Energy & Fuels 1991,5, 155-163 hence the corresponding moieties in coal, have not been completely elucidated from previous studies with diphenylalkanes. 2. The major primary pathway for BPP, PPN, and diphenylpropane pyrolysis involves a rapid Rice-Herzfeld free-radical chain mechanism. Thus, the mechanistic insights obtained from pyrolysis of diphenylpropane can be extrapolated to describe the analogous features of the pyrolysis of polycyclic diarylpropanes. Additionally, elementary reaction rate constants that are adjusted to account for resonance stabilization energy differences can describe the kinetics for the disappearance of diarylpropanes. 3. Aryl-alkyl C-C bond cleavage was important only in secondary reaction pathways for diarylpropane pyrolysis because the primary pathway involved a very rapid chain reaction. Note, however, that polycyclic diarylmethanes and diarylethanes, relevant coal model compounds for which rapid free-radical chain decomposition reactions are not available, could be expected to exhibit aryl-alkyl cleavage as a major primary pathway.
155
1. A selective hydrogenolysis mechanism (e.g., RHT or MD) appears to be responsible for aryl-alkyl C-C bond cleavage during BPP pyrolysis. 5. Extrapolating the present observations from BPP pyrolysis to coal suggests that very strong aryl-alkyl C-C bonds in coal can be thermally cleaved in appreciable numbers even at relatively low temperatures and in the absence of added H2or donor solvent.
Acknowledgment. We thank Pat Smith for performing some of the PPN neat pyrolyses and Sarah Mancini for experimental assistance with the nitrogen glovebox. This work was supported in part by the Shell Faculty Career Initiation Fund, an Energy Research Grant from the University of Michigan Office of the Vice-president for Research, and NSF Grant CTS-8906859. Registry No. PPN, 40739-96-6; BPP, 61549-24-4; 2-ethylnaphthalene, 939-27-5; 2-vinylnaphthalene, 827-54-3; toluene, 108-88-3; ethylbenene, 100-41-4; styrene, 100-42-5; 2-methylnaphthalene, 91-57-6; vinylpyrene, 50986-33-9; methylpyrene, 27577-90-8; ethylpyrene, 56142-12-2.
An Investigation of the Vitrinite Maceral Group in Microlithotypes Using Density Gradient Separation Methods Gary R. Dyrkacz,* C. A. A. Bloomquist, and L. Ruscic Chemistry Division, Argonne National Laboratory, 9700 South Cuss Avenue, Argonne, Illinois 60439
John C. Crelling Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 Received July 13, 1990. Revised Manuscript Received October 9, 1990 Several sets of microlithotypes derived from channel samples have been separated by density gradient centrifugation. Detailed density distribution data and elemental data for the separated vitrinites reveal that there are easily discernible differences in various types of vitrinites found in different microlithotypes. Moreover, the vitrinite in closely allied vitrite microlithotypes also can have different densities and chemical compositions. Although it appears that liptinite content may be an important marker for the aliphatic character of the coals, the results suggest that other factors, probably related to the paleoenvironment, may also play a strong role. Physical and chemical variations that can be seen, even within a single microlithotype such as vitrite, reenforce the view that there is a need for more concern about the physical state of a coal sample in coal research.
Introduction Although considerable work has been published on the chemical nature of the vitrinite maceral group in coal, it is not always clear what is really being studied. As Hatcher et al. have indicated, the study of vitrinite is complicated by the presence of nonvascular plant debris whose origins are often uncertain.l One possible way around this pitfall is to study coalified logs, where the contribution of nonvascular materials should be minimized.' Approaching the problem this way has certainly provided new insights into the process of coalification. However, there are many chemical and physical variations in the composition of (1) Hatcher, P. G.; Breger, I. A.; Szeversenyi, N.; Maciel, G. E. Org. Geochem. 1982,4,9-18.
0887-0624/91/2505-0155$02.50/0
Table I. Classification of the Vitrinite Maceral Group in Bituminous Coals maceral maceral tvDe telinite telinite / 1 telinite 2 vitrodetrinite desmocollinite / telocollinite collinite \ gelocollinite corpocollinite
'
these logs that still have not been adequately explained. Moreover, as helpful as the analysis of coalified logs has been, we are still faced with the problem that much of the vitrinite in coal cannot be unequivocally established as 0 1991 American Chemical Society
156 Energy & Fuels, Vol. 5, No. 1, 1991
arising solely from woody tissues. Often the complexity that can appear in the microscopic examination of coal is ignored by most coal chemists, and the term “vitrinite” becomes abused to the extent that it is considered in the same class as any simple reagent grade chemical. There are several reasons why it is necessary to be cautious in use of the term vitrinite. First, vitrinite is defined petrographically primarily on the basis of its reflectance and morphology, but as coal rank increases, the properties of vitrinite (including reflectance) change. Second, vitrinite is a maceral group composed of a number of different maceral types. The current international coal maceral classification system recognizes several maceral species of vitrinite.2 Table I is a list of the vitrinite macerals found in bituminous coals. Pure telinite actually exhibits the form of the original plant cell walls and plant structures, suggesting direct coalification of the original cell wall biopolymers. Pure collinite, on the other hand, is a completely amorphous vitrinite, suggesting repolymerization of small molecular weight moieties such as humic acids or tannins. Most vitrinites found in coals usually contain both forms as an intimate mixture, with the telinite cells being infilled with collinite (hence the name “tel~collinite”).~ Desmocollinite is another common form of vitrinite found associated with other macerals. In reflected light, it shows a much less uhomogeneow”texture than telocollinite and appears as a matrix for other macerals. Finally, there is evidence suggesting that differences in vitrinite can occur when it is associated with other macerals at the level of microlithotypes. It is this last case for vitrinite complexity that is the major interest of the current work. Third, the idea of “homogeneous vitrinite” is an artifact of the way petrography is routinely performed using polished coal sections with oil immersion objectives. There is no guarantee that, even if the vitrinite shows no textural or reflectance differences, the chemical or physical structure must be homogeneous. Also, the identification of vitrinite is dependent on a particular method of observation. In fact, etching of polished coal surfaces by various methods or examination of the polished coal surfaces with methylene iodide immersion objectives can enhance structural features within the ~ i t r i n i t e . ~Likewise, ?~ the use of fluorescence microscopy reveals vitrinites with distinctly different fluorescence ~ p e c t r a . ~ Microlithotypes are assemblages of macerals that occur in most banded coals. They are classified according to two conventions.6 First, the width of an individual band must be at least 50 pm wide when viewed perpendicular to the bedding plane, and the band must occupy an area of at least 50 X 50 pm. Second, macerals which have concentrations of less than 5% are disregarded when a microlithotype is defined. Table I1 shows the current classification of microlithotypes. Obviously, this classification system is not at all concise. For example, a clarite can be composed of as little as 6% liptinite and >89% vitrinite (remainder could be inertinite), or as much as 89% liptinite and 6% vitrinite. Often, the variation is described in terms (2)Stach, E.et al. Stach’s Textbook of Coal Petrology, 3rd ed.; Gebruder Borntraeger: Berlin 1982. (3)It should be noted that brown coals have a much larger array of macerals in the huminite group, which is the precursor of the vitrinite goup. The reason for this is the much larger variation in reflectance and morphology that is present in these low rank coals. (4)Stach, E.Brennstoff Chem. 1958,39,15-20. (5)Bensley, D. F.;Crelling, J. C. Advances in Coal Spectroscopy; Plenum: New York,in press. (6) International Committee for Coal Petrology. International Handbook of Coal Petrology, 2nd ed.,Centre Natisnal De La Recherche Scientifique: Paris, 1963.
Dyrkacz et al. Table 11. Partial Claosification List of the Microlithotypes microlithotypes maceralsa monomaceral v > 95% vitrite liptite L > 95% I > 95% inertite bimaceral L + v > 95% clarite v + I > 95% vitrinertite L + I > 95% durite trimaceral duroclarite v > I,L > 5% L > I,V > 5% vitrinertoliptite I > L,V > 5% clarodurite “The numbers are the percent of maceral that must be present to qualify for a particular microlithotype category, e.g., liptinite
and vitrinite must constitute at least 95% of the sample to qualify as clarite.
such as liptinite-poor clarite or liptinite-rich clarite, which only marginally improves the categorization. Nevertheless, beyond the classification problems, and for our purposes, the really important concept is that these microlithiotypes can represent different paleoenvironments. Usually other factors, such as the type of the minerals and the texturing of the layers, need to be taken into account, as well as the constituent macerals. Smith,’ Teichmueller,8 and, more recently, Hagemann and WolP have discussed the nature of microlithotype origins. The precoalification nature of several of the microlithotypes is still not really well understood, partially because there still is so little known about the physical and chemical nature of the individual vitrinite macerals. Nevertheless, there is little doubt that, within certain bounds of maceral composition and texture, microlithotypes represent a fossilized record of certain conditions of pH, redox potential, water level variation, and plant and microbial communities. What effect do these varying paleoecological conditions have on the resultant vitrinite in microlithotypes? Leighton examined two sets of British vitrinite samples.1° For the first set, consisting of handpicked vitrains, which were at least 93% pure and obtained from adjacent areas in the same seam, he concluded that small differences in volatile matter could be seen between the vitrinites. Variations in ultimate analysis were very small. The second set of samples were paired sets of vitrains and clarains; these sets showed more variability in their ultimate analysis, volatile matter, and Geiseler plasticity. Brown et al.” and Taylor12found that they could divide vitrinite into two categories: vitrinite A and B. Vitrinite A has a higher reflectance and has a homogeneous appearance even when examined by transmission electron microscopy. Vitrinite B tends to be less texturally homogeneous than A, exhibiting fine, “liptinite-like” laminations (KO.10 pm), and is intimately mixed with other macerals. The reflectance distinction between the two forms of vitrinite was lost in the low volatile bituminous coal stage (maximum reflectance 1.4;VM, 27%). Vitrinite A is similar to telocollinite and vitrinite B is similar to desmocollinite. Stach interpreted the difference in reflectances in terms of the different types of plant parts that made up the paleoenvironment of the peat.13 He proposed (7)Smith, A. H. V. Proc. Yorkshire Geol. SOC.1962, 33, part 4, 423-474. (8)Teichmuller, M. Reference 1, pp 285-294. (9)Hagemann, H.W.; Wolf, M. Int. J. Coal Geol. 1987,7,335-348. (10)Leighton, L. H.Fuel 1969,38, 155-164. (11)Brown, H.R.; Cook, A. C.; Taylor, G. H.Fuel 1964,43,111-124. (12)Taylor, G.H.ACS Symp. Ser. 1964,55,274-283. (13)Stach, E.Fortschr. Geol. Rheinl. Westfalen 1970,17, 439-460.
Energy & Fuels, Vol. 5, No. 1, 1991 157
Vitrinite Maceral Group i n Microlithotypes
coal name set 1 PSOC-592 PSOC-594
L, 12.5 5.1
Table 111. Petrographic Data (vol %) for Coal Samples' SP Re cu Bit Vit S-FUS Illinois No. 5 10.7 0.3
1.4 4.4
0.4 0.4
0 0
Id
Mi
Fus
53.0 88.5
13.4 2.1
11.2 1.0
0.3 4.8
3.4 5.0
97.2 71.0
0.6 5.8
0.8 2.5
0 2.1
0.2 0.8
74.3 92.6 89.1 96.6 55.2 89.7 74.1 98.4 82.6 98.2 84.1
3.9 0.6 0.2 0.8 9.7 1.6 9.7 0.2 3.1
7.7 0.2 1.0 0.2 4.9 1.4 4.1 0.4 3.7 0.2 1.9
0
3.9 0.8 0.4 0.2 7.6 0.2 5.5 1.0 3.6 0.4 2.5
Elkhorn No. 3 set 2 SIU-744A SIU-744B
1.2 15.4
1.0 13.6
0.2 1.2
0 0.6
0 0
Illinois No. 6 set 3 1. CMK3-16-L3 2. CMK3-3B-L3 3. CMK4-11-L12 4. CMK4-19-L3 5. CMK4-22-L32 6. CMK4-22-L9 7. CMK4-23-L25 8. CMK4-5-Ll9 9. CMK4-5-L30 10. CMK4-6-L21 11. CMK4-6-L29
10.2 4.8 8.5 1.6 21.8 7.2 5.8 0 5.6 1.2 7.8
2.0 3.2 1.4 1.6 15.6 4.4 3.6 0 3.6 1.0
4.2
6.8 1.6 0.7
0 0
2.9
3.5
0
0 0
0
2.0 2.4 1.4 0
0.6 0.2 3.0
0.4 0.8 0 1.4 0
0.6
1.4 0
4.2 0 0 0 0 0 0
0
2.2
a & = total liptinite; Sp = sporinite; Re = resinite; Cu = Cutinite; Bit = bituminite; Vit = vitrinite sinite; Id = inertodetrinite; Mi = micrinite; and Fus = fusinite.
that the various differences could be due to varying proportions of cellulose and lignin products in the sediments. Brown et al. also indirectly examined the hydrogen, nitrogen, and volatile matter contents of the vitrinites." They found quite large differences between the two types of vitrinite, although the variations were not as large as the differences between the maceral groups. Binder et al. examined a set of vitrinites from four coals: one pair of nearly the same reflectance, but different geological age, and a second pair from the same pillar sample, but slightly different reflectance 1 e ~ e l . lElemental ~ composition, Li/ethylamine reduction, and dehydrogenation experiments showed large variation between each pair of samples. Although the data is based on a very limited number of samples, the reaction differences between the same reflectance type vitrinites suggest that chemical reactivity and reflectance cannot always be related. Benedict et al. distinguished pseudovitrinite from normal vitrinite on the basis of ita higher reflectance and petrographic properties and showed that pseudovitrinite was not as reactive in coking as normal vitrinite.15 Crelling found that pseudovitrinite is the material occurring in vitrain layers (hence, the same as telocollinite or vitrinite A) and normal vitrinite is the material occurring in clarain layers (the same as desmocollinite or vitrinite B).le He also showed that the two types of vitrinite can be separated by density gradient centrifugation on the basis of their different densities and that they have measurable differences in their mode of occurrence, petrography, reflectance, thermodynamic properties, chemical composition, and reactivity. Hutton and Cook found that there was an inverse relationship between vitrinite reflectance and the amount of alginite associated with it." Their data led them to believe that the vitrinite was modified by the leakage of material from the alginites and its subsequent incorporation into the vitrinite. (14) Binder, C. R.; Duffy,L. J.; Given, P. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1968, 7,145-153. (15) Benedict, L. G.; Thompson, R. R.; Shigo 111, J. J.; Aikman, R. P. Fuel 1968,47, 125-143. (16) Crelling, J. C. Proc. Ironmaking Con/. 1988,47, 351-356. (17) Hutton, A. C.; Cook, A. C. Fuel 1980,59,711-714.
1.0 0.8 0.6 0.8 0 0.8 0 0.6 0 1.5
+ pseudovitrinite; S-Fus = semifu-
Shibaoka et al. found from microscopic observations that the swelling responses of the various forms of vitrinite were different for a variety of solvents.18 Thus,there is considerable evidence to support the view that there are indeed different forms of vitrinite, each with somewhat different chemistries. The differences can be found through direct observation, reflectance measurements, or response to solvents or reactions. In at least some of the cases, the evidence seems to suggest that the differences noted above are due to different forms of vitrinite, such as telocollinite and desmocollinite. However, one problem with the majority of these studies, particularly those discussing the chemical or thermal properties of bimacerites or trimacerites, is that it is difficult to factor out the interference of the other macerals. This has primarily been due to the experimental difficulty in physically separating the macerals. In a continuing study of the use of density gradient centrifugation (DGC) to explore the heterogeneity of coal, we felt that the high-resolution character of DGC could be used to examine this aspect of coal heterogeneity. We therefore obtained sets of microlithotypes and subjected them to density gradient separations. Experimental Section Coals and Microlithotypes. Data for the three seta of coals used in this work are given in Tables I11 and IV. The first coal set in the tables, from the Illinois No. 5 seam, was obtained from the Pennsylvania State University Coal Data Library. The last two sets of microlithotypes were handpicked samples collected a t the Coal Characterization Laboratory of Southern Illinois University. Set 2 is from the Elkhorn No. 3 seam and set 3 is from the Illinois No. 6 seam. All the coals are high-volatile bituminous coals. All the coals in set 3 had the same mean reflectance (0.77). For sets 2 and 3, the various lithotypes were first identified in polished blocks of the particular coal and then selected lithotypes were cut from the blocks by hand with a diamond saw. Each separated lithotype was then crushed, mixed, and split into subsamples. One subsample was used for chemical analysis, and another was made into a standard petrographic pellet which was used to determine the maceral composition of each lithotype. (18) Shibaoka, M.; Stephans, J. F.; Russell, N. J. Fuel 1979, 58,
515-522.
158 Energy & F u e l s , Vol. 5, No. 1, 1991
Dyrkacz et al.
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The elemental analysis of coals 1 and 5 in set 3 of Table IV shows that the percent carbon is very low. We believe that these results are due to the presence of very large amounts of mineral matter interfering with the microanalysis. In fact, chemically demineralized coal data show values which are more consistent with the other vitrinite microlithotype data in this set. Separation and Microscope Analysis. The density gradient separation technique and subsequent petrographic analysis have been previously de~cribed.'~The procedure consists of fluid energy mill grinding of the coal to one to 1-3 pm average particle size, chemically demineralizing, and then separating the coal sample by CsCl density gradient centrifugation. The gradient is then fractionated and the coal isolated. A modified maceral analysis is then performed to identify the maceral groups present in certain density fractions. From this, it is possible to derive the distribution of each maceral group as a function of density. For all the coals we have separated in this work, we have used what we called the "preparative I" method, where approximately 2 g of a coal is separated in a linear density gradient. Extraction of Vitrinite Samples. Approximately 80 mg of a density separated vitrinite sample was placed in a flask with 25 mL of benzene/methanol (3:1), and refluxed under nitrogen for 2 days. The residue was collected, washed with benzene/ methanol, and dried. FTIR Spectroscopy of Vitrinites. Standard methods were used for the preparation of KBr pellets for quantitiative FTIR comparisons of samples.20s1 The Coal samples, O.WI.10 mg, were weighed on a Cahn Model 27 electrobalance and the KBr was weighed on a standard analytical balance. After pressing, the pellets were dried overnight at 110 OC in vacuo and then weighed to determine the coal concentration. Spectra were recorded on an IBM 98/4A vacuum FTIR spectrometer at 4 cm-' resolution with 200 scans. The spectra were subjected to a linear baseline correction between 3650 and 1855 cm-'. The difference spectra were obtained by using the original absorbance spectra, rather than the corrected spectra. Elemental Analysis of Density Fractions. The analysis of the individual density fractions required a microanalysis because of the often limited amounts of material available. The procedure (19) Dyrkacz, G.R.; Horwitz, E. P. Fuel 1982, 61, 3-12. (20) Painter, P.; Starsinic, M.; Coleman, M. In Fourier Transform Infrared Spectroscopy; Ferraro, J., Basille, L. J., Eds.;Academic Press: New York, 1985; Vol. 4, p 169. (21) Painter, P. C.; Rimmer, S. M.; Snyder, R. W.; Davis, A. Appl. Spectrosc. 1981, 35, 102.
0 1.26
~ 1.80
0 1.5s
.
1
0
1.40
Denslty (g cm-9)
Denslty (g cm-?
Figure 1. Separation and chemical analysis of set 1,Illinois No. 5 coals (see Table 111): (a) density gradient distribution of the vitrinite from each coal; (b) elemental ratios of the corresponding fractions from the distribution in (a). The vertical bars represent the maxima in the density distribution of the vitrinite bands. The designation as clarain or vitrain is based on the Pennsylvania State University classification of these samples. Densities in all figures are given at 25 "C. The inset bar graphs display the original volume percent concentration of the three maceral groups.
0 1.20
Figure 2. Separation and chemical analysis of set 2, Elkhom No. 3 coals (see Table 111): (a) density gradient distribution of the vitrinite from each coal, (b) elemental ratios of the corresponding fractions from the distribution in (a). The vertical bars represent the maxima in the density distribution of the vitrinite bands. The inset bar graphs display the original volume percent concentration of the three maceral groups. 0.0% L
12% L
4.8% L
7 8% 1
10 2 %
L
IO
Density (g ~ m - ~ )
Figure 3. Vitrinite group density distributions for set 3, Illinois No. 6 coals (see Table 111). Numbers on the left indicate the amount of liptinite in each coal microlithotype. used has been described elsewhereen Ash analysis was according to the ASTM standard.
Results T h r e e sets of coal microlithotypes were used in this study. T a b l e s I11 and IV present the petrographic a n d elemental data. E a c h s e t of coal samples represents different microlithotypes from n o t only t h e same seam, but the same channel sample as well. T h u s , although the diagenesis of each microlithotype m a y be different, t h e (22) Dyrkacz, G. R.; Bloomquist, C. A. A.; Ruscic, L. Fuel 1984, 63, 1166-1173.
Vitrinite Maceral Group in Microlithotypes
coal name set 1 PSOC-592 demineralized PSOC-594 demineralized
Energy & Fuels, Vol. 5, No. 1, 1991 159
Table IV. Elemental Analysis (wt % (daf)) of Coals microlithotype C H N S Illinois No. 5 duroclarite duroclarite
75.86 77.10 72.15 74.60
5.12 4.92 5.03 4.79
Ob.c,d
ash (dry)
1.37 0.92 1.40 0.86
2.24 2.01 3.00 2.71
15.41 14.68 18.42 16.57
7.59 0.37 11.30 0.47
1.57 1.30 1.49 1.28
0.78 0.77 0.74 0.82
9.57 11.75 14.66 10.52
2.79 0.05 18.30 0.28
72.40
Elkhorn No. 6 set 2 SIU-744A demineralized SIU-744B demineralized
vitrinite duroclarite
80.42 81.40 81.70 81.89
4.83 4.73 6.38 5.21 Illinois No. 6
set 3 1. CMK3-16-L3
duroclarite
2. CMK3-3B-L3
vitrite
3. CMK4-ll-Ll2
clarite
9. CMK4-5-L30
vitrite
5. CMK4-22-L32
duroclarite
6. CMK4-22-L9
clarite
7. CMK4-23-L25
duroclarite
8. CMK4-5-Ll9
vitrite'
9. CMK-4-5-L30
duroclarite
10. CMK4-6-L21
vitrite
11. CMK4-6-L29
duroclarite
60.29 73.60 76.42 74.20 75.59 73.80 76.65 74.80 67.59 75.60 77.13 74.10 77.74 75.50
5.87 4.66 4.88 4.59 4.99 4.69 4.66 4.39 5.66 4.80 4.93 4.54 4.60 4.40
1.41 1.68 1.68 1.75 1.64 1.71 1.59 1.67 1.46 1.67 1.67 1.74 1.44 1.60
1.09 0.85 0.71 0.58 0.71 0.65 0.55 0.51 1.01 0.78 0.67 0.06 0.63 0.60
31.34 19.20 16.31 18.90 17.07 19.20 16.56 18.60 24.27 17.20 15.60 19.00 15.59 17.90
74.50 76.66 75.20 76.63 74.00 76.14 74.60
4.47 4.69 4.39 4.65 4.33 4.88 4.58
1.62 1.54 1.64 1.53 1.61 1.63 1.71
0.48 0.60 0.50 0.63 0.52 0.77 0.62
18.90 16.50 18.30 16.56 19.50 15.59 18.50
e
2.46 16.50 1.73 57.60 6.06 8.03
4.08 1.97 5.28
'This sample was inadvertently lost after separation. From the data on the vitrinite for this coal (see Table V), the data is similar to the values noted in the other vitrinite samples. *Oxygen by difference. Second values are for demineralized coals. For demineralized coals 0 = 0 + ash. e Ash values for demineralized coals in this set could not be reliably determined because of insufficient amounts of coal samples.
thermal history of coalification should be virtually the same for an individual sample set. Any differences that we note should be attributable either to maceral-maceral interactions, such as those suggested by Hutton and Cook,l' or to differences in the original paleoenvironment. The vitrinite density distribution patterns for the three sets of coals are shown in Figures la, 2a, and 3. The data shown in all of these plots represent, first, only the vitrinite in a coal, and second, only the monomaceral microscopically pure particles. We have discussed elsewhere how we derive the data for monomaceral particle^.'^*^ These data are obtained during the maceral analysis on the density fractions by noting whether a particle being counted contains more than 10% of a second maceral, and then, later, based on data, calculating distributions for the pure macerals only. This method of maceral counting generates idealized data, representing a near-perfect liberation and separation of macerals due to monomaceral properties. Because of the fine grinding we employ, very often there is not much difference between the real distribution and this idealized version (see ref 23). Pseudomonomaceral particles, that is, particles that become monomaceral in the process of polishing the pellets, can be generated and will be included along with true monomaceral particles with this procedure. However, we have determined that, for vitrinite bands, the potential distortion of the density distributions will be minor. Again, this is a consequence
of the high level of maceral liberation, especially for vit r i n i t e ~ . ~The ~ data are treated in this way because it allows us to discuss absolute changes in density patterns, independent of maceral liberation problems, in different coal samples. All of the plots have been normalized to the highest value for each peak, since our primary interest is in the position and distribution of the vitrinite densities. In the first set of coals (set 1)both samples are considered as duroclarites, but as can be seen from Table 11, there is quite a difference in the maceral contents: 53% versus 88% vitrinite. The PSOC-592 is closer to a vitroinertite. Figures la, 2a, and 3 show the density gradient centrifugation (DGC)data for each set of vitrinite samples. The major trend in the first two coal sets is that the coal with the larger amount of liptinite has the lower density vitrinite distribution. The third, more extensive set of microlithotypes shown in Figure 3 allows us to expand on the notion of an inverse relationship between density and liptinite. The data in Figure 3 have been plotted as a function of increasing liptinite content going toward the bottom of the figure. Each individual plot represents the data for a single microlithotype vitrinite. The peak density values for the last two coals, both containing over 10% total liptinite, show the same trend as noted with the first two microlithotype sets. However, it is not clear from this more extensive set of microlithotypes whether the trend with liptinite content should be described as monotonic
(23) Dyrkacz, G. R.; Bloomquist, C. A. A,; Ruscic, L. Fuel 1984,63, 1367-1373.
(24) Dyrkacz, G. R. To be published. Inertinitee will ala0 exhibit little band distribution, but this may not be true for liptinites.
Dyrkacz et al.
160 Energy & Fuels, Vol. 5, No. 1, 1991 0.755
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2ol
.................... 10.2% L
21.8%
L I
5
0
Q
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1.40
Figure 6. H/C and O/C versus density for each of vitrinite bands in set 3 coals. H/C (-) values range from 0.6 to 0.85; O/C (- - -) values range from 0.125 to 0.187.
Q
,
1.36
Density (g ~ m - ~ )
B 5-
1.30
" r
3
Figure 5. Percent liptinite versus the averaged density for each vitrinite density band for set 3 coals.
or stepwise. Although most of the other coals contain some liptinite, there is no indication of a density shift until we have coal samples with 10% liptinite or greater. There is one coal which is not consistent with this pattern: the microlithotype containing 4.8% liptinite (no. 2). This vitrinite has almost the same peak density shift as the microlithotypescontaining larger amounts of liptinite. The differences can be more clearly seen in Figure 4,which is a plot of the H/C ratio at the peak versus the peak density for the third coal set. Almost all of the coals fall in the same range, except for the same three coals that also exhibit the lowest density. A similar plot for O/C ratios shows no discernible pattern. From the work of Leighton on pure vitrinites, this might be expected.1° We do not have a solid rationale for the variation exhibited by the 4.8% liptinite microlithotype sample. Possibly this sample represents a portion of woody tissue that was either more decomposed than usual, or else it might represent a different type of plant tissue than the other vitrinites. Considering the data from a different approach, for set 3 we calculated the average density of the vitrinite bands. This was accomplished by using the known solution density and weight data for each fraction. Figure 5 presents the results. The two coals with the largest amount of liptinite stand out as expected, but the no. 2 sample no longer stands out. (A possible reason for this is suggested in the FTIR section.) Plotting the results as a function of the average density seems to suggest a monotonic inverse relationship with respect to liptinite content. However, because of the sparseness of data and the scatter in the values, we hesitate to firmly advocate such a relationship. Although we do not show the data here, plotting either the density distributions or the average denisty against vi-
trinite, inertinite, or mineral matter content shows no definite trends. If there are relationships, they must be of a second order nature. Dyrkacz et al. have noted that there is a distinct pattern between the elemental ratios of coals and the density.22 Therefore, C, H, and N microanalyticaldata were obtained for fractions from each of the vitrinite curves. Figures lb, 2b, and 5 present the data for those vitrinite fractions which were at least 90% pure. The majority of fractions were greater than 95% monomaceral. We were careful to watch that there were no trends in purity for the maceral composition data across the vitrinite bands. This could lead to the variations in elemental analysis being driven by sample maceral composition differences rather than by inherent chemical character of the vitrinite in the separated fraction. In fact, we found that, if we took the average liptinite elemental analysis, the average vitrinite analysis, and the maceral composition of fractions with 5% liptinite contamination (largest concentration of liptinite contaminated vitrinite in all the data), we could not reproduce the elemental composition of the actual fraction. Thus, our data represent real variations across the vitrinite density band, rather than merely contaminated vitrinite samples. In the first two sets of coals, Figures l b and 2b, the vertical dashed lines represent the peak concentrations of each vitrinite. The microlithotypes with the greater content of associated liptinite exhibit a higher H/C ratio at the peaks. An important question arises as to whether the H/C ratios for densities other than the peak are also shifted. For the third set of Illinois coals, simply plotting the H/C ratio versus density for each coal is not sufficient to answer this question. Figure 6 presents the H/C and O/C data as a function of density. As can be seen, not all of the coals have the same H/C to density relationship, and the data are too complex to clearly plot in a single graph. Therefore, the data were plotted in an altered form, where the H/C ratio is not directly related to density, but instead the weight distribution is related to the H/C ratio
Energy & Fuels, Vol. 5, No. 1, 1991 161
Vitrinite Maceral Group in Microlithotypes
Table V. Analysis (wt % ) of Extracted Vitrinite Microlithotype Residues
0.0% L '
'
'
,
~
'
'
'
I
~
'
~
'
coal
% ' liptinite
8. CMK4-5-Ll9 10. CMK4-6-L21 2. CMK3-3B-L3 11. CMK4-6-L29 1. CMK3-16-L3 5. CMK4-22-L32
0 1.2 4.8 7.8 10.2 21.8
C 76.8 77.4 76.1 75.8 76.0 77.4
H
N
0
H/C
4.40 4.46 4.72 4.51 4.56 4.84
1.62 1.62 1.74 1.86 1.68 1.70
17.2 16.5 17.4 17.8 17.8 16.1
0.690 0.688 0.739 0.709 0.715 0.745
* 5, L = 21.8%
L = 10.2%
LLJL
9cn t " " 0.60 , " " 0.86 ~ " ' ~0.70~ ' '0.76 '
0.66
I
'
"
'
I
'
'
0.80
.
.
(
L = 4.8%
,
HIC Ratio
Figure 7. Distribution by relative mass of the H/Cin the vitrinite bands. This data is a combination of data in Figures 2 and 6 and indicates the proportion of monomaceral vitrinite with a certain H/C.
4000
of material. Figure 7 is a plot of the normalized weight distribution for each microlithotype plotted against the H/C ratio. The plot represents only those vitrinite density fractions that have elemental analysis associated with them. The weight data are then normalized to the highest weight value and finally plotted versus the H/C ratio for the individual weight fractions. The overall effect of this manipulation is to give an idea of how the H/C ratio is distributed as a function of mass in each vitrinite band. Some of the individual plots show quite complex behavior, a result which we attribute to small errors in the elemental data and not to any real coal separation phenomena. What is more important for the present discussion is how these individual distributions compare to one another. Once again, vitrinites 2, 10, and 11 (Table 111) show different behavior than the other vitrinites. The high liptinite containing species have the highest H/ C ratio, but once again the 4.8% liptinite containing microlithotype also has a high H/C ratio distribution. Thus, there is a consistent similarity between variations in density and the H/C ratio of the microlithotypes. Plotting the O/C ratios in a similar'fashion resulted in a much more scattered plot, and we could observe no definite trend in the data. Extraction of Selected Vitrinite Samples. Since higher liptinite content leads to a higher H/C ratio, we decided to see if the variation in this ratio was due to the presence of small extractable species in the vitrinites. Accordingly, fractions of the vitrinite density distribution maximum from several coals in set 3 were extracted with benzene/methanol (3:1), and the carbon, hydrogen, and nitrogen content of the residues measured (Figure 3). Table IV shows the analytical results obtained for the residue portion of the extractions. The data on the extracts are not informative, because they were found to contain substantial amounts of Brij-35, the nonionic surfactant used in the density gradient separation procedure. The
iim v
L = 0.0%
3000
2000
1000
Wavenumber (cm-') Figure 8. FTIR of selected coals from set 3. Values on left are the liptinite contents of the original microlithotype samples.
extracted vitrinites show the same trend as the unextracted coals, higher H/C ratio with high liptinite content, and once again coal no. 2, with 4.8% liptinite, shows an unusually high H/C ratio. This result suggests that the variations in H/C ratio either are related to the fundamental macromolecular structure of the vitrinite or are at least due to large molecular weight species which are not readily extracted. FTIR of Selected Vitrinite Samples. In the hope of obtaining more information on the nature of the differences between the various vitrinites, a selected set of the vitrinite samples was examined by Fourier transform infrared spectroscopy (FTIR). The samples correspond to the extracted residues in the previous section (Table V) except that the CMK4-6-L21 sample was not used. Figure 8 shows the stacked absorbance spectra of the five samples obtained by using KBr pellets. The pellets were prepared on a quantitative basis, and all the spectra have been scaled to 1mg of coal. Although there are no major differences between the spectra, there are minor differences which hint at the nature of the variations that we observed in the elemental data. As an aid to deciphering Figure 8, difference spectra are reported in Figure 9. The spectra are all subtracted from sample CMK4-5-19. The main reason for using this sample as a base for subtraction is that this microlithotype has the least amount of liptinite associated with it. The samples have been arranged in increasing order of aliphatic hydrogen, as indicated by the 2923-cm-I band. The microlithotype samples associated with high liptinite content show the largest aliphatic hydrogen content when compared to the lower liptinite content samples. They also show a reduced aromatic character (900-700 cm-'), which can be more clearly seen in Figure 9.
Dyrkacz et al.
162 Energy & Fuels, Vol. 5, No. 1, 1991 L
-
21.8%
L = 10.2%
L = 7.0%
4000
3000
2000
1000
Wavenumber (cm-l)
Figure 9. Difference spectra for the coals in Figure 8. The coal sample with the least amount of liptinite was subtracted from the other coal samples (CMK-4-5-Ll9).
The one anomalous coal that was reported in the previous data comparisons, CMK3-B3-L3(4.8% liptinite), seill remains enigmatic. Although the liptinite content of this coal is between that of the two coals showing the least amount of aliphatic hydrogen, its IR spectra is more similar to that of the high liptinite containing microlithotypes. The aliphatic C-H stretching region shows two bands, at 2868 and 2964 cm-', which are not nearly as prominent in any of the other coals. The original unextracted sample of this coal contains three additional unexpected bands at 1086,1043, and 875 cm-'. These band positions are all at typical positions for clay minerals, such as montmor i l l ~ n i t e .Moreover, ~~ we found the higher density fractions showed slightly increasing contents of these bands. The presence of these bands is surprising, because all of the coal samples were chemically demineralized. It appears that the chemical demineralization of this coal was not adequately done. The likelihood of mineral matter still remaining in this sample may offer an explanation of why this vitrinite sample did not show anomalous behavior when the average density was plotted against H/C ratio (Figure 4). The mineral matter may have caused some shifting of the density band to higher densities. If only some of the vitrinite particles contain minerals, this would also explain why this particular coal also has the broadest density distribution (Figure 3) of all the samples studied. However, this does not alter the fact that this coal still has a high H/C ratio and also a high aliphatic hydrogen content relative to coals of similar liptinite content. Discussion The results we have found confirm and elaborate on the earlier reports that microlithotypes are composed of different vitrinites.lO-lE Because density gradient centrifugation is a high-resolution separation method for coal macerals, we have been able to isolate the vitrinites as almost pure species. Our conclusions are, therefore, based on samples that do not have the drawbacks associated with the older studies, where many of the vitrinite samples contained other macerals. We have observed density and (25) van der Marcel, H. W.; Beutelspacher, H. Atlas of Infrared Spectroscopy of Clay Minerals and the Admixtures; Elaevier: New York, 1976.
chemical differences between individual vitrinites that came from virtually pure vitrinite microlithotypes and from samples containing different amounts of liptinite. Our rationale behind using the liptinite content of the coal as a plotting parameter is based on the fact that the presence of liptinite is often associated with different types of vitrinite. Certainly, the presence of liptinite infers a more degraded and more heterogeneous origin for any associated vitrinite. Crelling has indicated that pseudovitrinite has a higher density than normal vitrinite and is probably related to or identical with the terms vitrinite A, telocollinite, or homocollinite.16 This would then be the higher density material that we observe in the current study. However, it is apparent from the one "odd" vitrain microlithotype sample in set 3 that there are subtle variations, even within a class such as pseudovitrinite. We are not in a position at this point to say exactly why the vitrinites exhibit these observed differences. Although it might be reasoned that the more aliphatic macerals, such as sporinite, "leak" low molecular weight aliphatic material into the surrounding vitrinite, we did not see evidence of this leakage in the extracted vitrinite samples. Davis et al. have reported the extraction of a series of density-isolated liptinites and telocollinites obtained from British coals.26 Their data show that the extracts for each of the maceral groups have quite different composition, something which would not be expected foi a highly mobile material. Also, why would the leakage be localized and not spread into adjacent vitrinite bands as is apparently the case presented by Hutton and Cook for alginite-containing samples?" We believe our data imply that we are observing residual differences in the type of plant communities, coupled with the different degradative processes that were present in the original peat environments. Thus, even at the highvolatile bituminous coal stage, we still can observe changes which are related to the diagenetic history of the coal. In the broadest sense, we can attribute the differences that we observe to the variations in character between telocollinite, which is associated with vitrinite bands, or to desmocollinite, which is typically associated with bimacerite or trimacerite microlithotypes. Unfortunately, this conclusion does not get us very far, because we do not really firmly understand the nature of origins of these two vitrinite macerals. Moreover, our results suggest that, by using simple density gradient centrifugation, we cannot completely isolate all of the different forms of vitrinite at the same time. The density properties are not that different. However, we can enrich a sample with respect to the content of each of the vitrinite macerals. In a general sense, we should not be surprised by the variation in the vitrinite group. Such variations might be expected, based on recent work with peats and brown c ~ a l s Large . ~ ~ and ~ ~ still ~ enigmatic variations in NMR, FTIR, and pyrolysis results can be found within peat samples taken from different regions and different depths or from different regions within a single coalified log. We believe that it is time for coal scientists to realize that there must be more concern for the nature of the vitrinite they are using. It is not too soon for separation (26) Davis, M. R.; Abbott, J. M.; Gaines, A. F. Fuel 1985, 64, 1362-1369. (27) Ryan, N.J.; Given, P. H.; Boon, J. J.; De Leeuw, J. Znt. J . Coal Geol. 1987,8, 85-98. (28) Given, P.H.Proceedings, NATO Advanced Study Institute on 'New Trends in Coal Science", Datca, Turkey, August 1987; to be published. (29) Given, P. H.In "Coal Science"; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: London, 1984; pp 63-252.
163
Energy & Fuels 1991,5,163-167
to become a standard part of coal research, especially when work is directed toward understanding the fundamental nature of coal. Understanding how coal is physically put together and how this relates to the heterogeneity of this chemistry can only be accomplished when we subject the coal sample itself to separation processes. The majority of past research on coal has traditionally established only the bulk or average properties of coal, leading to conclu-
sions that can be grossly m i ~ l e a d i n g . ~ ~
Acknowledgment. We express our gratitude to Christopher Kravits for the sample preparation and analysis of the set 3 coals. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US.Department of Energy, under contract no. W-31-109-ENG-38.
Hydrotreatment of 8-Hydroxyquinoline on a NiMO/A1203 Catalyst Chung M. Lee and Charles N. Satterfield* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received July 23, 1990. Revised Manuscript Received September 5, 1990
The hydrotreatment of 8-hydroxyquinoline was studied on a sulfided NiMo/A1203catalyst at 360 OC and 6.9 MPa total pressure of H2 in a vapor-phase tubular reactor. Some minor studies were performed at 385 "C. 8-Hydroxyquinoline undergoes hydrodeoxygenation via three routes to enter the hydrodenitrogenation network of quinoline. Route 1 is hydrogenation of the heterocyclic ring (8HTHQ) followed by hydrogenation of the aromatic ring to 8-hydroxy-1,2,3,4-tetrahydroquinoline to 8-hydroxydecahydroquinolinewhich undergoes C-O hydrogenolysis to decahydroquinoline. Route 2 is direct C-0 hydrogenolysis of 8HTHQ to form 1,2,3,4-tetrahydroquinoline(PyTHQ). Route 3 is hydrogenation of the benzene ring in 8-hydroxyquinoline to form 8-hydroxy-5,6,7,8-tetrahydroquinoline followed by removal of the oxygen to form 5,6,7,8tetrahydroquinoline.After oxygen removal, subsequent reactions follow the hydrodenitrogenation network of quinoline. The hydroconversion of 8-hydroxyquinoline is governed by the N center rather than the OH center.
Introduction Model compound hydrotreating studies in the past have generally focused on molecules containing one heteroatom only, although a few reports have appeared on the reaction of compounds containing both nitrogen and sulfur or oxygen and sulfur. These are referred to in the recent paper of Toropainen and Bredenberg' who studied three isomeric methoxythiophenols and p-thiomethoxyphenol. The removal of heterocyclic nitrogen compounds by hydrotreatment has received considerable attention and has been reviewed recently by Ho2 and Ledoux3 and earlier by Katzer and Sivasubramanian! However, Petrakis et aL5 found a substantial fraction of compounds with two heteroatoms, both oxygen and nitrogen, in SRC-I1 heavy distillate fractions. These compounds were classified as hydroxypyridines, hydroxyindoles, and hydroxyanilines. We are not aware of any study of hydrotreatment in which both oxygen and nitrogen are in the same molecule. The objective of this study was to determine how the hydrotreatment of a representative compound with both oxygen and nitrogen might differ from that of related compounds with one heteroatom only. We chose 8hydroxyquinoline (8HQ) as the model compound because it is readily available and its parent molecule, quinoline, has been studied extensively in the past by our group and by others.- It is a hydroxypyridine. We were specifically interested in determining the reaction network by which
8-hydroxyquinoline is converted to clean products.
Experimental Section The reactor consisted of a vertical 0.52-cm4.d. tube immersed in a fluidized sand bath to maintain isothermal conditions. Liquid was fed through a preheater coil to the top of the catalyst bed by a high-pressure liquid chromatography pump (Milton Roy Constametric 111). Similarly, hydrogen was fed through a preheater coil by a mass flow controller (Brooks). The liquid and hydrogen feeds were mixed at the reactor inlet, passed downward through the catalyst bed, and flashed through a back-pressure regulator (Grove). This equipment has been used in our laboratory in previous studies of hydrodenitrogenation, -deoxygenation, and -desulfurization (HDN, HDO, and HDS) and is described in more detail elsewhere.8 (1)Toropainen, P.; Bredenberg, ,J. B. Appl. Catal. 1989,52,57. (2) Ho, T. C. Catal. Rev.-Sci. Eng. 1988, 30, 117. (3) Ledoux, M. J. Catalysis; The Royal Society: London, 1988; Vol. 7, p 125. (4) Katzer, J. R.; Sivasubramanian, R. Catal. Reo.-Sci. Eng. 1979,20, 155. (5) Petrakis, L.; Young, D. C.; Ruberto, R. G.; Gates, B. C. Ind. Eng. Chem., Process Des. Deu. 1983, 22, 298. (6) Cocchetto, J. F.; Satterfield, C. N. Ind. Eng. Chem., Process Des. Deu. 1981, 20, 53. (7) Shih, S . S.;Katzer, J. R.; Kwart, H.; Stiles, A. B. P r e p . - A m . Chem. SOC.,Diu. Pet. Chem. 1977, 22, 919. (8) Satterfield, C. N.; Yang, S. H. Ind. Eng. Chem., Process Des. Deu. 1984. 23. 11. (9) Satterfield,C. N.; Smith, C. M. Ind. Eng. Chem., Process Des. Deu. 1986, 25, 942.
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* Author to whom correspondence should be addressed.
~~
0887-0624/91/2505-0163$02.50/0
Q 1991 American Chemical Society
