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Article

Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization

by
Yugao Wang
1,2,*,
Xiaochen Liu
2,
Zhilei Wang
2,
Chuan Dong
1,
Jun Shen
2 and
Xing Fan
3,*
1
Institute of Environmental Science, Shanxi University, Taiyuan 030006, China
2
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
3
College of Chemical and Biological Engineering, Shandong University of Science and Technology, Qingdao 266590, China
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(1), 32; https://doi.org/10.3390/en15010032
Submission received: 18 November 2021 / Revised: 13 December 2021 / Accepted: 15 December 2021 / Published: 21 December 2021
(This article belongs to the Special Issue Low-Grade Coal Beneficiation)
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Abstract

:
Oxidative depolymerization of low-rank coals is promising for obtaining benzene carboxylic acids (BCAs). However, it is hindered by the low yield of BCAs along with a large number of alphatic acids. Thermal dissolution could modify the physico-chemical structural features of low-rank coals, which is expected to improve the oxidation of LRCs. In this paper, lignite and subbituminous coal were firstly subjected to thermal dissolution with cyclohexane at 250 °C for 2 h. Then, the raw coal and the corresponding thermal insoluble portion (TIP) were oxidized by NaOCl under the same conditions. The residual yields of TIPs oxidation were both lower than those of raw coals oxidation, indicating that TIPs were more easily oxidized than the raw coals. The yield of BCAs obtained by TIPs oxidation was above 19% higher than that from the oxidation of raw coals. Meanwhile, the selectivity of BCAs was improved in the resulting oxidation products from TIPs compared with that from the raw coals. The relationship between BCAs generation and thermal dissolution of low rank coals was investigated by ultimate analysis, Fourier transform infrared spectroscopy, and nitrogen adsorption-desorption analysis. The results suggested that thermal dissolution could enrich aromatic portion in the remaining TIPs, resulting in an increasing of the yield and selectivity of BCAs. Simultaneously, thermal dissolution raised the specific surface area and expanded the looser space structure of TIPS, which were beneficial for the sufficient collision between aromatic structures and oxidant, facilitating the oxidative depolymerization of TIPs. This investigation would provide a novel route for promoting BCAs production by mild oxidative depolymerization of low-rank coals.

Graphical Abstract

1. Introduction

China is relatively rich in low-rank coals (LRCs) such as lignites and subbituminous coals, whose reserves account for about 50% of total coal reserves in China [1,2]. However, LRCs are characterized by high oxygen content, high moisture content, and low calorific value, which seriously affect combustion, coking, and liquefaction of LRCs, leading to poor utilization level of LRCs [3,4]. LRCs inherently contain abundant aliphatic structures and oxygen-containing functional groups, which are helpful for their oxidation. Therefore, it is promising for producing carboxylic acids by oxidation of LRCs [5,6,7]. Benzene carboxylic acids (BCAs) are the fundamental raw materials for developing novel chemical materials and high value-added fine chemical products [8,9]. As a typical representation, terephthalic acid is an important monomer for polyethylene terephthalate, whose worldwide demand exceeds 12.6 million tons [10]. At present, BCAs are mainly synthesized through a series of chemical reactions of petroleum-based materials. However, the shortage of petroleum resources in China limits the large-scale production of BCAs [11]. Thus, it is of great significance to produce BCAs with abundant LRCs in China.
Mild oxidation could depolymerize organic macromolecules in coals to hydrosoluble carboxylic acids, which could be used not only to study the structures of coals, but also to potentially obtain fine chemicals from coals [12]. Wu et al. [11,13] systematically studied the oxidative depolymerization of lignites and find that the yield of BCAs could be up to 20%. This fact powerfully indicates that lignite could be oxidized to obtain BCAs. There are many oxidants such as O2 [14], H2O2 [15,16], RuO4 [17,18] and NaOCl [19,20], among which NaOCl is widely adopted in the oxidative depolymerization of LRCs to prepare carboxylic acids due to its low cost, good reactivity and reusable electrolysis [21,22]. Shengli lignite [23] and Shenfu Subbituminous [24] were oxidized by NaOCl, respectively, and BCAs were detected in their resulting oxidation products. However, the yield of BCAs is low and a large number of alphatic acids with small molecules coexist in the water-soluble portion obtained by LRC oxidation, which are different from the resulting products from bituminous coal and anthracite oxidation. It is a huge challenge for the subsequent separation of BCAs [25]. BCAs are derived from oxidation of aromatic moieties in coals. With increasing coal ranks from lignite to anthracite, the yield of BCAs and the selectivity of BCAs gradually increase. Therefore, enhancing the aromatic portions in LRCs through pretreatments would facilitate the generation of BCAs [6,7].
Thermal dissolution could extract soluble small molecules mainly including aliphatic portions from coals by breaking the weak covalent and non-covalent bonds between molecules in a given solvent via heating [26,27]. Wei et al. [28,29,30,31,32] achieve a lot of work on thermal dissolution of LRCs, and systematically investigate the composition and structure of LRCs at the molecular level. After thermal dissolution of LRCs, the remained thermal insoluble portion enriches aromatic macromolecular structures, which is expected to improve the yield and selectivity of BCAs in oxidation products.
In this paper, the effect of thermal dissolution on oxidation of LRCs to prepare BCAs was studied, whilst the intrinsic relationship between the thermal pretreatment of LRCs and the yield and selectivity of BCAs were investigated by ultimate analysis, Fourier transform infrared (FTIR) spectroscopy, and nitrogen adsorption-desorption analysis.

2. Materials and Methods

2.1. Materials

Coal samples were collected from Zhaotong lignite (ZT) in Yunnan Province, China and Yining subbituminous coal (YN) in Xinjiang Autonomous Region, China. The two samples were pulverized to pass through a 100-mesh sieve (particle size <0.15 mm) followed by desiccation in a vacuum oven at 80 °C for 24 h before use. NaOCl solution (available chlorine 10%) and hydrochloric acid (36 wt%) used in the experiment were analytically pure.

2.2. Experimental Method

As shown in Figure 1, about 4 g coal sample and 30 mL cyclohexane were put into a into a 200 mL stainless-steel, magnetically stirred autoclave. The air inside the autoclave was purged with nitrogen. Then, the autoclave was heated to 250 °C and maintained at the temperature for 2 h. When the reaction was over, the autoclave was cooled down to room temperature in a water bath, and the reaction mixture was taken out from the autoclave and filtrated to get filtrate and filter cake. The filter cake was exhaustively extracted with cyclohexane in a Soxhlet extractor to afford extraction solution and thermal insoluble portion, which were labeled RZT and RYN, respectively. ZT and RZT, YN and RYN were analyzed by ultimate analysis, FTIR, and nitrogen adsorption-desorption analysis.
In order to improve the heating efficiency and the uniformity of temperature distribution, the oxidation of samples was carried out in a microwave reactor. Approximately 0.5 g ZT or RZT and 20 mL NaOCl solution were added to a 250 mL spherical flask and fully mixed by magnetical agitation at 50 °C for 3 h, while 0.5 g YN or RYN and 35 mL NaOCl solution were added to a 250 mL spherical flask and fully mixed by magnetical agitation at 70 °C for 10 h due to the inferior reactivity of subbituminous coal. As displayed in Figure 2, the reaction mixture was filtrated through a 0.45 µm membrane filter to afford Filtrate 1 and filter cake (i.e., oxidation residue). The oxidation residue was dried in a vacuum oven before weighing. Then, the lost weight could be obtained by the difference between the used sample and oxidation residue, and oxidation weightlessness rate was calculated by dividing the mass of the used sample with the lost weight. Filtrate 1 was acidified with aqueous HCl solution to pH < 2 for converting –COONa to –COOH and then separated into Filtrate 2, which was analyzed by high performance liquid chromatograph (HPLC) for quantitatively determining BCAs and alphatic acids mainly including formic acid, acetic acid and oxalic acid. The selectivity of BCAs in the resulting Filtrate 2 was determined by dividing the total yield of BCAs and alphatic acids with the yield of BCAs (i.e., SelectivityBCAs = YieldBCAs/(YieldBCAs + Yieldalphatic acids).

2.3. Analytical Method

BCAs in Filtrate 2 from oxidation of ZT, RZT, YN, and RYN were analyzed using HPLC according to the previous report [6]. Besides, Filtrate 2 was analyzed by an Agilent 1220 HPLC equipped with a ZORBAX SB-C18 column (5.0 um × 4.6 mm × 250 mm) for quantitatively analyses of formic acid, acetic acid and oxalic acid. A binary gradient elution procedure was used. The mobile phase flow rate was 1 mL·min−1, and the column temperature was at 40 °C. The mobile phase was comprised of methanol and 20 mmol·L−1 potassium dihydrogen phosphate buffer solution. Firstly, the volume ratio of methanol to aqueous buffer solution was 5:95, then linearly increased to 20:80 within 10 min and hold for 2 min, finally decreased to 5:95 over 2 min.
The ultimate analyses of ZT, RZT, YN, and RYN were carried out by Elementar UNICUBE® elemental analyzer. CHNS model was selected for determining the carbon, hydrogen, nitrogen, and sulfur content in samples, while the oxygen content was calculated by difference.
Nitrogen adsorption-desorption analyses of ZT, RZT, YN, and RYN were performed with a Micromeritics® ASAP 2460 gas sorption analyzer, in which using N2 as the adsorbate at −196 °C. Prior to the test, the prepared samples were degassed under vacuum at 110°C for 12 h. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method, the total pore volume was determined as the volume of liquid nitrogen adsorbed at a relative pressure of 0.99, while the pore size distribution was determined from the nitrogen adsorption isotherm by the Barrett-Joyner-Halenda method [33].
FTIR analyses of ZT, RZT, YN, and RYN were recorded with a Shimadzu IRAFFINITY-1 spectrometer using KBr pellet technology by collecting 50 scans at a resolution of 4 cm−1 in reflectance mode with a measuring region of 4000–400 cm−1. FTIR spectra were processed with Omnic software.

3. Results

ZT and YN were subjected to thermal dissolution to afford thermal insoluble portions (i.e., RZT and RYN) with macromolecular structures. The yield of soluble portion from ZT was 17.39%, while the one of YN was only 5.03%, suggesting more components were taken away from ZT than YN. As displayed in Table 1, ZT contained less carbon and C/H value but more oxygen than YN. The fact indicated that compared with YN, ZT had low coalification degree and rich alphatic structures, which could account for the higher yield of RYN than that of RZT. Aqueous NaOCl solution possessed strong oxidation ability, and it would be reduced to NaCl in the reaction process, which would cause less environmental pollution. Therefore, NaOCl was widely used in coal oxidation. Herein, raw coals and thermal insoluble portions were oxidized by aqueous NaOCl solution to investigate the effect of thermal dissolution on oxidation of LRCs.

3.1. Oxidation of LRCs and its Thermal Insoluble Portions

As Table 2 shows, the oxidative weightlessness rate of RZT was more than that of ZT under the same condition, indicating that the oxidative reactivity of RZT was higher than the one of ZT [34]. The fact suggested thermal dissolution pretreatment could improve the reactivity of LRCs, which was also proved by the result of higher oxidative weightlessness rate of RYN than that of YN. The yield of BCAs obtained by ZT oxidation was 10.86 mg·g−1, whereas the one of BCAs obtained by RZT oxidation was 13.44 mg·g−1. Similarly, the yield of BCAs from RYN oxidation was 19.01% more than the one from YN oxidation. These facts strongly evidenced the yield of BCAs from oxidation of LRCs could be greatly improved by thermal dissolution pretreatment. Different from the yield of BCAs, the yield of alphatic acids decreased after thermal dissolution pretreatment of ZT and YN. Therefore, the selectivity of BCAs obviously raised in the resulting products from oxidation of thermal insoluble portions compared with raw LRCs. It would be promising that BCAs were dominant in the oxidation products by further optimizing the thermal dissolution pretreatments of LRCs, which would greatly contribute to the subsequent fine separation of BCAs.
Comparing the oxidation of ZT and YN, lignite was easily oxidized, but the yield of BCAs obtained by oxidation was low, which could be due to its rich alphatic structures, small aromatic ring structures, and rich oxygen-containing functional groups. With increasement of coal rank, the condensed aromatic structures of sub-bituminous coal significantly increased, which led to that the yield of BCAs obtained by oxidation was high. However, the oxidability was low. The organic structures of LRCs could be modified via thermal dissolution pretreatment, resulting in that the yield of BCAs could be effectively increased as long as the oxidative reactivity. This investigation would provide a new idea for the mild oxidation of LRCs to prepare BCAs.
In order to accurately evaluate the effect of thermal dissolution pretreatment on oxidation of LCRs to prepare BCAs, BCAs distribution was investigated in the resulting oxidation products. As exhibited in Figure 3, benzene pentacarbonic acid dominated both in the resulting BCAs from oxidation of raw coals and their corresponding thermal insoluble portions. Moreover, the yield of benzene dicarboxylic acid, benzene tricarbonic acid, and mellitic acid grew more obviously than other BCAs from ZT oxidation to RZT oxidation as summarized in Figure 4. However, the yield of most BCAs except terephthalic acid increased with the similar extent from YN oxidation to RYN oxidation. Generally, the carboxyl group of BCAs could be generated by oxidative opening of benzene rings and chains attached to the aromatic clusters in LRCs, thus, which could reflect some information on aromatic clusters and the attached chain types to some extent. As mentioned above, the more yield of soluble portion from ZT than that from YN by pretreatment demonstrated that thermal dissolution more significantly affected the organic structures of ZT rather than extract soluble portion. It could be inferred that a large amount of thermal-instable bridged linkages connected to aromatic cluster in ZT were broken during thermal dissolution, which would afford plenty of aliphatic chains or aliphatic chains with oxygen-containing groups directly attached to aromatic cluster. The fact would result in varying degrees of increment for different BCAs from ZT oxidation to RZT oxidation. Nevertheless, YN had a relatively high degree of coalification, in which the number of thermal-instable bridged linkages was relatively low. Therefore, the growing yield of BCAs mainly could be attributed to the enriching aromatic portion for RYN oxidation.

3.2. Investigation on the Cause of Thermal Dissolution Improving Oxidation of LRCs

Modern direct characterization instruments could quickly and accurately reflect structural information of complex samples. Herein, ultimate analysis, FTIR and nitrogen adsorption-desorption analysis were used to afford structural features of raw LRCs and their thermal insoluble portions, aiming at gaining insight into the internal correlation between thermal dissolution pretreatment and the generation of BCAs.
As summarized in Table 1, the content of carbon, hydrogen, nitrogen, oxygen, and sulfur elements in samples were determined, based on which C/H and aromaticity (fa) were obtained. The content of carbon increased, while the one of oxygen decreased after thermal dissolution pretreatment for ZT and YN. Besides, the values of C/H and fa in RZT and RYN were both higher than those in ZT and YN, respectively. It could be inferred that the thermal dissolution could take away the low-carbon soluble structures, which could be oxidized into alphatic acids, whereas the remained thermal insoluble portions enriched the macromolecular aromatic structures, which could be oxidized to generate more BCAs.
FTIR could directly analyze the organic structures of complex mixture samples and obtain the overall structure information of the sample. As Figure 5 displays, the peaks at 2918 and 2855 cm−1 corresponded to the vibration of alphatic hydrogen, and the peak around 1595 cm−1 could be attributed to the vibration of aromatic C=C, which were similar for ZT and YN. While, C-O vibration peak at 1030 cm−1 was greatly stronger in ZT than YN. Besides, the vibration absorbance of aromatic hydrogen could be detected in the range of 720–860 cm−1, and their absorbances were weak, suggesting that aromaticity was low for the two LRCs. Notably, it seemed to be visually similar for FTIR spectra of these two LRCs before and after thermal dissolution, especially for aromatic C=C vibration absorbance. The fact might imply that thermal dissolution would not break the macromolecular aromatic structures of LRCs. However, the change of aromaticity of LRCs before and after thermal dissolution could be clearly exhibited using the value of index Ib. The index was obtained by quantificationally analyzing the ratio of the peak area of aromatic hydrogen to the peak area of aliphatic hydrogen [35,36] (i.e., Ib = Aaromatic hydrogen/Aaliphatic hydrogen), whose higher value reflected a higher aromaticity.
FTIR analyses were consistent with those of ultimate analyses. Compared with the corresponding raw LRCs, the values of C/H, fa, and Ib of RZT and RYN all increased as exhibited in Figure 6. This fact powerfully indicated that the aromaticity of the thermal insoluble portions was higher than that of raw coals, which would facilitate generating more BCAs by oxidation.
As exhibited in Figure 7a, the adsorption curve nearly overlapped desorption one for ZT, YN, and RYN, and their small hysteresis loops implied that closed and semi-closed pores existed in these samples. Conversely, there was a significant drop in the desorption curve of RZT when P/P0 was approximately equal to 0.5, and its large hysteresis loop illustrated that cylindrical pores and slit-shaped pores with all open sides were in RYN [33]. The difference of adsorption/desorption isotherm for ZT and RZT suggested thermal dissolution really markedly modified the structures of ZT.
Pore size distribution and the specific surface area were the important parameters demonstrating some information about pore structures of coal [37,38]. RZT possessed more pores with diameter between 2–100 nm than ZT, especially for the pores ranging from 2 to 10 nm, as displayed in Figure 7b. However, the mesopore size distribution were similar to each other for YN and RYN except for pores with 2–3 nm. Based on porous texture parameters calculated in Table 3, the specific surface area of RZT was expanded by nearly three times for ZT and the pore volume was expanded by nearly two times. However, the specific surface area of RYN was slightly increased, and the pore volume was basically unchanged. The different changes of porous features might result from the different thermal dissolution effects for ZT and YN. As mentioned above, the oxidative reactivity of LRCs was improved. The fact could be attributed to the increasing specific surface area and loosing spatial structure after thermal dissolution of LRCs, which would be helpful for the full interaction between the oxidant and aromatic structures [39].

4. Conclusions

The yield of BCAs significantly increased but the one of alphatic acids obviously decreased in the resulting oxidation products under the same conditions for the two dissolution insoluble portions compared with the corresponding raw LRCs, which would contribute to producing BCAs from oxidation of LRCs and subsequent fine separation of BCAs. Meanwhile, the oxidative reactivity of LRCs was improved after thermal dissolution pretreatment. Based on ultimate analysis, FTIR, and nitrogen adsorption-desorption analysis, we determined the intrinsic reason for the directional improvement of BCAs yield by thermal pretreatment. The increased yield and selectivity of BCAs were mainly attributed to enriching aromatic structures in dissolution insoluble portions. Moreover, the increasing specific surface area and expanding looser space structure could account for the improved oxidative reactivity.

Author Contributions

Conceptualization, C.D. and X.F.; data curation, Z.W. and X.L.; funding acquisition, Y.W. and J.S.; investigation, Y.W. and Z.W.; writing—original draft, Y.W.; writing—review and editing, J.S., C.D. and X.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Key Research and Development (R&D) Projects of Shanxi Province (201903D321061), State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2021-K79) and the NSFC-Shanxi Joint Fund for Coal-based Low Carbon (U1710102).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Thermal dissolution of ZT and YN in cyclohexane.
Figure 1. Thermal dissolution of ZT and YN in cyclohexane.
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Figure 2. Samples oxidation with NaOCl.
Figure 2. Samples oxidation with NaOCl.
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Figure 3. Yield of BCAs in resulting products from ZT and RZT oxidation (a) and YN and RYN oxidation (b).
Figure 3. Yield of BCAs in resulting products from ZT and RZT oxidation (a) and YN and RYN oxidation (b).
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Figure 4. Distribution of BCAs in resulting products from oxidation of coal samples.
Figure 4. Distribution of BCAs in resulting products from oxidation of coal samples.
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Figure 5. FTIR spectra of coal samples.
Figure 5. FTIR spectra of coal samples.
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Figure 6. Organic structural parameters of coal samples.
Figure 6. Organic structural parameters of coal samples.
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Figure 7. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) of coal samples.
Figure 7. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) of coal samples.
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Table
Table 1. Ultimate analysis of each component in ZT and RZT, YN and RYN a.
Table 1. Ultimate analysis of each component in ZT and RZT, YN and RYN a.
SamplesUltimate Analysis (wt%, daf)C/Hfa
CHNOdiffS
ZT57.575.711.7334.300.690.840.47
RZT65.765.942.1625.530.610.920.52
YN71.314.210.7323.380.371.410.74
RYN81.014.260.8613.250.621.590.78
a daf, dried ash-free base; diff, by difference; fa = 1.132–0.560 × [(H/1.0008)/(C/12.01)].
Table
Table 2. The results from oxidation of ZT, RZT, YN, and RYN a.
Table 2. The results from oxidation of ZT, RZT, YN, and RYN a.
SamplesOxidative Weightlessness Rate (wt%)Yield of BCAs (mg/g)Yield of Alphatic Acids (mg/g)Selectivity of BCAs (wt%)
ZT54.8810.86286.103.66
RZT58.9813.44266.394.80
YN70.6564.46369.1214.86
RYN77.8076.71320.5419.31
a Reaction conditions for ZT and RZT: 0.5 g sample, reaction temperature 50 °C, reaction time 3 h, 20 mL NaOCl solution; Reaction conditions for YN and RYN: 0.5 g sample, reaction temperature 70 °C, reaction time 10 h, 35 mL NaOCl solution.
Table
Table 3. Porous texture parameters of coal samples.
Table 3. Porous texture parameters of coal samples.
SamplesSpecific Surface Area (m2/g)Total Pore Volume (cm3/g)
ZT2.480.015
RZT6.460.029
YN2.130.0089
RYN2.350.0086
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Wang, Y.; Liu, X.; Wang, Z.; Dong, C.; Shen, J.; Fan, X. Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization. Energies 2022, 15, 32. https://doi.org/10.3390/en15010032

AMA Style

Wang Y, Liu X, Wang Z, Dong C, Shen J, Fan X. Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization. Energies. 2022; 15(1):32. https://doi.org/10.3390/en15010032

Chicago/Turabian Style

Wang, Yugao, Xiaochen Liu, Zhilei Wang, Chuan Dong, Jun Shen, and Xing Fan. 2022. "Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization" Energies 15, no. 1: 32. https://doi.org/10.3390/en15010032

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