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Characterization of vitrinite concentrates: 3. Correlation of FT-i.r. measurements to thermoplastic and liquefaction behaviour
Characterization
of vitrinite
concentrates
3. Correlation of FT-i.r. measurements and liquefaction behaviour J. T. Senftle*, P. C. Painter
D. Kuehn,
A. Davis,
B. Brozoski,
to thermoplastic
C. Rhoads and
The Pennsylvania State University, College of Earth and Mineral Science, University Park, PA 16802, USA (Received 4 June 7982)
Measurements of aliphatic C-H content by FT-i.r. have been correlated to the plastic properties and liquefaction behaviour of a set of coals and vitrinite concentrates obtained from the Lower Kittaning seam. It has been found that for vitrinites a measure of total aliphatic C-H content correlates with Gieselerfluidity, displaying an abrupt change over a narrow range of aliphatic C-H content. Liquefaction conversion does not correlate with FT-i.r. measurements of total aliphatic C-H content, but does correlate with the intensity of a band that is a measure of CH, content. These results confirm the general relationship between the mechanism of liquefaction conversion and plastic development, but also suggests there are some subtle differences. (Keywords:
coal; vitrinite;
liquefaction)
Recent work in this laboratory has focused on an extensive effort to obtain coal data from a single seam (the Lower Kittaning) to investigate systematic relationships between petrographic composition, chemical structure, liquefaction behaviour and thermoplastic properties. Initial papers in this series have dealt with the characterization of the chemical structure of vitrinite concentrates by FT-i.r.’ and solid state 13C n.m.r2. The determination of the thermoplastic properties and liquefaction behaviour of both vitrinite concentrates and their parent coals have also recently been reported3v4. A statistical analysis demonstrated a relationship between liquefaction conversion data and the temperature of maximum fluidity, as determined by a Gieseler plastometer (correlation coefficient, - 0.96). Although the correlation coefficient (determined by bivariate analysis) between temperature of maximum fluidity and liquefaction conversion is considerably higher than those reported by previous workers for any variables correlated against liquefaction conversion3, at least a qualitative understanding that there is a relationship between the mechanism of coal liquefaction and the mechanism of plastic development has been known for some time. Neavel’ discussed this correlation in some detail and pointed out that Wiser6 appeared to be the first to call attention to this aspect of coal properties. Brown and Water’ had previously proposed that the low molecular weight hydrogen rich bitumens, that can be extracted from coal using suitable solvents, act as solvating or plasticizing agent as well as hydrogen donors during pyrolysis. Summarizing such threads of evidence, Neavel’ noted that three conditions are apparently necessary for * Presentaddress: Union Science and Technology Division, Union Oil Company of California, 376 South Valencia Avenue, Bree, CA 92621, USA 0016-2361/84/020245~$3.00 @ 1984 Buttenvorth & Co. (Publishers) Ltd
plastic development; first, there should be lamellaebridging structures that can be thermally cleaved; second, there should be an indigenous supply of hydroaromatic hydrogen; third that the micelles and lamellae should have the inherent capability of becoming mobile. These conditions are very similar to those necessary for liquefaction, except that the hydrogen donating vehicle is supplied to the coal (e.g. as tetralin). With the extensive chemical and physical data that has now been amassed from coals and vitrinite concentrates obtained from the lower Kittaning seam it is possible to re-examine these concepts. Particular attention will be paid to the relation between the amount and type of aliphatic hydrogen and liquefaction and thermoplastic properties. EXPERIMENTAL The characteristics of the coals and vitrinite concentrates used in this study have been reported in detail elsewhere’ -4. Thermoplastic properties were measured using a Gieseler plastometer. In a typical run 5 g of sample is packed into the apparatus. ASTM D2639-77 calls for a 40g in torque upon the stirring rod. It was found, however, that the freshly obtained samples used in this study developed an extremely high degree of fluidity, which exceeded the measuring capability of the Gieseler plastometer (30000ddpm). It was therefore necessary to reduce the torque to 20 gin to obtain a reasonable measurement of fluidity. Unfortunately, this does not allow the values presented here to be compared with ASTM Gieseler data, but in this study the correlation of plastic properties to chemical structure is the principle aim and the modification in experimental procedure was considered a necessity.
FUEL,
1994,
Vol 63, February
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Characterization of vitrinite concentrates: J. T. Senftle et al.
The Gieseler fluidity tests were run in duplicate with representative splits from each sample. Five measurements are made in each run; the softening temperature, temperature of maximum fluidity, resolidillcation temperature, temperature range of fluidity and a relative measure of maximum fluidity (dial divisions/minute, ddpm). The results for a total of 81 samples of whole coals and vitrinite concentrates together with a statistical analysis have been reported in detai13s4 and will not be reproduced here. However, specific results that can be related to structural characteristics determined by FT-i.r. will be presented. The liquefaction experiments were performed under the following uniform conditions; 5 g of - 20 mesh (840 pm) coal sample were mixed with 11 ml of tetralin and rapidly heated (and agitated) in a tubing bomb reactor while being heated in a fluidized sand bath at 400°C for 1 h. The tubing bomb reactor contents were extracted with ethyl acetate for 24 h in a soxhlet apparatus. The degree of conversion was determined as the proportion of product converted to ethyl acetate soluble material. FT-i.r. data were obtained using a Digilab FTS 15B instrument. Spectra were recorded at a resolution of 2 cm-’ using 400 ‘scans’ (averaged interferograms). Details of the instrumentation and experimental procedures have been discussed in previous studies’ ’ 8. RESULTS AND DISCUSSION Before considering correlations between aliphatic hydrogen content and thermoplastic and liquefaction properties, it is important to discuss the types of aliphatic functional groups present in coal and the degree to which they can be distinguished and quantitatively measured by FT-i.r. Aliphatic C-H groups have characteristic absorption bands between 2700 cm - 1 and 3ooO cm - ‘. The CH stretching modes observed in this region have been assigned on the basis of group frequenciesg*iO. However, many of these assignments refer specifically to alkanes and there are a number of complications that have to be considered when discussing the spectrum of coal. The CH stretching region of the spectrum of a vitrinite concentrate is presented in Figure 1 as a basis for discussion. This spectrum can be curve-resolved with a reasonable degree of accuracy and confidence providing certain criteria and procedures are used, as discussed in detail by Maddams’ ‘. The application of these methods to coal has been discussed8 and the resolution of the spectral profile presented in Figure I into five bands has been accomplished. On the basis of band assignments most often made in this spectral region, it is tempting to assign the bands at 2956 and 2864cm-’ to the asymmetric and symmetric stretching modes of CH, groups, the bands at 2923 and 2849cm-’ to the asymmetric and symmetric vibrations of CHz groups, and the band near 2891cm-’ to lone C-H groups. Unfortunately, when dealing with coal two complications have to be considered, one involves the limitations of curve resolving and the other involves a more detailed examination of group frequency assignments. The limitations of curve-resolving are straightforward and well-known”. Depending on factors such as the relative intensities of the bands involved, it is not possible to separate or detect (using derivative methods) bands separated by less than about half their half-width. The bands present in coals and vitrinites (but not their
246
FUEL, 1984, Vol 63, February
2923 I
I
3000
2900
2800 cm-’
Figure 7 Results of curve resolving the aliphatic C-H stretching region of the i.r. spectrum of a vitrinite concentrate (PSMC 52)
extracts, as will be shown later) are inherently broad and the bands being resolved are composites of contributions from a number of different groups. The band at 2923 cm-’ certainly has a contribution from methylene bridges (but not certain hydroaromatic structures) but methyl groups attached directly to aromatic groups also have their strongest band near this frequencyg. The 2891 cm-’ band is not simply due to lone C-H groups but also has a contribution from the overtone and combination bands of lower frequency bending modes, probably intensity enhanced by Fermi resonance interactions. However, the most important consideration in this study is the assignment of bands due to hydroaromatic structures, since these groups are thought to play a key role in plastic development and liquefaction. Figure 2 compares the C-H stretching region of the spectrum of three hydroaromatic model compounds, 9,10_dihydroanthracene, 9,10-dihydrophenanthrene and indan. The regions between 3000 and 31OOcm-’ and 2800 and 2900 cm-’ (assigned to aromatic C-H stretching modes and symmetric aliphatic C-H stretching modes, respectively) are complicated by combination and overtone modes of lower frequency bending modes. This is not uncommon in such low molecular weight model compounds and is a severe limitation in using such materials for determining exteinction coefficients for use in quantitative coal studies. The key observation to be made from these spectra, however, is the frequency of the asymmetric CH, stretching mode, which appears near 2950cm-’ in both 9,10-dihydroanthracene and 9,10-dihydrophenanthrene, significantly shifted from 2925 cm - ’ frequency usually observed for CH, groups. In the spectrum of indan the shift is smaller, to 2935cm-‘. Corresponding shifts for sterically strained alkane ring compounds have been considered in some detail in classic group frequency texts, such as Bellamy’. The C-H stretching frequencies of CH, groups increase and their intensities decrease progressively with bond angle strain. A corresponding interpretation is clearly in order for the frequency shifts observed in hydroaromatic structures such as 9,10-
Characterization of vitrinite concentrates: J. T. Senftle et al.
C
27
b 3060
. 3065
a
3015
A 30:” I
3100
L
I
I
I
I
I
I
I
I
I
2700
3150
absorb near 2950cm-‘. Because of these complications the modes between 2900 and 3OOOcm-’ are of little use in distinguishing between various types of aliphatic structures. The symmetric stretching modes near 2850 and 2865cm-’ are not as sensitive to structure as their asymmetric counterparts and so are potentially of some value in providing at least an approximate measure of methylene and methyl group content (respectively). However, to obtain a measure of total aliphatic C-H content the entire integrated area between 2995 and 2750cm- ’ must be used, thus assuming that the average value of the extinction coefficient (relating band intensities to the concentration of the appropriate functional group) stays approximately constant from coal to coal. This is clearly unsatisfactory, but in an earlier study of vitrinite concentrates it seemed to give reasonable values’. In the context of these limitations the correlation of aliphatic band intensities to coal properties is examined; first, considering thermoplastic behaviour. It was mentioned in the introduction to this paper that two of the conditions considered necessary for plastic development are the presence of lamellae bridging structures that can be thermally cleaved and an indigenous supply of hydroaromatic hydrogen. Based on an examination of space-filled models of various proposed coal structures, Spiro” has noted that the bridging entities that are most readily cleaved are aliphatic and hydroaromatic groups that protrude from flat aromatic lamellae. Consequently, it is anticipated that the maximum fluidity measured by the Gieseler plastometer might be related to the amount of aliphatic hydrogen determined by FT-i.r., since these structures apparently provide a source of both transferable hydrogen and thermally cleavable bridges. The first observations were not encouraging. Figure 3 shows a plot of the maximum fluidity measured for 29 coals (from the Lower Kittaning seam) plotted against the integrated intensity of the aliphatic C-H stretching modes between 2995 and 2750cm-’ (these wavenumber limits were defined so as to obtain a consistent baseline). Obviously, the line that has been drawn through the data points can only be considered arbitrary. To a degree the scatter in the plot represents a measure of experimental error, but a more fundamental cause of such scatter relates to the nature of coal itself. These samples, although
cm-’
Figure 2 Comparison of the C-H stretching region of the i.r. spectrum of three hydroaromatic model compounds. A, Indan; B. 9,10-dihydroanlhracene; C, 9,10-dihydrophenanthrene
dihydroanthracene and 9,10-dihydrophenanthrene. The steric strain is anticipated to be less in indan. In materials such as tetralin, where bond angle rotations can relieve even more steric strain, the asymmetric CH, stretching modes appear at almost the same frequency as in the alkanes. To summzrise these results, the 2956cm-’ band that can be curve resolved in the spectra of coals must be considered to be a composite of various overlapping contributions, as well as the band near 2925 cm- 1 discussed previously. Methyl groups attached directly to aromatic rings have a band near 2950cm-’ (as well as a stronger mode near 2925 cm-‘), methyl groups attached to other alkyl groups have bands between 2960 and 2970cm-’ 9*10, while methylene groups in certain types of hydroaromatic structures
I
Figure 3 Plot of maximum fluidity (ddpm) versus the integrated intensity of the aliphatic C-H stretching modes (29952750 cm-‘) for coal samples from the Lower Kittaning seam
FUEL, 1964, Vol63,
February
247
Characterization of vitrinite concentrates: J. T. Senftle et al. Table 1 Results of FT-i.r.
liquefaction
and plasticity
measurements
of a set of vitrinite
concentrates
Vitrinite PSMC
%C (dmmf)
Total area aliphatic stretching modes! (2995-2750 cm-r)
Area curve resolved 2853 cm-r band
Gieseler fluidity (ddpm)
Conversion %
16 18 20 30 33 36 39 48 49 50 52 54 56 58 59 62 67 68 70 71 72
86.6 85.8 85.4 84.6 83.0 84.3 85.9 88.8 88.3 90.1 91.3 89.8 89.4 83.7 83.3 84.7 83.2 87.0 87.3 85.5 88.7
18.10 18.75 18.72 16.71 18.75 15.16 19.27 17.04 17.77 15.87 15.35 12.98 16.65 17.15 18.38 17.60 17.32 20.48 20.89 18.09 17.79
6.25 6.48 7.07 5.96 6.54 5.76 7.00 5.98 5.89 5.82 6.03 5.65 6.33 6.58 6.62 6.12 6.33 6.90 6.81 6.28 6.05
3108 15292 24 674 291 13918 425 30 000 276 867 884 16 N/A 9425 31 145 24 106 30 000 29 994 5921
37.1 48.8 53.5 57.6 54.9 46.5 51.9 20.0 14.9 12.0 8.3 N/A 24.2 65.1 67.6 50.1 67.1 44.7 45.6 36.6
1400
15.4
a To convert to %H as aliphatic hydrogen multiply by a conversion factor of 0.20
predominantly vitrinite, have significant proportions of macerals that do not display plastic properties (plasticity is only exhibited by vitrinite and exinite). This hypothesis is confirmed when considering the data for a set of 21 vitrinite concentrates, listed in Table I and plotted in Figure 4. There is now a clear relationship between aliphatic hydrogen, as measured by the integrated intensity of the whole aliphatic C-H stretching region, and the fluidity of the samples. Because the maximum dial division that can be measured is 30000, the curve levels out at this value for vitrinites with a high ahphatic hydrogen content. The key observation however, is that over a range of integrated intensity measurements between 18 and 19 absorbance units ( x 5% change) the thermoplastic properties change from essentially zero fluidity to the maximum that can be measured by the Gieseler plastometer. Using the extinction coefficients (or, more accurately in terms of the definitions used, conversion factors) recently determined, the aliphatic hydrogen content corresponding to the range of maximum change in plastic properties is between 3.6 and 3.8% (dmmf). These results clearly support the general points concerning the mechanism of plastic development outlined by Neavel’, but also raises some new questions. A progressive increase in plasticity with increasing aliphatic hydrogen content would be anticipated, rather than the abrupt change displayed in Figure 4. At this point it is not known if other factors play a role. A measure of total aliphatic C-H content has been obtained and no attempt has been made to distinguish between various types of aliphatic C-H groups. In view of the arguments presented previously, the only way to do this is to measure the 2860 and 2850 cm-’ bands, which should bear some relation to the concentration of CH, and CH, groups, respectively. Even so, this would not permit CH, groups present as thermally cleavable methylene bridges to be distinguished from those present in hydroaromatic entities. Nevertheless, the general observation of a sharp transition in
248
FUEL, 1984, Vol 63, February
thermoplastic behaviour of vitrinites over a narrow range of aliphatic C-H content is intriguing and leads to two further observations. The first point that should be made concerns the wellknown observation that even a low level of oxidation has an extremely adverse effect upon coal plasticity. It has often been assumed that this loss in plastic properties is related to cross links formed during the oxidation process. However, Figure 4 suggests that simple loss of aliphatic hydrogen could be a key factor. This is not to say that cross linking, for example by the formation of ethers, does
30000-
260000 C
22000z P 5 ~
18000-
1 .g z
0 0
14000.-z
10000-
0
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14
15
16
17
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I
78
19
20
;
Figure 4 Plot of maximum fluidity (ddpm) versus the integrated intensity of the aliphatic C-H stretching modes (29952750 cm-‘) for a set of vitrinite concentrates
Characterization of vitrinite concentrates: J. T. Senftle et al.
not occur, but a difference of only 5% in aliphatic hydrogen corresponds to a dramatic variation in fluidity. Methylene groups in the benzylic position are highly susceptible to autoxidation and a decrease in the concentration of these groups has been observed in FT4.r. studies of weathered and laboratory oxidized coals’ 3*’ 4. It is therefore suggested that a simple small decrease in aliphatic hydrogen upon oxidation could play a major role in the corresponding loss of plastic properties. The second point concerns the location and availability of the aliphatic hydrogen groups. Loss of the component known as bitumen upon solvent extraction leads to the elimination of plastic properties. It has often been assumed that the structure of solvent extracts, or more accurately the extracts obtained using solvents such as pyridine, are very similar to the parent coal, differing perhaps only in molecular weight. To a degree this conclusion is based upon i.r. studies of coal extracts performed more than 20 years ago by Brown”, although Friedel et ~1.‘~subsequently noted some differences in the aliphatic C-H stretching region. A pyridine extract of a coal (PSOC 1336) was obtained which displayed the maximum measurable fluidity (30 000, ddpm) when fresh. Although the spectra of the extract, residue and original coal display a certain similarity in their general features, there are distinct differences. Figure 5 compares the aliphatic C-H stretching region of these samples plotted on an absolute absorption scale. The spectra were normalized to the equivalent of 1 mg !2
dmmf in a 300 mg KBr pellet and the plots are offset for clarity of presentation. Clearly, there is a much higher concentration of aliphatic groups in the extract and it is anticipated that not only does this soluble phase act as a plasticizer but also as a source of hydrogen. Because of the soluble nature of this component, it is much easier to characterize using a variety of techniques. This study is being pursued using proton n.m.r. and solid state 13C n.m.r. in an attempt to obtain a more detailed understanding of the structure of this component, particularly the types and distribution of aliphatic functional groups. In view of the proposed relationship between the mechanisms of plastic development and liquefaction’, initially it is considered that a plot of total aliphatic C-H content of coals, or more likely, vitrinites, would bear some relation to liquefaction conversion. Surprisingly, this was not the case, at least under the conditions of liquefaction used in this study. On the right-hand side of Figure 6 the integrated area of the entire aliphatic C-H stretching region (2995-2750cm-‘) has been plotted for the 29 coal samples on which plasticity measurements were made. An enormous amount of scatter in the data points can be observed. In contrast to the plasticity studies, however, this scatter does not reduce to a coherent relation once the vitrinite concentrates alone are considered, as can be seen from Figure 7. However, if the area of the curve resolved CH, band at 2853 against % conversion is plotted, see Figures 6 and 7, a clear linear relation emerges for both sets of samples. In fact, the scatter in these plots is remarkably small considering the errors inherent in measuring the area of a weak, curve resolved band. The observation of this relationship is extremely gratifying in that proposed mechanisms of liquefaction involve cleavage of groups such as methylene bridges and transfer of hydrogen, not only from an externally supplied donor such as tetralin but also among groups within the coal. The 2853cm-’ band has to be considered a measure of total CH, groups, those present as bridges and hydroaromatic structures. In fact, this type of relationship is precisely what would be anticipated for these measurements of plastic properties. The observation of a relationship of total aliphatic C-H content as measured by FT-i.r. to Gieseler fluidity suggests that there are some subtleties to the mechanism of plastic development that have yet to be defined. Finally, in addition to correlating aliphatic C-H con-
70o
60I2053
506 s40h, 2 cj 30:20lo0
0’ ’ 3200
Figure 5 Aliphatic C-H stretching region of the i.r. spectrum of (bottom) a coal (PSOC 1336), (middle) residue and (top) pyridine extract. The spectra are plotted on the same absolute absorbance scale but the baselines have been offset for clarity
0
2
”
4
” ’ ’ ’ ” ” ” ’ I ’ ’ 6 8 10 12 14 16 18 20 Intensity, aliphatic C-H bands
’
’ ’ ‘1 22 24
Figure 6 Plot of % conversion in liquefaction reactions versus the intensity of aliphatic C-H stretching modes for a set of coal samples. 0, Integrated intensity 2993-2750 cm-‘; 0, area of curve-resolved 2853 cm-’ band
FUEL, 1984, Vol 63, February
249
Characterization of vitrinite concentrates: J. T. Senftle et al.
tent to measurements of fluidity and liquefaction conversion, other possible correlations were examined. Hydroxyl group content did not correlate with Gieseler fluidity, the samples with the maximum fluidity falling in about the middle of the range of OH values (see Table I). This is probably a simple reflection of the rank of the samples. However, there is a relation between y0 conversion in liquefaction and the OH group content of vitrinite concentrates, as shown in Figure 8. This observation supports the proposal of Larsen and Sams’ ’ that phenolic groups can act as hydrogen transfer agents, thus facilitating liquefaction conversion. However, it should be noted that this result is in sharp contrast to the lack of correlation of OH group content to Gieseler fluidity.
70-
In broad outline the relationship between liquefaction conversion and plastic development is supported by the correlation of FT-i.r. measurements to these properties. Both Gieseler fluidity and liquefaction conversion are related to aliphatic C-H content. However, within this context there are some differences. Plasticity can be correlated to a measure of total aliphatic C-H content for vitrinite concentrates alone and changes abruptly over a narrow range of C-H content. In contrast, liquefaction conversion for both coals and vitrinite concentrates correlates to CH, content as measured by the curveresolved 2853 cm-’ band. Furthermore, liquefaction conversion also correlates to OH-group content. It has previously been considered that the major difference in the mechanisms of plastic development and liquefaction is that in the latter process there is an additional external source of transferable hydrogen. The results presented here suggest that there are also some subtle differences that need to be investigated.
a
OD
IAl
(total
605Qa
1 oOrea)
a
a
a
ACKNOWLEDGEMENTS
aa
s z 40b z 8 30-
The authors gratefully acknowledge the financial support of the Department of Energy under contract No. DEAC22-OPC30013 and the Pennsylvania State Cooperative Program in Coal Research.
a ‘0
f 20-
8
a
10-
01 0
SUMMARY AND CONCLUSIONS
REFERENCES
a ’
0
’
2
’
’
4
’ ’ ’ ’ 0 ’ ’ ’ I ’ ’ 6 8 10 12 14 16 Intensity, allphattc C-H bands
0
0
18
0
’
20
’
’
Figure 7 Plot of % conversion in liquefaction reactions versus the intensity of the aliphatic C-H stretching modes for a set of vitrinite concentrates. 0, Integrated intensity 2995-2750 cm-‘; 0, area of curve resolved 2853 cm-’ band 4
5
6 7 8 9 10
11 12 13
01 ” 0
2
”
4
1’
6
’
” ’ 1” 8 I”” 8 10 12 14 16 Intensity, OH bands
18
““I 22
Figure 8 Plot of % conversion in liquefaction reactions versus bands assigned to OH groups
250
FUEL, 1984, Vol 63, February
14
20
15 16
Kuehn, D., Snyder, R. W., Davis, A. and Painter, P. C. Fuel 1982, 61,682 Painter, P. C., Kuehn, D. W., Starsinic, M., Davis, A., Havens, J. R. and Koenig, J. L. Fuel 1983,62, 103 Senftle. J. T. Ph.D. thesis in Geoloav. ‘Relationshio between coal constitution, thermoplastic properties and liquefaction behavior of coals and vitrinite concentrates from the Lower Kittaning seam’, The Pennsylvania State University, November 1981 Senftle, J. T. and Davis, A. ‘A Data Base for the analysis of compositional characteristics of coal seams and mace&, Final Report-Part 1,1982, DOE contract No. DOE-30013-2 Neavel, R. C. ‘Coal Plasticity Mechanism: Inferences from Liquefaction Studies’, Proc. Coal Agglom. Conversion Symp. West Virginia Geol. Econ. Survey, Morgantown, 5-6 May 1975, published 1976 Wiser, W. Fuel 1968,47,475 Brown, H. R. and Water, P. L. Fuel 1966,45, 17 Painter, P. C., Snyder, R. W., Starsinic, M., Coleman, M. M., Kuehn, D. and Davis, A. Appl. Spectrosc. 1981,35(S), 475 Bellamy, L. J. ‘The infrared spectra of complex molecules’, Third Edition Chapman and Hall, London, 1975 Colthup, N. B., Daly, L. H. and Wiberley, S. E. ‘Introduction to Infrared and Raman Spectroscopy’, 2nd Edition, Academic Press, New York, 1975 Maddams, W. F. Appl. Spectrosc. 1980,34,245 Spiro, C. L. Fuel 1981,60, 1121 Painter, P. C., Snyder, R. W., Pearson, D. E. and Kwong, J. Fuel 1980,59,282 Painter, P. C., Coleman, M. M., Snyder, R. W., Mahajan, O., Kamatsu, M. and Walker, P. L. Appl. Spectrosc. 1981, 35, 106 Brown, J. K. Fuel 1959,38, 55 Friedel, R. A., Retcofsky, H. L. and Queiser, J. A. US Bur. Mines Bull. 1967,640
17
Larsen, J. W. and Sams, T. L. Fuel 1981,60,272
of vitrinite
concentrates
3. Correlation of FT-i.r. measurements and liquefaction behaviour J. T. Senftle*, P. C. Painter
D. Kuehn,
A. Davis,
B. Brozoski,
to thermoplastic
C. Rhoads and
The Pennsylvania State University, College of Earth and Mineral Science, University Park, PA 16802, USA (Received 4 June 7982)
Measurements of aliphatic C-H content by FT-i.r. have been correlated to the plastic properties and liquefaction behaviour of a set of coals and vitrinite concentrates obtained from the Lower Kittaning seam. It has been found that for vitrinites a measure of total aliphatic C-H content correlates with Gieselerfluidity, displaying an abrupt change over a narrow range of aliphatic C-H content. Liquefaction conversion does not correlate with FT-i.r. measurements of total aliphatic C-H content, but does correlate with the intensity of a band that is a measure of CH, content. These results confirm the general relationship between the mechanism of liquefaction conversion and plastic development, but also suggests there are some subtle differences. (Keywords:
coal; vitrinite;
liquefaction)
Recent work in this laboratory has focused on an extensive effort to obtain coal data from a single seam (the Lower Kittaning) to investigate systematic relationships between petrographic composition, chemical structure, liquefaction behaviour and thermoplastic properties. Initial papers in this series have dealt with the characterization of the chemical structure of vitrinite concentrates by FT-i.r.’ and solid state 13C n.m.r2. The determination of the thermoplastic properties and liquefaction behaviour of both vitrinite concentrates and their parent coals have also recently been reported3v4. A statistical analysis demonstrated a relationship between liquefaction conversion data and the temperature of maximum fluidity, as determined by a Gieseler plastometer (correlation coefficient, - 0.96). Although the correlation coefficient (determined by bivariate analysis) between temperature of maximum fluidity and liquefaction conversion is considerably higher than those reported by previous workers for any variables correlated against liquefaction conversion3, at least a qualitative understanding that there is a relationship between the mechanism of coal liquefaction and the mechanism of plastic development has been known for some time. Neavel’ discussed this correlation in some detail and pointed out that Wiser6 appeared to be the first to call attention to this aspect of coal properties. Brown and Water’ had previously proposed that the low molecular weight hydrogen rich bitumens, that can be extracted from coal using suitable solvents, act as solvating or plasticizing agent as well as hydrogen donors during pyrolysis. Summarizing such threads of evidence, Neavel’ noted that three conditions are apparently necessary for * Presentaddress: Union Science and Technology Division, Union Oil Company of California, 376 South Valencia Avenue, Bree, CA 92621, USA 0016-2361/84/020245~$3.00 @ 1984 Buttenvorth & Co. (Publishers) Ltd
plastic development; first, there should be lamellaebridging structures that can be thermally cleaved; second, there should be an indigenous supply of hydroaromatic hydrogen; third that the micelles and lamellae should have the inherent capability of becoming mobile. These conditions are very similar to those necessary for liquefaction, except that the hydrogen donating vehicle is supplied to the coal (e.g. as tetralin). With the extensive chemical and physical data that has now been amassed from coals and vitrinite concentrates obtained from the lower Kittaning seam it is possible to re-examine these concepts. Particular attention will be paid to the relation between the amount and type of aliphatic hydrogen and liquefaction and thermoplastic properties. EXPERIMENTAL The characteristics of the coals and vitrinite concentrates used in this study have been reported in detail elsewhere’ -4. Thermoplastic properties were measured using a Gieseler plastometer. In a typical run 5 g of sample is packed into the apparatus. ASTM D2639-77 calls for a 40g in torque upon the stirring rod. It was found, however, that the freshly obtained samples used in this study developed an extremely high degree of fluidity, which exceeded the measuring capability of the Gieseler plastometer (30000ddpm). It was therefore necessary to reduce the torque to 20 gin to obtain a reasonable measurement of fluidity. Unfortunately, this does not allow the values presented here to be compared with ASTM Gieseler data, but in this study the correlation of plastic properties to chemical structure is the principle aim and the modification in experimental procedure was considered a necessity.
FUEL,
1994,
Vol 63, February
245
Characterization of vitrinite concentrates: J. T. Senftle et al.
The Gieseler fluidity tests were run in duplicate with representative splits from each sample. Five measurements are made in each run; the softening temperature, temperature of maximum fluidity, resolidillcation temperature, temperature range of fluidity and a relative measure of maximum fluidity (dial divisions/minute, ddpm). The results for a total of 81 samples of whole coals and vitrinite concentrates together with a statistical analysis have been reported in detai13s4 and will not be reproduced here. However, specific results that can be related to structural characteristics determined by FT-i.r. will be presented. The liquefaction experiments were performed under the following uniform conditions; 5 g of - 20 mesh (840 pm) coal sample were mixed with 11 ml of tetralin and rapidly heated (and agitated) in a tubing bomb reactor while being heated in a fluidized sand bath at 400°C for 1 h. The tubing bomb reactor contents were extracted with ethyl acetate for 24 h in a soxhlet apparatus. The degree of conversion was determined as the proportion of product converted to ethyl acetate soluble material. FT-i.r. data were obtained using a Digilab FTS 15B instrument. Spectra were recorded at a resolution of 2 cm-’ using 400 ‘scans’ (averaged interferograms). Details of the instrumentation and experimental procedures have been discussed in previous studies’ ’ 8. RESULTS AND DISCUSSION Before considering correlations between aliphatic hydrogen content and thermoplastic and liquefaction properties, it is important to discuss the types of aliphatic functional groups present in coal and the degree to which they can be distinguished and quantitatively measured by FT-i.r. Aliphatic C-H groups have characteristic absorption bands between 2700 cm - 1 and 3ooO cm - ‘. The CH stretching modes observed in this region have been assigned on the basis of group frequenciesg*iO. However, many of these assignments refer specifically to alkanes and there are a number of complications that have to be considered when discussing the spectrum of coal. The CH stretching region of the spectrum of a vitrinite concentrate is presented in Figure 1 as a basis for discussion. This spectrum can be curve-resolved with a reasonable degree of accuracy and confidence providing certain criteria and procedures are used, as discussed in detail by Maddams’ ‘. The application of these methods to coal has been discussed8 and the resolution of the spectral profile presented in Figure I into five bands has been accomplished. On the basis of band assignments most often made in this spectral region, it is tempting to assign the bands at 2956 and 2864cm-’ to the asymmetric and symmetric stretching modes of CH, groups, the bands at 2923 and 2849cm-’ to the asymmetric and symmetric vibrations of CHz groups, and the band near 2891cm-’ to lone C-H groups. Unfortunately, when dealing with coal two complications have to be considered, one involves the limitations of curve resolving and the other involves a more detailed examination of group frequency assignments. The limitations of curve-resolving are straightforward and well-known”. Depending on factors such as the relative intensities of the bands involved, it is not possible to separate or detect (using derivative methods) bands separated by less than about half their half-width. The bands present in coals and vitrinites (but not their
246
FUEL, 1984, Vol 63, February
2923 I
I
3000
2900
2800 cm-’
Figure 7 Results of curve resolving the aliphatic C-H stretching region of the i.r. spectrum of a vitrinite concentrate (PSMC 52)
extracts, as will be shown later) are inherently broad and the bands being resolved are composites of contributions from a number of different groups. The band at 2923 cm-’ certainly has a contribution from methylene bridges (but not certain hydroaromatic structures) but methyl groups attached directly to aromatic groups also have their strongest band near this frequencyg. The 2891 cm-’ band is not simply due to lone C-H groups but also has a contribution from the overtone and combination bands of lower frequency bending modes, probably intensity enhanced by Fermi resonance interactions. However, the most important consideration in this study is the assignment of bands due to hydroaromatic structures, since these groups are thought to play a key role in plastic development and liquefaction. Figure 2 compares the C-H stretching region of the spectrum of three hydroaromatic model compounds, 9,10_dihydroanthracene, 9,10-dihydrophenanthrene and indan. The regions between 3000 and 31OOcm-’ and 2800 and 2900 cm-’ (assigned to aromatic C-H stretching modes and symmetric aliphatic C-H stretching modes, respectively) are complicated by combination and overtone modes of lower frequency bending modes. This is not uncommon in such low molecular weight model compounds and is a severe limitation in using such materials for determining exteinction coefficients for use in quantitative coal studies. The key observation to be made from these spectra, however, is the frequency of the asymmetric CH, stretching mode, which appears near 2950cm-’ in both 9,10-dihydroanthracene and 9,10-dihydrophenanthrene, significantly shifted from 2925 cm - ’ frequency usually observed for CH, groups. In the spectrum of indan the shift is smaller, to 2935cm-‘. Corresponding shifts for sterically strained alkane ring compounds have been considered in some detail in classic group frequency texts, such as Bellamy’. The C-H stretching frequencies of CH, groups increase and their intensities decrease progressively with bond angle strain. A corresponding interpretation is clearly in order for the frequency shifts observed in hydroaromatic structures such as 9,10-
Characterization of vitrinite concentrates: J. T. Senftle et al.
C
27
b 3060
. 3065
a
3015
A 30:” I
3100
L
I
I
I
I
I
I
I
I
I
2700
3150
absorb near 2950cm-‘. Because of these complications the modes between 2900 and 3OOOcm-’ are of little use in distinguishing between various types of aliphatic structures. The symmetric stretching modes near 2850 and 2865cm-’ are not as sensitive to structure as their asymmetric counterparts and so are potentially of some value in providing at least an approximate measure of methylene and methyl group content (respectively). However, to obtain a measure of total aliphatic C-H content the entire integrated area between 2995 and 2750cm- ’ must be used, thus assuming that the average value of the extinction coefficient (relating band intensities to the concentration of the appropriate functional group) stays approximately constant from coal to coal. This is clearly unsatisfactory, but in an earlier study of vitrinite concentrates it seemed to give reasonable values’. In the context of these limitations the correlation of aliphatic band intensities to coal properties is examined; first, considering thermoplastic behaviour. It was mentioned in the introduction to this paper that two of the conditions considered necessary for plastic development are the presence of lamellae bridging structures that can be thermally cleaved and an indigenous supply of hydroaromatic hydrogen. Based on an examination of space-filled models of various proposed coal structures, Spiro” has noted that the bridging entities that are most readily cleaved are aliphatic and hydroaromatic groups that protrude from flat aromatic lamellae. Consequently, it is anticipated that the maximum fluidity measured by the Gieseler plastometer might be related to the amount of aliphatic hydrogen determined by FT-i.r., since these structures apparently provide a source of both transferable hydrogen and thermally cleavable bridges. The first observations were not encouraging. Figure 3 shows a plot of the maximum fluidity measured for 29 coals (from the Lower Kittaning seam) plotted against the integrated intensity of the aliphatic C-H stretching modes between 2995 and 2750cm-’ (these wavenumber limits were defined so as to obtain a consistent baseline). Obviously, the line that has been drawn through the data points can only be considered arbitrary. To a degree the scatter in the plot represents a measure of experimental error, but a more fundamental cause of such scatter relates to the nature of coal itself. These samples, although
cm-’
Figure 2 Comparison of the C-H stretching region of the i.r. spectrum of three hydroaromatic model compounds. A, Indan; B. 9,10-dihydroanlhracene; C, 9,10-dihydrophenanthrene
dihydroanthracene and 9,10-dihydrophenanthrene. The steric strain is anticipated to be less in indan. In materials such as tetralin, where bond angle rotations can relieve even more steric strain, the asymmetric CH, stretching modes appear at almost the same frequency as in the alkanes. To summzrise these results, the 2956cm-’ band that can be curve resolved in the spectra of coals must be considered to be a composite of various overlapping contributions, as well as the band near 2925 cm- 1 discussed previously. Methyl groups attached directly to aromatic rings have a band near 2950cm-’ (as well as a stronger mode near 2925 cm-‘), methyl groups attached to other alkyl groups have bands between 2960 and 2970cm-’ 9*10, while methylene groups in certain types of hydroaromatic structures
I
Figure 3 Plot of maximum fluidity (ddpm) versus the integrated intensity of the aliphatic C-H stretching modes (29952750 cm-‘) for coal samples from the Lower Kittaning seam
FUEL, 1964, Vol63,
February
247
Characterization of vitrinite concentrates: J. T. Senftle et al. Table 1 Results of FT-i.r.
liquefaction
and plasticity
measurements
of a set of vitrinite
concentrates
Vitrinite PSMC
%C (dmmf)
Total area aliphatic stretching modes! (2995-2750 cm-r)
Area curve resolved 2853 cm-r band
Gieseler fluidity (ddpm)
Conversion %
16 18 20 30 33 36 39 48 49 50 52 54 56 58 59 62 67 68 70 71 72
86.6 85.8 85.4 84.6 83.0 84.3 85.9 88.8 88.3 90.1 91.3 89.8 89.4 83.7 83.3 84.7 83.2 87.0 87.3 85.5 88.7
18.10 18.75 18.72 16.71 18.75 15.16 19.27 17.04 17.77 15.87 15.35 12.98 16.65 17.15 18.38 17.60 17.32 20.48 20.89 18.09 17.79
6.25 6.48 7.07 5.96 6.54 5.76 7.00 5.98 5.89 5.82 6.03 5.65 6.33 6.58 6.62 6.12 6.33 6.90 6.81 6.28 6.05
3108 15292 24 674 291 13918 425 30 000 276 867 884 16 N/A 9425 31 145 24 106 30 000 29 994 5921
37.1 48.8 53.5 57.6 54.9 46.5 51.9 20.0 14.9 12.0 8.3 N/A 24.2 65.1 67.6 50.1 67.1 44.7 45.6 36.6
1400
15.4
a To convert to %H as aliphatic hydrogen multiply by a conversion factor of 0.20
predominantly vitrinite, have significant proportions of macerals that do not display plastic properties (plasticity is only exhibited by vitrinite and exinite). This hypothesis is confirmed when considering the data for a set of 21 vitrinite concentrates, listed in Table I and plotted in Figure 4. There is now a clear relationship between aliphatic hydrogen, as measured by the integrated intensity of the whole aliphatic C-H stretching region, and the fluidity of the samples. Because the maximum dial division that can be measured is 30000, the curve levels out at this value for vitrinites with a high ahphatic hydrogen content. The key observation however, is that over a range of integrated intensity measurements between 18 and 19 absorbance units ( x 5% change) the thermoplastic properties change from essentially zero fluidity to the maximum that can be measured by the Gieseler plastometer. Using the extinction coefficients (or, more accurately in terms of the definitions used, conversion factors) recently determined, the aliphatic hydrogen content corresponding to the range of maximum change in plastic properties is between 3.6 and 3.8% (dmmf). These results clearly support the general points concerning the mechanism of plastic development outlined by Neavel’, but also raises some new questions. A progressive increase in plasticity with increasing aliphatic hydrogen content would be anticipated, rather than the abrupt change displayed in Figure 4. At this point it is not known if other factors play a role. A measure of total aliphatic C-H content has been obtained and no attempt has been made to distinguish between various types of aliphatic C-H groups. In view of the arguments presented previously, the only way to do this is to measure the 2860 and 2850 cm-’ bands, which should bear some relation to the concentration of CH, and CH, groups, respectively. Even so, this would not permit CH, groups present as thermally cleavable methylene bridges to be distinguished from those present in hydroaromatic entities. Nevertheless, the general observation of a sharp transition in
248
FUEL, 1984, Vol 63, February
thermoplastic behaviour of vitrinites over a narrow range of aliphatic C-H content is intriguing and leads to two further observations. The first point that should be made concerns the wellknown observation that even a low level of oxidation has an extremely adverse effect upon coal plasticity. It has often been assumed that this loss in plastic properties is related to cross links formed during the oxidation process. However, Figure 4 suggests that simple loss of aliphatic hydrogen could be a key factor. This is not to say that cross linking, for example by the formation of ethers, does
30000-
260000 C
22000z P 5 ~
18000-
1 .g z
0 0
14000.-z
10000-
0
I
14
15
16
17
I
In
I
I
78
19
20
;
Figure 4 Plot of maximum fluidity (ddpm) versus the integrated intensity of the aliphatic C-H stretching modes (29952750 cm-‘) for a set of vitrinite concentrates
Characterization of vitrinite concentrates: J. T. Senftle et al.
not occur, but a difference of only 5% in aliphatic hydrogen corresponds to a dramatic variation in fluidity. Methylene groups in the benzylic position are highly susceptible to autoxidation and a decrease in the concentration of these groups has been observed in FT4.r. studies of weathered and laboratory oxidized coals’ 3*’ 4. It is therefore suggested that a simple small decrease in aliphatic hydrogen upon oxidation could play a major role in the corresponding loss of plastic properties. The second point concerns the location and availability of the aliphatic hydrogen groups. Loss of the component known as bitumen upon solvent extraction leads to the elimination of plastic properties. It has often been assumed that the structure of solvent extracts, or more accurately the extracts obtained using solvents such as pyridine, are very similar to the parent coal, differing perhaps only in molecular weight. To a degree this conclusion is based upon i.r. studies of coal extracts performed more than 20 years ago by Brown”, although Friedel et ~1.‘~subsequently noted some differences in the aliphatic C-H stretching region. A pyridine extract of a coal (PSOC 1336) was obtained which displayed the maximum measurable fluidity (30 000, ddpm) when fresh. Although the spectra of the extract, residue and original coal display a certain similarity in their general features, there are distinct differences. Figure 5 compares the aliphatic C-H stretching region of these samples plotted on an absolute absorption scale. The spectra were normalized to the equivalent of 1 mg !2
dmmf in a 300 mg KBr pellet and the plots are offset for clarity of presentation. Clearly, there is a much higher concentration of aliphatic groups in the extract and it is anticipated that not only does this soluble phase act as a plasticizer but also as a source of hydrogen. Because of the soluble nature of this component, it is much easier to characterize using a variety of techniques. This study is being pursued using proton n.m.r. and solid state 13C n.m.r. in an attempt to obtain a more detailed understanding of the structure of this component, particularly the types and distribution of aliphatic functional groups. In view of the proposed relationship between the mechanisms of plastic development and liquefaction’, initially it is considered that a plot of total aliphatic C-H content of coals, or more likely, vitrinites, would bear some relation to liquefaction conversion. Surprisingly, this was not the case, at least under the conditions of liquefaction used in this study. On the right-hand side of Figure 6 the integrated area of the entire aliphatic C-H stretching region (2995-2750cm-‘) has been plotted for the 29 coal samples on which plasticity measurements were made. An enormous amount of scatter in the data points can be observed. In contrast to the plasticity studies, however, this scatter does not reduce to a coherent relation once the vitrinite concentrates alone are considered, as can be seen from Figure 7. However, if the area of the curve resolved CH, band at 2853 against % conversion is plotted, see Figures 6 and 7, a clear linear relation emerges for both sets of samples. In fact, the scatter in these plots is remarkably small considering the errors inherent in measuring the area of a weak, curve resolved band. The observation of this relationship is extremely gratifying in that proposed mechanisms of liquefaction involve cleavage of groups such as methylene bridges and transfer of hydrogen, not only from an externally supplied donor such as tetralin but also among groups within the coal. The 2853cm-’ band has to be considered a measure of total CH, groups, those present as bridges and hydroaromatic structures. In fact, this type of relationship is precisely what would be anticipated for these measurements of plastic properties. The observation of a relationship of total aliphatic C-H content as measured by FT-i.r. to Gieseler fluidity suggests that there are some subtleties to the mechanism of plastic development that have yet to be defined. Finally, in addition to correlating aliphatic C-H con-
70o
60I2053
506 s40h, 2 cj 30:20lo0
0’ ’ 3200
Figure 5 Aliphatic C-H stretching region of the i.r. spectrum of (bottom) a coal (PSOC 1336), (middle) residue and (top) pyridine extract. The spectra are plotted on the same absolute absorbance scale but the baselines have been offset for clarity
0
2
”
4
” ’ ’ ’ ” ” ” ’ I ’ ’ 6 8 10 12 14 16 18 20 Intensity, aliphatic C-H bands
’
’ ’ ‘1 22 24
Figure 6 Plot of % conversion in liquefaction reactions versus the intensity of aliphatic C-H stretching modes for a set of coal samples. 0, Integrated intensity 2993-2750 cm-‘; 0, area of curve-resolved 2853 cm-’ band
FUEL, 1984, Vol 63, February
249
Characterization of vitrinite concentrates: J. T. Senftle et al.
tent to measurements of fluidity and liquefaction conversion, other possible correlations were examined. Hydroxyl group content did not correlate with Gieseler fluidity, the samples with the maximum fluidity falling in about the middle of the range of OH values (see Table I). This is probably a simple reflection of the rank of the samples. However, there is a relation between y0 conversion in liquefaction and the OH group content of vitrinite concentrates, as shown in Figure 8. This observation supports the proposal of Larsen and Sams’ ’ that phenolic groups can act as hydrogen transfer agents, thus facilitating liquefaction conversion. However, it should be noted that this result is in sharp contrast to the lack of correlation of OH group content to Gieseler fluidity.
70-
In broad outline the relationship between liquefaction conversion and plastic development is supported by the correlation of FT-i.r. measurements to these properties. Both Gieseler fluidity and liquefaction conversion are related to aliphatic C-H content. However, within this context there are some differences. Plasticity can be correlated to a measure of total aliphatic C-H content for vitrinite concentrates alone and changes abruptly over a narrow range of C-H content. In contrast, liquefaction conversion for both coals and vitrinite concentrates correlates to CH, content as measured by the curveresolved 2853 cm-’ band. Furthermore, liquefaction conversion also correlates to OH-group content. It has previously been considered that the major difference in the mechanisms of plastic development and liquefaction is that in the latter process there is an additional external source of transferable hydrogen. The results presented here suggest that there are also some subtle differences that need to be investigated.
a
OD
IAl
(total
605Qa
1 oOrea)
a
a
a
ACKNOWLEDGEMENTS
aa
s z 40b z 8 30-
The authors gratefully acknowledge the financial support of the Department of Energy under contract No. DEAC22-OPC30013 and the Pennsylvania State Cooperative Program in Coal Research.
a ‘0
f 20-
8
a
10-
01 0
SUMMARY AND CONCLUSIONS
REFERENCES
a ’
0
’
2
’
’
4
’ ’ ’ ’ 0 ’ ’ ’ I ’ ’ 6 8 10 12 14 16 Intensity, allphattc C-H bands
0
0
18
0
’
20
’
’
Figure 7 Plot of % conversion in liquefaction reactions versus the intensity of the aliphatic C-H stretching modes for a set of vitrinite concentrates. 0, Integrated intensity 2995-2750 cm-‘; 0, area of curve resolved 2853 cm-’ band 4
5
6 7 8 9 10
11 12 13
01 ” 0
2
”
4
1’
6
’
” ’ 1” 8 I”” 8 10 12 14 16 Intensity, OH bands
18
““I 22
Figure 8 Plot of % conversion in liquefaction reactions versus bands assigned to OH groups
250
FUEL, 1984, Vol 63, February
14
20
15 16
Kuehn, D., Snyder, R. W., Davis, A. and Painter, P. C. Fuel 1982, 61,682 Painter, P. C., Kuehn, D. W., Starsinic, M., Davis, A., Havens, J. R. and Koenig, J. L. Fuel 1983,62, 103 Senftle. J. T. Ph.D. thesis in Geoloav. ‘Relationshio between coal constitution, thermoplastic properties and liquefaction behavior of coals and vitrinite concentrates from the Lower Kittaning seam’, The Pennsylvania State University, November 1981 Senftle, J. T. and Davis, A. ‘A Data Base for the analysis of compositional characteristics of coal seams and mace&, Final Report-Part 1,1982, DOE contract No. DOE-30013-2 Neavel, R. C. ‘Coal Plasticity Mechanism: Inferences from Liquefaction Studies’, Proc. Coal Agglom. Conversion Symp. West Virginia Geol. Econ. Survey, Morgantown, 5-6 May 1975, published 1976 Wiser, W. Fuel 1968,47,475 Brown, H. R. and Water, P. L. Fuel 1966,45, 17 Painter, P. C., Snyder, R. W., Starsinic, M., Coleman, M. M., Kuehn, D. and Davis, A. Appl. Spectrosc. 1981,35(S), 475 Bellamy, L. J. ‘The infrared spectra of complex molecules’, Third Edition Chapman and Hall, London, 1975 Colthup, N. B., Daly, L. H. and Wiberley, S. E. ‘Introduction to Infrared and Raman Spectroscopy’, 2nd Edition, Academic Press, New York, 1975 Maddams, W. F. Appl. Spectrosc. 1980,34,245 Spiro, C. L. Fuel 1981,60, 1121 Painter, P. C., Snyder, R. W., Pearson, D. E. and Kwong, J. Fuel 1980,59,282 Painter, P. C., Coleman, M. M., Snyder, R. W., Mahajan, O., Kamatsu, M. and Walker, P. L. Appl. Spectrosc. 1981, 35, 106 Brown, J. K. Fuel 1959,38, 55 Friedel, R. A., Retcofsky, H. L. and Queiser, J. A. US Bur. Mines Bull. 1967,640
17
Larsen, J. W. and Sams, T. L. Fuel 1981,60,272