Effect of Ionic Liquid Surfactants on Coal Oxidation and Structure

The effects of six ionic liquids with surfactant property (1-hydroxyethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([HOEtMIm][NTf2]), 1-hydroxyethyl-3-methyl imidazolium tetrafluoroborate ([HOEtMIm][BF4]), 1-dodecyl-3- methyl imidazolium bromide ([C12MIm]Br), 1-tetradecyl-3- methyl imidazolium bromide ([C14MIm]Br), trioctyl methyl ammonium chloride ([N8,8,8,1])Cl, and tetraethyl ammonium chloride ([N2,2,2,2]Cl)) on the oxidation characteristics and functional groups of coal were studied by means of critical micelle concentration, surface tension, thermogravimetric analysis, temperature-programmed oxidation, and Fourier transform infrared spectroscopy (FTIR) measurements. The lower critical micelle concentration for the ionic liquids except the [N2,2,2,2]Cl suggests the favorable surface activity of these ionic liquids. The surface activities of [N8,8,8,1]Cl, [C14MIm]Br, [C12MIm]Br, and [HOEtMIm][NTf2] were high, while that of [N2,2,2,2]Cl was relatively lower. The thermal stabilities of [HOEtMIm][NTf2] and [HOEtMIm][BF4] were high, while those of [N8,8,8,1]Cl and [N2,2,2,2]Cl were lower. The oxidation activities of ionic liquid-mixed coals were weakened to different degrees except [N8,8,8,1]Cl-mixed coal, because of the poor thermal stability and decomposition of [N8,8,8,1]Cl accelerating the coal oxidation. The other five ionic liquids were suitable for inhibiting coal oxidation, particularly the [HOEtMIm][BF4] and [HOEtMIm][NTf2] with higher inhibition rate, longer inhibition time, and also better thermal stabilities. The activation energy results further confirmed such inhibition effect. The functional group results showed that treatment of ionic liquids on coal can change the contents of hydrogen bonds, aliphatic groups, and aromatic groups in coal. It was inferred that the [HOEtMIm][BF4], [HOEtMIm][NTf2], and [C14MIm]Br were more effectively to affect coal structure and decrease coal oxidation activity.


Introduction
e property of low temperature oxidation of coal can result in coal spontaneous combustion, which seriously threatens the safety and sustainable development of coal industry. At present, the study of inhibition agents to change the oxidation activity of coal becomes the hot spot of coal spontaneous combustion control, including the inorganic salts [1,2] and organic matters [3,4]. ese chemical inhibitors show favorable inhibition effects on low temperature oxidation of coal.
Ionic liquids (ILs), as the hot spots in the field of green chemistry, consist of bulky and asymmetric organic cations and organic or inorganic anions [5,6]. ILs have melting point below 100°C, negligible volatility, nonflammability, and excellent dissolving and swelling capacity to coal [7,8]. Such capacity can influence the coal structure and change the coal oxidation property. Wang and Zhang et al. found that imidazolium-based ILs can partially change the oxygencontaining and aliphatic functional groups in coal and affect the oxidation properties of the coal [9][10][11][12]. Zhang et al. concluded that the phosphonium-based ILs can affect coal oxidation activity and inhibit coal oxidation process [13].
ese results showed that ILs can affect coal microstructure as well as change coal oxidation activity. e interaction between coal and inhibition agents is solid-liquid surface contact so that the inhibition agents with surface activity can interact with coal surface better and promote the inhibition effect. e surface tension of ILs is normally 21-60 mN/m [14,15], which is lower than that of water (72 mN/m) so that the ILs with surface activity may promote the interaction of coal and ILs and better inhibit coal oxidation. Our previous research investigated the effect of some imidazolium-based and phosphonium-based ILs on coal wettability and microcosmic structure [16], indicating the wetting action of these ILs and the effect on functional groups of coal.
Herein, the authors further analyzed the effect of the typical ILs with surfactant property on coal oxidation and structure to provide more evidence for searching new materials which can significantly weaken the coal oxidation activity.

Sample Preparation.
e concentration of the six IL surfactant solutions was 20% in distilled water (w/v), which is the inhibition concentration usually used in coal spontaneous combustion suppression study. e coal sample was ground in a mortar, sieved to a particle size of 74 μm, and then vacuum-dried in an oven at 27°C for 48 h. e dried particulate coal (∼5 g) was vigorously mixed with the six IL surfactant solutions (∼3 ml) separately. e mixed coal was vacuum-dried in oven at 27°C until the weight of the mixture unchanged, which was named as IL-mixed coal (IL-mc).
Part of the IL-mixed coal samples were washed with distilled water and filtered to separate the coal until the filtrate was transparent and neutral. en, the washed coal was dried in a vacuum oven at 27°C until the weight of the coal samples unchanged, which was named as IL-treated coal (IL-tc).
In addition to the IL-mixed coals and IL-treated coals, a sample of the untreated particulate coal was washed only with distilled water to enable a comparison to be made with the IL-mixed coals and IL-treated coals; this sample was denoted as IL-untreated coal (IL-untc).

Experimental Procedures.
e critical micelle concentration (CMC) of the six ILs was measured by ultravioletvisible (UV-Vis) spectrophotometry ( ermo Biomate 3S) using a solvatochromic dye fluorescein. One drop of a 100 mL anhydrous ethanol solution containing 0.1 g of fluorescein was added in the cuvette for the experiments. e UV-Vis spectra were recorded between 200 and 600 nm with a spectral bandwidth of 1 nm. e surface tensions of the six IL solutions were tested by the ring tear-off method using a Force Tensiometer (K100, KRÜSS GmbH, Germany). e thermal stabilities of the six ILs were tested by the thermogravimetric analysis (TGA) using a thermogravimetric analyzer (Diamond TG/DTA 6300, PerkinElmer, UK) in dry air flow of 50 cm 3 /min at a heating rate of 5°C/ min over the temperature range 30-800°C. e oxidation properties of the IL-untc and six IL-mcs were measured using a temperature-programmed (TP) testing system [17] with 3 g coal samples in dry air flow of 20 cm 3 /min at a heating rate of 2°C/min over the temperature range 30-200°C. e gaseous products were analyzed by an indicator gas analysis system including a highprecision CO sensor, an O 2 sensor, and a signal processing module. e functional groups of the IL-untc and six IL-tcs were subjected to Fourier-transform infrared spectroscopy (FTIR) measurement. FTIR spectra were recorded between 3800 and 400 cm −1 and were accumulated for 32 scans at a resolution of 4 cm −1 on a FTIR spectroscope (Vertex 80v, Bruker, Germany).

CMC Results of the ILs.
e CMC is a key characteristic for the IL surfactants. e surface tension changes strongly with the concentration changing before the CMC, then remains relatively constant after the CMC. e determination of the ILs CMC was based on the changes of the wavelength of the absorption maximum (λ max ) [18][19][20], which is a semiquantitative method. Figure 1 shows the λ max of [C 12 MIm]Br at 0.1 mol/L (∼230 nm) and fluorescein (∼490 nm). e λ max changed suddenly before and after the CMC, such as the λ max of [HOEtMIm][NTf 2 ] changed from 217 nm at 0.0008 mol/L to 227 nm at 0.001 mol/L, and remained relatively unchanged with the concentration increasing ( Figure 2(a)). e results provided the CMC value of [HOEtMIm][NTf 2 ] was 0.001 mol/L. Similarly, the λ max belonged to fluorescein changed from 490 nm at 0.005 mol/L [C 12 MIm]Br to 500 nm at 0.008 mol/L [C 12 MIm]Br and remained unchanged with the increasing concentration ( Figure 2(b)). Such results gave the CMC value of [C 12 MIm] Br was 0.008 mol/L. Table 1 lists the CMC values of the six ILs surfactants. e CMCs were found to be from 1 to 10 mM for the ILs except the [N 2,2,2,2 ]Cl. is suggests the favorable surface activity of these ILs. Further, the concentration 20%, which was the inhibitor concentration usually used in coal spontaneous combustion suppression, was higher than the six CMCs so that the IL solutions (20%) used below can play both inhibitor and surfactant roles.

Surface Tension Results of the ILs.
e surface tension results of the ILs solutions are shown in Table 2.
Generally, the critical surface tension of coal is 45 mN/m so that when the surface tension of ILs is lower than 45 mN/ m, the ILs can wet coal surface [21]. Table 1 shows that the surface tension of the distilled water was 72.55 mN/m, while the surface tensions of the six IL solutions were lower than that of distilled water, indicating the high surface activity of    Figure 3 shows the TG and DTG results of the six ILs. According to the results, the ILs with better thermal stability can be chosen and further used for inhibition agent.

ermal Stability Results of the ILs.
From Figure 3( Figure 4 showed the gas product CO of the six IL-mixed coals by TP oxidation measurement. e CO yield of the IL-mixed coals was less than that of IL-untreated coal after 110°C except that of [N 8,8 [22,23], where R is the inhibition rate (%), A is the CO yield of the IL-untreated coal (ppm), and B is the CO yield of the IL-mixed coals (ppm). e results are shown in Figure 5. e inhibition rate results were mainly analyzed between 100 and 200°C because the significant change of CO product occurred from 100°C. From Figure 3, the inhibition rate for different ILs was different and the inhibition rate reduced with increasing temperature except that of [N 8,8

Activation Energy Results of the IL-Mixed Coals.
According to the Arrhenius formula, the activation energy can be calculated by the equation ln(f(c)/T 2 ) � −(E/RT) + ln(AR/βE) [24,25] when the reaction series is set as 1, where E is the apparent activation energy (J/mol), R is the gas constant (R � 8.314 J/(K·mol)), T is the coal sample temperature (K), f(c) � ln(c 0 /c) (c 0 is the initial oxygen concentration, 21%; c is the oxygen concentration at temperature T (mol/cm 3 )), A is the former factor, and β is the heating rate (β � dT/dt � 274.15 K/min). e relationship According to the linear relationship, the whole temperature range can be divided into two stages for fitting the data points, which is shown in Figure 6. According to the fitting results, the apparent activation energy values of ILuntreated coal and six IL-mixed coals can be obtained, which are shown in Table 3. e activation energy represents the difficult degree of the oxidation reaction, where the greater the activation energy is, the more difficult the reaction is. e activation energy at the first temperature stage was generally small, which is because only easy reactions with small activation energies occurred [10,26]. e activation energies of ILmixed coals were slightly larger than those of IL-untreated coal, indicating the reaction for IL-mixed coals happened more difficultly. e maximum activation energy was for [HOEtMIm][BF 4 ]-mixed coal. All the activation energies at the second temperature stage were larger than those at the first temperature stage, indicating the reactions with larger activation energies occurring. e activation energies of the IL-mixed coals were also larger than those of the IL-untreated coal in the second temperature stage, demonstrating the more          Figure 7 shows the FTIR spectra of IL-untreated coal and ILtreated coals at room temperature at the Common Scale.

Functional Group Results of the IL-Treated Coals.
From Figure 7, the peaks of the main functional groups of all coal samples appeared, indicating that the main structure of coal was not changed. However, the absorption strength of some groups changed, such as the hydrogen bond interactions (3600-3100 cm −1 ), the aliphatic C-H stretching (3000-2800 cm −1 ), the ether bonds (1300-1000 cm −1 ), and the aromatic C-H bending (900-650 cm −1 ) [27]. e aromatic C�C stretching at 1600 cm −1 was nearly unchanged, indicating the polyaromatic system of coal was unaffected. e hydrogen bonds associated by hydroxyl reduced remarkably in [N 8,8 In order to quantitatively analyze these changes, the FTIR spectra were fitted and the peak area ratios of A CH 2 /A CH 3 and A ar /A al were chosen to characterize the change degree of coal structure. e two ratios were calculated by the equations A CH 2 /A CH 3 � A (2853cm −1 ) /A (2918cm −1 ) and A ar /A al � A (900−650cm −1 ) /A (3000−2800cm −1 ) [27]. e A CH 2 /A CH 3 characterizes the length and degree of the aliphatic side chains and the compact of coal structure. e lower A CH 2 /A CH 3 represents shorter aliphatic side chains in coal and more compact structure of coal. e A ar /A al represents the aromaticity and reactivity of coal. e higher A ar /A al represents higher aromatic degree and lower oxidation activity of coal [27,28]. Figure 8 presents the fitting spectra of IL-untreated coal at 3000-2800 cm −1 . Table 4 shows the results of the A CH 2 /A CH 3 and A ar /A al .
From Table 4, all the ratios of A CH 2 /A CH 3 of IL-treated coals were lower than those of IL-untreated coals in different degrees, indicating the shorter aliphatic side chains and more compact structure of the IL-treated coals. e lowest ratios of A CH 2 /A CH 3

Conclusions
e effects of six IL surfactants on the oxidation characteristics and functional groups of coal were studied by means of critical micelle concentration, surface tension, thermogravimetric analysis, temperature-programmed oxidation, and FTIR measurements.
Firstly, the lower critical micelle concentration for the ILs except the [N 2,2,2,2 ]Cl suggests the favorable surface activity of these ILs so that the ILs used can play both inhibitor and surfactant roles. e surface activity of the six IL solutions from strong to weak was [N 8,8 Figure 8: Fitting spectra of IL-untreated coal at 3000∼2800 cm −1 (the five fitting spectra at 2960, 2928, 2896, 2871, and 2854 cm −1 from left to right belong to methyl asymmetrical stretching vibration, methylene asymmetrical stretching vibration, methylene stretching vibration, methyl symmetrical stretching vibration, methylene symmetrical stretching vibration, respectively).   Journal of Analytical Methods in Chemistry