Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Oxidation of Coal Pitch by H2O2 under Mild Conditions Yao-Ling Wang,† Xin-hua Chen,‡ Ming-jie Ding,*,† and Jia-Zhen Li† †
School of Materials and Chemical Engineering, Henan University of Urban Construction, Pingdingshan, 467036, China Department of Light Industry, Luohe Vocational and Technical College, Luohe, 462000, China
‡
ABSTRACT: In order to expand the application of coal pitch, the H2O2 oxidation of coal pitch was employed to obtain some small molecular oxygen-containing aromatic compounds and to increase high-value-added products of coal pitch. The optimum oxidation conditions were determined by the oxygen conversion rate. The oxidation products were investigated with FTIR spectroscopy and gas chromatography/mass spectrometry (GC/MS) method. The results show that the coal pitch had been oxidized and depolymerized by H2O2 under mild conditions. The oxygen conversion rate was 59.06% at optimum condition of 60 °C, 6 h. The small molecular oxygen-containing aromatic compounds derived from this H2O2 oxidation are mainly grouped into alkanoic acids (AAs), aliphatic carboxylic acids (APCAs), benzene polycarboxylic acids (BPCAs), and a small amount of alkylbenzenes (ABs). Finally, the oxidation process of coal pitch by H2O2 is speculated.
1. INTRODUCTION Oxidative depolymerization of coal is an important method for obtaining organic compounds, especially for the production of oxygenated organic compounds and condensed aromatic hydrocarbons. Among them, benzene polycarboxylic acids are of particular importance as organic chemical raw materials, which have been widely used in the polymer field to fabricate drugs, preservative agent, plasticizing agents as well as some other chemical industries.1−3 At present, many researches have been conducted on the oxidation of coal, whose results are considered valuable and instructive on the study of coal pitch oxidation.4−9 Coal pitch is a byproduct residue from the distillation of high-temperature coal tar, accounting for about 50%. It is mainly used for fabricating the electrodes, carbon materials, carbon fiber, needle coke, adhesives, road construction materials, fuel, etc.10−14 Martin et al.15 studied the structural characterization of several coal pitches. Garciá et al.16 researched the carbonization in order to study the mesophase development of coal pitch. Sidorov17 investigated the carbonization of coal pitch with additives. As can be seen, carbonization, mesophase and structural characterization of coal pitch were concerned by researchers. Recently, pyrolysis, adsorption, activated carbon, and microstructure were highfrequency keywords of coal pitch research.18−22 With further study, it was found that the application of coal pitch was one of the main sources of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in the environment.23−25 Coal pitch could release carcinogen into the atmosphere in the process of machining, which not only polluted the environment but also caused great harm to human body. Now some measures have been taken to prevent and control the pollution of coal pitch during the processing, especially some harmless treatment of coal pitch in the initial stage,26−28 i.e., pretreatment of coal pitch as raw materials before using. Therefore, pollution prevention and control have become non-negligible issues in the utilization of coal pitch. In addition, there are still a lot of untapped residues after other processing which also arouse the problems such as resource waste, ecological destruction, and environmental pollution. All of the above-mentioned issues © XXXX American Chemical Society
greatly restrict the application of coal pitch. Consequently, the new methods of developing processing technology, reducing carcinogenic emissions, and extracting valuable products for utilizing of coal pitch have become imperative. Coal pitch is composed primarily of three or more condensed ring aromatic hydrocarbons. The structure can be broken down into corresponding aldehydes, ketones, aromatic acids, quinones, epoxides, and peroxides, which are essential chemical intermediates. With the rapid development of aromatic polymer engineering, the demand of aromatic compounds is increasing rapidly, especially the oxygen-containing compounds that make up the polymer monomers, such as phenols, monocyclic and polycyclic aromatic acids, aldehydes, ketones, alcohols, and so on. However, little studies have been carried out on the oxidation and depolymerization of coal pitch to obtain these chemicals. If the compounds can be obtained directly from coal pitch oxidation, the production cost can be greatly reduced. Moreover, they are also important ways to improve the economic and social benefits of the coal and coke industry. Oshika29 studied the two-step oxidation of coal pitch with oxygen in water and in alkaline solution. The optimum conditions were determined and the highest yield of watersoluble aromatic acids was 79 wt %. It is well-known that these aromatic carboxylic acids can be obtained by the oxidation of coal.30−36 Wang37 studied the oxidation of coal pitch by ozone in formic acid. The result showed that ozone could make the fused aromatic ring of coal pitch oxidized and depolymerized to obtain small molecular and oxygen-containing aromatic compounds. These studies prove it is a potential way to obtain some small molecular oxygen-containing aromatic compounds by oxidization of coal pitch. In this paper, the oxidation of coal pitch by hydrogen peroxide under mild conditions was studied. The products were separated and analyzed with gas chromatography/mass spectroscopy (GC/MS) and Fourier transform infrared spectrosReceived: July 12, 2017 Revised: November 23, 2017 Published: December 4, 2017 A
DOI: 10.1021/acs.energyfuels.7b02010 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
increase of oxidation degree of the reaction. The oxidative depolymerization products may be oxygen-containing aldehyde, ketone, acid, alcohol, and other aromatic compounds. The percentage of carbon is reduced, which may subsequently cause the release of CO2. 2.3. Analysis and Methods. 2.3.1. Influence of Oxidation Condition. In order to determine the optimum reaction conditions, the H2O2 oxidation of coal pitch at different temperatures and reaction times was studied. The effects of different conditions on oxidation were investigated by the oxygen conversion rate (X). X was calculated by using eq 1
copy (FTIR). The study has shown that coal pitch oxidation with hydrogen peroxide can generate small molecular aromatic organic chemicals, especially alkanoic acids (AAs) and benzene polycarboxylic acids (BPCAs).
2. EXPERIMENTAL SECTION 2.1. Coal Pitch Sample and Solvents. The coal pitch was collected from Kaifeng Carbon Co. Ltd., PingMei ShenMa Group in Henan Province, China. It was pulverized to pass through a 180-mesh screen and extracted with toluene to remove small molecules possibly existing in it, followed by desiccation in a vacuum at 40 °C for 24 h, and after that stored in a desiccator as coal pitch sample (CPS) for the experiment. Hydrogen peroxide solution (30%), CH2N2, acetone, ethanol, anhydrous MgSO4, benzene, and toluene used in the experiment are commercially purchased analytically pure reagents. All the organic solvents were distilled before use. 2.2. Oxidation General Procedures. The coal pitch sample (CPS, 1 g) and 75 mL of 30% hydrogen peroxide solution were mixed and shocked by an ultrasonic oscillator for 15 min. The reaction mixture was placed into a three-neck flask equipped with a magnetic stirrer, a thermometer, and a reflux condenser. The reaction system was heated to 60 °C for 30 min and an additional 25 mL of hydrogen peroxide solution was added in the reaction process. The above operation was repeated once again. Then, the mixture was kept at 60 °C and stirred for 6 h in a water bath. After the reaction, the product mixture was separated by filtration to afford filtrate (F) and filter cake (FC). The FC was washed with distilled water at least 3 times. The final sediment, i.e., the oxidized coal pitch (OCP), was dried to constant weight in vacuum under room temperature and analyzed by using FTIR techniques. The F was extracted with benzene. The extraction solutions (ES) were dried over anhydrous MgSO4. After removal of the solvents from the dried solution with a rotary evaporator under reduced pressure, the products were esterified with CH2N2 to enable the corresponding methyl esterified products (MEPs) to be analyzed with GC/MS and FTIR. The experimental procedure is shown in Figure 1. The properties of CPS and OCP are shown in Table 1. As can be seen from Table 1, the significant increase of oxygen percentage and O/C ratio after oxidation was observed, indicating the
X = (m0 − m1)/m0
(1)
where m0 and m1 are the weights of CPS and OCP, respectively. 2.3.2. FTIR Analysis. The raw coal pitch and oxidized samples were analyzed with a Nicolet iS10 FTIR spectrometer. KBr pellets were prepared by grinding about 1 mg of dried sample with 100 mg of KBr. IR spectra of the sample in the 4000−400 cm−1 region were studied. The accuracy was 0.01 cm−1 and the frequency was 32 times per second. 2.3.3. GC/MS Analysis. GC/MS analysis was performed on an Agilent 7890/5975C which was equipped with a DB-35MS capillary column and a quadrupole mass analyzer with a flow velocity of 1.0 mL/s and a diffluent rate of 20:1, operated in electron impact (70 eV) mode. The mass scanning range was from 30 to 500 amu. in an electron bombarding voltage of 70 eV. The column was heated at a raising rate of 10 °C/min from 100 to 300 °C, and then kept at 300 °C until there was no peak flowing out on the GC. The data were acquired and processed using software of GC/MS, and the compounds were identified by comparing the mass spectra with NIST08 spectral library data and referring to the available reference.
3. RESULTS AND DISCUSSION 3.1. Influence of Temperature. In order to study the influence of reaction temperature on oxidation, CPS was oxidized with 30% aqueous hydrogen peroxide at 30, 40, 50, 60, and 70 °C for 6 h. X of the oxidation at different temperatures are listed in Table 2. The results showed that X increased and then decreased with the increase of reaction temperature. When the temperature increased from 30 to 60 °C, X increased gradually. The maximum of X was 59.06% at 60 °C. It indicated that, with the increase of temperature, the intensity of oxidative depolymerization became larger. After that, X was declining. This could be attributed to the partial decomposition of hydrogen peroxide with the increase of oxidation temperature. Therefore, the optimum reaction temperature was 60 °C. 3.2. Influence of Reaction Time. In order to study the effect of reaction time on oxidation, CPS was oxidized with 30% aqueous hydrogen peroxide at 60 °C for 2, 3, 4, 5, 6, and 7 h. X of the oxidation are shown in Table 3. Clearly X increased with increasing reaction time up to a certain level, similar to that increase of X with the temperature. Before 6 h, X gradually increased from 43.78% to 59.06%. The maximum of X was 59.06% at 60 °C for 6 h. Then, X decreased with the reaction time rising. Therefore, the proper reaction time was 6 h. In view of the above results, CPS was oxidized at 60 °C for 6 h in the following discussions. 3.3. FTIR Analysis of Coal Pitch. Figure 2 shows the IR spectra of CPS and OCP. In the FTIR spectrum of CPS, some characteristic absorbances are observed, i.e., the aromatic ring CC structures at around 1600 cm−1, the aromatic C-H stretching vibration at 3040 cm−1, and adjacent hydrogen bending vibrations at 876, 810, and 742 cm−1. The presence of polycyclic aromatic hydrocarbons in CPS is implied from these
Figure 1. Procedure for oxidation of CPS with H2O2, subsequent treatments, and products analysis. B
DOI: 10.1021/acs.energyfuels.7b02010 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Properties of Coal Pitch coal pitch
C (wt %)
H (wt %)
O (wt %)
N (wt %)
S (wt %)
O/C
CPS OCP
93.37 90.29
4.356 4.043
0.525 4.081
1.04 0.97
0.709 0.616
0.0056 0.0452
absorption were observed in the FTIR spectrum of OCP. For the OCP sample, first, the CO stretching vibration of carboxyl functional groups at 1707 cm−1 and the C-O-C stretching vibration of ethers at 1028 cm−1 can be obtained. Second, the aromatic CC structures at around 1600 cm−1, the aromatic C-H stretching vibration at 3040 cm−1, and adjacent hydrogen bending vibrations at 742 cm−1 became weaker. This reveals that the aromatic ring skeleton still constructs the main structure of OCP, while the macromolecular structures of CPS were oxidized by H2O2 and consequently converted to oxygen-containing functional groups of some new substances. 3.4. GC/MS Analysis. Figure 3 shows the total ion chromatogram (TIC) of the MEPs. The corresponding compounds detected are listed in Table 4. According to the
Table 2. Oxygen Conversion Rate (X) in Different Temperatures temp. (°C)
m0 (g)
m1 (g)
X (%)
30 40 50 60 70
1.0380 1.0304 1.0983 1.0279 1.0309
0.7780 0.7396 0.6017 0.4208 0.5955
25.05 28.22 45.21 59.06 42.23
Table 3. X of Different Reaction Times time (h)
m0 (g)
m1 (g)
X (%)
2 3 4 5 6 7
1.0281 1.0142 1.053 1.0144 1.0279 1.0093
0.5780 0.5690 0.5838 0.5616 0.4208 0.6652
43.78 43.90 44.56 44.64 59.06 34.09
Table 4. Compounds Detected in the MEPs no.
compound
no.
compound
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
butyric acid pentanoic acid dimethylbenzene isobutyric acid dimethylbenzene butyrolactone dimethylbenzene ethylbenzene trimethylbenzene succinic acid glutaric acid methylethylbenzene diethyl succinate diethyl glutarate diethyl adipate
16 17 18 19 20 21 22 23 24 25 26 27 28 29
benzene dicarboxylic acid benzene tricarboxylic acid benzene tricarboxylic acid benzene tetracarboxylic acid benzene tetracarboxylic acid benzene pentacarboxylic acid tricosanoic acid benzene hexacarboxylic acid tetracosanoic acid pentacosanoic acid hexacosanic acid trimethyltetracarboxylic acid octacosanoic acid hexacosanedioic acid
results by GC/MS analysis, a total of 29 compounds were detected in the products from MEPs of coal pitch. The organic compounds could be largely grouped into alkanoic acids (AAs), aliphatic carboxylic acids (APCAs), benzene polycarboxylic acids (BPCAs), and a small amount of alkylbenzenes (ABs). The detected AAs and APCAs included two categories. One was small molecule acid from four to six carbons, i.e., butyric
Figure 2. FTIR spectra of CPS and OCP.
characteristic absorbance. In addition, the absorption of oxygen-containing functional groups cannot be revealed in the FTIR spectrum of CPS. In contrast, significant changes in
Figure 3. TIC of MEPs. C
DOI: 10.1021/acs.energyfuels.7b02010 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 4. Possible structures of BPCAs.
Figure 5. Oxidation process of coal pitch by H2O2.
macromolecular polycyclic aromatic system was destroyed and converted to some small molecule oxygen-containing compounds. The detected ABs included 6 compounds, which were C8 and C9 aromatics. The existence of C8 and C9 aromatics illustrated that coal pitch was oxidized acutely in hydrogen peroxide solution, which broke the condensed structure of aromatic compounds of coal pitch. Different bond scission of the coal pitch structure resulted in the production of small molecules ABs, AAs, APCAs, and BPCAs. As can be seen from Figure 3, AAs were the most abundant, followed by BPCAs and APCAs. AAs produced by deep oxidation of coal pitch; BPCAs may be the result of moderate oxidation. However, it was unexpected to find APCAs in our investigation. We expect to carry out another research on this
acid, pentanoic acid, succinic acid, glutaric acid, and a little of their ester. Ten compounds listed in Table 4 were classified to this small molecule acid category. Meanwhile, 6 compounds were subjected to the other category, namely, long-chain aliphatic carboxylic acid which ranged from tricosanoic acid (C23) to octacosanoic acid (C28). All of these results suggested that the polycyclic aromatic hydrocarbons of coal pitch were partly depolymerized by H2O2 and converted into some small acids. The detected BPCAs contained 7 compounds including a benzene dicarboxylic acid, 2 benzene tricarboxylic acids, 2 benzene tetracarboxylic acids, a benzene pentacarboxylic acid, and a benzene hexacarboxylic acid. The possible structures are shown in Figure 4.3 The results indicated that MEPs of coal pitch were abundant in polycyclic aromatic structures. BPCAs were generated by selective oxidation of coal pitch. The D
DOI: 10.1021/acs.energyfuels.7b02010 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
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subject. Thus, the hydrogen peroxide oxidation of coal pitch could depolymerize the condensed ring and mainly produce a large number of small molecular carboxylic acids AAs, APCAs, and BPCAs. The dominant splitting mainly displayed in the generation of monocyclic BPCAs and small molecule AAs. It could be considered as a promising alternative to obtain corresponding aromatic carboxylic acids by the oxidation of coal pitch with H2O2. The possible oxidation process could be expressed in Figure 5.
4. CONCLUSIONS The oxidation of coal pitch by H2O2 was conducted, and subsequently, the oxidation products were analyzed by FTIR and GC/MS in this study. Furthermore, the possible oxidation process was speculated. The main conclusions are as follows: (1) The maximum of oxygen conversion rate was 59.06% at the optimum reaction conditions of 60 °C and 6 h. (2) FTIR analysis of the CPS and OCP showed that the macromolecular structures of CPS were depolymerized by H2O2 and converted to some new substances with oxygen-containing functional groups. GC/MS analysis of liquid product showed that the detected organic compounds mainly consisted of AAs, APCAs, BPCAs, and a small amount of ABs. The dominant splitting mainly displayed in the generation of small molecule BPCAs and AAs. (3) The coal pitch was oxidized acutely in hydrogen peroxide solution, which broke the condensed structure of aromatic compounds and produced small molecules ABs, AAs, APCAs, and BPCAs. This can be a potential approach to prepare some small molecular AAs or BPCAs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86-375-2089043. ORCID
Yao-Ling Wang: 0000-0001-8053-8895 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge funding support from the Key Project of Science & Technology of Universities of Henan Province (Grant No. 14A530003), and the Natural Science Foundation of Henan Province of China (Grant No. 152102310090). The authors are also grateful to Dr. Lei Bai of West Virginia University for his help of polishing the expressions and valuable suggestions.
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DOI: 10.1021/acs.energyfuels.7b02010 Energy Fuels XXXX, XXX, XXX−XXX
