stability and break down products of

*Address correspondence to this author at the Faculty of Chemistry, University of Gdask, ul. Sobieskiego 18, PL 80-952 Gdask; Tel: +48 58 523 5437; Fax: +48 58 523 54 72; E-mail: [email protected] 1385-2728/11 $58.00+.00

Fig. (1). Applications of ionic liquids.

ILs to get more information on the fate of these compounds within technical applications and especially on their fate in the environment. To describe the environmental fate and the environmental lifetime of ionic liquid structures also abiotic transformation processes have to be considered. Abiotic mechanisms like oxidation, hydrolysis or photolysis represent important pathways for the degradation of man-made chemicals in the environment. Furthermore, it is crucial to include degradation products formed during technical © 2011 Bentham Science Publishers Ltd.

Stability of Ionic Liquids in Application Conditions

Current Organic Chemistry, 2011, Vol. 15, No. 12 1975

Table 1. Full Names and Abbreviations for Cations and Anions Mentioned in this Paper Cations Abbreviation [IM11]

+

Name

Abbreviation +

1-methyl-3-methylimidazolium

[IM1-10]

[IM12]+

1-ethyl-3-methylimidazolium

[IM12OCOO1]+

[IM13]+

1-propyl-3-methylimidazolium

[Pyr13]+

[IM1i3]

+

[IM14]+ [IM16]

+

[IM18]

+

[IM46]+ [IM11-2Me-4Me-5Me]

+

Name 1-decyl-3-methylimidazolium 1-[2-(methoxycarbonyloxy) ethyl]-3-methylimidazolium 1-methyl-1-propylpyrrolidinium

1-isopropyl-3-methylimidazolium

[Pyr14]

+

1-methyl-1-butylpyrrolidinium

1-butyl-3-methylimidazolium

[Pyr18]+

1-methyl-1-octylpyrrolidinium

1-hexyl-3-methylimidazolium

[Pip13]

+

1-methyl-1-propylpiperidinium

1-octyl-3-methylimidazolium

[Pip14]

+

1-methyl-1-butylpiperidinium

1-hexyl-3-butylimidazolium

[Pip18]+

1-methyl-1-octylpiperidinium

+

1,2,3,4,5-pentamethylimidazolium

[Py4]

[IM12O1]+

1-(2-methoxyethyl)-3-methylimidazolium

[Py4-3Me]+

1-butyl-3-methylpyridinium

[IM12O2O2O1]+

1-[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]-3methylimidazolium

[Py4-4Me]+


[IM1A]+

1-n-alkyl-3-methylimidazolium

[Py6-3Me]+

1-hexyl-3-methylpyridinium

[IM13-2Me]

+

1,2-dimethyl-3-propylimidazolium

1-butylpyridinium

[N1444]

+

tributylmethylammonium

Anions Abbreviation [C(CN)3]

-

[BF4][PF6]

-

Name

Abbreviation

tricyanomethide

[CF3COO]

tetrafluoroborate

[1COO]-

hexafluorophosphate

[1SO3]

-

Name trifluoroacetate acetate

-

methanesulfonate

[PhCOO]-

benzoate

[(CF3SO2)2N]-

bis(trifluoromethylsulfonyl) imide

[(CF3SO2)3C]-

tris(trifluoromethylsulfonyl) methide

[N(CN)2]-

dicyanamide

[(C5F11)COO]-

perfluorohexanoate

triflate

[(C9F17)COO]

-

perfluorodecanoate

trifluorophenylborate

[(C7F15)COO]-

perfluorooctanoate

[CF3SO3]

-

[BF3(Ph)][(C4F9)SO3]

-

[AsF6][(C2F5SO2)2N]

-

-

nonaflate

[1OSO3]

hexafluoroarsenate

[2OSO3] -

bis(pentafluoroethylsulfonyl)imide

application to the hazard assessment of ionic liquids, because degradation products have their own characteristic ecotoxicological profiles and it may well happen that they are more toxic than their parent compound. In this article we summarize and discuss available data to IL’s stability under application conditions (high temperature, presence of water, irradiation, oxidation, and reduction) and during wastewater treatment. The full names and abbreviations of ionic liquids cations and anions used in this text are summarized in Table 1. THERMAL STABILITY Temperature has a capacious influence on the liquid state, the emission of toxic vapors and the degradation of ILs. The most significant thermal parameters are the melting point (Tm), the glass transition temperature (Tg), the crystallization point (Tc), and the decomposition temperature (Td). The liquid range is limited by the glass transition temperature for amorphous ILs and by the melting point for crystalline ionic liquids. Phase transitions of ionic liquids

[B(Ph)4]

-

methylsulfate ethylsulfate tetraphenylborate

can be measured using Differential Scanning Calorimetry (DSC) [46-47]. In this study we would like to focus on the degradation temperatures of ionic liquids. At first it was thought that ionic liquids are stable over a very wide range of temperatures and this view was verified by many experiments. Meanwhile it is known that the thermal stability range is much lower than the difference between the melting point/glass transition temperature and the decomposition temperature. This is affected by many parameters, e.g. the cation and anion type, structural modifications of the cation (alkyl chain length, different functionalities in the alkyl chain), impurities of the ILs (water, chlorides, etc.) and the type (isothermal or gradient) and the time of exposition to high temperatures. A very simple method to identify IL degradation is through visual observation of color changes but this is useful only for a few ILs [47-49]. Other methods are UV/Vis and fluorescence spectroscopy but the most popular method for determining the thermal stability of ILs is Thermal Gravimetric Analysis (TGA), which can be used in parallel with absorption methods [46-47].

1976 Current Organic Chemistry, 2011, Vol. 15, No. 12

Siedlecka et al.

Table 2. Degradation Temperature of Typical Bromides, Tetrafluoroborates, Benzoates, and bis(trifluoromethylsulfonyl)imides Ionic Liquids (TGA Analysis) ANION

CATION

Tm /Tg [°C]

Td [°C]

REF

-37 gt

237

[50]

-

250

[51]

1-hexyl-3-methylimidazolium

-49 gt

276

[50]

1-methyl-1-butylpyrrolidinium

78 mp

310

[52]

1-octyl-3-methylpyridinium

-70 gt

310

[53]

1-methyl-1-octylpiperidinium

92 mp

325

1-hexyl-3-methylpyridinium [Br]-

butyl-(2-hydroxyethyl)-dimethylammonium

[BF4]-

tetraethylammonium 1-butyl-3-methylimidazolium

[PhCOO]

-

383 , 412

[54]

-85 gt

424, 361

[17,55]

11 mp/-73 gt

234

[56]

butyltriphenylphosphonium

19 mp/-62 gt

236

[56]

-72 gt

280

[56]

12 mp/-74 gt

290

[53]

-76 gt

315

[52]

15 mp/-58 gt

319

[50]

1-octyl-2-methylpyridinium 1-methyl-1-octylpiperidinium [(CF3SO2)2N]

72 mp

[52] b


1-butyl-3-methylimidazolium

-

a

1-butyl-nicotinic acid butyl ester 1-methyl-1-octylpyrrolidinium

-81 gt

325

[52]

1-methyl-3-octylimidazolium

-80 (-84) gt

425 (427)

[50,57]

Where: a aluminum pan, b Al2O3 pan; - not found

The first discussed factor which could affect the decomposition temperature is the cation type. Degradation temperatures of typical cations are presented in Table 2. Based on experimental results it was found that imidazolium ILs are more stable than pyridinium, phosphonium, ammonium, pyrrolidinium, and piperidinium ILs. Pyridinium cations are less stable than pyrrolidinium cations. The difference between the degradation temperature (Td) of imidazolium ILs and the rest is appreciable but differences between Td values for other cations are not so significant [50-51,54,56]. Although some trends are observable, the correlation between the degradation temperature and the type of IL cation is not very predictive. One experiment showed that the pyrrolidinium cation has a higher thermal stability compared to that of the imidazolium cation ([IM14][(CF3SO2)2N] Tg = 423 °C < [Pyr14][(CF3SO2)2N] Tg = 431 °C) [58], while the results obtained for the same IL in different investigations from other labs led to opposite conclusions [52,57,59]. The thermal stability of geminal dicationic ionic liquids (bis(trifluoromethylsulfonyl)imides) was investigated, too. It was proven that the thermal stability range for them is much higher compared with traditional monocationic ILs. Comparing different dicationic ionic liquids, the most resistant to high temperature was the 1-methylpyrrolidinium cation linked by a nonyl alkyl chain (over 400 °C), less resistant was the 1-methylimidazolium dication, and the least resistant was the 1-butylpyrrolidinium dication (330 °C) [60]. In Table 3 the temperatures of decomposition onset for several types of ionic liquid cations, which differ in their alkyl chain length, are collected. A trend between side chain length and stability could be identified to some extent. The shorter the substituted alkyl chain the higher the decomposition temperature. This intercorrelation was observed for imidazolium, pyrrolidinium, pyridinium, piperidinium and ammonium ionic liquids [50-51,56,59]. The difference in val-

ues is more significant for shorter alkyl chains, with increasing alkyl chain length only small changes in decomposition temperature are observed. For bis(trifluoromethylsulfonyl)imides of [IM11], [IM12], [IM14], [IM16], [IM18] the degradation temperatures are 444 °C, 439 °C, 427 °C, 428 °C and 425 °C, respectively [57]. In the case of 1-alkyl-3-methylimidazolium chlorides the experimental results correspond well with calculated activation energies of the thermal breakdown reactions [61]. With increasing alkyl chain length also the ILs liquid range (Td- Tg) decreases [56]. The impact of ether functionalized side chains on the thermal stability of ILs is shown in Table 4. After analyzing TGA data it was concluded that nonfunctionalized 1-alkyl imidazolium ILs have higher decomposition temperatures than 1-alkyl ether ones for all tested anion types and alkyl chain lengths [46]. In case of imidazolium-based ILs ([IM12OCOO1][(CF3SO2)2N]), the presence of ester functionalities in the alkyl chain is responsible for two decomposition temperatures (203 and 364 °C). The lower Td value is connected with the decomposition of the carbonate group [62]. In another experiment the influence of two different functionalities in the imidazolium cation alkyl chain was compared. For these investigations 3-oxobutylimidazolium and 1-methyl-3-oxobutylimidazolium bis(trifluoromethylsulfonyl)imides were chosen. For both compounds the hydrogenation reaction was performed, which resulted in the hydroxylated compounds. The thermal stability of the oxo compound has been compared with the hydroxylated ones. It was found that the hydrogenated products were more stable. The reason is possibly the retro-Michael reaction, which causes decomposition of keto-functionalized ionic liquids. In the case of hydrogenated products this kind of degradation pathway is suppressed (Fig. 2) [63]. It was noticed that modifications could appear not only in the alkyl chain but also in the imidazolium ring, for example substitu-



Fig. (2). Possibility of the retro-Michael reaction as a decomposition pathway for oxobutyl functionalized compared to the hydroxyl-functionalized imidazolium cations. Table 3. Interdependence between Thermal Decomposition Value and the Alkyl Chain Length of imidazolium, Pyrrolidinium, Piperidinium, and Pyridinium ILs CATION

R

IONIC LIQUID

Td [°C]

REF

CH3(CH2) 2-

[IM13][(CF3SO2)2N]

452a,453b

[54]

CH3(CH2) 3-

[IM14][(CF3SO2)2N]

439 (423,427)

[57,58,59]

CH3(CH2) 7-

[IM18][(CF3SO2)2N]

427 (425)

[50,57]

CH3(CH2) 2-

[Pyr13][(CF3SO2)2N]

343

[52]

CH3(CH2) 3-

[Pyr14][(CF3SO2)2N]

340 (431)

[52,58]

CH3(CH2) 7-

[Pyr18][(CF3SO2)2N]

325

[52]

CH3(CH2) 2-

[Pip13][(CF3SO2)2N]

341

[52]

CH3(CH2) 3-

[Pip14][(CF3SO2)2N]

320

[52]

CH3(CH2) 7-

[Pip18][(CF3SO2)2N]

315

[52]

CH3(CH2) 3-

[Py4-3Me][(CF3SO2)2N]

397

[50]

CH3(CH2) 5-


399

[50]

CH3(CH2) 7-


394

[50]

Where: a aluminum pan, b Al2O3 pan

Table 4. Melting Point and Thermal Decomposition Temperatures of Several 1-alkyl and 1-alkyl ether Functionalized Imidazolium Ionic Liquids CATION

ANION

Tm / Tg [°C]

Td [°C]

[1SO3]-

32 mp

n.d.

[46]

[BF4]-

-81 mp/-85gt

403, 424, 361

[17, 55,59]

10 mp

349

[59]

[PF6][1SO3]

-

REF

-56 gt

250

[46]

[BF4]-

-75 gt

252

[46]

[PF6]-

10 mp/-70 gt

156

[46]

[1SO3]-

32 mp

295

[46]

[BF4]

-

0 mp/-35 gt

310

[46]

[1SO3]-

-59 gt

196

[46]

[BF4]-

-86

266

[46]

Where: “-“- no melting point observed; n.d. – not detected

tions. The replacement of hydrogen (especially acidic C2 hydrogen) from the ring by linear alkyl groups causes increasing thermal stability of imidazolium ILs, for example from 332 °C for [IM1i3][PF6] to 401 °C for [IM11-2Me-4Me-5Me][PF6]. When branched chains are substituted to the core-nitrogen instead of linear alkyl groups the thermal stability decreases [54]. Dialkylimidazolium cations with fluorinated alkyl chains are characterized to have a higher degradation temperature. The presence of amino and piperidino groups in pyridinium cations (in the 4-position of the ring) are responsible for a higher thermal stability [50]. Conversely to the observed limited cation influence, the thermal stability of ionic liquids is highly anion dependent. The effect of the

anion type has been investigated many times and several gradation schemes have been created: a. [(C2F5SO2)2N]- > [(CF3SO2)2N]- > [AsF6]- > [(C4F9)SO3]- > [CF3SO3]- ~ [BF4]- > [N(CN)2]- > [BF3(Ph)]- > [B(Ph)4]- (when cation was [Py8-4Me]+) [53]; b. [PF6]- > [BF4]- > [(CF3SO2)2N]- (when cation was [IM16]-) [64]; c. [PF6]- > [(C2F5SO2)2N]- > [(CF3SO2)2N]- [BF4]- > [(CF3SO2)3C]- [AsF6]- >> [Cl]-, [Br]-, [I]- (for imidazolium cations) [54];


Siedlecka et al.

Table 5. Degradation Temperature of 1-butyl-3-methylimidazolium, 1-octyl-4-methylpyridinium, 1-methyl-1-butylpiperidinium, and tetraethylammonium Ionic Liquids with Different Anions CATION

ANION [CF3COO] [1COO]

-

-

Tm / Tg [°C]

Td [°C]

REF

-78 gt

170, 176

[50, 65]

-

220

[50]

[Cl]-

41 mp/- 69 gt

254, 264

[55,59]

[I]-

-72 mp

265

[59]

[Br]-

-50 gt

273

[55]

[PhCOO ]

-

[N(CN)2][PF6][CF3SO3]

-

[(C2F5SO2)2N][(C4F9)SO3]

-

[(CF3SO2)3C][BF4]

-

[(CF3SO2)2N][B(Ph)4]-

-72 gt

280

[56]

-6 mp/-90 gt

300

[55]

10 mp/-77 gt

349, 370, 433

[59,61,65]

13 mp

392

[55]

-84 gt

402

[65]

17 mp

409

[65]

-65 gt

413

[55]

-81 mp/-85 gt

424, 403, 361

[17,55,59]

-3(-2) mp/-87(-86) gt

439,427, 422

[55,57,59]

130 mp

190

[53]

-

-76 gt

205

[53]

[N(CN)2]-

-69 gt

228

[53]

-64 gt

290

[53]

-65 gt

296

[53]

68 mp

298

[53]

16 mp/-77 gt

300

[53]

[BF3(Ph)]

[BF4]

-

[CF3SO3][(C4F9)SO3]

-

[(CF3SO2)2N][AsF6]

-

[(C2F5SO2)2N][N(CN)2][BF4]

-

-20 mp/-64 gt

305

[53]

2 mp/-75 gt

325

[53]

-

283

[49]

78 mp

310

[52]

-

319

[49]

[AsF6]-

66 mp

325

[52]

[(CF3SO2)2N]-

-81 gt

325

[52]

[C(CN)3]-

[Cl]

-

[BF4][PF6]

-

[(CF3SO2)3C]

-

264

72 mp

383a, 412b

70 mp -

[(C2F5SO2)2N][(CF3SO2)2N]

-

b

388

[54]

b

[54]

46 mp

411 , 397

b

[54]

83 mp

423a, 397b

[54]

104 mp

a

[54]

a

439 , 399

b

[54]

Where: a aluminum pan, b Al2O3 pan; - not found

d. e.

f. g. h.

[BF4]- > [1SO3]- > [PF6]- (when cation was [IM12O2O2O1]+ or [IM12O1]+]) [46]; [PF6]- > [BF4]- [(CF3SO2)2N]- > [(C4F9)SO3]- > [(C2F5SO2)2N]- >> [CF3SOO]- (when cation was [IM14]+] [65]; [PF6]- > [(CF3SO2)2N]- [BF4]- >[Cl]-, [I]- (when cation was [IM1A]+) [59]; [PF6]- > [BF4]- > [AsF6]- >> [Cl]-, [Br]-, [I]- (for imidazolium cations) [3]; [(CF3SO2)2N]- > [(CF3SO2)3C]- > [CF3SO3]- > [BF4]> [N(CN)2]- > [Br]- (for imidazolium cations) [55].

In Table 5 the typical degradation temperatures of 1-butyl-3methylimidazolium, 1-octyl-4-methylpyridinium, 1-methyl-1butylpiperidinium, and tetraethylammonium ionic liquids with different anions are presented. Based on these data we can draw several conclusions. Firstly, we notice that independently of the cation, halides are the least thermally stable anions of all ILs. The anions most resistant to high temperatures are the inorganic ones such as hexafluorophosphate and tetrafluoroborate or organic anions such as bis(pentafluoroethylsulfonyl)imide and bis(trifluoromethylsulfonyl)imide. This is related to the coordinating nature of the anionic moiety. The degra-



liquid which can be related to different analysis conditions, the used method, the type of instrument or within TGA, on the position of the thermocouple relative to the sample. From quantum chemical calculations thermal degradation pathways of ionic liquids can be predicted by creating energy profiles of the process, and calculating activation barriers of involved reactions [61]. By applying mass spectrometric methods degradation products and pathways can be identified and followed experimentally. In most of the cases the theoretically derived results are in good agreement with experimental data. The possible thermal decomposition pathways of several ionic liquid cations and anions are collected in Tables 6 and 7, respectively.

dation temperature decreases with an increase of the coordinating properties of the anion. Td is the lowest for halide (highly coordinating anions) containing ILs while it is very high for anions with sulfonyl groups [50]. The presence of halide anions causes a decrease in Td of about 100 °C compared with ionic liquids with nonhalide anions. The thermal stability of ILs decreases with an increased hydrophilicity and increases with increasing anion size [55,59]. The anion type determines also the thermodynamic character of the degradation process. Organic anions undergo exothermic decomposition while for inorganic anions it is an endothermic process [3,54]. Occasionally it is not possible to estimate the same range of increasing/decreasing temperature for experiments with one ionic

Table 6. Possible Thermal Degradation Pathways of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, N-alkyl-N-methylpyrrolidinium, 1butylpyridinium, and Tetraethylammonium Cations IONIC LIQUID CATION

PRODUCED IONS

REF [66]

N

N

N

H2C

N

CH2

CH3X, CH3 CH2 X

N

[61,66]

N

N

N

N

N

+

N

N -

H2C

CH2

X CH3X, CH3 (CH2) 3 X


Siedlecka et al. Table 6. contd…

Ionic Liquid Cation

Produced Ions

Ref.

[49,61]

N

N

R

N

+ N

N R

R

R N

X CH3X, RCH2 X

R [61]

+ N

N

CH3(CH 2)3 X

N

[61]

+ N CH3CH2 X Where: X –ILs anion

For imidazolium ionic liquids, the SN2 mechanism predominates in forming thermal degradation products. The most favorite one is the dealkylation by C-N bond cleavage. This is true for halides, dicyanamide, hexafluorophosphate and tetrafluoroborate but not for bis(trifluoromethylsulfonyl)imide. In this case, the anion will more readily decompose. Imidazolium halides which are treated under high temperature conditions are converted to haloalkanes and 1-alkylimidazoles, as a result of the nucleophilic attack of the halide anion on the alkyl group. Methyl group substitution in ring C2-position blocks this mechanism (due to a lack of hydrogen),

automatically causing higher thermal degradation temperatures. Under experimental conditions of 550 °C the C2 substituted imidazolium ring was still resistant to thermal degradation. In the case of tetrafluoroborate and hexafluorophosphate dialkylimidazolium ILs alkenes were mainly formed as well as small amounts of haloalkanes [3,61,65–67]. For trialkylimidazolium-based ILs or dialkylimidazolium bis(trifluoromethylsulfonyl)imides thermal degradation proceeds according to the SN1 mechanism [57,68]. It was found that also 1-alkylpyridinium, dialkylpyrrolidinium, and tetraalkylammonium-based ILs are decomposed via the same



Table 7. Possible Thermal Decomposition Pathways of Ionic Liquid Anions PARENT ANION IN IONIC LIQUID [X]

-

PRODUCT IONS

REF

CH3X, (CH2)n CH3 X

[61,66,67]

[BF4]-

CH3 F, BF3, (CH2)n CH3 F

[61,66]

[PF6]-

HF, PF 3, PF5,CH3 F, (CH2)nCH3 F

[61,66]

CF3H, SO2

[66]

(CF3SO 2)CF3 N -, SO 2

[61]

[(C4F9)SO3][(CF3SO2)2N]-

[61]

[N(CN)2][NO3]-

N2O, CH3CHO, CO2, NO, H 2CO -

-

Where: [X] = [Cl] , [Br]

[67]

-

SN2 mechanism as imidazolium salts are [61]. For quaternary ammonium ionic liquids the most popular mechanisms for the thermal decomposition process are reverse Menschutkin and Hofmann reactions. The first one provides an amine and the other an alkyl compound. Hofmann elimination reaction products are alkenes, amines, and trialkylammonium cations. In the case of a strongly nucleophilic anion (X-) thermal degradation causes the formation of olefins and R3NH+X- [3]. The thermal degradation of cyano containing ionic liquids was also tested. It was found that anions containing a cyano group as well as N-based cations are sensitive for polymerization under high temperature conditions while phosphonium ILs form volatile products [49]. Beside the structural constitution also the presence of impurities is assumed to influence the thermal stability of ionic liquids. The influence of water, chloride, silver and sodium ions was investigated for tetrafluoroborates of [IM12], [IM14], and [IM13-2Me]. For all compounds a weight loss due to water evaporation (at 100°C) was found. However, TGA onset doesn’t change a lot because traces of water have only a minimal impact on it [17]. Nevertheless, hydrolytic processes have to be kept in mind for tetrafluoroborate ILs when water is present. Hydrolytic formation of HF and borates could lead to a decrease in decomposition temperature based on the basicity of the fluoride ions formed [17,61]. In the case of imidazolium cations substituted in the 2 position by a methyl group instead of hydrogen, thermal stability is higher. The presence of a methyl group makes the reaction leading to the formation of HF much less likely [61]. The water contaminant effect depends on either the concentration or the cation and anion characters. Also, the influence of chloride contamination was tested. The safe concentration of chlorides not affecting thermal stability of ILs is 10 mol% [17,61]. In the case of silver ions and sodium ions no significant effect on thermal degradation was observed [17]. However, the presence of many types of contaminants could decrease IL degradation temperatures and, depending on the kind of the impurity, the mechanism responsible for this reduction will differ. As mentioned earlier, the most popular source of knowledge about IL degradation temperature is thermal gravimetric analysis (TGA). There are many factors influencing TGA results. One of them is the type of the used sample pan (platinum, aluminum, alu-

mina, silver) [54-55,64-65,67,69]. Experiments on tetrafluoroborates and hexafluorophosphate ionic liquids using alumina crucibles found higher decomposition temperatures compared to aluminum pans. The difference range was even 100 °C. This can be explained by aluminum acting as a catalyst in the degradation process [5455,65,67,69]. The decomposition proceeds exothermically for aluminum crucibles whereas it becomes endothermic for crucibles made from alumina [54]. Comparing results obtained with aluminum oxide pans as well as silver pans, no significant differences were found [69]. The effect of nitrogen, air and oxygen on thermal degradation was also tested. In the presence of air the mass loss appeared earlier but differences are little enough to find them insignificant [54,69,70]. The effect of ceramic powder (silica, alumina, and anatase) was studied. It was found that silica quartz or amorphous silica accelerated the thermal degradation of [IM14][CF3SO3], [IM1-10][CF3SO3], [IM14][PF6], [IM16][PF6], and [IM18][PF6] [69]. When testing the thermal decomposition of ionic liquids two temperature parameters can be measured: the onset temperature (Tonset = Td) and the start temperature (Tstart). The onset temperature or degradation temperature is the intersection of the baseline weight, and the tangent of the weight dependence on the temperature curve as decomposition occurs. The start temperature defines the value when sample degradation begins [50,55,64]. Thermal decomposition can be tested isothermally or with different rates of heating [64,69,71]. An increase in the heating rate doesn’t significantly influence the thermal degradation although it decreases Tstart [64]. It was experimentally proved that the optimal heating rate for ILs is 10 °C x min-1. From the IL’s applicability point of view, the more important point is the long-term thermal stability, which can be investigated using isothermal TGA. Mainly, ionic liquids, which are stable for a very long time at high temperatures are needed. That is why the ILs’ thermal stability is tested isothermally at different temperatures for a dozen hours or even days [69,71]. There are also other methods for determining ILs’ thermal degradation besides thermal gravimetric analysis. One of them is based on inverse gas chromatography. In particular, the inner wall of a fused silica capillary is coated with a thin film of the ionic liquid. The column is heated and the IL’s degradation products are de-


Siedlecka et al.

Table 8. Abundant by-Products of Radiolysis of Ionic Liquid Anions [75, 76] Parent Anion in Ionic Liquid [Cl]

-

Primary by-Products Cl•, Cl2•

[NO3][BF4] [PF6]

-

-

[(CF3SO2)2N]-

-

Secondary by-Products Cl-

•NO3-

NO2-, O 2

F•

RF

F•

RF

F•, CF3•, NSO 2-•, NSO2 CF3 -•, N(CF3SO 2)SO2- •

RF, RCF3, NHRSO 2H, NHRSO2 CF3

R = trialkylamonium or dialkylimidazolium residue

tected using a flame ionization detector (FID) or, even better, via a mass spectrometer. The limit of detection is between 10 ppm and 10 ppb [60]. Additionally, thanks to mass spectrometry detection we can identify individual products of thermal degradation [60,6667]. RADIOLYSIS OF IONIC LIQUIDS Ionic liquids may be successfully applied as solvents for highly radioactive materials, so they must not undergo significant degradation due to radiolysis on exposure to high radiation doses. On the one hand, typically, a nitrogen heterocyclic compound such as imidazole [hydrogen yields G(H2)=0.03 molecules per 100 eV] demonstrates the stabilizing effect of aromaticity upon a molecule’s tendency to undergo radiolysis. Therefore it was assumed that the aromatic nature of [IM1A][X] ionic liquids would make them highly resistant to radiolysis. On the other hand, alkanes give rise to hydrogen yields [G(H2)] of about 5–6 molecules per 100 eV [72], which may suggest a disadvantageous influence on the stability of ionic liquids because of the substituted N-alkyl chain. The initial interactions of radiation with chemical systems is by ionization (Eq. 1) and excitation (Eq. 2): R R+ +e(1) R R* (2) Ionization may be followed by geminate recombination (Eq. 3) to give more excited states or by ion dissociation (Eq. 4): R+ + e- R* (3) (4) RH + R•++ H• A range of recombination and hydrogen abstraction processes can follow these reactions. Additionally, H2 is found as a radiolysis product from irradiated organics (Eq. 5): (5) RH + H• R• + H2 Previous investigations have focused on the understanding of radiolytic behavior of imidazolium and tetraalkylammonium ionic liquids. Studies of the effect of irradiation on 1-alkyl-3methylimidazolium cations with [NO3]-, [Cl]-, [(CF3SO2)2N]-, [CF3SO3]-, [PF6]-, [BF4] as counter ions indicated that they appeared to be relatively radiation resistant under 0.4 MGy of radiation [73–77]. Less than 1% of these ionic liquids were depleted and the stability of them was close to that of benzene [73]. It was observed that the stability of the imidazolium cation depends on the anion in the ionic liquid and the relative degradation of the [IM14]+ cation follows in order: [IM14][(CF3SO2)2N]< [IM14][CF3SO3]< [IM14][PF6]< [IM14][BF4] [78]. The irradiation stability of the investigated anions in imidazolium ionic liquids was also different. [(CF3SO2)2N]- and

[CF3SO3]- have approximately the same stability. [PF6]- appears to have approximately the same stability compared to [BF4]- which undergoes a lower radiolytic damage [76]. However differences in the radiolytic yields of [PF6]- and [BF4]- confirm results previously reported [79-80], and can be explained by the lower energy of dissociation of the P-F bond as compared to the B-F bond. Several publications also demonstrate the high stability of the tetraalkylammonium-based ionic liquids under irradiation [78,81– 83]. Despite the fact that aromatics are known to absorb energy and relax non-dissociatively compared to aliphatics under ionizing radiation, the relative degradation of [IM14]+ and [N1444]+ cations appears to be similar under high irradiation. Additionally, [(CF3SO2)2N]- seems to show the same radiolytic stability regardless of the nature of the cation ([IM14]+ or [N1444]+) [76,78]. Due to the presence of water in many room-temperature ionic liquids, even after a moderate drying procedure, the effect of water and the radiolysis atmosphere was taken into account in several studies [75-76,78]. Irradiation of [N1444][(CF3SO2)2N] indicated that the degradation of the [N1444]+ cation in both air and argon was significant when the ionic liquid was initially in contact with water. In contrast to this, the concentration of the [(CF3SO2)2N]anion after irradiation in air was appreciably lower than that of the ionic liquid irradiated in air which was in contact with water before. Generally, the presence of water slightly modified [N1444][(CF3SO2)2N] radiolytic yields of disappearance from [N1444]+ and [(CF3SO2)2N]-. Similar investigations carried out on imidazolium-based ionic liquids, showed that water, does not have a significant influence on the degradation efficiency of [IM14][(CF3SO2)2N] [76]. Nevertheless, the degradation pathways of ionic liquids in the presence of water need further investigation. It is known that the imidazolium ring as well as alkyl and amine chains are very sensitive to •OH radicals, which are generated during irradiation of water. However, the analytical techniques currently used by researchers (NMR, ESI-MS, liquid chromatography) did not reveal new products formed by irradiation due to the presence of water [7576,78]. It was supposed that the formation of hydrogen peroxide by water radiolysis or dehydrogenation of ionic liquids by sparging with argon or air would be the reason [78]. Investigations of the effect of the initial water concentration on hydrogen yields (eqn. 5) have been carried out at higher radiation doses (up to 1.0 MGy) but no differences were observed on hydrogen yields between “dry” and water pre-equilibrated [IM14][(CF3SO2)2N] [78]. More particularly, some researchers focused on ionic liquids’ stabilities under high gamma irradiation doses (up to 2.0 MGy), and on the characterization of the intermediates of room-temperature ionic liquids. These investigations have been performed using different analytical techniques: HPLC, ESI-MS, GC-MS, 1H-NMR,



Table 9. Abundant by-Products of Radiolysis of Tetraalkylammonium Ionic Liquids [78] PARENT CATION IN IONIC LIQUID

PRIMARY BY-PRODUCTS

SECONDARY BY-PRODUCTS

+

+

N

N

.

H

H

.+ +

N

+

N

N

+

N

.

+

N

+

N

R +

N

R Where R=NHSO 2CF 3, CF3 , 19

F-NMR spectroscopy, IR, UV-Vis, and EPR spectroscopy for quantitative and qualitative studies. In the literature, the behavior of ionic liquids under radiation is considered as the effect of primary and secondary processes during radiolysis. The primary effect of radiation in ionic liquids was studied using pulse radiolysis of: imidazolium ([IM14][PF6], [IM14][BF4]), pyridinium ([Py4-4Me][PF6]), ([Py4][BF4]) and ammonium ([NAAAA][(CF3SO2)2N]) ionic liquids [73-74,8182,84-85]. It was shown that electrons produced by ionization (eqn. 1) are very rapidly captured by [IM14]+ cations followed by the formation of imidazoyl radicals [86]. Secondary reactions of these radicals may lead to the formation of carbene and imidazoyl dimerization products. Behar et al. showed spectroscopic evidence of the formation of the neutral radical [IM14]• and dication radical [IM14]•2+ upon irradiation of [IM14][PF6] ionic liquid [73]. Marcinek et al. also reported the formation of the [IM14]• radical and comment upon the ability of ionic liquids to stabilize radical ions [86]. Under UV-laser irradiation it was found that the neutral radical [IM14]• was relatively stable and that the excited state [IM14]+* underwent degradation efficiently [87].

In the case of [Py4]+ and [Py4-3Me]+ the electrons are not completely trapped, and it is possible to follow the electron transfer from the cation radicals to various solutes (CCl4, benzophenone, quinones). In contrast to imidazolium and pyridinium cations solvated electrons produced in tetraalkylammonium-based ionic liquids react much more slowly with the solvent cation [84,88]. Some investigations indicated also a correlation between irradiation degradation of ionic liquids and changes in their physical properties. The influence of (137Cs) radiation on [IM14][Cl], [IM14][PF6], and [IM14][(CF3SO2)2N], on the physical properties of ionic liquids as solvents, was first revealed by Berthon et al. [75]. They observed darkening after irradiation of imidazolium ionic liquids [75,77,89]. According to Berthon et al., gamma radiolysis by doses ranging from 100 to 1200 kGy caused a significant increase in the viscosities and a decrease in the conductivities. The formation of colored products was generally related to the formation of carbene as a result of dialkylimidazolium cation radiolysis. Electrospray ionization mass spectroscopy and NMR measurement indicated that the abstraction of the H• atom from the 2carbon position of the imidazolium ring, ([IM14]•+), or from the


Siedlecka et al.

Table 10. Abundant by-Products of Radiolysis of Imidazolium Ionic Liquids [75, 76] Parent Cation in Ionic Liquid

Primary by-Products

•

Secondary by-Products

CH3 •C4H9 H•

C4H8, C2 H4 C4H10, CH4 , C2 H6 ,H2 where: R =(CH3 •,C4H9•, CF3•, F•, SO2 CF 3•)

carbon position of the butyl chain, ([IM14•]+), and the splitting of [IM14]+ into a butyl radical [4]• and the 1-methylimidazolium radical cation [IM1]•+, were the primary processes of ionic liquid gamma radiolysis [75]. The secondary reactions of these radicals yield numerous stable products at very low concentrations. Recombination of the primary radicals produced dialkylimidazolium cations and two different dimers (Table 10). -Radiolysis of [IM14][BF4] carried out by another research group, also induced a detectable destruction of the imidazolium cation, but relatively small changes of the [BF4]- anion [80]. The Raman spectra presented in that paper indicate that radiation at doses of 20 and 100 kGy induces a detectable destruction of alkyl chains of imidazolium cations. The authors considered that significant darkening even at low doses is accounted for by the recombi-

nation of imidazolium cation radicals which form H- or F-deficient dimers and oligomers which are known to have high molar absorption UV-Vis coefficients. Finally, radiolysis of [IM14][BF4] may generate H2, HF, and F2 as gaseous products and nitrogenous heteroaromatics. Under identical conditions, the degree of darkening and the amount of pungent fumes containing toxic and corrosive HF, for [IM14][BF4] is lower than for [IM14][PF6]. Despite that, the stability of [IM14][BF4] is higher than [IM14][PF6]; however, the authors concluded that the studied ionic liquids seemed to be unsuitable as an extractant for the use in nuclear processes [79]. Allen et al. presume that in radiolysis of dialkylimidazolium cations the aromatic ring serves to inhibit radiolysis of the alkyl chain, so the inhibition process would be more efficient for the shorter alkyl chain in imidazolium cations [74]. Additionally the


study of the effect of irradiation on the 1-alkyl-3-methylimidazolium cation (where the alkyl group length was increased from ethyl to octyl) with nitrate or chloride as counter ions indicated that similar mechanisms of radiolysis might be applied to all studied dialkylimidazolium cations [74]. However, analyzing the anions in terms of radiolysis of [IM1A][NO3] was different to [IM1A][Cl]. Ionization of [Cl]- would form Cl•, which could react with [Cl]- to form the radical anion Cl2•-. This entity would not oxidize the imidazolium cation by hydrogen abstraction but would react with an electron adduct to give [Cl]-. In contrast to the chloride anion, [NO3]- forms [NO2]- and O(3P) directly after the energy absorption. Further, O(3P) may be expected to react with [NO3]- to form [NO2]- and O2, although other reactions may occur in the presence of organic compounds. Experiments carried out with other imidazolium ionic liquids indicated that the radiation induced the fragmentation of the anions such as [(CF3SO2)2N]-, [BF4]- and [PF6]- [73,75,78]. The [(CF3SO2)2N]- anion produces •CF3 radicals through the dissociation of the intermediate [(CF3SO2)2N]•, “hole” radicals or excited [(CF3SO2)2N]-* anions. The primary radiolysis products of [BF4]-, [PF6]-, and [(CF3SO2)2N]- anions was F• (Table 9). In contrast to aromatic cations (imidazolium and pyridinium), with the electrons are attached to the ring, and therefore forming imidazoyl and pyridinyl radicals, the aliphatic cations (phosphonium, ammonium, pyrrolidinium) generally formed terminal and penultimate C-centered radicals (Table 9). The authors confirmed that this pattern was indicative of efficient deprotonation of a hole trapped in the aliphatic cation (the radical dication) that competes with rapid electron transfer from a nearby anion. This charge transfer, the prevalent route for aromatic cations, leads to the formation of stable N- or O-centered radicals. The dissociation of parent anions is a minor pathway [90]. HYDROLYSIS OF IONIC LIQUIDS To report the water content, drying procedures or impurity levels (for example HF) in ionic liquids used in experiments is crucial, because the presence of especially water may affect properties and reactivity of ionic liquids. In general, most ionic liquids can be considered as not susceptible to hydrolysis. Maybe, a higher water sensitivity could be observed for some functionalized side chains containing e.g. ester groups but this was not investigated so far. Hydrolytic processes are more related to the anion moiety of an ionic liquid like [PF6]- or [BF4]-. For both anions hydrolytic processes were observed [91,92] and a higher susceptibility for the [BF4]- anion was found. In case of [IM14][PF6] hydrolysis was found during the extraction process in acid conditions [93]. The authors supposed that HNO3 might act as a catalyst in the reaction: H+ + PF6- + 6H2O + HNO3 H3PO4 + 6HF + HNO3 + 2 H2O which promotes the degradation of [PF6]- to [PO4]3-. Initially biphasic systems of aqueous HNO3 and [IM14][PF6] become monophasic as the decomposition of counter anions proceeds. The catalyzed formation of [PO4]3- is also accelerated in the presence of SiO2. There is also formation of SiF4 and H+, further driving the reaction towards the production of H3PO4. Additionally, ionic liquids containing the counter ion [PF6]- are unstable, because of hydrolysis forming volatiles, including HF, POF3 or other species, which are very toxic and corrosive [94]. The hydrolytic stability of [IM14][1OSO3] and [IM14][2OSO3] also depends on the anions. In the presence of


water, methyl sulfate and ethyl sulfate anions form the corresponding alcohol at elevated temperatures. For alkyl sulfates with longer alkyl chains (from 8 to 18 carbon atoms), the sensitivity to hydrolysis is much lower [95]. From the environmental point of view the hydrolysis of manmade compounds represent the most important abiotic degradation pathway in the environment. The products formed by such abiotic conversions may be biodegraded further by microorganisms or are naturally found in the environment. For instance, the hydrolysis of [BF4]- and [PF6]- results (as mentioned above) in the release of HF. This acute toxic and corrosive compound is problematic with respect to operational safety and the application in technical processes. Regarding ecotoxicity and bioaccumulation, it is of minor concern because the evolving compounds (fluoride, boric acid and phosphoric acid) are not harmful to the environment in moderate concentrations. The hydrolysis data from the ionic liquid anions trifluorotris(pentafluoroethyl)phosphate and bis(trifluoromethylsulfonyl)imide [92] suggest that these anions are resistant to hydrolytic cleavage under environmental conditions. OXIDATIVE DEGRADATION Lately, the attention has been focused on the application of advanced oxidation processes (AOPs) for industrial waste water treatment, especially for the removal of hazardous organic compounds from contaminated water. AOPs rely on the generation of a very reactive oxidizing agent, i.e. free radicals such as the hydroxyl radical. •OH can initiate oxidative degradation reactions of refractory synthetic and natural organic compounds and it is capable of mineralizing them ultimately to CO2 and H2O owing to its high oxidation potential (+2.8 eV NHE) in aqueous solutions. There are several oxidative processes involving direct photolysis of H2O2 (H2O2/UV), of O3 (O3/UV), photocatalyzed processes (TiO2/UV), the reaction of H2O2 with ferrous (Fe2+) and ferric (Fe3+) iron in acidic aqueous solutions (Fenton and Fenton-like reactions). Among them, for the degradation of ionic liquids oxidation by H2O2/UV [96], Fenton’s reagent [97], or ultrasonication [98] in aqueous solutions have been used successfully. Advanced oxidative degradation in the presence of reactive peroxides generated by UV irradiation for removal of ILs from water was used first by Stepnowski and Zalewska [96]. In this study the H2O2/UV system was found to be more effective than a TiO2/UV system. Regarding the degradation of four imidazolium ionic liquids: 1-butyl-, 1-hexyl-, 1-octyl3-methylimidazolium chlorides, and 1-ethyl-3-ethylimidazolium tetrafluoroborate, their stability correlates directly with the length of the N-1 n-alkyl substituent. 1-Octyl compounds proved to be the most stable; the least stable entity was the 1-butyl compound. The authors consider that despite the evident correlation, the results obtained on the stability were net observations of several effects, depending on the type and number of atoms but also on the conformation of the molecule [96]. The reaction of ozone with a pyridinium IL containing 1-alkoxymethyl and 1-alkylthiomethyl substituents and imidazolium ILs with lactate anions was investigated by Pernak et al. [99-100]. Both types of salts undergo the decomposition very quickly and were quantitatively removed. Additionally, the investigations showed that ozonation was strongly dependent on the kind and the positions of the substituents on the pyridinium ring. The most favorable were the 3-position on the ring and the substituents including hydroxyl or dimethylamino groups [99-100].


Siedlecka et al.

Table 11. Abundant by-Products of Imidazolium Ionic Liquids Degradation by Advanced Oxidation Processes Parent Cation in Ionic Liquid

Aop Method

Possible by-Products

CH3COOH/H2O2 / ultrasound [98]

Fenton-like reaction [107] H2O2/UV [108]

Where R= from C(2) to C(8) aliphatic chain

Further study of the degradation of ionic liquids by AOP showed that [IM14][Cl], was very effectively degraded in the Fe3+/H2O2 system [97]. Fenton’s reagent forms hydroxyl radicals: (6) Fe2+ + H2O2 Fe3+ + •OH + OHwhich react with most organic pollutants and oxidize them to organic by-products and leading to carbon dioxide, different inorganic ions and water. In comparison, the depletion efficiency of [IM14][Cl] and the degradation of three other imidazolium ionic liquids with longer

N-alkyl chains ([IM16][Cl], [IM18][Cl] ) indicated that the most stable compound in this system was 1-octyl-3-methylimidazolium chloride, with the least stable being the 1-butyl entity [101]. As in the photodegradation study, the Fenton reaction also revealed that a lengthening of the substituent at an imidazolium ionic liquid in the position 1-N increased the resistance to chemical degradation. Replacing the imidazolium head group with pyridinium ([Py4-3Me][Cl]) slightly increased the resistance of ionic liquids to degradation. Surprisingly, [Py4-3Me][Cl] was degraded to a lesser extend than [IM14][Cl], when the dose of H2O2 increased up to 400 mM (a


Fenton-like process with a large excess of H2O2 called a vigorous Fenton-like process). The authors consider that the enhanced resistance of [Py4-3Me][Cl] to degradation, and the decrease of the resistance of the imidazolium ionic liquids with increasing H2O2 concentration, was probably indicative of a change in the degradation mechanism. The vigorous Fenton-like system in contrast to the Fenton-like system (where the oxidation by •OH radicals is the main reaction) generated different radicals (•OH, HO2•, O2•, HO2-) [102]. In this system, organic compounds can react in many different ways: they can be oxidized, reduced, or attacked by nucleophiles. However, ultrasonic chemical oxidative degradation of 1-alkyl3 methylimidazolium ionic liquids showed that altering the nature of the side chains and the type of anions used, surprisingly, does not affect the degradation process [98]. The authors consider that, under ultrasound, the reactions in the ionic liquid mixture /H2O2/CH3COOH took place rapidly between the radicals and specific sites of the molecule as H2O2 is in excess while the new radicals were generated continuously with the aid of ultrasonic treatment leading to exhaustive degradation. A comparative study of 1-butyl-3-methylimidazolium salts with different anions as a counter ion and as a background in Fenton-like systems indicated that the stability of ionic liquids depends on the strength of the interactions between the positively charged imidazolium ring and the negatively charged anion in aqueous solution [103]. It was observed that the anion exchange process in [IM14][Cl] in the presence of [(C5F11)COO]-, [(C7F15)COO]-, or [(C9F17)COO]- significantly elevated the stability of the imidazolium cation. The resistance of [IM14]+ to degradation was as follows: [Cl]- < [(C5F11)COO]- < [(C9F17)COO]- < [(C7F15)COO]-. The interaction between [IM14]+ and [(C7F15)COO]- ions was stronger than that of [IM14]+ and [(C9F17)COO]- ions, because the concentration of [(C9F17)COO]- was probably near the critical micellar concentration and a stronger association with the protons could take place in acidic solution (pH=3) [103]. Additionally, the study showed that the degradation of imidazolium cations decreases when the counter ion competes with the cation for hydroxyl radicals [103]. Cl- ions react rapidly with •OH radicals at low pH (the optimal pH for Fenton’s reaction is 3–4) to form Cl2•- radicals, which in contrast to •OH radicals will not oxidize [IM14]+. Similar to [Cl]-, [C(CN)3]- reacts with •OH radicals leading to a decrease in the amount of •OH radicals accessible for imidazolium cation oxidation. Therefore, among the studied ionic liquids the resistance to oxidation decreased in the following order: [IM14][Cl] > [IM14][C(CN)3] > [IM14][CF3SO3]. Additionally, in Fenton-like reactions the anions were undergoing significantly complex reactions with ferric ions, inhibiting the generation of •OH radicals [103]. The investigators were also interested in intermediates generated during degradation of ionic liquids by AOPs. The previous study with imidazolium compounds shows that hydroxyl radicals react with them very rapidly via addition [104-105]. Further, the OH adducts react fast with O2 in accordance with eqn. 7 (the rate of the reaction 7 is ~109 M-1s-1) [106]: (7) OH•Im+H + O2 OH•Im+HO2 On the one hand, addition of •OH to [IM14]+ is expected to take place at all free ring carbons to produce three isomeric adducts. The constant rate for the reaction of •OH radicals with [IM14]+ was estimated to be 3.7x109 Lmol-1s-1, and it is slightly lower than that for imidazolium (5x109 Lmol-1s-1) [73,106]. On the other hand, the


hydroxyl radicals generated in AOP can react non-selectively with the n-alkyl substituted group in [IM1A]+ cations. Investigation of the degradation of imidazolium ionic liquids by AOPs, suggest that the first stage in all studied processes was similar – generation of OH-adduct radicals, which undergo further reactions. The main intermediates which were found during the degradation of ionic liquids by different AOPs depend on the experimental conditions (the process of •OH radical generation, the concentration of •OH or if they were supported by ultrasound). 1 H-NMR studies of mixtures performed subsequently after Fenton-like reactions were carried out on a [IM14][Cl] solution. They confirm that in the first stage of degradation, the radical attack is nonspecific, with any of the carbon atoms in the ring and the nalkyl chain being susceptible to attack. Further degradation leads to the generation of mono- and dicarboxylic acids and amino acids as by-products of imidazolium and pyridinium ionic liquid degradation [97,101]. The identification of ionic liquid by-products in both advanced oxidation systems, H2O2/UV and Fe3+/H2O2 [107-108], showed that AOP gives a large number of degradation by-products. In spite of the different methods used for the identification of intermediates in H2O2/UV (LC-MS) and Fenton-like system (GC-MS), similar intermediates were found. In the H2O2/UV system one of the main by-products is formed via the substitution of hydroxyl groups in the imidazolium ring. Products, which were also identified, are ones with carboxyl groups instead of methyl groups in the alkyl chain, a carbonyl group in the alkyl chain, and various numbers of hydroxyl groups instead of hydrogen atoms in the alkyl chain. During a single HPLC run, two or more compounds with the same mass but slightly different retention times were also observed; these probably represent compounds substituted with the same number of hydroxyl or carbonyl functionalities but in a different position of the alkyl chain or in the imidazolium ring [107]. The authors suggest that the main degradation products of [IM14][Cl] at the first stage of Fenton-like reaction are imidazolones (m/z=154). Additionally, there are probably compounds substituted with one hydroxyl group in different positions of the alkyl groups of imidazolone derivatives or substituted with the hydroxyl group on the ring. These intermediates are expected to be unstable under further oxidation processes, and they were decomposed by oxidative cleavage of the N-C bond in an N-alkyl side chain, as well as due to the ring opening. The authors consider that the study offers preliminary insights into the degradation process of imidazolium ionic liquids by OH radicals [108]. However, the most important factor for the wastewater treatment process seems to be that the intermediates of ionic liquids generated by AOPs are more degradable than the parent compounds. In experiments carried out in an H2O2/CH3COOH system aided by ultrasound, other intermediates were identified [98]. The degradation mechanism proposed by the authors implies three primary steps. Firstly, the 1,3-dialkylimidazolium ring backbone was oxidized generating transient or intermediate 1,3-dialkylimidazolidine2,4,5-trione. Secondly, the cleavage of N-alkyl side chains together with the destruction of either the N1-C5 or the N3-C4 bond simultaneously formed secondary transient/intermediate products such as alkyl aldehyde and ring open degradation products. Further cleavage of the remaining C-N bond generates species forming acetoxyacetic acid and biuret [98].


The fact that sonochemical oxidation of ionic liquids in the presence of H2O2 and acetic acid didn’t show any differences in the efficiency of degradation and the nature of by-products indicates that imidazolium ionic liquids decompose in various ways: by attack of •OH radicals, the indirect effect of ultrasound and temperature. The sonochemical oxidation of ionic liquids was probably carried out in more drastic conditions than the oxidation in H2O2/UV and Fenton-like systems. This might be the reason why different intermediates were generated. The studies mentioned before are indicating that AOP can in principle be applied to remove ionic liquids from the technical wastewater stream. For a final assessment of the utility of the used AOP to remove ionic liquids in an environmentally compatible manner, further investigations are necessary to analyze the biodegradability and toxicity of the formed solutions. Using the example of imidazolium-based compounds, here the main transformation products formed in the AOP applied are related to the imidazolium core and functional groups are introduced into the side chain [107]. In particular this N-substituted head group is known to be poorly biodegradable [109] but the introduction of hydroxyl/carbonyl groups associated with a ring-opening reaction or the removal of aromaticity are very promising attempts to increase biodegradability. The products formed (amines, imines, aldehydes, and ketones) can undergo further chemical or enzymatic reactions. In general, it is known that the introduction of hydroxyl groups into the side chain of ionic liquid cations results in a significantly decreased acute toxicity toward rat cells [110], bacteria (Vibrio fischeri), duckweed (Lemna minor) and algae (Scenedesmus vacuolatus) [111] compared to the unsubstituted alkyl compounds. However, it has to be kept in mind that the reactive intermediates formed during degradation may still cause toxic effects on the acute or chronic scale, which have to be investigated in more detail. ELECTROCHEMICAL STABILITY The electrochemical stability of an ionic liquid is manifested by the width of the electrochemical window. This is the range of voltages in which the ionic liquid is electrochemically inert. Many research groups have measured the electrochemical window of a wide variety of ionic liquids [112,114–120]. They found that phosphonium-, ammonium-, pyrrolidinium-, and piperidinium-based ionic liquids with triflate or triflyl anions show the largest electrochemical windows [112–114]. Additionally the cathodic stability limit is generally determined by the cation, while the anodic limit is determined by the anion. However, the type of counter ion may influence the cathodic stability limit [121]. The electrochemical stability of aprotic quaternary ammonium salts, determined at a glassy carbon (GC) or platinum electrode is within a range of 4–6 V. Imidazolium and morpholinium salts show stability of ca. 4 V, while piperidinium and pyrrolidinium salts, especially based on imide anions, show stability of ca. 6 V. [122]. Quaternary phosphonium ionic liquids exhibit also the wide electrochemical window of approximately 6 V [123]. However, a shift to a lower potential of the anode limit with an increase in the length of the side-chain in tri-n-butylalkylphosphonium fluorohydrate (alkyl= methyl, butyl or octyl) was observed. This trend is not observed for phosphonium ionic liquids with the amide ion [124]. The branching of the alkyl group in pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids leads to a moderate increase of the electrochemical stability in passing from n-butyl to iso-butyl while a dramatic decrease was observed from the sec-butyl group. The

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progressive increase of the cathode limit potential was also observed with the increase of the side chain length [125]. Halide counter ions such as [F]- or [Br]-, which undergo anodic oxidation at relatively low potentials, are responsible for the narrow stability of ionic liquids of the order of ca. 2–3 V. The amide anion on the contrary, is oxidized at relatively high anodic potentials, which implies the broad stability of ionic liquids based on this anion [126]. However, it may be difficult to compare stability limits since the voltammetry curves showing the stability range of ionic liquids were recorded for different materials: GC [122,127], Pt, W [115,128], Au [129], titanium oxide bronze [119] and against different references: ferrocene, Ag/Ag+, Li/Li+, I3/I-, in some cases not electrochemically defined [126]. Stability windows obtained at Pt and GC electrodes are comparable, while the ionic liquid/tungsten interface shows a much enhanced stability in the range of 6.5–7 V. It was found that impurities like residual halides and water have a profound impact on the electrochemical stability of ionic liquids [130]. Some researchers used quantum chemical calculations to predict the electrochemical stability of ionic liquids [127]. It has been shown that reductive stability can be correlated to the lowest unoccupied molecular orbital (LUMO) energy level of the cation. The intermediates, which occurred at the cathodic limit of the pyrrolidinium- and imidazolium ionic liquids were predicted by quantum chemical calculations and verified by experiments [127]. The indirect reduction of ionic liquids at an electrode results in unstable radicals. These radicals decompose into a neutral fragment and a smaller, more stable radical. Secondly, two radicals can react with each other in the formation of one neutral molecule (radicalradical coupling) or two neutral molecules (disproportionation). Finally, radicals can react with alkenes, where the radical combines with one of the electrons of the -bond in the formation of a larger radical (radical addition reaction). GC-MS studies confirm the above described route of ionic liquid reduction initiated on the cathode. It was detected in [Pyr14]+ electrochemically decomposed on 1-methylpyrrolidine, octanes, octenes and 2-butanol, which indicates that at the first stage the [Pyr14]• radical decomposes into 1-methylpyrrolidine and a butyl radical. In the case of [IM14+] all the expected decomposition reaction products were found: (i) dimer of 1-butyl-3-methylimidazolium formed in the coupling reaction of two 1-butyl-3methylimidazolium radicals, (ii) 1-butyl-2-hydro-3-methylimidazole formed in a disproportionation reaction. The cathodic reduction has been thought to yield neutral [IM1A]• radicals that subsequently undergo disproportionation or recombination. Shkrob and Wishart imply the formation of a dimer product (IM1A)2 in electrochemical decomposition of [IM14]. Their study suggests that these decomposition products are more likely to be formed via deprotonation of the dimer radical cation than by the reaction of these unstable [IM1A]• radicals [131]. An electrochemical treatment – acting as an alternative disposal strategy for non-biodegradable compounds – was applied for an aqueous solution of 1-butyl-3-methylimidazolium chloride using boron-doped diamond-coated (BDD) electrodes to facilitate the compounds breakdown [132]. In general, the BDD coating exhibits a high oxygen overpotential, which promotes the electro-oxidation of organics via electrogenerated hydroxyl radicals due to their high standard redox potential of 2.8 V and reduces the side reaction of oxygen evolution.


With these electrodes an almost complete primary degradation of the [IM14] cation was determined via HPLC–UV measurements within 4 h. In general, no UV signals after the electrochemical reaction could be found in the chromatogram leading the authors to the conclusion that the chromophore - the positively charged imidazolium core structure (considered mainly to be responsible for the non-biodegradability) - has been destroyed. The remaining solution after the electrolysis process has been analysed in a biodegradation test. For the solutions coming out of both chambers of the electrophoretic setup an oxygen consumption through the test microorganisms used was found after eight days, suggesting an improved biodegradability of the transformation products in comparison to [IM14]. These preliminary investigations indicate that an electrochemical treatment could be an appropriate technique to remove non-degradable ionic liquids from wastewater. However, detailed examinations including the identification of transformation products and a toxicity assessment for the generated solutions are needed for a final appraisal. CONCLUSIONS This review emphasizes the stability of ionic liquids under different physicochemical conditions such as high temperatures, irradiation, electrical currents or oxidation agents. It describes the influence of structural modifications (type of cation, anion, alkyl chain length, etc.) in helping or limiting the applicability of ILs. The stability of ionic liquids is important from a technical point of view (performance, lifetime etc.) but also from the operational safety (exposure of employees) and environmental (if they are released e.g. via wastewaters) point of view. A good understanding of hazards associated with degradation products (especially if they are volatile) is crucial, but in respect to this no information is available so far and needed to be investigated, when ILs will finally be used in large-scale applications. Moreover, advanced oxidation processes seem to be an alternative disposal strategy for non-biodegradable ionic liquids. Nevertheless, the design of inherently biodegradable ionic liquids should be preferred due to a reduced hazard for man and the environment and considering the energy consumption and the need for further chemicals and apparatus when AOP are applied.

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ACKNOWLEDGMENT Financial support was provided by Polish Ministry of Research and Higher Education under grant N N523 42 3737, DS 8270-40093-1 and DS 8200-4-0085-1. The authors gratefully thank Marianne Matzke and Jürgen Arning for helpful discussions.

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Received: 14 January, 2010

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Revised: 19 March, 2010

Accepted: 19 March, 2010