Inhibitory Effect of Phosphonium-Based Ionic Liquids on Coal

Jun 19, 2014 - E-mail: [email protected]., *E-mail: [email protected]. ... The deployment of the [P4,4,4,2][DEP] and tributylmethylphospho...
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Inhibitory Effect of Phosphonium-Based Ionic Liquids on Coal Oxidation Weiqing Zhang,*,† Shuguang Jiang,*,†,‡ Christopher Hardacre,*,§ Peter Goodrich,§ Kai Wang,‡ Zhengyan Wu,‡ and Hao Shao‡ †

State Key Laboratory of Coal Resources and Safe Mining, and ‡School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China § School of Chemistry and Chemical Engineering, Queen’s University Ionic Liquid Laboratories (QUILL), Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom ABSTRACT: To develop a chemical inhibitor that can efficiently suppress coal oxidation, nine tetraalkylphosphonium-based ionic liquids (ILs) and one imidazolium-based IL [1-allyl-3-methylimidazolium chloride ([AMIm]Cl)] were examined as additives. These ILs were used to treat and investigate the inhibitory effect on the oxidation activity and the structure of lignite coal. Characterization using thermogravimetric analysis showed that phosphonium-based ILs are able to inhibit coal oxidation up to 400 °C with the tributylethylphosphonium diethylphosphate ([P4,4,4,2][DEP]) found to be the most effective. In contrast to the tetraalkylphosphonium-based ILs, inhibition using [AMIm]Cl was only found to be effective at temperatures below 250 °C, indicating that the tetraalkylphosphonium-based ILs may be more suitable for the future application of suppressing coal spontaneous combustion over a wide range of temperatures. Fourier transform infrared spectroscopic data showed that the various functional groups change in the coal following IL treatment, which are a decrease in the minerals and hydrogen bonds in all treated coals, while decreased aliphatic hydrocarbon and increased carbonyl bonds only appeared in some samples. During the oxidation of coal, the decomposition of aliphatic hydrocarbon groups is inhibited and the formation of carbonyl groups is delayed, so that the evolved gas concentration decreased, as shown by the temperature-programmed oxidation−mass spectrometry results. The deployment of the [P4,4,4,2][DEP] and tributylmethylphosphonium methylsulfate ILs as additives also show good inhibitory effect on coal oxidation over the temperature range studied, and a relatively stronger interaction between [P4,4,4,2][DEP] and coal is demonstrated by the additive model. [BF4]).14,15 Geng et al. reported that the pretreatment of coal by 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIm][BF4]) resulted in the breaking of weak covalent bonds in coal.16 Painter et al. demonstrated that a range of [BMIm]+ ionic liquids with triflate ([CF3SO3]−), tetrafluoroborate ([BF4]−), hexafluorophosphate ([PF6]−), Cl−, and I− anions used as neat solvents or as co-solvents with N-methylpyrollidine or pyridine were capable of disintegrating, dispersing, and solubilizing a range of coal samples at ambient temperatures.17,18 Pulati et al. thermally treated Illinois No. 6 coal in IL [BMIm][CF3SO3] in the presence of tetralin and hydrogen, leading to significant fragmentation and a dramatic increase in pyridine solubility of coal.19 Qi et al. investigated the solubility of Victorian brown coal in “distillable” ILs formed by the reaction of carbon dioxide with low-molecular-weight amines and found that solubilities up to 23 wt % could be obtained.20 Nie and co-workers demonstrated that it was possible to use a range of protic, imidazolium, pyridinium, and dialkylphosphate anion-containing ILs to extract asphaltenes from a direct coal liquefaction residue.21−26 Wang et al. studied the dissolution of bituminous coal in six imidazolium-based ILs and found that ILs can partially change the functional groups in coal, which affects the oxidation properties of the coal.27 Zhang et al. also

1. INTRODUCTION The spontaneous combustion of coal after long-term exposure to air is a well-known phenomenon. This effect is associated with exothermic decomposition during the oxidation of coal and is mainly due to the large number of surface active sites, which strongly interact with dioxygen, releasing heat and resulting in significant temperature increases in coal piles.1−3 As well as the obvious hazards involved with coal fires, there is also a large financial incentive to mitigate spontaneous combustion. To prevent spontaneous combustion from occurring, inhibitors are often used to suppress the coal oxidation.4−9 The role of some physical inhibitors, such as mud grout5 and three-phase foam,5 is to prevent oxygen from interacting with surface active sites on the coal. Other inhibitors, such as sodium chloride,6 calcium chloride,7,8 and sodium phosphate,9 also play a chemical role by suppressing the autocatalytic coal oxidation process. Therefore, chemical inhibitors have been a main area of research in coal mine safety. Ionic liquids (ILs) have physicochemical characteristics, such as low melting point and negligible volatility, are nonflammable, and can dissolve a wide range of organic and inorganic materials.10−13 Coal is a composite material composed of organics and inorganics, and thus, the aforementioned properties of ILs as novel solvents in coal chemistry have attracted significant attention. Cao et al. studied how the molecular structure of coal changed after treatment using 1ethyl-3-methyl imidazolium tetrafluoroborate ([EMIm]© 2014 American Chemical Society

Received: November 12, 2013 Revised: June 17, 2014 Published: June 19, 2014 4333

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Figure 1. TG profiles of IL-untreated and IL-treated coals: (a) 25−250 °C and (b) 250−400 °C for IL-untc and [P4,4,4,2][DEP]-tc, [P4,4,4,1][MeSO4]-tc, [P6,6,6,14][Bis]-tc, [P4,4,4,4,][Br]-tc, and [AMIm]Cl-tc and (c) 25−250 °C and (d) 250−400 °C for IL-untc and [P6,6,6,14]Cl-tc, [P6,6,6,14]Br-tc, [P6,6,6,14][NTf2]-tc, [P6,6,6,14][N(CN)2]-tc, and [P4,4,4,1][NTf2]-tc (“tc” denotes treated coal). mide ([P4,4,4,4]Br) were supplied by Cytec Industry, Inc. (Canada) and used as received. The ILs tributyl(methyl)phosphonium bis(trifluoromethylsulfonyl) ([P4,4,4,1][NTf2]) and 1-allyl-3-methylimidazolium chloride ([AMIm]Cl) were synthesized in house according to the protocols reported by Zhang et al.29 and Luska et al.,30 respectively. 2.2. Sample Preparation. The coal sample was ground in a mortar and pestle within a glovebox and sieved to a particle size of 150−250 μm. The sieved particulate coal (∼0.5 g) was vigorously mixed with each of the ILs using a weight ratio of 1:1 over 24 h at room temperature. Thereafter, the 10 mixtures were washed with dichloromethane (100 cm3) and filtered to separate the coal. The coal was further washed with extra portions of dichloromethane (3 × 25 cm3) until the filtrate was transparent. Then, the washed coal was dried at room temperature by flowing air for 4 h over the sample, which is named as IL-treated coal. In addition to the IL-treated coals, a sample of the untreated particulate coal was washed with dichloromethane to enable a comparison to be made with the IL-treated coal; this sample is denoted as IL-untreated coal. For the experiments taking ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4] as inhibitors for suppressing the coal oxidation study, each IL (10 mg) was dissolved in 5 cm3 dichloromethane and then blended equably with the raw coal particles (150−250 μm, 190 mg) in a glass vial. The mixture was dried at room temperature by flowing air for 48 h over the sample to ensure that dichloromethane was removed. The evaporated samples are denoted as 5 wt % [P4,4,4,2][DEP] and 5 wt % [P4,4,4,1][MeSO4]. Using the same procedure, a 5 wt % CaCl2 sample

studied the low-temperature oxidation of coal after being treated by 1-allyl-3-methylimidazolium chloride ([AMIm]Cl) and concluded that the IL could inhibit the oxidation.28 These results showed that ILs can affect the coal structure as well as suppress coal oxidation. To date, current studies have mainly concentrated on dialkylimidazolium-based ILs. This paper reports the treatment of coal with a range of cationic tetraalkylphosphonium-based ILs. In particular, the effect of these ILs on the oxidation stability and functional groups of coal were investigated to find a better inhibitor to prevent the spontaneous combustion of coal.

2. EXPERIMENTAL SECTION 2.1. Coal Sample and ILs. The coal sample used in this study was lignite according to the China Standard GB/T 5751-2009. The moisture content, volatility, and ash content of the sample on an airdried basis are 17.75, 36.22, and 20.86%, respectively. The IL samples of trihexyltetradecylphosphonium chloride ([P6,6,6,14]Cl), trihexyltetradecylphosphonium bromide ([P6,6,6,14]Br), trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][Bis]), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) ([P6,6,6,14][NTf2]), trihexyltetradecylphosphonium dicyanamide ([P6,6,6,14][N(CN)2]), tributylethylphosphonium diethylphosphate ([P4,4,4,2][DEP]), tributylmethylphosphonium methylsulfate ([P4,4,4,1][MeSO4]), and tetrabutylphosphonium bro4334

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Table 1. Parameters Characterizing the Relative Oxidation Activity of Coal between 125 and 400 °C coal sample

percentage of mass decrease (%)

relative decrease (%)

IL-untc [P4,4,4,2][DEP]-tc [P4,4,4,4]Br-tc [P6,6,6,14][Bis]-tc [P4,4,4,1][MeSO4]-tc [AMIm]Cl-tc

39.2 17.2 22.3 22.9 24.7 32.0

0 56.1 43.2 41.6 37.1 18.4

coal sample

percentage of mass decrease (%)

relative decrease (%)

[P6,6,6,14]Br-tc [P4,4,4,1][NTf2]-tc [P6,6,6,14]Cl-tc [P6,6,6,14][NTf2]-tc [P6,6,6,14][N(CN)2]-tc

33.2 33.8 34.2 36.1 36.3

15.4 13.8 12.8 8.0 7.4

400 °C. In comparison, all of the IL-treated coals showed lower mass loss over the temperature range of 250−400 °C. To eliminate the moisture effect during the initial mass loss stage (25−125 °C), a parameter was defined to evaluate the oxidation activity of different coal samples, which is the percentage of mass decrease between 125 and 400 °C. The results are reported in Table 1. It is evident that the percentage of mass loss under oxidation between 125 and 400 °C is smaller because of the treatment of ILs on coal; however, the results vary greatly. For example, the [P4,4,4,2][DEP]-treated coal shows the smallest decrease in the mass loss from 39.2 to 17.2%, with a relative decrease in mass loss compared to the untreated coal of 56.1%. [P4,4,4,4]Br-, [P6,6,6,14][Bis]-, and [P4,4,4,1][MeSO4]-treated coals showed similar decreases in mass loss between 22 and 25% with a relative change compared to the untreated coal of ∼40%, which is over twice that found for the [AMIm]Cl-treated coal. Coal samples treated with the other five ILs showed a smaller relative decrease in mass loss, with [P6,6,6,14][N(CN)2]-treated coal showing the least effect. Although significant differences in mass loss were observed at temperatures above 250 °C, the same trend was not observed at lower temperatures. For example, only the [P4,4,4,2][DEP]-, [AMIm]Cl-, [P6,6,6,14][Bis]-, and [P4,4,4,1][MeSO4]-treated coals showed any significant difference compared to the IL-untreated coal. The mass change during this temperature range is mainly caused by the different moisture contents for each coal sample, with only small changes as a result of the low-temperature oxidation activity of coal. These differences are fundamentally caused by the different effects of ILs on coal structures, which will be discussed later. As reported previously,28 [AMIm]Cl is effective at limiting the oxidation of coal below 180 °C and showed comparative behavior to that for [P4,4,4,2][DEP] in this study. However, at temperatures of >250 °C, the mass loss for the [AMIm]Cltreated coal increased and exceeded four phosphonium ILtreated coal samples (Figure 1a). Such results indicate that the inhibitory effect of [AMIm]Cl is perhaps limited to the lowtemperature oxidation. Overall, for the same cationic species [P6,6,6,14]+, the inhibitory effects of anion [Bis]− on the coal oxidation significantly outperforms all of the other anions Br−, Cl−, [NTf2]−, and [N(CN)2]−, which show similar oxidation curves. For the same anions Cl−, Br− and [NTf2]−, the smaller cation shows better inhibitory effect on coal oxidation, i.e., [AMIm]+ > [P6,6,6,14]+, [P4,4,4,4]+ > [P6,6,6,14]+, and [P4,4,4,1]+ > [P6,6,6,14]+. In addition, the hydrophilic ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4] show a relatively better inhibitory effect compared to the hydrophobic ILs. 3.2. FTIR Study of All Coal Samples at Room Temperature. The main active groups in coal, such as aliphatic hydrocarbon groups and oxygen-containing groups,

was also prepared. To eliminate the effect of dichloromethane on the experiment, a sample of 190 mg of raw coal was blended with 5 cm3 dichloromethane and the mixture was dried for 48 h at room temperature, which was 0 wt % sample. 2.3. Experimental Procedures. The IL-untreated coal and 10 ILtreated coals were subjected to thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) measurements. TGA measurements were carried out using a thermogravimetric analyzer (TGA/DSC 1 Star System, Mettler Toledo, Switzerland) in a dry air flow of 50 cm3/min at a heating rate of 5 °C/min over the temperature range of 25−400 °C. FTIR spectra were recorded between 3800 and 650 cm−1 and were accumulated for eight scans at a resolution of 8 cm −1 on a PerkinElmer Spectrum 100 FTIR spectrometer (PerkinElmer, U.K.). All of the spectra were auto-baseline-corrected using software Omnic 8.0. The IL-untreated coal, [P4,4,4,2][DEP]-treated coal, [P4,4,4,1][MeSO4]-treated coal, and [AMIm]Cl-treated coal were further measured using FTIR after oxidation of the sample at 225, 345, and 400 °C. To obtain these samples, 20 mg of the coal was heated in a tube furnace (Carbolite MTF 1200 °C) at a heating rate of 5 °C/min from 25 to 225 °C, from 25 to 345 °C, and from 25 to 400 °C in a dry air flow of 50 cm3/min. At the end of the oxidation treatment, the sample was cooled to room temperature in N2 and then measured using FTIR. These four samples were also measured by temperatureprogrammed oxidation−mass spectrometry (TPO−MS). In the TPO−MS experiment, ∼50 mg of sample was placed in a quartz reactor tube and held in place between plugs of quartz wool. The reactor tube was then installed in a horizontal tube furnace (Carbolite MTF 10/15/130), and a thermocouple was placed in the center of the coal. Subsequently, the coal was heated in a 21 vol % oxygen/argon/ krypton mixture with a total flow rate of 50 cm3/min up to 700 °C at a rate of 10 °C/min. The outlet gas concentrations were performed by a Hiden Analytical HPR 20 mass spectrometer. The following mass/ charge ratios were monitored as a function of the coal temperature: 2 (H2), 28 (CO), 30 (C2H6), 44 (CO2), and 84 (Kr). Quantification was carried out with reference to the Kr signal. The 0 wt % sample and 5 wt % [P4,4,4,2][DEP], 5 wt % [P4,4,4,1][MeSO4], and 5 wt % CaCl2 samples were subjected to TGA measurement at the same experimental conditions as mentioned above. The pure ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4] were also measured using the same TGA method.

3. RESULTS AND DISCUSSION 3.1. TGA Results. Thermogravimetry (TG) data may be used to examine the oxidation stability of coal.27,31 Figure 1 shows the TG results for the IL-untreated and IL-treated coal samples. It is clear that all of the coals show similar mass loss trends but different degrees of mass loss for each of the ILtreated coals at 400 °C, indicating that the oxidation activity of the coal was strongly affected by IL treatment. From Figure 1, at temperatures below 125 °C, all of the coal samples initially undergo a small decrease in mass (∼2.0−3.5 wt %), mainly ascribed with the loss of moisture. Above 125 °C, the mass remains relatively constant up to ∼225 °C. At temperatures higher than 225 °C, rapid decomposition of ILuntreated coal was observed with more than 60 wt % loss by 4335

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have been identified previously by FTIR spectroscopy,32 and Figure 2a shows the FTIR spectra of IL-untreated coal and all

kaolinite, the decrease is lowest for the IL [P4,4,4,1][MeSO4], while the ILs [P 4,4,4,2 ][DEP], [P 4,4,4,4, ]Br, [P 6,6,6,14 ]Br, [P6,6,6,14]Cl, and [P6,6,6,14][N(CN)2] show significant decreases. The broad vibration band between 3600 and 3100 cm−1 is believed to be caused by association of a large number and variety of hydroxyls in coal through hydrogen bonds. The hydrogen bonds are reduced to different degrees in IL-treated coals, except [P4,4,4,1][NTf2]-, [P6,6,6,14][NTf2]-, and [P4,4,4,4,]Brtreated coals. The obvious decrease of hydrogen bonds in [P6,6,6,14][Bis]-, [AMIm]Cl-, and [P4,4,4,2][DEP]-treated coals indicates that these three ILs can break the associated hydroxyl bonds in coal effectively. This has been observed previously for [AMIm]Cl.28 This decrease in hydrogen bonding leads to a lower mass loss for these three IL-treated coals below 225 °C (Figure 1a). The decrease in the degree of hydrogen bonds in the [P4,4,4,1][MeSO4]-treated coal is smaller compared to the three IL-treated coals mentioned above; therefore, the mass loss of [P4,4,4,1][MeSO4]-treated coal below 225 °C is higher than the three IL-treated coals but still less than that of ILuntreated coal (Figure 1a). Specifically, the positive bands in the 3600−3100 cm−1 region of [P4,4,4,4,]Br-treated coal could be ascribed to moisture in the sample, so that the mass loss of [P4,4,4,4,]Br-treated coal is larger than IL-untreated coal during the low-temperature range (Figure1a). No significant changes are observed in the hydrogen-bonding regions after treatment with [P4,4,4,1][NTf2] and [P6,6,6,14][NTf2]. In contrast with the other IL-treated samples, upon treating the coal with [AMIm]Cl, one obvious band at ∼3150 cm−1 appears, which is ascribed to the =N−H antisymmetric stretching vibration in the imidazole ring of [AMIm]Cl,32 indicating an IL residue within the treated coal. In addition, a strong feature at 1570 cm−1 of [AMIm]Cl-treated coal ascribed to N−H scissor bending vibration35 also clearly indicates the presence of residual [AMIm]Cl in the coal, even following washing with DCM. As found for the [AMIm]Cl-treated coal, the stronger band intensity for the C−H stretching vibration in the region of 3000−2800 cm−1 for [P6,6,6,14][Bis]- and [P4,4,4,2][DEP]-treated coals could also indicate residual IL [P6,6,6,14][Bis] and [P4,4,4,2][DEP] present in each IL-treated coal. In contrast, the bands associated with −CH2 groups at 2920 and 2850 cm−1 decreased upon treatment with [P4,4,4,1][MeSO4], [P6,6,6,14][NTf2], [P6,6,6,14]Br, and [P6,6,6,14][N(CN)2], while the −CH3 bands at 2968 and 2870 cm−1 increased slightly in the [P4,4,4,1][MeSO4]- and [P4,4,4,4]Br-treated coals. Such results show that the ILs can affect the aliphatic hydrocarbon groups in coal. The band at 1702 cm−1 attributed to CO of carboxylic acids in coal increased significantly in [P4,4,4,1][MeSO4]-, [P4,4,4,2][DEP]-, and [P6,6,6,14]Cl-treated coals, together with the observation of a negative band at 1550 cm−1 ascribed to carboxyl groups COO−. These changes demonstrate a conversion of the charged carboxyl groups COO− into carboxylic acid COOH. The same results were observed by Painter et al. from the infrared spectra of a coal sample before and after washing with HCl.36 Wang et al.28 also observed more carboxyl groups occurring during imidazolium-based ionic liquids interacting with coal. However, the carboxylic acid CO band shows a decrease in [P6,6,6,14][NTf2]- and [P6,6,6,14][N(CN)2]-treated coals, showing that the two ILs result in a decrease in the −COOH groups. In general, the IL-treated coals showing a larger reduction in the oxidation activity, such as [P4,4,4,2][DEP]-, [AMIm]Cl-, and

Figure 2. (a) FTIR spectra of all coal samples (from top to bottom are the FTIR spectra of IL-untc, [P6,6,6,14][Bis]-tc, [AMIm]Cl-tc, [P4,4,4,1][MeSO4]-tc, [P4,4,4,2][DEP]-tc, [P4,4,4,1][NTf2]-tc, [P6,6,6,14][NTf2]-tc, [P4,4,4,4,]Br-tc, [P6,6,6,14]Br-tc, [P6,6,6,14]Cl-tc, and [P6,6,6,14][N(CN)2]tc). (b) Difference FTIR spectra for all IL-treated coals (from top to bottom are the FTIR spectrum of [P6,6,6,14][Bis]-tc, [AMIm]Cl-tc, [P4,4,4,1][MeSO4]-tc, [P4,4,4,2][DEP]-tc, [P4,4,4,1][NTf2]-tc, [P6,6,6,14][NTf2]-tc, [P4,4,4,4,]Br-tc, [P6,6,6,14]Br-tc, [P6,6,6,14]Cl-tc, and [P6,6,6,14][N(CN)2]-tc).

IL-treated coals. To highlight the change between the FTIR spectra, the spectrum of IL-untreated coal was subtracted from each IL-treated coal, with the stable aromatic CC band at 1600 cm−1 being used to normalize the intensites. The differential spectra are shown in Figure 2b. The FTIR spectra of coal in the fingerprint region (1500−650 cm−1) arises from very complex deformations of molecules; therefore, the functional group changes in the region of 3800−1500 cm−1 were analyzed in detail. The sharp peaks between 3800 and 3600 cm−1 are ascribed to O−H stretching vibrations of the silicate/clay mineral and kaolinite in coal,33 which show significant decreases in a number of the IL-treated coals. However, only a small change is observed for the [P6,6,6,14][Bis]- and [AMIm]Cl-treated coals. Because the dehydration of kaolinite begins at 550−600 °C to form metakaolin (well above the maxmum oxidation temperature of 400 °C employed), we suspect that the ILs employed within can modify the mineral moisture.34 Among the eight ILs, which can effectively break and dissolve the minerals and 4336

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Figure 3. FTIR spectra of IL-untreated coal and [P4,4,4,2][DEP]-, [P4,4,4,1][MeSO4]-, and [AMIm]Cl-treated coals at 25, 225, 345, and 400 °C.

relatively higher oxidation stability compared to the [P4,4,4,1][MeSO4]- and [AMIm]Cl-treated coals. These results are consistent with those previously obtained with TGA. Interestingly, for the [AMIm]Cl-treated coal, the bands at 3148 (ascribed to =N−H antisymmetric stretching vibration in the imidazole ring of IL [AMIm]Cl) also disappear at 345 °C, showing that the residual IL decomposes under these conditions. This is believed to be the main reason for the rapid increase in mass loss between 345 and 400 °C for the [AMIm]Cl-treated coal compared to that found for the [P4,4,4,2][DEP]- and [P4,4,4,1][MeSO4]-treated coals at the same temperatures (Figure 1b). At 400 °C, the CO band (1850−1700 cm−1) is significantly smaller in the IL-untreated coal and [AMIm]Cltreated coal compared to that in the [P4,4,4,2][DEP]- and [P4,4,4,1][MeSO4]-treated coals. This reduced carbonyl reactivity observed in [P4,4,4,2][DEP]- and [P4,4,4,1][MeSO4]-treated coals provides further evidence for the overall reduced oxidation activity of the IL-treated coals. The absorption bands associated with the minerals (1050− 1000 cm−1) also changed as a function of oxidation after IL treatment. For example, the [P4,4,4,2][DEP] treatment significantly reduces the mineral content in the coal, while the treatment with [P4,4,4,1][MeSO4] and [AMIm]Cl does not affect the mineral content significantly. Phosphate esters also stretch around this wavenumber; therefore, this could be residual IL that is being decomposed and leaching as the temperature increases. Accordingly, the oxidation ability of [P4,4,4,2][DEP]-treated coal is lower than that of [P4,4,4,1][MeSO4]- and [AMIm]Cltreated coals, as indicated by TGA results. Such results show that the minerals in coal lead to increased oxidation of coal. As

[P4,4,4,1][MeSO4]-treated coals, show significant decreases in the hydrogen-bonding features but smaller changes in the mineral and kaolinite contents. The IL-treated coals that retain relatively high oxidation activity, such as [P6,6,6,14][NTf2]- and [P6,6,6,14][N(CN)2]-treated coals, show decreased methylene group absorption and a large decrease in the mineral and kaolinite contents. Interestingly, the presence of increased carboxyl group absorption is not shown for all of the IL-treated coals. 3.3. FTIR Spectra Change during Oxidation of Coal. Figure 3 reports the FTIR spectra of IL-untreated coal and [P4,4,4,2][DEP]-, [P4,4,4,1][MeSO4]-, and [AMIm]Cl-treated coal before and after oxidation at 25, 225, 345, and 400 °C. In general, the features in the hydrogen-bond region (3600−3100 cm−1) and aliphatic hydrogen region (3000−2800 and 1450 cm−1) decrease with an increasing oxidation temperature, while the carbonyl stretching region (1850−1700 cm−1) increases initially before decreasing. In addition, the aromatic C−H band at ∼3070 cm−1 is more evident relative to the carbonyl stretching region for all coals at 345 °C compared to that at 25 and 225 °C, showing increasing aromaticity of these coals following deep oxidation. These results show that the hydroxyl and aliphatic hydrocarbons are the most active sites in the coal structure, with the reaction with oxygen forming aldehydes, ketones, and carboxyl esters.2,3,28,37−39 In comparison to the FTIR spectrum of IL-untreated coal after oxidation at 345 °C, the bands in the aliphatic hydrocarbon regions at 3000−2800 cm−1 are still visible following oxidation of the [P4,4,4,2][DEP]-, [AMIm]Cl-, and [P4,4,4,1][MeSO4]-treated coals, showing reduced oxidation of these coals. The slightly stronger absorption (3000−2800 cm−1) found in the [P4,4,4,2][DEP]-treated coal also indicates a 4337

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Figure 4. TPO−MS results of IL-untreated coal and [P4,4,4,2][DEP]-, [P4,4,4,1][MeSO4]-, and [AMIm]Cl-treated coals.

Figure 5. TGA profiles for raw coal and coals containing 0 wt % additive, 5 wt % [P4,4,4,2][DEP], 5 wt % [P4,4,4,1][MeSO4], and 5 wt % CaCl2.

the temperature increases, the intensity of the mineral bands in [P4,4,4,1][MeSO4]- and [AMIm]Cl-treated coals decreased significantly up to 225 °C, increasing the oxidation of these IL-treated coals compared to that of the [P4,4,4,2][DEP]-treated coal. 3.4. TPO−MS Change during Oxidation of Coal. Figure 4 shows the evolved hydrogen, carbon monoxide, carbon

dioxide, and ethane profiles as a function of the temperature for the IL-untreated coal and [P4,4,4,1][MeSO4]-, [P4,4,4,2][DEP]-, and [AMIm]Cl-treated coals. All four samples showed similar H2, CO, CO2, and C2H6 profiles. The main peaks were observed with the onset temperatures of ∼214 °C for ILuntreated coal and ∼235 °C for [P4,4,4,2][DEP]-, [P4,4,4,1][MeSO4]-, and [AMIm]Cl-treated coals. While it is a relatively 4338

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small difference, this increase in the temperature does show the inhibitory effect of IL treatment on coal oxidation. The concentrations of the evolved combustion gases CO and, in particular, CO2 were greater in the untreated and [AMIm]Cltreated coals compared to that of the [P4,4,4,2][DEP]- and [P4,4,4,1][MeSO4]-treated coals. This could be, in part, due to the formation of the more stable aromatic systems observed in the latter two IL-treated coal samples, as previously observed by FTIR experiments. Similarly, under pyrolysis, the production of H2 was significantly decreased in both the [P4,4,4,2][DEP]- and [P4,4,4,1][MeSO4]-treated coals compared to the untreated and [AMIm]Cl-treated coals. Because of the small signal count observed for the evolution of ethane, only slight reductions were observed for both phosphonium ILs. Such results show that the pyrolysis and oxidation of the coal structure were inhibited greatly by ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4] but only to a small extent by [AMIm]Cl. 3.5. IL Additive as an Inhibitor for Coal Oxidation. 3.5.1. TGA Study of Coals Containing an IL Additive. The TGA data for raw coal containing 0 wt % additive, 5 wt % [P4,4,4,2][DEP], and 5 wt % [P4,4,4,1][MeSO4] are shown in Figure 5. These systems were compared against the conventional system using 5 wt % CaCl2. With an increasing temperature, all coals show similar mass loss trends. Under high-temperature oxidation at 400 °C, all of the coals containing an additive decreased in comparison to that of coal without an additive, indicating the inhibitory effect of these additives on coal oxidation. However, different coal samples showed different mass loss trends. The coal containing [P4,4,4,1][MeSO4] showed the smallest mass loss over the whole temperature range, while the coal containing [P4,4,4,2][DEP] exhibited a slightly higher mass loss than that of coal without an additive between 250 and 310 °C. This is attributed to the decomposition of IL [P4,4,4,2][DEP] over this temperature range (Figure 6). The CaCl2 additive sample

Table 2. Parameters Characterizing Oxidation Activity of Coal coal sample 0 5 5 5

wt wt wt wt

% % % %

additive [P4,4,4,2][DEP] coal [P4,4,4,1][MeSO4] coal CaCl2 coal

percentage of mass decrease (%)

relative decrease (%)

32.1 19.1 21.9 28.7

0 40.5 31.8 10.4

effect than [P4,4,4,1][MeSO4]. Such results are related to the interaction between the coal and IL as well as the thermal stability properties of the IL itself. Furthermore, similar mass losses were observed for coal samples treated with additive (5 wt %) amounts of the IL compared to the 100 wt % IL shown in Figure 1. 3.5.2. Interaction between Coal and IL. To assess whether the variations observed are true changes in the oxidation reactivity of the coal and not the result of the relative mass losses of each individual component in the mixtures, an additive model was used to compare the experimentally determined mass loss (Mexpt) to a calculated value based on a weight average for each component (Mcalculated) at a fixed temperature.40 Mcalculated is calculated by the method that the total mass of a blend at a certain temperature is the sum of the mass of each pure component by its fraction at that temperature, which is expressed as Mcalculated = X1M1 + X2M2, where Xi is the mass fraction and Mi (wt %) is the mass loss. The values of Mexpt and Mcalculated of coal samples with 5 wt % ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4] are presented by TG curves in Figure 7. Clearly, under decomposition conditions for the coal sample with 5 wt % ILs, the Mexpt mass loss is less than the Mcalculated over the whole oxidation process and the two curves possess a significant deviation of >275 °C. Such results indicate that an interaction between the coal and IL does exist and shows an inhibitory effect on coal oxidation, especially during the hightemperature oxidation stage. To investigate the degree and variation of the influence of the interaction on the oxidation of coal, ΔM is introduced as follows ΔM = Mexpt − Mcalculated,41 where ΔM is the mass difference that can be assumed as an indicator of the degree of interaction existing between coal and IL. The relationship of ΔM and temperature is presented in Figure 8. It is found that ΔM of coals with [P4,4,4,2][DEP] is slightly higher than that of coal with [P4,4,4,1][MeSO4] during the oxidation process, indicating a relatively better inhibitory effect of [P4,4,4,2][DEP] on the oxidation of coal. The inhibitory effect during 25−250 °C may be mainly related to the coating effect by IL on the coal surface, which will hinder access of oxygen to the coal surface and, thus, reduce the rate of the oxidation reaction. During this temperature range, the two ILs are relatively stable according to the mass loss change of them in Figure 8, showing that the IL film is stable. Between 280 and 330 °C, the [P4,4,4,2][DEP] additive shows a higher inhibitory effect on coal oxidation than [P4,4,4,1][MeSO4], which may be caused by the intermediate products of thermal decomposition of [P4,4,4,2][DEP]. From ∼330 to 400 °C, ΔM rises sharply and reaches 17.5% for the coal containing [P4,4,4,2][DEP] and 15.3 wt % for the coal containing [P4,4,4,1][MeSO4]. This significant change at the higher temperatures may be related to the decomposition of the IL. As a result of the decomposition of the IL, gas products are formed as well as some low-volatility components. The gas products can dilute the oxygen

Figure 6. Mass loss of ILs [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4].

shows much more mass loss than coal without an additive until temperatures of >380 °C. Such a TG curve trend is mainly affected by the initial mass loss stage between 25 and 125 °C ascribed to moisture in CaCl2. Table 2 shows the percentage of mass decrease during 125−400 °C. It can be seen that the coal containing CaCl2 shows less relative reduction in mass compared to the coal containing the ILs, indicating a much higher inhibitory effect of the ILs on the oxidation than found for CaCl2. The [P4,4,4,2][DEP] additive shows better inhibitory 4339

dx.doi.org/10.1021/ef402229z | Energy Fuels 2014, 28, 4333−4341

Energy & Fuels

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Figure 7. Mexpt and Mcalculated of coal samples with 5 wt % IL [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4]: (a) coal samples with 5 wt % IL [P4,4,4,2][DEP] and (b) coal samples with 5 wt % [P4,4,4,1][MeSO4].

the hydrogen bond region and mineral absorption, while the decrease of methylene groups and an increase in the carbonyl groups are only found in some IL-treated coals. In addition, despite washing the treated coal samples, strongly bound ILs in the cases of [P6,6,6,14][Bis], [P4,4,4,2][DEP], and [AMIm]Cl were found to be present in the coal samples. During the coal oxidation process, the decomposition of aliphatic hydrocarbon groups is inhibited, while the formation of carbonyl groups is delayed in IL-treated coals compared to the IL-untreated coal. The [P4,4,4,2][DEP] and [P4,4,4,1][MeSO4]-treated coals showed lower evolved gas concentration compared to that of the IL-untreated coal, which is consistent with the reduced low-temperature oxidation activity for the coals. Using an IL as an additive can inhibit oxidation of coal with [P4,4,4,2][DEP], showing a more significant inhibitory effect than [P4,4,4,1][MeSO4], especially during a high-temperature stage. This difference may be related to the stronger interaction between [P4,4,4,2][DEP] and coal, as demonstrated by the additive model.

Figure 8. ΔM of coal samples with 5 wt % [P4,4,4,1][MeSO4] and [P4,4,4,2][DEP].

concentration on the coal surface, and the residual components may accumulate on the coal surface, reducing the interaction between the coal and oxygen. A similar reaction mechanism was reported for the co-pyrolysis of microalgae Chlorella vulgaris and coal.40 The better inhibitory effect of IL [P4,4,4,2][DEP] on the oxidation of coal compared to [P4,4,4,1][MeSO4] above ∼330 °C may be caused by the difference in the residual components following decomposition in each case and their effect on the coal−oxygen interaction.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-516-83885753. Fax: +44-28-90974592. Email: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

4. CONCLUSION The inhibitory effect of 10 tetraalkylphosphonium-based ILs and one imidazolium-based IL [AMIm]Cl on coal oxidation and structure were investigated by TGA, FTIR, and TPO−MS techniques. [P4,4,4,2][DEP] shows the greatest inhibitory effect on coal oxidation, as determined by the mass loss in TGA measurements. These effects are seen over a wide temperature range, which is in contrast with the inhibitory effect of [AMIm]Cl, which only works better at a low-temperature oxidation stage, so that phosphonium-based ILs may be more suitable for future application of suppressing coal spontaneous combustion. In general, the IL can affect the functional groups in the coal structure, as shown by the FTIR spectra. More detailed changes in the chemical structure are indicated by difference FTIR spectra, where the common changes are found in a decrease in

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51074159 and 51134023), the Project of the State Key Laboratory of Coal Resources and Mine Safety in China University of Mining and Technology (SKLCRSM09X04), and the National Natural Science Foundation of Jiangsu Project (BK20130203). The authors gratefully acknowledge the help of technicians in Analytical Services and Environmental Projects (ASEP) and QUILL Research Centre for TGA measurements and Dr. Sarayute Chansai of Queen’s University of Belfast for TPO−MS measurements. Special acknowledgement also goes to Cytec Industry, Inc. (Canada) for supplying the phosphonium-based IL samples. 4340

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(24) Li, Y.; Zhang, X. P.; Lai, S. Y.; Dong, H. F.; Chen, X. L.; Wang, X. L.; Nie, Y.; Sheng, Y.; Zhang, S. J. Ionic liquids to extract valuable components from direct coal liquefaction residues. Fuel 2012, 94, 617−619. (25) Wang, J. L.; Yao, H. W.; Nie, Y.; Bai, L.; Zhang, X. P.; Li, J. W. Application of iron-containing magnetic ionic liquids in extraction process of coal direct liquefaction residues. Ind. Eng. Chem. Res. 2012, 51, 3776−3782. (26) Bai, L.; Nie, Y.; Li, Y.; Dong, H. F.; Zhang, X. P. Protic ionic liquids extract asphaltenes from direct coal liquefaction residue at room temperature. Fuel Process. Technol. 2013, 108, 94−100. (27) Wang, L. Y.; Xu, Y. L.; Jiang, S. G.; Yu, M. G.; Chu, T. X.; Zhang, W. Q.; Wu, Z. Y.; Kou, L. W. Imidazolium based ionic liquids affecting functional groups and oxidation properties of bituminous coal. Saf. Sci. 2012, 50, 1528−1534. (28) Zhang, W. Q.; Jiang, S. G.; Wang, K.; Wu, Z. Y.; Shao, H. An experimental study of the effect of ionic liquids on the low temperature oxidation of coal. Int. J. Min. Sci. Technol. 2012, 22, 687−691. (29) Zhang, H.; Wu, J.; Zhang, J.; He, J. S. 1-Allyl-3methylimidazolium chloride room temperature ionic liquid: A new and powerful nonderivatizing solvent for cellulose. Macromolecules 2005, 38, 8272−8277. (30) Luska, K. L.; Moores, A. Ruthenium nanoparticle catalysts stabilized in phosphonium and imidazolium ionic liquids: Dependence of catalyst stability and activity on the ionicity of the ionic liquid. Green Chem. 2012, 14, 1736−1742. (31) Kok, M. An investigation into the combustion curves of lignites. J. Therm. Anal. Calorim. 2001, 64, 1319−2323. (32) Ibarra, J. V.; Munoz, E.; Moliner, R. FTIR study of the evolution of coal structure during the coalification process. Org. Geochem. 1996, 24, 725−735. (33) Strydom, C. A.; Bunt, J. R.; Schobert, H. H.; Raghoo, M. Changes to the organic functional groups of an inertinite rich medium rank bituminous coal during acid treatment processes. Fuel Process. Technol. 2011, 92, 764−770. (34) Bellotto, M.; Gualtieri, A.; Artioli, G.; Clark, S. M. Kinetic study of the kaolinite−mullite reaction sequence. Part I: Kaolinite dehydroxylation. Phys. Chem. Miner. 1995, 22 (4), 207−217. (35) Li, W. Z.; Ju, M. T.; Wang, Y. N.; Liu, L.; Jiang, Y. Separation and recovery of cellulose from Zoysia japonica by 1-allyl-3methylimidazolium chloride. Carbohydr. Polym. 2013, 92, 228−235. (36) Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. Concerning the application of FT-IR to the study of coal: A critical assessment of band assignments and the application of spectral analysis programs. Appl. Spectrosc. 1981, 35, 475−485. (37) Gethner, J. S. Thermal and oxidation chemistry of coal at low temperatures. Fuel 1985, 64, 1443−1446. (38) Yurum, Y.; Altuntas, N. Air oxidation of Beypazari lignite at 50 °C, 100 °C and 150 °C. Fuel 1998, 77, 1809−1814. (39) Calemma, V.; Rausa, R.; Margarit, R.; Girardi, E. FTIR study of coal oxidation at low temperature. Fuel 1988, 67, 764−770. (40) Chen, C. X.; Ma, X. Q.; He, Y. Co-pyrolysis characteristics of microalgae Chlorella vulgaris and coal through TGA. Bioresour. Technol. 2012, 117, 264−273. (41) Zhou, L. M.; Wang, Y. P.; Huang, Q. W.; Cai, J. Q. Thermogravimetric characteristics and kinetic of plastic and biomass blends co-pyrolysis. Fuel Process. Technol. 2006, 87, 963−969.

REFERENCES

(1) Wang, H. H.; Dlugogorski, B. Z.; Kennedy, E. M. Coal oxidation at low temperatures: Oxygen consumption, oxidation products, reaction mechanism and kinetic modeling. Prog. Energy Combust. 2003, 29, 487−513. (2) Baris, K.; Kizgut, S.; Didari, V. Low-temperature oxidation of some Turkish coals. Fuel 2012, 93, 423−432. (3) Wang, D. M.; Zhong, X. X.; Gu, J. J.; Qi, X. Y. Changes in active functional groups during low-temperature oxidation of coal. Min. Sci. Technol. 2010, 20, 35−40. (4) Smith, A. C.; Miron, Y.; Lazzara, C. P. Inhibition of Spontaneous Combustion of Coal; Bureau of Mines, U.S. Department of the Interior: Washington, D.C., 1988; Report of Investigations 9196. (5) Zhou, F. B.; Ren, W. X.; Wang, D. M.; Song, T. L.; Li, X.; Zhang, Y. L. Application of three-phase foam to fight an extraordinarily serious coal mine fire. Int. J. Coal Geol. 2006, 67, 95−100. (6) Sujanti, W.; Zhang, D. A laboratory study of spontaneous combustion of coal: The influence of inorganic matter and reactor size. Fuel 1999, 78, 549−556. (7) Taraba, B.; Rudolf, P.; Václav, S. Calorimetric investigation of chemical additives affecting oxidation of coal at low temperatures. Fuel Process. Technol. 2011, 92, 712−715. (8) Slovák, V.; Taraba, B. Urea and CaCl2 as inhibitors of coal lowtemperature oxidation. J. Therm. Anal. Calorim. 2012, 110, 363−367. (9) Zhan, J.; Wang, H. H.; Song, S. N.; Hu, Y.; Li, J. Role of an additive in retarding coal oxidation at moderate temperatures. Proc. Combust. Inst. 2011, 33, 2515−2522. (10) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc. 2002, 124, 4974−4975. (11) Winterton, N. Solubilization of polymers by ionic liquids. J. Mater. Chem. 2006, 16, 4281−4293. (12) Zakrzewska, M. E.; Bogel-Łukasik, E.; Bogel-Łukasik, R. Solubility of carbohydrates in ionic liquids. Energy Fuels 2010, 24, 737−745. (13) Lu, F.; Zhang, S. H.; Zheng, L. Q. Dispersion of multi-walled carbon nanotubes (MWCNTs) by ionic liquid-based phosphonium surfactants in aqueous solution. J. Mol. Liq. 2012, 173, 42−46. (14) Cao, M.; Gu, X. H.; Zhang, A. Y.; Ma, M. Study on rheological properties about ionic liquid and coal slurry. Coal Convers. 2009, 32, 40−43 (in Chinese). (15) Cao, M.; Gu, X. H.; Zhang, A. Y.; Ma, M. Swelling capacity of coal sample in ionic liquids. Coal Convers. 2009, 32, 58−60 (in Chinese). (16) Geng, S. C.; Fan, T. B.; Liu, Y. Y. Application of ionic liquid [Bmim]BF4 in swelling pretreatment of Shen-Hua coal. Coal Convers. 2010, 33, 35−38 (in Chinese). (17) Painter, P.; Pulati, N.; Cetiner, R.; Sobkowiak, M.; Mitchell, G.; Mathews, J. Dissolution and dispersion of coal in ionic liquids. Energy Fuels 2010, 24, 1848−1853. (18) Painter, P.; Cetiner, R.; Pulati, N.; Sobkowiak, M.; Mathews, J. Dispersion of liquefaction catalysts in coal using ionic liquids. Energy Fuels 2010, 24, 3086−3092. (19) Pulati, N.; Sobkowiak, M.; Mathews, J. P.; Painter, P. Lowtemperature treatment of Illinois No. 6 coal in ionic liquids. Energy Fuels 2012, 26, 3548−3552. (20) Qi, Y.; Verheyen, V.; Ranganathan, V.; MacFarlane, D.; Chaffee, A. L. Solubility of brown coal in ionic liquid. Chemeca 2012: Quality of Life through Chemical Engineering; Wellington, New Zealand, Sept 23− 26, 2012; pp 249−258. (21) Nie, Y.; Bai, L.; Li, Y.; Dong, H. F.; Zhang, X. P.; Zhang, S. J. Study on extraction asphaltenes from direct coal liquefaction residue with ionic liquids. Ind. Eng. Chem. Res. 2011, 50, 10278−10282. (22) Nie, Y.; Bai, L.; Li, Y.; Dong, H. F.; Zhang, X. P.; Zhang, S. J. Extraction of asphaltenes from direct coal liquefaction residue by dialkylphosphate ionic liquids. Sep. Sci. Technol. 2012, 47, 386−391. (23) Li, Y.; Zhang, X. P.; Dong, H. F.; Wang, X. L.; Nie, Y.; Zhang, S. J. Efficient extraction of direct coal liquefaction residue with the [bmim]Cl/NMP mixed solvent. RSC Adv. 2011, 1, 1579−1584. 4341

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