Energy & Fuels 1995,9, 359-363
369
A Comparison of Drift and KBr Pellet Methodologies for the Quantitative Analysis of Functional Groups in Coal by Infrared Spectroscopy Maria Sobkowiak and Paul Painter* Polymer Science Program, Penn State University, University Park, Pennsylvania 16802 Received June 22, 1994. Revised Manuscript Received November 14, 1994@
A set of pyridine soluble coal extracts, previously characterized by Fourier transform infrared spectroscopy using the KBr pellet technique, are now characterized using diffuse reflectance. The ratios of the aromatic to aliphatic CH band areas are plotted against the ratio’s of the aromatic to aliphatic CH contents determined by proton NMR, thus allowing the determination of the ratio of average absorption coefficients. This quantity varies with coal origin but also differs from that obtained using KBr pellets. This latter procedure produces superior correlations and is to be preferred for accurate quantitative work.
Introduction In a recent publication’ we have described the characterization of a large set of pyridine-soluble coal extracts by Fourier transform infrared spectroscopy (FTIR). The purpose of this work was t o obtain absorption coefficients for the aromatic and aliphatic CH bands that could then be used in structural studies of the parent coals. There are two major problems with this approach. First, the extracts may have a different distribution of functional groups relative to their parent coals and hence different absorption coefficients. However, the results we obtained for the extracts of bituminous coals were very similar to the absorption coefficients determined for whole coals of the same rank by Solomon and Carangelo2 using a very different methodology. This, together with the marked similarity of the spectra of the extracts and parent coals, indicates that we can have a good deal of confidence in the accuracy of our results and their applicability to bituminous coals in general. Unfortunately, lignites and subbituminous coals were a different matter: the spectra of the extracts displayed the sharp aliphatic bands characteristic of the presence of long-chain alkanes. As a result, the absorption coefficients differed considerably from those determined by Solomon and Carangelo for whole coals of this rank. Clearly, we must develop a different methodology to characterize these types of coals. Here, we address a different question, however, relating to the second major problem involved in applying FT-IR to the study of coal, sampling and sample preparation. Practically all quantitative infrared studies of coal have employed the KBr pellet methodology. There are several disadvantages to this procedure, the most important being that samples must be ground to a fine enough size that maximum absorption is reached and that only 1-3 mg of coal is used in each pellet. In our preceding study,l we found it necessary to average the results from the spectra obtained from at least three Abstract published in Advance A C S Abstracts, January 1, 1995. (1)Sobkowiak, M.; Painter, P. C. Fuel 1992,71, 1105. (2) Solomon, P. R.; Carangelo, R. M. Fuel 1988,67,949.
0887-0624/95/2509-0359$09.00/0
KBr pellets, and we usually prepared five pellets of each sample to ensure reproducibility. Clearly, a sample preparation method that is less demanding and involves a larger and hence more representative sample size would be useful. Diffuse reflectance Fourier transform spectroscopy (DRIFT) is potentially such a technique and appears to be a much simpler method. It was first applied to coal in the pioneering work of Fuller and Griffith~~ and - ~has subsequently been applied in a wide Although the intensities of various range of bands have been plotted as a function of rank, degree of oxidation, etc., no attempt has yet been made to relate intensities to functional group concentrations. The availability of a set of coal extracts, whose aromatic and aliphatic CH contents have been independently determined by proton NMR, now allows us t o do this and also allows us to compare the two methodologies, DRIFT and KBr pellets, thus forming the content of this paper. One final note; the DRIFT sampling methodology, employed here is the older “standard procedure” (see Experimental Section) currently used in most laboratories. There are some recent advances in instrumenta(3)Fuller, M. P.; Griffths, P. R. Anal. Chem. 1978,50,1906. (4)Fuller, M. P.; Griffiths, P. R. Am. Lab. 1978,10 (lo),69. (5) Fuller, M. P.; Griffiths, P. R. Appl. Spectrosc. 1980,34,533. (6) Fuller, M. P.; Hamadeh, I. M.; Griffths, P. R.; Lowenhaupt, D. E.Fuel 1982,61. (7) Smyrl, N. R.; Fuller, E. R. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Ed.; 1982; p 77. (8) Fuller, E. L.; Smyrl, N. R. Fuel 1985,64,1143. (9) Brimmer, P. J.; Griffiths, P. R. Anal. Chem. 1986,58,2179. (10)Brimmer, P. J.;Griffths, P. R. Appl. Spectrosc. 1987,41,791. (11)Fuller, M. P., personal communication as cited in ref 9, p 2. (12) Painter, P. C.; Bartges, B.; Plasczynski, D.; Plaszynski, T.; Lichtus, A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,31 (11, 65. (13) Painter, P. C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987,66,973. (14)Larsen, J . W.; Baskar, A.Energy Fuels 1987,1, 230. (15)Smyrl, N. R.; Fuller, E. L. Appl. Spectrosc. 1987,41,1023. (16) Wang, S. H.; Griffths, P. R. Fuel 1985,64,229. (17) Fuller, E. L.; Smyrl, N. R. Appl. Spectrosc. 1990,44,451. (18)Vidine, D. M. Appl. Spectrosc. 1990,44,451. (19) Smyrl, N. R.; Fuller, E. L.; Powel, G. L. Appl. Spectrosc. 1983, 37,38. (20) Painter, P. C.; Starsinic, M.; Coleman, M. Fourier Transform Infrared Spectrosc. 1985,4,169. (21) Mastral, A. M.; Rubio, B.; Membrano, L. Fuel 1989,68, 1584. (22) Cai, M. F.; Smart, R. B. Energy Fuels 1994,8, 369.
0 1995 American Chemical Society
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360 Energy & Fuels, Vol. 9, No. 2, 1995
tion and ampl ling^^-^^ that we are not equipped t o test (we are grateful to a reviewer for alerting us t o these advances).
Quantitative Analysis Using DRIFT The general theory for diffuse reflectance at scattering layers within powered samples was developed by Kubelka and M ~ n k . It~ gives ~ , ~an~equation that is similar in form to the Beer-Lambert law and (for an “infinitely thick layer) can be written as
3800.0
3000.0
2200.0
1400.0
600.0
WAVENUMBERS CM-1
f(RJ = (1 - RJ2/2R, = k I S
(1)
Figure 1. Comparison of the IR (KBr pellet) and DRIFT spectra a pyridine extract of coal PSOC 821.
where R, is the absolute reflectance of the layer, s a scattering coefficient, and k the molar absorption coefficient. Fuller and Griffiths3pointed out that in practice a perfect diffuse reflection standard has not been found, and R, is replaced by R‘,, where
R‘, = R’J sampleYR’J s tandard)
(2)
Here, R’,(sample) is the measured reflectance of the sample, whereas R’dstandard) is the measured reflectance of a selected nonabsorbing standard. We used KBr as a reference material. The Kubelka-Munk theory predicts a linear relationship between the molar absorption coefficient k and the peak value f(Rm)for each band, provided that s remains constant. The parameter k is related t o the molar absorptivity a and the molar concentration c by
k =2 . 3 0 3 ~ ~
(3)
for dilute samples in low-absorbing or nonabsorbing matrices. Hence,
f(R,)
(1- RJ2/2R, = c 1k’
(4)
where k‘ = sl2.303a. If the diffuse reflectance spectra are converted to the Kubelka-Munk function Ah!,), they should therefore appear similar to absorbance spectra and within limits, be used for quantitative analysis. As Fuller and Griffths3 pointed out, however, for intense absorption bands large deviations from linearity are observed, even at low concentrations, and the spectra of neat materials may be considerably different from the spectra of the same material diluted in an alkali halide matrix. Furthermore, particle size is a particularly important parameter (just as in the preparation of KBr pellets) and bandwidths and intensities can change dramatically as this parameter is r e d u ~ e d .If~this were not enough, there is also the problem that the specular component of the reflected radiation (which does not penetrate the sample) is measured along with the diffused reflected light (which does penetrate the sample). Generally, the change in specular reflectance with frequency for bands of low absorptivity is small, but for bands of strong and intermediate absorption, where the anomalous dispersion leads t o “Reststrahlen” bands in the specular reflection spectrum, the effect is much (23) Christy, A. A,; Tvedt, J. E.; Karstang, T. V.; Velapoldi, R. A. Rev. Sci. Instrum. 1988,59,423. ( 2 4 ) TeVrucht, M. L. E.; Griffiths, P. R. Appl. Spectrosc. 1989,43, 1492. ( 2 5 ) Iwanski, P.; Griffiths, P. R. Energy Fuels 1990,4 , 589. (26) Kubelka, P.; Munk, F. Z . Tech. Phys. 1931,12, 593. (27) Kubelka, P.; Munk, F. J . Opt. SOC.Am. 1948,38, 448.
bigger. When the Reststrahlen bands are observed the absorption bands can appear inverted at their center and their frequencies can be shifted. This effect makes quantitative measurement on samples with strong absorptivities very difficult. To perform quantitative work we must therefore “dilute” the sample with a nonabsorbing material. As we mentioned in the Introduction to this paper, DRIFT appears at first sight to provide a simpler method of sample preparation than KBr pellets, but as the above discussion indicates, there are problems when attempting quantitative measurements. We will now describe the methodology we employed in an attempt to overcome some of these difficulties and then proceed to examine the results.
Experimental Section The characteristics of the coals and extracts used in this study (origin, elemental analysis, etc.) were tabulated in our previous paper’ and will not be reproduced here. Spectra were obtained with a Barnes DRIFT accessory placed in a Digilab FTS 60 spectrometer using a minimum of 400 “scans” (interferograms) at 2 cm-’ resolution. To ensure and check reproducibility at least five samples of each extract were prepared for infrared analysis. Each sample consisted of 15 mg of coal, which was initially ground with 20 mg of KBr for 1 min in a Wig-L-Bug. A further 280 mg of KBr was then added and the sample ground for an additional minute. Pure ground KBr was used to obtain a reference spectrum. Samples were loaded into the DRIFT cup and the surface leveled with a glass slide.
Results and Discussion It is useful to start by comparing the infrared spectrum of a coal pyridine extract (PSOC 821, see ref 1for characteristics), obtained in absorbance using a KBr pellet, to that obtained using DRIFT. The spectra are shown Figure 1. This figure immediately demonstrates the appeal of using diffuse reflectance. The baseline is flatter (the sloping baseline in the spectrum of the KBr pellet is predominantly due to scattering) and some of the bands appear somewhat more intense and better resolved from their neighbors (e.g. the aromatic out-ofplane bending modes between 900 and 700 cm-l). In the spectra of the parent coals some of these effects are much more pronounced and Figure 2 compares the spectra of two coals (PSOC 821 and PSOC 11951,where it can be seen that the bands due t o mineral matter are particularly enhanced in intensity when using DRIFT, for reasons we mentioned above. Clearly, even grinding the coal particles to a fine size and diluting them in KBr
Quantitative Analysis of Functional Groups in Coal KBrPELLET
Energy & Fuels, Vol. 9, No. 2, 1995 361
A
PSOC821
DRIFT
3800 KBrPELLET
PSOC 833
i“l 3000
2200
1400
600
I
3 0’00
2200
1 4’0 0
600
30’00
2200
14’0 0
600
i\
PSOC 815 86.6 %C
DRIFT
3800
3 8’0 0
3000
2200
1400
600
38’00
WAVENUMBERS CM-1
Figure 2. Comparison of the IR (KBr pellet) and DRIFT spectra of two US.coals (top, PSOC 821, 84.2 wt % C and 6 wt % MM; bottom, PSOC1195, 87% C and 12.1 wt % MM). k
does not remove or limit some of the problems associated with the change in specular reflectance with frequency. However, if we confine our attention to the aromatic and aliphatic CH stretching modes, which lie close in frequency (3100-3000 and 3000-2800 cm-l, respectively), we would anticipate that these problems should be minimized, particularly if we ratio the intensities of these bands and thus at least partially “cancel” such effects. We will assess the extent to which this can be achieved below. The intensities of the aromatic and aliphatic CH stretching modes vary systematically with coal rank and t o illustrate the trends the infrared spectra of three coals and their pyridine-soluble extracts are shown in Figure 3. This figure also illustrates the problem with low-rank coals, where the extract from PSOC 833 (70% C content) has the intense sharp bands characteristic of relatively long chain alkanes. The absorption coefficients of such groups is much larger than shorter chain (methylene and ethylene) linkages that are most likely found between aromatic units20 and we would not anticipate that absorption coefficients determined from such extracts would apply t o their parent coals. Accordingly, we will focus our attention on the bituminous coals, which are a different matter, as the spectra of the extracts correspond closely to the spectra of the parent coals (the intensities of the CH stretching modes are somewhat more intense, reflecting the higher hydrogen contents of the extracts, but the band shapes and relative intensities are very similar.) Having obtained the spectra of our samples, the next step is t o separate the overlapping bands of the various vibrational modes by curve-resolving. This is a potential source of considerable error and we described and discussed our methodology (and various alternatives) in ref 1. Here we will simply summarize the basic steps, which are illustrated in Figure 4. The first step is to remove the contribution from the very broad bands due to various types of hydrogen-
PSOC 1198 91.1 K C
3800
-A 3000
2200 1400 WAVENUMBERS CM-1
660
Figure 3. Comparison of the DRIFT spectra of the three US. coals and their pyridine-soluble extracts.
bonded and free OH stretching modes. Our procedure uses four bands to fit the OH profile. These bands may not be (in fact, they probably are not) an accurate reflection of the OH groups present in the sample, but that is not important. All that is necessary is that the bands we use, when added together, fit the overall profile of the OH modes, as this contribution is then subtracted from the parent spectrum, as illustrated in Figure 4. Because of the enormous difference in the bandwidths of the OH and CH stretching vibrations this can be accomplished accurately, leaving the bands due to aromatic and aliphatic CH groups sitting on a flat baseline. The step that is much more prone to error is the next one, where the partially overlapping aromatic and aliphatic CH modes are curve-resolved. As mentioned above, we compared various methodologies in our previous work1 and the method we use provides a fit t o the profile to a set of carefully chosen bands (the choice being based on studies of the second derivatives of the spectra). The band shapes are assumed to be a sum of Lorentzian and Gaussian contributions, the relative contributions being determined in the least-squares fit. The band shapes are usually determined to be largely Gaussian (some of the sharp bands in the low-rank coal extracts can be more Lorentzian in their shapes). Figure 4 also shows the results of summing separately the aromatic and aliphatic CH modes to give a profile characteristic of each type of group. It can be seen that the overall profile is accurately reproduced by this methodology, but more crucially this procedure also illustrates where errors are most likely to arise, namely
362 Energy & Fuels, Vol. 9, No. 2, 1995
Sobkowiak and Painter Table 1. Results of Curve-&solving the Aromatic and Aliphatic Stretching Modes of Pyridine Extract of U.S. Coal PSOC739 sample 1 2 3 4
5 6 av SD
b
ar
KBr pellet a1 ar/al
DRIFT ar
a1
aria1
2.10 2.65 2.33 2.54 2.70
20.95 22.64 23.25 24.27 22.24
0.10 0.12 0.10 0.11 0.12
0.40 0.74 0.34 1.32 0.57 0.98
3.57 5.23 2.67 9.21 4.27 7.04
0.11 0.14 0.13 0.14 0.13 0.14
2.46 *0.21
22.67 k1.20
0.11 +0.01
0.67 *0.35
4.99 &0.35
0.13 *0.01
Table 2. Ratio of A,JAA,lObtained from Curve-Resolving of Spectra of Different Concentration Pyridine Extract of U.S. Coal PSOC 708=
a - b 3800
3400
2200
2600
3000
A 3300
3100
2900
a
2700
WAVENUMBERS CM-1
Figure 4. Results of curve-resolvingthe OH and CH stretching region of the DRIFT spectrum of a pyridine extract of U.S. coal PSOC 739. in the relative overlap of the two sets of bands. Changing the band shape from largely Gaussian to largely Lorentzian can lead to significant differences in the overall areas of the aromatic and aliphatic modes, particularly the former, as being much weaker than the aliphatic modes their calculated area is much more affected by the degree of mutual overlap of the tails of the bands.l However, because of the heterogeneity of coal structure we are essentially observing for each group (e.g., aromatic CH) the average of the contributions from various chemical structures in various local environments. In the limit we would therefore expect the band shape to approach a Gaussian profile, as our fitting procedure determines, so we believe that we do not introduce large errors as a result of this curveresolving methodology. We prepared and curve-resolved the spectra of five samples of each extract and the overall areas of the aromatic and aliphatic stretching modes of a typical sample are presented in Table 1. Also included for comparative purposes are the results obtained from the absorption spectra of Kl3r pellets of the same sample. The areas were normalized to 1mg of sample. As might be expected, the areas of the aromatic and aliphatic CH stretching modes of samples of the same extract showed large differences because of variations in packing, etc., associated with the usual DRIFT procedure. Because these modes are separated by only 100 cm-l, however, we postulated that such effects would cancel if we
mg of sample in 300 mg KEh-
ar
a1
ar/al
5 10 15 30 50 100 150 300
0.20 0.31 0.48 0.51 0.65 0.97 0.45 0.64
1.94 2.35 3.18 4.67 6.08 2.47 3.07 4.05
0.10 0.13 0.15 0.14 0.16 0.15 0.14 0.16
Areas normalized to 1 mg of coal.
ratioed the areas of the bands. The data presented in Table 1 suggests that this is largely true and as a further check we prepared eight samples of the one coal, this time varying the weight of the coal relative to the KBr matrix. These results are shown in Table 2 and it can be seen that as long as we use more than 10 mg of coal in our coaVKBr mixture we obtain values of the band ratios that are consistent, within a range of about 3 3 % . However, if we were completely eliminating the various factors that affect scattering, the ratios of the areas determined by DRIFT would correspond to those determined from KBr pellets (the contribution of the scattering coefficient s in eqs 1-4 would be eliminated). They differ (0.13 for DRIFT vs 0.11 for KEir pellets), presumably because of the variation of scattering with frequency which seems to come into play even for bands separated by only 100 cm-l or so. Nevertheless, the reasonable degree of reproducibility of the values of band ratio areas would suggest that we could still obtain parameters that could be used for quantitative analysis and we now turn our attention to this part of the problem. In our previous work1 we obtained the ratio of “absorption coefficients” or “conversion factors” by plotting the ratio of the areas of the aromatic and aliphatic CH stretching modes against the ratio of aromatic to aliphatic hydrogen (Hafial) determined by proton NMR. Of course, what we are calculating is an average of such parameters for the various bands that contribute separately t o the aliphatic and aromatic CH stretching regions. What we determine is actually a correlation parameter, no more, but if the relative distribution of functional groups stays approximately the same throughout a sample set then we should obtain linear plots and an average “absorption coefficient” that can be used in structural studies. If this assumption is incorrect, then the plots should be nonlinear. A plot of the ratio of DRIFT band intensities against Har/Halis shown in Figure 5 (top). There is an obvious
Quantitative Analysis of Functional Groups in Coal
Energy & Fuels, Vol. 9, No. 2, 1995 363 0.20 r E@' RM
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correlation but also a disappointingly large degree of scatter in the data. In contrast, the equivalent plots obtained using KI3r pellets, shown in Figure 6, are much more tightly clustered. In our previous work we also separated the data according to coal origin (the samples we studied are from the Eastern and Interior region of the US, the Rocky Mountain region, and there is also a set of Polish coals). This separation was based on the grounds of chemical intuition, rather than a statistical analysis of the data. Nevertheless, for the Kl3r pellets there is now a very clear linear correlation between the ratio of the aromatic to aliphatic CH stretching modes and Ha,./Hal. The difference between Polish and Rocky Mountain coals is probably not statistically meaningful, but these samples clearly differ from Eastern and Interior US coals, demonstrating that average "absorption coefficients" for bituminous coals vary with coal origin. If the spectra obtained by the DRIFT methodology employed here are separated according to origin in the same manner, the individual plots again show an improved linear correlation relative to the data as a whole, but this correlation is again not as good as that obtained using KJ3r pellets. Furthermore, the slope of the lines, which are related to the ratios of average absorption coefficients, are different t o those obtained using KBr pellets. If factors such as the scattering coefficient were identical for the two sets of bands, then we should have obtained the same results (see eqs 1-4). Presumably, this variation of scattering coefficient between the aliphatic and aromatic modes is one source of the scatter in the data, in that different degree's of packing of samples in the DRIFT cup, variations in
RATIO Har/Hal,NMR
Figure 6. Ratio of the IR (KBr pellet) band areas of the aromatic and aliphatic stretching modes vs ratio Hama] from proton NMR for pyridine extracts of bituminous coals.
particle size, etc., will result in varying intensities from sample to sample. We had hoped that ratioing the areas of the aliphatic and aromatic bands would eliminate problems associated with scattering variations and to a large degree it does, but not t o the extent that the correlations are as good as those obtained from KBr pellets. In this regard, Iwanski and G r i f f i t h ~have ~ ~ shown the advantage of using DRIFT cells with an off-axis optical configuration. Also, a sample packing accessory devised by TeVrucht and G r i f f i t h ~has ~ ~been shown to give highly reproducible quantitative data. These newer techniques must be employed if more accurate results are t o be obtained.
Conclusions
A linear correlation between the ratio of the areas of the aromatic to aliphatic CH stretching modes and the ratio of aromatic to aliphatic CH content (as determined by proton nmr) has been obtained for a set of coal extracts characterized by conventional DRIFT sampling methods. The correlations obtained are not as good as those based on studies of Kl3r pellets. Some of the more recent DRIFT methodologies and instrumentation should give superior results and will be tested in future work. Acknowledgment. The authors gratefully acknowledge the support of the Office of Chemical Sciences, U.S. Department of Energy, under grant No. DE-FGOB86ER13537. EF940119R
