A

Applications of Ionic Liquidsin Clean and ValuableUtilization of Coal: FromAspects of Environment

Huacong Zhou1,2, Quansheng Liu1,2 andLimin Han1,21College of Chemical Engineering, InnerMongolia University of Technology, Hohhot,China2Inner Mongolia Key Laboratory of High-ValueFunctional Utilization of Low Rank CarbonResources, Hohhot, China

Imidazole-Based ILs

[AMIM]Cl 1-Allyl-3-methylimidazoliumchloride

[AMIM]BF4 1-Allyl-3-methylimidazoliumtetrafluoroborate

[AOEMIM]BF4

1-((Ethoxycarbonyl)methyl)-3-methylimidazoliumtetrafluoroborate

[BDMIM]Cl 1-Butyl-2,3-dimethylimidazolium chloride

[BMIM]Cl 1-Butyl-3-methylimidazoliumchloride

[BMIM]I 1-Butyl-3-methylimidazoliumiodide

[BMIM]OH 1-Butyl-3-methylimidazoliumhydroxide

[BMIM]AC 1-Butyl-3-methylimidazoliumacetate

[BMIM]BF4 1-Butyl-3-methylimidazoliumtetrafluoroborate

[BMIM]BrO3

1-Butyl-3-methylimidazoliumbromate

[BMIM]NO3 1-Butyl-3-methylimidazoliumnitrate

[BMIM]PF6 1-Butyl-3-methylimidazoliumhexafluorophosphate

[BMIM]OTf 1-Butyl-3-methylimidazoliumtrifluoromethanesulfonate

[BMIM]H2PO4

1-Butyl-3-methyl-imidazoliumdihydrogen phosphate

[BMIM]FeCl4

1-Butyl-3-methylimidazoliumtetrachloroferrate

[BMIM]CF3SO3

1-Butyl-3-methylimidazoliumtrifluoromethanesulfonate

[BMIM]TCM

1-Butyl-3-methylimidazoliumtricyanomethanide

[B(SO3H)MIM]OTf

1-Sulfonic acid butyl-3-methylimidazoliumtrifluoromethanesulfonate

[EMIM]Cl 1-Ethyl-3-methylimidazoliumchloride

[EMIM]I 1-Ethyl-3-methylimidazoliumiodide

[EMIM]AC 1-Ethyl-3-methylimidazoliumacetate

[EMIM]BF4 1-Ethyl-3-methylimidazoliumtetrafluoroborate

[EMIM]DCM

1-Ethyl-3-methylimidazoliumdicyanamide

© Springer Nature Singapore Pte Ltd. 2019S. Zhang (ed.), Encyclopedia of Ionic Liquids,https://doi.org/10.1007/978-981-10-6739-6_99-1

[EMIM]NTf2 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide

[EOMIM]BF4

1-Methoxyethyl-3-methylimidazoliumtetrafluoroborate

[HOEtMIM]BF4

1-Hydroxyethyl-3-methylimidazoliumtetrafluoroborate

[HOEtMIM]NTf2

1-Hydroxyethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[MMIM]I 1,3-Dimethylimidazolium iodide[PMIM]I 1-Propyl-3-methylimidazolium

iodide[PMIM]BF4 1-Propyl-3-methylimidazolium

tetrafluoroborate

Pyridine- and Pyrrole-Based ILs

[BMP]Cl 1-Butyl-4-methylpyridiniumchloride

[BPy]FeCl4

N-Butylpyridiniumtetrachloroferrate

[BPYD]Cl

1-Butylpyridinium chloride

[BMP]FeCl4

1-Butyl-1-methylpyrroliumtetrachloroferrate

[Py]BF4 Pyridinium tetrafluoroborate

Ammonium Carbamate-Based ILs

DACARB N,N-Diallylammonium N',N'-diallylcarbamate

DBCARB N,N-Bisethylhexylammonium N',N'-bisethylhexylcarbamate

DECARB N,N-Diethylammonium N',N'-diethylcarbamate

DIMCARB N, N-Dimethylammonium N',N'-dimethylcarbamate

DPCARM N,N-Dipropylammonium N',N'-dipropylcarbamate

Tetraalkylphosphonium-Based ILs

[P4,4,4,1]MeSO4

Tributylmethylphosphoniummethylsulfate

[P4,4,4,1]NTf2

Tributyl(methyl)phosphonium bis(trifluoromethylsulfonyl)imide

[P4,4,4,2]DEP

Tributylethylphosphoniumdiethylphosphate

[P4,4,4,4]Br Tetrabutylphosphonium bromide[P6,6,6,14]Cl Trihexyltetradecylphosphonium

chloride[P6,6,6,14]Br Trihexyltetradecylphosphonium

bromide[P6,6,6,14]Bis Trihexyltetradecylphosphonium

bis(2,4,4-trimethylpentyl)phosphinate

[P6,6,6,14]NTf2

Trihexyltetradecylphosphoniumbis(trifluoromethylsulfonyl)

[P6,6,6,14]N(CN)2

Trihexyltetradecylphosphoniumdicyanamide

Amino Acid-Based ILs

[Ch]ALA Choline alaninate[Ch]ARG Choline argininate[Ch]ASP Choline aspartate[Ch]GLY Choline glycinate[Ch]LEU Choline leucinate[Ch]LYS Choline lysinate[Ch]PHE Choline phenyl-alaninate[Ch]VAL Choline valaninate

Introduction

Low-rank coals (LRCs) are important carbonresources on earth and have some specific prop-erties different from high-rank coals (HRCs), suchas the high content of water, volatile components,and oxygen, but low calorific value. These defectsmake LRCs not suitable for direct combustion asprimary energy sources like HRCs due to the lowenergy efficiency and pollution problems. There-fore, the clean and valuable utilization of LRCsbecomes significantly important. The specificstructures and properties of LRCs make it possibleto be used as non-energy sources. Therefore, thefundamental and practical studies on utilizingLRCs can be gradually categorized into two direc-tions, (1) improving the efficiency and decreasingpollutions for the traditional utilizing approaches,such as pyrolysis, liquefaction, gasification, and

2 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

combustion, and (2) exploring novel, clean, andvaluable utilization of coals or their derivatives.As the alternative of traditional approaches, LRCscould be used as the sources of valuable chemicalcompounds and/or functional materials [1, 2].

Water and organic solvents are inevitably andcommonly used during the utilization processes.Due to the volatility, inflammability, and/or toxic-ity, the large-scale use of organic solvents has highpotential risk of pollution and safety. Ionic liquids(ILs), as well-known new type of solvents withspecific properties different from traditionalorganic solvents, were applied in the clean utili-zation processes of coals in recent years. The useof ILs as solvents for CO2 adsorption or capture,desulfurization, and denitration was welldiscussed or reviewed in many researches andwill not be the topic herein [3–5]. The focus ofthis entry will be critically paid on the use of ILs inthe emerging techniques of treating or pretreatingcoals for the clean and valuable utilization of coalresources since around 2010 pioneered by Painteret al. [6, 7]. As emerging applications, ILs couldbe used as solvents in the following aspects,according to the reported studies, but not limitedto those (Fig. 1): (1) pretreatment before thermalconversion (pyrolysis, gasification, or liquefac-tion), (2) pretreatment to inhibit spontaneouscombustion or low-temperature oxidation pro-cesses, (3) pretreatment to decrease coal dust,and (4) extraction valuable components in coals.Like other emerging technologies, the introduc-tion of ILs in the field of coal utilization hasexhibited huge potential to promote the cleanand valuable utilization of coals, especiallyLRCs, but it also brings new issues concerningILs cost problems, energy consumption, moreimportantly environmental effects, etc. Theseimportant factors codetermine whether ILs couldbe applied in the industrial scale for a certainapplication technique. To the best of our knowl-edge, the reviews and comprehensive analysisconcerning these aspects especially the environ-mental effects are rare up to now. In this entry, wewill first give a short review on the applications ofILs in LRC utilization. And then special focuseswill be paid on the analysis of potential effects onthe environment and the existing problems in the

present studies. At the end, the outlook and futureperspective of the applications of ILs in the cleanand valuable utilization of LRCs will bediscussed.

Emerging Applications of ILs DuringUtilization of LRCs

IL Pretreatment Before Thermal ConversionPyrolysis, gasification, and liquefaction are effi-cient ways to obtain light components like gasesand tar for the use of coal resources. Also, thequality of the char can be improved through pyrol-ysis. IL pretreatment is an emerging approach tochange the composition of tar and improve thequality of char [8, 9]. Lei et al. used 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) as thesolvent to pretreat lignite and found that the pre-treatment could increase the total yield of liquidproducts and oil fractions compared to the originallignite [10]. The pretreatment temperature and theratio of [BMIM]Cl to lignite played key roles onthe total liquid yield and the change of tar com-position. Further characterization indicated thatthe pretreatment by [BMIM]Cl could change thecomposition of OFGs in lignite. Functional ILswere also reported to be used for pretreatmentof lignite before pyrolysis. Acidic Brønsted IL1-sulfonic acid butyl-3-methylimidazoliumtrifluoromethanesulfonate ([B(SO3H)MIM]OTf)was reported to be efficient even at low IL/ligniteratio of 1:1 at pretreatment temperature of 200 �C,improving the tar yield, light fraction content,arene content, and phenolic content in tar for1.6, 2.4, 6.3, and 3.2 times higher than that fromraw SL pyrolysis, respectively [11]. Besides,other acidic IL 1-butyl-3-methyl-imidazoliumdihydrogen phosphate ([BMIM]H2PO4) was alsoproved to be efficient for the treatment beforepyrolysis [12]. Structure analysis indicated thatIL pretreatment led to the fracture of the coalnetwork, resulting in the increase of the contentof C=O and C-O bonds and the disruption ofhydrogen bonds. The effects of ILs depended onboth the combination of cations and anions in ILsand the coal types [13]. For coal gasification,[BMIM]Cl pretreatment could improve the H2

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 3

and CO yield during fixed-bed gasification withsteam and CO2 [14]. The dissociation behaviorsof coal-related model compounds indicated thatchemical reactions indeed occurred for the com-ponents in coals promoted by IL pretreatment[15]. However, it still lacked the strict comparisonanalysis of ILs with the traditional organic sol-vents or simple water pretreatment in most of thereported studies. If these contents wereconducted, it would be beneficial for further prov-ing the superior effects of ILs.

IL Pretreatment to Inhibit SpontaneousCombustionSpontaneous combustion under natural conditionsis a serious shortcoming for LRCs during trans-portation or storage. As an emerging approach,ILs were used to pretreat coals to reduce the low-temperature oxidation reactivity and inhibit thespontaneous combustion [16–18]. Zhang et al.found that 1-allyl-3-methylimidazolium chloride([AMIM]Cl) could effectively inhibit the low-temperature oxidation of coals [16]. ILs couldindeed inhibit or accelerate the spontaneouscombustion behavior depending on the structuresof ILs [19, 20]. 1-butyl-3-methylimidazoliumtrifluoromethanesulfonate ([BMIM]OTf) and 1-butyl-3-methylimidazolium acetate ([BMIM]AC)had better effects than that of other ILs used for

depressing the oxidation rate of bituminous coal[19]. FTIR and TG analysis indicated that ILsplayed their roles mainly via changing the compo-sition of oxygen-containing groups in coal, breakingthe associated hydroxyls into dissociated hydroxyls,and dissolving the easily reductive components incoal. Bai and Shu et al. studied the effects of imid-azole ILs on the inhibition of low-temperatureoxidation using kilogram-scale coal samples[21]. The results showed that ILs could destructthe functional groups in the coal molecular struc-ture, with the benzene rings barely affected. Theinhibiting effect of ILs on low-temperature oxi-dation followed the order as 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) > 1-butyl-3-methylimidazolium nitrate([BMIM]NO3) > 1-ethyl-3-methylimidazoliumtetrafluoroborate ([EMIM]BF4) > 1-butyl-3-methylimidazolium iodide ([BMIM]I). Shu andJiang et al. also proved that 1,3-dimethylimi-dazolium iodide ([MMIM]I) could efficientlyinhibit the spontaneous combustion risk of lig-nite by reducing the -OH groups, and 1-ethyl-3-methylimidazolium iodide ([EMIM]I) couldincrease -COOH groups [22]. Nine imidazole-based ILs were compared by Zhang and Jianget al., and it was found that 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HOEtMIM]BF4) and 1-hydroxyethyl-3-methylimidazolium bis

Applications of IonicLiquids in Clean andValuable Utilization ofCoal: From Aspects ofEnvironment,Fig. 1 Emergingapplications of ILs in theclean and valuableutilization of coals

4 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

(trifluoromethylsulfonyl) imide ([HOEtMIM]NTf2)gave the strongest effect for decreasing oxidation[23]. The possiblemechanismswere deduced as thatILs destroyed the associated hydroxy in coals andchanged the content of carbonyl and ether bonds.Phosphonium-based ILs were deduced to be moresuitable for suppressing coal spontaneous combus-tion than imidazole-based IL ([AMIM]Cl) [18].

IL Pretreatment to Inhibit Coal DustCoal dust is one of the most difficult hazards toprevent and control, which can cause occupa-tional disease and coal dust explosion. The for-mation of coal dust has an important relationshipwith the surface structures and properties of thecoal particle. Recently, IL pretreatment was usedattempting to change the surface structures andproperties of the coal particle and decreasing theformation of coal dust [13, 24, 25]. The studies ofJiang et al. revealed the better wettability of coaltreated with ILs compared to the distilled water bycontact angle experiment [24]. [HOEtMIM]NTf2showed excellent effects on enhancing wettabilityof coals by increasing the hydrophilic functionalgroup content and decreasing the hydrophobicfunctional group content [24]. The cases on theapplications of ILs in this aspect were relativelyfewer than those in other aspects, and more explo-rations are needed in future.

Extraction and Dissolution of LRCs in ILsLRCs contain various valuable structure units,such as the aromatic nucleus structures, aliphaticside chains, and OFGs. These structure units oftenexist in the LRCs as complex spatial networkstructures connected by hydrogen bonds or cova-lent bonds. How to break these interaction anddepolymerize the spatial network structures areimportant to obtain valuable components fromLRCs. ILs were proved to be an efficient solventto achieve this target due to the specific physicaland chemical properties. Lei et al. used [BMIM]Cl as the solvent to extract lignite under micro-wave assistance and found that the extractionyield (up to 80%) increased with the increasingof temperature and the ratios of IL to lignite[26]. Their results also indicated that [BMIM]Clcould break the non-covalent interactions and

extract the aromatic fractions in lignite. Furtherstudies showed that [BMIM]BF4mainly broke theweak hydrogen bonds, while [BMIM]Cl broke thestronger hydrogen bonds, and the extraction yieldwas strongly dependent on non-covalent interac-tions, such as ionic cross-links and hydrogenbonds [27]. Zhang et al. studied magnetic ILs withdifferent cations, including imidazole-, pyridine-,and pyrrolidine-based cations, to dissolve coal directliquefaction (CDLR) and to obtain asphaltene frac-tions [28]. The anions of ILs were one significantfactor to affect the extraction yield of lignite andthe chemical characteristics of the extracts. Theperformances of imidazolium-based ILs toward thedissolution of lignite followed the order asCl�>H2PO4

�>Br�=OH�>BF4�>BrO3

�>PF6�

[29]. The results indicated that the anion structurescould affect the compositions of the extracts, andpyridine-based IL was the most effective toextract asphaltene. The use of ILs for coal disso-lution and extraction was also discussed in therecent review [30].

The commonly used ILs in the above studiesand the coal types applied were summarized inTable 1.

Potential Effects of ILs on Environment

Recycling of ILs and Long-Term Performancesof ILsLike any other chemicals, the use of ILs inevitablybrings potential risk of environmental pollutionespecially considering the large amounts ofusage. Decreasing the usage and fulfilling therecovery of ILs are the efficient approaches forlowering the risk of ILs pollution on environmentin the view of green chemistry. Unfortunately,only small fraction of the reported literatures men-tioned the recycling of ILs. Lei et al. proved that[BMIM]Cl could be recovered by water washingand subsequent evaporation under a reduced pres-sure with recovery yield up to 97.7% after onecycle [10, 27, 31]. The recovery yield of the ILsdecreased with the increasing of the pretreatmenttemperature [31]. Shu and Jiang et al. recoveredILs via similar methods from lignite, and therecovery yield could reach higher than 92% [22].

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 5

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal: From Aspects of Environment,Table 1 Common ILs used in the reported literatures among the emerging applications stated above

Application fields IL structures Coal types Refs.

Pretreatment beforethermal conversion(mainly pyrolysis)

[BMIM]Cl Chinese Shengli lignite [10]

[B(SO3H)MIM]OTf Chinese Shengli lignite [11]

[BMIM]H2PO4 Chinese Shengli lignite [12]

[EMIM]DCM, [BMIM]Cl, [BMIM]TCM,

Australian subbituminous coals(liquefaction)

[13]

[BMIM]Cl Indonesian low-rank coal (steamand CO2 gasification)

[14]

[EMIM]AC Chinese Shenhua bituminous coal [31]

Inhibition ofspontaneouscombustion and low-temperature oxidation

[AMIM]Cl Not given [16]

[BMIM]BF4, [BMIM]AC, [BMIM]OTf Chinese XiShaHe coal mine [17]

[AMIM]Cl, [P6,6,6,14]Cl, [P6,6,6,14]Br,[P6,6,6,14]Bis, [P6,6,6,14]NTf2, [P6,6,6,14]N(CN)2, [P4,4,4,2]DEP, [P4,4,4,1]MeSO4,[P4,4,4,4]Br, [P4,4,4,1]NTf2,

Not given [18]

[AOEMIM]BF4, [AMIM]Cl, [BMIM]AC, [EMIM]AC, [HOEtMIM]BF4,[BMIM]OTf

Chinese bituminous coal fromWenZhang coal mine

[19]

[EMIM]BF4, [BMIM]BF4, [BMIM]NO3, [BMIM]I, [PMIM]I, [EMIM]I,[MMIM]I

Chinese bituminous coal fromShilawusu (Inner Mongolia) andDingji (Anhui), lignite (fromSelian)

[20–22]

[EMIM]BF4, [EMIM]AC, [BMIM]BF4,[BMIM]AC, [AMIM]BF4, [HOEtMIM]BF4, [HOEtMIM]NTf2, [AOEMIM]BF4,[EOMIM]BF4,

Chinese lignite [23]

Inhibition of coal dust [HOEtMIM]BF4, [HOEtMIM]NTf2,[BMIM]BF4,

Chinese bituminous coal fromDatun Longdong coal mine

[24]

[EMIM]NTf2, [EMIM]BF4, [PMIM]BF4, [BMIM]BF4, [HOEtMIM]BF4,[HOEtMIM]NTf2, [P4,4,4,1]MeSO4,[P4,4,4,2]DEP

Chinese lignite from Dongtan coalmine

[25]

Extracts anddissolution of coals

[BMIM]Cl, [BMIM]OTf, [BMIM]BF4 American Illinoisno. 6 subbituminous coals

[6, 7]

[BMIM]Cl, [BPYD]Cl, [EMIM]DCM,[BMIM]TCM,

Australian subbituminous coals [8, 9]

[BMIM]Cl, [EMIM]AC, [B(SO3H)MIM]OTf

Coal model compounds:diphenylmethane, diphenyl ether,diphenyl ketone

[15]

[BMIM]Cl, [BMIM]BF4, [BMIM]Br,[BMIM]OH, [BMIM]H2PO4, [BMIM]BrO3, [BMIM]PF6, [Py]BF4

Chinese Xianfeng lignite [26, 27,29]

[BMIM]FeCl4, [BPy]FeCl4, [BMP]FeCl4

Coal direct liquefaction residues [28]

DIMCARB, DACARB, DPCARM,DECARB, DBCARB

Australian Victorian brown coals [32–34]

[BMIM]Cl, [EMIM]Cl, [BMP]Cl,[BDMIM]Cl

Turkish lignite and bituminouscoals

[35]

[BMIM]OTf Chinese Xianfeng lignite [36]

(continued)

6 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

Although recycling ILs through water washingand subsequent rotary evaporation under lowerpressure are efficient, this approach consumeslots of water and energy to remove water again.Therefore, other approaches are still required torecycle ILs. Zhang et al. developed electrodialysis(ED) method to recycle ILs from dilute aqueoussolutions [38]. The effects of initial concentration,applied voltage, and initial volume of the dilutesolutions on the overall current efficiency, recov-ery ratio, and concentration ratio were systemati-cally investigated. The highest recovery ratio ofIL could reach 85.2%, and the highest overallcurrent efficiency could reach 80.9%. The energyconsumption was evaluated about 1350 g/kW h.Due to the response property to an additionalmagnetic field, magnetic ILs are potential to beused as extractant. Zhang et al. used magnetic ILsto treat coal derivatives, and it was a pity that themagnetic recovery performance of ILs was notseen in their report [28].

In order to facilitate the reuse of ILs, new ILswith specific structures were required. The

“distillable” IL such as N,N'-dimethylcarbamate(DIMCARB) was used to solubilize brown coalsand could be recovered by the condensation oftheir volatile precursor components formed uponheating at relative low temperature[32–34]. These kinds of ILs are promising interms of separating soluble products from coalsand recovering the nonvolatile ILs for further useor disposal. More new ILs with specific structuresfacile for recovering are still very rare in thereported work, and more attentions should bepaid on the designing of new ILs easy forrecycling. Magnetic ILs are another kind of func-tional ILs as extractants with the response prop-erty to an additional field [28]. How to increasethe response ability and recycle ILs in an indus-trial reactor is critical topic for this aspect. Theapproaches for recovery and purification of ILswere well reviewed and analyzed in the reportedliterature [39]. However, if these approaches weresuitable for recovering ILs derived from coal,pretreatment still needs to be studied and evalu-ated in future researches.

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal: From Aspects of Environment,Table 1 (continued)

Application fields IL structures Coal types Refs.

[Ch]ALA, [Ch]ARG, [Ch]ASP, [Ch]GLY, [Ch]LEU, [Ch]LYS, [Ch]PHE,[Ch]VAL

Australian lignite and bituminouscoals

[37]

Notes: [AMIM]BF4, 1-allyl-3-methylimidazolium tetrafluoroborate; [AOEMIM]BF4, 1-((ethoxycarbonyl)methyl)-3-methylimidazolium tetrafluoroborate; [BDMIM]Cl, 1-butyl-2,3-dimethylimidazolium chloride; [BMIM]OH, 1-butyl-3-methylimidazolium hydroxide; [BMIM]BrO3, 1-butyl-3-methylimidazolium bromate; [BMIM]PF6, 1-butyl-3-methylimidazolium hexafluorophosphate; [BMIM]FeCl4, 1-butyl-3-methylimidazolium tetrachloroferrate; [BMIM]TCM, 1-butyl-3-methylimidazolium tricyanomethanide; [EMIM]Cl, 1-ethyl-3-methylimidazolium chloride; [EMIM]AC, 1-ethyl-3-methylimidazolium acetate; [EMIM]DCM, 1-ethyl-3-methylimidazolium dicyanamide; [EMIM]NTf2, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [EOMIM]BF4, 1-methoxyethyl-3-methylimidazoliumtetrafluoroborate; [PMIM]I, 1-propyl-3-methylimidazolium iodide; [PMIM]BF4, 1-propyl-3-methylimidazolium tetra-fluoroborate; [BMP]Cl, 1-butyl-4-methylpyridinium chloride; [BPy]FeCl4, N-butylpyridium tetrachloroferrate; [BPYD]Cl, 1-butylpyridinium chloride; [BMP]FeCl4, 1-butyl-1-methylpyrrolium tetrachloroferrate; [Py]BF4, pyridinium tetra-fluoroborate; DACARB, N,N-diallylammonium N0,N0-diallylcarbamate; DBCARB, N,N-bisethylhexylammonium N0,N0-bisethylhexylcarbamate; DECARB, N,N-diethylammonium N0,N0-diethylcarbamate; DIMCARB, N, N-dimethylammonium N0, N0-dimethylcarbamate; DPCARM, N,N-dipropylammonium N0,N0-dipropylcarbamate; [P4,4,4,1]MeSO4, tributylmethylphosphonium methylsulfate; [P4,4,4,1]NTf2, tributyl(methyl)phosphonium bis(tri-fluoromethylsulfonyl)imide; [P4,4,4,2]DEP, tributylethylphosphonium diethylphosphate; [P4,4,4,4]Br, tetra-butylphosphonium bromide; [P6,6,6,14]Cl, trihexyltetradecylphosphonium chloride; [P6,6,6,14]Br,trihexyltetradecylphosphonium bromide; [P6,6,6,14]Bis, trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate; [P6,6,6,14]NTf2, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl); [P6,6,6,14]N(CN)2, tri-hexyltetradecylphosphonium dicyanamide; [Ch]ALA, choline alaninate; [Ch]ARG, choline argininate; [Ch]ASP, cholineaspartate; [Ch]GLY, choline glycinate; [Ch]LEU, choline leucinate; [Ch]LYS, choline lysinate; [Ch]PHE, choline phenyl-alaninate; [Ch]VAL, choline valaninate

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 7

Another important question concerning therecovery of ILs is the contamination of ILs bycoal components. After contacting with coals,ILs may be contaminated by the soluble compo-nents from coals, which has been indicated bysome researches. For example, Shah et al. showedthat the color of the recycled ILs after pretreatingcoals had changed compared with the fresh ILs[13]. Lei and Shui et al. found that the TG profilesof the fresh and regenerated [BMIM]Cl were dif-ferent, which was attributed the contamination ofILs by coals [10]. The impurities in ILs from coalsmay further affect the performances of ILs insubsequent uses. However, detailed researchesare rare in the present literatures, which shouldbecome critical focuses in the future studies inviewpoint of industrial applications.

Pollution Risk by the Residual ILs in CoalsAnother issue concerning the effects of ILs on theenvironment is that the possible pollution causedby the residual ILs in coals during the pre-treatment or extraction processes of coals usingILs. IL residue is a critical problem because itcannot only lead to the loss of ILs, but moreimportantly, it also results in the residue of theelements of N, S, F, Cl, P, etc. in ILs themselves.These residual ILs may be converted into pollut-ants during subsequent coal conversion. Most ofthe ILs used in the reported work were imidazole-based ILs, and some of them contained sulfonategroups [11, 12]. If some of these ILs left in thecoals, the N, S, F, etc. elements in ILs may beconverted into NOx-, SOx-, or F-containing spe-cies released into air during combustion or pyrol-ysis processes. In theory, the residue of ILs incoals is nearly inevitable because coals haveplenty of natural pore structures and functionalgroups, which may limit or interact with ILs.However, up to now, only very few reported liter-atures mentioned the residue of ILs in coals afterthe contact of coals with ILs. The IL residue inlignite was estimated to be 0.186 g/g coal underpretreatment 150 �C based on the N element anal-ysis, and the residue amounts increased with theincreasing of pretreatment temperature and ratiosof ILs to lignite [10]. IL containing sulfonic acidgroups in cation and trifluoromethanesulfonate as

anion with high contents of N, S, and F was pro-ved to be efficient for pretreatment before pyroly-sis, but the residue of IL was not mentioned in thereported work and needed to be evaluated [11,36]. Overall, more attentions should be paid onthe residue of ILs in coals and the life evolution ofthe residual ILs during subsequent coal conver-sion processes in the future studies.

Life Cycle Effects on the Environment of ILsIn most of the reported literatures, the dosages ofILs were usually very large, with mass ratios ofILs to coals around 1:1–20:1. This indicated thatlarge amounts of ILs would be needed in thepotential industrial applications. One of themethods to decrease the amounts of ILs was touse ILs together with traditional organic solventsor water as mixtures [35, 37]. For such large usageof ILs, one problem inevitably requires to beconsidered and evaluated carefully, i.e., the envi-ronmental effects of the whole life cycle of ILsincluding the synthesis of ILs, the using processesof ILs, and the recycling and posttreatment of thewaste ILs (Fig. 2). From the view of green andsustainable chemistry, only one single step of thewhole technical routes is green or highly efficientwhich cannot support the industrial applicationsof ILs in coal utilization. Therefore, the compre-hensive evaluation should be conducted fromaspects of economics and environment. As far asenvironment analysis, special attentions should bepaid to the evaluation of the use of organic sol-vents during the synthesis of ILs, the solvents(water or organic solvents) and energy consumedduring the recycling of ILs, and the subsequentinfluences of the residual ILs. When applying ILsinto the clean and valuable utilization of coals, theresearchers should keep in mind that the wholeroutes should meet the requirement of being eco-friendly and efficient as much as possible, ratherthan just emphasizing the efficiency of ILs in onesingle step and neglecting others. Up to now, noenough attention has been paid on the whole lifecycle analysis of ILs concerning environment dur-ing their applications in coal industry, which arehighly desirable in future.

8 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

Problems in the Reported Studies

Many of the techniques of using ILs to treat coalto realize the clean and valuable utilization of coalresources discussed above are emerging and havejust developed for around 10 years. Comparedwith other applications of ILs, the exact casesare much fewer. The reported studies have provedthat the introduction of ILs could indeed changethe reaction results, overcome (or partially) theproblems faced by the traditional organic sol-vents, and exhibit the huge potentials for indus-trial applications. But also, some problemsexposed in the reported studies, which should bepaid more attentions in the future studies. Some ofthese problems have been mentioned above. Formore clearly, they will be briefly explained andsummarized in detail herein. The potential effectsof the use of ILs on environment during coalutilization together with the problems in the pre-sent studies were summarized in Fig. 2.

(1) Are ILs really efficient and superior tothe traditional solvents for certain processes orcertain coal types? According to the experimen-tal data in the reported literatures, some ILs could

indeed influence the structures of coals and playpositive effects for coal utilizations. But for someother ILs, the effects of ILs were not as “signifi-cant” as the researchers claimed. Sometimes theword “significant” was abused to describe thepositive effects of ILs, which may mislead otherresearchers for conducting subsequent studies.Therefore, the researchers should pay enoughattentions on the choices of the ILs and the realeffects of ILs, which should also be extensivelycompared to the common and traditional solvents.

(2) The influences of ILs on the structures ofcoals and the mechanism of ILs playing theirroles are not consistent or even contradictory.In the present studies, FTIR and TG techniqueswere often adopted to study the structures beforeand after ILs treatment and disclose the mecha-nism [11, 19, 21, 22]. It was reported that ILscould break the non-covalent interactions suchas hydrogen bonds during microwave-assisted ILextraction of lignite by FTIR characterization[26]. During thermal pretreatment under theimproved temperature, ILs might interact withOFGs (especially carboxylic groups and hydro-gen bonding) and change the composition of

Applications of Ionic Liquids in Clean and ValuableUtilization of Coal: From Aspects of Environment,Fig. 2 Potential effects of the use of ILs during coalutilization and the problems summarized from the present

studies in the viewpoint of environmental effects. The shortdash line in the figure means that this step may havenegative effects on the environment

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 9

OFGs, indicating the chemical reactions occurredduring pretreatment process [10]. Based on thelimited characterization methods, general mecha-nisms were concluded that IL influenced the struc-tures of coals by breaking coal network(non-covalent bonds especially hydrogen bonds),triggering the chemical reaction such as dehydra-tion, and changing the composition of OFGs incoals [10, 11, 31]. But the conclusions were notvery consistent and sometimes contradictory. Pos-sible reasons could be attributed to (a) the com-plexity of coals, (b) various structures andproperties of ILs, (c) different operational condi-tions, and (d) the inaccuracy of the characteriza-tion techniques (such as FTIR and TG) inquantitative analysis of functional groups. There-fore, the revelation of the mechanism of how ILsplay their roles faces huge challenges. More char-acterization techniques are needed to reflect theinfluences of ILs. The structure-function relation-ship analysis of ILs is also important for disclos-ing the mechanism and providing guidance for thechoice of ILs.

(3) The recycling approaches and the long-term performances of the recycled ILs have notbeen well investigated.Water washing and evap-oration of water are efficient ways to recover ILs[10]. However, it consumes large amounts ofwater and energy. New and more efficient modesto recover ILs are needed. Besides, the long-termperformances of the recycled ILs have not drawnenough attention in the present studies. Althoughsome of the reported studies gave the results of therecycled ILs in the second use, the performancesof ILs during more repeated uses, i.e., long-termperformances, are still rare and highly desirable[10]. Also, few attentions were paid on the post-treatment of the waste ILs.

(4) No enough attentions have been paid onthe cost analysis and the potential effects of ILson the environment when choosing ILs. Itseemed that ILs were randomly chosen for manyof the reported literatures without definite crite-rion. IL costs of raw materials and the synthesisprocesses were seldom analyzed. The potentialeffects of ILs on the environment have drawn

few attentions. ILs prepared using natural andsustainable materials were reported only in lim-ited literatures, which should be a potential way tolower the cost of ILs and decrease the effects ofILs on the environment [37]. Fundamental data ofthe life evolution of the residual ILs in coals areseriously blank but critically important in the viewof environment.

Conclusions and Future Prospects

Above all, the emerging applications of ILs duringthe clean and valuable utilization of coals werebriefly reviewed. Compared to the use of the tra-ditional solvents, the introduction of ILs shedslights on the utilization of coals and has showngreat potentials for real applications. Togetherwith the chances brought by ILs, some new issuesalso emerge and need to be considered in thesubsequent studies. To well promote the applica-tion of ILs in the clean and valuable utilization ofcoals, the following aspects may need to be paidmore attentions, and detailed studies should beconducted in the future researches (Fig. 3):(1) more kinds of ILs should be applied to studythe real effects of ILs and disclose the structure-function relationship of ILs; (2) the mechanismsof ILs playing their roles should be further ana-lyzed via more characterization techniques;(3) the general criterion of choosing ILs shouldgradually be drawn out; (4) new and efficientapproaches of recovering ILs still require to bedeveloped, and the quality of the recycled ILs andlong-term performances should be analyzed indetail; (5) new ILs with the properties of goodstability under high temperature, easy synthesisand recycling, and being obtained from greenmaterials should be developed, especially thosecould be prepared by natural and sustainablematerials; (6) the life evolution of the residualILs should be studied; (7) the posttreatment ofthe waste ILs should be considered; and (8) thewhole life cycle effects of ILs on the environmentshould be evaluated. There are series of funda-mental issues to be solved before the large-scale

10 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

applications of ILs, most of which concerning thecosts of ILs and the possibly negative effects onthe environment. By solving these problems ratio-nally, it is believed that the applications of ILs forclean and valuable utilization of coals have abright future.

Acknowledgments This entry was supported by theNational Natural Science Foundation of China(21606134, 21676149, and 21566029) and the Innovativeand Entrepreneurial Talents Grassland Talents Engineeringof Inner Mongolia.

References

1. Liu FJ, Wei XY, FanMH, Zong ZM (2016) Separationand structural characterization of the value-addedchemicals from mild degradation of lignites: a review.Appl Energ 170:415–436. https://doi.org/10.1016/j.apenergy.2016.02.131

2. Sha YF, Xiao ZH, Zhou HC, Yang KL, Song YM,Li N, He RX, Zhi KD, Liu QS (2017) Direct use ofhumic acidmixtures to construct efficient Zr-containingcatalysts for Meerwein–Ponndorf–Verley reactions.Green Chem 19(20):4829–4837. https://doi.org/10.1039/c7gc01925d

3. Sreedhar I, Nahar T, Venugopal A, Srinivas B (2017)Carbon capture by absorption – path covered andahead. Renew Sust Energ Rev 76:1080–1107. https://doi.org/10.1016/j.rser.2017.03.109

4. Ibrahim MH, Hayyan M, Hashim MA, HayyanA (2017) The role of ionic liquids in desulfurization offuels: a review. Renew Sust Energ Rev 76:1534–1549.https://doi.org/10.1016/j.rser.2016.11.194

5. Kunov-Kruse AJ, Thomassen PL, Riisager A,Mossin S, Fehrmann R (2016) Absorption and oxida-tion of nitrogen oxide in ionic liquids. Chem-EurJ 22(33):11745–11755. https://doi.org/10.1002/chem.201601166

6. Painter P, Pulati N, Cetiner R, Sobkowiak M,Mitchell G, Mathews J (2010) Dissolution and disper-sion of coal in ionic liquids. Energy Fuel24(3):1848–1853. https://doi.org/10.1021/ef9013955

7. Pulati N, Sobkowiak M, Mathews JP, Painter P (2012)Low-temperature treatment of Illinois no. 6 coal inionic liquids. Energy Fuel 26(6):3548–3552. https://doi.org/10.1021/ef3002923

8. Cummings J, Kundu S, Tremain P, Moghtaderi B,Atkin R, Shah K (2015) Investigations into physico-chemical changes in thermal toals during low-temperature ionic liquid treatment. Energy Fuel29:7080–7088. https://doi.org/10.1021/acs.energyfuels.5b01824

9. Cummings J, Tremain P, Shah K, Heldt E,Moghtaderi B, Atkin R, Kundu S, VuthaluruH (2017) Modification of lignites via low temperatureionic liquid treatment. Fuel Process Technol155:51–58. https://doi.org/10.1016/j.fuproc.2016.02.040

10. Lei ZP, Hu ZQ, Zhang H, Han LN, Shui HF, Ren SB,Wang ZC, Kang SG, Pan CX (2016) Pyrolysis oflignite following low temperature ionic liquid pre-treatment. Fuel 166:124–129. https://doi.org/10.1016/j.fuel.2015.10.059

Applications of Ionic Liquids in Clean and ValuableUtilization of Coal: From Aspects of Environment,Fig. 3 Potential perspectives in the future researches for

the in-depth applications of ILs during the clean and valu-able utilization of coals

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 11

11. YuWH, ZhangH, Lei ZP, Shui HF, Kang SG,Wang ZC,Ren SB, Pan CX (2019) Pretreatment of lignite byacidic bronsted ionic liquid [B(SO3H)mim]OTf forlignite pyrolysis. Fuel 236:861–869. https://doi.org/10.1016/j.fuel.2018.07.150

12. Lei ZP, Zhang K, Hu ZQ, Zhang H, Shui HF, Ren SB,Wang ZC, Kang SG, Pan CX (2016) Effect of ionicliquid 1-butyl-3-methyl-imidazolium dihydrogenphosphate pretreatment on pyrolysis of Shengli lig-nite. Fuel Process Technol 147:26–31. https://doi.org/10.1016/j.fuproc.2016.01.016

13. Cummings J, Shah K, Atkin R, Moghtaderi B (2015)Physicochemical interactions of ionic liquids withcoal: the viability of ionic liquids for pre-treatmentsin coal liquefaction. Fuel 143:244–252. https://doi.org/10.1016/j.fuel.2014.11.042

14. S-p Y, Deng L, Namkung H, Fan S, Kang T-J, KimH-T (2017) Coal structure change by ionic liquidpretreatment for enhancement of fixed-bed gasifica-tion with steam and CO2. Korean J Chem Eng35(2):445–455. https://doi.org/10.1007/s11814-017-0296-6

15. Lei ZP, Dong L, Kang SG, Huang YQ, Li ZK, Yan JC,Shui HF, Wang ZC, Ren SB, Pan CX (2019) Dissoci-ation behaviors of coal-related model compounds inionic liquids. Fuel 241:1019–1025. https://doi.org/10.1016/j.fuel.2018.12.117

16. ZhangWQ, Jiang SG,WangK,Wu ZY, ShaoH (2012)An experimental study of the effect of ionic liquids onthe low temperature oxidation of coal. Int J Min SciTechnol 22(5):687–691. https://doi.org/10.1016/j.ijmst.2012.08.016

17. ZhangWQ, Jiang SG,Wu ZY, ShaoH,WangK (2013)Effect of ionic liquids on low-temperature oxidation ofcoal. Int J Coal Prep Util 33(2):90–98. https://doi.org/10.1080/19392699.2013.763231

18. Zhang WQ, Jiang SG, Hardacre C, Goodrich P,Wang K, Wu ZY, Shao H (2014) Inhibitory effect ofPhosphonium-based ionic liquids on coal oxidation.Energy Fuel 28(7):4333–4341. https://doi.org/10.1021/ef402229z

19. Wang L-Y, Xu Y-L, Jiang S-G, Yu M-G, Chu T-X,Zhang W-Q, Wu Z-Y, Kou L-W (2012) Imidazoliumbased ionic liquids affecting functional groups andoxidation properties of bituminous coal. Safety Sci50(7):1528–1534. https://doi.org/10.1016/j.ssci.2012.03.006

20. Xiao Y, Lü H-F, Yi X, Deng J, Shu C-M(2018) Treating bituminous coal with ionic liquids toinhibit coal spontaneous combustion. J Therm AnalCalorim 135(5):2711–2721. https://doi.org/10.1007/s10973-018-7600-5

21. Deng J, Bai Z-J, Xiao Y, Shu C-M, Laiwang B (2019)Effects of imidazole ionic liquid on macroparametersand microstructure of bituminous coal during low-temperature oxidation. Fuel 246:160–168. https://doi.org/10.1016/j.fuel.2019.02.066

22. Cui F-S, Laiwang B, Shu C-M, Jiang J-C(2018) Inhibiting effect of imidazolium-based ionic

liquids on the spontaneous combustion characteristicsof lignite. Fuel 217:508–514. https://doi.org/10.1016/j.fuel.2017.12.092

23. Zhang WQ, Jiang SG, Wu ZY, Wang K, Shao H,Qin T, Xi X, Tian HB (2018) Influence ofimidazolium-based ionic liquids on coal oxidation.Fuel 217:529–535. https://doi.org/10.1016/j.fuel.2017.12.056

24. Xi X, Jiang SG, Zhang WQ, Wang K, Shao H, Wu ZY(2019) An experimental study on the effect of ionicliquids on the structure and wetting characteristics ofcoal. Fuel 244:176–183. https://doi.org/10.1016/j.fuel.2019.01.183

25. Zhang WQ, Jiang SG, Sun JF, Wu ZY, Qin T, XiX (2018) Wettability of coal by room temperatureionic liquids. Int J Coal Prep Util:1–10. https://doi.org/10.1080/19392699.2018.1488692

26. Lei ZP,Wu L, Zhang YQ, Shui HF,Wang ZC, Pan CX,Li HP, Ren SB, Kang SG (2012) Microwave-assistedextraction of Xianfeng lignite in 1-butyl-3-methyl-imidazolium chloride. Fuel 95:630–633. https://doi.org/10.1016/j.fuel.2011.12.006

27. Lei ZP, Wu L, Zhang YQ, Shui HF, Wang ZC, Ren SB(2013) Effect of noncovalent bonds on the successivesequential extraction of Xianfeng lignite. Fuel ProcessTechnol 111:118–122. https://doi.org/10.1016/j.fuproc.2013.02.004

28. Wang JL, Yao HW, Nie Y, Bai L, Zhang XP, Li JW(2012) Application of iron-containing magnetic ionicliquids in extraction process of coal direct liquefactionresidues. Ind Eng Chem Res 51(9):3776–3782. https://doi.org/10.1021/ie202940k

29. Lei ZP, Zhang YQ, Wu L, Shui HF, Wang ZC, Ren SB(2013) The dissolution of lignite in ionic liquids. RSCAdv 3(7):2385. https://doi.org/10.1039/c2ra23097f

30. Sharma DK, Dhawan H (2018) Separative refining ofcoals through solvolytic extraction under milder condi-tions: a review. Ind Eng Chem Res 57(25):8361–8380.https://doi.org/10.1021/acs.iecr.8b00345

31. Yu WH, Zhu P, Lei ZP, Shui HF, Kang SG, Wang ZC,Ren SB, Pan CX (2018) Study of pyrolysis behavior ofShenhua coal pretreated by ionic liquid 1-Ethyl-3-Methylimidazolium acetate. Int J Chem React Eng16(7). https://doi.org/10.1515/ijcre-2017-0244

32. Jin LJ, Qi Y, Chaffee AL (2018) Effect of temperatureon the solubility of Victorian brown coal in the ionicliquid DIMCARB. Fuel 216:752–759. https://doi.org/10.1016/j.fuel.2017.11.133

33. Qi Y, Verheyen TV, Tikkoo T, Vijayaraghavan R,MacFarlane DR, Chaffee AL (2015) High solubilityof Victorian brown coal in ‘distillable’ ionic liquidDIMCARB. Fuel 158:23–34. https://doi.org/10.1016/j.fuel.2015.04.060

34. Qi Y, Verheyen TV, Vijayaraghavan R,MacFarlane DR,Chaffee AL (2016) Ambient temperaturesolubilisation of brown coal in ammonium carbamateionic liquids. Fuel 166:106–115. https://doi.org/10.1016/j.fuel.2015.10.064

12 Applications of Ionic Liquids in Clean and Valuable Utilization of Coal

35. Sönmez Ö, Yıldız Ö, Çakır MÖ, Gözmen B, Giray ES(2018) Influence of the addition of various ionic liq-uids on coal extraction with NMP. Fuel 212:12–18.https://doi.org/10.1016/j.fuel.2017.10.017

36. Z-q H, S-f Z, Z-p L, H-f S, Z-cW, S-b R (2015) Study onthe thermal extraction of Xianfeng lignite in ionic liquid1-butyl-3-methyl-imidazolium trifluoromethanesulfonate.J Fuel Chem Technol 43(5):513–518. https://doi.org/10.1016/s1872-5813(15)30014-1

37. To TQ, Shah K, Tremain P, Simmons BA,Moghtaderi B, Atkin R (2017) Treatment of lignite

and thermal coal with low cost amino acid based ionicliquid-water mixtures. Fuel 202:296–306. https://doi.org/10.1016/j.fuel.2017.04.051

38. Wang XL, Nie Y, Zhang XP, Zhang SJ, Li JW(2012) Recovery of ionic liquids from dilute aqueoussolutions by electrodialysis. Desalination 285:205–212.https://doi.org/10.1016/j.desal.2011.10.003

39. Zhou JJ, Sui H, Jia ZD, Yang ZQ, He L, Li XG(2018) Recovery and purification of ionic liquidsfrom solutions: a review. RSC Adv 8:32832–32846.https://doi.org/10.1039/c8ra06384b

Applications of Ionic Liquids in Clean and Valuable Utilization of Coal 13