48
Energy & Fuels 1988,2, 48-58
595-46-0; 11, 111-14-8; 12, 98-89-5; 13, 110-15-6; 14, 498-21-5; 15, 124-07-2; 16, 597-43-3; 17,65-85-0; 18, 13545-04-5; 19, 110-94-1; 20,112-05-0; 21,617-62-9; 22,681-57-2; 23,17179-91-8; 24,124-04-9; 25,334-48-5;26,626-70-0; 27,3058-01-3;28,763-06-4; 29, 111-16-0; 30, 112-37-8; 31, 10200-27-8; 32, 10200-31-4; 33, 505-48-6; 34, 111324-47-1; 35,100-21-0; 36,88-99-3; 37, 121-91-5; 38,123-99-9; 39, 65891-27-2; 40, 37102-74-2; 41, 4316-23-8; 42, 111-20-6; 43,
66021-66-7; 44, 111324-41-5; 45, 111324-42-6; 46, 1852-04-6; 47, 111324-43-7; 48, 693-23-2; 49, 110063-44-0; 50, 27252-21-7; 53, 505-52-2; 54, 111348-90-4; 55, 67595-78-2; 58, 821-38-5; 60, 70174-69-5; 61, 111324-44-8; 62, 111324-45-9; 63, 1460-18-0; 65, 505-54-4; 66, 476-73-3; 67,89-05-4; 68,479-47-0; 69,67595-79-3; 71,111324-46-0; 74, 1585-40-6; H&C02H, 64-19-7; H&CH&02H, 79-09-4; R u O ~20427-56-9. ,
Dipolar-Dephasing 13C NMR Studies of Decomposed Wood and Coalified Xylem Tissue: Evidence for Chemical Structura1 Changes Associated with Def unctionalization of Lignin Structural Units during Coalification Patrick G. Hatcher US. Geological Survey, 923 National Center, Reston, Virginia 22092 Received July 30, 1987. Revised Manuscript Received October 5, 1987
A series of decomposed and codified gymnosperm woods was examined by conventional solid-state 13Cnuclear magnetic resonance (NMR) and by dipolar-dephasing NMR techniques. The results of these NMR studies for a histologically related series of samples provide clues as to the nature of coalification reactions that lead to the defunctionalization of lignin-derived aromatic structures. These reactions sequentially involve the following: (1)loss of methoxyl carbons from guaiacyl structural units with replacement by hydroxyls and increased condensation; (2) loss of hydroxyls or aryl ethers with replacement by hydrogen as rank increases from lignin to high-volatile bituminous coal; (3) loss of alkyl groups with continued replacement by hydrogen. The dipolar-dephasing data show that the early stages of coalification in samples examined (lignin to lignite) involve a decreasing degree of protonation on aromatic rings and suggest that condensation is significant during coalification at this early stage. An increasing degree of protonation on aromatic rings is observed as the rank of the sample increases from lignite to anthracite.
Introduction The study of coal’s chemical structure has traditionally relied on detailed characterization of whole coal by several chemical and spectroscopic techniques. Despite such extensive studies having been conducted over several decades, we still have only a limited knowledge of coal’s chemical composition, because coal is such a complex substance composed of a multitude of different plant remains, each of which introduces heterogeneity and complexity to coal’s chemical structure. To simplify the task of defining coal’s chemical composition and the processes that transform plant remains to coal, several earlier studies1s2focused only on woody xylem tissue. This tissue is commonly coalified to a maceral known as vitrinite, a major component of most coals. Examining the chemical structural evolution of xylem tissue as it is converted to vitrinite provides an insight into the processes that can collectively be described as coalification. In most cases, the early diagenetic phase of coalification, the peat stage, involves degradation of cellulosic components of wood and selective preservation of lignin-like components.l” However, specific examples can (1) Hatcher, P. G.; Breger, I. A.; Earl, W. L. Org. Geochem. 1981,3,
be found where cellulosic substances survive the early stages of decomposition in coalified wood and persist over geologic time.2v6p7 No cases have been reported where cellulosic substances survive in coals having ranks greater than subbituminous coal. Thus, while cellulosic materials can rarely be found in lignitic coals, or brown coals, these substances are geologically unstable and are usually selectively degraded at the peat stage. In most cases lignin is altered to vitrinite during coalification; consequently, we must seek to examine the chemical structural alteration of lignin. Nuclear magnetic resonance studies of a sample series of coalified logs and/or stems that increase in degree of coalification from peat to lignite and to higher ranks have shown that lignin-like components of wood become defunctionalized, first losing methoxyl groups and then losing aryl ether and phenolic groups.2 With coalification increasing to high-volatile bituminous coal, the xylem tissue is converted to aromatic and aliphatic structures containing little, if any, substitution by oxygen-containing functional groups. This study focuses on the nature of changes that lead to the defunctionalization of lignin. The solid-state 13C NMR data provide direct evidence that defunctionalization is the major alteration, but details of the reactions are
49.
(2) Hatcher, P. G.; Breger, I. A.; Szeverenyi, N. M.; Maciel, G. E. Org. Geochem. 1982,4, 9. (3) Fischer, F.; Schrader, H. Brennst.-Chem. 1921, 2, 37. (4) Hedges, J. J.; Cowie, G. L.; Ertel, J. R.; Barbour, R. J.; Hatcher, P. G. Geochim. Cosmochim. Acta 1985, 49, 701.
(5) Spiker, E. C.; Hatcher, P. G. Geochim. Cosmochim.Acta. 1987,51, 1385. (6) Mitchell, R. L.; Ritter, G. J. J. Am. Chem. SOC.1934, 56, 1603. (7) Russel, N. J.; Barron, P. F. Int. J . Coal Coal. 1984, 4 , 119.
This article not subject to U S . Copyright. Published 1988 by the American Chemical Society
Dipolar-Dephasing 13C NMR Studies
Energy &Fuels, Vol. 2, No. 1, 1988 49 Table I. Origin of Samples
sample no. 1
2 3
3a
4
5
6 7 8 9 10 11
12 13 14 15
16 17 18
19
origin Decomposed Xylem Tissue decomposed Douglas fir tree, fallen in a storm, from Mt. Rainier, WA. The sample, described by Hatcher,ls had apparently lost most of its cellulose, but its cellular morphology is nearly intact. The tree was lying on the grovnd and was entirely waterlogged. Atlantic white cedar log buried in peat from the Dismal Swamp, VA. The sample was isolated from a brown, decomposed area of the log. Chemical analysis indicates that most of the cellulose has been degraded and lost and that cellular morphology is retained. periodate lignin from spruce wood, described by Hatcher et aL2 periodate lignin from sample 3, hydrolyzed by refluxing in 6 N HCl for 2 h. Brown Coal Samples coalified log (lignite B or brown coal) of Miocene age from Oczkowice, Poland, collected by J. H. Medlin (US. Geological Survey). This log, thought to have been derived from the Sequoia family of trees, was discussed in a previous paper.2 piece of xylem tissue from a large Podocarpaceae log from the Yallourn seam in the Yallourn open-cut mine near Morwell, Victoria, Australia. The sample was provided by T. V. Verheyen (Coal Corp. of Victoria). Lignite Samples xylem tissue from a log collected in Cretaceous age rocks (Potomac Group) near Stafford, VA by W. L. Newel1 and R. W. Stanton (U.S. Geological Survey). coalified log, lignite, collected by B. D. Stone (US. Geological Survey) from probable Pleistocene sediments at the Nassau Brick Company clay pit on Long Island, NY. The sample probably was reworked from the Magothy Formation (Cretaceous). coalified log, lignite, collected by R. Thomas (U.S. Geological Survey) l/s-mi east of the 1-95/1-695interchange at Landsdowne, MD, from the Patapsco Formation (Cretaceous). Paleocene coalified log, lignite in rank, collected by W. S. Marsalis (Bendix Corp.) from the Nanafalia Formation, Clay County, GA. coalified log collected from the Morwell Seam, Morwell open-cut mine, in Victoria, Australia. The sample was described as being gelified, in that it displayed a vitreous appearance in a hand specimen. coalified log collected by the late J. M. Schopf (U.S. Geological Survey) from the Brunner coal, a lignite (Eocene), Burley's Mine, Buller Gorge, New Zealand. Subbituminous Coal Samples coalified log, subbituminous rank, collected by T. Ryer (U.S. Geological Survey) from the Ferron Sandstone Member of the Cretaceous Mancos Shale in Willow Springs Wash, central Utah. coalified log, subbituminous rank, collected by B. R. T. Simoneit (Oregon State University) from the Posidonia Shale (Jurassic) near Doterhausen, West Germany. coalified stem, subbituminous rank, collected by C. E. Turner-Peterson (US.Geological Survey) from the Johnny M Mine in the Ambrosia Lake Uranium District near Gallup, NM. The log was from a mudstone unit of the Brushy Basin Shale Member of the Morrison Formation (Jurrassic). coalified log from the Falling Creek Member of the Doswell Formation (Upper Triassic) collected by A.J. Froelich (US. Geological Survey) near Taylorsville, VA. The sample had a vitrinite reflectance value of 0.44," which provided the basis for classifying its rank as subbituminous coal. The analysis was from Hatcher and Rowankiw.20 High-Volatile Bituminous Coal Samples coalified log, high-volatile C bituminous rank, collected by J. Hatch (US. Geological Survey) from a coal bed in the Vermejo Formation (Cretaceous), Hastings mine, Fremont, CO. Vitrinite reflectance was 0.44.b coalified stem, high-volatile C bituminous rank, collected from the Midland Formation (Lower Jurrassic), Licking Creek locale, Midland, VA. The stem was embedded in a black lacustrine shale and had a vitrinite reflectance value of 0.6." The analysis was from Hatcher and Romankiw.20 coalified log (Waynesburg log), high-volatile A bituminous rank as determined by its vitrinite reflectance value of O A b Collected by R. W. Stanton (U.S. Geological Survey) from the Connellsville Sandstone Member of the Conemaugh Formation (Upper Pennsylvanian), 4 km north of the intersection on Routes 70 and 48 in western Virginia. The analysis was from Hatcher et a1.2 Anthracite Sample coalified stem, anthracite rank, as determined by its vitrinite reflectance value of 2 . P collected from a sandstone unit of the Lockatong Formation (Upper Triassic) at the H and K quarry near Chalfont, PA. The analysis was from Hatcher and Romankiw.20
Bostick, N., U. S. Geological Survey, personal communication.
lacking. Dipolar-dephasing NMR studies have demonstrated that important structural information can be obtained612 over and above that obtained from conventional solid-state 13C NMR spectra. In particular, Wilson and Vassallog have shown that the number of protonated carbons per aromatic ring can provide information about (8) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J.Am. Chem. SOC.1983,105,6697. (9)Wilson, M.A.; Vassallo, A. M. Org. Geochem. 1985,8, 299. (10)Wilson, M.A.; Vassallo, A. M.; Russell, N. J. Org. Geochem. 1983, 5, 35. (11) Wilson, M.A.; Vassallo,A. M.; Collin, P. J.; Rottendorf, H. Anal. Chem. 1984,56,433. ( 1 2 ) Wilson, M.A.; Alemany, L. B.; Woolfenden, W. R.; Pugmire, R. J.; Given, P. H.; Grant, D. M.; Karas, J. Anal. Chem. 1984,56, 933.
Stanton, R. W., U.S. Geological Survey, personal communication.
specific reactions of aromatic centers during coalification. The same approach can be used to examine the degree of protonation as aromatic rings become defunctionalized during coalification of woody tissue. This approach can provide clues to the processes that lead to the loss of methoxyl and phenolic groups from lignin. The approach can also provide information on the nature of the resulting chemical structure of a major component of most coals, vitrinite. Methodology All samples were air dried after collection, ground to a powder in a mortar and pestle, and stored in glass bottles. The samples were collected from the numerous localities listed in Table I, and most of the samples have been described by Hatcher et a1.'
50 Energy & Fuels, Vol. 2, No. 1, 1988
Hatcher
Attempts were made t o establish the origin of the decomposed woods and coalified woody tissue that originated from gymnosperm or related woods. Scanning electron microscopy and reflected light microscopy were used to discern wood texture that would betray a gymnospermous origin (i.e. bordered pita). In many instances, the woody tissue had lost cellular morphology, and no determination could be made of the wood type. In such instances, only the macroscopic appearance provided a clue that the sample was, in fact, a piece of coalified woody tissue; therefore, verifying the gymnosperm origin of the sample was not possible. The coalified wood samples were present either in coal beds or in clastic sediments, as noted in Table I. Elemental Analyses. Direct determinations were made of the C, H, N, and 0 contents with a Carlo Erba Model 1106 elemental analyzer after powdered samples were dried at 105 OC. Mineral matter contents were calculated by difference between the sum of C, H, N, and 0 contents and 100%. The C, H, N, and 0 contents were reported on a dry mineral-matter-free (DMMF) basis. 13C NMR. Conventional solid-state 13C NMR analyses were performed by using the cross-polarization and magic-anglespinning (CPMAS) technique. The spectrometer,a Chemagnetica Inc. CMC-100S-200L instrument, was operated at a field strength of 2.35 T (25.2 M H z for l%). Spectra were recorded in the Fourier transform mode by acquiring between 3000 and 15000 scans having a frequency sweep width of 10 kHz each, separated by 0.5-1-5 delays. A cross-polarization contact time of 11119 was used for all samples. Exactly 512 data points were used to define the free induction decay, and the data were zero-filled to 4096 data points before Fourier transformation. Dipolar-dephasing NMR spectra were acquired by the pulse sequence described originally by Opella and Frey13 and modified by Alemany et al.* After the protons are spin-locked and cross-polarization is induced, as in the conventional CPMAS experiment, a variable time, Tdd, is inserted during which the high-power decoupler is turned off. During this period, which lasts from 5 to 200 ps, carbon magnetization becomes influenced (dephased) by the strong dipolar interactions between 13C and 'H spins. Carbons directly bonded to protons dephase much more rapidly than those without attached protons. Murphy et al.," Alemany et al.? Wilson et al.,12and Wilson and Vassallogshowed that most protonated carbons dephase a t an exponential rate relative to Tdd2, and the decay can be described by the following Gaussian equation:
I = IOae-Tdd2/2T'a2
(1)
where Tis the signal intensity, Io,is the signal intensity for rapidly dephasing carbons (a) at Td = 0, and T a is the dipolar-dephasing time constant for the rapidly dephasing carbons or protonated carbons. Nonprotonated carbons (b) dephase at an exponential rate relative to Tdd according to the following Lorentzian equation:
I = IObe-Tdd/T2b
(2)
Methyl groups, even though protonated, dephase at a rate similar to that of nonprotonated carbons, because their rapid molecular motion reduces the lH-13C dipolar interactions.*Jl In spectra where significant overlap of signals for protonated, methyl, and nonprotonated carbons occurs (e.g., CPMAS spectra of coal), the decay of signal intensity during dipolar dephasing is a mixed function, which can be a weighted s u m of eq 1and 2. The following equation can then be written:
(3) Wilson et al.12have shown that eq 3 can be deconvoluted and that values for IO,, Iob,Tk,, and T'2b can be calculated from the dipolar-dephasing data. Determining values for Ioa and I o b allows one to calculate the relative contribution of protonated and nonprotonated (and methyl) carbons to the total intensity at Tdd = 0, respectively. Thus, by a focus on the dipolar-dephasing behavior of aromatic carbons (region between 100 and 160 ppm), (13) Opella, S. J.; Frey, M. R.
J. Am. Chem. SOC.1979, 101, 5854.
(14) Murphy, P.D.;Gerstein, B. C.; Winberg, V. L.; Yen, T. F. Anal. Chem. 1982,54, 522.
the degree of substitution on aromatic rings can be estimated. Such an approach was used by Hatcher15to delineate the substitution pattern on aromatic rings from hardwood and softwood lignin. The scaled NMR spectra were integrated by using a Numonics Model 253 digitizer. Areas for aromatic carbons were determined by integrating the peaks between 100 and 160 ppm. The intensities of aromatic spinning sidebands were usually so low that they accounted for less than 5% of the signal and therefore ignored. The peak for methoxyl carbon a t about 56 ppm was also integrated. The precision with which areas can be measured with the integrator is less than 1 % . The errors associated with accurately quantifying a structural parameter (e.g., amount of a r y l 4 carbons or aromaticity) are directly related to the ability to accurately separate a region of the spectrum from other overlapping regions. Because such determinations are subjective, even with the aid of line-shape analysis routines on computers, it is virtually impossible to determine the accuracy of such measurements. In this study, the areas for particular components were obtained by drawing vertical lines to the base line between peaks. Errors in calculating dipolar-dephasing data are on the relative order of M% as determined by multiple replicate analyses on a few samples. Thus, to obtain this value, replicate series of dipolar dephasing data were obtained and the calculated parameters (e.g. fa*, T i , etc.) were subjected to statistical treatment. The relative standard deviation of is% is an upper limit for all calculated parameters. Quantitative Reliability of 13C NMR Spectra. Although several reports have addressed the quantitative reliability of conventional CPMAS spectra for coal (see review by Axelson16 and ref 17), less is known about the quantitative reliability of dipolar-dephasing data. In this study, the assumption is made that if the CPMAS spectra are reasonably representative of the carbon structures in coal, then the dipolar-dephasing data are equally as representative, because the dipolar-dephasing NMR technique is a variation of the CPMAS NMR technique and uses the same polarization transfer to induce the NMR signals. The quantitative reliability of CPMAS spectra depends on several factors that deal with the intensity of carbon magnetization, which is derived from the proton magnetization. Thus, the decay of proton magnetization (Tip) in the spin-locked coordinate system has a direct bearing on whether full intensity will be realized for carbons. T l pmust be sufficiently long to allow maximum transfer of magnetization to all carbon centers during cross-polarization. Measurements of T1, have been made for many samples of coal and lignin (see Wilson and V d o S and references cited therein) and have been found to be on the order of 4-16 ma, sufficiently longer than the criteria mentioned above. Another factor that is as critical as T1, is the time needed to induce polarization transfer from protons to carbons (TCH). The efficiency of magnetization transfer is dependent on the proximity of protons to carbons. For directly bonded protons, carbon magnetization builds rapidly, on the order of 0.1-0.3 ms. If contact times of 1ms are employed, then full transfer occurs. However, for carbons that are nonprotonated and distal to protons, the values for T m may be higher than 0.3 1119 and possibly in the range of 1-2 ms. This means that a contact time of 1 ms may not be sufficient to gain full magnetization for these carbons, and disproportionate signal intensities may be observed when they are compared with protonated carbons. Another important consideration when attempting to achieve quantitative signal intensities is the spin-lattice relaxation time of protons (TIH). Sufficient delays must be inserted between pulses to allow all the protons to fully relax. For coal examples, T1H values are usually less than 300 ms; these values imply that the commonly used pulse delays of 1 s are sufficient to allow almost full proton r e l a ~ a t i o n . ~ Although direct measurements of T C H , T1,, and T I H were not made for all samples in this study, a few samples for lignin, lignite, and subbituminous coal were examined to insure that the most quantitative signals possible were being generated. First, spectra (15) Hatcher, P. G.Org. Geochem. 1987,11, 31. (16) Axelson, D.E.Solid State NMR of Fossil Fuels; Multiscience Publications: Montreal, 1985. (17) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987,1,173.
Dipolar-Dephasing 13C NMR Studies
no. 1 2 3 3a 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Energy & Fuels, Vol. 2, No. 1, 1988 51
Table 11. Elemental Compositions of Samples (Reported on a Dry sample %C %H %N Douglas fir log 60.6 5.70 0.36 Atlantic cedar log 58.4 5.79 0.72 periodate spruce lignin 6.10 63.0 co.1 hydrolyzed lignin 4.66 64.4 0.13 Miocene brown coal xylitea 5.74 64.8 0.22 Podocarpaceae log 5.22 67.3 H occur as a function of rank. The are plotted with carbon content (DMMF) values for in Figure 6A. In lignin and decomposed wood, fa* values are approximately 0.5 indicative of the fact that, on the average, three of the six aromatic carbons are protonated. This average is expected of samples whose lignin is relatively unaltered. As the lignin is altered through coalification and the carbon content increases, the value for f2H begins to decrease, to a low of 0.37 at a rank equivalent to lignite. This decrease means that the aromatic rings are losing protons so that they eventually have an average of only about two of the six carbons protonated. This loss of protons is consistent with the trends in chemical shift data that show loss of methoxyl groups and replacement with hydroxyl groups and condensation at one of the sites of proton substitution. Equation 5 depicts this alteration of aromatic structures in the transformation of lignin to lignite, and the dipolar-dephasing data for the samples examined are consistent with such alteration. The hydrolytic condensation of lignin involves loss of methoxyl carbons, not unlike the loss of methoxyl carbon during coalification of xylem tissue samples in this study. The values of fa* for hydrolyzed lignin is 0.36,significantly lower than that of periodate spruce lignin or lignin in decomposed wood (Table IV). This value is a clear indication that hydrolysis has induced loss of protonated aromatic carbons and of methoxyl carbon. The trend is remarkably similar to the trend for f2Hand for defunc-
Energy & Fuels, Vol. 2, No. 1, 1988 57
/ .19
8.
55
eo
70
A
80
90
% C
(faaiH).
D
0.5
a.H fa 0.4
0.3
B
0.04
0.08
0.12
0.16
OCHJAr
0.5
l5.
L
14.
13
a.n fa 0
.
0.4
faaIH
0.0
C
01
02
03
Ar-O/Ar
Figure 6. Plots of the degree of protonation on aromatic rings (fa"'H) as a function of various rank parameters: (A) % carbon, DMMF; (B) methoxyl carbon content normalized to total aromatic carbon (OCH,/Ar); (C) the aryl-0 carbon content normalized to total aromatic carbon content (Ar-O/Ar). Sample numbers are listed in Table I. Solid lines represent visual trends in data and dotted lines represent the trend betweeh lignin and hydrolyzed 1ign in
.
tionalization apparently associated with coalification of xylem tissue to a rank of lignite in the samples examined. The major spectral change was the loss of methoxyl carbon, as lignin is converted to lignite. The chemical shift data suggest that the methoxyl group is replaced by a hydroxyl. If this reaction proceeds as a function of increasing coalification,then loss of methoxyl groups can be used as a rank parameter. The value for faa,H is plotted in Figure 6B against the methoxyl carbon content normalized to the aromatic carbon content (OCH,/Ar). In essence this is a plot of as methoxyl carbons are stripped from aromatic structures in lignin. The trends noted in Figure 6B show that fcfi remains constant or may faarH,
Hatcher
58 Energy & Fuels, Vol. 2, No. 1, 1988
slightly increase until about half of the methoxyl carbons have been lost from aromatic rings, or until the brown coal stage has been reached. As brown coal is converted to lignite, fa4Hdecreases, and most of the remaining methoxyl is lost. After going through a minimum, the values begin to increase as the last few percent of methoxyl carbon are lost (Tables I11 and IV). This reversal may not necessarily be associated with the loss of methoxyl carbon but may represent reactions occurring on aromatic rings that have already been demethylated. In short, the reversal may represent reactions on aromatic rings that now have two hydroxyl groups. These reactions may increase and thereby compete with the demethylation reactions that decrease faaVH
faaVH
faa,H.
Figure 6A shows increasing with carbon content or rank increasing from that of lignite to anthracite, similar to the trend noted by Wilson and Vassal10.~This trend accompanies the gradual loss of aryl-0 carbons from aromatic rings, as indicated by the ratio of Ar-O/Ar in versus Ar-O/Ar in Figure Table I11 and the plot of 6C. Thus, the increase in faa,H for low-rank coal samples appears to correlate more with the loss of phenolic hydroxyls or aryl ethers from aromatic rings than with the loss of alkyl groups that was suggested by Wilson and V a ~ ~ a lLoss l ~ .of~ alkyl carbon may be important at a rank greater than high-volatile bituminous coal. For the transition from lignite to subbituminous and bituminous coal the loss of phenolic hydroxyls must involve the following process: faaVH
faavH
c-c-c
c-c-c
(7)
0
0
The removal of a phenolic hydroxyl group is accompanied by protonation, so that only aromatic structures with one or no a r y l 4 groups remain. This trend is consistent with the trend in O/C and H/C on the van Krevelen diagram and, more importantly, with the chemical shift data that show aryl-0 carbons having a resonance line at 154 ppm rather than at 146-148 ppm. The limited number of samples having a rank higher than subbituminous coal in this study show an increasing value for faGH even though most a r y l 4 carbons have been lost. A similar increase was noted by Wilson and Vassallo? Because the concentration of a r y l 4 carbon is low, it is not realistic to call upon the further loss of a r y l 4 to cause the continued increase in It is more likely that other reactions are responsible for the increasing fa@. The major change in the CPMAS spectra for xylem tissue samples above the rank of subbituminous coal is a precipitious increase in a r ~ m a t i c i t y . ~ ~ , ~ ~ , ~ ~ The coalified wood sample having a rank of anthracite (sample 19) is essentially entirely aromatic, in contrast to faaiH.
(26) Boudou, J. P.; Pelet, R.; Letolle, R. Geochim. Cosmochim. Acta 1984,48, 1357.
values of about 0.60 for high-volatile A bituminous coal. The loss of aliphatic structures associated with coalification at high ranks is the primary structural change that is most likely responsible for changes in faa,H, although conversion of aliphatic to aromatic structures is also possible. The loss of alkyl groups from aromatic rings, followed by protonation of the site of substitution as suggested by Wilson and Vassal10,~is consistent with the above trends of increasing f2H. However, significant amounts of aromatic ring condensation can occur without a significant effect or with an increase in on
fa
faaiH
faaVH.
Conclusions The NMR results presented here for decomposed wood and coalification of xylem tissue show some chemical structural trends that reveal some coalification reactions. With the assumption that lignin is the precursor of aromatic structures in coalified xylem tissue (an assumption that is reasonably well justified) the chemical shift information, dipolar-dephasing NMR data, and elemental data permit the following reactions to be proposed for defunctionalization of lignin structural units during coalification: 1. Guaiacyl units in gymnosperm lignin lose about half of their methoxyl carbons, which are replaced by hydroxyl groups at early stages (brown coal and decomposed wood). 2. As demethylation progresses, aromatic rings undergo condensation reactions, primarily by carbon linkage. 3. As coalification of xylem tissue proceeds to higher rank, all methoxyl carbons are lost, and residual aromatic structures are essentially composed of carbon-linked ortho hydroxyl phenols (catechols) or phenolic ethers. The Ar-0 carbon content begins to decrease. 4. Continued loss of the aryl-0 groups is accompanied by an increase in faa,H or in degree of protonation on aromatic rings. This increase implies that the lost aryl-0 group is replaced by a hydrogen. This loss of hydroxyl or aryl-0 groups continues through subbituminous coal to bituminous coal, where the phenolic or aryl-0 groups are significantly diminished so that they can no longer be distinguished by NMR. 5. The value of increases from bituminous coal to anthracite. Such an increase occurs concomittantly with increasing aromaticity brought about by loss of aliphatic alkyl carbons, indicating that dealkylation of aromatic rings proceeds with possible replacement by hydrogen, as suggested by Wilson and Vassal10.~ faaPH
Acknowledgment. I thank all my colleagues at the U S . Geological Survey who assisted me in collection of samples or who provided samples for this study. I also thank Gary E. Maciel, Francis P. Miknis, Elliott C. Spiker, C. Blaine Cecil, and A1 J. Froelich for valuable comments and discussions. I am grateful to Dr. Geoffery J. Perry and T. Vincent Verheyen of the Coal Corp. of Victoria for providing samples of Victorian brown coal. I also thank Lisa A. Romankiw and Diana Mills for assistance in data processing and Ronald W. Stanton and Neeley Bostick for vitrinite reflectance measurements. Registry No. Lignin, 9005-53-2.
