# Conversion of Organic Matter in Coal by Photocatalytic Oxidation with H2O2 over SFC/TiO2 in Isolated Oxygen System

Authors: Heng-Shen Xie; Zhi-Min Zong; Yu-Gao Wang; Pei-Zhi Zhao; Shi-Hua Zhang; Jun-Qiang Wang; Hong Fu; Shuang-Hua Liang; Ling Su; Xian-Yong Wei
Journal: Journal of Chemistry
 (2019)

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## Abstract

Shenfu bituminous coal (SFBC), Geting coal (GTC), Shengli lignite (SLL), and Holingola coal (HLGLC) were oxidized by UV light radiation with aqueous H2O2 over SFC/TiO2 in a closed suspension system (CSS) to understand structural characteristics of 4 typical Chinese coals. Raw and oxidized coals were dried and extracted with acetone thoroughly to ensure residue extraction. Meanwhile, the extracts were analyzed using a gas chromatograph/mass spectrometer. The results show that organic matters (OMs) in coals can be converted into a large number of oxygen-containing organic compounds (OCOCs), mainly containing ketones, esters, alcohols, etc. Oxidizing species such as hydroxyls, hydroperoxyl, and alkyl radicals are excited by light irradiation and substitute for hydrogen atoms of methyls and methylenes of acenes or branched-chain alkanes in coals. Acetic acid and acetaldehyde can be formed and dissolved in aqueous solution in the oxidation reaction. The yields can be improved with the enhancement of the oxidation effect.


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## Body

## 1. Introduction

In understanding the composition and structure of organic matters (OMs) in coals and in using coals effectively, the mild oxidation and the separable and nondestructive analysis method of coals at the molecular level coals have increasingly been the focus of scientific study over the past decades [1–13]. Meanwhile, the oxidation products of coals are usually viewed as an important source of high value-added products [14–19]. However, the existing oxidation methods are less satisfactory due to their high dosages, high costs, and harsh conditions. Photocatalytic oxidation (PCO), as one of the advanced oxidation processes, has attracted extensive attention since 1972 [20–27] because of its low temperature, high efficiency, and low selectivity. In recent years, the PCO of coals including solid-gas mixed systems (UV/O2/TiO2) and solid-gas-liquid heterogeneous systems (H2O/UV/air/TiO2) has received considerable attention [28–30]. Nevertheless, the component analysis of oxidized coals is often complicated by many kinds of oxidizing particles detected in the systems. Especially, previous studies have found that oxygen molecules can be stimulated by light to generate singlet or triplet oxygen, the main oxidizing substances, which can enhance the oxidation of OMs in coal and inhibit the recombination of electron hole and pairs due to the existence of the photoelectron capture center [31–33]. Meanwhile, the oxygen-related oxidation may increase the complexity of products and thus make the mechanism analysis of coal oxidation more difficult [34–37]. Hence, an isolated oxygen system was designed to oxidize four species coals, and results were analyzed by using gas chromatograph/mass spectrometer (GC/MS). In the system, the inducted OMs conversion rule proved that it is completely possible to realize coal mild oxidation effectively and utilize coal efficiently by controlling the species and quantities of reactants. In addition, the Fourier transform infrared spectroscopy analysis of coal PCO is scarcely noted in the literature.


## 2. Experiment

### 2.1. Samples and Reagents

Shenfu bituminous coal (SFBC), Geting coal (GTC), Shengli lignite (SLL), and Holingola coal (HLGLC) were, respectively, collected from Shaanxi Province, Shandong Province, Shaanxi Province, and Inner Mongolia Autonomous Region. Placed in a desiccator, all coals were pulverized to pass through a standard sieve of 0.074 mm. Their proximate and ultimate analyses are shown in Table1. Purchased as commercially available, analytical reagents acetone and ether were distilled in a Büchi R-134 rotary evaporator prior to use. The catalyst (SFC/TiO2) was a man-made nanometer powder [32]. The mass percentage of the commercially purchased H2O2 was 30%.Table 1
Proximate and ultimate analyses (wt.%) of coals.
Coal
Proximate analysis
Ultimate analysis (daf)
St,d (wt.%)
M
ad
A
d
V
daf
C
H
N
SFBC
10.21
6.50
37.70
80.51
4.82
0.90
0.40
GTC
8.51
5.71
35.00
82.95
5.47
1.13
1.00
SLL
20.07
11.35
46.77
38.99
2.78
0.19
1.00
HLGLC
17.31
23.45
43.42
42.35
2.68
0.43
0.57
daf: dry and ash-free base;Mad: moisture (air-dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); St,d: total sulfur (dry base).


### 2.2. Experimental Methods and Procedures

As shown in Figure1, dried coal (2 g) was mixed with the catalyst (0.01 g), and H2O2 (30 ml) was added dropwise to the 100 ml quartz tube to obtain a reaction mixture system. After 10 minutes of low-power ultrasonic dispersion, the mixture was illuminated with a 500 W low-pressure UV mercury lamp for 4 hours under magnetic stirring. Thereafter, the mixture was filtered in a sand core funnel in vacuum. Filtrate portions (FPs) were extracted exhaustively with ether to separate ether-soluble fractions (ESFs) and ether-insoluble fractions (EIFs). EIFs were extracted with acetone to gain acetone-soluble fraction (ASFIs). The weight of acetone-soluble fraction (WASFI) can be obtained by ASFIs volatile naturally. Filter cakes (FCs) were dried in vacuum at 50°C to 60°C for 2 hours to obtain drying filter cakes (DFCs) and weighted to get weight acetone-soluble fraction (WASFII). FCs were dispersed by using carbon disulfide for 2 minutes and extracted exhaustively with 200 ml acetone to obtain acetone-soluble fractions (ASFIIs). ESFs and ASFs were concentrated through rotary evaporation and analyzed with GC/MS. The extract yields (EYs) were marked as EY, EYI, EYII, EYIII, and EYIV corresponding to the blank sample and different oxidation systems.Figure 1
Procedure for coal oxidation in SSC.The experiment was completed through four systems, in which the same amount of H2O2 and coal but different amounts of catalyst were employed during different illumination hours. In the first oxidation system, 0.1 g of catalysts were used during 4 hours of irradiation to get the extracts of oxidized coals, marked, respectively, as ASFOSFBCI, ASFOGTCI, ASFOSLLI, and ASFOHLGLI. In the second oxidation system, 0.1 g of catalysts were used during 12 hours of illumination to get the extracts, labeled as ASFOSFBCII, ASFOGTCII, ASFOSLLII, and ASFOHLGLCII. In the third oxidation system, 0.05 g of catalysts were used during 12 hours of irradiation to get the extracts, labeled as ASFOSFBCIII, ASFOGTCIII, ASFOSLLIII, and ASFOHLGLCIII. In the fourth oxidation system, no catalyst was used and 4 hours of reaction time brought forth the extracts, marked as ASFOSFBCIV, ASFOGTCIV, ASFOSLLIV, and ASFOHLGLCIV.


### 2.3. GC/MS Analysis

GC/MS analysis was conducted with a Hewlett-Packard 6890/5973GC/MS, which was equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 30 m × 0.25 mm i.d., and 0.25μm film thickness) and a quadrupole analyzer in the electron impact mode at 70 eV. The injection and detector temperatures were set to 300°C. The column temperature was programmed to range from 100°C to 200°C at a rate of 15°C·min−1 for 5 minutes before being increased to 300°C at a rate of 8°C·min−1 for 15 minutes. Data were obtained and processed by the Chemstation software. The compounds were identified by comparing their mass spectra with the NIST05 library data. The residues were dried in vacuum for 10 hours and weighed to calculate the extraction yield by difference.


## 2.1. Samples and Reagents

Shenfu bituminous coal (SFBC), Geting coal (GTC), Shengli lignite (SLL), and Holingola coal (HLGLC) were, respectively, collected from Shaanxi Province, Shandong Province, Shaanxi Province, and Inner Mongolia Autonomous Region. Placed in a desiccator, all coals were pulverized to pass through a standard sieve of 0.074 mm. Their proximate and ultimate analyses are shown in Table1. Purchased as commercially available, analytical reagents acetone and ether were distilled in a Büchi R-134 rotary evaporator prior to use. The catalyst (SFC/TiO2) was a man-made nanometer powder [32]. The mass percentage of the commercially purchased H2O2 was 30%.Table 1
Proximate and ultimate analyses (wt.%) of coals.
Coal
Proximate analysis
Ultimate analysis (daf)
St,d (wt.%)
M
ad
A
d
V
daf
C
H
N
SFBC
10.21
6.50
37.70
80.51
4.82
0.90
0.40
GTC
8.51
5.71
35.00
82.95
5.47
1.13
1.00
SLL
20.07
11.35
46.77
38.99
2.78
0.19
1.00
HLGLC
17.31
23.45
43.42
42.35
2.68
0.43
0.57
daf: dry and ash-free base;Mad: moisture (air-dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); St,d: total sulfur (dry base).


## 2.2. Experimental Methods and Procedures

As shown in Figure1, dried coal (2 g) was mixed with the catalyst (0.01 g), and H2O2 (30 ml) was added dropwise to the 100 ml quartz tube to obtain a reaction mixture system. After 10 minutes of low-power ultrasonic dispersion, the mixture was illuminated with a 500 W low-pressure UV mercury lamp for 4 hours under magnetic stirring. Thereafter, the mixture was filtered in a sand core funnel in vacuum. Filtrate portions (FPs) were extracted exhaustively with ether to separate ether-soluble fractions (ESFs) and ether-insoluble fractions (EIFs). EIFs were extracted with acetone to gain acetone-soluble fraction (ASFIs). The weight of acetone-soluble fraction (WASFI) can be obtained by ASFIs volatile naturally. Filter cakes (FCs) were dried in vacuum at 50°C to 60°C for 2 hours to obtain drying filter cakes (DFCs) and weighted to get weight acetone-soluble fraction (WASFII). FCs were dispersed by using carbon disulfide for 2 minutes and extracted exhaustively with 200 ml acetone to obtain acetone-soluble fractions (ASFIIs). ESFs and ASFs were concentrated through rotary evaporation and analyzed with GC/MS. The extract yields (EYs) were marked as EY, EYI, EYII, EYIII, and EYIV corresponding to the blank sample and different oxidation systems.Figure 1
Procedure for coal oxidation in SSC.The experiment was completed through four systems, in which the same amount of H2O2 and coal but different amounts of catalyst were employed during different illumination hours. In the first oxidation system, 0.1 g of catalysts were used during 4 hours of irradiation to get the extracts of oxidized coals, marked, respectively, as ASFOSFBCI, ASFOGTCI, ASFOSLLI, and ASFOHLGLI. In the second oxidation system, 0.1 g of catalysts were used during 12 hours of illumination to get the extracts, labeled as ASFOSFBCII, ASFOGTCII, ASFOSLLII, and ASFOHLGLCII. In the third oxidation system, 0.05 g of catalysts were used during 12 hours of irradiation to get the extracts, labeled as ASFOSFBCIII, ASFOGTCIII, ASFOSLLIII, and ASFOHLGLCIII. In the fourth oxidation system, no catalyst was used and 4 hours of reaction time brought forth the extracts, marked as ASFOSFBCIV, ASFOGTCIV, ASFOSLLIV, and ASFOHLGLCIV.


## 2.3. GC/MS Analysis

GC/MS analysis was conducted with a Hewlett-Packard 6890/5973GC/MS, which was equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 30 m × 0.25 mm i.d., and 0.25μm film thickness) and a quadrupole analyzer in the electron impact mode at 70 eV. The injection and detector temperatures were set to 300°C. The column temperature was programmed to range from 100°C to 200°C at a rate of 15°C·min−1 for 5 minutes before being increased to 300°C at a rate of 8°C·min−1 for 15 minutes. Data were obtained and processed by the Chemstation software. The compounds were identified by comparing their mass spectra with the NIST05 library data. The residues were dried in vacuum for 10 hours and weighed to calculate the extraction yield by difference.


## 3. Results and Discussion

### 3.1. GC/MS Analysis of Oxidized Coal

#### 3.1.1. Effect Analysis of ASFSFBC and ASFOSFBC

SFBC and OSFBC were extracted with acetone and analyzed by GC/MS to understand OM hydrothermal conversion in coals in the process. Results show that no OMs were detected in ASFOSFBCII and ASFOSFBCIII, except in ASFOSFBCI and ASFOSFBCIV, indicating that ASFs can be oxidized completely during 12 hours of illumination despite the decreasing amount of photocatalyst from 0.1 g to 0.05 g. Thus, only the analysis results of ASFOSFBCI and ASFOSFBCIV are presented in Table 2. As listed in Table 2, the depolymerization of some part of the OMs is indicated by the result that 24 OMs were confirmed in ASFSFBC, but only 13 OMs in ASFOSFBCI and 9 OMs in ASFOSFBCIV. Oxygen atoms were introduced in the process, according to the result that the relative content (RC) of 7-isopropyl-1-methylphenanthrene (peak 76) is the highest in ASFSFBC, whereas the proportion of 2,6-dimethylhepta-2,5-dien-4-one is the highest in ASFOSFBCI and ASFOSFBCIV. According to the analysis, the oxygen atoms may have two sources: the compounds self-depolymerization in coal resulted from the cleavage of C–O and the oxidation of hydroxyl radicals. Among the 13 OMs in ASFOSFBCI, 1 branched alkane (BA), 1 alkylarene, 1 arenol, 4 alkanols, 4 ketones, and 2 nitrogen-containing compounds (NCOCs) were detected. By contrast, in ASFOSFBCIII, 2 NAs, 1 BA, 1 ketone, 1 NCOC, 1 alkanol, 1 ester, and 1 ketone were detected. Quantitative analysis verified that OMs have been converted partly in the process. Possible evidences are as follows: firstly, the RCs of NAs and the number of carbon atoms in long-chain alkanes decrease. 8 NAs in ASFSFBC but no NA in ASFOSFBCI and 2 NAs in ASFOSFBCIV were detected, and their RCs have been reduced from 25.20% to 5.00% before and after oxidation (as shown in Figure 2). It can be interpreted that almost all NAs have been converted successfully, and no more NAs may be formed despite the extension of illuminating time [31]. Secondly, the amount of branched-alkanes (BAs) in ASFOSFBCI and ASFOSFBCIV decreases significantly compared with those in ASFSFBC. As listed in Tables 2 and 3, BAs and 5 alkylarenes containing α-H atoms account for 31.50% in ASFSFBC after oxidation, only 1 BA and 1 alkylarene in ASFOSFBCI and 1 BA in ASFOSFBCIV are verified, and the RCs of the BAs and alkylarenes in ASFOSFBCI and in ASFOSFBCIV decrease to 4.80% and 1.80%, respectively. These findings show that the majority of BAs or alkylarenes have been oxidized through α-H atoms substitution of photoelectron and photogenic-free radicals. Thirdly, there is no change in the number of oxygen-containing organic compounds (OCOCs) in ASFOSFBCI and ASFOSFBCIV after the oxidation. However, their RCs increased significantly from 43.00% in ASFSFBC to 92.80% in ASFOSFBCI and 93.20% in ASFOSFBCIV. More importantly, 2,6-dimethylhepta-2,5-dien-4-one is detected both in ASFOSFBCI and in ASFOSFBCIV, and theirs RCs are the highest (accounting for 44.60% and 50.20%, respectively), which indicate that the formation of ketones is the main process of photooxidation conversion.Table 2
Organic compounds detected in AEFSFBC, AEFOSFBCI, and AEFOSFBCIV.
Peak
Parent compound
Detected in
ASFSFBC
ASFOSFBCI
ASFOSFBCIV
NAs
60
Nonacosane
2.0
64
Octacosane
2.0
71
Heneicosane
3.2
74
Tetracosane
5.6
78
Heptadecane
3.0
79
Tetratetracontane
1.8
2.1
80
Docosane
5.6
82
Tritetracontane
2.9
84
Tetratriacontane
2.0
BA
51
2,6,10,14-Tetramethyl-hexadecane
1.9
Branched cyclane
41
9-Dodecyl-tetradecahydro-anthracene
1.8
Alkenes
20
1-Methyl-5,6-divinylcyclohex-1-ene
3.6
26
4a,8,9,9-Tetramethyl-1,2,3,4,4a,5,6,8a-octahydro-1,4-methano-naphthalene
2.3
44
1,2-Dimethyl-cyclohexene
2.1
CA
58
Phthalic acid mono-(1-butyl-2-ethyl-hexyl) ester
6.8
Alkylarenes
43
1,1,2,3,3-Pentamethyl-indane
2.7
66
7-Butyl-1-hexyl-naphthalene
8.4
76
7-Isopropyl-1-methyl-phenanthrene
11.0
1.2
77
1-Propyl-4-(2-p-tolylethynyl)benzene
1.7
81
8-Isopropyl-1,3-dimethyl-phenanthrene
1.4
Alkanols
6
2-Methyl-hexan-2-ol
2.3
7
Octan-3-ol
1.7
13
3-Isopropenyl-2-methyl-cyclohexanol
2.5
16
Octanol
2.3
32
2-Isopropenyl-5-methyl-cyclohexanol
1.4
Arenols
19
2-tert-Butyl-4,6-dimethyl-phenol
2.0
Ketones
1
Cyclohexanone
14.8
9
2,6-Dimethyl-hepta-2,5-dien-4-one
44.6
50.2
11
3,5,5-Trimethyl-cyclohex-2-enone
2.7
18
1-(4,7,7-Trimethyl-bicyclo[4.1.0]hept-3-en-3-yl)-ethanone
5.1
28
4-(2,6,6-Trimethyl-cyclohex-1-enyl)-but-3-en-2-one
4.9
59
2,4-Dimethyl-hexan-3-one
3.2
Esters
4
Acetic acid 1,2-dimethyl-propyl ester
9.0
30
Acetic acid 17-(5-methoxy-1,5-dimethyl-4-oxo- hexyl)-4,4,13,14-tetramethyl-tetradecahydro-cyclopropa[9,10]cyclopenta[a]phenanthren-3-yl ester
2.1
72
Methoxy-acetic acid 1-methyl-tridecyl ester
2.6
NCOCs
5
N-(5-Oxo-2,5-dihydro-furan-2-yl)-acetamide
1.3
12
2,6-Dimethyl-6-nitro-hept-2-en-4-one
6.3
16.1
33.5
67
3-(2,4,6-Trimethyl-phenyl)-pyridine
2.0
68
7-Ethyl-2,4-dimethyl-10H-benzo[b][1,8]naphthyridin-5-one
4.5
CCOC
10
2-Chloro-4-isopropyl-1-methyl-benzene
1.2
OCOC
40
2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene
8.2Figure 2
Component analysis of ASFSFBC, ASFOSFBCI, and ASFOSFBCIV.Table 3
Organic compounds detected in ASFGTC and ASFOGTCI.
Peak
Parent compound
Detected in
ASFGTC
ASFOGTCI
NAs
46
Heneicosane
2.9
56
Heptadecane
3.9
1.9
57
Heptacosane
2.2
59
Tetratriacontane
3.7
Bas
29
2,6,10,14-Tetramethyl-pentadecane
2.9
Alkenes
20
(3-Methyl-penta-1,2,4-trienyl)-benzene
1.6
Alkylarenes
13
1-Methyl-naphthalene
2.2
16
2,3-Dimethyl-naphthalene
1.6
18
1,4-Dimethyl-naphthalene
3.1
19
2,6-Dimethyl-naphthalene
2.6
22
2,3,6-Trimethyl-naphthalene
1.6
1.2
23
4,6,8-Trimethyl-azulene
2.3
25
1,6,7-Trimethyl-naphthalene
3.5
26
1,5,7-Trimethyl-naphthalene
2.7
27
1,4,5-Trimethyl-naphthalene
5.6
30
4-Isopropyl-1,6-dimethyl-naphthalene
8.5
3.0
34
7-Isopropyl-1-methyl-naphthalene
2.5
35
7-Ethyl-1,4-dimethyl-azulene
4.2
44
2-Methyl-anthracene
7.3
52
2-Methyl-octadecane
2.6
54
1-Methyl-pyrene
1.5
55
2-Methyl-fluoranthene
2.7
NSAs
43
Naphtho[2,3-b]norbornadiene
1.3
49
Pyrene
1.9
51
Fluoranthene
6.3
2.4
Esters
1
3-Methyl-5H-furan-2-one
20.7
9
Acetic acidm-tolyl ester
1.8
10
Acetic acidp-tolyl ester
1.3
12
Acetic acid 2,3-dimethyl-phenyl ester
3.6
32
Acetic acid 3,4-diacetoxy-6,8-dioxa-bicyclo[3.2.1]oct-2-yl ester
3.9
60
Phthalic acid 1-(2,2-dimethyl-propyl)ester 2-(2-ethyl-hexyl) ester
1.1
Ketones
2
Hexane-2,5-dione
18.3
4
2-Methylcyclopent-2-enone
1.3
6
2,6-Dimethyl-hepta-2,5-dien-4-one
7.0
8.7
7
3,5,5-Trimethyl-cyclohex-2-enone
3.5
15
1-(3,5,5-Trimethyl-cyclohex-2-enylidene)-propan-2-one
3.5
40
(2,4-Dimethyl-phenyl)-phenyl-methanone
1.4
NCOC
8
2,6-Dimethyl-6-nitro-hept-2-en-4-one
24.9
3.5
Alkanols
5
4-Methyl-heptan-3-ol
1.4
11
Octan-3-ol
3.3
Arenol
41
4-Cyclohexyl-benzene-1,3-diol
1.4According to the basic principle of photolysis, the excited electronic transition from valence band to conduction band on the surface of the photocatalyst causes the formation of electron hole pairs. The pairs bring about hydrogen peroxide and water cracking to produce hydroxyl radicals, which oxidize OMs in coal and form various free radicals. All radicals in the aqueous system react with OMs each other to obtain depolymerization products. Thus, the formation of alkanols, ketones, and esters should result in NA and BA oxidation conversion and can also lead to the dramatic reduction or diminishing of reactant quantity. The decreasing trend of alcohol conversion indicates that the oxidation susceptibility of the third oxidation system is greater than that of the first system. Therefore, the reaction process must be optimized to obtain a desirable result.In addition, many chromophores including the conjugated double bond and heterocyclic groups with nitrogen, sulfur, phosphorus, and chloride as bridged bonds were considered as important factors in determining the color of the compound and were excited easily by UV irradiation. Therefore, the photocatalytic decomposition of heteroatomic-containing compounds (HCCs) in coal is most likely to occur. As listed in Table2, 2,6-dimethyl-6-nitro-hept-2-en-4-one was detected in ASFOSFBCI and ASFOSFBCIV, accounting for 16.10% and 33.50%, respectively, but only 6.30% in ASFSFBC, indicating that macromolecular heteroatom compounds in coals have been excited by light through chemical bond fracture between carbon atom and heteroatom firstly. 2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene and 1,2-dimethyl cyclohex-1-ene can be excited only in ASFSFBC, whereas 1-methyl-5,6-divinylcyclohex-1-ene was confirmed in ASFOSFBCI (ca. 3.60%), indicating that photooxidation depolymerization must have occurred in the process of photolysis. It has proven that the branched hydrocarbons can be converted preferentially and the polycyclic aromatic compounds can be depolymerized and converted into other small molecule compounds. And it is a proof of ring opening or cleavage link, suggesting that chromophore groups are favorable for photoelectron delivery and light energy utilization.


#### 3.1.2. Effect Analysis of ASFGTC and ASFOGTC

OMs in SFBC could be converted into OCOCs under CSS as the upper experiment. GTCs were oxidized in the similar condition as the oxidation of SFBC to verify the coal oxidation laws and to understand the composition and structure of coals distributed in different regions. The results were analyzed by GC/MS and listed in Table3. No OM was detected in ASFOGTCII, ASFOGTCIII, and ASFOGTCIV, except in ASFOGTCI, which indicates that OMs in GTC are more difficult to obtain than those in SFBC and the oxidation systems exhibit different properties.Table3 lists 22 OMs, including 4 NAs, 9 alkylarenes, 2 ketones, 2 NSAs, 1 NCOC, 1 BA, 1 alkanol, 1 arenoland, and 1 alkene in ASFGTC. Among them, the abundance of 2,6-dimethyl-6-nitrohept-2-en-4-one is the highest. However, 25 OMs, including 1 NA, 9 alkylarenes, 2 NSAs, and 13 OCOCs, were detected in ASFOGTCI, and their RCs are consecutively 1.9%, 20.1%, 3.7%, and 74.5% (as shown in Figure 3). Notably, in comparison with those in ASFGTC, OCOCs in ASFOGTCI increase but NAs and BAs decrease. The OMs in GTC have been oxidized successfully because of the active hydrogen atoms replaced by radicals. Nonetheless, the oxidation degree is different from that of SFBC because of the difference between the species and the quantity of OCOCs detected in ASFOGTCI and in ASFOSFBCI. Most probably, the coals have different structures and compositions resulting from different geographical positions and formation mechanisms. The experimental data also verified that GTC is more difficult to be oxidized by UV irradiation compared with SFBC. It is proved that GTC, which is formed in high humidity marine climate environment, has a more dense structure and fine compositions as a result of its high coalification degree.Figure 3
Component analysis of ASFGTC and ASFOGTCI.


#### 3.1.3. Effect Analysis of ASFSLL, ASFOSLLI, and ASFOSLLIV

SLL is generally considered to be oxidizable in mild conditions. It was oxidized under the same conditions as those of the upper two coals. The results are listed in Table4. No peaks were detected in ASFOSLLII and ASFOSLLIII.Table 4
Organic compounds detected in ASFSLL, ASFOSLLI, and ASFOSLLIV.
Peak
Parent compound
Detected in
ASFSLL
ASFOSLLI
ASFOSLLIV
NA
32
Tetratetracontane
5.1
Bas
17
4-Methyl-octane
2.0
30
2,3,8-Trimethyl-decane
4.9
NCOCs
1
3-Methyl-3-nitro-butyric acid ethyl ester
1.8
2
Pyrrolidin-3-ol
1.4
3
(1Z)-1-((E)-Pent-3-en-2-ylidene)semicarbazide
9.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
34.4
39.4
32.4
8
4,6-Dimethyl-pyrimidin-2-ylamine
2.4
Esters
4
Oxalic acid cyclohexyl ester hexyl ester
8.9
23
Phthalic acid diisobutyl ester
9.2
26
7-Isopropenyl-3,6-dimethyl-6-vinyl-hexahydro-benzofuran-2-one
2.8
Alkanols
5
Octan-3-ol
1.4
12
(2-Hydroxymethyl-cyclohexyl)methanol
2.4
Ketones
6
2,6-Dimethyl-hepta-2,5-dien-4-one
49.9
58.8
21
6,10-Dimethylundec-9-en-2-one
2.7
29
Crinan-1-one
1.5
OCOC
10
2,2,4-Trimethyl-oxetane
5.2
Cas
15
4-Methyl-2-oxo-pentanoic acid
3.4
25
Phthalic acid mono-(1-methyl-pentyl)ester
1.5
33
Phthalic acid mono-[3-ethyl-1-(3-methyl-butyl)-heptyl]ester
2.8
NSA
22
Anthracene
7.6
Alkylarenes
24
Diphenyl-penta-1,3-diene
2.6
28
Hexa-1,3-dienyl-benzene
2.3
31
7-Isopropyl-1-methyl-phenanthrene
2.1
Branched cyclane
34
17-(1,5-Dimethyl-hexyl)-10,13-dimethyl-2,3,4,5,6,7,8,9,10,11,12,13,16,17-tetradecahydro-1H-cyclopenta[a]phenanthrene
1.8As shown in Table4, 25 OMs in total were detected in ASFSLL, ASFOSLLI, and ASFOSLLIV. 2,6-Dimethyl-6-nitro-2-hepten-4-one was confirmed in the three samples simultaneously and contained the maximum amount of OMs in ASFSLL. Yet, 2,6-dimethyl-hepta-2,5-dien-4-one, the highest content compound in the two oxidation coals, was merely detected in ASFOSLLI and ASFOSLLIV, and their proportions are 49.9% and 58.8%, respectively. It indicates that 2,6-dimethyl-hepta-2,5-dien-4-one is the major conversion product in oxidation SLL and its formation mechanism should be heteroatom detachment because of the breaking of C-N bonds as in Scheme 1. In addition, the confirmation of only four OMs in ASFOSLLI and three OMs in ASFOSLLIV indicates that a large number of OMs in SLL have been oxidized and converted into OCOCs. Compared with SFBC and GTC, large numbers of heterogeneous compounds (containing N, P, and O), as photosensitive substances, were detected in ASFSLL and degraded largely in ASFOSLLI and ASFOSLLIV. Analysis results also proved that SLL is a type of lignite with relatively low coalification degree and high activity. Given that only H2O2 takes part in the closed system, the oxidation of SLL and the formation of OCOCs should also have active hydrogen substituted by hydroxyl radicals rather than oxygen atoms or other oxidizing ions. Certainly electrophilic reaction is also the main process in the oxidation of SLL.Scheme 1
Possible pathways of oxidation product formation in coals in PCO.


#### 3.1.4. Effect Analysis of ASFHLGLC and ASFOHLGLCI

HLGLC, a typical lignite distributing in Northern China, was oxidized in the same condition as that of the upper coals. The results showed that no peaks were observed in the total ion chromatograms of ASFOHLGLCII, ASFOHLGCLCIII, and ASFOHLGLCIV, except for those of ASFHLGLC and ASFOHLGLCI. Thus, only the analysis data of ASFHLGLC and ASFOHLGLCI are listed in Table 5.Table 5
Organic compounds detected in ASFHLGLC and ASFOHLGLCI.
Peak
Parent compound
Detected in
ASFHLGLC
ASFOHLGLCI
NCOCs
1
Pent-3-en-2-ylidene-semicarbazide
27.2
6
2,6-Dimethyl-pyridin-3-ol
54.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
62.7
18.5
OCOCs
4
1-(1-Methoxy-ethoxy)-hex-3-ene
2.1
Ketones
5
2,6-Dimethyl-hepta-2,5-dien-4-one
18.8
12
Bicyclohexyl-2′-en-2-one
1.4
Alkanol
8
Octan-3-ol
8.5
Esters
9
Acetic acid 3-methyl-1-methylene-but-2-enyl ester
1.1
13
Phthalic acid diethyl ester
1.4As shown in Table5, seven OMs were detected in ASFHLGLC, but only three OMs were detected in ASFOHLGLCI. Notably, 2,6-dimethyl-6-nitro-hept-2-en-4-one exists both in ASFHLGLC and ASFOHLGLCI, but its amount decreases from 62.7% to 18.5% before and after oxidation. As a result of the fracture of the carbon nitrogen bond and the introduction of oxygen atoms by hydroxyl substituting, OCOCs have become the compounds with the highest abundance. Pent-3-en-2-ylidene-semicarbazide accounts for 27.2% in ASFOHLGLCI, indicating that more NCOCs tend to be converted into the compound containing the peptide bond through ring opening and restructuring. From the previous analysis, HLGLC is more easily oxidized than SFBC and GTC. Although OCOCs are also the species with the highest abundance in HLGLC and SLL, the number of their species decreases in OHLGLC and OSLL, indicating that two coals were formed in oxygen-abundant regions or evolved from oxygen-rich substances.In addition, the color of the extract differs because of the variation in the oxidation degree. In turn, the extract colors of ASFGTC, ASFSFBC, ASFSLL, and ASFHLGLC deepened consecutively to yellow to red to rufous. Those of their oxidized coals present the same tendency, indicating that a large number of photosensitive materials exist in lignite and can be oxidized through degradation and decolorization in PCO.


### 3.2. Analysis of ESFs in Oxidized Coals

As FPs were extracted with ether rapidly and exhaustively, ESFs were collected and analyzed by GC/MS. The results are shown in Figure4. EIFs were analyzed with the UV-visible spectrum, and two peaks (with wavelengths of 220 and 275 nm) were detected. Their intensities were stronger than those of the blank sample. The aqueous solution is yellow, and the result of potassium permanganate titration is less than that of deionized water. Therefore, the aqueous solution may only contain inorganic salt.Figure 4
Water-soluble ether extract fraction analysis of oxidation coals.As shown in Figure4, carbon dioxide, acetic acids, and acetate can be confirmed in ESFs. Among them, acetate mainly contains Mg or Mn atoms and only exists in ESFOGTCI and ESFOGTCII, indicating that the oxidation efficiency is different from our coals for different compositions and structures. Meanwhile, the RC of carbon dioxide has the highest abundance among the three peaks of ESFOSFBCI, ESFOGTCI, ESFOSLLI, and ESFOHLGLCI, but acetic acid has the highest abundance in ESFOSFBCII, ESFOSLLII, and ESFOHLGLCII, except in ESFOGTCII. Carbon dioxide as a primary product exists in the weak oxidation system, but acetic acid is considered as an intermediate product in the stronger oxidation system. The data reveal that OMs in coals have been depolymerized, particularly OMs oxidized in an environment with strong polarity, although GTC is more difficult to oxidize than the other coals. Certainly, GTC has a stable structure and composition due to its distribution area and formation time. The exchange of oxocompounds is the main feature in coal oxidation even in CSS. Furthermore, the peak time of the reactant in OMs was postponed and the dispersibility of the resultants was improved. Most OMs in coals and oxidized coals were confirmed by GC/MS, and their conversion rules were deduced in Scheme 1.


### 3.3. Effect of EYs

EYs can indirectly explain the construction and composition of coals according to the quantity of OM conversion. Thus, the EYs of coals and oxidized coals were analyzed through the differential method. The results are shown in Figure5.Figure 5
Extract yield of coals and oxidation coals.As shown in Figure5, the EYs of coals exhibit an increasing tendency with the optimization of the reaction condition. The EYs of SLL and HLGLC are higher than those of the other coals, indicating that lignite and bituminous coal were more oxidizable and more OMs were possibly converted into OCOCs in the oxidation system. The study has proven that SLL and HLGLC are composed of OMs with low coalification degrees. Due to the different formation periods and the geological formation environment of SFBC and GTC, the structure and composition of OMs in SFBC and GTC are different from those in SLL and HLGLC. The conversions are significantly different even in the same oxidation condition.According to the different species and number of OMs, coals with different coalification degrees are oxidized with different susceptibilities and can be converted into different species of OCOCs successfully. Furthermore, the second system (II) has the strongest oxidation resistance, and the first (I), third (III), and fourth (IV) systems have weaker oxidation resistance. In other words, the EYs of SFBC, GTC, and SLL in the second system are higher than those of the other three systems, and the numerical values of which are separately by 18%, 8.8%, and 37.4%. This study has proven that OMs in coals can be converted into stronger polar compounds if 0.1 g catalysts give that 12 hours of reaction time are available. Meanwhile, partly oxidized products may also be formed and dissolved in an aqueous solution, rarely leading to ESF. Compared with the open system, the isolated oxygen system involving only H2O2 OMs oxidation conversion laws is easier to analyze.Finally, the weights of ESFs, ASFIs, and ASFIIs were considered (with SFBC as representative), in which the values of tube weight with dried extract and the weight of the plain tube are different. The results show that the value of WASFI is 0 and the weights of ESFs and ASFIIs are, respectively, 0.0756 and 0.1559 g in SFBC (ca. 2 g), indicating that the conversion rate of SFBC is 11.57% in the process.


## 3.1. GC/MS Analysis of Oxidized Coal

### 3.1.1. Effect Analysis of ASFSFBC and ASFOSFBC

SFBC and OSFBC were extracted with acetone and analyzed by GC/MS to understand OM hydrothermal conversion in coals in the process. Results show that no OMs were detected in ASFOSFBCII and ASFOSFBCIII, except in ASFOSFBCI and ASFOSFBCIV, indicating that ASFs can be oxidized completely during 12 hours of illumination despite the decreasing amount of photocatalyst from 0.1 g to 0.05 g. Thus, only the analysis results of ASFOSFBCI and ASFOSFBCIV are presented in Table 2. As listed in Table 2, the depolymerization of some part of the OMs is indicated by the result that 24 OMs were confirmed in ASFSFBC, but only 13 OMs in ASFOSFBCI and 9 OMs in ASFOSFBCIV. Oxygen atoms were introduced in the process, according to the result that the relative content (RC) of 7-isopropyl-1-methylphenanthrene (peak 76) is the highest in ASFSFBC, whereas the proportion of 2,6-dimethylhepta-2,5-dien-4-one is the highest in ASFOSFBCI and ASFOSFBCIV. According to the analysis, the oxygen atoms may have two sources: the compounds self-depolymerization in coal resulted from the cleavage of C–O and the oxidation of hydroxyl radicals. Among the 13 OMs in ASFOSFBCI, 1 branched alkane (BA), 1 alkylarene, 1 arenol, 4 alkanols, 4 ketones, and 2 nitrogen-containing compounds (NCOCs) were detected. By contrast, in ASFOSFBCIII, 2 NAs, 1 BA, 1 ketone, 1 NCOC, 1 alkanol, 1 ester, and 1 ketone were detected. Quantitative analysis verified that OMs have been converted partly in the process. Possible evidences are as follows: firstly, the RCs of NAs and the number of carbon atoms in long-chain alkanes decrease. 8 NAs in ASFSFBC but no NA in ASFOSFBCI and 2 NAs in ASFOSFBCIV were detected, and their RCs have been reduced from 25.20% to 5.00% before and after oxidation (as shown in Figure 2). It can be interpreted that almost all NAs have been converted successfully, and no more NAs may be formed despite the extension of illuminating time [31]. Secondly, the amount of branched-alkanes (BAs) in ASFOSFBCI and ASFOSFBCIV decreases significantly compared with those in ASFSFBC. As listed in Tables 2 and 3, BAs and 5 alkylarenes containing α-H atoms account for 31.50% in ASFSFBC after oxidation, only 1 BA and 1 alkylarene in ASFOSFBCI and 1 BA in ASFOSFBCIV are verified, and the RCs of the BAs and alkylarenes in ASFOSFBCI and in ASFOSFBCIV decrease to 4.80% and 1.80%, respectively. These findings show that the majority of BAs or alkylarenes have been oxidized through α-H atoms substitution of photoelectron and photogenic-free radicals. Thirdly, there is no change in the number of oxygen-containing organic compounds (OCOCs) in ASFOSFBCI and ASFOSFBCIV after the oxidation. However, their RCs increased significantly from 43.00% in ASFSFBC to 92.80% in ASFOSFBCI and 93.20% in ASFOSFBCIV. More importantly, 2,6-dimethylhepta-2,5-dien-4-one is detected both in ASFOSFBCI and in ASFOSFBCIV, and theirs RCs are the highest (accounting for 44.60% and 50.20%, respectively), which indicate that the formation of ketones is the main process of photooxidation conversion.Table 2
Organic compounds detected in AEFSFBC, AEFOSFBCI, and AEFOSFBCIV.
Peak
Parent compound
Detected in
ASFSFBC
ASFOSFBCI
ASFOSFBCIV
NAs
60
Nonacosane
2.0
64
Octacosane
2.0
71
Heneicosane
3.2
74
Tetracosane
5.6
78
Heptadecane
3.0
79
Tetratetracontane
1.8
2.1
80
Docosane
5.6
82
Tritetracontane
2.9
84
Tetratriacontane
2.0
BA
51
2,6,10,14-Tetramethyl-hexadecane
1.9
Branched cyclane
41
9-Dodecyl-tetradecahydro-anthracene
1.8
Alkenes
20
1-Methyl-5,6-divinylcyclohex-1-ene
3.6
26
4a,8,9,9-Tetramethyl-1,2,3,4,4a,5,6,8a-octahydro-1,4-methano-naphthalene
2.3
44
1,2-Dimethyl-cyclohexene
2.1
CA
58
Phthalic acid mono-(1-butyl-2-ethyl-hexyl) ester
6.8
Alkylarenes
43
1,1,2,3,3-Pentamethyl-indane
2.7
66
7-Butyl-1-hexyl-naphthalene
8.4
76
7-Isopropyl-1-methyl-phenanthrene
11.0
1.2
77
1-Propyl-4-(2-p-tolylethynyl)benzene
1.7
81
8-Isopropyl-1,3-dimethyl-phenanthrene
1.4
Alkanols
6
2-Methyl-hexan-2-ol
2.3
7
Octan-3-ol
1.7
13
3-Isopropenyl-2-methyl-cyclohexanol
2.5
16
Octanol
2.3
32
2-Isopropenyl-5-methyl-cyclohexanol
1.4
Arenols
19
2-tert-Butyl-4,6-dimethyl-phenol
2.0
Ketones
1
Cyclohexanone
14.8
9
2,6-Dimethyl-hepta-2,5-dien-4-one
44.6
50.2
11
3,5,5-Trimethyl-cyclohex-2-enone
2.7
18
1-(4,7,7-Trimethyl-bicyclo[4.1.0]hept-3-en-3-yl)-ethanone
5.1
28
4-(2,6,6-Trimethyl-cyclohex-1-enyl)-but-3-en-2-one
4.9
59
2,4-Dimethyl-hexan-3-one
3.2
Esters
4
Acetic acid 1,2-dimethyl-propyl ester
9.0
30
Acetic acid 17-(5-methoxy-1,5-dimethyl-4-oxo- hexyl)-4,4,13,14-tetramethyl-tetradecahydro-cyclopropa[9,10]cyclopenta[a]phenanthren-3-yl ester
2.1
72
Methoxy-acetic acid 1-methyl-tridecyl ester
2.6
NCOCs
5
N-(5-Oxo-2,5-dihydro-furan-2-yl)-acetamide
1.3
12
2,6-Dimethyl-6-nitro-hept-2-en-4-one
6.3
16.1
33.5
67
3-(2,4,6-Trimethyl-phenyl)-pyridine
2.0
68
7-Ethyl-2,4-dimethyl-10H-benzo[b][1,8]naphthyridin-5-one
4.5
CCOC
10
2-Chloro-4-isopropyl-1-methyl-benzene
1.2
OCOC
40
2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene
8.2Figure 2
Component analysis of ASFSFBC, ASFOSFBCI, and ASFOSFBCIV.Table 3
Organic compounds detected in ASFGTC and ASFOGTCI.
Peak
Parent compound
Detected in
ASFGTC
ASFOGTCI
NAs
46
Heneicosane
2.9
56
Heptadecane
3.9
1.9
57
Heptacosane
2.2
59
Tetratriacontane
3.7
Bas
29
2,6,10,14-Tetramethyl-pentadecane
2.9
Alkenes
20
(3-Methyl-penta-1,2,4-trienyl)-benzene
1.6
Alkylarenes
13
1-Methyl-naphthalene
2.2
16
2,3-Dimethyl-naphthalene
1.6
18
1,4-Dimethyl-naphthalene
3.1
19
2,6-Dimethyl-naphthalene
2.6
22
2,3,6-Trimethyl-naphthalene
1.6
1.2
23
4,6,8-Trimethyl-azulene
2.3
25
1,6,7-Trimethyl-naphthalene
3.5
26
1,5,7-Trimethyl-naphthalene
2.7
27
1,4,5-Trimethyl-naphthalene
5.6
30
4-Isopropyl-1,6-dimethyl-naphthalene
8.5
3.0
34
7-Isopropyl-1-methyl-naphthalene
2.5
35
7-Ethyl-1,4-dimethyl-azulene
4.2
44
2-Methyl-anthracene
7.3
52
2-Methyl-octadecane
2.6
54
1-Methyl-pyrene
1.5
55
2-Methyl-fluoranthene
2.7
NSAs
43
Naphtho[2,3-b]norbornadiene
1.3
49
Pyrene
1.9
51
Fluoranthene
6.3
2.4
Esters
1
3-Methyl-5H-furan-2-one
20.7
9
Acetic acidm-tolyl ester
1.8
10
Acetic acidp-tolyl ester
1.3
12
Acetic acid 2,3-dimethyl-phenyl ester
3.6
32
Acetic acid 3,4-diacetoxy-6,8-dioxa-bicyclo[3.2.1]oct-2-yl ester
3.9
60
Phthalic acid 1-(2,2-dimethyl-propyl)ester 2-(2-ethyl-hexyl) ester
1.1
Ketones
2
Hexane-2,5-dione
18.3
4
2-Methylcyclopent-2-enone
1.3
6
2,6-Dimethyl-hepta-2,5-dien-4-one
7.0
8.7
7
3,5,5-Trimethyl-cyclohex-2-enone
3.5
15
1-(3,5,5-Trimethyl-cyclohex-2-enylidene)-propan-2-one
3.5
40
(2,4-Dimethyl-phenyl)-phenyl-methanone
1.4
NCOC
8
2,6-Dimethyl-6-nitro-hept-2-en-4-one
24.9
3.5
Alkanols
5
4-Methyl-heptan-3-ol
1.4
11
Octan-3-ol
3.3
Arenol
41
4-Cyclohexyl-benzene-1,3-diol
1.4According to the basic principle of photolysis, the excited electronic transition from valence band to conduction band on the surface of the photocatalyst causes the formation of electron hole pairs. The pairs bring about hydrogen peroxide and water cracking to produce hydroxyl radicals, which oxidize OMs in coal and form various free radicals. All radicals in the aqueous system react with OMs each other to obtain depolymerization products. Thus, the formation of alkanols, ketones, and esters should result in NA and BA oxidation conversion and can also lead to the dramatic reduction or diminishing of reactant quantity. The decreasing trend of alcohol conversion indicates that the oxidation susceptibility of the third oxidation system is greater than that of the first system. Therefore, the reaction process must be optimized to obtain a desirable result.In addition, many chromophores including the conjugated double bond and heterocyclic groups with nitrogen, sulfur, phosphorus, and chloride as bridged bonds were considered as important factors in determining the color of the compound and were excited easily by UV irradiation. Therefore, the photocatalytic decomposition of heteroatomic-containing compounds (HCCs) in coal is most likely to occur. As listed in Table2, 2,6-dimethyl-6-nitro-hept-2-en-4-one was detected in ASFOSFBCI and ASFOSFBCIV, accounting for 16.10% and 33.50%, respectively, but only 6.30% in ASFSFBC, indicating that macromolecular heteroatom compounds in coals have been excited by light through chemical bond fracture between carbon atom and heteroatom firstly. 2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene and 1,2-dimethyl cyclohex-1-ene can be excited only in ASFSFBC, whereas 1-methyl-5,6-divinylcyclohex-1-ene was confirmed in ASFOSFBCI (ca. 3.60%), indicating that photooxidation depolymerization must have occurred in the process of photolysis. It has proven that the branched hydrocarbons can be converted preferentially and the polycyclic aromatic compounds can be depolymerized and converted into other small molecule compounds. And it is a proof of ring opening or cleavage link, suggesting that chromophore groups are favorable for photoelectron delivery and light energy utilization.


### 3.1.2. Effect Analysis of ASFGTC and ASFOGTC

OMs in SFBC could be converted into OCOCs under CSS as the upper experiment. GTCs were oxidized in the similar condition as the oxidation of SFBC to verify the coal oxidation laws and to understand the composition and structure of coals distributed in different regions. The results were analyzed by GC/MS and listed in Table3. No OM was detected in ASFOGTCII, ASFOGTCIII, and ASFOGTCIV, except in ASFOGTCI, which indicates that OMs in GTC are more difficult to obtain than those in SFBC and the oxidation systems exhibit different properties.Table3 lists 22 OMs, including 4 NAs, 9 alkylarenes, 2 ketones, 2 NSAs, 1 NCOC, 1 BA, 1 alkanol, 1 arenoland, and 1 alkene in ASFGTC. Among them, the abundance of 2,6-dimethyl-6-nitrohept-2-en-4-one is the highest. However, 25 OMs, including 1 NA, 9 alkylarenes, 2 NSAs, and 13 OCOCs, were detected in ASFOGTCI, and their RCs are consecutively 1.9%, 20.1%, 3.7%, and 74.5% (as shown in Figure 3). Notably, in comparison with those in ASFGTC, OCOCs in ASFOGTCI increase but NAs and BAs decrease. The OMs in GTC have been oxidized successfully because of the active hydrogen atoms replaced by radicals. Nonetheless, the oxidation degree is different from that of SFBC because of the difference between the species and the quantity of OCOCs detected in ASFOGTCI and in ASFOSFBCI. Most probably, the coals have different structures and compositions resulting from different geographical positions and formation mechanisms. The experimental data also verified that GTC is more difficult to be oxidized by UV irradiation compared with SFBC. It is proved that GTC, which is formed in high humidity marine climate environment, has a more dense structure and fine compositions as a result of its high coalification degree.Figure 3
Component analysis of ASFGTC and ASFOGTCI.


### 3.1.3. Effect Analysis of ASFSLL, ASFOSLLI, and ASFOSLLIV

SLL is generally considered to be oxidizable in mild conditions. It was oxidized under the same conditions as those of the upper two coals. The results are listed in Table4. No peaks were detected in ASFOSLLII and ASFOSLLIII.Table 4
Organic compounds detected in ASFSLL, ASFOSLLI, and ASFOSLLIV.
Peak
Parent compound
Detected in
ASFSLL
ASFOSLLI
ASFOSLLIV
NA
32
Tetratetracontane
5.1
Bas
17
4-Methyl-octane
2.0
30
2,3,8-Trimethyl-decane
4.9
NCOCs
1
3-Methyl-3-nitro-butyric acid ethyl ester
1.8
2
Pyrrolidin-3-ol
1.4
3
(1Z)-1-((E)-Pent-3-en-2-ylidene)semicarbazide
9.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
34.4
39.4
32.4
8
4,6-Dimethyl-pyrimidin-2-ylamine
2.4
Esters
4
Oxalic acid cyclohexyl ester hexyl ester
8.9
23
Phthalic acid diisobutyl ester
9.2
26
7-Isopropenyl-3,6-dimethyl-6-vinyl-hexahydro-benzofuran-2-one
2.8
Alkanols
5
Octan-3-ol
1.4
12
(2-Hydroxymethyl-cyclohexyl)methanol
2.4
Ketones
6
2,6-Dimethyl-hepta-2,5-dien-4-one
49.9
58.8
21
6,10-Dimethylundec-9-en-2-one
2.7
29
Crinan-1-one
1.5
OCOC
10
2,2,4-Trimethyl-oxetane
5.2
Cas
15
4-Methyl-2-oxo-pentanoic acid
3.4
25
Phthalic acid mono-(1-methyl-pentyl)ester
1.5
33
Phthalic acid mono-[3-ethyl-1-(3-methyl-butyl)-heptyl]ester
2.8
NSA
22
Anthracene
7.6
Alkylarenes
24
Diphenyl-penta-1,3-diene
2.6
28
Hexa-1,3-dienyl-benzene
2.3
31
7-Isopropyl-1-methyl-phenanthrene
2.1
Branched cyclane
34
17-(1,5-Dimethyl-hexyl)-10,13-dimethyl-2,3,4,5,6,7,8,9,10,11,12,13,16,17-tetradecahydro-1H-cyclopenta[a]phenanthrene
1.8As shown in Table4, 25 OMs in total were detected in ASFSLL, ASFOSLLI, and ASFOSLLIV. 2,6-Dimethyl-6-nitro-2-hepten-4-one was confirmed in the three samples simultaneously and contained the maximum amount of OMs in ASFSLL. Yet, 2,6-dimethyl-hepta-2,5-dien-4-one, the highest content compound in the two oxidation coals, was merely detected in ASFOSLLI and ASFOSLLIV, and their proportions are 49.9% and 58.8%, respectively. It indicates that 2,6-dimethyl-hepta-2,5-dien-4-one is the major conversion product in oxidation SLL and its formation mechanism should be heteroatom detachment because of the breaking of C-N bonds as in Scheme 1. In addition, the confirmation of only four OMs in ASFOSLLI and three OMs in ASFOSLLIV indicates that a large number of OMs in SLL have been oxidized and converted into OCOCs. Compared with SFBC and GTC, large numbers of heterogeneous compounds (containing N, P, and O), as photosensitive substances, were detected in ASFSLL and degraded largely in ASFOSLLI and ASFOSLLIV. Analysis results also proved that SLL is a type of lignite with relatively low coalification degree and high activity. Given that only H2O2 takes part in the closed system, the oxidation of SLL and the formation of OCOCs should also have active hydrogen substituted by hydroxyl radicals rather than oxygen atoms or other oxidizing ions. Certainly electrophilic reaction is also the main process in the oxidation of SLL.Scheme 1
Possible pathways of oxidation product formation in coals in PCO.


### 3.1.4. Effect Analysis of ASFHLGLC and ASFOHLGLCI

HLGLC, a typical lignite distributing in Northern China, was oxidized in the same condition as that of the upper coals. The results showed that no peaks were observed in the total ion chromatograms of ASFOHLGLCII, ASFOHLGCLCIII, and ASFOHLGLCIV, except for those of ASFHLGLC and ASFOHLGLCI. Thus, only the analysis data of ASFHLGLC and ASFOHLGLCI are listed in Table 5.Table 5
Organic compounds detected in ASFHLGLC and ASFOHLGLCI.
Peak
Parent compound
Detected in
ASFHLGLC
ASFOHLGLCI
NCOCs
1
Pent-3-en-2-ylidene-semicarbazide
27.2
6
2,6-Dimethyl-pyridin-3-ol
54.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
62.7
18.5
OCOCs
4
1-(1-Methoxy-ethoxy)-hex-3-ene
2.1
Ketones
5
2,6-Dimethyl-hepta-2,5-dien-4-one
18.8
12
Bicyclohexyl-2′-en-2-one
1.4
Alkanol
8
Octan-3-ol
8.5
Esters
9
Acetic acid 3-methyl-1-methylene-but-2-enyl ester
1.1
13
Phthalic acid diethyl ester
1.4As shown in Table5, seven OMs were detected in ASFHLGLC, but only three OMs were detected in ASFOHLGLCI. Notably, 2,6-dimethyl-6-nitro-hept-2-en-4-one exists both in ASFHLGLC and ASFOHLGLCI, but its amount decreases from 62.7% to 18.5% before and after oxidation. As a result of the fracture of the carbon nitrogen bond and the introduction of oxygen atoms by hydroxyl substituting, OCOCs have become the compounds with the highest abundance. Pent-3-en-2-ylidene-semicarbazide accounts for 27.2% in ASFOHLGLCI, indicating that more NCOCs tend to be converted into the compound containing the peptide bond through ring opening and restructuring. From the previous analysis, HLGLC is more easily oxidized than SFBC and GTC. Although OCOCs are also the species with the highest abundance in HLGLC and SLL, the number of their species decreases in OHLGLC and OSLL, indicating that two coals were formed in oxygen-abundant regions or evolved from oxygen-rich substances.In addition, the color of the extract differs because of the variation in the oxidation degree. In turn, the extract colors of ASFGTC, ASFSFBC, ASFSLL, and ASFHLGLC deepened consecutively to yellow to red to rufous. Those of their oxidized coals present the same tendency, indicating that a large number of photosensitive materials exist in lignite and can be oxidized through degradation and decolorization in PCO.


## 3.1.1. Effect Analysis of ASFSFBC and ASFOSFBC

SFBC and OSFBC were extracted with acetone and analyzed by GC/MS to understand OM hydrothermal conversion in coals in the process. Results show that no OMs were detected in ASFOSFBCII and ASFOSFBCIII, except in ASFOSFBCI and ASFOSFBCIV, indicating that ASFs can be oxidized completely during 12 hours of illumination despite the decreasing amount of photocatalyst from 0.1 g to 0.05 g. Thus, only the analysis results of ASFOSFBCI and ASFOSFBCIV are presented in Table 2. As listed in Table 2, the depolymerization of some part of the OMs is indicated by the result that 24 OMs were confirmed in ASFSFBC, but only 13 OMs in ASFOSFBCI and 9 OMs in ASFOSFBCIV. Oxygen atoms were introduced in the process, according to the result that the relative content (RC) of 7-isopropyl-1-methylphenanthrene (peak 76) is the highest in ASFSFBC, whereas the proportion of 2,6-dimethylhepta-2,5-dien-4-one is the highest in ASFOSFBCI and ASFOSFBCIV. According to the analysis, the oxygen atoms may have two sources: the compounds self-depolymerization in coal resulted from the cleavage of C–O and the oxidation of hydroxyl radicals. Among the 13 OMs in ASFOSFBCI, 1 branched alkane (BA), 1 alkylarene, 1 arenol, 4 alkanols, 4 ketones, and 2 nitrogen-containing compounds (NCOCs) were detected. By contrast, in ASFOSFBCIII, 2 NAs, 1 BA, 1 ketone, 1 NCOC, 1 alkanol, 1 ester, and 1 ketone were detected. Quantitative analysis verified that OMs have been converted partly in the process. Possible evidences are as follows: firstly, the RCs of NAs and the number of carbon atoms in long-chain alkanes decrease. 8 NAs in ASFSFBC but no NA in ASFOSFBCI and 2 NAs in ASFOSFBCIV were detected, and their RCs have been reduced from 25.20% to 5.00% before and after oxidation (as shown in Figure 2). It can be interpreted that almost all NAs have been converted successfully, and no more NAs may be formed despite the extension of illuminating time [31]. Secondly, the amount of branched-alkanes (BAs) in ASFOSFBCI and ASFOSFBCIV decreases significantly compared with those in ASFSFBC. As listed in Tables 2 and 3, BAs and 5 alkylarenes containing α-H atoms account for 31.50% in ASFSFBC after oxidation, only 1 BA and 1 alkylarene in ASFOSFBCI and 1 BA in ASFOSFBCIV are verified, and the RCs of the BAs and alkylarenes in ASFOSFBCI and in ASFOSFBCIV decrease to 4.80% and 1.80%, respectively. These findings show that the majority of BAs or alkylarenes have been oxidized through α-H atoms substitution of photoelectron and photogenic-free radicals. Thirdly, there is no change in the number of oxygen-containing organic compounds (OCOCs) in ASFOSFBCI and ASFOSFBCIV after the oxidation. However, their RCs increased significantly from 43.00% in ASFSFBC to 92.80% in ASFOSFBCI and 93.20% in ASFOSFBCIV. More importantly, 2,6-dimethylhepta-2,5-dien-4-one is detected both in ASFOSFBCI and in ASFOSFBCIV, and theirs RCs are the highest (accounting for 44.60% and 50.20%, respectively), which indicate that the formation of ketones is the main process of photooxidation conversion.Table 2
Organic compounds detected in AEFSFBC, AEFOSFBCI, and AEFOSFBCIV.
Peak
Parent compound
Detected in
ASFSFBC
ASFOSFBCI
ASFOSFBCIV
NAs
60
Nonacosane
2.0
64
Octacosane
2.0
71
Heneicosane
3.2
74
Tetracosane
5.6
78
Heptadecane
3.0
79
Tetratetracontane
1.8
2.1
80
Docosane
5.6
82
Tritetracontane
2.9
84
Tetratriacontane
2.0
BA
51
2,6,10,14-Tetramethyl-hexadecane
1.9
Branched cyclane
41
9-Dodecyl-tetradecahydro-anthracene
1.8
Alkenes
20
1-Methyl-5,6-divinylcyclohex-1-ene
3.6
26
4a,8,9,9-Tetramethyl-1,2,3,4,4a,5,6,8a-octahydro-1,4-methano-naphthalene
2.3
44
1,2-Dimethyl-cyclohexene
2.1
CA
58
Phthalic acid mono-(1-butyl-2-ethyl-hexyl) ester
6.8
Alkylarenes
43
1,1,2,3,3-Pentamethyl-indane
2.7
66
7-Butyl-1-hexyl-naphthalene
8.4
76
7-Isopropyl-1-methyl-phenanthrene
11.0
1.2
77
1-Propyl-4-(2-p-tolylethynyl)benzene
1.7
81
8-Isopropyl-1,3-dimethyl-phenanthrene
1.4
Alkanols
6
2-Methyl-hexan-2-ol
2.3
7
Octan-3-ol
1.7
13
3-Isopropenyl-2-methyl-cyclohexanol
2.5
16
Octanol
2.3
32
2-Isopropenyl-5-methyl-cyclohexanol
1.4
Arenols
19
2-tert-Butyl-4,6-dimethyl-phenol
2.0
Ketones
1
Cyclohexanone
14.8
9
2,6-Dimethyl-hepta-2,5-dien-4-one
44.6
50.2
11
3,5,5-Trimethyl-cyclohex-2-enone
2.7
18
1-(4,7,7-Trimethyl-bicyclo[4.1.0]hept-3-en-3-yl)-ethanone
5.1
28
4-(2,6,6-Trimethyl-cyclohex-1-enyl)-but-3-en-2-one
4.9
59
2,4-Dimethyl-hexan-3-one
3.2
Esters
4
Acetic acid 1,2-dimethyl-propyl ester
9.0
30
Acetic acid 17-(5-methoxy-1,5-dimethyl-4-oxo- hexyl)-4,4,13,14-tetramethyl-tetradecahydro-cyclopropa[9,10]cyclopenta[a]phenanthren-3-yl ester
2.1
72
Methoxy-acetic acid 1-methyl-tridecyl ester
2.6
NCOCs
5
N-(5-Oxo-2,5-dihydro-furan-2-yl)-acetamide
1.3
12
2,6-Dimethyl-6-nitro-hept-2-en-4-one
6.3
16.1
33.5
67
3-(2,4,6-Trimethyl-phenyl)-pyridine
2.0
68
7-Ethyl-2,4-dimethyl-10H-benzo[b][1,8]naphthyridin-5-one
4.5
CCOC
10
2-Chloro-4-isopropyl-1-methyl-benzene
1.2
OCOC
40
2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene
8.2Figure 2
Component analysis of ASFSFBC, ASFOSFBCI, and ASFOSFBCIV.Table 3
Organic compounds detected in ASFGTC and ASFOGTCI.
Peak
Parent compound
Detected in
ASFGTC
ASFOGTCI
NAs
46
Heneicosane
2.9
56
Heptadecane
3.9
1.9
57
Heptacosane
2.2
59
Tetratriacontane
3.7
Bas
29
2,6,10,14-Tetramethyl-pentadecane
2.9
Alkenes
20
(3-Methyl-penta-1,2,4-trienyl)-benzene
1.6
Alkylarenes
13
1-Methyl-naphthalene
2.2
16
2,3-Dimethyl-naphthalene
1.6
18
1,4-Dimethyl-naphthalene
3.1
19
2,6-Dimethyl-naphthalene
2.6
22
2,3,6-Trimethyl-naphthalene
1.6
1.2
23
4,6,8-Trimethyl-azulene
2.3
25
1,6,7-Trimethyl-naphthalene
3.5
26
1,5,7-Trimethyl-naphthalene
2.7
27
1,4,5-Trimethyl-naphthalene
5.6
30
4-Isopropyl-1,6-dimethyl-naphthalene
8.5
3.0
34
7-Isopropyl-1-methyl-naphthalene
2.5
35
7-Ethyl-1,4-dimethyl-azulene
4.2
44
2-Methyl-anthracene
7.3
52
2-Methyl-octadecane
2.6
54
1-Methyl-pyrene
1.5
55
2-Methyl-fluoranthene
2.7
NSAs
43
Naphtho[2,3-b]norbornadiene
1.3
49
Pyrene
1.9
51
Fluoranthene
6.3
2.4
Esters
1
3-Methyl-5H-furan-2-one
20.7
9
Acetic acidm-tolyl ester
1.8
10
Acetic acidp-tolyl ester
1.3
12
Acetic acid 2,3-dimethyl-phenyl ester
3.6
32
Acetic acid 3,4-diacetoxy-6,8-dioxa-bicyclo[3.2.1]oct-2-yl ester
3.9
60
Phthalic acid 1-(2,2-dimethyl-propyl)ester 2-(2-ethyl-hexyl) ester
1.1
Ketones
2
Hexane-2,5-dione
18.3
4
2-Methylcyclopent-2-enone
1.3
6
2,6-Dimethyl-hepta-2,5-dien-4-one
7.0
8.7
7
3,5,5-Trimethyl-cyclohex-2-enone
3.5
15
1-(3,5,5-Trimethyl-cyclohex-2-enylidene)-propan-2-one
3.5
40
(2,4-Dimethyl-phenyl)-phenyl-methanone
1.4
NCOC
8
2,6-Dimethyl-6-nitro-hept-2-en-4-one
24.9
3.5
Alkanols
5
4-Methyl-heptan-3-ol
1.4
11
Octan-3-ol
3.3
Arenol
41
4-Cyclohexyl-benzene-1,3-diol
1.4According to the basic principle of photolysis, the excited electronic transition from valence band to conduction band on the surface of the photocatalyst causes the formation of electron hole pairs. The pairs bring about hydrogen peroxide and water cracking to produce hydroxyl radicals, which oxidize OMs in coal and form various free radicals. All radicals in the aqueous system react with OMs each other to obtain depolymerization products. Thus, the formation of alkanols, ketones, and esters should result in NA and BA oxidation conversion and can also lead to the dramatic reduction or diminishing of reactant quantity. The decreasing trend of alcohol conversion indicates that the oxidation susceptibility of the third oxidation system is greater than that of the first system. Therefore, the reaction process must be optimized to obtain a desirable result.In addition, many chromophores including the conjugated double bond and heterocyclic groups with nitrogen, sulfur, phosphorus, and chloride as bridged bonds were considered as important factors in determining the color of the compound and were excited easily by UV irradiation. Therefore, the photocatalytic decomposition of heteroatomic-containing compounds (HCCs) in coal is most likely to occur. As listed in Table2, 2,6-dimethyl-6-nitro-hept-2-en-4-one was detected in ASFOSFBCI and ASFOSFBCIV, accounting for 16.10% and 33.50%, respectively, but only 6.30% in ASFSFBC, indicating that macromolecular heteroatom compounds in coals have been excited by light through chemical bond fracture between carbon atom and heteroatom firstly. 2,4,5,5,8a-Pentamethyl-6,7,8,8a-tetrahydro-5H-chromene and 1,2-dimethyl cyclohex-1-ene can be excited only in ASFSFBC, whereas 1-methyl-5,6-divinylcyclohex-1-ene was confirmed in ASFOSFBCI (ca. 3.60%), indicating that photooxidation depolymerization must have occurred in the process of photolysis. It has proven that the branched hydrocarbons can be converted preferentially and the polycyclic aromatic compounds can be depolymerized and converted into other small molecule compounds. And it is a proof of ring opening or cleavage link, suggesting that chromophore groups are favorable for photoelectron delivery and light energy utilization.


## 3.1.2. Effect Analysis of ASFGTC and ASFOGTC

OMs in SFBC could be converted into OCOCs under CSS as the upper experiment. GTCs were oxidized in the similar condition as the oxidation of SFBC to verify the coal oxidation laws and to understand the composition and structure of coals distributed in different regions. The results were analyzed by GC/MS and listed in Table3. No OM was detected in ASFOGTCII, ASFOGTCIII, and ASFOGTCIV, except in ASFOGTCI, which indicates that OMs in GTC are more difficult to obtain than those in SFBC and the oxidation systems exhibit different properties.Table3 lists 22 OMs, including 4 NAs, 9 alkylarenes, 2 ketones, 2 NSAs, 1 NCOC, 1 BA, 1 alkanol, 1 arenoland, and 1 alkene in ASFGTC. Among them, the abundance of 2,6-dimethyl-6-nitrohept-2-en-4-one is the highest. However, 25 OMs, including 1 NA, 9 alkylarenes, 2 NSAs, and 13 OCOCs, were detected in ASFOGTCI, and their RCs are consecutively 1.9%, 20.1%, 3.7%, and 74.5% (as shown in Figure 3). Notably, in comparison with those in ASFGTC, OCOCs in ASFOGTCI increase but NAs and BAs decrease. The OMs in GTC have been oxidized successfully because of the active hydrogen atoms replaced by radicals. Nonetheless, the oxidation degree is different from that of SFBC because of the difference between the species and the quantity of OCOCs detected in ASFOGTCI and in ASFOSFBCI. Most probably, the coals have different structures and compositions resulting from different geographical positions and formation mechanisms. The experimental data also verified that GTC is more difficult to be oxidized by UV irradiation compared with SFBC. It is proved that GTC, which is formed in high humidity marine climate environment, has a more dense structure and fine compositions as a result of its high coalification degree.Figure 3
Component analysis of ASFGTC and ASFOGTCI.


## 3.1.3. Effect Analysis of ASFSLL, ASFOSLLI, and ASFOSLLIV

SLL is generally considered to be oxidizable in mild conditions. It was oxidized under the same conditions as those of the upper two coals. The results are listed in Table4. No peaks were detected in ASFOSLLII and ASFOSLLIII.Table 4
Organic compounds detected in ASFSLL, ASFOSLLI, and ASFOSLLIV.
Peak
Parent compound
Detected in
ASFSLL
ASFOSLLI
ASFOSLLIV
NA
32
Tetratetracontane
5.1
Bas
17
4-Methyl-octane
2.0
30
2,3,8-Trimethyl-decane
4.9
NCOCs
1
3-Methyl-3-nitro-butyric acid ethyl ester
1.8
2
Pyrrolidin-3-ol
1.4
3
(1Z)-1-((E)-Pent-3-en-2-ylidene)semicarbazide
9.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
34.4
39.4
32.4
8
4,6-Dimethyl-pyrimidin-2-ylamine
2.4
Esters
4
Oxalic acid cyclohexyl ester hexyl ester
8.9
23
Phthalic acid diisobutyl ester
9.2
26
7-Isopropenyl-3,6-dimethyl-6-vinyl-hexahydro-benzofuran-2-one
2.8
Alkanols
5
Octan-3-ol
1.4
12
(2-Hydroxymethyl-cyclohexyl)methanol
2.4
Ketones
6
2,6-Dimethyl-hepta-2,5-dien-4-one
49.9
58.8
21
6,10-Dimethylundec-9-en-2-one
2.7
29
Crinan-1-one
1.5
OCOC
10
2,2,4-Trimethyl-oxetane
5.2
Cas
15
4-Methyl-2-oxo-pentanoic acid
3.4
25
Phthalic acid mono-(1-methyl-pentyl)ester
1.5
33
Phthalic acid mono-[3-ethyl-1-(3-methyl-butyl)-heptyl]ester
2.8
NSA
22
Anthracene
7.6
Alkylarenes
24
Diphenyl-penta-1,3-diene
2.6
28
Hexa-1,3-dienyl-benzene
2.3
31
7-Isopropyl-1-methyl-phenanthrene
2.1
Branched cyclane
34
17-(1,5-Dimethyl-hexyl)-10,13-dimethyl-2,3,4,5,6,7,8,9,10,11,12,13,16,17-tetradecahydro-1H-cyclopenta[a]phenanthrene
1.8As shown in Table4, 25 OMs in total were detected in ASFSLL, ASFOSLLI, and ASFOSLLIV. 2,6-Dimethyl-6-nitro-2-hepten-4-one was confirmed in the three samples simultaneously and contained the maximum amount of OMs in ASFSLL. Yet, 2,6-dimethyl-hepta-2,5-dien-4-one, the highest content compound in the two oxidation coals, was merely detected in ASFOSLLI and ASFOSLLIV, and their proportions are 49.9% and 58.8%, respectively. It indicates that 2,6-dimethyl-hepta-2,5-dien-4-one is the major conversion product in oxidation SLL and its formation mechanism should be heteroatom detachment because of the breaking of C-N bonds as in Scheme 1. In addition, the confirmation of only four OMs in ASFOSLLI and three OMs in ASFOSLLIV indicates that a large number of OMs in SLL have been oxidized and converted into OCOCs. Compared with SFBC and GTC, large numbers of heterogeneous compounds (containing N, P, and O), as photosensitive substances, were detected in ASFSLL and degraded largely in ASFOSLLI and ASFOSLLIV. Analysis results also proved that SLL is a type of lignite with relatively low coalification degree and high activity. Given that only H2O2 takes part in the closed system, the oxidation of SLL and the formation of OCOCs should also have active hydrogen substituted by hydroxyl radicals rather than oxygen atoms or other oxidizing ions. Certainly electrophilic reaction is also the main process in the oxidation of SLL.Scheme 1
Possible pathways of oxidation product formation in coals in PCO.


## 3.1.4. Effect Analysis of ASFHLGLC and ASFOHLGLCI

HLGLC, a typical lignite distributing in Northern China, was oxidized in the same condition as that of the upper coals. The results showed that no peaks were observed in the total ion chromatograms of ASFOHLGLCII, ASFOHLGCLCIII, and ASFOHLGLCIV, except for those of ASFHLGLC and ASFOHLGLCI. Thus, only the analysis data of ASFHLGLC and ASFOHLGLCI are listed in Table 5.Table 5
Organic compounds detected in ASFHLGLC and ASFOHLGLCI.
Peak
Parent compound
Detected in
ASFHLGLC
ASFOHLGLCI
NCOCs
1
Pent-3-en-2-ylidene-semicarbazide
27.2
6
2,6-Dimethyl-pyridin-3-ol
54.3
7
2,6-Dimethyl-6-nitro-hept-2-en-4-one
62.7
18.5
OCOCs
4
1-(1-Methoxy-ethoxy)-hex-3-ene
2.1
Ketones
5
2,6-Dimethyl-hepta-2,5-dien-4-one
18.8
12
Bicyclohexyl-2′-en-2-one
1.4
Alkanol
8
Octan-3-ol
8.5
Esters
9
Acetic acid 3-methyl-1-methylene-but-2-enyl ester
1.1
13
Phthalic acid diethyl ester
1.4As shown in Table5, seven OMs were detected in ASFHLGLC, but only three OMs were detected in ASFOHLGLCI. Notably, 2,6-dimethyl-6-nitro-hept-2-en-4-one exists both in ASFHLGLC and ASFOHLGLCI, but its amount decreases from 62.7% to 18.5% before and after oxidation. As a result of the fracture of the carbon nitrogen bond and the introduction of oxygen atoms by hydroxyl substituting, OCOCs have become the compounds with the highest abundance. Pent-3-en-2-ylidene-semicarbazide accounts for 27.2% in ASFOHLGLCI, indicating that more NCOCs tend to be converted into the compound containing the peptide bond through ring opening and restructuring. From the previous analysis, HLGLC is more easily oxidized than SFBC and GTC. Although OCOCs are also the species with the highest abundance in HLGLC and SLL, the number of their species decreases in OHLGLC and OSLL, indicating that two coals were formed in oxygen-abundant regions or evolved from oxygen-rich substances.In addition, the color of the extract differs because of the variation in the oxidation degree. In turn, the extract colors of ASFGTC, ASFSFBC, ASFSLL, and ASFHLGLC deepened consecutively to yellow to red to rufous. Those of their oxidized coals present the same tendency, indicating that a large number of photosensitive materials exist in lignite and can be oxidized through degradation and decolorization in PCO.


## 3.2. Analysis of ESFs in Oxidized Coals

As FPs were extracted with ether rapidly and exhaustively, ESFs were collected and analyzed by GC/MS. The results are shown in Figure4. EIFs were analyzed with the UV-visible spectrum, and two peaks (with wavelengths of 220 and 275 nm) were detected. Their intensities were stronger than those of the blank sample. The aqueous solution is yellow, and the result of potassium permanganate titration is less than that of deionized water. Therefore, the aqueous solution may only contain inorganic salt.Figure 4
Water-soluble ether extract fraction analysis of oxidation coals.As shown in Figure4, carbon dioxide, acetic acids, and acetate can be confirmed in ESFs. Among them, acetate mainly contains Mg or Mn atoms and only exists in ESFOGTCI and ESFOGTCII, indicating that the oxidation efficiency is different from our coals for different compositions and structures. Meanwhile, the RC of carbon dioxide has the highest abundance among the three peaks of ESFOSFBCI, ESFOGTCI, ESFOSLLI, and ESFOHLGLCI, but acetic acid has the highest abundance in ESFOSFBCII, ESFOSLLII, and ESFOHLGLCII, except in ESFOGTCII. Carbon dioxide as a primary product exists in the weak oxidation system, but acetic acid is considered as an intermediate product in the stronger oxidation system. The data reveal that OMs in coals have been depolymerized, particularly OMs oxidized in an environment with strong polarity, although GTC is more difficult to oxidize than the other coals. Certainly, GTC has a stable structure and composition due to its distribution area and formation time. The exchange of oxocompounds is the main feature in coal oxidation even in CSS. Furthermore, the peak time of the reactant in OMs was postponed and the dispersibility of the resultants was improved. Most OMs in coals and oxidized coals were confirmed by GC/MS, and their conversion rules were deduced in Scheme 1.


## 3.3. Effect of EYs

EYs can indirectly explain the construction and composition of coals according to the quantity of OM conversion. Thus, the EYs of coals and oxidized coals were analyzed through the differential method. The results are shown in Figure5.Figure 5
Extract yield of coals and oxidation coals.As shown in Figure5, the EYs of coals exhibit an increasing tendency with the optimization of the reaction condition. The EYs of SLL and HLGLC are higher than those of the other coals, indicating that lignite and bituminous coal were more oxidizable and more OMs were possibly converted into OCOCs in the oxidation system. The study has proven that SLL and HLGLC are composed of OMs with low coalification degrees. Due to the different formation periods and the geological formation environment of SFBC and GTC, the structure and composition of OMs in SFBC and GTC are different from those in SLL and HLGLC. The conversions are significantly different even in the same oxidation condition.According to the different species and number of OMs, coals with different coalification degrees are oxidized with different susceptibilities and can be converted into different species of OCOCs successfully. Furthermore, the second system (II) has the strongest oxidation resistance, and the first (I), third (III), and fourth (IV) systems have weaker oxidation resistance. In other words, the EYs of SFBC, GTC, and SLL in the second system are higher than those of the other three systems, and the numerical values of which are separately by 18%, 8.8%, and 37.4%. This study has proven that OMs in coals can be converted into stronger polar compounds if 0.1 g catalysts give that 12 hours of reaction time are available. Meanwhile, partly oxidized products may also be formed and dissolved in an aqueous solution, rarely leading to ESF. Compared with the open system, the isolated oxygen system involving only H2O2 OMs oxidation conversion laws is easier to analyze.Finally, the weights of ESFs, ASFIs, and ASFIIs were considered (with SFBC as representative), in which the values of tube weight with dried extract and the weight of the plain tube are different. The results show that the value of WASFI is 0 and the weights of ESFs and ASFIIs are, respectively, 0.0756 and 0.1559 g in SFBC (ca. 2 g), indicating that the conversion rate of SFBC is 11.57% in the process.


## 4. Conclusions

In CSS, the process of PCO of OMs in coals can be oxidized and converted into many OCOCs via ring-opening and bond-breaking reactions through free radical substitution reaction and electrophilic reaction over SFC/TiO2 under UV light illuminating the strong polar environment.(1)
OCOCs mainly include esters, ketones, aldehydes, and acids, and the final products are carbon dioxide and water. Acetic acid is the main generated intermediate species and is easy to dissolve in an aqueous solution. The number and species of products depend on the oxidation intensity and the polarity of the solution.(2)
Lignite can oxidize more easily than bituminite, which indicate that the OMs in the coal possibly have more active hydrogen molecules or bridge bond hydrogen as a result of the distribution in different regions with different formation environments and formation periods.(3)
The EY of coals can be improved in the process of PCO. The improvement facilitates the structure analysis and efficiency utilization of coal through the solution polarity and the dispersibility of OM enhancement, also including the decoloring of coal.



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*Source: 1768351-2019-10-24.xml*