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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Molecules 2015, 20, 22241–22256
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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Molecules 2015, 20, 22241–22256
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.
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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
5.50
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
Molecules 2015, 20, 22241–22256
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
Molecules 2015, 20, 22241–22256
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
22253
Molecules 2015, 20, 22241–22256
(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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Sample Availability: Samples of the compounds are not available from the authors.
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