Energy & Fuels 1992,6,813-820
813
A Structural Model for Lignin-Derived Vitrinite from High-Volatile Bituminous Coal (Coalified Wood) Patrick G. Hatcher,**tJJean-Loup Faulon,? Kurt A. Wenzel,? and George D. Cody$ Fuel Science Program, Department of Materials Science and Engineering, and Energy and Fuels Research Center, Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802 Received July 6,1992. Revised Manuscript Received September 18, 1992
Using data from elemental analyses, solid-state 13C NMR, and pyrolysis/gas chromatography/ mass spectrometry, a structural model for coalified wood of high-volatile bituminous coal rank was constructed. Considering the fact that the vitrinite was derived from lignin, we utilized a model of lignin as a template upon which the coal structure is based. The chemical structural information derived from comparisons among coalified gymnospermous wood samples ranging in rank from subbituminous to high-volatile C bituminous coal provided the basis for constructing the model. Modifications of a previously published structural model for subbituminous coal, originally derived from the lignin template, involved condensation of phenols to diary1 ethers and cyclization of alkyl side chains to form naphthalenic structures. A computer software program was used to construct the structural model in three dimensions whereby an energy minimization program was subsequently applied to represent the structure in it lowest energy conformation.
Introduction In recent years, several models have been proposed for the chemical structure of coal.'+ Some have been visualized in three dimensionsby the use of computer graphic^.^ The models have been constructed by considering elemental, spectroscopic, liquefaction, and pyrolysislgas chromatography/mass spectrometry data. While the models have provided a visual framework for evaluating the kinds of structural elementsthat are contained in coal macromolecules, they fail to depict the chemical heterogeneitythat exists in coal due to the many varied macer&. Developing structuralmodels for individual macer& such as vitrinite would limit some of the heterogeneity, but vitrinite, a petrographically defined component, can also have a heterogeneous composition. There are numerous petrographic forms of vitrinitees The approach toward defining a more homogeneous maceral component of coal used in our laboratory is one which has focused on coalified wood as a representative for vitrinite derived from xylem in ancient trees.'-1° Structural models were developed from a lignin template, because lignin has been determined to be the major source * Author to whom correspondence should be addreseed. t Fuel Science Program, Department of
Materials Science.
t Energy and Fuels Research Center, Department of Geosciences. (1) Given, P. H. Fuel, 1960,39, 147. ( 2 )Wiser, W. H. h o c . Electric Power Res. Zmt. Conf. Coal Catal.
1971.
(3) Solomon, P. R. New Approaches in Coal Chemistry; ACS Symp. Seriea No. 16%AmericanChemical Society: Washington,DC, 1981;p 61. (4) Shinn, J. H. Fuel 1981,63,1187. (5) Carlson, G.A.; Granoff, B. Coal Sci. I1 1990,461, 159. (6)Stach, E.; Taylor, G. H.; Mackowski, M.-Th.; Chandra, D.; Teichmuer, M.; TeichmWer, R. Stach's Textbook of Coal Petrology, Gebdder BomMger: Berlin, 1975. (7) Hatcher, P. G.;Lerch,H. E.;111, Kotra, R. K.; Verheyen, T. V. Fuel 1988,67, 1069. (8) Hatcher, P. G. Energy Fuels 1988,2, 48. (9) Hatcher, P. G.;Lerch, H. E., III; Verheyen, T. V. Znt. J. Coal Ceol. 1989,13,65. (10) Hatcher, P. G. Org. Ceochem. 1990,16,959. (11) Hanna, J. V.; V d o , A. M.; Wilson, M. A. Energy Fuels 1992, 6, 28.
of chemical structures in coalified wood. By examining the chemistry of a series of woods from peat to coalified woods from ancient rocks and coal seams, we have been able to discern changes in the lignin framework induced by coalification to the rank of subbituminous coal. The models were then developed by applying the observed changes to the lignin template. The model for lignin was that proposed by Adler.12 Detailed examination of coalified wood samples of higher rank, high-volatile bituminous coal, has allowed us to extend the model to this rank range. This paper presents the data for high-volatile bituminous coalified wood samples, comparing them to similar data for subbituminous coal to develop an understanding of the reactions that have transformed subbituminous coal to the next higher rank level, and develops a model for vitrinite from coalified wood of high-volatile C bituminous coal rank. The model is constructed from elemental, solid-state 13C NMR, and flash pyrolysis/gas chromatography/mass spectrometric data. Although the samples are possibly different from vitrain that would be isolated from most coal seams in that they represent pure coalified wood buried in clastic sediments, the chemical data presented in this paper demonstrate that they share common characteristics to vitrinite from many coal samples examined by other investigators. The one primary feature of the coalifiedwoods that is important for structural model development is the common origin for the gymnospermous wood samples. This feature allows us to develop a model from one specifickind of lignin that is primarily composed of guaiacyl structural units. Gymnosperms and their predecessors or relatives are believed to be the major contributors to vitrinite in many North American coals. Methodology Several samples of coalified gymnospermous wood having ranks ranging from subbituminousto high-volatile ~~
(12) Adler, E. Wood Sci. Technol. 1977, 11, 69.
0887-0624192/2606-0813$03.00/0 0 1992 American Chemical Society
Hatcher et al.
814 Energy &Fuels, Vol. 6, No. 6,1992 Table I. Elemental and Solid-state W NMR Data for Coalified Wood Samples parameter Midland stem Taylorsville log Ferron log vitrinite reflectance 0.6 0.44 0.4 85.8 82.8 11.5 % carbod 6.5 6.38 5.28 % hydrogena % oxygen' 5.9 8.7 13.9 2.2 1.0 % nitrogen 2.2 0.64 (0.61)* 0.69 0.65 carbon aromaticity BDLC 0.003 carbonyVtotal carbon BDL BDL 0.009 carboxyVtotal carbon BDL 0.12 0.17 arYl-O/arYl 0.11 (0.13) NAd 0.33 methyl/total aliphatic 0.33 (0.28) 0.44 0.47 0.47 ~ 1W a- r ~ l 0.31 0.35 0.37 H,
*
a Moisture and ash free. Values in parenthesesare for Blochdecay data. BDL = below detection levels. d NA = not available.
I
I
FERRON LOG CPYAS
I
FERRON LOG BLOCH DECAY
1
TAYLORWLLE LOG
cations were based on mass spectral signatures which were compared with mass spectral information from the literature, the NBS/Wiley library of mass spectra, and mass spectra of model compounds where possible. The solid-state 13CNMR data were obtained by both the method of cross polarization with magic angle spining (CPMAS) and by a Bloch decay experiment. The spectrometer is a Chemagnetics Inc. CMC-100S-200Linstrument, operated at a field strength of 2.35 T. The frequency sweep width is 10 kHz, the pulse delay was 1.00 8, and the contact time during cross polarization was 1 ma. The CPMAS and dipolar dephasing conditions were similar to those described previously.8 The cycle time for the Bloch decay was 45 8. In this experiment, the sample is spun at the magic angle and the high-powerdecoupling commonly used in the CPMAS experiment is applied during data acquisition. A simple 90' pulse with data acquisition is used to acquire the data. The Fourier-transformed NMR spectra from both CPMAS and Bloch decay experiments were transferred to a PC computer by use of Unplotit software from Silk Scientific, and the peaks were deconvoluted by Lab Calc software available from Galactica Industries, Inc. Deconvolution of peaks was performed by assuming Gaussian line shapes. Lorentzian and mixed Lorentzian-Gaussian line shapes were also tested but proved to be less satisfactory than pure Gaussian.
Rssults
MIDLAND STEM BLOCH DECAY
200
150
100
50
0
1
-50 ppm
Figure 1. Solid-state 13C NMR data for the coalified woods obtained by the CPMAS and Bloch decay methods. bituminous coal have been analyzed in previously published studies.9 Three samples were selected for detailed structural characterization. These samples represent coalified gymnoepermouswood and appear to demonstrate some subtle changes in elemental composition that are particularly characteristic of changes expected as coalification increases, e.g., an increase in carbon content and a decrease in oxygen content (Table I). The sample with the lowest rank is from the Ferron Sandstone Member of the Mancos Shale in Willow Springs Wash, UT. The second sample, with a slightly higher rank, is from the Falling Creek Member of the Doswell Formation (Upper Triaesic)collectednear Taylorsville,VA. The samplewith the highest rank is a stem embedded in a lacustrine shale from the Midland Formation (Triassic) near Culpepper, VA. Elemental and 13CNMFt data for these samples have been previously publisheds but are reevaluated here for purposes of developing a structural model. Flash pyrolyeia/gaschromatography/massspectrometry was employed in a manner analogous to that described previ~usly.~!~ Pyrolysis products were quantified by integrating the peaks from the total ion chromatogram (TIC),assuming equivalentresponse factors for individual components, and normalizing the concentrations to the total peak area for all peaks in the pyrogram. Identifi-
The elemental and NMR data for the coalified wood samples are shown in Table I. The carbon contents range from 77.5 to 85.8% and vitrinite reflectance values from 0.4 (Stanton, R. W., personal communication) to 0.6 (Bostick, N., personal communication), indicating that the ranks of these samples range from subbituminous to high-volatile C bituminous coal. The oxygen content of 5.9 % for the Midland stem, measured by direct analysis? is significantly lower than the oxygen content (13.9%) of the coalifiedlogfrom the Ferron Sandstone.8 A significant amount of nitrogen, 2.2 % ,is observed in the two samples of Triassic age. Comparison of the NMR data obtained by CPMAS and by Bloch decay indicates that the two methods yield virtually identical spectra. Figure 1shows a comparison of the spectra obtained by CPMAS and Bloch decay for two of the samples. The Bloch decay does show slightly lower aromaticity and possibly a higher yield of phenolic carbon (Table I). In both types of spectra the broad peaks for aromatic (100-160 ppm) and aliphatic (0-60 ppm) carbons dominate. Discerniblepeaks/shoulders at 140and 153 ppm are observed in the aromatic carbon region, and these can be assigned to carbon-substituted aromatic carbons and aryl-0 carbons, respectively. Carbon aromaticities are very similar for all samples, ranging from 0.65 for the subbituminous Ferron log to 0.69 for the highvolatile C bituminous Taylorsville log. The Midland stem appears to be less aromatic than the Taylorsvillelog even though vitrinite reflectance values (Table I) indicate a higher rank. This could be due to the fact that aliphatic materials of the nature described below have been incorporated in the sample during burial. It is of particular significance that we determine the contribution of aryl-0 carbonsin order to assess the nature of coalification reactions affectingthe aromatic rings. The ratio of aryl-0 carbons to total aromatic carbons provides an indication of the average degree of oxygen substitution
Structural Model for Lignin-Derived Vitrinite on aromatic rings. With a ratio of aryl-0 to total aromatic carbon of about 0.17 for the subbituminous Ferron log, it appears that nearly all aromatic rings have at least one phenolic OH or phenolic ether carbon. The high-volatile C bituminous Midland stem contains about half the amount of oxygen, 5.996, as the Ferron log (0 = 13.9%), but the aryl-O/total aromatic ratio is only slightly less, about 0.13. The Taylorsville log is intermediate in composition with an oxygen content of 8.7 % ,but it has an aryl-O/aryl value, 0.12, that is similar to that of the Midland stem and, again, only slightly lower than that of the Ferron log. This large diminution of oxygen followed by only a small change in aryl-O/aryl is indicative of dehydration reactions of phenols to form diaryl ethers. Under such conditions, a large change in oxygen would be observed but the ratio of a r y l 4 to aryl carbons would remain unchanged, because we would still have one oxygen per aromatic ring. Formation of diaryl ethers from phenols can also be expected to change the chemical shift patterns of the aromatic carbons. For example, phenolic aryl-0 carbons normally resonate at 155 ppm, and in the corresponding phenolic ether, the aryl-0 carbons will resonate at higher field or at chemical shift values of less than 155 ppm. Dipolar dephasing data for these samples provide an indication of the relative proportion of nonprotonatedand methyl carbons to total carbons. If dipolar dephasing of only the aromatic region is considered, we can calculate the average number of protonated or nonprotonated aromatic carbons. Normalizing the average number of protonated carbons to the total aromatic carbons, arylH/aryl, provides a measure of the average degree of protonation for aromatic rings. It is clear that the average degree of protonation for the three samples is nearly the same, with aryl-H/aryl values ranging from 0.44to 0.47. Using the dipolar dephasing data (aryl-H/aryl), elemental data (molar values of C and H), carbon aromaticities (f3, and eq 1, we can calculate the fraction of in each sample. aromatichydrogen per total hydrogen (Ha) Hanna et al.ll have shown that calculating Hafrom dipolar dephasing data provides information that is as accurate as that measured directly by the CRAMPS lH NMR method. The Ha ranging from 0.31 to 0.37 are consistent with similar data for most coal samples of this rank." The Havalues also indicate that approximately one-thud of the hydrogen in coalified logs of these low ranks are attached to aromatic carbons.
Ha= (C/H)f,(aryl-H/aryl)
(1) The aliphatic carbon region also shows fine structure with a distinct shoulder/peak at 17 ppm which can be assigned to methyl carbons. Dipolar dephasing studies confiim that this peak is that of methyl carbons.8 Deconvolution of the aliphatic region shows that approximately one-third of the aliphatic carbons are methyl carbons in the Midland stem. It was more difficult to estimate the contribution of methyl carbons by deconvolution of the aliphatic region for the other two samples. In the case of the Ferron log, dipolar dephasing data was used for this calculation. The methyl carbons have a greater molecular mobility than other aliphatic carbons and, consequently, do not dephase as rapidly as other aliphatic carbons. By extrapolation of the dephasing behavior of the methylgroups to time zero, we can calculate their relative contribution to aliphatic signal intensity.
Energy & Fuele, Vol. 6, No. 6,1992 816
250
IS0
W
-W
Figure 2. Solid-state N M R data for two vitrinite-richcoals, the Pittsburgh No. 8 coal (Penn State coal samples DECS-13) and the Illinois No.6 coal (DECS-3). Such a calculation indicates that approximatelyone-third of the aliphatic carbons are methyl carbons. Due to insufficient spinning speeds of the sample rotor, spinning side bands are observed at 260 and 0 ppm. It is important to mention at this point that the NMR spectrumfor the high-volatileC bituminousMidland stem is virtually identical to the NMR spectra of vitrinite and vitrains from coal samplesof approximately the samerank. Figure 2 shows some solid-state 13C NMR spectra for samplesfrom the Pittsburgh No. 8 and Illinois No. 6 coals. Thus, the NMR data show a great deal of similarity between the coalifiedwood studied here and from vitriniterich coals, indicating that the coalified wood is representative of most vitrinite which is derived from wood. Flash pyrolysis data for the coalified wood samples are shown in Figures 3-5, and the peaks are identified and quantified in Table 11. Phenol and alkylphenols are the most readily visible pyrolysis products in the threesamples. Phenol, the three cresol isomers, 4-ethylphenol, and 2,4dimethylphenol predominate. The major isomers of CZ phenols are the 2,4-dimethylphenol and Cethylphenol as observed for other low-rank coalified wood samples.80ther isomers of CZ phenols are apparently minor or trace components. Only four isomem of CSphenols predominate, trimethylphenol, two isomers of ethylmethylphenols, and propylphenolare present. The specific substitution sites have yet to be determined for the C3 phenols. As a whole, the phenols account for approximately60% of the aromatic pyrolysis products (obtained by normalizing to total pyrolysis products with the alkanes and alkenes removed) and 40 96 of the total pyrolyzates in the sample of high-volatileC bituminousMidland stem (Table 11). Benzene and alkylbenzenes are the second most prominent aromatic components, accounting for about 17% of the aromatic pyrolyzates. CI, CZ,and CSbenzenes with undetermined substitution patterns comprise prominent Components eluting in the 0-10-min retention time window (Figure 5). Other pyrolysis producta which account for numerous other peaks in the pyrogram for the high-volatile C bituminous Midland stem (Figure 5) are naphthalenes, alkyldibenzofurans,alkyldibenzopyrans,and n-alkanelnalkene pairs. C1,C2, and C3 alltylnaphthalenes are present in significant amounb (17%) as various, as yet undeter-
Hatcher et al.
816 Energy & Fuels, Vol. 6, No.6, 1992
lool
Ferron log
901
P2
70 6050
-
40-
301I
P
1
20
5
10
P2
15
P3
25
20
50-
4030-
N5 0
30
35
40
45
50
RITFN'l'lON 'Tlh4E I MIN ) Figure 3. Flash pyrolysis GC/MS data showing the total ion chromatogram (TIC) for the coalified wood sample Ferron log. Pyrolysis was conductadat 610 O C and the J&W DB17 column (30 m X 0.25 mm i.dJ was temperature programmed from 40 to 280 O C at 4 OC/min. Other than the n-alkanes which are identified by numbers reflecting their carbon number, peak identifications are listed according to the code used in Table I1 and were made by use of mass spectrometric information.
mined isomers. The n-alkaneln-alkene pairs show a range of carbon numbers ranging from CSto C22. The lower molecular weight homologs predominate and the distribution tapers off with increasing carbon number. Quantitatively, the n-alkanesln-alkenes contribute to 33 % of the total pyrolyzate, a rather large percentage as a whole. At higher retention times in the pyrogram, peaks for alkyldibenzofurans and alkyldibenzopyrans are found. These contribute to only 3.5 % of the pyrolyzate and 5.2 9% of the aromatic products. The pyrogram for the subbituminous Ferron log is similarto that of the other two samples in that the phenols and alkylphenols predominate (Figure 3); however, this sample contains only trace levels of naphthalenes, dibenzofurans, dibenzopyrans, and n-alkaneln-alkene pairs. Also, the benzene and alkylbenzenes are minor in comparison to the phenols and alkylphenols, unlike the pyrogram for the Midland stem where the alkylbenzenes are as intense as the phenols. The pyrolysis of the Taylorsville log (Figure 4) yields a similar suite of products as the Midland stem but relative proportions are different (Table 111, due mostly to the effecta of lower rank. The n-alkanesln-alkenesare present in only trace amounts, probably due to the fact that the aliphatic materiale contributing to the Midland stem were not present in this sample of equivalent age. The phenols and alkylphenols predominate the aromatic pyrolysis products and alkylbenzenes are significant contributors,
but not as significant as they are in the pyrolysis products of the Midland stem. At higher retention times in the pyrogram, peaks identified as alkyldibenzofurans and alkyldibenzopyranscontribute significantly,more so than observed for the other two samples.
Discussion The quantitative information or carbon types afforded by the NMR data and the molecular-level information supplied .by the flash pyrolysis data provide sufficient detail to allow construction of a molecular model from a lignin template. It is clear that the original lignin structures composed of glyceryl methoxyphenolic structures have been modified by coalification, because the coalified wood samples examined here do not show any characteristicsof the lignin-derivedmethoxyphenol structures. In previous reporta,BJOit was suggested that lignin undergoes a series of coalification reactions that include (1)(3-0-4 aryl ether cleavage, (2) demethylation to form catechol-likestructures, (3)dehydroxylation of the threecarbon side chain, and (4) dehydroxylation of catechols to form phenols. A structural model was developed for ranks of brown coal, lignite, and subbituminous coal, using the lignin template published by Adler12and modifying the aromatic structuresaccordingto the coalificationreactions observed for each rank level. It is a logical progression to take the model developed in this prior study for subbituminous coal and to alter it
Energy & Fuels, Vol. 6, No.6, 1992 817
Structural Model for Lignin-Derived Vitrinite 100 90
Taylorsville log
80 PI 70
B2
0
60 50 40
P3 P2
5 7
30
20
10 0 ,
~ i ~ q ~ " r ~ ~ ~ ~ l ~ " l " P 1 " r m l " " l ' m y l m p , l r m
columns used 'at different times.
in a way which would reflect the changes in chemistry observed between the high-volatile C bituminous coalified woods in the present study and the subbituminous log. The major changes between the coalified woods in going towards higher rank include (1) a decrease in oxygen content from 13.9 to 5.9 % with a corresponding increase in carbon content, (2) a small decrease in the ratio of aryl-0 to total aromatic carbons, (3) a significant increase in benzene and alkylbenzenes in pyrolyzates, and (4) the emergence of a series of n-alkaneln-alkene pairs upon pyrolysis. Interestingly, the carbon aromaticity and the fraction of protonated aromatic carbons to total aromatic carbons do not change greatly. The most significant change observed between subbituminousand high-volatile C bituminous logs is the decrease in oxygen contents. These were measured directly: consequently,this diminution is real and not a function of analytical error. The structural model depicted for subbituminouscoallo indicatea that the coal is composed of the original remnants of lignin backbone structures (Figure 6). Each aromatic ring is a phenol with an average degree of substitution which is commensurate with the dipolar dephasing data showing an average of 3.3 substituents per ring. Because no significant intensity was observed in the region of alkyl-0 carbons in the NMR spectrum, virtually all the oxygen in the subbituminous Ferron log is present as phenol or diphenyl ether carbon. The small amount of intensity in the NMR spectrum in the region of carboxyl
and carbonyl carbons accounts for only a small proportion of the oxygen, 2.1 % . Assuming that the remainder of the oxygen, 11.8%, is associated with phenolic structures having a three-carbon side chain, or nine-carbon unite, we can estimate that approximatelyone of nine carbons is an aryl-0 carbon. This would imply that most of the aryl-0 carbons are monohydric phenols in the subbituminous log. It is important to mention that the structural model for subbituminous coalified wood shown in Figure 6 is a modified version of that presented in a previous paperelo The previous model was originally constructed without specific consideration for the high three-dimensional strains that would be introduced by the high degree of cross-linkingbetween aromatic rings and the three-carbon side chains contained within the depicted structure. Such high strains were deduced from application of the energy minimization program discussed below. The modified model maintains the high cross-linkdensity of the previous model but cross-linking is accomplished by interactions with other macromolecules adjacent to the one depicted. These cross-links are identified by a bond drawn to a circled letter in Figure 6. In the high-volatile C bituminous logs, the oxygen content decreases to 5.9 % . The lack of intensity in the carboxyl region of the NMR spectrumfor both the Midland stem and Taylorsville log attests to the fact that oxygen is not present in these functional groups. Thus, aryl-0
818 Energy
I%
Hatchet et al.
Fuels, Vol. 6, No. 6,1992 P2
'"1
904
eo -1!
Midland stem
PI mP
~
70:
I
60: 50
-Ii
PI
BI
404
1
0
5
0
1
02
10
15
25
20
i
50;
30
I f3
25
30
35
40
45
50
RETENTION TIME ( MIN ) Figure 5. Flash pyrolysis GC/MS data showing the TIC for coalified wood sample Midland stem. Pyrolysis conditions were the same aa in Figure 3, but chromatographic conditions were changed to expand the elution time for low-boiling components. Thus, the column waa temperature programmed from 25 to 280 "C at a rate of 2 "C/min. Peak labeling is the same as in Figure 3.
compound benzene toluene C-2 benzenes C-3 benzenes btal benzenes
Table 11. Flash Pyrolysis Data Coalified Wood Samples Midland stem normalized Taylorsville Ped wt% wt %" log, wt % B 2.2 3.3 tr B1 3.2 4.8 1.0 5.2 2.6 B2 3.4 3.9 1.6 B3 2.6 11 17 5.2
trb
tr 1.9
phenol o-cresol m- p-cresol 2,4 dimethylphenol other C-2 phenols C-3 phenols C-4 phenols total phenols
P P1 P1 P2 P2 P3 P4
2.3 3.7 8.9 8.2 6.8 7.2 3.2 40
3.6 5.7 13 12 11 11 4.8 61
2.4 4.1 11.7 10.3 3.3 4.8 1.5 38.6
10 6.2 36 13 4.4 6.3 1.0 77
alkylnaphthalenes alkyldibenzofwans
N1, N2, N3 F1, F2, F3
11 3.4
17 5.2
4.3 1.4
tr tr
+
cd22
0
Ferron log, wt % 1.3 0.6
33 tr n-alkanehlkenes 4-22 Normalized to total aromatic pyrolysis products, exclusive of n-alkaneslalkenes. * tr = trace levels.
accounts for all the oxygen, and we can calculate that an averageof one aryl-0 carbon per 19carbon atoms is present. If we assume that the nine-carbon structural backbone of lignin is the basic building block, as in the case of subbituminouscoal, then this translates to approximately one oxygen per two building blocks. The NMR data, however, indicate that 11-13% of the aromatic carbons are aryl-0 carbons. Clearly, the aryl-0 carbons must be mostly diaryl ethers with one oxygen bridging two phenylpropane building blocks. Such a model is a sharp contrast to the monohydric phenol model for subbitumi-
tr
nous coalified wood,indicating that coalification of the wood has involved condensation of the phenols to diaryl ethers. The pyrolysis data substantiate the above transformation from subbituminousto high-volatile bituminouscoal. Phenols are the dominant pyrolysis products in subbituminous coalified wood; in high-volatile C bituminous wood benzene and alkylbenzenes become more prominent than they are in subbituminouswood. If we consider that pyrolysis would induce cleavage of the diphenyl ether bonds, then such a cleavage will produce both phenols1
Energy & Fuels, Vol. 6, No. 6,1992 819
Structural Model for Lignin-Derived Vitrinite
JMLG -.I .
v
HC-
I
Y2
.cn2
I
C b
@-y
- : 2"
HC
on
OH
SUBBITUMINOUS COAL
1'87
"78'14
I
HvC BITUMINOUS COAL
I
c 8 s % 2 Os.5
Figure 6. A structural model for subbituminous coalified wood modified from a previous report.10 The circled letters indicate cross-link sites to other circled letters. Sites C and D are crosslinks to other coal macromolecules.
Figure 7. Structuralmodel for HvC bituminouscoal constructed from the model in Figure 6 by chemical reactions mentioned in the text. Circled letters are cross-link sites as in Figure 6.
alkylphenols and benzene/alkylbenzenes. If the coal structures are already phenolic as in the case of subbituminous coal, then pyrolysis will not yield substantial benzene/alkylbenzenes but principally phenols/alkylphenols. The presence of n-alkaneln-alkene doublets extending in carbon number to n-C22 in the pyrolyzates of the Midland stem is indicative of the presence of aliphatic biopolymers derived from materials other than lignin. Lignin does not contain these substances and it is unlikely that they are produced from lignin, so they must have been introduced postdepositionally. Microbial remains have been detected in coalified and it is possible that these aliphatic materials were introduced by microorganisms,bacteria, or algae. There is also the possibility that these substances were adsorbed to the degradinglignin from fluids passing through the wood as it was coalified. It is well-knownthat ground waters carry dissolved organic materials, some of which are macromolecular such as the humic materials. It is possible that hydrophobic humic substances carried by ground waters were adsorbed to the wood and incorporated and coalified along with the wood. The presence of alkane/alkene pairs from flash pyrolysis of humic material has been 110ted.l~Yet another explanation for the Occurrence of alkane/alkene pyrolysis products is that lipids were introduced or bound to the macromolecular structure of coalified wood by a vulcanization process involving reaction with sulfur species produced in the sediment during diagenesis. These bound lipids became incorporated into the macromolecular structure and released as alkanes and alkenes through flash pyrolysis. The presence of n-alkaneln-alkene in bituminous coal as significant contributors (30%)to the pyrolysis products is a bit disconcerting. The NMR data does not indicate a large contribution from long-chain material which normally yields a sharp peak a t 30 ppm,14 albeit the Midland stem which appears to contain more aliphatic carbon than the other samples of lower rank. Thus, we conclude that these aliphatic materials are only
minor components in the coalified wood and that the pyrolysis technique appears to be overly sensitive with regard to their contribution. Such sensitivity may reflect the possibility that they are somehow associated with sulfur-containing species, because it is well-known that activation energies for thermal breakdown of such species are generally lower than other non-sulfur-containing fossil fuels. The structural model depicted in Figure 7 is one drawn by inducing the above-mentioned changes to the structure for subbituminous coalified wood shown in Figure 6. These changes include (1)the formation of diaryl ethers, dibenzofurans, and dibenzopyrans via condensation of phenols, (2) the inclusion of naphthalene by cyclization of a fourcarbon side chain with an attached aromatic structure, and (3)reduction of the total number of oxygen functional groups. Energy minimization of this structure by use of the MMX which was operated under PCMODEL software on a Sun Spark Workstation provided an averagevalue of less than 6 kcal/atom. While the depicted structureagrees in a general sense with elemental data for coalified wood samples of HVC bituminous rank,8 with 13C NMR data, and pyrolysis data, there are some inadequacies which relate to the inability to explain certain pyrolysis products (e.g. the n-alkanelalkene pairs) and the propensity of coal to contain more naphthalenic structures and three or more condensed ring systems. It is unclear how we might generate the condensed aromatic systems from the model depicted in Figure 7. The characteristic features of the model which are consistent with the NMR and pyrolysisdata are the diaryl ether structures, most of which are depicted as either dibenzofuransor dibenzopyrans. These types of structures are inferred mostly from the presence of alkyldibenzofurans and pyrans in pyrolysis products. We expect the alkyldibenzofuransand pyrans either to be produced intact by cleavage from the macromolecular network at alkyl side-chain sites or to be cleaved to produce alkylphenols. Thus, the high yields of alkylphenols in the pyrolysis can be explained as pyrolytic fragments produced from the alkyldibenzofurans and pyrans. The cresols and 4-
(13)Taylor, G.H.;Liu, S. Y. Fuel 1987,66, 1269. (14)Saiz-Jimenez, C.;deLeeuw, J. W. J . Anal. Appl. Pyrol. 1987.11, 367.
(15)Alinger, N. L. J. Am. Chem. Soc. 1977,99,8127.
I
820 Energy & Fuels, Vol. 6, No. 6,1992
ethylphenol could be produced as pyrolysis products of the alkyldibenzofuranswhile 2,Gdimethylphenolis a likely pyrolysis product of alkyldibenzopyrans. The model shown in Figure 7 depicts the alkyldibenzofurans and pyrans as structures bonded to other macromolecularunit structures external to the model (shaded regions). This was done as a matter of convenience so as to induce minimum distortion of the originallignin framework used as the template, while recognizingthat other unit structures adjacent to the one depicted would have phenolic structures capable of undergoingcondensation reactionsto form the furans and pyrans. Another feature of the model shown in Figure 7 is the lack of oxygenated species other than ethers. The NMR data infer that carbonyl and carboxyl groups are not present as they are in subbituminous coalified wood. Accordingly they must have been lost by either reductive processes or decarboxylation. We have little evidence to explain the mechanisms for the loss of these functional groups at this time. We do not incorporate macromolecular aliphatic materials into our model, even though we recognize their contributionto the chemistryof the wood. As we discussed previously, we do not feel that these structures are lignin-
Hatcher et al.
derived and cannot predict their occurrence due to their extraneous origin. There is also the problem with the depiction of nitrogen-containing structures. It is wellknown that nitrogen is not present to any significant amount in lignin. Thus, nitrogen which exists in the HvC bituminous logs must have been introduced during coalification through a process not known at this time. The specific form of this nitrogen is unknown because we have been unable to identify nitrogen-containing compounds in pyrolyzates and we have not addressed ourselves vigorously to identifying the organic nitrogen in this sample. This is an area for future work. At the present time, we would rather omit nitrogen-containingstructures than introduce structures that may not be representative of these compounds.
Acknowledgment. Financial support for this study was provided by the U.S.Department of Energy, Sandia National Laboratories, under contracts DE-AC0476DP00789 and 12-5543. We also acknowledge financial support from DOE contract DE-AC22-91PC91042from the Pittsburgh Energy Technology Center. We thank Jackie M. Bortiatynski for assistance in assembling the data and manuscript.
