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Energy & Fuels 1998, 12, 446-449
Articles Observation of the Type of Hydrogen Bonds in Coal by FTIR Chong Chen, Jinsheng Gao,* and Yongjie Yan Department of Chemical Engineering for Energy Resource, East China University of Science and Technology, Shanghai 200237, China Received July 1, 1997. Revised Manuscript Received January 23, 1998
It is hard to distinguish the type of hydrogen bonds formed by hydroxyl groups by normal FTIR methods because of the interference of water in samples. In this work, a method is proposed for the clear observation of the types of hydrogen bonds by FTIR. The method involves the evaporation of water in a vacuum chamber with CaF2 windows (vacuum level, 4 × 10-3 kPa) followed by in situ scanning of the IR spectrum. Five types of hydrogen bonds were found in coal and its extracts. The hydroxyl groups in coal mainly exist as self-associated OH and OH-N hydrogen-bonded pairs. The thermal stability of these hydrogen bonds was investigated by means of in-situ pyrolysis FTIR. It follows the order of OH-ether O > self-associated OH ≈ cyclic OH > OH-N > OH-π.
Introduction It is widely known that there are several types of hydrogen bonds in coal and that the most important one is formed by hydroxyls with various hydrogen-bonding acceptors.1-7 However, it is not easy to observe clearly these types because water molecules have an infrared absorption in the same region as coal hydroxyl absorptions. So the hydroxyl absorption often appears as a broad overlapping band with its center at 3400 cm-1. Although the water can be removed by vacuum-drying, it is resorbed easily on KBr pellet during the FTIR determination under ambient humid condition. The interference of water makes determining the quantity and type of hydroxyls difficult.1,2,4,8 Indirect methods have been developed to solve this problem. Painter and co-workers4 proposed a procedure to avoid the water problem and distinguish the types of hydroxyl groups. It employs acetylation of coal and analysis of the difference between spectra of raw coal and acetylated coal. Miura et al.2 separated the overlapping bands of hydroxyl into five peaks by using a band-fit program. These methods are time consuming and of low accuracy. * Corresponding author. (1) Painter, P. C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987, 66, 973978. (2) Miura, K.; Mae, K.; Morozumi, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 209-213. (3) Cai, M. F.; Smart, R. B. Energy Fuels 1994, 8, 369-374. (4) Snyder, R. W.; Painter, P. C.; Havens, J. R.; Koenig, J. L. Appl. Spectrosc. 1983, 37, 497. (5) Supaluknari, S.; Larkins, F. P. Fuel Procss. Technol. 1988, 19, 123-140. (6) Miura, K.; Mae, K.; Sakurada, K.; Hashimoto, K. Energy Fuels 1992, 6, 16-21. (7) Petersen, J. C. Fuel 1967, 46, 295-305. (8) Solomon, P. R.; Carangelo, R. M. Fuel 1982, 61, 663-669.
Table 1. Ultimate Analysis of Coals coals (abbrev)
C
H
N
S
Oa
Shenbei (SB) Yanzhou (YZ) Shuangyashan (SY) Zaozhuang (ZZ) preasphaltenesb asphaltenesb
73.1 78.9 82.3 88.5 77.9 76.7
5.01 5.20 5.69 5.63 5.50 6.08
1.94 1.33 1.03 1.41 1.31 0.850
0.350 1.77 0.182 0.572 1.85 1.39
19.6 12.8 10.8 3.89 13.4 15.0
a
By difference. b Extracts from YZ coal.
Table 2. Peaks Assignments of Various Hydrogen Bonds Formed by Hydroxyl in Coal and Extract wavenumber (cm-1) types of hydrogen bonds
coals
extracts
OH-π self-associated OH OH-ether O cyclic OH OH-N
3530 3410
3500 3370 3300 3240 3170
3220 3150
In this work, vacuum FTIR method is proposed in which the types of hydrogen bonds are observed directly. Four types of hydrogen bonds in raw coals and five types in extracts were observed clearly. The thermal stability of these hydrogen bonds was investigated by in situ pyrolysis FTIR. Experimental Section Four coals with different carbon content and extracts from YZ coal were selected. The ultimate analysis of the coals are listed in Table 1. The extraction of ZZ coal with NMP-CS2 mixed solvent and the preparation of preasphaltenes and asphaltenes from the cyclohexanone extract of YZ coal are
S0887-0624(97)00100-X CCC: $15.00 © 1998 American Chemical Society Published on Web 04/07/1998
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Figure 1. Comparison of normal FTIR spectra under ambient conditions (a) and vacuum FTIR spectra (b). YZ, YZ coal; PA, preasphaltenes; A, asphaltenes. described in refs 9 and 10 respectively. FTIR spectra were obtained on a Bio-Rad FTS-25ps FTIR spectrometer. A normal KBr disk technique was used with a ratio of sample/KBr, 1.5 mg/200 mg. The spectra were collected at a resolution of 4 cm-1 with 64 “scans” over the range of 4000-1000 cm-1. A KBr sample pellet was placed in a vacuum chamber with CaF2 windows. After evaporation of the sorbed water in a sample or KBr at 100 °C and 4 × 10-3 kPa, in situ scanning of the FTIR spectrum was accomplished. In situ pyrolysis FTIR was carried out in the same chamber and under the same conditions. Temperatures ranged from 25 to 500 °C with a heating rate 25 °C/min.
Results and Discussion A comparison of normal FTIR spectra and vacuum FTIR spectra is shown in Figure 1. Only one strong absorption at 3400 cm-1 was observed for YZ coal and its extracts by normal FTIR measurement under ambient conditions. However, several hydroxyl absorption bands were observed for the same samples by means of vacuum FTIR method. The absorption intensity of hydroxyl is much stronger in the normal FTIR spectra than in the vacuum FTIR spectra taking the intensity of νar,C-H at 3050 cm-1 as a reference. The difference in the two sets of spectra is obviously caused by sorbed water in the sample. In this work, the water problem was diminished or eliminated by the vacuum FTIR method. Five peaks were observed in the extracts and four peaks in the raw coal spectra in the hydroxyl region, as shown in Figures 1 and 2. In addition, the absorption bands of minerals at around 3600 cm-1 can also be observed clearly using this method (Figure 2). They are often mixed with water peaks in normal FTIR spectra. Each peak in the hydroxyl region is assigned to an individual hydrogen bond formed by hydroxyl groups with different hydrogen-bonding acceptors. Painter et al.1 has classified the various hydroxyl hydrogen-bonding absorption peaks based on model compounds, but no clear FTIR spectra relating these types of hydrogen bonds were observed. Following the data (9) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639-1647. (10) Chen, C.; Gao, J. S.; Yan, Y. J. J. Fuel Chem. Technol. (Ch.) 1997, 25, 60-64.
Figure 2. Vacuum FTIR spectra of coal with different rank (for abbreviations of coals see Table 1), ZZ-ER, NMP-CS2extracted ZZ coal.
of Painter et al.,1 the assignment of hydroxyl absorption peaks in extracts and raw coal is summarized in Table 2. The complicated coal structure may not exactly match with the OH tetramer structure of novolak estimated by Painter et al.,1 but similar cyclic OH hydrogen bonds are possible. The band at 3220 cm-1 in the raw coal or 3240 cm-1 in the extracts is considered to be the hydrogen bond between cyclic OH groups, which is similar to intramolecular hydrogen bonds. The wavenumber of the peaks in the extracts was slightly different from that in the raw coal. Most peak positions observed in this work are close to those estimated by Painter et al., except for the OH-N hydrogen bond (3100-2800 cm-1, estimated by Painter et al.1). Our previous work11 showed that this peak enlarged with the increase of N content in sample. It should be assigned to OH-N hydrogen bond. The vacuum FTIR spectra of four raw coals with different rank are shown in Figure 2. The type of hydrogen bonds changes with the coal rank. The subbituminous coal (SB) only shows a strong self-associated hydroxyl absorption peak at 3410 cm-1 and a serious overlapping absorption in the region of 3100-3400 cm-1. Vacuum evaporation is only valid for weakly bound sorbed water. Some water, especially that in low-rank coal and that tightly bound to mineral hydroxyl groups, is hard to remove by vacuum evaporation. The seriously overlapping bands from 3100 to 3400 cm-1 in SB coal indicate that the interference of water remains. As the carbon content increases, three peaks are observed in the spectra of YZ coal, i.e., 3410, 3220, and 3150 cm-1. The last two peaks are somewhat overlapped. As the
448 Energy & Fuels, Vol. 12, No. 3, 1998
carbon content further increases, the cyclic hydrogen bond (3220 cm-1) disappears, but the self-associated OH and OH-N hydrogen bond remain. Besides the two commonly existing types of hydrogen bonds, i.e., selfassociated OH at 3410 cm-1 and OH-N at 3150 cm-1, a new peak at 3530 cm-1 appears in the spectrum of ZZ coal, a high carbon content bituminous coal. It is reasonable that as the aromaticity of coal increases with the coal rank, the density of π electrons on aromatic rings in ZZ coal is high enough to form OH-π hydrogen bonds with hydroxyls. Similarly, preasphaltenes have stronger OH-π absorption than asphaltenes because of their higher aromaticity. Hydrogen bonds can be changed by extraction. ZZ coal has high solubility in NMP-CS2 mixed solvents.9 Its OH-π and OH-N hydrogen bonds disappear after NMP-CS2 mixed solvent extraction, but self-associated OH does not, as illustrated in Figure 2. Since the strongest hydrogen bond of the five types, OH-N hydrogen bond,2 can be broken by NMP during solvent extraction, all the other hydrogen bonds with weaker bond strengths certainly can be broken. However, no free OH groups absorption around 3620 cm-1 appears in the FTIR spectrum of residue. Most OH groups in the NMP-extracted coal exist as the self-associated type. Rearrangement of the coal structure to the lower free energy state is possible when solvent is removed from the swollen coal. The change of type of hydrogen bonds of ZZ coal demonstrates that self-associated OH may have lower free energy among these types of hydrogen bonds. Self-associated OH and OH-N hydrogen bonds are the two main types of hydrogen bonds in coals. The hydrogen bond between self-associated OH groups with a linear chain structure12 appears as a dominant hydroxyl hydrogen bond both in raw coals and in extracted coals. This result implies that coal molecular segments are mainly associated by the hydrogen bonding of selfassociated OH. The appearance of OH-N hydrogen bonds indicate the presence of the base/acid structure proposed by Sternberg et al.13 in coal. The weak absorption of the OH-N hydrogen bond is in accord with the low content of N in coal. The types of hydrogen bonds in extracts is different from that in raw coals because the hydrogen bonds in extracts rearrange to minimize free energy when the solvent is removed. Selfassociated OH, OH-ether O, and cyclic OH hydrogen bonds are the three main hydrogen bonds in preasphaltenes and asphaltenes of YZ coal, as shown in Figure 1. No OH-ether O hydrogen bond is observed in raw coals perhaps due to low concentration of ether structures or steric limits. The cyclic OH hydrogen bond at 2200 cm-1 disappears with increasing of coal rank as shown in Figure 2. It is reasonable that hydroxyl content decreases with increasing coal rank, which increases the distance between the nearby hydroxyls. Consequently, hydroxyl groups can seldom form cyclic OH hydrogen bond in higher rank bituminous coal. In order to investigate the thermal stability of these (11) Chen, C. PhD Disseratation. East China University of Science and Technology, Shanghai, P. R. China, 1997. (12) Painter, P. C.; Nowak, J.; Sobkowiak, M.; Youtcheff, J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32, 576-582. (13) Sternberg, H. W.; Raymond, R.; Schweighardt, F. K. Science 1975, 188, 49-51.
Chen et al.
Figure 3. In situ pyrolysis FTIR spectra of preasphaltenes (a) and asphaltenes (b) from cyclohexanone extracts of YZ coal.
hydrogen bonds, in situ pyrolysis-FTIR is employed for preasphaltenes and asphaltenes from the cyclohexanone extracts of YZ coal. The change of five hydrogen bonds related to OH groups is shown in Figure 3. OH-π hydrogen bond at 3500 cm-1 is the most unstable one, which disappears above 100 °C for asphaltenes and 200 °C for preasphaltenes. The other four types are stable enough below 500 °C. The absorption center shifts from 3370 cm-1 at room temperature to 3300 cm-1 on heating. As the temperature rises, the OH-ether O hydrogen bond becomes the major one with the highest thermal stability, because its absorption intensity becomes stronger compared with the intensity of other hydrogen bonds at 3370, 3240, and 3170 cm-1. The thermal stability of the other four hydrogen bonds below 500 °C can be evaluated by their normalized absorption intensity, Pi, Pi ) Ii/(I3370 + I3300 + I3240 + I3170), where Ii is the absorption intensity of individual hydrogen bond at 3370, 3300, 3240, and 3170 cm-1, respectively. The higher thermal stability, the larger is Pi as the pyrolysis temperature increases. The change of Pi for preasphaltenes and asphaltenes with pyrolysis temperature is described in Figure 4. An increase of P3300 and a decrease of P3170 with pyrolysis temperature above 100 °C is observed for both preasphaltenes and asphaltenes. These results imply that the thermal stability of OH-ether O hydrogen bond is good but the OH-N one is poor. P3370 and P3240 almost do not change with increase of pyrolysis temperature, indicating that the thermal stability of self-associated OH and cyclic OH hydrogen bonds is close, and the thermal stability of both of them is higher than that of OH-N hydrogen bond but lower than that of OH-ether O one. Although the hydrogen-bonding strength decreases as the absorption peak shifts to lower wavenumber,2,14,15 the thermal stability of these hydrogen bonds does not change with the same order. On the basis of our results, the thermal stability of the four (14) Arnett, E. M.; Mitchell, E. J.; Murty, T. S. S. R. J. Am. Chem. Soc. 1974, 96, 3875-3891. (15) Purcell, K. F.; Drago, R. S. J. Am. Chem. Soc. 1967, 89, 28742879.
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Energy & Fuels, Vol. 12, No. 3, 1998 449
the stronger is the interaction between OH and π electrons.16 OH-ether O hydrogen bond is the most stable during pyrolysis. It is probably because it is hard for the OH group hydrogen bonded with oxygen atom in ether structure to get a nearby proton and form water. Similarly, OH-π and OH-N hydrogen bonds also do not have a nearby proton around OH group, but their thermal stability is very poor. The weak bond strength of OH-π hydrogen bonding may result in its low thermal stability, while the low thermal stability of OH-N hydrogen bond may be involved in the low N content in coal. The mechanism of release and conversion of these hydrogen bonds during pyrolysis is complicated and not fully understood. Both hydrogenbonding strength and the surrounding environment of the hydroxyl groups will affect their thermal stability. The correlation of the thermal stability with the structure of these hydrogen bonds will be examined in our future works. Conclusions
Figure 4. Thermal stability of various hydrogen bondsschange of Pi with pyrolysis temperature: (a) preasphaltenes; (b) asphaltenes.
hydrogen bonds follows the order of OH-ether O > selfassociated OH ≈ cyclic OH > OH-N. However, all the four hydrogen bonds are more thermally stable than OH-π one. The decrease of the absorption intensities of hydrogenbonded hydroxyl during pyrolysis may be attributed to two reasons: loss of hydroxyl by dewatering, and conversion of the weak hydrogen bond to more stable ones. It is obvious that disappearance of OH-π hydrogen bond may not be due to dewatering of hydroxyl at such a low temperature (100-200 °C), but due to the conversion of OH-π hydrogen bond to other more stable ones. The OH-π hydrogen bond in preasphaltenes disappearing at higher temperatures than in asphaltenes may be attributed to the higher aromaticity of preasphaltenes. The larger the size of an aromatic ring,
Four types of hydrogen bonds formed by OH groups in raw coal and five types in extracts are observed clearly by vacuum FTIR, which diminishes the influence of water by evaporation in a vacuum sample chamber, and in situ scanning the IR spectra. The types of hydrogen bonds change with coal rank, but the selfassociated OH and OH-N hydrogen bonds are observed in all the coals examined. The thermal stability of the five hydrogen bonds in extracts follows the order of OHether O > self-associated OH ≈ cyclic OH > OH-N > OH-π. Acknowledgment. The authors are grateful to the State Education Commission of China for the financial support of this work. We are also grateful to Mr. W. Li in State Key Laboratory of Coal Conversion for some experimental works. EF970100Z (16) Stenberg, V. I.; Baltisberger, R. J.; Patal, K. M.; Raman, K.; Woolsey, N. F. In Coal Science; Gorbaty, M. L., Larsen J. W., Wender, I., Eds.; Academic Press: New York, 1983; Vol. 2, pp 125-173.
