Low-temperature oxidation of Methylene bridge bond in coal by model compound

Article abstract of DOI:10.14233/ajchem.2014.15546

In order to find out characteristics of active structures in coal at early phase of oxidation, diphenylmethane was adopted as the model compound to study the oxidation properties of bridge bond (methylene) in coal with low-temperature experimental condition (293 K-423 K), according to structure of molecular coal. Mean while, the generated oxidation products under different reaction conditions were analyzed qualitatively via FTIR spectrometer and GC-MS. The results show that the new structures which involve C-O, C=O, Ar-O and Ar-OH are constantly generated with temperature rise in the process of low-temperature oxidation of diphenylmethane, especially after the 393 K. Besides, the phenols, benzaldehyde, diphenylmethanone, phenyl benzoate, 2-benzyl phenols and 2-phenolic-diphenylmethanone were detected in oxidation products, which demonstrated this oxidation is a multi-step reaction. Meantime, the substitution between O and H is more easily occurs in -CH₂-than in benzene ring. The temperature and time are major factors which affecting low-temperature oxidation of diphenylmethane. In conclusion, this study provides reference for explaining and controlling the low-temperature oxidation of coal from chemical perspective.

Full text of DOI:10.14233/ajchem.2014.15546

Asian Journal of Chemistry; Vol. 26, No. 3 (2014), 795-798
http://dx.doi.org/10.14233/ajchem.2014.15546
Low-Temperature Oxidation of Methylene Bridge Bond in Coal by Model Compound
1
2
3
1
YIBO TANG1,2,*, ZENGHUA LI , JERRY C. TIEN , WENXUAN YIN and DONGJUAN MA
1School of Safety Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, P.R. China
2Mining & Nuclear Engineering, Missouri University of Science and Technology, Rolla, MO 65401, USA
3School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, P.R. China
*Corresponding author: E-mail: tangyibo11@126.com
Received: 26 March 2013;
Accepted: 27 July 2013;
Published online: 30 January 2014;
AJC-14639
In order to find out characteristics of active structures in coal at early phase of oxidation, diphenylmethane was adopted as the model
compound to study the oxidation properties of bridge bond (methylene) in coal with low-temperature experimental condition (293 K-423
K), according to structure of molecular coal. Mean while, the generated oxidation products under different reaction conditions were
analyzed qualitatively via FTIR spectrometer and GC-MS. The results show that the new structures which involve C-O, C=O, Ar-O and
Ar-OH are constantly generated with temperature rise in the process of low-temperature oxidation of diphenylmethane, especially after
the 393 K. Besides, the phenols, benzaldehyde, diphenylmethanone, phenyl benzoate, 2-benzyl phenols and 2-phenolic-diphenylmethanone
were detected in oxidation products, which demonstrated this oxidation is a multi-step reaction. Meantime, the substitution between O
and H is more easily occurs in -CH2- than in benzene ring. The temperature and time are major factors which affecting low-temperature
oxidation of diphenylmethane. In conclusion, this study provides reference for explaining and controlling the low-temperature oxidation
of coal from chemical perspective.
Keywords: Low-temperature oxidation, Methylene, Diphenylmethane, Model compound, Spontaneous combustion.
inducing spontaneous combustion of coal. As a result, well
targeted study of oxidation properties of bridged bond and
active group in coal in the process of low-temperature oxidation
has become the top priority of study concerning spontaneous
combustion of coal.
INTRODUCTION
The spontaneous combustion of coal is a worldwide
disaster1, which often occurs in the coal-related industries. Coal
fire raised by self-heating of coal also responsible for the
environmental problems such as surface subsidence, toxic gas
emissions and water pollution2-3. Since the 17th century,
numerous reasons4-7 were considered to be inducing factors
which effect on spon-combustion of coal. Although many
scholars attempted to reveal variations of various coal mole-
cular structure in the process of self-ignite with multiple testing
means8-11, its mechanism is still unknown. At present, making
use of model compound to study complicated chemical system
is a effective research approach which widely applied in proce-
sses such as coal liquefaction12-13, coal desulfuration14 and
petroleum refining15.
The low-temperature oxidation of coal is the most impor-
tant initial part19-21 for the whole process on self-ignite. Meth-
ylene is typical bond in coal molecule21,22. Therefore, studying
low-temperature oxidation properties of methylene is of certain
guiding significance to reveal spontaneous combustion of coal.
Thus, model compound comprised by methylene and its
coupled aromatic ring was adopted as experimental subject in
this paper.
EXPERIMENTAL
Experimental set-up: The schematic diagram of experi-
mental apparatus was shown in Fig. 1. In experimental system,
dry air uniformly passes through pipeline. The temperature
control system assures that the sample can enables to be conduc-
ted by temperature programming and stabilized at the setting
temperature.
In line with molecular structure of coal, the main cons-
truction units of coal are aromatic rings which interrelated via
bridged bond (aliphatic hydrocarbons bond, thioether bond
and ether bond etc.) and active group16. Besides, chemical
characteristics of aromatic ring are relatively stable. Bridged
bonds and active groups in coal molecular structure are firstly
oxidized17-18. In other words, the oxidation of such structures
as bridged bonds and active groups play an important role in
Experimental procedure: The diphenylmenthane (50 g)
was placed into glass bottle and the air (2 L/m) passed into it

796 Tang et al.
Asian J. Chem.
stretching vibration in alkane, at 1601 cm-1 representing C=C
skeletal vibration on aromatic ring, at 1492 and 1444 cm-1
representing C-H flexural vibration in alkane, at 729 and 681
cm-1 representing C-H out-of-plane deformation vibration of
aromatic hydrocarbon where H was replaced.
The activity structure in dipenylmethane was oxidized
continuously with the ceaseless temperature increase. After
oxidation, there lies in larger variation of activity structure in
dipenylmethane. To be specific, when the reaction temperature
was 333 K, absorption peak representing C-O stretching
vibration occurred at 1072 and 1034 cm-1 after low-temperature
oxidation for 48 h, which indicated that C-O bond was gradually
generated. Meanwhile, a small adsorption peak representing
C=O (ketone) also emerged at 1076 cm-1.And there were fewer
changes among other adsorption peaks. When reaction tempe-
rature rose to 353 K, the adsorption peak representing ether
bond gradually appeared at 1319 and 1277 cm-1, which showed
that stretching vibration of aliphatic ether structure and aromatic
ether structure started to appear along with temperature rise.
Besides, at 1706 and 1746 cm-1 signified that adsorption peak
of C=O gradually enhanced. When the temperature reached
373 K, at 2907 and 2843 cm-1 representing CH2 antisymmetric
stretching vibration in naphthene or aliphatic series involved
slight enhancement aside from adsorption peaks represented
by previously generated structures.
Fig. 1. Experimental set-up
continuously for oxidation. Keep oxidizing 24 h. under diffe-
rent setting temperatures (333 K-393 K) and collect the sample
of 1 mL at intervals of 4 h. Afterwards, products constituents
were qualitatively analyzed through GC-MS (split ratio of
30:1). The concentration variation of its oxidation products
under different reaction conditions was analyzed quantitatively
via gas chromatograph. Meanwhile, the variation features of
its functional group under different reaction conditions were
tested by FTIR.
RESULTS AND DISCUSSION
Infrared spectroscopy is a kind of test method which
could effectively reflect the changes of functional groups in
organics23. As shown in Fig. 2, the main characteristic peaks
of infrared spectrogram of diphenylene include absorption
peak at 3023 cm-1 representing C-H stretching vibration on
aromatic ring, at 2907 cm-1 and 2843 cm-1 representing C-H
Finally, when reaction temperature reached 393 K, the
adsorption peak of Ar-OH was relatively obvious showing at
3487 cm-1.Yet, this adsorption peak under reaction temperature
of 333 K exhibited in the spectrogram. Its intensity gradually
increased along with temperature rise, which indicated that
phenolic compounds were generated in low-temperature
oxidation process of diphenylmethane.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
O
(c)
2500000
OH
(d)
(a)
2000000
O
(e)
O
O
1500000
OH
500
1000
1500
Wavenumber (cm–
3000
3500
1
)
(b)
1000000
500000
(f)
(a) Diphenyl methane
OH
O
Undefined
3.2
2.4
1.6
0.8
0
(g)
393K
4.00
6.00
8.00
10.00
12.00 14.00
Time
Fig. 3. Chromatograms for composition of oxidation products from
diphenylmethane at 393 K
373K
353K
333K
The liquid oxidation products of diphenylmethane were
qualitatively analyzed via GC-MS. It can be clearly shown
from Fig. 3 that (d) diphenylketone was generated by
diphenylmethane oxidation. Meanwhile, such substances as
(a) phenol, (b) benzaldehyde, (d) diphenylketone, (e) phenyl
benzoate, (f) 2-benzylphenol, (e) 2-phenolic -diphenylketone
were tested. As we all know, the active group included in
500
1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm–1
)
(b) Diphenyl methane after oxidized 24 h
Fig. 2. Infrared spectrum of diphenyl methane and its oxidation

Vol. 26, No. 3 (2014)
Low-Temperature Oxidation of Methylene Bridge Bond in Coal by Model Compound 797
diphenylmethane is methylene on which αC atom and bilateral
benzene rings involve δ-π hyperconjugation. As a result, the
electron cloud between αC atom and αH atom and conjugated
system of aromatic ring could be integrated into one. In this
way, electron delocalization extent enlarged where free radical
can be easily formed.And, bridged bond among benzene rings
was easier than benzene ring itself to make adsorption with
oxygen. Combined with oxidation products detected by experi-
ment, it can be inferred that the reaction course of diphenyl-
methane and O2 is as follows (Fig. 4):
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
4
8
12
Time (h)
(a) Phenol
16
20
24
•
O
OH
O
•
O
O
O
•
O
12
•
O
•
O
10
8
•
O
•
O
OH
O
O
OH
6
and
4
Fig. 4. Simple oxidation reaction pathway of diphenyl methane
2
This pathway suggested that the supplant primarily hap-
pened between oxygen atom and hydrogen atom on bridge
bond of methylene during the oxidation of diphenylmethane.
Meantime, some oxygen atoms replaced the hydrogen atom
on the benzene ring. In the process of reaction, the generation
of carbon monoxide and carbon dioxide along with the breaking
of bonds.
0
5
10
15
20
25
Time (h)
(b) Diphenyl ketone
Fig. 5. Effect of temperature and time on the oxidation of diphenyl methane
Air flow also exerts evident impact on oxidation of
diphenylmethane. To be specific, the larger the air flow is, the
higher the contact efficiency of diphenylmethane and oxygen
in air is.
In the experiment the production rate of diphenyketone
was the highest among diverse oxidation products. Thus, it
was deemed as the vital product to make qualitative analysis.
As shown in Fig. 5, at 333 K, phenol concentration generated
in reaction increased slightly as time increased. But, it still
remained at lower level (0.33 %). As the temperature cons-
tantly escalated, phenol concentration generated in reaction
increased continuously. When reaction temperature was 353
K, phenol concentration slightly increased compared with that
under the temperature of 333 K. But when reaction temperature
was 373 K, the generated phenol concentration clearly rose
through oxidation for 24 h. (0.82 %). Especially, when reaction
temperature was 393 K, the generated phenol concentration
sharply rose. And phenol concentration reached 0.95 % after
reacting for 12 h. However, the phenol output had no substantial
increase along with constant increase of reaction time. It just
rose slowly over time. The phenol concentration reached 1.25 %
after reacting for 24 h. By contrast, the concentration of the
other major product, diphenyl-ketone in reaction rose stably as
temperature and reaction time increased. However, the rising
rate of product concentration was higher. Based on the above,
it can be found that reaction time is one influential factor.
Besides, it also proves the significant influence of temperature
on this reaction.
9
1 L/m
2 L/m
3 L/m
8
7
6
5
4
3
2
0
4
8
12
16
20
24
Time (h)
Fig. 6. Yield of diphenyl methanone from oxidation of the diphenylmethane
under different airflow at 353 K
As show in Fig. 6, oxidation rate of diphenylmethane
increased constantly along with air flow increase. Under the same
reaction time, the proportion of diphenylketone generated in

798 Tang et al.
Asian J. Chem.
reaction was increased with the growth of air mass flow. When
air flow increased from 1 to 3 L/min, the proportion of gene-
rated diphenylketone rose from 5.53 to 7.48 % after reacting
for 24 h. In the early stage of reaction, the impact of increasing
ventilation mass flow on diphenylmethane was hardly obvious.
However, as time passed by, the impact of increasing ventilation
mass flow on productivity of diphenyl-ketone was more and
more obvious. The concentration difference of diphenylketone
generated in reaction was larger and larger, which indicates
air flow is one of factors affecting low temperature oxidation
of diphenylmethane.
These studies demonstrate that the oxidation of methylene
bridge bond is prone to occur at low temperature conditions
and also an important part of the radical chain reaction caused
self-heating of coal. More importantly, the detail information
on oxidation properties and reaction pathway of methylene
bridge bond supply a foundation for selecting inhibitors used
in hindering self-ignite of coal, while the targeted retardant
usually has better inhibiting effect.
ACKNOWLEDGEMENTS
This work is supported by the Project of China National
Natural Science Foundation (NO.51074158) and the Funda-
mental Research Funds for the Central Universities (No.
2012LWBZ10).
Conclusions
The oxidation process of diphenylmethane is complicated
multistep reaction ranging from the initial chemisorptions, fi-
nal multistep decomposition reaction to eventual generation
of various products.
REFERENCES
1. K. Benfell, B.B. Beamish and K.A. Rodgers, Thermochim. Acta, 298,
119 (1997).
• It can be deduced that methylene in diphenylmethane
can make composite oxidation with oxygen under normal
temperature. Even its oxidative activity is weaker under normal
temperature, it would constantly improve along with tempe-
rature rise.
2. D.L. Carpenter and D.S. Giddings, Fuel, 43, 247 (1964)
3. H. Wang, B.Z. Dlugogorski and E.M. Kennedy, Fuel, 81, 1913 (2002)
4. J.D. Davis and J.F. Byrne, J. Am Ceram. Soc., 7, 809 (1924).
5. Y. Tang, Z. Li and Y. Yang, Asian J. Chem., 25, 441 (2013).
6. D.M. Wang, Mine Fire, China University of Mining and Technology
Press, Xuzhou, pp. 45 (2006).
• The substitution is easily happens between O and -CH2-
connected two benzene rings and then two benzene rings will
lose connection through a further multi-step oxidation. Only
small part of the oxygen will directly substitute the H on the
benzene ring. No matter what kind of reaction happened,
various intermediate products such as aldehydes, phenols and
ketones were generated in this oxidation, which demonstrated
the transfer of O free radicals is very active in this reaction.
• Furthermore, reaction time exerts the certain impact on
activity structure oxidation in diphenylmethane. But the impact
of reaction temperature is much larger. More absorption peaks
(C-O, C=O, Ar-O and Ar-OH) were increasely appeared as
long as the temperature reached 393 K, which meant this
reaction efficiency would rise greatly after 393 K.
7. Y. Tang Z. Li and Y. Yang, Asian J. Chem., 25, 3384 (2013).
8. J.N. Carras and B.C.Young, Progr. Energy Combust. Sci., 20, 1 (1994).
9. J.J. Pis, G. de la Puente, E. Fuente, A. Morán and F. Rubiera,
Thermochim. Acta, 279, 93 (1996).
10. J.C. Jones, K.P. Henderson, J. Littlefair and S. Rennie, Fuel, 77, 19 (1998).
11. B.B. Beamish, M.A. Barakat and J.D. St. George, Int. J. Coal Geol.,
45, 217 (2001).
12. M.J. Trewhella and A. Grint, Fuel, 67, 1135 (1988).
13. X. Wei, E. Ogata, Z. Zong, S. Zhou, Z. Qin, J. Liu, K. Shen and H. Li,
Fuel Process. Technol., 62, 103 (2000).
14. P. Yi J. Liu and H. Zhao, Coal Conversion, 20, 27 (1997).
15. I. Mochida, K. Sakata, K. Maeda, H. Fujitsu and K. Takeshita, Fuel
Proc. Technol., 3, 207 (1980).
16. J.W. Smith, B.D. Batts and T.D. Gilbert, Org. Geochem., 14, 365 (1989).
17. J. Wang and C. Deng, J. China Coal Soc., 24, 78 (2001).
18. T. Shi, J. Deng and X. Wang, J. Fuel Chem. Technol., 32, 6 (2004).
19. Y. Tang Z. Li and D. Ma, Asian J. Chem., 25, 8660 (2013).
20. Y. Tang Z. Li and D. Ma, Asian J. Chem., 25, 8667 (2013).
21. J.H. Shinn, Fuel, 63, 83 (1984).
• Besides, air flow involved in reaction is also another
factor that affects reaction efficiency. In the early stage of
reaction, the influence of air flow on reaction is not apparent.
However, as reaction time grows continuously, this influence
of air flow on reaction efficiency is increasingly obvious.
22. J.P. Mathews, A.C.T. van Duin and A.L. Chaffee, Fuel Proc. Technol.,
92, 718 (2011).
23. P. Larkin, Infrared and Raman Spectroscopy, Elsevier Inc: Amsterdam,
Vol. 1, pp. 1-5 (2011).