Article
Studies on the Low-Temp Oxidation of Coal Containing Organic Sulfur and the Corresponding Model Compounds Lanjun Zhang 1,2 , Zenghua Li 1,2, *, Jinhu Li 1,2 , Yinbo Zhou 1,2 , Yongliang Yang 1,2 and Yibo Tang 3 Received: 19 October 2015 ; Accepted: 3 December 2015 ; Published: 11 December 2015 Academic Editor: Derek J. McPhee 1
2 3
*
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China;
[email protected] (L.Z.);
[email protected] (J.L.);
[email protected] (Y.Z.);
[email protected] (Y.Y.) School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China College of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-0516-8359-0598
Abstract: This paper selects two typical compounds containing organic sulfur as model compounds. Then, by analyzing the chromatograms of gaseous low-temp oxidation products and GC/MS of the extractable matter of the oxidation residue, we summarizing the mechanism of low-temp sulfur model compound oxidation. The results show that between 30 ˝ C to 80 ˝ C, the interaction between diphenyl sulfide and oxygen is mainly one of physical adsorption. After 80 ˝ C, chemical adsorption and chemical reactions begin. The main reaction mechanism in the low-temp oxidation of the model compound diphenyl sulfide is diphenyl sulfide generates diphenyl sulfoxide, and then this sulfoxide is further oxidized to diphenyl sulphone. A small amount of free radicals is generated in the process. The model compound cysteine behaves differently from diphenyl sulfide. The main reaction low-temp oxidation mechanism involves the thiol being oxidized into a disulphide and finally evolving to sulfonic acid, along with SO2 being released at 130 ˝ C and also a small amount of free radicals. We also conducted an experiment on coal from Xingcheng using X-ray photoelectron spectroscopy (XPS). The results show that the major forms of organic sulfur in the original coal sample are thiophene and sulfone. Therefore, it can be inferred that there is none or little mercaptan and thiophenol in the original coal. After low-temp oxidation, the form of organic sulfur changes. The sulfide sulfur is oxidized to the sulfoxide, and then the sulfoxide is further oxidized to a sulfone, and these steps can be easily carried out under experimental conditions. What’s more, the results illustrate that oxidation promotes sulfur element enrichment on the surface of coal. Keywords: coal; organic sulfur; model compounds; low-temp oxidation; XPS
1. Introduction Spontaneous coal combustion is a severe disaster ocurring widely in coal industry. It not only occurs during the coal exploitation process, but also during the processes of coal transport and storage. Meanwhile, it causes great economical losses, as well as safety and environmental issues [1–3]. Some scholars believe that deep coal beds have a high sulfur content while shallow coal beds have low sulfur content [4]. As massive scale exploitation and utilization of coal resources continues, the sulfur content in coal is increasing. Sulfur in coal can be classified into two categories: inorganic and organic sulfur. Inorganic sulfur is present in the form of pyrite and sulfate, mainly the
Molecules 2015, 20, 22241–22256; doi:10.3390/molecules201219843
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Molecules 2015, 20, 22241–22256
former. Often, organic sulfur and the macromolecular structure of coal join together in the form of covalent bonds with complex structure and hard to separate. It mainly includes the form of mercaptans, sulfide, disulphides, thioethers, sulfoxides, sulphones, thiophene, sulfoacids, etc. [5–7]. It is generally acknowledged that pyrite as the main component of inorganic sulfur exerts an important effect on coal spontaneous combustion. In the presence of water, pyrite reacts with oxygen to form sulfate, H2 O2 , and hydroperoxides, and thereby oxidation is initiated [8–11]. Nearly all coals contain more or less organic sulfur. According to Chinese statistics, low sulfur coal whose total sulfur content is lower than 0.5%, mainly contains organic sulfur. Organic sulfur in high sulfur coal, whose average sulfur content registers 2.76%, accounts for 1.04% of the total amount and 37.7% of the total sulfur [12–14]. The organic sulfur content in some superhigh organosulfur coals such as Spanish lignite from Mequinenza and Croatian coal from Raša is over 10%, while, the organic sulfur content in Zelanian coal from Charming Creek (New Zealand) and Indian coal from Tipong is 5% [15]. Because organic sulfur in coal has a complex structure and cannot be separated as it often exists in macromolecular form, its form and content cannot be determined directly so far [16]. People usually use destructive methods to study the forms of organic sulfur in coal. The principle is that HS is produced by pyrolysis and catalytic reduction or SO by oxidation reaction [5]. The analyses mainly involve temperature programmed reduction (TPR) [17], temperature programmed pyrolysis (TPP) [17], temperature programmed oxidation (TPO) [18,19], fast pyrolysis [20] etc. Nowadays, some non-destructive technologies such as XANS [21], or XPS [22,23] are widely applied in the determination of sulfur forms in coal. It is noteworthy that the information about surface sulfur in coal samples detected by surface-analysis technologies is not identical to forms of bulk sulfur found [22,24,25]. The oxidation characteristics of organic sulfur and inorganic sulfur in coal during the coal spontaneous combustion process are different. People often ignore the role of organic sulfur during the process of coal spontaneous combustion. From modern chemistry knowledge, knowing that sulfur uses 3p orbital overlap with other atoms to form π bonds, the π bond overlap among 2p and 3p orbitals or between 3p orbitals is smaller, the distance between π electrons and the nucleus is also farther, and C-S bond and S-H bonds are prone to break. Besides, the atomic volume of sulfur is larger, while its electronegativity is smaller, so the valence shell is far from the nucleus, and less influenced by the nucleus. Compared to carbon atoms, sulfur atoms is more easily oxidized [26], so the organic sulfur functional groups in coal are often more active, although the oxidation characteristics of the various forms of organic sulfur differ greatly. For example, the mercapto group in mercaptans has strong reductibility, and it can be instantly oxidized to disulfide with cryogenic air. Under moderate conditions, this reaction can take place and disulfide can be further oxidized to a sulfonic acid [27]. Sulfides such as thioethers also can be oxidized by air to sulfoxides and sulphones. However, –SO2 bond connected with aliphatic and aromatic functional groups are prone to C–S bond breakage under low temperature heating conditions, and this releases SO2 gas. Under a higher temperature sulfoxides and sulphones with β hydrogens, can be oxidized to sulfinic acids and sulfonic acids [28]. Organic sulfur in thiophene is very stable, even at 500 ˝ C [29]. LaCount et al., reported that the model compound thiophene could release SO2 at 450 ˝ C [30] and the reaction can take place slowly with water at 300 ˝ C [31]. From the perspective of dissociation energy, in the structure of R–S–H, the dissociation energy of C–S bond is 20 kcal lower than that of the S–H bond, because the sulfur atom function weakens the bond energy. The C–S bond energy when connected with a phenyl is higher than in the corresponding aliphatic hydrocarbon compound. This is due to the fact that the unpaired electrons and the aromatic structure produce a conjugation effect [32]. Dark et al., claimed that at 20–25 ˝ C the C–S bond easily breaks and produces free radicals, so radicals appear during low-temp oxidation. As the aliphatic carbon atom content increases, the C–S bond dissociation energy decreases significantly [33]. On the contrary, the dissociation energy of an aromatic disulfide S–S bond is lower than an aliphatic disulfide one because it is undermined by the phenyls [32]. The technology for oxidative desulphurization of organic sulfur in coal is based on the characteristic that C–S bonds
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and S–S bonds are easy to break during low-temp oxidation. At present, this technology has been widely applied in the coal chemical industry. Borah et al., believe that desulphurization of organic sulfur in coal is due to the release of sulfur free radicals under 50 ˝ C, but when temperature reaches 50 ˝ C, along with free radicals, volatile sulfur compounds begins to participate [6]. After oxidation pretreatment, the organic sulfur in coal will be broken down into smaller molecules, thus making further desulphurization easier. They also believe that low-temp oxidation can convert organic sulfur in coal to S=O and –SO2 groups [6,30,34]. Gorbaty et al., conducted oxidation experiments on three coal samples with air in 125 ˝ C, the XPS and XANS results showed that aliphatic sulfides are more easily oxidized than aromatic sulfides. The aliphatic sulfides were then converted into sulfoxides, sulfones and sulfonic acids. One important thing is that sulfonic acid is easier to produce in solution than by air oxidation [35,36]. Pietrzak and Grzybek did series of oxidation experiments using different rank coals by using O2 /Na2 CO3 , PPA, and their XPS results showed that the oxidation sequence of the organic sulfur in coal is: sulphide Ñ sulfoxide Ñ sulfone. They also found the sulfur enrichment on the surface of coal during oxidation is probably due to the opening and expansion of coal pores [23,37,38]. All in all, the organic sulfur groups in coal show strong reactivity, and some organic sulfur presents itself as mercaptan, thioether and other groups, which can be oxidized at normal temperature. During oxidation, free radicals can be produced. Due to the easily-oxidized features of organic sulfur, people have conducted plenty of studies on desulphurization that have yielded fruitful results [34,39], but scholars have seldom researched the changes in organic sulfur functional groups during the process of coal low-temp oxidation in air [6], let alone the effect of organic sulfur on the characteristics of spontaneous coal combustion. In light of the above, this paper selects two representative organic sulfur compounds as model compounds. By analyzing the chromatograms of their gaseous low-temp oxidation products and GC/MS scans of the extractable matter in the oxidation residues, we tried to figure out what happens to these sulfur model compounds during low temp-oxidation. On this basis, we conducted an experiment against coal samples containing organic sulfur from Xingcheng, Guizhou Province (China). By adopting XPS technology, we investigated the sulfur form changes in coal samples before and after low-temp oxidation so as to further study the role organic sulfur plays in spontaneous coal combustion. 2. The Result and Discussion 2.1. The Result of Experiments with Organic Sulfur Model Compounds As mentioned previously, the organic sulfur in coal has a complex composition and structure, therefore, nowadays, people still cannot conduct thorough studies on it. However, coal spontaneous combustion is a very complex physicochemical process and its mechanism remains to be unveiled. Therefore, in order to better understand the mechanism of organic sulfur reactions during the spontaneous coal combustion process, this paper selected two representative organic sulfur model compounds and conducted an experimental study on their low-temp oxidation. The use of model compounds to study the complex chemical reaction processes in coal is widely accepted, and it has been applied in the international coal chemistry study field. Model compounds possess some specific functional groups. Therefore, we try to reveal the complex reaction mechanisms of coal molecules by studying the reaction mechanisms of model compounds [40–42]. Nowadays, people mainly use organic sulfur model compounds to study the mechanisms of the pyrolysis and desulfurization processes and sulfur’s transformation and transportation. Yan et al., conducted pyrolysis analysis of many sulfur-containing model compounds and concluded the rules of the generation of gases from thioethers, mercaptans, disulphides, and thiophene [43]. Mullens et al. conducted an AP-TPR-MS experiment on the model compounds of benzyl thiofuran and dibenzothiophene and found that the corresponding H2 S release peaks occurred at 545 ˝ C and 690 ˝ C [44], respectively.
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2.1.1. The Oxygen Consumption of Model Compounds during Low-Temp Oxidation The relationship between the oxygen consumption and temperature during the low-temp oxidation of the diphenyl sulfide and cysteine model compounds is shown in Figure 1. It can be seen from the figure that the oxygen consumptions of both the diphenyl sulfide and cysteine model compound and temperature show an exponential relation. The oxygen consumption of the cysteine model compound is much larger than the oxygen consumption of the diphenyl sulfide model compound, which changes little between 30 ˝ C to 80 ˝ C. When the temperature reaches 90 ˝ C, Molecules 2015, 20, page–page the oxygen concentration gradually decreases while the oxygen consumption increases. During the low-temp oxidation consumption increases from 0 mL/min to 0.5to mL/min, which oxidationprocess, process,the theoxygen oxygen consumption increases from 0 mL/min 0.5 mL/min, can be explained by the fact thatfact diphenyl sulfide mainly byreacts physical up to 80 °C. which can be explained by the that diphenyl sulfidereacts mainly by adsorption physical adsorption up Above 80 Above °C, it starts to it undergo and react chemically with oxygen, andoxygen, the oxygen to 80 ˝ C. 80 ˝ C, starts tochemisorption undergo chemisorption and react chemically with and consumption increases slowly. Theslowly. cysteine compound’s oxygen consumption registers the oxygen consumption increases Themodel cysteine model compound’s oxygen consumption ˝ 0.418 mL/min 30 °C, at which shows that the active group in group cysteine be oxidized at room registers 0.418 at mL/min 30 C, which shows that the active in can cysteine can be oxidized ˝ C, temperature. Under 140Under °C, the140 slope curve demonstrates a gradual increase of oxygen consumption, at room temperature. the slope curve demonstrates a gradual increase of oxygen which reflects which the slow reaction speedreaction of the active in cysteine. the contrary, thecontrary, oxygen consumption, reflects the slow speed group of the active groupOn in cysteine. On the consumption increases sharply above 140 °C, which to attributed a strong reaction between the the oxygen consumption increases sharply above 140is ˝attributed C, which is to a strong reaction active groups and oxygen. between the active groups and oxygen. 2.5 Diphenyl sulfide cysteine
Oxygen Consumption/mL.min-1
2.0
1.5
1.0
0.5
0.0 20
40
60
80
100
120
140
160
180
200
Temperature/℃
Figure 1. The relationship between the O2 consumption and oxidation temperature. Figure 1. The relationship between the O2 consumption and oxidation temperature.
2.1.2. Gaseous Products Generated in the Low-Temp Oxidation Process of Organic Sulfur 2.1.2. Gaseous Products Generated in the Low-Temp Oxidation Process of Organic Sulfur Model Model Compounds Compounds CO, gas chromatograph chromatograph CO, CO CO22 and and SO SO22 are are not not detected detected in in the the low-temp low-temp oxidation oxidation process process when when aa gas and infrared gas analyzer are used to determine the gaseous product of diphenyl sulfide oxidation. and infrared gas analyzer are used to determine the gaseous product of diphenyl sulfide oxidation. Yet, as shown in Figure 2, when the above instruments are used to determine the Yet, as shown in Figure 2, when the above instruments are used to determine the gaseous gaseous products of cysteine, the results demonstrate that CO, SO 2 and CO2 are detected during the products of cysteine, the results demonstrate that CO, SO2 and CO2 are detected during the low-temp CO2 appears first at the lowest temperature and with the highest low-temp oxidation, oxidation, among among which which CO 2 appears first at the lowest temperature and with the highest concentration, the concentration concentration of of both both CO CO and and SO SO2 are are rather rather low. low. According concentration, while while the According to to the the 2 relationship between the amount generated and the oxidation temperature of CO, SO 2 and CO2 the relationship between the amount generated and the oxidation temperature of CO, SO2 and CO2 gas generation shows an an exponential relationship with the gas generation shows exponential relationship withoxidation oxidationtemperature, temperature,with with aa correlation correlation index over 0.99. When the temperature is 30 °C, CO 2 can be detected at a concentration of 0.00336%, ˝ index over 0.99. When the temperature is 30 C, CO2 can be detected at a concentration of 0.00336%, which come from from the the outside outside atmosphere. atmosphere. The which may may come The amount amount of of CO CO22 generated generated begins begins to to increase increase gradually when the temperature reaches 50 °C and increases more slowly from 50 °C to 140 140 ˝°C. gradually when the temperature reaches 50 ˝ C and increases more slowly from 50 ˝ C to C. Finally, it rises sharply above 140 °C. The concentration of CO 2 can be up to 1.07% when the ˝ Finally, it rises sharply above 140 C. The concentration of CO2 can be up to 1.07% when the temperature reaches 180 °C. The oxidation temperature of the model compound cysteine is 120 °C when CO emerges, while the rate of release slows down a little from 120 °C to 140 °C and then it 22244 experiences a larger increase when the temperature is above 140 °C. In general, the concentration of CO during the whole process is not high. SO2 appears when the oxidation temperature is 130 °C. The amount of SO2 generated increases slightly when the temperature ranges from 130 °C to 140 °C and
Molecules 2015, 20, 22241–22256
temperature reaches 180 ˝ C. The oxidation temperature of the model compound cysteine is 120 ˝ C when CO emerges, while the rate of release slows down a little from 120 ˝ C to 140 ˝ C and then it experiences a larger increase when the temperature is above 140 ˝ C. In general, the concentration of CO during the whole process is not high. SO2 appears when the oxidation temperature is 130 ˝ C. The amount of SO2 generated increases slightly when the temperature ranges from 130 ˝ C to 140 ˝ C and experiences a larger increase as the temperature increase continues. When the temperature goes ˝ up to 180 concentration of SO2 reaches 93 ppm. Molecules 2015,C, 20,the page–page 60
30
ExpGro1
Equation
y = A1*exp(x/t 1) + y0
Reduced ChiSqr
0.21822
Adj. R-Square
0.99804
CO
y0
CO
A1
CO
t1
fitting curve of SO2
100
Value
20
SO2
120
Model
Standard Erro
0.20776
0.12943
SO2/ppm
40
CO/ppm
140
CO fitting curve of CO
50
7.53466E-1 7.65595E-12 6.13294
80 60
10
ExpGro1
Equation
y = A1*exp(x/t 1) + y0
Reduced ChiSqr
1.7074
Adj. R-Square
0.99712 Value
40
0.21254
Model
y0
0.0908
0.38891
SO2
A1
7.66983E-
4.00383E-6
SO2
t1
11.03343
0.35513
20
0
0 0
50
100
150
200
0
25
50
75
Temperature/℃ 1.2
100
125
Temperature/℃
150
175
200
CO2 fitting curve of CO2
1.0
0.8
CO2/%
Standard Erro
SO2
Model
ExpGro1
Equation
y = A1*exp(x/t1) + y0
Reduced Chi-S qr
0.6
6.49908E-4
Adj. R-Square
0.99212 Value
0.4
Standard Error
CO2
y0
0.0014
0.00765
CO2
A1
1.7922E-7
1.47099E-7
CO2
t1
11.52109
0.60914
0.2
0.0 0
50
100
150
200
Temperature/℃
Figure 2. The oxidation product curve of the model compound cysteine. Figure 2. The oxidation product curve of the model compound cysteine.
2.1.3. GC/MS Analysis Results of Organic Sulfur Model Compounds 2.1.3. GC/MS Analysis Results of Organic Sulfur Model Compounds In this paper, the acetone extract from the low-temp oxidation residues of diphenyl sulfide and In this paper, the acetone extract from the low-temp oxidation residues of diphenyl sulfide and cysteine are analyzed using GC/MS. Table 1 shows the name and structure of the peaks marked in cysteine are analyzed using GC/MS. Table 1 shows the name and structure of the peaks marked in Figures 3 and 4. Figures 3 and 4. It can be seen from the GC/MS analysis results of the model compound diphenyl sulfide in It can be seen from the GC/MS analysis results of the model compound diphenyl sulfide in Figure 3 that diphenyl sulfide, the compound represented by peak 2, displays the highest content, Figure 3 that diphenyl sulfide, the compound represented by peak 2, displays the highest content, followed by the content of diphenyl sulfone represented by peak 5. In addition, other substances like followed by the content of diphenyl sulfone represented by peak 5. In addition, other substances like diphenyl disulfide, diphenyl sulfoxide and benzene are also detected in the extract. It can be seen diphenyl disulfide, diphenyl sulfoxide and benzene are also detected in the extract. It can be seen from the GC/MS analysis results of the model compound cysteine in Figure 4 that the compound from the GC/MS analysis results of the model compound cysteine in Figure 4 that the compound represented by peak 6 contains the highest content of mesityl oxide, while other content ranked in represented by peak 6 contains the highest content of mesityl oxide, while other content ranked in descending order are the compounds represented by peak 8, peak 7, peak 10, peak 9 and peak 11. descending order are the compounds represented by peak 8, peak 7, peak 10, peak 9 and peak 11. 12 11
abundance(105)
10 9
5
2
8 7 6
22245
5 4 3 2
1
3
4
Figures 3 and 4. It can be seen from the GC/MS analysis results of the model compound diphenyl sulfide in Figure 3 that diphenyl sulfide, the compound represented by peak 2, displays the highest content, followed by the content of diphenyl sulfone represented by peak 5. In addition, other substances like diphenyl disulfide, diphenyl sulfoxide and benzene are also detected in the extract. It can be seen from the GC/MS analysis results of the model compound cysteine in Figure 4 that the compound Molecules 2015, 20, 22241–22256 represented by peak 6 contains the highest content of mesityl oxide, while other content ranked in descending order are the compounds represented by peak 8, peak 7, peak 10, peak 9 and peak 11. 12 11
abundance(105)
10 9
5
2
8 7 6 5 4 3 2
1
1
4
3 2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
Retention time/min Figure 3. GC-MS of the oxidation products of diphenyl sulfide.
Figure 3. GC-MS of the oxidation products of diphenyl sulfide. Molecules 2015, 20, page–page
5
18
Molecules 2015, 20, page–page 17 Molecules 2015, Molecules 2015, 20, page–page Molecules 2015, 20, page–page Molecules 2015, 20, 20, page–page page–page Molecules 2015, 20, page–page Molecules 2015, 20, page–page 16 15 14 13 12 11 10
6 abundance(1055))) abundance(10 abundance(10
9
555
5
17 17 17 1717 16 16 16 1616 15 15 15 1515 14 14 14 1414 13 13 13 1313 12 12 12 1212 11 11 11 1111 10 10 10 1010 9 99 99 8 88 88 7 77 77 6 66 66 5 55 55 4 44 44 3 33 33 2 22 22 1 11 11
abundance(10 )
5
abundance(1055555)) abundance(10 abundance(10 abundance(10 abundance(10 )))
18 18 18 1818
8
666
7 6 5 4 3 2 1
777
8 8
888 6 6 7
2.50
2.50 2.50 2.50 2.50 2.50
18 18 18 17 17 17 16 16 16 15 15 15 14 14 14 13 13 13 12 12 12 11 11 11 10 10 10 999 888 777 666 555 444 333 222 111
3.00 3.00 3.00 3.00 3.00
10
9 7 3.00
3.50
10 10 10
999
9
11 4.00
10
4.50
5.00
10 9 Retention time/min
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11 11 11
2.50 3.50 2.50 3.50 2.50 3.50 3.50 3.50
3.00 4.00 3.50 4.50 4.00 5.00 4.50 5.50 3.00 4.00 4.50 4.00 4.50 5.00 of cysteine. 5.50 4.50 3.00 4.00 3.50 4.00 5.00 4.50 5.50 4.00 4.50 5.00 5.50 4.00 4.50 5.00 5.50 Figure 4. GC-MS of the3.50 oxidation products
5.00 6.00 5.00 6.00 5.00 6.00 6.00 6.00
6.00
11
5.50 5.50 5.50
6.00 6.00 6.00
Figure 4. GC-MS of the oxidation products of cysteine. Retention time/min Retention time/min Retention time/min Retention time/min Retention time/min Retention time/min Table 1. GC/MS-detectable compounds in organic sulfur compound extracts. Figure the products 4. GC-MS of the of oxidation products of cysteine. Figure4. 4.GC-MS GC-MSof ofFigure theoxidation oxidation products ofcysteine. cysteine. Figure GC-MS of the oxidation products of cysteine. Figure 4.4. GC-MS of the oxidation products of cysteine.
TableChemical 1. GC/MS-detectable compounds sulfur compound extracts. Peak Peakin organic Chemical
Structural Formula Structural Formula Number Name Number Name Table 1.1.GC/MS-detectable sulfur extracts. Table 1.compounds GC/MS-detectable compounds in organic sulfur compound extracts. Table GC/MS-detectable compoundsininorganic organic sulfurcompound compound extracts.
Peak Peak Peak Peak Peak Peak Number
Table 1.compounds GC/MS-detectable compounds in organicextracts. sulfur compound extracts. Table1.1.GC/MS-detectable GC/MS-detectable compoundsin inorganic organic sulfurcompound compound extracts. Table sulfur O Chemical Structural Peak
Chemical Peak Chemical Chemical Name Structural Formula Chemical Peak Chemical Peak Peak Chemical Chemical Peak Chemical PeakStructural Chemical Peak Chemical Chemical Peak Chemical Chemical Peak Chemical diacetone Chemical Formula Structural Formula Name benzene Formula Number Formula Formula OH Structural Formula Structural Structural Formula Structural Structural Formula Structural Formula Structural Formula Structural Formula Structural Formula Structural Formula Structural Formula Structural Formula Number Number Name Number Name Number 1Name Name Number Number 7 Name Name Number Name Number Name Number Name Number Name H3C Number Name Number Name Number Name Number Name alcohol O O OOO
1 11111
2
2 22222
4 44444
5 55555
diphenyl sulfone
diphenyl diphenyl diphenyl diphenyl diphenyl diphenyl sulfone sulfone sulfone sulfone sulfone sulfone 6
6 66666
mesityl mesityl mesityl mesityl mesityl mesityl oxide oxide oxide oxide oxide oxide
S
O S
O O O OO O S
O
O O O OO O O O OO S S SSS
diphenyl diphenyl O sulfone sulfone
55
mesityl oxideO O OOO 66 H CC C 3 HHH 3C 3HC 33
H 3C
O O O
5-hexen-2-one
H C H C HH33H C33C 3C 3
CH 3 CH CH CH CH 33 33 3
O O O CH3
O H C H H333C C
O O OOO
H C H H333C C
OH OH OH CH CH CH333
O O O
CH3
H C H H222C C
H C H22C C HH22H C 2C
2 5-hexen-2-one 88 5-hexen-2-one 5-hexen-2-one 5-hexen-2-one CH3 5-hexen-2-one 5-hexen-2-one 5-hexen-2-one 5-hexen-2-one CH3 O 4-methoxy-4-m CH CH CH CH 3 CH CH333 CH CH 3333 9 ethyl-2-pentan H3C CH CH CH CH 3 CH333 CH CH CH one CH 3 3 O 33 33 CH CH CH 3 CH CH CH 3333 3 O H3C 4-methoxy-4-m O O O 4-methoxy-4-m 4-methoxy-4-m 4-methoxy-4-m O OO 4-methoxy-4-m 4-methoxy-4-m 4-methoxy-4-m S S S ethyl-2-pentan H C ethyl-2-pentan H C ethyl-2-pentan C 99 H ethyl-2-pentan HHH CC 3 H333C C ethyl-2-pentan C ethyl-2-pentan 9 99999SS 4-methoxy-4-methyl-2-pentanone 3ethyl-2-pentan 3H 33 one H3C one O one4-mercapto-4one one O O O CH3 one one O OO H C H C CC 3 H333C C 3C 3H 33 10 methyl-2-penta HHH H S O O none CH3 H H C H C HHH C3C 3 HO C 333C 3 CH 33C 4-mercapto-4CH CH 4-mercapto-4CH 3 4-mercapto-44-mercapto-4CH333 CH CH 4-mercapto-44-mercapto-44-mercapto-43333 S S 10 methyl-2-penta 10 methyl-2-penta 10 methyl-2-penta 10 methyl-2-penta methyl-2-penta 1010 4-mercapto-4-methyl-2-pentanone 10 methyl-2-penta S 10 methyl-2-penta H S H H S HHH S HS S CH3 S S Snone none none none none O none CH CH CH 3 O 3,3,5,5-tetramet noneCH CH333 O CH CH O OO 3333 O 11 hyl-1,2,4-trithi H3C CH 3 S O S O S S S S SS olane CH O CH O CH S CH 3 CH333 CH S CH S SSS 3333 S S 3,3,5,5-tetramet S H3C 3,3,5,5-tetramet 3,3,5,5-tetramet 3,3,5,5-tetramet 3,3,5,5-tetramet 3,3,5,5-tetramet 3,3,5,5-tetramet hyl-1,2,4-trithi 11 hyl-1,2,4-trithi 11 hyl-1,2,4-trithi 1111 3,3,5,5-tetramethyl-1,2,4-trithiolane 11 hyl-1,2,4-trithi H C 11 hyl-1,2,4-trithi 11 hyl-1,2,4-trithi 11 hyl-1,2,4-trithi H C CH33 C H CC CH 3 H HH 333 CH CH CH 3C 3H CH CH 33 33C S S 3 33 S olane olane olane olane olane SSSS olane olane
8 88888
S
S S SSS diphenyl diphenyl sulfoxide sulfoxide
44
S S
S S SSS diphenyl diphenyl sulfane sulfane
OH OH OH OH OH
3 3H C
2
diphenyl S diphenyl S SSS S S SSS disulfide disulfide
33
diphenyl sulfoxide
diphenyl diphenyl diphenyl diphenyl diphenyl diphenyl sulfoxide sulfoxide sulfoxide sulfoxide sulfoxide sulfoxide 5
22
7 77777
S
8
diphenyl disulfide
diphenyl diphenyl diphenyl diphenyl diphenyl diphenyl disulfide disulfide disulfide disulfide disulfide disulfide 4
benzene benzene
diphenyl sulfane
diphenyl diphenyl diphenyl diphenyl diphenyl diphenyl sulfane sulfane sulfane sulfane sulfane sulfane 3
3 33333
11
benzene benzene benzene benzene benzene benzene
H3C
diacetone diacetone diacetone diacetone diacetone diacetone diacetone diacetone alcohol 77 alcohol H C alcohol alcohol alcohol H C HH33H CC3C alcohol alcohol alcohol
CH 3
mesityl H 3 C mesityl oxide CH H C CH oxide CH H CH CH H3333333C C 3
_ _____
C 3 HHH C3C 3H 33C
H H333C C
_
________
H C C HHH CC 3 3C 3H 33
H H33C C
33 _____________
________ ________ _____________ ________________ _____________ __ ________ ________ _____________ ________ _____________ ________ _____________ CH CH CH333
_____________ _____________ _____________
2.1.4. The Low-Temp Oxidation of ModelHH Compounds Containing Organic Sulfur C H C C The mechanism of the low-temp oxidation process of the model compound diphenyl sulfide 2.1.4. Low-Temp Oxidation of Model Compounds Containing Organic Sulfur 2.1.4. The Low-Temp Oxidation of low-temp Model Compounds Containing 2.1.4.The The Low-Temp Oxidation ofby Model Compounds Containing Organic Sulfur asOrganic can be generally inferred analyzing the oxidation products well asSulfur the oxidation 2.1.4. The Low-Temp Oxidation of Model Compounds Containing Organic Sulfur 2.1.4. The Low-Temp Oxidation of Model Compounds Containing Organic Sulfur residue extract. As shown in Figure 5, in the temperature range of 30 °C~80 °C, the adsorption between The mechanism of the low-temp oxidation process of the model compound diphenyl sulfide The mechanism of the low-temp oxidation process of the model compound 22246 The mechanism of the low-temp oxidation process of the model compound diphenyl sulfide The mechanism mechanism of of the the low-temp low-temp oxidation oxidation process process of of the the model model compound compound diphenyl diphenyl sulfide sulfidediphenyl sulfide The diphenyl sulfide and oxygen is mainly physical, and during this time both the oxygen consumption can by the low-temp oxidation products as as can be generally inferred analyzing the low-temp oxidation as well as the oxidation canbe begenerally generallyinferred inferred byanalyzing analyzing theby low-temp oxidation products aswell wellproducts asthe theoxidation oxidation can be generally inferred by analyzing the low-temp oxidation products as well as the oxidation can be generally inferred by analyzing the low-temp oxidation products as well as the oxidation and theAs oxygen concentration change are small. However, afterrange 80 °C, chemical adsorption and between residue extract. As5, shown in Figure 5, in in the temperature of 30 °C~80 °C, °C, the adsorption adsorption residue shown in in range of adsorption between residueextract. extract.As As shownextract. inFigure Figure 5, inthe thetemperature temperature range of30 30°C~80 °C~80°C, °C,the the adsorption between residue As shown in Figure 5, the temperature range of 30 °C~80 the between residue extract. As shown in Figure in the temperature range of 30 °C~80 °C, the adsorption between residue extract. shown in Figure 5,5, in the temperature range of 30 °C~80 °C, the adsorption between reactions begin to take place andphysical, oxygenisconsumption increases, thenthe diphenyl sulfoxide isoxygen generated diphenyl sulfide and oxygen is mainly and during this time both oxygen consumption diphenyl sulfide and oxygen mainly physical, and during this time both the consumption diphenylsulfide sulfideand andoxygen oxygenisis ismainly mainlyphysical, physical,and andduring duringthis thistime timeboth boththe theoxygen oxygenconsumption consumption diphenyl sulfide and oxygen mainly physical, and during this time both the oxygen consumption diphenyl the reaction oxidized diphenyl sulphone. This process is the main mechanism and and the finally oxygen concentration change are after small. after 80 reaction °C, chemical adsorption and and theinoxygen concentration change aretosmall. However, 80However, °C, chemical adsorption and
Molecules 2015, 20, 22241–22256
2.1.4. The Low-Temp Oxidation of Model Compounds Containing Organic Sulfur The mechanism of the low-temp oxidation process of the model compound diphenyl sulfide can be generally inferred by analyzing the low-temp oxidation products as well as the oxidation residue extract. As shown in Figure 5, in the temperature range of 30 ˝ C~80 ˝ C, the adsorption between diphenyl sulfide and oxygen is mainly physical, and during this time both the oxygen consumption and the oxygen concentration change are small. However, after 80 ˝ C, chemical adsorption reactions begin to take place and oxygen consumption increases, then diphenyl Molecules 2015,and 20, page–page sulfoxide is generated in the reaction and finally oxidized to diphenyl sulphone. This process is the main reactionoxidation mechanism of the low-temp of diphenyl sulfide. C–C of the low-temp of diphenyl sulfide.oxidation In this process, the C–C bondsIninthis theprocess, benzenethe ring, as bonds the bonds benzene ring, as well as C–H bonds only aofseries of oxidation well asinC–H remain unchanged, with only remain a seriesunchanged, of oxidationwith reactions the sulfur atom in reactions the sulfur atomThis in diphenyl ocurring. This isreactivity due to the larger nucleophilic diphenyl of sulfide ocurring. is due to sulfide the larger nucleophilic of the sulfur atoms in reactivity of the sulfur atoms insulfoxide, diphenylwhich sulfideare and diphenyl which aretomore easily diphenyl sulfide and diphenyl more easily sulfoxide, oxidized compared the carbon oxidized to the carbon atoms on the benzene ring. Besides, is alsoofa small amount atoms oncompared the benzene ring. Besides, there is also a small amount of there homolysis the C–S bond of of homolysis of the C–S bondto ofthe diphenyl sulfide, leading to theWe formation freetoradicals. attribute diphenyl sulfide, leading formation of free radicals. attributeofthis the lowWe dissociation this to the energy of C–S bondFinally, in phenyl Finally, collision among energy of low C–Sdissociation bond in phenyl compounds. the compounds. collision among freethe radicals generates free radicals generates disulfide and benzene. Asbond the C–C bond and ring C–Hremain bond in the diphenyl disulfide and diphenyl benzene. As the C–C bond and C–H in the benzene intact benzene ring remain intact during the low-temp oxidation process of diphenyl sulfide, naturally gases during the low-temp oxidation process of diphenyl sulfide, naturally gases such as CO, CO2 and SO2 such as be CO, CO2 and cannot be detected in its products. cannot detected inSO its2products. O S
+O2
-
+
S
O S
+O2
O S
bond rupture
C
+O2
O
S
S
S
+
Figure 5. 5. The The oxidation oxidation pathway pathway of of diphenyl diphenyl sulfide. sulfide. Figure
The low-temp oxidation of the model compound cysteine can be generally inferred from The low-temp oxidation of the model compound cysteine can be generally inferred from analyzing the low-temp oxidation products as well as the oxidation residue extract. As shown in analyzing the low-temp oxidation products as well as the oxidation residue extract. As shown in Figure 6, the reducibility of the sulphydryl in cysteine determines that cysteine can be oxidized to Figure 6, the reducibility of the sulphydryl in cysteine determines that cysteine can be oxidized disulfide cystine in low-temp air. This reaction can take place when the temperature reaches 30 °C. to disulfide cystine in low-temp air. This reaction can take place when the temperature reaches The model compound cysteine begins to consume oxygen at a rate of 0.418 mL/min. disulfide cystine 30 ˝ C. The model compound cysteine begins to consume oxygen at a rate of 0.418 mL/min. continues to be oxidized in the air to sulfenic acid, sulfinic acid and sulfonic acid, among which the disulfide cystine continues to be oxidized in the air to sulfenic acid, sulfinic acid and sulfonic acid, sulfonic acid generates pyruvic acid, sulfurous acid and ammonia through desulfonation and among which the sulfonic acid generates pyruvic acid, sulfurous acid and ammonia through deamination. Then the sulfurous acid is decomposed into SO2 at 130 °C, while the pyruvic acid generates CO2 and then acetone after decarboxylation. The condensation of acetone forms diacetone 22247 alcohol which generates mesityl oxide after dehydration. The reaction of mesityl oxide with oxygen and methylene generates 5-hexen-2-one, which is accompanied by of a small amount of CO generated at the highest temperature of 120 °C. This reaction also generates 4-methoxy-4-methyl-2-pentanone,
Molecules 2015, 20, 22241–22256
desulfonation and deamination. Then the sulfurous acid is decomposed into SO2 at 130 ˝ C, while the pyruvic acid generates CO2 and then acetone after decarboxylation. The condensation of acetone forms diacetone alcohol which generates mesityl oxide after dehydration. The reaction of mesityl oxide with oxygen and methylene generates 5-hexen-2-one, which is accompanied by of a small amount of CO generated at the highest temperature of 120 ˝ C. This reaction also generates 4-methoxy-4-methyl-2-pentanone, the reaction of which with H2 S generates 4-mercapto-4-methyl-2-pentanone whose chemical bonds break under its reaction with oxygen to generate isopropyl mercaptan free radicals. These isopropyl mercaptan free radicals combine with sulfur free radicals, generating 3,3,5,5-tetramethyl-1,2,4-trithiolane. In general, the large number of active groups in cysteine, such as sulphydryl, amino and carboxy, leads to a more complex Molecules 2015, 20, page–page oxidation reaction process, with more cross reactions and side reactions, therefore, it can be substantially concluded from the reaction process that the oxidation mechanism of organic sulfur therefore, it can be substantially concluded from the reaction process that the oxidation mechanism is that mercaptan is oxidized in low-temp air to disulfide, which is further oxidized to sulfinic acid of organic sulfur is that mercaptan is oxidized in low-temp air to disulfide, which is further oxidized and sulfonic acid. Though desulfonation, the sulfonic acid generates sulfurous acid, which releases to sulfinic acid and sulfonic acid. Though desulfonation, the sulfonic acid generates sulfurous acid, SO2 by pyrolysis. C–S bond cleavage and sulphydryl detachment can also occur in the mercaptan, which releases SO2 by pyrolysis. C–S bond cleavage and sulphydryl detachment can also occur in the leading to the generation of H2 S gas. S–S bond and C–S bond cleavage in the disulfide can also mercaptan, leading to the generation of H2S gas. S–S bond and C–S bond cleavage in the disulfide can produce sulfur free radicals. also produce sulfur free radicals. O
H2N
O
+O2 −−NH2
CH3
HO
O
− −SH
S
Δ
HO
OH
O
H2N
NH2
O
H2N
+Ο2
+Ο2
O
H2N
O
+Ο2
O
O S
NH2
S
SH
HO
OH
O
+O2 -H2O
S
OH
OH
S
HO
OH
HO
O
O
H2SO3
+ NH + H C 3
3
O
O
-CO2
self-aldol OH
OH
H3C
O
H3C
CH3
-H2O
SO2 O
O
CH3
+Ο2 + −CH2−
+Ο2 + −CH2− −CO
H2C
H3C
H 3C
O
CH3
H3C
CH3
CH3
CH3
O H3C
H 3C
+H2S
H 3C HS
CH3
CH 3 +O bond rupture 2 CH 3
HS
O
S H3C
S
S
CH3
C
CH3
+O2 + S:
CH3
H3C
Figure 6. The oxidation pathways of cysteine. Figure 6. The oxidation pathways of cysteine.
2.2. The XPS Results Before and After the Low-Temp Oxidation of Coal Samples As has been mentioned, XPS technology, the most effective method of surface elemental analysis so far, is widely employed in the research on the sulfur forms of coal [19,45–48]. Using this technology, this paper examined the changes in the surface composition of coal samples from Xingcheng, Guizhou Province, before and after low-temp oxidation 22248 in order to further understand the characteristics and mechanism of the spontaneous combustion of coal containing organic sulfur. Figures 7 and 8 are XPS peak separation fitting result charts of element S in the XF sample before
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2.2. The XPS Results Before and After the Low-Temp Oxidation of Coal Samples As has been mentioned, XPS technology, the most effective method of surface elemental analysis so far, is widely employed in the research on the sulfur forms of coal [19,45–48]. Using this technology, this paper examined the changes in the surface composition of coal samples from Xingcheng, Guizhou Province, before and after low-temp oxidation in order to further understand the characteristics and mechanism of the spontaneous combustion of coal containing organic sulfur. Figures 7 and 8 are XPS peak separation fitting result charts of element S in the XF sample before and after oxidation. Tables 2 and 3 are the XPS analysis results of various forms of sulfur in the coal samples before and after oxidation. It can be seen from Figure 7 and Table 2 that there are five S 2p peaks on the surface of the XF original coal samples. As represented by peak 0, the total content of sulfide sulfur and pyrite sulfur is 29.77%. It can be inferred from the higher pyrite content of the coal samples that the coal surface only contains trace amounts of sulfide sulfur. Apart from sulfide sulfur (sulfide sulfur and pyrite sulfur cannot be distinguished due to the low resolution of the experiment) the content of thiophene sulfur, the major component of organic sulfur, is 27%. In addition, there is Molecules 2015, 20, page–page also sulfone, a small amount of sulfoxide and 17.01% of sulfate detected in the surface of coal samples. Differences exist which show show that the Differences exist between between the the XPS XPS results results and and the the chemical chemical analysis analysis results, results, which that the content of pyrite sulfur in coal is 44.77%, while the total content of pyrite sulfur and sulfate is 29.77% content of pyrite sulfur in coal is 44.77%, while the total content of pyrite sulfur and sulfate is 29.77% in the the XPS XPS results. The reason reason for for this this phenomenon phenomenon is is that that the the pyrite pyrite in in the the coal coal particle particle surface surface is is in results. The easily oxidized to sulfate, leading to the increase of sulfate sulfur content to 17.01% in the XPS results easily oxidized to sulfate, leading to the increase of sulfate sulfur content to 17.01% in the XPS results while there there is By adding adding the the content content of of while is only only 0.03% 0.03% of of sulfate sulfate in in coal coal in in the the chemical chemical analysis analysis results. results. By pyritic sulfur, sulfate and sulphide sulfur in the XPS results, it can be obtained that these three forms pyritic sulfur, sulfate and sulphide sulfur in the XPS results, it can be obtained that these three forms of sulfur sulfur make make up up 42.7% 42.7% of of the the total total sulfur sulfur ratio, ratio, while while in in chemical chemical analysis analysis results, results, the the sum sum of of pyritic pyritic of sulfur and and sulfate make up up 44.38% The two two results results are are therefore with the sulfur sulfate make 44.38% of of the the total total sulfur sulfur ratio. ratio. The therefore close, close, with the ratio in the XPS results being slightly smaller than that of the chemical analysis results. The reasons ratio in the XPS results being slightly smaller than that of the chemical analysis results. The reasons for for this phenomenon are that, on the one hand, as a surface analysis technology, XPS only detects the this phenomenon are that, on the one hand, as a surface analysis technology, XPS only detects the molecular layer layer 2–20 2–20 in in the the surface surface [37], [37], so so there there is is aa gap gap between between it it and and bulk bulk analysis analysis results; results; on on the the molecular other hand, the “particle effect” of pyrite might weaken the XPS signal. other hand, the “particle effect” of pyrite might weaken the XPS signal.
Figure 7. S 2p spectrum of the XF original coal sample. Figure 7. S 2p spectrum of the XF original coal sample.
22249
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Figure 7. S 2p spectrum of the XF original coal sample.
Figure 8. S 2p spectrum of the XF oxidized coal sample.
Figure 8. S 2p spectrum of the XF oxidized coal sample.
Peak 0 1 2 3 4
Table 2. The S 2p spectrum analysis of the XF original coal sample. 9 Sulfur Form Postion Area FWHM (Ev) %GL (%) Pyritic sulfur + sulphide Thiophene Sulfoxide Sulphone Sulfate
163.018 164.297 166.049 168.443 169.620
123.098 111.669 31.065 94.254 53.456
1.2 1.2 1.2 1.2 1.2
0 0 0 0 0
W (%) 29.77 27.00 7.51 22.79 12.93
Table 3. The S 2p spectrum analysis of the XF oxidized coal sample. Peak
Sulfur Form
Postion
Area
FWHM (Ev)
%GL (%)
W (%)
0 1 2 3
Pyritic sulfur + sulphide Thiophene Sulphone Sulfate
162.991 164.335 168.267 169.509
195.719 114.139 193.329 121.942
1.2 1.2 1.2 1.2
0 0 0 0
31.31 18.26 30.93 19.51
As illustrated by Figure 8 and Table 3 that there are four S 2p peaks on the surface of the XF sample after oxidation. As represented by peak 0, the total content of sulfide sulfur and pyrite sulfur is 31.3%. The content of sulfone, the major component of organic sulfur, is 30.93%, followed by the content of thiophene sulfur, 18.26% and the content of sulfate, 19.51%. Sulfoxide is not detected on the coal surface. The conclusions can be drawn by making a comparison of XPS test results of the XF sample before and after oxidation: the sulfur peak intensity and area of the oxidized sample are higher than the original one, which shows sulfur element enrichment on the surface of coal during oxidation, which is consistent with Graybek’s conclusions. This may be due to the expansion of pores on the oxidized sample surface, which enables sulfur compounds to migrate to the surface [37]. Another reason is the oxidation makes C elements on the surface be consumed via gasification, while less S element is consumed, then the content of coal surface S is relatively increased [22]. In addition, by comparing the XPS peak parameters of sulfur S 2p before and after oxidation, it can be found that: (1) the results show that sulfide and pyrite sulfur increased slightly (from 29.77% to 31.31%), which may result from S element enrichment on the surface; (2) a very interesting phenomenon is the observation of the relative content reduction of thiophenic sulfur (from 27% to 18.26%). This may be due to the large thiophenic sulfur molecules being very stable, and compared with other small molecules find it harder to migrate to the surface of coal; (3) there is no sulfoxide detected on the 22250
Molecules 2015, 20, 22241–22256
surface of the coal after oxidation, yet the content of sulfone increases significantly (from 22.79% to 30.93%), indicating that the form of organic sulfur in the coal particle surface changes after low-temp oxidation. That is, sulfide sulfur is oxidized to the sulfoxide, and then the sulfoxide is further oxidized to sulfone. The step where sulfoxide is oxidized to sulfone can be easily carried out under these experimental conditions; (4) sulfate content increased (from 12.93% to 19.15%), suggesting that the pyrites are transformed to sulfate during oxidation on the coal surface; (5) SO2 has not been detected throughout the low-temp oxidation process of the XF samples, so it can be inferred by combining the low-temp oxidation mechanism of the above two organic sulfur model compounds that there is no or little mercaptan and thiophenol in the organic sulfur composition of the XF sample, with thiophene and sulfone being its main ingredients. It also contains traces of sulfoxide and sulfide. 3. Experimental Section 3.1. Low-Temp Oxidation Experimental Facilities This paper adopts the low-temp oxidation experimental facilities shown in Figure 9. This system is composed of an air inlet system, coal sample can, an oven for programmed temperature adjustment, temperature control system, gas chromatograph, infrared gas analyzer, and GC/MS. The analysis of the gas product composition of the organic sulfur model compounds under low-temp oxidation conditions has been done using a FUL9790 gas chromatograph and an XLZ-1090 infrared gas analyzer, respectively. This paper uses the FUL9790 gas chromatograph to examine the gas products Molecules 2015, 20, page–page coming from low-temp oxidation, such as O2 , CO, CO2 . We adopted an external reference method to have a quantitative analysis of measured samples. Besides, use the XLZ-1090 infrared gas analyzer respectively. This paper uses the FUL9790 gas chromatograph to examine the gas products coming to conduct real-time sensing and monitoring of the concentration of SO2 under low-temp oxidation from low-temp oxidation, such as O2, CO, CO2. We adopted an external reference method to have a conditions. The component analysis of the model compound oxidation residue has done on an quantitative analysis of measured samples. Besides, use the XLZ-1090 infrared gas analyzer tobeen conduct real-time sensing and monitoring of the concentration of SO 2 under low-temp oxidation conditions. Agilent 6890/5975 GC/MS refering to the standard spectra database (NIST05) to make comparative component analysis of the model compound oxidation residue has been done on an Agilent analyses The of the detected compounds. 6890/5975 GC/MS refering to the standard spectra database (NIST05) to make comparative analyses Agilent 6890/5975 GC/MS test condition: DB-5 capillary chromatographic (column 30 m ˆ of the detected compounds. ˝ 0.25 mm ˆ 0.25 um); Gasification the carrier gas helium; the carrier Agilent 6890/5975 GC/MS temperature test condition: 280 DB-5 C; capillary chromatographic (column 30 m × gas flow rate 1 mL/min; ratio 5:1; sample quantity 2 the uL;carrier ionization methods EI;gas Ionization 0.25 mm × splite 0.25 um); Gasification temperature 280 °C; gas helium; the carrier flow rate energy: 1 mL/min; spliterang ratio 20–650. 5:1; sample quantity 2test uL; condition: ionization methods EI; Ionization energy:70 5ev;˝ C–35 ˝ C; 70 ev; Amu scanning FUL9790 environment temperature Amu scanning rang 20–650. FUL9790 test condition: environment temperature 5 °C–35 °C; relative relative humidity ď85%; carrier gas nitrogen; fuel gas hydrogen; detector temperature 200 ˝ C; thermal humidity ≤85%; carrier gas nitrogen; fuel gas hydrogen; detector temperature 200 °C; thermal conductivity temperature 150 ˝ C, oven temperature 80 ˝ C; the nitrogen pressure is 0.4–0.5 Mpa; conductivity temperature 150 °C, oven temperature 80 °C; the nitrogen pressure is 0.4–0.5 Mpa; the the hydrogen pressure Mpa;thethe air pressure is 0.1 Mpa; TCD current set to “60”. hydrogen pressureisis 0.1 0.1 Mpa; air pressure is 0.1 Mpa; TCD current is set to “60”. is XLZ-1090 infraredXLZ-1090 ˝ C; ≤90%; test condition: ambient temperature 0 °C–40 °C; relative humidity the minimum infrared gas gasanalyzer analyzer test condition: ambient temperature 0 ˝ C–40 relative humidity ď90%; limit (0~1000) × 10−6. the minimum limit (0~1000) ˆ 10´6 .
6 3 7
2 1
4
8
5
Figure 9. Experimental device of model compound oxidation. 1-Dry Air 2-Pressure Reducing
Figure 9. Experimental device of model compound oxidation. 1-Dry Air 2-Pressure Reducing Valve 3-Flow Controller 4-Sample pot 5-Gas Chromatography and Mass spectrometry 6-FUL9790 Valve 3-Flow Controller 4-Sample pot 5-Gas Chromatography and Mass spectrometry 6-FUL9790 Gas Chromatography Apparatus & Infrared Gas Analyser 7-Integrated Temperature Sensor Gas Chromatography Apparatus & Infrared Gas Analyser 7-Integrated Temperature Sensor 8-Temperature Programmed Furnace 8-Temperature Programmed Furnace. 3.2. Preparation of Laboratory Samples This paper selects diphenyl sulfide and cysteine as organic sulfur model compounds to perform an experimental study on low-temp oxidation.22251 Table 4 shows the details of the model compounds. These two model compounds separately contain an organic sulfur thioether bond and a mercapto active functional group.
44
55
88
Molecules 2015, 20, 22241–22256 Figure 9. Experimental device of model compound oxidation. 1-Dry Air 2-Pressure Reducing Figure 9. Experimental device of model compound oxidation. 1-Dry Air 2-Pressure Reducing Valve 3-Flow Controller 4-Sample pot 5-Gas Chromatography and Mass spectrometry 6-FUL9790 Valve 3-Flow Controller 4-Sample pot 5-Gas Chromatography and Mass spectrometry 6-FUL9790 Gas Chromatography Apparatus & Infrared Gas Analyser 7-Integrated Temperature Sensor Gas Chromatography Apparatus & Infrared Gas Analyser 7-Integrated Temperature Sensor 3.2. Preparation of Laboratory Samples 8-Temperature Programmed Furnace 8-Temperature Programmed Furnace
This paper selects diphenyl sulfide and cysteine as organic sulfur model compounds to perform 3.2. 3.2. Preparation Preparation of of Laboratory Laboratory Samples Samples an experimental study on low-temp oxidation. Table 4 shows the details of the model compounds. This paper selects diphenyl sulfide cysteine as sulfur model compounds to This papercompounds selects diphenyl sulfide and andcontain cysteine an as organic organic sulfur model compounds to perform perform Theseantwo model separately organic sulfur thioether bond and a mercapto experimental study on low-temp oxidation. Table 4 shows the details of the model compounds. an experimental study on low-temp oxidation. Table 4 shows the details of the model compounds. activeThese functional group. two model compounds separately contain an organic sulfur thioether bond and a mercapto These two model compounds separately contain an organic sulfur thioether bond and a mercapto active active functional functional group. group.
Table 4. Model organic sulfur compounds.
Table 4. Model organic sulfur compounds. Table 4. Model organic sulfur compounds. Model Compound Structural Formula Physical and Chemical Properties Model Compound Structural Formula Physical and Chemical Properties Model Compound
Diphenyl Diphenyl SulfideSulfide
Diphenyl Sulfide Cysteine
Cysteine Cysteine
Structural Formula
Physical and Chemical Properties Molecular weight 186.27, light yellow liquid, malodorous Molecular weightcolorless 186.27, or colorless or light yellow liquid, malodorous Molecular weight 186.27, colorless or light yellow liquid, malodorous Molecular weight 121.15, colorless crystals Molecular weight 121.15, colorless Molecular weight 121.15,crystals colorless crystals
First, First, we we put put organic organic sulfur sulfur model model compound compound on on the the carrier carrier and and distribute distribute it it evenly evenly on on the the surface. The carrier should support the model compounds firmly and have good chemical inertness First, we organic model compound onfirmly the carrier and distribute it evenly on the surface. Theput carrier shouldsulfur support the model compounds and have good chemical inertness with good thermal stability which it not the compounds and does surface. carrier should support the model and have good withThe good thermal stability which means means it does does compounds not react react with with firmly the model model compounds andchemical does not not inertness take part in any reactions or otherwise disturb the low-temp oxidation of the model compound. take part in any reactions or otherwise disturb the low-temp oxidation of the model compound.
with good thermal stability which means it does not react with the model compounds and does 11 not take part in any reactions or otherwise disturb 11 the low-temp oxidation of the model compound. After several experiments and analysis, this paper finallyselected selected6201 6201 support (the particle size After several experiments and analysis, this paper finally support (the particle size is is 0.18 mm) as the carrier of model compounds of these experiments. As shown in Figure 10, 0.18 mm) as the carrier of model compounds of these experiments. As shown in Figure 10, 6201 6201 support is usually to fill chromatographic columns andis iscomposed composedofof calcined calcined natural natural support is usually usedused to fill chromatographic columns and diatomite. Because it has a small amount of ferric oxide, it has pale red color. Its specific surface area diatomite. Because it has a small amount of ferric oxide, it has pale red color. Its specific surface area 2 is large large (specific (specific 4.0 4.0 m m2/g), /g),its itsaverage averagepore poresize sizeisis11μm, µm,and andits itsmechanical mechanicalstrength strengthisisgood. good. is Using an electronic scale the above model compounds (3 g) were weighed out, acetone (10 g) g) Using an electronic scale the above model compounds (3 g) were weighed out, acetone (10 was added and they were separately placed in a beaker. After mixing them evenly with a glass was added and they were separately placed in a beaker. After mixing them evenly with a glass rod rod they and they are divided into acetone and mixed compound liquor. 6201 (30 support (30 g) is and are divided into acetone and mixed model model compound liquor. 6201 support g) is weighed weighed out and poured into the previous beaker. Then the ingredients are mixed and the mixed out and poured into the previous beaker. Then the ingredients are mixed and the mixed liquor is liquor is distributed evenly on the surface of the support. The mixture of acetone, model compound distributed evenly on the surface of the support. The mixture of acetone, model compound and 6201 and 6201insupport in the beakeron is placed on athe tray and the acetone is allowed to completely support the beaker is placed a tray and acetone solution is solution allowed to completely evaporate, evaporate, and the model compound will thus be evenly attached on the surface of the 6201 support. and the model compound will thus be evenly attached on the surface of the 6201 support.
Molecules 2015, 20, page–page
Figure 10. 6201-Type Supports. Figure 10. 6201-Type Supports.
3.3. Experimental Procedures 3.3. Experimental Procedures A temperature setting range of 30~180 °C was used in this paper for the low-temp oxidation ˝ C was used in this paper for the low-temp oxidation A temperature experiments. The airsetting flow isrange set toof2030~180 mL/min. Firstly, the 6201 support with the attached organic experiments. The air flow is set to the 20 mL/min. Firstly, thesample 6201 support with the attached organic sulfur model compound is put into sample can, then the can is placed in the programmed sulfur modeladjustment compoundoven, is putand intothe theair sample sampleNext, can is in the the programmed programmed temperature flow iscan, set then to 20the mL/min. weplaced turn on temperature adjustment oven, and the air flow is set to 20 mL/min. Next, we turn on the temperature adjustment oven which is heated in 10 °C temperature intervals. When theprogrammed temperature ˝ C temperature intervals. When the temperature temperature adjustment oven which is heated in 10 of the can reaches the setting temperature, we hold that state for 20 min. After that, we analyze the of the can reaches the setting temperature,and we hold thatgas state for 20 min. After that, we analyze the gas composition using the chromatograph infrared analyzer. When the temperature reaches 180 °C, then programmed temperature cycle is stopped and the oven is shut down. After waiting 22252 until the sample cools down to room temperature, the sample can is opened and 1 g of 6201 support is removed. Next, we put this 1 g of 6201 support into a breaker, and add 2 g of acetone. After mixing with a glass rod and allowing to stand for 20 min we use a dropper to transfer the extract liquor from the upper layer into a glass bottle, annotating and properly sealing it before using GC/MS to conduct
Molecules 2015, 20, 22241–22256
gas composition using the chromatograph and infrared gas analyzer. When the temperature reaches 180 ˝ C, then programmed temperature cycle is stopped and the oven is shut down. After waiting until the sample cools down to room temperature, the sample can is opened and 1 g of 6201 support is removed. Next, we put this 1 g of 6201 support into a breaker, and add 2 g of acetone. After mixing with a glass rod and allowing to stand for 20 min we use a dropper to transfer the extract liquor from the upper layer into a glass bottle, annotating and properly sealing it before using GC/MS to conduct component analysis. 3.4. XPS Experimental Samples and Handling The experimental coal sample is fat coal, Xingcheng Feimei (XF), from Weng’an, Guizhou Province, China. The industry analysis, elemental analysis, and various forms of sulfur analysis of this coal are shown in Tables 5 and 6. Experimental coal samples (30 g) are taken from a sealed bag and then placed in a vacuum drying oven at 50 ˝ C to dry for 12 h. After they cool down to room temperature, 2 g dried coal sample is ground to 74 µm or less for the XPS test. We put the rest into the sample can and allow it to undergo low-temp oxidation in the experimental apparatus shown in Figure 9 to test for SO2 gas emission during the whole process. The test results shows no evidence of SO2 during the low-temp oxidation. Afterwards, the 2 g oxidized samples were tested by XPS to conduct a comparison of the XPS results before and after the oxidation. Table 5. The proximate and ultimate analysis of XF coal samples. Proximate Analysis (wt % as Received) Mad Aad Vdaf FCd 1.39
16.61
40.17
St.d
49.89
2.39
Ultimate Analysis (wt % Daf) Odaf Cdaf Hdaf Ndaf 7.11
83.11
5.47
1.43
Table 6. The forms of sulfur in XF coal samples.
Content Absolute (wt %) Relative (wt %)
Forms of Sulfur (wt % db) Pyritic Sulfate 1.07 44.77
0.03 1.26
Organic 1.29 53.97
XPS measurements are completed on an ESCALAB250 X-ray Photoelectron Spectrometer (Thermo Fisher, Waltham, MA, USA), which uses Al K alpha with power registering 200 W, and the spot size is 900 µm. The pass energy is 20 eV and the base vacuum is 10´7 Pa. C 1 s (284.6 eV) is set as the calibration standards. The S 2p XPS spectra obtained receives peak separation fitting by the special software, XPSEAK. As for the specific binding energies of sulfur, 163.1 ˘ 0.3 eV is attributable to sulfide sulfur and pyrite sulfur, 164.1 ˘ 0.3 eV to thiophene sulfur, 166.0 ˘ 0.3 eV to sulfoxide, 168.4 ˘ 0.3 eV to the sulfone and 169.3 ˘ 0.3 to sulfates [19,25,35]. 4. Conclusions (1)
(2)
From 30 ˝ C to 80 ˝ C, the adsorption between diphenyl sulfide and oxygen is mainly physical, at which time both the oxygen consumption and the change in the oxygen concentration are small. However, after 80 ˝ C, chemical adsorption and reactions begin to take place and oxygen consumption increases, then diphenyl sulfoxide is generated in the reaction and finally oxidized to diphenyl sulphone. This process is the main reaction mechanism in low-temp oxidation of the model compound diphenyl sulfide. Besides, some free radicals emerge. The reducibility of the sulphydryl group in cysteine determines that cysteine can be oxidized to cystine, a kind of disulfide, in low-temperature air. This reaction, which is also a common biochemical reaction, can take place under mild conditions. Cystine, a disulfide, continues to
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(3)
(4)
be oxidized in air to sulfenic acid, sulfinic acid and sulfonic acid, among which sulfonic acid generates pyruvic acid, sulfurous acid and ammonia through desulfonation and deamination. Then the sulfurous acid is decomposed into SO2 at a temperature of 130 ˝ C. Besides, C–S sulphydryl bond cleavage can also occur, leading to the generation of H2 S gas, while S–S bond and C–S bond cleavage in disulfides may also produce sulfur free radicals. There are five fitting S 2p peaks on the surface of XF original coal samples. The major inorganic sulfur forms are pyrite and sulfate and the major forms of organic sulfur are thiophene and sulfone. It also contains traces of sulfoxide and sulfide. In accordance with the low-temp oxidation mechanism of the above two model compounds, it can be inferred that there is none or little mercaptan and thiophenol in the coal. Differences exist between the XPS results and chemical analysis results, the reason for which is that the pyrite in the coal particle surface is easily oxidized to sulfate, leading to a much higher content of in the coal particle surface than the bulk phase content of coal samples. There are four fitting S 2p peaks according to XPS analysis of the oxidized XF sample. The main forms of inorganic sulfur on the surface are still pyrite and sulfate. The major organic sulfur component is sulfone, followed by thiophene sulfur. Sulfoxide is not detected on the coal surface. By comparing the XPS peak parameters of sulfur S 2p before and after oxidation, it can be found that sulfide and pyrite sulfur increased slightly, thiophenic sulfur is reduced, there is no sulfoxide detected on the surface of the coal after oxidation, yet the content of sulfone increases, and the sulfate content also increases too. This phenomenon shows that the form of organic sulfur in the coal particle surface changes after low-temp oxidation. To be more specific, sulfide sulfur is oxidized to sulfoxide, and then this sulfoxide is further oxidized to sulfone. The sulfoxide oxidization step to sulfone can be easily carried out. Sulfur peak intensity and area of the oxidized sample are higher than in the original one, which means there is a sulfur element enrichment on the surface of coal during oxidation.
Acknowledgments: This research was supported by the National Natural Science Foundation of China (No. 51304189), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13098). This work is also a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Author Contributions: Lanjun Zhang performed the experiments, and analyzed the date Zenghua Li conceived the study, participated in its design and coordination, and helped to draft the manuscript Jinhu Li; Yinbo Zhou; Yongliang Yang and Yibo Tang drafted the manuscript. All the authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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