Relationship between rank and composition of aromatic hydrocarbons for coals of different origins

Org. Geochem. Vol.6, pp. 423-430.1984 Printcdin Great Britain.Allrightsreserved

0146-6380/84 $03.00+(X/.00 Copyright © 1984PergamonPress Ltd

Relationship between rank and composition of aromatic hydrocarbons for coals of different origins MATTHIASRADKE, DETLEV LEYTHAEUSERand MARLIESTEICHMULLER* Institute for Petroleum and Organic Geochemistry (ICH-5), KFA-Jfilich, P.O. Box 1913, D-5170 J01ich, Federal Republic of Germany *Geologisches Landesamt Nordrhein-Westfalen, De-Greiff-Str. 195, D-4150 Krefeld, Federal Republic of Germany

Abstract--Distributions of di- and tricyclic aromatics were determined for Upper Carboniferous to Oligocene coals of various origins. In addition to Upper Carboniferous Ruhr coals, Wealden (Lower Crctaceous) coals from North-west Germany, Lower Cretaceous coals from Western Canada, and Tertiary coals from Japan and Colombia were studied. The coals ranged from sub-bituminous to anthracite in rank and showed vitrinite reflectance from 0.4 to 4.9% Rm. The previously established relationship between the rank of coal in terms of vitrinite reflectance and the distribution patterns of individual aromatic hydrocarbon isomers has been verified and refined. It is demonstrated how the above relationships can be used to calculate vitrinite reflectance from the Methylphenanthrene Index (MPI 1). The "'chemical"rank levelsof coals can now be defined in terms of the calculated vitrinite reflectance (R,,). Key words: aromatic hydrocarbons, coal rank, methylnaphthalenes, methylphenanthrenes, methylphenanthrene index, vitrinite reflectance

INTRODUCTION A widely accepted model for the average chemical structure of coal is that it consists mainly of aromatic and hydroaromatic "clusters". Most of the "clusters" are interlinked by short methylene chains and ether linkages to form a macromolecular network while others exist as individual molecules (Green et al., 1982). Recent studies using solid state ~3C NMR and f.t.i.r, revealed a minimum average aromatic "cluster" size of three for bituminous coals (Gerstein et al., 1982). Though the ring size may vary depending on maceral type, phenanthrene structures appear to be prominent constituents of the average coal. Methyl groups are the dominant aryl-alkyl substituents (Deno et al., 1982). Our previous studies on Upper Carboniferous coals from the Ruhr area, West Germany, show that m e t h y l p h e n a n t h r e n e h o m o l o g u e s are m a j o r components of the solvent extracts from bituminous coals (Radke et al., 1982b). This does not necessarily imply that the isomer distribution of the extract is representative of the coal structure. However, a striking relationship exists between coal rank in terms of vitrinite reflectance and the isomer distribution of methyl- and dimethylphenanthrenes. Similar relationships were established for the corresponding naphthalenes (Radke et al., 1982b). The objectives of this study are to: (1) verify the previously established relation between rank and the isomer ratios of aromatic hydrocarbons in coal extracts; (2) study changes of the isomer ratios around the third "coalification jump" of liptinites at 1.3% Rm ("coalification jump" according to E. Stach, as discussed by Teichmtiller and Durand, 1983);

(3) extend and verify the established relation between the methylphenanthrene index (MPI 1) and the mean vitrinite reflectance (Rm), which is used for determination of the calculated vitrinite reflectance (Re); (4) e x a m i n e t h e r e l a t i o n b e t w e e n t h e m e t h y l p h e n a n t h r e n e r a t i o ( M P R ) a n d the fluorescence spectral quotient (O) of sporinite.

423

EXPERIMENTAL

The samples are listed in Table 1. Twenty-four of the Ruhr coals were previously analysed for aromatics (Radke et al., 1982b). The data are included in this study. New data on aromatics are presented for six Ruhr coal samples of Westphalian A age from the "Katharina" seam, which previously were analysed for saturates only (Alteb/iumer, 1983; Alteb/iumer et al., 1983), one coal of Westphalian A age from the "Wasserfall" seam, and one coal of Westphalian B age from the "Zollverein 6" seam. These samples were intended to improve data control in the rank interval 1.1-1.4% R m. The coals from Western Canada in Table 1 were all from the Elmworth gas field area. They represent samples of the Fourth Coal from the Falher member of the Lower Cretaceous age Spirit River formation. Nine samples were selected for evaluation of their aromatic distributions from a series of 18 Lower Cretaceous age Wealden coals from North-west Germany. The fluorescence parameters of these coals are described in Teichmiiller (1982). They were d e r i v e d from the Lower Saxony Basin, the coalification conditions of which were studied previously by Bartenstein et al. (1971). The coal

424

MATTHIAS RADKE et al.

Table 1. Description of samples Number of samples Location 24*

Rank range (% Rm)

Geological age

Ruhr Basin, W Germany

Upper Carboniferous Westphalian A,B,C Westphalian A,B

0.63-1.70 1.06-1.4(I

Deep Basin, W Canada

Lower Cretaceous

0.92-1.51

9

Lower Saxony Basin, NW Germany

Lower Cretaceous Wealden

0.45-1.00

3

Kyushu, Hokkaido, Japan

Eocene, Oligocene

0.65-0.91

5

Titiribi-Bolombolo depression, Colombia

Oligocene

0.43.4.85

8i 11

*Radke et al., 1982b. +This study.

samples from Colombia were derived from the Oligocene age Antioquia formation (Departments Antioquia and Caldas). The two anthracites represent thermal altered coals from the Sabaletas Mine. A petrographic study of most of these samples was published by Ramirez Castro (1980). Powdered samples were extracted twice with a mixture of chloroform, acetone and methanol (47 : 30 : 23, w/w/w) using the "flow-blending" method (Radke et al., 1978). The highest rank anthracite from Colombia had to be re-extracted twice with xylene to recover sufficient amounts of polycyclic aromatic hydrocarbons. The de-asphalted extracts were separated into saturated and aromatic hydrocarbon fractions by medium-pressure liquid chromatography (Radke et al., 1980). Aliquots of the aromatic fractions were further separated according to the number of aromatic rings and molecular structure into four subfractions (AF1-4) by s e m i p r e p a r a t i v e h i g h - p e r f o r m a n c e liquid chromatography. Total aromatics and aromatic subfractions containing tricyclic and perifused tetracyclic aromatics not seriously affected by evaporative loss and obtained in sufficient amounts (AF2) were analysed by glass capillary GC (Radke et al., 1982b, Radke and Welte, 1983). Using capillary columns of 25 m minimum length and a slightly polar stationary phase, such as SE-54 or CP-Sil-8, the dimethylphenanthrenes generally show a characteristic pattern of two separate groups of four peaks (first group) and of six peaks (second group). The peaks were numbered according to their order of elution. Vitrinite reflectance was measured on polished coal blocks using oil immersion and a wavelength of 546 nm. Liptinite fluorescence spectra were measured on polished coal blocks under 365 + 30 nm wavelength u.v. irradiation (Radke et al., 1982b). The spectral quotient was determined for sporinites

in all but the Western Canada coals. For the Carboniferous coals, only Q values of microspores are considered in this study. Fluorescence intensities of sporinites were not measurable above 1.3% R m. RESULTS AND DISCUSSION

Methylnaphthalene and dimethylnaphthalene ratios

The carbon-normalized yields of individual methylnaphthalene homologues extracted from lower-rank coals (R m values ranging from 0.4 to 0.8%) were generally below the detection limit of 0.1 Ixg g Corg- 1. Consequently, isomer ratios could not be calculated for these samples. Only the lower-rank Wealden coals exhibited higher carbon-normalized yields of the methylnaphthalene homologues. This could be due to their terpenoid-rich source material, as indicated by a high proportion of eudalene (1-methyl-7-isopropylnaphthalene) and cadalene (1,6-dimethyl-4-isopropylnaphthalene) in the diaromatic fractions of these coals. Sesquiterpenoids are considered potential precursors of naphthalenes in petroleum (Mair, 1964). The methylnaphthalene ratios (MNR) of the lower-rank Wealden coals average about 1.0 (Fig. 1). This relatively high value is reached by the Ruhr coals only above 0.9% Rm possibly due to the influence of different source materials. The regression lines of MNR on Rm (a) and of Rm on MNR (b) are shown in Fig. 1. At ranks c o r r e s p o n d i n g to 0 . 7 - 1 . 5 % Rm, where the concentrations of methylnaphthalenes are generally enhanced, a linear relationship between MNR and R m seems to exist. Although the correlation is significant (correlation coefficient r = 0.75) it is not useful for the accurate determination of coal rank. There are systematic deviations related to differing origins of the coals. For example, in the range

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Fig. 1. Relationship of methylnaphthalene ratio (MNR) to mean vitrinite reflectance (Rm) for coals of different origins. MNR =

[2-methylnaphthalene] [1-methylnaphthalene]

The correlation coefficient (r) and the regression lines are based on n data points in the range 0.7-1.5% Rm. (a) MNR = 3.31 Rm - 2.14. (b) Rm = 0.17 MNR + 0.82.

0.9-1.3% R m, Cretaceous coals from Western Canada show higher MNR values than Wealden or Carboniferous Ruhr coals of equivalent rank. Thus, source-effects as well as maturation may influence the isomer distributions of the methylnaphthalenes. The source-effects are more important at lower ranks where the molecules released from the coal matrix are structurally more closely related to individual biological precursor molecules. As the methylnaphthalene population is in flux with sources and sinks of these compounds, an MNR increase c o u l d be d u e to s e l e c t i v e g e n e r a t i o n of 2-methylnaphthalene and/or selective degradation or condensation of 1-methylnaphthalene. Generally, methyl substitution on s-carbon atoms leads to the greater reactivity of the respective naphthalene homologues. For example, in pyrolysis experiments the condensation rate of 1-methylnaphthalene was several times greater than that of 2-methylnaphthalene (Madison and Roberts, 1958). Although selective condensation reactions are adequate to explain the MNR increase at higher ranks, methyl-shift reactions resulting in a reduction of steric strain would have a corresponding effect on MNR, and hence may contribute to the MNR increase (Radke et al., 1982b).

Between 0.7 and 1,5% R m the rank trend of the dimethylnaphthalene ratio (DNR) resembles that of the MNR values, showing a linear relationship with R m (Fig. 2). The D N R values show a closer correlation with the R m values than the MNR values. In particular, systematic deviations related to differing origins of the coals are less pronounced. Thus, the DNR is a better tool for determination of the rank stages of coal extracts than the MNR. However, the data from the lignite and anthracite samples indicate that the relationship between DNR and Rm does not apply to Rm values outside a restricted R m range (0.7-1.5% Rm). It is unlikely that there is a predominance of the [3,[3-type isomers in the starting materials (Radke et al., 1982b). Again, differential destruction or condensation of the 1,5- vs the 2,6- and 2,7-isomers by processes subject to reactivity differences between the ~t- vs [3-substituted positions would result in a DNR increase. Likewise, methyl-shift reactions induced by steric strain possibly contribute to the shift with rank in the dimethylnaphthalene distribution. For 1 , 5 ( c ~ u ) - d i m e t h y l n a p h t h a l e n e , h y d r o g e n s at positions 4 and 8 sterically interfere with hydrogens of the methyl groups at 5 and 1, respectively. On the other hand, hydrogens of the methyl groups on the

426

MATTHIAS RADKE et al.

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[2,6-dimethylnaphthalene + 2,7-dimethylnaphthalene] [ 1,5-dimethylnaphthalene]

The correlation coefficient (r) and the regression lines are based on n data points in the range 0.7-1.5% Rm. (a) DNR = 14.8 Rm - 12.0. (b) R,, = 0.046 DNR + 0.89.

2,6(1313)- and 2,7(1313)-isomers do not interfere with adjacent hydrogens.

Methylphenanthrene and dimethylphenanthrene ratios In contrast to MNR and DNR values the methylphenanthrene (MPR) and dimethylphenanthrene (DPR) ratios generally can be measured at lower ranks corresponding to 0.4-0.8% Rm. This resulted in an extension of these maturity trends into lower vitrinite reflectance intervals (Figs 3 and 4). The relationship between MPR and Rm values is nonlinear; i.e. the MPR values exhibit an exponential increase with rank up to 1.7% Rm. The MPR values of the anthracites do not differ significantly from that of the low-volatile bituminous coals. Thus, a gradient change must occur between 1.7 and 2.5% R,,. Below 1.7% Rm the most pronounced change in gradient occurs between 0.8 and 0.9% R m where the second "coalification jump" (Teichmfiller and Durand, 1983) of liptinites is observed. Thus, the previously established strong increase of the MPR values with coal rank beyond 0.9% Rm (Radke et al., 1982b) is verified. The good correlation between MPR and Rm values occurs because at equal ranks coals of different origins do not show systematic differences of their MPR values.

MNR and MPR exhibit very similar minimum values of 0.43 and 0.42, respectively. This suggests that at the lower rank stages methylaromatics may be generated via a common reaction pathway, e.g. kinetically controlled methylation reactions. Mechanistically, the increase of MPR with rank appears to be due to a thermal rearrangement of the ~x-type 1and 9-methylphenanthrenes to yield the sterically less hindered 13-type 2- and 3-methylphenanthrenes (Radke et al., 1982b). The DPR is sensitive to changes in coal rank only at ranks above 0.8% Rm where it shows an exponential increase (Fig. 4), i.e. the sensitivity of this parameter increases continuously beyond the second "coalification jump". Again, the previously established relation between DPR and R~ values (Radke et al., 1982b) is verified. The DPR values of the Cretaceous Wealden coals and the Tertiary coals from Japan at equal ranks do not differ significantly from those of the Ruhr coals. O b v i o u s l y , the d i s t r i b u t i o n of the dimethylphenanthrenes used in the calculation of the DPR must be more dependent on maturation than source. For example, the ratio has not been influenced by the diterpenoid-rich source material of the Wealden coals. Although the genetic relationship between diterpenoid precursors and pimanthrene (1,7-

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428

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Fig. 6. Relationship of m e t h y l p h e n a n t h r e n e ratio (MPR) to spectral quotient (Q 650 nm/500 nm, sporinite) for coals of different origins.

Q =

relative fluorescence intensity at 650 n m relative fluorescence intensity at 500 n m

The correlation coefficient (r) and the regression lines are based on n data points. (a) M P R = 0.29 Q + 0.38. (b) Q = 1.78 M P R + 0.29.

Rank and composition of aromatic hydrocarbons dimethylphenanthrene) is well established (LaFlamme and Hites, 1978; Hayatsu et al., 1978) the bulk of dimethylphenanthrenes may be generated from other precursors. Successful study of product precursor relationships is impeded by the coalification process. Thermal rearrangement reactions tend to progressively complicate the original distribution of compounds. On the other hand, systematic differences do exist between the D P R values for coals of different origins. For example, between 0.9 and 1.3% Rm, Cretaceous coals of equivalent rank from Western Canada show higher DPR values than the Wealden and Carboniferous Ruhr coals. For this reason the correlation with Rm is worse for DPR than for MPR. Corresponding deviations are observed for the MNR values, which likewise affected the correlation with R,,1.

Calculated vitrinite reflectance A good linear correlation between the methylphenanthrene index (MPI 1) and the mean vitrinite reflectance (R~) was previously established for type III kerogen in rocks and for bituminous coals (Radke et al., 1982a,b; Radke and Welte, 1983). The c o r r e l a t i o n is p o s i t i v e at m a t u r i t y s t a g e s corresponding to the liquid window (0.65-1.35% Rm) and negative at higher maturities (1.35-2.1% Rm). Thus, the two equations in the legend of Fig. 5 are used alternatively for the calculated vitrinite reflectance (Re), depending on which vitrinite reflectance range is considered. If the Rm value is unknown or questionable, the above established relations between Rm and DNR, MPR and DPR will normally allow the decision whether equation (1) or (2) is to be used. The Rc is a useful rank parameter, as indicated by the good correlation with Rm between 0.4 and 1.7% Rm (Fig. 5). Regression analysis of the data suggests that for the given coals Rc somewhat underestimates the measured vitrinite reflectance.

Spectral quotient The MPR (Fig. 3) and the fluorescence spectral q u o t i e n t ( Q ) of s p o r i n i t e s h o w n o n l i n e a r r e l a t i o n s h i p s with R~. The c o r r e l a t i o n s are characterized by an abrupt gradient increase at the s e c o n d "coalification j u m p " ( 0 . 8 - 0 . 9 % Rm). Nevertheless, MPR and Q are not closely correlated (r = 0.71, Fig. 6). The data from the Tertiary and Mesozoic coals are scattered and show systematic deviations with coal type. For example, at a given MPR value the corresponding Q value of Wealden c o a l is g e n e r a l l y h i g h e r t h a n t h a t of t h e Carboniferous coals. A corresponding discrepancy in the rank trends of Q exists between Carboniferous coals and Tertiary or Mesozoic coals, which has been attributed to the different origins of the sporinites (Teichmiiller, 1982). Carboniferous sporinites are from spores, whereas Tertiary and Mesozoic

429

sporinites generally are from pollen remains. This difference in source materials of the sporinites did not influence the MPR values (Fig. 3), indicating that the bulk of the methylphenanthrenes was generated from vitrinite rather than sporinites. CONCLUSIONS The established relations between coal rank in terms of vitrinite reflectance and the isomer ratios of i n d i v i d u a l a r o m a t i c h y d r o c a r b o n s have been verified. The results for coals of different origins confirm the nonlinear rank trends of the isomer ratios of methylphenanthrene homotogues, MPR and DPR. The MPR increases strongly above 0.9% R m, whereas DPR exhibits a corresponding increase only above 1.1% R m. Based on regression analysis of the d a t a calibrations were o b t a i n e d for the two parameters vs mean vitrinite reflectance. A good correlation with Rm was observed. However, the isomer ratios of the methylnaphthalene homologues, MNR and DNR, do not correlate well with R m. Their rather poor rank trends indicate linear rather than nonlinear relationships. There is good correlation between the calculated v i t r i n i t e r e f l e c t a n c e ( R e , b a s e d on t h e m e t h y l p h e n a n t h r e n e index, MPI 1) and the measured vitrinite reflectance (Rm) in the range 0.4-1.7% Rm. Thus, the Rc value is a reliable "chemical" rank parameter. Due to a trend reversal of the MPI 1 at the third "coalification jump" (1.3% Rm), the calculated Rc value for a given coal is ambiguous unless additional information is available indicating that its rank is lower or higher than 1.3% Rm. This information can be derived from the relations between Rm and DNR, MPR and DPR shown in Figs 2-4. In the given Rm ranges these relationships allow the unambiguous determination of coal rank from the aromatic distribution only.

Acknowledgements--The authors thank P. F. Ramirez Castro, Universidad Nacional de Colombia, Medellin, R. Takahashi, Kyushu University, Fukuoka, the Ruhrkohle AG, Essen and the Canadian Hunter Exploration Ltd., Calgary, Canada, for providing coal samples. Special thanks are due to K. Ottenjann, who carried out the fluorescence and reflectance measurements of all but the Western Canada coals and to P. K. Mukhopadhyay, who measured the vitrinite reflectance of the Western Canada coals. Technical assistance by the following members of KFA/ICH-5 is gratefully acknowledged: H. Willsch, A. Fischer, B. Kammer and W. Laumer. This paper benefited greatly from the thorough review of K. E. Peters. REFERENCES

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