Energy & Fuels 2003, 17, 791-793
791
A New Hydrogen Bond in Coal Dongtao Li, Wen Li,* and Baoqing Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China Received July 24, 2002
The hydrogen bonds in four coals with different rank and different sulfur content were checked using the in situ diffuse reflectance FTIR (DRIFT) method. Besides the hydrogen bonds formed by -OH and -COOH groups, there exists a new kind of hydrogen bond (SH-N) formed by the -SH in the thiophenols or mercaptans and the nigrogen in pyridine-like compounds in coal. The absence of this shoulder peak in the IR spectra of low rank coals (even high sulfur) can be attributed to the high content of -COOH groups, and consequently the strong and broad absorbance in carboxylic acid dimers in this range covers the weak absorbance of SH-N. However, when the low rank coals were heated to 620 °C in Ar atmosphere and the -COOH disappears, the peak of SH-N clearly occurs. To summarize all the work on this new hydrogen bond, we assign it to the position of about 2514 cm-1.
1. Introduction The discovery of hydrogen bond in coal should be ascribed mainly to the application of infrared spectrometry. When Cannon et al.1,2 applied this technique in coal research for the first time, they found that there was a broad band centered at 3300 cm-1, and assigned it to the hydrogen bond in coal. Afterward, there were many evidences proposed to prove that this band was not an artifact of scattering correction. Friedel and Queiser3 observed similar band shapes using thin section of coal which do not exhibit scattering; Using Nujol mulls, Brown4 obtained the similar results also; Similar band shapes were also observed in the non scattering spectra of vacuum distillated coal tar melted onto a KBr blank.5 Spectra of coal obtained by diffuse reflectance spectroscopy6,7 and by photoacoustic spectroscopy8 which do not require KBr also proved that this band is true. After the hydrogen in coal is exchanged by deuterium, this broad adsorption peak of OH was drastically reduced and a new O-D peak appeared at 2600 cm-1.9 The broadness of this band is also drastically reduced after the coal samples were acetylated or alkylated.10,11 As expected for hydrogen bonds, spectra of tar from coal coated on KBr pellet * Corresponding author. Tel/Fax: 86-351-4048967. E-mail: liwen@ sxicc.ac.cn. (1) Cannon, C. G.; Sutherland, G. B. B. M. Nature 1945, 156, 240. (2) Cannon, C. G.; Sutherland G. B. B. M. Trans. Farady Soc. 1945, 41, 279-288. (3) Friedel, R. A.; Queiser, J. A. Anal. Chem. 1956, 28, 22. (4) Brown, J. K. J. Chem. Soc. 1955, 744. (5) Solomon, P. R. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1979, 24 (2), 184. (6) Fuller, M. P.; Griffiths, P. R. Appl. Spectrosc. 1980, 34, 533. (7) Fuller, M. P.; Griffiths, P. R. Anal. Chem. 1978, 50, 1906. (8) Vidrine, D. W. Appl. Spectrosc. 1980, 34, 314. (9) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M. Application of Fourier Transform IR Spectroscopy in Fuel Science. In Coal and Coal Products: Analytical Characterization Techniques; Symposium Series; Fuller, E. L., Ed.; American Chemical Society: Washington, DC, 1982; Vol. 205, pp 85-86. (10) Durie, R. A.; Sternhell, S. Aust. J. Chem. 1959, 12, 205. (11) Liotta, R. Fuel 1979, 58, 724.
Figure 1. The scheme of hydrogen bonds in coal.17
obtained at elevated temperatures showed that this absorption decreases and the peak for free hydroxyl group increases. Hence, it is undeniable that a hydrogen bond exists in coal.5 Now, it has been well-known that hydrogen bonding in coal affects the properties, chemical structure, and conversion of coal significantly. In the two widely accepted structural models of coal (host-guest model and associated model), the influence of noncovalent bonds such as van der Waals forces, hydrogen bonding, ionic linkages, and π-π interactions on the chemical structure and properties of coal (in particular low rank coal) are fully considered. Larsen12 estimated that such intersegment secondary bonds exceed the number of covalent crosslinks by a factor of 4. Brenner13 has pointed out that hydrogen bonding probably accounts for the high Tg and glassy properties of coal. Larsen et al.14 thought hydrogen (12) Larsen, J. W. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1985, 30(4), 444-449. (13) Brenner, D. Fuel 1985, 64, 167-173. (14) Larsen, J. W.; Green, T. K.; Kovac J. J. Org. Chem. 1985, 50, 4729-4735.
10.1021/ef020169t CCC: $25.00 © 2003 American Chemical Society Published on Web 04/30/2003
792
Energy & Fuels, Vol. 17, No. 3, 2003
Communications
Figure 2. The DRIFT spectra of coals with different sulfur content. Table 1. The Band Assignments of Hydrogen Bonds in Coal16,17 band assignments of hydrogen bonds in coal free OH groups OH-π hydrogen bonds self-associated n-mers (n > 3) OH-ether O hydrogen bonds tightly bound cyclic OH tetramers OH-N(acid/base structure) COOH dimmers17
abbreviations free OH groups OH‚‚‚π OH‚‚‚OH OH‚‚‚ether cyclic OH tetramers OH‚‚‚N COOH‚‚‚COOH
positions (cm-1) 3611 3516 3400 3300 3200 3100-2800 264017
boning in coal closely related with the rock properties and brittleness of coal. Miura and co-workers15 found that the cross linking of hydrogen bonding in coal was suppressed by preheatment, and the conversion of coal increased consequently. Moreover, hydrogen bonding in coal also plays important roles in the swelling, pyrolysis and reactivity of coal. Therefore, it is extremely important to study the hydrogen bonding in coal for effective utilization of coal and better understanding of its structure. Up to now, there are mainly six types of hydrogen bonds, including the hydrogen bonds formed by hydroxyl groups and carboxylic acids dimmers in coal. Based on the study of model compounds of coal, Painter et al.16 classified the hydrogen bonds formed by hydroxyl group in coal into five types, and their assignments and chemical structure are illustrated in Table 1 and Figure 1. Moreover, Miura et al.17 thought that the hydrogen bond formed by COOH dimmers should also be taken into account and assigned it at 2640 cm-1 (Figure 1 and Table 1). During our study on hydrogen bond in coal by diffuse reflectance IR, we found that a weak peak at 2514 cm-1 always occurred for some coals. Hence, we paid more attention to this band and suggested this band was attributed to the new hydrogen bond in coal. The aim of this work is to make sure the existence of this new hydrogen bond and supply the evidence of this point. 2. Experimental Section 1. Samples. Four coals with different carbon content were used, and the ultimate analysis of them are listed in Table 2. All samples -100 mesh were further ground under argon for 30 min in a glovebox with an agate mortar. 2. Apparatus and Procedure. The diffuse reflectance spectra of all samples were measured on EQUINOX 55 FTIR (15) Miura, K.; Mae K.; Sakurada K. et al. Energy Fuels 1992, 6, 1621. (16) Painter, P. C.;, Sobkowiak, M.; Youtcheff, J. Fuel 1987, 66, 973978. (17) Miura, K.; Mae, K.; Li W. et al. Energy Fuels 2001, 15 (3), 599610.
Figure 3. The second derivative of IR spectrum of YZ coal. Table 2. Analysis of Samples ultimate analysis (%, daf) sample HLH YZ ST ZB
C
H
N
S
O (diff)
73.11 79.30 89.14 91.09
4.70 5.04 4.95 3.57
1.22 1. 19 1.57 1.31
0.44 3.04 0.46 3.00
20.53 11.43 3.88 1.03
spectrometer (BRUKER) with the 0030-102 high temperature/ high-pressure accessory of Thermo Spectra-Tech using ZnSe windows. The detector was liquid nitrogen cooled MCT (mercury cadmium telluride). A mirror that is believed not to absorb water was used as the background.17 The diffuse reflectance spectra were collected with the co-additions of 200 scans at 4 cm-1 resolutions when the samples were heated to 140 °C in argon to eliminate the influence of moisture. The resultant spectra were converted to Kubelka-Munk function.
3. Results and Discussion When we studied the infrared spectra of coals with different sulfur and nitrogen content, it was found that the IR spectra of the high sulfur coals with high rank have shoulder peaks near 2514 cm-1 (Figure 2). This band is located between 2538 cm-1 and 2475 cm-1, which is believed to be a kind of hydrogen bond as observed by Gordy et al. in the mixture of mercaptans and pydine.18 In view of this, we thought there was a new hydrogen bond, which were not found before in coal. It is the hydrogen bond formed by the -SH in mercaptans or thiophenols and the nitrogen in pyridine-like compounds (SH-N). The evidence are as following. The spectra in Figures 2 and 3 give powerful evidence. A shoulder peak at 2514 cm-1 is clearly recognized in the spectra of high rank coal (YZ, ST, and ZB). The second (18) Gordy, W.; Stanford, S. J. Am. Chem. Soc. 1940, 62 (1), 497-505.
Communications
Energy & Fuels, Vol. 17, No. 3, 2003 793
Figure 4. The DRIFT spectrum of HLH and YZ coal at 620 °C.
derivative spectra of them (Figure 3) have also negative peaks at this position, which cannot be assigned to the contribution of noise. Figure 3 only shows the case of YZ coal, but ST and ZB coal also have the similar phenomena. The absence of this shoulder in the IR spectra of low rank coals (even high sulfur) such as HLH can be attributed to the high content of -COOH of them, and consequently, the strong and broad absorbance of carboxylic acid dimmers in this range covers the weak absorbance of SH-N. Moreover, when we studied the IR spectra obtained by Painter16 and Solomon,9 we found that this peak also appeared in their spectra. Unfortunately, probably because of the limitation of the coal used in their research, this peak was not noted by them although its existence is undeniable if we study their spectra more carefully. Using IR, Gordy et al.18 studied the hydrogen bonds between mercaptans or thiphenols and pyridine, R-picoline, dibenzylamine, and came to the conclusion that thiophenols form hydrogen bonds with substances of a rather narrow range of basicities. For substances with basicity constants much lower than the basicity constant of pyridine, no appreciable interaction occurs. For substances with basicities much stronger than that of pyridine, the thiophenols appear to be completely ionized by the base to form a salt. Although the aliphatic mercaptans have low acidity constants, and low tendency to share their proton in hydrogen bonds formation, the evidence for the hydrogen bonding between them and pyridinelike compounds is certain. From the spectra they provided, the position of this hydrogen bond is at around 2500 ( 30 cm-1. According to the basic knowledge of coal science,19 about 50-75% of the nitrogen exists in the form of pyridine or quinoline, and the rest is amino, imino, cyan, and five-membered ring organic nitrogen compounds. On the other hand, the sulfur in coal has the forms such as thiophenols, mercaptans, thioether, and thiophene etc.
Hence, it is not strange that coal is able to form the hydrogen bond of SH-N. In addition, the absorbance of IR spectra in this range is rarely affected by other absorbance.20 Moreover, when we study the spectrum of HLH and YZ at 620 °C (Figure 4), we fortunately found that this peak is very clear and relatively strong when the content of carboxylic acid dimers is removed and hence its disturbance is eliminated at this high temperature. After measurement, we found that the half width of this peak is almost the same as that measured from the spectra in Gordy’s work.18 When we study the spectra of macerals of other coals at high temperature, we also found that this peak exists. In view of the above evidences, we suggest there is a new hydrogen bond formed by the -SH in thiophenols or mercaptans and the nitrogen in pyridine-like compounds in coal, which was not found before. 4. Conclusions The evidences of diffuse reflectance FTIR proved that a new hydrogen bond formed by the -SH in thiophenols or mercaptans and the nitrogen in pyridine-like compounds exists in coal. To summarize all the work on this new hydrogen bond, we assign it at the position of 2514 cm-1. Acknowledgment. This work was financially supported by the Natural Science Foundation of China (29906012). EF020169T
(19) Guo, C. T., Ed. Coal Chemistry; Chemical Industry Press: Beijing, 1992; p 75. (20) Wu, J. G., Ed. The Theory and techniques of Fourier Transform Spectroscopy; ; Science and technology Press of China: Beijing, 1994; p 629.
