Structural relationships between a brown coal and its kerogen and humic acid fractions

Org. Geochem. Vol. 8, No. 5, pp. 375-388, 1985 Printed in Great Britain. All fights reserved

0146-6380/85 $3.00+0.00 Copyright © 1985 Pergamon Press Ltd

Structural relationships between a brown coal and its kerogen and humic acid fractions T. V. VERHEYEN*,A. G. PANDOLFO and R. B. JOHNS Department of Organic Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia (Received 28 August 1984; accepted 10 May 1985) Abstraet--A South Australian Tertiary brown coal is fractionated into humic acid and kerogen fractions. These related samples are then subjected to a number of different analytical techniques including infrared and 13C-Nuclear Magnetic Resonance spectroscopies and pertrifluoroacetic acid oxidation. Structural conclusions are drawn from an integrated consideration of the data. Brown coal aliphatic structure is concentrated in the kerogen and the solvent soluble polar acid fractions. The humic acids are the most aromatic and contain a high degree of hydrogen bonding. Only very minor amounts of long polymethylene chain structures are observed in the humic acids, in complete contrast to the kerogen fraction. Different organic detrital origins are proposed for the coal fractions.

Key words: brown coal, 13C-NMR

humic acid,

kerogen,

INTRODUCTION Chemical structural investigations of humic (soluble at high pH) and kerogenous (insoluble) organic matter are widely reported (Welte, 1973; Schnitzer, 1976; Tissot and Welte, 1978; Durand and Monin, 1980). This interest in kerogen and humic acid results from their dominant contribution to the recycling of sedimentary organic carbon within the carbon cycle. However, the structural elucidation of these materials is complex due to the number of inherent variables. These are: (1) their structure is dependent on the method of isolation--solvent pH, extraction time and extraction energy (Worobey and Webster, 1981); (2) their structure is influenced by the origin of the parent organic material--terrigenous, lacustrine, marine (Tissot and Welte, 1978; Durand and Monin, 1980); (3) their structure is amorphous and highly functionalized giving a tendency to form colloidal polyelectrolytes with a variable, possibly high, molecular weight. Australia contains extensive brown coal deposits-sedimentary systems which are rich in organic carbon primarily as humic material, resulting from terrestrial input during the Tertiary period. Their generally shallow burial depth and low seam temperatures since deposition provide organic detritus with a high degree of structural preservation and result in a low rank classification. These factors make the brown coal an excellent sediment for the study of humic acid/kerogen/coal structural relationships.

*Present address: Coal Corporation of Victoria Research Facility, c/o G.I.A.E., Switchback Road, Churchill, Victoria 3842, Australia.

peroxytrifluoroacetic acid oxidation,

The exhaustive extraction of coals with mixed organic solvents provides extracts which are often dominated by polar material (on a weight basis). This polar material is not readily amenable to chromatographic analysis and is therefore generally ignored in favour of hydrocarbon and alkanoic acid classes. The structure of this organic solvent soluble polar material is poorly understood and is investigated in this article particularly with respect to its relationship to humic acids. Direct structural investigation of humic acids and kerogen without prior separation or degradation results in average macromolecular characterization only, i.e. information such as aromaticity and carboxyl content (e.g. Hagaman and Woody, 1982; Wilson et al., 1983). Attempts at providing structural information on humic substances at the molecular level i.e., alkanoic acids, hydroxy-benzene poly carboxylic acids, have been summarized by Chaudry (1984). Oxidative degradation has been moderately successful (Chaudry, 1984) in providing representative products amenable to chromatographic analysis. This article combines several analytical techniques to provide an improved structural characterization of brown coal humic acids and kerogens by investigating both their organic structure at a macromolecular level and at a molecular chemical level. At the macromolecular level solid state ~3CNuclear Magnetic Resonance spectroscopy (SS13C-NMR) and infrared spectroscopy (IR) are employed as analytical techniques. Solid state 13C - N M R provides a direct measure of aromatic content (Maciel et al., 1979). Low field SS-13C-NMR has been used extensively to probe the structure of humic acids and kerogens by Hatcher (1980). In this article the use of high field 50.3 MHz ~3C-NMR coupled with

375 O.G. 8/5--F

infra-red,

376

T.V. VERHL~'ENet al.

spinning side band elimination (TOSS) techniques (Dixon, 1982) provides high resolution spectra showing considerable additional detail in regions associated with oxygen functional groups, i.e. phenols and carboxyl groups. Infrared spectroscopy provides a complementary technique to SS-t3C-NMR by providing more specific, though less quantitative, information on organic functional groups. The use of infrared spectroscopy in the structural characterization of coals, kerogens and humic compounds is well founded in the literature (Brown, 1955; Stevenson and Goh, 1971; Rouxhet and Robin, 1978). The use of spectral subtraction routines (Liotta et aL, 1981) enables a better assessment of subtle differences between IR spectra and is particularly suited to examining variations in oxygen-containing functional groups. At the molecular level, two analytical techniques are employed (i) pyrolysis gas chromatography and (ii) pertrifluoroacetic acid (perTFA) oxidation. PerTFA is a reagent which was originally applied to the oxidation of American coals by Deno et al. (1978). It is reported (Deno et al., 1978) to selectively attack aromatic moieties producing total dissolution thus enabling the quantitation of aliphatic proton content by 1H-NMR. Recent developments in the isolation and chromatography of these aliphatic oxidation products have led to a better understanding of brown coal structure (Verheyen and Johns, 1983). These properties make perTFA oxidation a suitable probe for the investigation of the aliphatic content in humic compounds. This paper will use the above techniques to examine: (i) which inherent structural features are responsible for the solubility differences between solvent soluble polar material, humic acids and kerogens; (ii) how the structures of these fractions are inter-related and dependent on different detrital inputs. EXPERIMENTAL

Coal sample

The Tertiary brown coal was provided by the South Australian Department of Mines and Energy. It originates from the 75.3 m depth of the V122 core taken in the Port Wakefield area of South Australia. Fractionation procedure

Details of the procedure used to isolate humic acid and kerogen from the brown coal are provided in Verheyen and Johns (1981). Briefly, the coal was extracted with CHCI3/MeOH (2:1 v/v) and toluene/MeOH (3:1 v/v) by Soxhlet and sonication procedures. After removal of solvent both the combined extractable fractions and the insoluble residue were subsequently extracted separately with aqueous 0.5 N NaOH + 1% Na4P207 under N 2. Upon acidification of these extracts the organic solvent soluble acidic material (derived from the extract) and humic acid (derived from the organic solvent insoluble

residue) were precipitated and purified by ionexchange and dialysis, The residue from humic acid extraction was demineralised using HF/HC1 and washed with deionised water and CHC13/MeOH. The completely insoluble material was then termed kerogen. Techniques

Elemental analysis was performed under contract with the Australian Mineral Development Laboratories and the State Electricity Commission of Victoria (SECV). Solid state ~aC-NMR aromaticity data was obtained at 50.33 MHz using cross polarization, magic angle spinning (CP/MAS) techniques outlined below. Approximately 0.35 g of dried sample was contained in a zirconia rotor and spun at 3 kHz. The magic angle was set using internal KBr. A Bruker CXP 200 spectrometer was employed for the solid state NMR analyses using the following parameters: Relaxation delay Contact time Proton 90 ° pulse width Scan number Line broadening

2 sec 1.5 msec 5/zsec 103-3 × 103 50 Hz

Chemical shifts were measured relative to external adamantane and corrected to tetramethylsilane. Carbon aromaticity was calculated by chemical shift/area integration on conventional CP/MAS spectra and correcting for spinning side band intensity (Verheyen et al., 1984). Total side band suppression (TOSS) spectra were obtained using Standard Bruker software incorporating the Dixon sequence (Dixon, 1982). Infrared spectra were measured with a Hitachi 270-30 computer controlled spectrophotometer using the KBr pellet technique outlined in Verheyen and Johns (1981). Petrifluoroacetic acid oxidation was performed using the method described by Deno et al. (1978). The work up and fractionation of the perTFA oxidation products has also been published (Verheyen and Johns, 1983). A brief summary of the oxidation technique is included below: 0.5 g of sample (< 120 mesh) is reacted with perTFA (2 ml CF3 CO2 H + 4 ml H 2 0 2 + 2 m l H2SO4) for 16hr at 60°C in a small stirred reflux apparatus. Solid and liquid products were then separated and the total liquids analyzed by tH-NMR (Verheyen and Johns, 1983) using a calibrated internal reference capillary. Liquid oxidation products were further separated from the aqueous reagent into Neutrals, Acids 1 and Polar Acids 2 fractions by organic solvent extraction under basic and acidic pH. The polar Acids 2 fraction was isolated by adsorption/elution chromatography on Amberlite XAD-8 resin. The liquid oxidation product fractions were subsequently derivatised using BF3.MeOH and N,O-bis (trimethylsilyl) trifluoroacetamide. Gas chromatography and gas chromato-

Brown coal kerogen and humic acid

377

Table 1. V122 series analytical data % Bond equivalent composition

Elemental analyses

Weight % contribution to parent coal

H

Oa

N

S

Ash

C

H

O

Aromaticity fa b

66.9

5.3

22.6

0.6

4.6

11.2

71.77

11.06

11.17

0.52

I00.0

58.4

4.2

33.1

0.7

3.6

6.1

68.70

14.82

16.48

0.60

40.8

62.7

6.3

27.1

0.5

3.4

3.0

67.27

20.28

12.46

0.43

27.2

66.5

7.3

22.8

0.4

3.0

12.2

67.69

22.29

10.02

0.36

3.1

Sample

C

Coal Humic Acid Kerogen Solvent Soluble Acids

~Calculated by difference. ~Calculated by solid state ~3C-NMR as the percentage of the total carbon which is aromatic.

graphy/mass spectrometry were employed to analyse the mixtures using "on column" injection and nonpolar bonded-phase quartz capillary columns. Pyrolysis gas chromatography was completed via a Chemical Data Systems (C.D.S.) Pyroprobe system coupled to a gas chromatograph. The sample was flash heated at I°C •s e c -3 up to 700°C in quartz tube using helium as a carrier gas (Chaffee et aL, 1983). RESULTS A~D DISCUSSION

Coals from the Wakefield area on the Gulf of St Vincent have characteristically high sodium, chloride and sulphur contents (Muir and Krener, 1982). Bed moisture content is approx. 58% and is highly saline which contributes to the 11-12% ash yield listed in Table 1. Naturally, the coal does not contain ash but inorganic material present as both counter ions attached to acidic functional groups (Kiss and King, 1979) and as mineral matter dispersed throughout the coal matrix. The combined contribution of the three fractions investigated here to the parent coal totals 71.1% (Table 1). This value is lower than the average 80% value obtained for Victorian brown coals (Verheyen and Johns, 1981). The predominance of humic acid material in the composition of the coal places the coal towards the lighter end of brown coal lithotype classification (Verheyen and Johns, 1981). The unaccounted 28.9% of the parent coal is expected to be present as inorganic counterions, neutral lipid and fulvic acid material. Petrographic examination of brown coal and its kerogen and humic acid fractions reveals wide differences in their optical properties. The kerogen fraction is very heterogenous and concentrates the liptinite material present in the brown coal. The maximum vitrinite reflectance (R0.~x%) is significantly lower (0.18) than that of the parent coal (0.28). The humic acid fraction is optically homogenous with reflectance at the high end (0.3) of the range of reflectances found in brown coals. Elemental analysis

The elemental data in Table 1 are included in standard weight % form. The humic acid fraction is characterized by its high oxygen and low hydrogen

content. This implies, in gross structural terms, a reduction in hydrogen linkages and a significant increase in oxygen bonds. The kerogen structure contains intermediate oxygen and hydrogen contents though both are higher than in the parent coal. The organic solvent soluble acidic fraction, despite its being isolated via its solubility in aqueous base and insolubility at acidic pH, contains the lowest oxygen and highest hydrogen contents. The unit C, H, O structures of all the brown coal fractions contain less carbon than the parent coal. The carbon contents of the kerogen and organic solvent soluble acidic fractions are very similar, however, they are readily distinguished by the higher hydrogen and lower proportion of oxygen linkages in the unit structure of the solvent soluble acidic fraction. 13C - N M R Analysis

The aromaticity values (proportion of the total carbon which is aromatic) listed in Table 1 show a good correlation with hydrogen content. The kerogen and solvent soluble acidic fractions are both considerably more aliphatic than their parent coal with only 36% of the solvent soluble acid carbon being aromatic. To counterbalance this decrease in aromatic structural contribution to the parent brown coal, the humic acid fraction reports (Table 1) an aromaticity significantly higher (0.60) than that for coal (0.52). The 13C-NMR spectra presented in Fig. 1 allow an objective comparison of the skeletal carbon structure between the parent coal and its fractions. All the spectra exhibit considerable resolution associated with oxygen functional groups and consistent with the low rank of the brown coal. Carboxyl (170 ppm), phenolic (150ppm) and methoxyl (55ppm) resonances are present to varying degrees in all the spectra. The two dominant bands in the spectra are assigned to aromatic (130ppm) and aliphatic (30 ppm) carbons. The spectrum of the humic acid fraction (Figure 1) is notable for its high concentration of carboxylic and phenolic moieties. Polymethylene carbons identified by their sharp resonances (30ppm) in the other spectra are not resolved, suggesting only a minor contribution of this species (or that the substituents are short chain) in the humic acid structure. A broad resofiance centred at 50 ppm dominates the high field

378

T.V. VEarmYEr~et al. although phenolic carbon content appears low judging by the minor intensity of the 150 ppm band in the solvent soluble acid spectrum. Infrared analysis

The IR spectra of the samples are complementary to their ~3C-NMR data in displaying the expected broad bands (Fig. 2) associated with heterogenous low rank organic material. Infrared spectroscopy has the advantage of greater sensitivity towards functional groups especially those containing oxygen. The absence of an absorption band near 3030 cmand the presence of weak absorptions between 900-700 cm -t (excluding the solvent soluble acids) indicate that the aromatic carbons present in these structures are highly substituted (Durie et aL, 1960). The V122 coal spectrum in Figure 2 shows only a minor carboxyl (1710cm -1) absorption despite the significant carboxyl contribution to the 13C-NMR coal spectrum and a large 1600 cm -1 band. The high salt content of the Wakefield seam is consistent with carboxyl groups occurring as their carboxylate salts. This would explain the reduction in the 1710 cm -~ absorption and result in carboxylate absorption at 1580 cm -t .

I 3O0

I 2O0

I 100

I 0

I - 100

I -2OO

ppm

Fig. I. 50.33 MHz CP/MAS ~3C-NMR spectra obtained using spinning side band suppression of the V122 series. (1) Parent brown coal; (2) humic acid; (3) kerogen; (4) solvent soluble acid. re#on of the humic acid ~3C-NMR spectrum. This band is assigned to aliphatic carbons present in short chains which are bonded to oxygen molecules. The broad nature of this band is indicative of a complex distribution of carbon types all possessing slightly different structural environments. The comparison of kerogen and humic acid spectra (Fig. 1) reveals a significant reduction in carboxyl and phenolic groups and increase in polymethylene structure for the kerogen fraction. The kerogen spectrum has a resemblance to that of the parent coal, the former being distinguished by its relative decrease in carboxyl and polymethylene groups and increase in alkyl oxygen moieties (80-50 ppm). The organic solvent soluble acid fraction, as with the humic acid, is strikingly different in its ~3C-NMR spectrum relative to the parent coal. The spectrum of the solvent soluble acids is consistent with their average carbon structure being dominated by long chain carboxylic acids. The high contribution of carboxyl carbon (170 ppm), methylene carbon adjacent to carboxyl/carbonyl functions (41 ppm) and polymethylene (30 ppm) resonance bands support this analysis. Aromatic structures are also present

°-4 u

o ta ¢Z

4000

I

J

I

2000

10(30

400

Wovenumber (cm -1)

Fig. 2. Infrared absorbance spectra of the V122 series. (I) Brown coal; (2) humic acid; (3) kerogen; (4) solvent soluble acid.

Brown coal kerogen and humic acid The IR spectra of the humic acid in Fig. 2 contains a very broad hydroxyl band centred at ~ 3150 crncharacteristic of carboxylic acid groups involved in both intra- and intermolecular H-bonding. Hence, the aromatic 3030cm -~ band would not be seen unless of strong intensity. The dominant 1710 and 1630cm -l, but minor aliphatic 2920crn -t absorptions in the humic acid spectra suggest that its structure contains an aromatic backbone heavily substituted by carboxylic and hydroxyl groups. The solvent soluble acidic and humic acid fractions report similar intensities for carboxyl (1710 cm - ~), aromatic (1630 cm -~) and oxygenated (1000 cm -~) bands. The IR spectrum of the kerogen fraction is distinguished by its dominant carboxyl/carbonyl absorption and the high resolution in its aliphatic structural bands. The intense 1740 cm- ~ band in the kerogen is consistent with the presence of carboxylic acid methyl esters (this band increases markedly in intensity in the derivative of the parent coal when the coal is methylated with dimethyl sulphate). To facilitate spectral comparison, IR difference spectra are presented in Fig. 3 for the coal and Fig. 4 for the fractions. All possible subtraction combina-

379

c u .a to

I 4000

1 2000

Wavenumber m

__

I 400

( c r n "1 )

1

Fig. 4. Infrared V122 component difference spectra. (1) Humic acid--kerogen; (2) humic acid--solvent soluble acid; (3) kerogen--solvent soluble acid.

./ c

o ---

I 4000

I 100G

I 2(300 Wovenumber

I 1000

3

I 400

( c r n -1 )

Fig. 3. Infrared V122 brown coal difference spectra. (1) Coal--humic acid; (2) coal--kerogen; (3) coal--solvent soluble humic acid. Note that the ordinate scale is individually adjusted for each spectrum to produce comparably sized spectra.

tions for the coal and its individual fractions are covered in these two figures. Coal--humic acid (1). As expected, the difference spectrum illustrates by its strong negative absorptions the concentration of hydrogen bonding (acidic-OH) and carboxyl/carbonyl functional groups into the humic acid fraction. The strong negative CO2 peak at ~2400 cm -~ indicates a greater affinity for gaseous CO2 in the humic acid fraction than in the coal. The difference spectrum (1) contains an unusually strong negative band centred at 1200 cm-~. The structural origin of this symmetrical band is open to speculation but may describe an oxygenated linkage such as in an ester. Coal--kerogen (2). This difference spectrum confirms the greater aliphatic structural content of the kerogen fraction from its strongly negative aliphatic bands. The positive 3400 cm -~ shoulder suggests the parent coal is richer in intermolecularly hydrogen bonded species such as phenols. The kerogen appears to concentrate carboxyl/carbonyl moieties; however, the strong negative 1710cm -~ absorption is exaggerated due to the carboxylate content of the parent coal. Coal--solvent soluble acids (3). The appearance of this difference spectrum supports the view that the

380

T.V. V

structure of the solvent soluble acids is much less complex than the parent coal and is predominantly composed of aliphatic acidic compounds. Thus IR structural comparison between the brown coal and its fractions reveals distinct variation in their macromolecular composition. These structural variations can be further investigated with the IR difference technique by comparison between the humic acid, kerogen and solvent soluble acids fractions (Fig. 4). Humic acid--kerogen (1). The positive absorptions in this difference spectrum imply greater H-bonding and aromatic character in the humic acid. Both structures appear to have similar carboxyl/carbonyl contents. The kerogen produces strong negative bands indicative of greater aliphatic CH2 and aliphatically bonded oxygen content. The large positive 1600 cm -1 band is consistent with the humic acid structure containing a high degree of aromatic ring substitution by hydroxyl and carboxyl groups. These groups enhance the aromatic ring "breathing" mode responsible (Painter et al., 1983) for the 1600crn -t band. Humic acid--solvent soluble acid (2). As could be expected from the previous discussion, the humic acid structure is richer in all group resonance frequencies excluding aliphatic moieties. Kerogen--solvent soluble acid (3). These spectra are generally similar and as a result their difference spectra required higher ordinate expansion to achieve a size comparable with that of the other spectra. The difference spectrum contains a number of narrow spectral bands. The hydrogen bonding region (3750-2500 cm -1) has been divided into a number of positive and negative bands. This indicates that the functionality responsible for hydrogen bonding in the solvent soluble acidic component differs from those in the kerogen and contains a larger diversity of strongly hydrogen bonded species. The t3C-NMR data discussed previously suggest that these differences in hydrogen bonding are due to the higher carboxyl and lower phenolic carbon content of the solvent soluble acids relative to the kerogen. The aliphatic CH2, C H 3 bands are stronger in the solvent soluble acid fraction, however, the ratio of the two resolved components in the 2800cm -~ region is

~

et aL

different. The structure of the solvent soluble acids contains a higher proportion of carboxyl/carbonyl structures responsible for the strongly negative 1710 and 900cm -l bands. This high concentration of aliphatic acidic moieties and low phenolic content will assist in solubilising the fraction. In summary, the macromolecular techniques indicate that in comparison to the parent brown coal there is for the progression: Humic acid---, Kerogen--, Solvent soluble acid • an increase in C and H content at the expense of oxygen; • a decrease in weight percentage contribution of the respective fraction; • a decrease in aromaticity; • a decrease in phenolic content; • an increase in polymethylene groups; • a reduction in hydrogen bonding.

Proton NMR analysis of PerTFA oxidation products 1H-NMR analysis of the total reaction mixtures (after removal of inorganic matter by filtration) allowed quantitative data to be obtained on the concentration of succinic and malonic acids, as well as products such as acetic acid. These data, together with quantitative information on the general aliphatic resonance bands, are presented in Table 2. Reproducibility of these analyses was checked by duplicate work-up through the oxidation and 1H-NMR stages. Reproducibility as gauged by IH-NMR peak areas, is generally better than 10~o for the broad regions (due to baseline variation) and 5~ for sharp individual acid peaks. The separate samples were then combined for extraction and gas chromatographic analyses. ~H-NMR spectra of the aliphatic regions of the V122 sereis perTFA oxidation products are illustrated in Fig. 5. The spectra are not normalised; however, consideration of individual intensities relative to the internal reference capillary peak (0 ppm) allows comparison between spectra. The spectra highlight the concentrating of aliphatic structure in the coal into its solvent soluble acid and kerogen fractions. This observation agrees with independent

Table 2. Proton magnetic resonance data of V122 series coal and pertrifluoroacetic acid oxidation product mixtures (dry ash free basis) Weight percentagea Sample Coal Humic Acid Kerogen Solvent Soluble Acids

0.3-1.45 ppm

1.46--2.20 ppm

2.21-3.34 ppm

Total aliphatic

Acetic acid

Succinic acid

Malonic Total acid spectrum

11.3

8.8

8.6

28.7

0.9

--

0.7

30.3

5.4

6.9

11.7

24.0

4.5

1,3

0.5

30.3

10.3

11.7

12.1

34.1

2.8

1.7

0.8

39.4

15.3

12.2

10.9

38.4

3.8

1.2

0.3

43.7

~Weight percentage yield of hydrogen appearing in aliphatic regions of the proton NMR spectrum--relative to the total hydrogen content in the parent coal. - - N o t detected.

Brown coal kerogen and humic acid

381

D

I

I

I

4

2

o

Chemical shift (ppm)

Fig. 5. Staggered 200MHz IH-NMR spectra (aliphatic region) of V122 series pertrifluoroacetic acid oxidation product mixtures. (1) brown coal; (2) humic acid; (3) kerogen; (4) solvent soluble acid. Peaks identified in spectrum 4 are: (A) malonic acids; (B) succinic acid; (C) acetic acid; (D) TMS contained in calibrated internal reference capillary.

evidence provided by elemental analysis, 13C-NMR and IR data discussed previously. The spectral data in Table 2 are corrected for the differing total hydrogen contents of the samples. Hence, attenuation of 1H-NMR spectral intensity due to varying hydrogen concentration are eliminated. Listed below are the structural assignments for three broad chemical shift regions listed in Table 2. Chemical shift 0.3-1.45ppm 1.45-2.20 ppm 2.20-3.34 ppm

Assignment Primary and secondary protons RCI-I3, RCH2 R Tertiary protons R2CHR Benzylic acids and esters Ar--CH, RO2C--CHR2 Carbonyl compounds HCR2C(=O)R

Aryl methyl structures, which produce acetic acid on oxidation, are concentrated in the humic acid fractions (Table 2) and are less important in the kerogen. The decreased amount of aryl methyl or terminal methyl groups in the kerogen is consistent

with an increase in molecular weight as large molecules have a lower external surface/internal volume ratio. High molecular weight is a factor which would contribute to kerogen insolubility. Current views on the source of succinic and malonic acids (Jones, 1984) are that their structural origins remain unresolved. The preservation of long chain aliphatic structures in pertrifluoroacetic acid oxidations (Verheyen et al., 1985) suggests that succinic and malonic acids derive from short aliphatic segments within the coal matrix. The kerogen fraction provides the highest concentration of succinic and malonic acids (Table 3). This implies that the kerogen is relatively rich in short chain aliphatic "connectors". In contrast, the humic acid fraction reports the lowest production of succinic and malonic acids (Table 2) possibly indicating a relatively minor contribution of short aliphatic segments within its structure. The low yield of acetic and succinic acids from the parent brown coal relative to its fractions (Table 2)

Table 3. V122 pertrifluoroacetic acid oxidation products weight composition Sample

Insoluble products

Total neutrals

Acids I

Acids 2

Acetic acid

Succinic acid

Malonic acid

Total recovery

Coal PK H A Kerogen SE H A

3.0 -2.0 2.5

0.80 0.18 1.21 2.22

2.26 0.91 5.18 5.13

11.6 7.0 18.8 33.6

0.96 3.81 3.56 5.60

-1.62 3.19 2.61

0.10 0.06 0.09 0.06

19 13 34 52

- - N o t detected.

382

T.V. Vmtrm'WNet al.

suggests that the yield of individual perTFA oxidation products is dependent on more factors than the concentration of parent or source molecules. It is not likely that these individual concentrations would change during fractionation. However, their accessibility and proximity would be altered due to changes in the three dimensional coal network by solvent disruption. These structural changes would affect the efficiency of pertrifluoroacetic acid oxidation in the production and perhaps destruction of products. Examination of the proton concentrations present in polymethylene and methyl structures (Table 2) show that the kerogen and particularly the humic acid fractions contain less of these structures than the parent coal. These reductions result from a combination of the humic acid structure containing predominantly short alkyl chain species, and the prior removal by solvent extraction of the rich aliphatic extractable material (often referred to as bitumen) during kerogen isolation. The total concentration of aliphatic protons in the samples is consistent with earlier ~3C-NMR aromaticity assessments in that the more aromatic fractions produce lower concentrations of aliphatic structures in their oxidation products. The observation that between 30-44~o of the initial hydrogen content of the sample remains as aliphatic oxidation products supports the stated (Deno et al., 1978) selectivity of the perTFA as an oxidation reagent. Weight percentage composition To obtain further structural information from the oxidation product mixtures the reagent was removed and the products fractionated as outlined in the experimental section. The distribution of principal products is presented in Table 3. Total recoveries of non-volatile oxidation products varied from 13.6 to 51.6wt~ of starting material. This wide range in product yields reflects: (i) the aromatic content of the starting material; (ii) its structural susceptibility; (iii) accessibility to pertrifluoroacetic acid oxidation. This conclusion results from: (i) aromatic centres being the primary sites of perTFA attack; (ii) the nature of aromatic ring substituents affecting their degradability; (iii) that 3D structural configurations may block access by the reagent to internal sites. The sum of the total oxidation product recoveries from the fractions is less than total recovery when the whole coal is oxidised. The low oxidation product recovery from the humic acids is consistent with a considerable number of its aromatic carbons being linked to hydroxyl, carboxyl and other short chain structures containing heteroatoms. These activate the aromatic ring (Shadle and Given, 1982) to electrophilic attack by the reagent (OH + ) and result in their complete decomposition to oxalic acid, H20 and C02. The total neutral products were isolated by npentane extraction of the aqueous product mixture

after it was adjusted to pH 12. Hence they can be characterized as not containing acidic moieties which are not extractable under these conditions. The total neutral product concentrations (Tables 1 and 3) are dependent on the aromaticity of the starting material. The origin of this fraction was initially proposed to be "trapped" or solvent extractable moieties present in the parent structure (Johns et aL, 1984). Although the solvent soluble acidic fraction produces the highest concentration of neutral material, the insoluble kerogen (despite its rigorous isolation) also produces a significant amount. Therefore, the structural origin of this neutral material is not solely due to trapped extractable molecules, but also to those released by oxidative cleavage of the macromolecular network (i.e. one pathway would be cleavage of Ar-(CH2), R via destruction of the aromatic ring). Considering the mechanism of the perTFA oxidation (formation of carboxyl groups at the site of aromatic rings) it is not surprising that the Acids 1 fractions form a higher weight percentage of the oxidation products (Table 2) than the neutrals. The kerogen products are particularly rich in acidic material. The high Acids 1/ Neutrals ratio reported for the humic acid is consistent with the very low contribution of neutral material to the humic acid oxidation products. The quantitatively most important fractions within the product mixtures are the polar "Acids 2" material. These components from their content of polyhydroxylated and carboxylated short chain components, are thought to derive primarily from the oxidation of "skeletal" structures (Verheyen and Johns, 1983). It follows from the total recovery product data discussed previously that the solvent soluble acids produce the highest and the humic acids the lowest concentration of Acids 2 material (Table 3). The contributions of acetic, succinic and malonic acids were calculated from the IH-NMR data (Table 2). Methoxyl groups were considered (Deno et al., 1978) to produce methanol during perTFA oxidation. Traces of methanol are visible in the ~H-NMR spectra (3.7 ppm) of the fractions, but are absent from the parent coal spectrum. This observation is confirmed by ~3C-NMR (Fig. 1) in that methoxy carbons (55 ppm) are not resolved in the V122 brown coal spectrum but appear to concentrate in the kerogen fraction. Gas chromatographic~mass spectrometric analyses The complex mixture responsible for the broad NMR-aliphatic resonances illustrated in Figs 1, 2 and 5 and quantified in Tables 2 and 3 were analyzed by GC/MS. The generally small amounts of material available precluded further separation into individual molecular classes. The polar Acids 2 fraction does not produce satisfactory gas chromatograms due to the very high contribution of unresolved material which affects their baseline. These problems occur despite derivatization, which still does not render the material completely soluble in C H C I 3 . Despite these limit-

Brown coal kerogen and humic acid

383

ations the number of resolved components (primarily significant contribution to its structure. The humic aliphatic moieties) are significantly higher in the acid fraction produces relatively very minor amounts products from the kerogen and whole coal samples. of neutral material showing a similar distribution to Oxirane polycarboxylic acids were found in the Acids that from the other samples. These two conflicting 2 fraction from each sample, however, the poor results suggest that despite careful preparation/ quality of their chromatograms precluded further isolation and washing, traces of these neutral comanalysis. In contrast to the Acids 2 fraction, the ponents are adventitiously adsorbed into the humic Neutrals and Acids 1 fractions possess excellent acid structure. A c i d s - - l . As predicted by the data in Table 3, the chromatographic properties and are discussed in concentration of carboxylic acids (Table 4, Fig. 7) is detail below. Total neutrals. Gas chromatograms attenuated to much higher than that of alcohols in the perTFA bring the major individual neutral components oxidation products. Three major series of long chain to approximately the same height are presented in carboxylic acids are present: n-mono-carboxylic, Fig. 6. Labelled peaks are identified within the legend co-hydroxy monocarboxylic and n-dicarboxylic acids. and their concentrations included in Table 4. The Along with these aliphatic species, some aromatic chromatograms are dominated by two series of long carboxylic acids are also present, principally benzoic chain hydroxylated components, viz., n-alcohols and and phthalic acids. Aromatic structures, as in the n-diols. Minor series of diois in which one hydroxyl neutrals fraction, are found chiefly in the products of group is present at intermediate positions along the the humic acid fraction. The overall appearance of polymethylene chain were also observed by single ion the Acids-1 chromatograms shown in Fig. 6 is similar monitoring. Small contributions by C1-C3 phenols to those of the total Neutrals (Fig. 6); evidently the and C0-C2 dihydroxy benzenes are concentrated in humic acids are deficient, the kerogen similar and the solvent soluble acids rich in long chain acidic comthe products of the humic acid fraction. Several other groups have applied this oxidation to ponents. Blanks of the sample work-up procedure coals but none have been able to report the presence revealed only minor concentrations--<150ppm total of C~2-C~s n-monocarboxylic acids. These comof long chain alcohols as described here. This results from both the generally higher rank of coals in- ponents are thought to derive from minor contamination in both the oxidation and methylation vestigated (see Verheyen et al., 1985) and the author's reagents and could influence the bimodal distribution failure to isolate the minor concentration of neutral patterns as discussed later. oxidation products as a separate fraction. The kerogen fraction produces the highest yields The two Neutral series both exhibit bimodal distri(Table 4, Fig. 7) of acidic products in all series. This butions with distinct even/odd chain length predominance (Table 4, Fig. 6). The n-alcohols are con- dominance by the kerogen products is particularly centrated in the solvent soluble acid fraction predom- noticeable for the co-OH and n-dicarboxylic acids. inantly as > C20 chain length components. The humic The n-monocarboxylic acids show a bimodal distriacid fraction produces the lowest amount of bution, except in the solvent soluble acid fraction which has a single maximum at C28. In accord with n-alcohols on perTFA oxidation. The range and the neutrals pattern there is a distinct even over odd maxima for the alcohols are included in Table 4 and are virtually sample independent. The chromato- chain length predominance in the n-monocarboxylic acids. The ~o-hydroxy and n-dicarboxylic acids are grams of the neutral fractions from the kerogen and solvent soluble acids are distinguished from one much more restricted in their concentration, range another by the kerogen revealing no particular bias and maxima compared with the n-monocarboxylics with carbon chain length in the concentration of (Table 4, Fig. 6). Their maxima decrease to C~0-C~2 components. In contrast, the solvent soluble acids with a distribution range confined to C6-C26 comneutral product distribution is strongly biased to- pared with C8-C36 for the n-monocarboxylics. No wards long chain products (Fig. 6). even/odd predominance of vice-versa is found in The diol series data (Table 4, Fig. 6) produces a either the og-OH nor dicarboxylic acids. These observations suggest a different structural greater discrimination between the parent coal and its fractions. Again the carbon length range and maxima origin for the co-OH and n-dicarboxylic acids to that are relatively similar in the various samples. The for the monocarboxylics. Oxidative cleavage of neutral perTFA oxidation product mixture from longer chain material by perTFA is not a likely humic acids is notable for the near absence of explanation, a view confirmed by comparison of the c¢,co-diols. The solvent soluble acids fraction again V122 coal pyrogram shown in Fig. 8 with the chroproduces the highest concentration of diols. This matograms of its perTFA oxidation products (Figs 6 behaviour confirms that the trapped or solvent ex- and 7). The pyrogram exhibits a very similar bimodal tractable material from V122 brown coal is domi- carbon chain length distribution to the perTFA nated by long chain material as implied by macro- oxidation products. Both chromatograms have molecular techniques discussed previously. The maxima in the C~4 and C28 regions despite the obvious kerogen fraction produces a higher proportion of differences due to volatiles, aromatics and alkene shorter chain length species indicative of their production during pyrolysis.

384

T . V . VBtH]EYENet al.

17

li

19

21

25

t5

I

10

.

8 3

4

5

6

9

23

12 13 1,4

26

11

7

{1)--

4. o

8 == (2)-D E3

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B

LL

H

I I L I

E

(3) --J

K

J

!1 LILJil IIUIIIJII/Eq~, 'I('~'~L

( 4 ) -J L

Retention

time

Fig. 6. Expanded gas chromatograms of the total neutral product fractions derived from the pertrifiuoroacetic acid oxidation of the V 122 series: (I) brown coal; (2) humic acid; (3) kerogen; (4) solvent soluble acid. Components labelled: 1-26 were identified as a consecutive series of n-alcohols with a carbon number range of (1) C8-(26) C33. A - Y were identified as a consecutive series of terminally hydroxylated n-diols with a carbon number range of (A) C4-(Y) C2s.

Brown coal kerogen and humic acid

385

17

15

11

19

21 23

2~ 16

13

2

s

1/.

I

[2/, 25

12

(I)

C 8Q. (2)

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time

Fig. 7. Expanded gas c h r o m a t o g r a m s of the total Acids-1 fractions as in Fig. 6. C o m p o n e n t s labelled: 1-26 were identified as a consecutive series of n-monocarboxylic acids with a carbon n u m b e r range of (1) C8-(26 ) C33. a - n were identified as a consecutive series o f n-terminally hydroxylated n-monocarboxylic acids with a carbon n u m b e r range o f (a) Cs-(n) C2~. A - P were identified as consecutive series o f n-dicarboxylic acids with the carboxyl groups located at the opposite ends of the polymethylene chain with a carbon range o f (A) C9-(P ) C24. Benzoic acid and m-phthalic acids can be identified by - and = respectively.

386

T. V. VERI-mYENet al. Table 4. Gas chromatographic analyses--V122 oxidation products Total neutrals

Sample Coal Humic acid Kerogen Solvent soluble acid

Range 8-36 8-34 8-36 8-36

n-Alcohols M a x i m u m Concentration 10 & 26 1626 12 & 28 225 12 & 30 1619 10 & 28

4850

Range 5-30 5-30 4-34

(=,to)n-Diols M a x i m u m Concentration 9 & 22 375 12 & 21 101 12 & 21 2276

5-30

7 & 21

4478

Acids-I

n-Carboxylic acids Range Maximum Cone. 8-34 16 & 28 3162 8-32 16 & 28 1086 8-34 18 & 28 15820

to-Hydroxy n-carboxylic acids R a n g e Maximum Cone. 6-18 10 472 6-18 11 403 6-24 11 4309

n-Dicarboxylic acids Sample Range Maximum Cone. Coal 7-20 12 699 Humic 7-20 11 422 acid Kerogen 7-26 12 4301 Solvent soluble 8-36 28 14463 6-17 10 644 10-20 12 844 acids Note: the experimental work-up procedures did not allow accurate quantitation below C~-alcohol and monocarboxylic acid C6--OH and C9 dicarboxylic acids. Range and maximum refer to carbon chain length. Concentration is expressed as #g/g DAF coal basis.

Product distributions from the p e r T F A oxidation o f the V122 coal are markedly different from those reported earlier (Verheyen and Johns, 1983) for a Victorian brown coal. These differences are thought to result from an unusual maceral composition for the Victorian coal resulting in a virtual absence of long chain ( > C20) p e r T F A oxidation products. However, several other Victorian brown coals (Johns et al., 1984) have produced similar product distributions to V122 coal. CONCLUSIONS The results of the various characterization procedures used here provide in combination, consistent data on the role of aliphatic materials in the struc-

o L.,j tD

~

~

tural differences between V122 brown coal and its humic acid and kerogen fractions. Both the solvent extractable acidic and kerogenous coal fractions are distinguished from the humic acid by their high aliphatic contents. The aliphatic structure of the solvent soluble acids is biased towards long chain moieties whilst the aliphatic portion o f the kerogen has a wide distribution of carbon lengths. The higher aromatic carbon content of the kerogen fraction also distinguishes it from the solvent soluble acids. The aromatic kerogen carbon possibly derives from heavily degraded lignin moieties and microbially induced aromatization of aliphatic substrates. The p o o r resolution of phenolic groups in the kerogen 13C-NMR spectrum supports an absence of direct lignin input to the kerogen fraction. The well

m

'~

O

N

~

e~

C3

Retention

time

Fig. 8. Pyrolysis-gas chromatogram of V122 brown coal. Numbered peaks identify the carbon chain length of the alkane/alkene pairs. Alphabetically labelled peaks are identified as: (a) C]-C 5 hydrocarbon gases; (b) toluene; (c) xylenes; (d) C2-benzene; (e) phenol; (f) C3-benzene; (g) indane; (h) Crphenol; (i) C2-phenol.

Brown coal kerogen and humic acid resolved t 3 C - N M R peak at 5 1 p p m and strong I R 1740 cm-~ band in the kerogen spectra are assigned to methyl ester functional groups. Hence the insolubility of kerogenous material in aqueous base and organic solvents probably results from its relative lack of phenolic groups, high polymethylene content, the presence of carboxylic functionalities as methylesters and its large molecular weight. These features are consistent with a significant input of cutin, suberin and sporopollenin to its organic source material. H u m i c acids make a major contribution to the V122 brown coal structure. They are significantly more aromatic than the whole coal and contain higher concentrations of acidic hydroxyl in the form of phenolic and carboxyl groups. On p e r T F A oxidation the humic acid structure is heavily degraded and the observed traces of long chain aliphatic products are not structural components of this fraction. These structural features are consistent with degraded lignins being the organic source for humic acid structure. A major conclusion we would draw from our data is that the humic acid and kerogen components are so structurally dissimilar that their interconversion and particularly the conversion of humic acid to kerogen is virtually impossible at the brown coal maturation stage. During coalification the fate of these two d o m i n a n t sedimentary organic matter fractions are most probably still independent. Defunctionalisation of the humic acids and aliphatic cracking within the kerogen ultimately produce an amorphous aromatic carbon. Acknowledgements--The authors wish to thank the South

Australian Department of Mines for the provision of the coal sample; D. Cookson and P. Lloyd of B.H.P. Melbourne Research Laboratories for access to NMR facilities; R. Western of C.S.I.R.O. Division of Materials Science for access to GC/MS facilities and NERDDC for financial support. P. Given and R. Ishiwatari are thanked for their review of this manuscript and the provision of some unpublished data. REFERENCES

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