Investigations into Physicochemical Changes in Thermal Coals during

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Investigations into Physico-chemical Changes in Thermal Coals during Low Temperature Ionic Liquid Treatment Joshua Cummings, Sazal Kundu, Priscilla Tremain, Behdad Moghtaderi, Rob Atkin, and Kalpit V Shah Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01824 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015

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Investigations into Physico-chemical Changes in Thermal Coals during Low Temperature Ionic Liquid Treatment Joshua Cummingsa, Sazal Kundub, Priscilla Tremainb, Behdad Moghtaderib, Rob Atkina, and Kalpit Shahb*

a

Discipline of Chemistry, The University of Newcastle, Callaghan, NSW 2308, Australia.

b

Discipline of Chemical Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308, Australia. Keywords: Coal, Ionic Liquids, Solvent treatment, Coal Conversion, Coal treatment

Abstract Two Australian thermal coals were treated with four different ionic liquids (ILs) at temperatures as low as 100 °C. The ILs used were 1-butylpyridinium chloride [Bpyd][Cl], 1ethyl-3-methylimidazolium

dicyanamide

[Emim][DCM],

1-butyl-3-methylimidazolium

chloride [Bmim][Cl] and 1-butyl-3-methylimidazolium tricyanomethanide [Bmim][TCM]. Visual comparisons were made between the raw and IL treated coals via optical microscopy. Changes in thermal behaviour of these treated coals were compared against raw coals via pyrolysis experiments in a thermogravimetric analyser (TGA). Changes in functional group composition in the treated coals were probed via Fourier Transform Infrared (FTIR) spectroscopy. The recovered ILs were also analysed via FTIR and Nuclear Magnetic Resonance (NMR) spectroscopy in order to observe any changes after recovery. Low temperature IL treatment of each of the coals resulted in fragmentation and fracturing, reducing the average particle size. An increase in mass loss in the treated coals was also observed when compared to each raw coal, indicating an increase in lower molecular weight fragments after treatment. This was corroborated by a large increase in aliphatic

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hydrocarbons being observed in the treated coals, along with a decrease in oxygenated functional groups and mineral matter in one coal. The recovered ILs were shown to be unchanged by this treatment process, indicating their potential recyclability. These results indicate the potential for ionic liquids to be implemented as solvent treatments for coal conversion processes.

1.0 Introduction Coal is a fuel source of great importance in Australia, both as an export commodity and a source of energy. The structure of coal is inherently complex.1,

2

A

comprehensive understanding of this complex structure would allow coal to be utilised optimally, however, the heterogeneous structure of coal is still not well understood, and thus is still widely studied.3,

4

Interactions between coals and a wide range of

solvents have been studied in regards to coal conversion applications.5-7 Typical organic solvents such as, n-methyl-2-pyrrolidone (NMP), pyridine and carbon disulfide (CS2) have been shown to partially dissolve, swell and fragment coals.8 It has been suggested that this is due to the ability of these solvents to disrupt certain types of secondary interactions, such as hydrogen bonds, aromatic-aromatic interactions and ionic interactions, which are known to contribute significantly to the structure of coal.9-11 Ionic liquids are a class of solvents composed entirely of ions and have a melting point below 100 °C.12 Ionic liquids possess a range of favourable properties, including negligible vapor pressure, high thermal stability and most importantly high tenability.13 ILs have been studied as applied solvents extensively in recent times.14 Previous investigations have shown their applicability for the solvent extraction and 2 ACS Paragon Plus Environment

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deconstruction of cellulose and lignocellulosic biomass.15-18 It was proposed in these studies that ILs disrupt hydrogen bonding between biomass units which facilitates dissolution. In comparison, studies on coal and IL interactions are still quite preliminary.19-21 Initial investigations carried out by Painter et al.19 observed the ability of various ionic liquids to disperse and dissolve Illinois No. 6 coal. Lei et al.22 carried out

solvent

extractions

of

lignite

in

1-butyl-3-methylimidazolium

chloride

([Bmim][Cl]). They found that the efficiency of extraction was dependent on extraction temperature and the ratio of IL to coal, with higher temperatures and higher IL/coal ratios resulting in an increase in extract yield recovered. Shah et al.20 showed that the interactions occurring between coals and ionic liquids appear to be maceral specific. It was observed that vitrinite rich coal became swollen during the IL treatment, whilst inertinite rich coal was severely fragmented.

Moreover, the

dissolution of vitrinite rich coal in IL was found to be 30% greater than intertinite rich coal. In our previous investigation, it was shown that certain ILs, specifically [Bmim][Cl], were able to interact with thermal coals favourably for coal conversion processes. Extensive fragmentation was observed after treatment, along with an observed increase in mass loss during pyrolysis, indicating the presence of lower molecular weight fragments after IL treatment.23 The aim of the present investigation is to study the interactions occurring between two thermal coals with four different ILs with a specific aim to evaluate the recyclability of ILs as well as the physic-chemical alterations in coal.


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2.0 Materials and Methods Two Australian sub-bituminous coals, coal A and coal B, were used in this investigation. The coal samples were milled and sieved to a particle size range of 150 - 212 µm. The proximate and ultimate analyses of coal A and coal B are presented in Table 1. Four ILs used in this investigation: 1-butylpyridinium chloride [Bpyd][Cl], 1-ethyl-3methylimidazolium dicyanamide [Emim][DCM], 1-butyl-3-methylimidazolium chloride [Bmim][Cl] and 1-butyl-3-bethylimidazolium tricyanomethanide [Bmim][TCM], with the chemical structure of each presented in Table 2. All ILs were purchased from io-li-tec at 99% purity and were selected based upon previous work carried out on biomass/IL pretreatments,16, 18, 24, 25 and preliminary work by Painter et al.19, 26 The preparation and recovery process of ILs and coal are detailed in Figure 1. Samples were made up in a 1:5 mass ratio of coal to ionic liquid, respectively. The IL and coal mixtures were magnetically stirred in sealed jars that were placed in an oil bath at 100 °C for 3 hours. After 3 hours, samples were removed from the oil bath, and the coal-IL mixture placed in a centrifuge tube. The jar, in which the coal and IL were stirred, was rinsed with distilled water to ensure maximum coal recovery. The resultant coal/IL/water mixture was then sealed in the centrifuge tube and centrifuged for 10 min at 550 rpm, the tube was then removed carefully, and the supernatant was then removed from the tube with a pipette. Distilled water was then added to the coal and centrifuged again, this was done 5 times. The water/IL mixture was placed in an oven at 80 °C to evaporate water and recover the IL. The coal samples were placed in an oven at low temperatures (60-80 °C) to dry. The dried coal was then subjected to multiple distilled water washes to ensure the removal of any IL adsorbed to the coal surface, or within its pores. The coal sample was added to a beaker with 20 ml of distilled water, this mixture was then stirred with a magnetic stirrer for ~10 min. This water/coal mixture was then separated via the use of filter paper. The distilled 4 ACS Paragon Plus Environment

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water was recovered after each wash and its electrical conductivity (EC) measured in order to observe if any residual IL was present on the recovered coal. The washing and EC measurement process was repeated until the recovered water had a conductivity close to that of distilled water (i.e. ≤ 10 µS/m). The coal was then dried at 100 °C before being characterised. In order to corroborate this, microanalysis measurements were carried out on an Elemental Analyser, Model PE2400 CHNS/O (PerkinElmer, Shelton, CT, USA) with a PC based data system, PE Datamanager 2400 for WindowsTM and a PerkinElmer AD-6 Ultra Micro Balance. This was implemented in order to compare the treated and raw coal’s nitrogen content, the nitrogen contents of both the raw and treated sample were found to be similar. 2.1 Optical Microscopy Optical micrographs at 20x magnification were obtained on a Zeiss Optical Microscope Axioskope 40 at room temperature. Each sample was analysed by smearing a small amount of the IL/coal mixture in between a glass slide and a coverslip. 2.2 Thermogravimetric (TG) Analysis A thermogravimetric analyzer (Model Q50 V20.10 Build 36, TA Instruments, U.K.) was used to observe the changes in the thermal behaviour of the coals after IL treatment. Each treated coal sample (10 - 15 mg) was placed in a platinum pan in a nitrogen atmosphere with a flow rate of 20 mL/min. Pyrolysis was then carried out from 0 to 650 °C with a temperature ramp rate of 10 °C/min. Each raw coal sample was also analysed under the same conditions for comparison. Multiple runs were performed to ensure the repeatability of results. 2.3 Fourier Transform Infrared (FTIR) Spectroscopic Analysis FTIR analysis was carried out on a PerkinElmer Spectrum Two spectrometer, with a universal ATR sampling accessory and a scan range of 400 – 4000 cm-1. The raw and IL


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treated coals were both analysed. The four ILs used in this investigation were also analysed before and after the treatment process via the same method. 2.4 Nuclear Magnetic Resonance (NMR) Spectroscopic Analysis 13

C NMR analysis was carried out on a on a Bruker AVANCE III 400MHz with a sample

changer on both the pure, and recovered IL samples using deuterated dimethyl sulfoxide (DMSO-d6) as the solvent.

3.0 Results and Discussion The treated coal was recovered after the IL treatment process, the average recovery percentages for each of these treated coal samples can be seen in Table 3. The highest recovery was observed for the [Bmim][TCM] treated coals, in both coals A and B, and the lowest recovery was observed for the [Bmim][Cl] treated coals. The following sections present the characterization of the recovered coals and ILs.

3.1 Effect of ILs on Particle Size Optical microscopy was used to observe changes in particle size after the treatment process. Optical micrographs of coal A and B treated by [Bmim][Cl] and treated by water, for comparison, at 20x magnification are presented in Figure 2. The micrograph of coal A in water (Fig. 2a) show particles that are primarily 150 - 200 µm in size, with a small amount of fine particles present. In comparison with the micrographs of coal A in [Bmim][Cl] (Fig. 2b), an immediate difference can be observed. There is a far higher amount of finer particles suspended in the IL. The treated coal particles appear to be severely fragmented and almost fluidized, with the average particle size appearing to be greatly decreased. The majority of the coal A particles appear to be roughly 10 µm in size after the IL treatment process. These results are similar to 6 ACS Paragon Plus Environment

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the observations made by Painter et al.,19, 21 where [Bmim][Cl] was observed to fracture and fragment the bituminous, Illinois No. 6 coal. Micrographs of coal B (Fig. 2c & d) also showed particle size changes after treatment with [Bmim][Cl] at low temperatures, although to a lesser degree than was observed for coal A. The particles that were mixed with water appear to have an average size of 100 - 150 µm, with a small amount of fine particles being present. Similarly to coal A, the micrographs of coal B in [Bmim][Cl] showed an increase in the amount of fine particles present with a significant amount of the treated particles appearing to be roughly 50 µm in size. The observations of each IL with coals A and B can be seen in Table 4. Agglomeration of coal particles was observed in coal A, with it occurring in the [Bpyd][Cl], [Emim][DCM] and [Bmim][Cl] treated coals. For coal B, agglomeration was not observed in any of the treated samples, and the degree of fragmentation observed was far lesser than that observed in coal A. This may be due to higher oxygen content observed in coal A when compared to coal B. [Bmim][TCM] appeared to be the least effective in altering the morphological properties of both coals B and A, and [Bmim][Cl] appeared to be the most effective. No swelling was observed in any of the treated coal samples, which illustrates that the coal-IL interactions are as much coal specific as they are IL specific. 3.2 Effect of ILs on Coal Thermochemical Behaviour Pyrolysis of each of the raw and treated coal samples was conducted via TGA. Figure 3 shows the TG profiles for coal B and its IL treated counterparts. Raw coal B, over the 100 650 °C range, lost roughly 25% of its mass during pyrolysis. The majority of this mass loss occurred at 400 °C. This is the temperature at which the coal’s macromolecular structure begins to decompose, resulting in the large mass loss. The IL treated coals exhibited a different mass loss trend during pyrolysis. The [Bpyd][Cl], [Bmim][TCM] and [Bmim][Cl] treated coals lost roughly 30% of their mass over the same temperature range. This mass loss 7 ACS Paragon Plus Environment

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also begins at lower temperatures when compared to the raw coal, with mass losses beginning at 300 °C. Similarly to the raw coal, the majority of the mass loss occurs at the same temperature of 400 °C. A larger mass loss was observed for the [Emim][DCM] treated coal; roughly 34 % over the 100 - 650 °C range. This mass loss occurred at lower temperatures, beginning at ~ 250 °C. However, the majority of the mass loss was at the same decomposition temperature of 400 °C. The larger mass losses in the treated coals show that the ILs were able to alter the thermal properties of coal B after being mixed for only 3 hr at 100 °C. This indicates that during the low temperature treatment, the ILs may be altering the cross-linked macrostructure of the coal, breaking it apart, as was observed in the optical micrographs. Hence, the fragmented coal is then able to decompose at lower temperatures, resulting in the higher mass losses observed. Figure 4 shows the TG profiles for coal A and its IL treated counterparts. Raw coal A, over the 100 - 650 °C range, lost roughly 16% of its mass during pyrolysis. This mass loss occurred after 400 °C. Similar to coal B, the IL treated coals exhibit a different trend in mass loss during pyrolysis. The IL treated coals lost more mass when compared to the raw coal, and this mass loss began at lower temperatures. The [Emim][DCM], [Bmim][TCM], [Bpyd][Cl] and [Bmim][Cl] treated coals lost between 21 - 29% of their mass over the same temperature range during pyrolysis. The IL treated coals began to lose mass at lower temperatures of 250 - 300 °C; this was a decrease of 150 °C when compared to the profile given by the raw coal. This large decrease in temperature for mass loss indicates the presence of lower molecular weight fragments. This further suggests that coal A’s macromolecular structure was broken apart by the ILs during the low temperature treatment.


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The TG profiles for each of the IL treated coals demonstrate that the water washing procedure is effective in removing residual IL from the coal surface, as no significant mass losses were observed at 250 °C, the temperature at which these ILs begin to thermally decompose. In order to confirm this fact, the nitrogen content of both raw coal A, and coal treated with [Bmim][Cl], was compared. The [Bmim][Cl] treated coal had a nitrogen content of 1.4%, compared with a nitrogen content of 1.1% for the raw coal. This further illustrates that the washing step is effective in removing and recovering residual IL. 3.3 Effect of ILs on Coal Composition FTIR Spectrometry was used to probe compositional changes in the coals after IL treatment at low temperatures. Figure 5 shows the FTIR spectra of coal B and its IL treated counterparts. In the raw coal spectra, the peak observed at 3100 – 3400 cm-1 was assigned to OH stretching, which was most likely caused by residual moisture.27 The peak at 2925 cm-1 was assigned to aliphatic C-H stretching and the peak at 1600 cm-1 was assigned to aromatic C=C groups.27 These two peaks are present at approximately a 1:1 ratio in the raw coal, with the C=C peak appearing slightly more intense. In the IL treated coals, this trend was not observed. Each of the IL treated coals in Figure 5 had a far more intense aliphatic C-H peak, indicating that the coals had become more aliphatic after IL treatment. The 2900/1600 cm-1 peak ratio increased to 1.5:1. This corroborates the trend established previously; the macromolecular network of coal B appears to have been broken apart by the treatment process, resulting in an increase in lower molecular weight volatiles. Peak shifting in the 2900 cm-1 region for the IL treated coals was also observed. The peaks present in the raw coal were at 2923 cm-1 and 2875 cm-1, whereas in the IL treated coals they appeared at 2981 cm-1 and 2890 cm-1. This peak shifting may be caused by a reduction in aromatic rings present in the treated coals. When the IL interacts with the coal structure, it


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appears to break it apart resulting in severe fragmentation and fracturing, this reduction in aromatic carbons bonded to the aliphatic carbons may explain this slight shifting present in the 2900 cm-1 region. This peak shifting may also be due to a reduced OH content, as there appears to be a significant amount of water present in the raw coal spectra. Since the broad OH peak appears to overlap with the aliphatic peak in the raw coal, it may be possible that when this OH peak is reduced, it causes the aliphatic peak to shift.

Figure 6 shows the FTIR spectra of coal A and its IL treated counterparts. Unlike coal B, raw coal A did not have any peak present at 2900 cm-1. An aromatic C=C peak was observed at 1600 cm-1, along with an intense peak at 1010 cm-1, which was assigned to Si-O linkages, while an intense set of peaks at 3620 - 3690 cm-1, were assigned to OH stretching bands. The peaks at 3620 - 3690 cm-1 and 1010 cm-1 were identified as kaolinite (Al2Si2O5(OH)4) peaks.28 This mineral content in coal A is further observed when referring to its high ash content in the proximate analysis present in Table 1. In the IL treated coals, a change in the aliphatic C-H stretching peak at 2900 cm-1 was immediately apparent. The IL treated coals appeared to have an intense aliphatic present at 2900 cm-1, indicating that the aliphatic carbon content of the coal had been greatly increased after IL treatment. This again appeared to be indicative of the cross-linked, macromolecular network of coal A being broken apart, resulting in an increase in lower molecular weight volatiles. Along with this, there appeared to be a decrease in the two kaolinite peaks after treatment, specifically in the [Bmim][TCM] and [Bmim][Cl] treated coals. This may indicate that these two ILs were able to disrupt the Si-O and Al-OH interactions contributing to kaolinite’s structure, possibly due to their charge density or cation exchange ability. Altering the mineral matter of certain coals is a key area of recent coal research itself.29,

30

The

removal of these oxygenated mineral peaks is advantageous as mineral matter and certain oxygen functional groups can be a major hindrance for coal conversion and utilization 10 ACS Paragon Plus Environment

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processes.5,

31, 32

The exception to these trends was the [Emim][DCM] treated coal, which

appeared to have a less significant increase in aliphatic content at 2900 cm-1 and an increase in the 1600 cm-1 aromatic peak. 3.4 Recoverability of ILs after Coal Treatment The recoveries of the ILs varied between runs, the average recovery percent for each IL after the treatment process can be seen in Table 5. The average recovery percent for the ILs was in between 77 - 86% for all of the ILs used in this investigation. The recovered ILs were characterised using FTIR to probe any structural changes that result from the treatment process, or if any coal extract was present in the IL after recovery. Figure 7 shows [Bpyd][Cl], [Emim][DCM], [Bmim][Cl] and [Bmim][TCM] before and after treating coals A and B. Figure 7(a) shows the FTIR of [Bmim][Cl] after treating each coal. There were few changes between the pure and the recovered [Bmim][Cl]. The main difference was the appearance of the peak at 3400 - 3500 cm-1. This was almost certainly caused by residual water, or water soluble compounds from the coal in the recovered ILs, as [Bmim][Cl] is known to be hygroscopic. This is supported by the increase in the peak at 1650 cm-1, which was also attributed to water. Figure 7(b, c and d) reveal similar trends for [Bpyd][Cl], [Emim][DCM] and [Bmim][TCM], with little difference between the recovered and pure IL. These ILs are also hygroscopic, and water is indicated by the peaks at 3400 - 3500 cm-1 and 1650 cm-1. 13

C NMR analysis was also performed on the recovered ILs as presented in Figure 8.

13

C

NMR analysis showed the recovered IL to be the same as the pure IL, and no impurities were observed in the recovered samples. The lack of change in the IL after the treatment process indicates that the IL is not denatured during this process. The absence of extracted material could be due to only a small


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concentration of extract being present in the IL, although the same IL was recovered and reused four times in the treatment process, yielding the same results in coal alteration. The FTIR analysis of these reused samples still yielded the same trend (see Fig. 7).

3.5 Mechanism for IL-coal interactions It is posited that the ILs are able to break apart the macrostructure of these coals by disrupting secondary interactions occurring in these thermal coals, particularly hydrogen bonding. ILs are known to be able to disrupt hydrogen bonds occurring in analogous systems such as cellulosic and lignocellulosic biomass.33-35 It is known that the cross linked coal network contains clusters which are extensively hydrogen bonded to one another.36 When this hydrogen bonding is disrupted, it places a high amount of stress on the cross-linked network, resulting in swelling and eventually fracturing and fragmentation. A variety of groups form hydrogen bonds in coal.37 Lower rank coals have higher amounts of carboxyl groups, whereas higher rank coals have more phenolic hydroxyl groups and ether linkages.38 It is postulated that when the IL is mixed with the coal at low temperatures, it is able to interact with the hydrogen bonding network in the coals, causing its macrostructure to depolymerise. This mechanism is quite similar to those put forward for the dissolution of cellulose in ILs,33, 35 as many of the same hydrogen bonding groups present in cellulose are also present in coal. It is also assumed that other factors such as steric effects also affect the ability of the IL to interact with the coal structure. Additional investigations need to be carried out in order to further confirm this mechanism. This disruption of secondary interactions contributing to the network in coal can explain the differing trends observed between coal A and B in the characterization techniques used in this study. In the micrographs, coal A was observed to fracture and fragment to a far higher degree when treated with [Bmim][Cl], this may be attributed to its higher oxygen and mineral 12 ACS Paragon Plus Environment

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matter content, as ILs are able to interact via hydrogen bonding and electrostatically. A similar trend was observed with the pyrolysis of both coals, with the IL treated coal A samples appearing to lose up to 13% more mass than the raw coal, compared to the 5% present between the IL treated and raw coal B samples. This higher mass loss can also be related to the higher oxygen content of coal A as hydrogen bonding groups present in the coal structure may cross-link during the early stages of pyrolysis, these cross-linking reactions then suppress the formation of volatiles during pyrolysis.

37

Hence, disruption of hydrogen

bonding by IL treatment would reduce cross-linking reactions and enable volatile formation during the early stages of pyrolysis to occur. 3.6 Applications for ILs in Coal Conversion Processes The ILs used in this investigation have been shown to alter the chemical and physical properties of coals B and A at low temperatures. FTIR analysis indicated that the ILs were able to break apart the cross-linked macrostructure of coal, altering the amounts of oxygenated functional groups and the mineral matter content. This may be beneficial in a variety of coal thermochemical conversion processes, such as coking, pyrolysis or gasification, where the suitable coals are required to have low amounts of oxygen and mineral matter

39-41

. The higher amounts of lower molecular weight volatiles observed after

IL treatment in TGA experiments may also be beneficial. Primarily due to the greater ease of handling these species in comparison to high molecular weight volatiles and tars. Optical microscopy showed that all of the ILs used were able to reduce the particle size of the coals used to differing degrees, with [Bmim][Cl] performing the best; reducing the particle size of the coals used by as much as 10 times. This may be beneficial for coal conversion processes such as liquefaction, as it allows for a higher surface area to be available for catalyst impregnation and solvent interactions to occur.7, 42, 43 Finally the recoverability of ILs after coal treatment, evidenced by FTIR and NMR analysis, suggests that the ILs may be reused in 13 ACS Paragon Plus Environment

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the treatment process. This fact is essential in ensuring the economic viability of otherwise expensive solvents like ILs.

4.0 Conclusions Four ILs were investigated for solvent pretreatment in coal conversion processes. Optical microscopy indicated ILs were able to fracture and fragment the two thermal coals used in this investigation to different degrees, with [Bmim][Cl] shown to reduce particle size by as much as 10 times. Greater mass loss during the early stages of pyrolysis for IL treated coal in comparison to the raw coal indicated the presence of lower molecular weight volatiles in the IL treated coals. This compositional change was confirmed via FTIR with an increase in aliphatic hydrocarbons observed for both coals. FTIR also revealed IL treated Coal A lost oxygenated functional groups and altered the mineral matter content. Combined, the morphological and compositional changes to the two thermal coals after IL treatment indicate that the ILs used are able to disrupt the macromolecular network of Coal A and Coal B. Moreover, it was posited that hydrogen bonding ILs disrupts hydrogen bonding in each of the coals which breaks apart the coal macrostructure. This liberates previously trapped lower molecular weight volatiles. FTIR and NMR analysis revealed that the ILs were recycled ILs were largely unaffected by coal treatment process. Ultimately the morphological and compositional changes to the two thermal coals and the recoverable nature of ILs offer potential for IL pretreatments process to be integrated with a variety of coal thermochemical conversion processes.


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Acknowledgements: The authors wish to acknowledge the financial support provided by the University of Newcastle Australia for the work presented in this paper. The authors would also like to acknowledge Dr Christopher McRae from the Department of Chemistry and Biomolecular Sciences at Macquarie University for carrying out the microanalysis of the raw and treated coal samples. In addition, the authors appreciate the assistance provided by technical staff members Mr Neil Gardner, Mr Tejas Patel and Dr Ron Roberts from the University of Newcastle in the laboratory.


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Energy & Fuels

List of Figures

Figure 1. Flow diagram of the IL treatment process. Figure 2. Micrographs of coal A treated with water (a) and and [Bmim][Cl] at 20x magnification and coal B treated with water (c) and [Bmim][Cl] (d) 20x magnification. Figure 3. TG profiles of coal B and each of its IL treated counterparts. Figure 4. TG profiles of coal A and each of its IL treated counterparts. Figure 5. FTIR spectra of raw coal B, and coal B treated by [Bpyd][Cl], [Emim][DCM], [Bmim][Cl] and [Bmim][TCM]. Figure 6. FTIR spectra of raw coal A, and coal A treated by [Bpyd][Cl], [Emim][DCM], [Bmim][Cl] and [Bmim][TCM]. Figure 7. FTIR spectra of (a) [Bmim][Cl], (b) [Bpyd][Cl] (c) [Emim][DCM], and (d) [Bmim][TCM] before and after the treatment process with each coal. Figure 8. 13C NMR spectra of (a) pure [Bmim][Cl]; (b) [Bmim][Cl] after treating coal B; and (c) [Bmim][Cl] after treating coal A; along with (d) the assigned structure of [Bmim][Cl].


Energy & Fuels

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Figure 1.


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Figure 2.


Energy & Fuels

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Figure 3.


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Figure 4.


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Figure 5.


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Energy & Fuels

Figure 6.


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Figure 7.


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Energy & Fuels

Figure 8. 3

7 5

4

1

2

8

(a)

6

3

7 5

4

2 1

(b)

8

6

(c)

6

7 5

8 3

4

2

1

DMSO-d6 (d)


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Page 30 of 35

List of Tables Table 1. Proximate and ultimate analyses of the coals used in this study. Table 2. Names, abbreviated names, physical states and molecular structures of the ILs used in this study. Table 3. Average recovery percentages of coals A and B after the treatment process. Table 4. Visual observations of each coal treated with each IL. Table 5. Average recovery percentages of each IL after the treatment process.


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Energy & Fuels

Table 1. Coal

Proximate analysis

a

Ultimate analysis

b c

M (%)

A (%)

VM (%)

FC (%)

C (%)

H (%)

N (%)

S (%)

O (%)

A

3.9

24.5

27.1

48.4

73.8

4.3

1.1

0.3

20.5

B

3.7

8.9

32.7

58.4

77.2

5.2

2

0.7

15

a

dry basis, M: Moisture content; A: Ash content; VM: Volatile Matter; FC: Fixed Carbon dry ash free basis c O calculated by difference b


Energy & Fuels

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Table 2. Name

Abbreviation

1-Butylpyridinium Chloride

[Bpyd][Cl]

1-Ethyl-3Methylimidazolium Dicyanamide

Liquid at Room Temperature

Cation

Anion

No

Butylpyridini um

Chloride

[Emim][DCM]

Yes

1-Ethyl-3Methylimida zolium

Dicyanamide

1-Butyl-3Methylimidazolium Chloride

[Bmim][Cl]

No

1-Butyl-3Methylimida zolium

Chloride

1-Butyl-3Methylimidazolium Tricyanomethanide

[Bmim][TCM]

Yes

1-Butyl-3Methylimida zolium

Structure

Tricyanomethanid e


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Table 3.

Ionic Liquid Treatment [Bpyd][Cl] [Emim][DCM] [Bmim][Cl] [Bmim][TCM]

Recovery (%) Coal A Coal B 73 81 79 83 70 76 82 84


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Table 4. Coal

A

B

Solvent

Fragmentation

Agglomeration

Swelling

Water

✕

✕

✕

[Bpyd][Cl]

✓

✓

✕

[Emim][DCM]

✓

✓

✕

[Bmim][Cl]

✓

✓

✕

[Bmim][TCM]

✓

✕

✕

Water

✕

✕

✕

[Bpyd][Cl]

✓

✕

✕

[Emim][DCM]

✓

✕

✕

[Bmim][Cl]

✓

✕

✕

[Bmim][TCM]

✓

✕

✕

✕ = No; ✓ = Yes


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Energy & Fuels

Table 5.

Ionic Liquid [Bpyd][Cl] [Emim][DCM] [Bmim][Cl] [Bmim][TCM]

Recovery (%) 83 77 80 86


Investigations into Physicochemical Changes in Thermal Coals during

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