Under different metamorphic environments, coal will form different types of tectonically deformed coal (TDC) by tectonic stress and even the macromolecular structure can be changed. The structure and composition evolution of TDC have been investigated in details using Fourier transform infrared spectroscopy and Raman spectroscopy. The ductile deformation can generate strain energy via increase of dislocation in molecular structure of TDC, and it can exert an obvious influence on degradation and polycondensation. The brittle deformation can generate frictional heat energy and promote the metamorphism and degradation, but less effect on polycondensation. Furthermore, degradation affects the structural evolution of coal in lower metamorphic stage primarily, whereas polycondensation is the most important controlling factor in higher metamorphic stage. Tectonic deformation can produce secondary structural defects in macromolecular structure of TDC. Under the control of metamorphism and deformation, the small molecules which break and fall off from the macromolecular structure of TDC are replenished and embedded into the secondary structural defects preferentially and form aromatic rings by polycondensation. These processes improved the stability of macromolecular structure greatly. It is easier for ductile deformation to induce secondary structural defects than in brittle deformation.
The study of macromolecular structure and complicated composition of coal is the most difficult and important topic in coal chemistry [
The TDC samples with different deformation and metamorphism (
All samples are pretreated through demineralization and vitrinite centrifugation processes in order to better delineate the characteristics of the deformation and metamorphism of TDC samples. The demineralization process utilized HCl and HF to reduce the proportion of mineral matter in each sample (<2%). The vitrinite centrifugation process used benzene and CCl4 to increase the composition percentage of vitrinite to 80–90%. The group maceral and vitrinite reflectance
The FTIR analysis on 32 samples and Raman analysis on 19 samples are conducted to further understand the evolution of macromolecular structure affected by deformation and metamorphism. It can be determined that the type of functional group and its change correlated with deformation and metamorphism in coal by analysis of absorption band, shown on infrared spectrum [
Part of experiment results are listed in Table
Part of experiment results of TDC samples.
Series of deformation | Sample ID |
|
|
|
FTIR | Raman2 | |||||
---|---|---|---|---|---|---|---|---|---|---|---|
CH3CH2 | C=C | CH3CH2 | C–H |
|
| ||||||
2923 | 2826 | 1600 | 1442 | 749 | |||||||
Brittle deformation coal | LHM06 | 0.98 | 0.83 | 0.15 | 0.557 | 0.326 | 0.991 | 0.815 | 0.317 | 113967 | 55594 |
HZM03 | 1.93 | 1.67 | 0.13 | 0.344 | 0.220 | 0.994 | 0.822 | 0.348 | 515720 | 286732 | |
S |
1.00 | 0.91 | 0.09 | 0.522 | 0.372 | 0.998 | 0.920 | 0.369 | 115158 | 58378 | |
HZM02 | 1.93 | 1.67 | 0.13 | 0.323 | 0.199 | 0.998 | 0.935 | 0.420 | — | — | |
LHM12 | 1.37 | 1.13 | 0.18 | 0.579 | 0.383 | 0.998 | 0.933 | 0.385 | 215980 | 131925 | |
STM02 | 1.41 | 1.12 | 0.21 | 0.647 | 0.403 | 0.994 | 0.806 | 0.380 | 272570 | 153720 | |
TYM04 | 0.95 | 0.8 | 0.16 | 0.504 | 0.354 | 1.000 | 0.895 | 0.176 | 137498 | 64296 | |
S |
0.98 | 0.88 | 0.10 | 0.363 | 0.264 | 0.996 | 0.886 | 0.548 | 174251 | 78056 | |
| |||||||||||
Ductile deformation coal | LHM04 | 1.40 | 1.18 | 0.16 | 0.657 | 0.414 | 0.988 | 0.893 | 0.450 | 394537 | 202143 |
LHM09 | 1.39 | 1.12 | 0.19 | 0.599 | 0.406 | 0.996 | 0.951 | 0.362 | — | — | |
LLM04 | 0.83 | 0.60 | 0.28 | 0.547 | 0.378 | 0.999 | 0.918 | 0.282 | 324725 | 164211 | |
HZM10 | 2.62 | 2.02 | 0.23 | 0.671 | 0.406 | 0.996 | 0.913 | 0.301 | 461139 | 157064 | |
LHM02 | 1.38 | 1.08 | 0.22 | 0.582 | 0.38 | 0.998 | 0.946 | 0.385 | — | — | |
LHM03 | 1.58 | 1.18 | 0.25 | 0.555 | 0.351 | 0.988 | 0.840 | 0.406 | 420961 | 229994 | |
STM05 | 1.66 | 1.12 | 0.33 | 0.491 | 0.312 | 0.997 | 0.992 | 0.455 | 511926 | 306591 | |
XTM08 | 1.92 | 1.63 | 0.15 | 0.260 | 0.180 | 1.000 | 0.910 | 0.500 | 499840 | 260745 |
1
The types of functional group in TDC samples collected from Huaibei coal field are basically the same with other researchers [
The infrared spectrum of TDC with different metamorphic stage.
The characteristic frequency of aromatic structure includes the absorption strength of (1) 3049 cm−1 related to stretching vibration of CH in aromatic ring, (2) 1600 cm−1 related to vibration of C=C in aromatic ring, and (3) 749 cm−1, 810 cm−1, and 871 cm−1 related to the plane deformation vibration of CH in aromatic rings. With the increase of metamorphic grade, the absorption strength of 1600 cm−1 has little change in brittle deformational coal but decreases first and increases later in ductile deformational coal, and with the range from 0.994 to 1 (Figure
Relationship of aromatic absorbance peaks of TDC with different metamorphic and deformation stages. (a) and (b) relationship between aromatic absorbance peaks and metamorphic stages. (c) and (d) relationship between aromatic absorbance peaks and deformation stages.
Generally, the change of absorption strength of 1600 cm−1 was not so obvious compared with the other frequencies. The absorption strength of 749 cm−1, 810 cm−1, and 871 cm−1 related to the plane deformation vibration of CH in aromatic ring is correlated with independent, two and more adjacent hydrogen atoms state, respectively. The strongest absorption strength of these frequencies is in the middle metamorphic grade and then in the higher and lower metamorphic grades it is the weakest (Figure
With the increase of deformational intensity (brittle and ductile deformation), the absorption strength of 1600 cm−1 increases at first and then decreases, which is inversed with the variation of absorption strength of 749 cm−1 (Figures
The frequency of aliphatic structure includes absorption strength of (1) 2923 cm−1 and 2862 cm−1 related to the asymmetric stretching vibration of CH2 and symmetrical stretching vibration of CH3,shown as shoulder absorption of 2923 cm−1 in aliphatic structure. These frequencies are the weakest in the middle metamorphic grade and increase in the lower and higher metamorphic grades and (2) 1442 cm−1 related to the asymmetric deformation vibration of CH2 and CH3 in alkane structure. With the increase of metamorphic grade, the change of absorption strength of 1442 cm−1 range from 0.8 to 0.99.
With the increase of metamorphic grade, the absorption strength of aliphatic structure is much more complicated (Figures
Relationship of aliphatic absorbance peaks of TDC with metamorphic and deformation stages. (a) and (b) relationship between aliphatic absorbance peaks and metamorphic stages. (c) and (d) relationship between aliphatic absorbance peaks and deformation stages.
With the increase of deformational intensity, under the lower metamorphism grade, the absorption strength of 2923 cm−1 increases first and decreases later, which is contrary to the absorption strength variation of 1442 cm−1 in brittle and ductile deformational coal (Figures
Nakamizo studied the grinded graphite and graphitized coke applying Raman spectrum and found that the peak
Two peaks (
The Raman spectrum of TDC with different deformation mechanisms (from [
Peak
Relationship of
The Peak
Relationship of
Previous research discussed that there were two types in
With the increase of deformational intensity, the value of
Compared with primary structure coal [
With the increase of metamorphic grade, the aliphatic structure and functional groups break off, and the aromatic structure is enriched in ductile deformational coal. However, for brittle deformational coal, the increase of aromatic structure is not obvious. It is indicated that the ductile deformation could produce apparent effect on degradation and polycondensation, and the increased metamorphic grade could promote the ductile deformation and the polycondensation process. The brittle deformation can only produce apparent effect on degradation and has little influence on polycondensation. The absorption strength of aromatic structure is developed as complementary to the aliphatic structure bands compared to the absorption peaks of aromatic and aliphatic structure. In lower metamorphic grade, there is much more aliphatic structures in TDC. Because of the fracture, abscission and cyclization of aliphatic functional groups and alkane branched chains made the aromatic structure increase with the metamorphic grade rise.
With the increase of deformational intensity, the brittle deformation transforms the stress into frictional heat energy [
Raman data show that the
To summarize, different deformational mechanisms change the chemical structure and produce the secondary structural defects, which are the main reason for various structure evolution of TDC compared with primary structure coals. Based on the XRD test about those TDC samples we studied early [
With the increase of deformation and metamorphism, the change of FTIR and Raman spectrum shows different ways. The tectonic deformation made a very important role which affects the macromolecular structure of TDC. Different deformational mechanism induced different evolution process of macromolecular structure of TDC. The ductile deformation can produce apparent effect on degradation and polycondensation, but brittle deformation has little influence on polycondensation in lower metamorphic grade. In higher metamorphic grade, polycondensation is the main reaction in macromolecular structure of coal. It means that the degradation is the main effect under brittle deformation and the polycondensation under ductile deformation. Tectonic dmaineformation can produce the secondary structural defects in macromolecular structure of TDC. The increase of total number of aromatic rings and secondary structural defects is mainly caused by brittle deformation in lower metamorphic grade, but ductile deformation in higher metamorphic grade. Furthermore, the ductile deformation can produce the secondary structural defects easier than brittle deformation. The existence of secondary structural defects reduces the stability of macromolecular structure in TDC. Brittle deformation promotes the degradation and makes the aliphatic functional groups and alkane branched chains break off selectively in lower metamorphic grade. With the increase of deformation and metamorphism, more secondary structural defects are produced and small molecules are dropped; the ductile deformation promotes the polycondensation, so the dropped small molecules splice and embed preferentially in secondary structural defects or residual aromatic structures and form aromatic rings to make the macromolecular structure of much stability.
This work is supported by the National Natural Science Foundation of China (Grants nos. 40772135, 40972131, and 41030422), the National Basic Research Program of China (Grants nos. 2009CB219601 and 2006CB202201), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA05030100).