In situ analytical pyrolysis of coal macerals and solid bitumens by laser micropyrolysis GC–MS

Journal of Analytical and Applied Pyrolysis58–59 (2001) 237–253

In situ analytical pyrolysis of coal macerals andsolid bitumens by laser micropyrolysis GC–MS

Paul F. Greenwood a,*, Simon C. George a, Walter Pickel a,Yangming Zhu b, Ningning Zhong c

a CSIRO Petroleum, PO Box 136, N. Ryde, NSW 1670, Australiab Petroleum Geochemistry Research Center, Jianghan Petroleum Uni6ersity, Jingzhou,

Hubei 434102, People’s Republic of Chinac Applied Geochemistry Laboratory, Uni6ersity of Petroleum, Changping,

Beijing 102200, People’s Republic of China

Received 5 May 2000; accepted 2 November 2000

Abstract

Specific macerals in coals and distinct solid organic bitumens within limestones fromnorthern China were separately analysed in situ by laser micropyrolysis gas chromatogra-phy–mass spectrometry. The major pyrolysates from the resinite and cutinite maceralsincluded aromatic hydrocarbons such as alkylnaphthalenes, alkylbenzenes, alkyltetralins (inthe resinite) and alkylphenols (in the cutinite), and aliphatic hydrocarbons such as n-alk-1-enes, n-alkanes, pristane and phytane. The same classes of hydrocarbons were also detectedin co-occurring vitrinites, including very similar relative distributions of alkylnaphthalenes inthe case of the resin-rich coal. Several observed quantitative differences are consistent withprevious molecular data from maceral concentrates. The qualitative similarity of the maceraldata suggest (1) extensive assimilation of free hydrocarbons by all macerals within the coals;or (2) inefficient targeting of the macerals by the laser, although microscopic observationshowed that the laser craters were generally constrained within surface boundaries of themaceral. The organic composition of a solid bitumen within a Carboniferous limestone wascharacterised by a distinctive distribution of C0–C4 alkyldibenzofurans, as well as alkylben-zenes, alkylnaphthalenes and a low molecular weight distribution of n-alk-1-enes andn-alkanes. These data suggest the solid bitumen comprises a strongly aromatic matrix withterrestrially-derived oxygen functionality and alkyl linkages. The same aromatic andaliphatic hydrocarbon products were also detected in the morphologically homogenous

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* Corresponding author. Present address: Isotope and Organic Geochemistry Laboratory, AustralianGeological Survey Organisation (AGSO), GPO Box 378, Canberra 2601, Australia.

E-mail address: [email protected] (P.F. Greenwood).

0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0165-2370(00)00208-4

238 P.F. Greenwood et al. / J. Anal. Appl. Pyrolysis 58–59 (2001) 237–253

carbonates hosting the solid bitumens, indicating the presence of sub-microscopic organicmoieties throughout the limestone. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Analytical pyrolysis; Laser pyrolysis; Micropyrolysis; Gas chromatography–mass spectrome-try; Organic petrography; Coal maceral; Solid bitumen

1. Introduction

Many different organic entities arising from a vast array of precursory plantmaterial are typically present in organic-rich sediments such as coals. The heteroge-neous nature of coals is apparent from the large variety of optically homogenousaggregates called macerals [1,2]. Sedimentary rocks also occasionally contain solidbitumens (also called migrabitumens) which are described as solid, amorphous,asphaltic constituents [3]. They can be produced by thermal cracking of oil, by thedeasphalting of oil by gas, or by biodegradation, and are subject to maturitychanges on subsequent burial [3].

Analytical pyrolysis has been used extensively to provide useful molecularinformation from coals and kerogens [4–7]. Whilst it can be considered as aminimally destructive technique (i.e. analysis can be performed on as little asmilligram amounts of material), analytical pyrolysis is typically applied to bulksamples and does not account for the heterogeneous morphology of organicsediments. Various isolation techniques have been used in conjunction with analyt-ical pyrolysis to study chemically and/or physically derived sample fractions.Physical isolation methods include simple, but tedious, hand-picking of discretecomponents from complex organic materials, and density gradient centrifugation[2,8–10], a technique which has had reasonable success in producing pure maceralconcentrates based on their density characteristics.

Laser microprobes have recently been used in combination with various tradi-tional analytical methods to address the physical heterogeneity of complex organicmaterials. Laser micropyrolysis gas chromatography–mass spectrometry (GC–MS)is one of these new techniques and it has been developed to facilitate the molecularanalysis of microsample quantities of organic matter [11–15]. The laser micropyrol-ysis GC–MS apparatus used for the present study [14,15] offers the followingadvantages over conventional pyrolysis methods: (1) the directional nature of thelaser output can be focussed to tightly constrained, localised areas to pyrolyse verysmall samples or very small parts of samples; and (2) the custom-built inlet systemwas designed for optimum transfer of pyrolysates to the GC and provides a highlevel of detection sensitivity. The molecular integrity of laser pyrolysis data hasbeen established by scrutiny against conventional pyrolysis data obtained fromcomparative samples [12–15].

To test the spatial resolution and in situ analytical capabilities of laser micropy-rolysis GC–MS, the technique was used here to molecularly characterise severalorganic microparticles. Macerals in coals were analysed and compared to theorganic composition of the co-occurring vitrinite matrix. Two solid bitumens in

239P.F. Greenwood et al. / J. Anal. Appl. Pyrolysis 58–59 (2001) 237–253

Chinese limestones were also analysed and compared to the morphologicallyhomogenous carbonate matrix.

2. Experimental

2.1. Sample details

Two Chinese coals of different age and composition were analysed. These were acoal from Fushun (Liaoning Province, N.E. China; Eocene; Rf=0.55%, Tmax=429°C) and a coal from Hequ (Jiuxian, Shanxi Province, N. China; Carboniferous;Rf=0.73%). In situ analysis was performed on the resinite and vitrinite macerals ofthe Fushun coal and the cutinite and vitrinite macerals of the Hequ coal. Severalrelatively large solid organic bitumens within Carboniferous limestones fromPingdingshan, Henan Province, N. China were also analysed.

Small cylindrical cores (�5 mm diameter×3 mm long) were cut from largerblocks of the coal and limestone samples. One surface of each core was handpolished to assist with petrographical examination.

2.2. Laser micropyrolysis gas chromatography–mass spectrometry

The main components of the instrument are a laser microprobe and a GC–MS,which are interfaced via an intricate gas inlet system. The laser microprobe consistsof a laser applications continuous wave Nd:YAG laser (using the l064 nm nearinfrared fundamental beam) and an Olympus reflected light/fluorescence micro-scope. The microscope enables sample viewing, selection and pyrolysis. The sampleis housed in a sample chamber external to the GC. The laser gains access to thechamber and the sample inside through a window on top of the chamber. The areaof the sample to be pyrolysed is targeted by appropriate manual positioning of thepyrolysis chamber under the microscope objective.

The samples were pyrolysed with laser energy of �10–25 Ws−1 for periods of�5–10 s. The surface area of the pyrolysis craters increased with increasing laserenergy and ranged from less than 50 mm diameter to greater than 400 mm diameter.The spatial resolution of the pyrolysis was further controlled by selection of themicroscope objective. The microscope is equipped with l0× , 20× and 50×objectives; pyrolysis resolution increasing with magnification such that the smallestcraters were generated with the 50× objective. The resinite macerals and severalcutinite macerals were of sufficient size (0.1–1 mm) to accommodate relatively largelaser pyrolysis craters. Larger craters are preferred where possible as they result ingreater abundance of pyrolysates. Detection sensitivity is of considerable concernwhen analysing such small amounts of sample. Less intense laser conditions wereused to selectively analyse the smaller sized solid bitumens (diametersB100 mm).

The inlet system uses an elaborate two-flow routine encompassing an initial fastflow (\100 ml min−1) to efficiently transfer the pyrolysates to an external coldtrap followed by a slower flow (1.5 ml min−1) more suitable for GC. A 6-port


switching valve allows selection of the flows. More detailed descriptions of theapparatus and methodology can be found elsewhere [14,15].

Two different GC–MS instruments were used throughout the course of thisstudy. These were (1) a HP 6890 GC interfaced to a HP 5973 mass selectivedetector, and (2) a HP 5890/Series II GC interfaced to a Micromass-AutoSpecUltimaQ mass spectrometer. A 25 m DB-5 capillary column (5% phenyl methyl-polysiloxane, 0.32 mm i.d., 0.52 mm film thickness) was used on both GCs. Atypical GC oven temperature programme was an initial temperature of 40°C, heldfor 2 min, then increased at 4°C min−1 to a final temperature of 300°C, held for 25min. Full scan mass spectra (m/z 50–550) were obtained using standard detectionparameters (electron energy=70 eV; source temperature=250°C). Additional Au-toSpec GC–MS parameters were: filament current=200 mA; electron multiplier=200 V; mass resolution=1000.

3. Results and discussion

3.1. In situ analysis of the resinite and 6itrinite macerals in the Fushun coal

The total ion chromatograms (TICs) from the in situ laser micropyrolysisGC–MS analysis of resinite and the co-occurring vitrinite in the Fushun coal areshown in Fig. 1(a) and (b), respectively. The pyrolysis distributions from both theresinite and vitrinite macerals are qualitatively similar, with alkylnaphthalenes andalkyl-1,2,3,4-tetrahydronaphthalenes (i.e. alkyltetralins) dominating each analysis.The major aromatic products are listed in Table 1 with assignments based on GCretention time and/or 70 eV MS data. Several polycyclic aromatic hydrocarbonsdetected in high abundace (e.g. indene, acenaphthalene) are probably artefacts ofthe fast pyrolysis mechanism. A homologous series of n-alk-1-enes/n-alkanesextending out to �C30 is also evident in the vitrinite data.

Plant resins are composed of a rich array of mono- and sesqui-terpenoid basedbiopolymers. The molecular nature of extant and fossil resins has attracted consid-erable attention [16–26] from researchers who have often utilised data fromanalytical pyrolysis methods. A classification scheme based on the polymericstructure of resins and resinites has been developed [18,20]. Type 1 resins are mostcommon and consist of polymers based on a diterpenoid labdatriene monomer (e.g.labdane, abietane and pimarane). These resins mostly originate from gymnosperms.Type 2 resins contain sesquiterpenoid polymers based on a cadinane skeleton andmostly originate from angiosperms.

On maturation, the indigenous aliphatic biopolymers undergo several structuralchanges. Clifford and Hatcher [26] studied the resinite in three coals of increasingage (Miocene to Eocene) and maturity (lignite B to subbituminous). Over thismaturity range the terpenoid biopolymers experienced olefinic reduction (losing oneof their original two double bond groups) and lose carboxylic acid and alcoholfunctionalities. Extensive cyclisation and cross-linking reactions produce a tightlybound, multicyclic aliphatic network. An increased abundance of alkyltetralins was


also observed with increasing maturity, indicating the occurrence of aromatic-yield-ing condensation processes. The present resinite data similarly showed a highconcentration of alkyltetralins (Fig. 1(a)), as well as a very high concentration ofalkylnaphthalenes. A high abundance of alkylnaphthalenes have been previouslydetected in the liquid hydrocarbon products from the artificial maturation ofresinite [27]. Increasing aromatisation has been proposed to account for thedecreased abundance of alkyltetralins observed with increasing maturity in otherhigher maturity sequences [27,28].

Selected ion chromatograms specific to the C1–C4 alkylnaphthalenes (m/z 142+156+170+184), C2–C4 alkyltetralins (m/z 160+174+188) and C9–C27 n-alkanes(m/z 57) are shown in Fig. 2 for both the resinite and vitrinite analysis. The relative

Fig. 1. TICs from the in situ laser micropyrolysis GC–MS analysis (25 W for 5 s; 20× objective) of (a)resinite and (b) vitrinite macerals of Fushun coal. Numbered peak assignments are listed in Table 1,Cn=n-alkane.


Table 1Aromatic products detected from in situ pyrolysis of resinite, cutinite and vitrinite macerals in Chinesecoals and the solid organic bitumen in a Chinese limestone

Product assignmentPeak

Benzene12 Toluene

C2-Alkylbenzene3Styrene4C3-Alkylbenzene5Phenol6

7 MethylsterateIndene8Cresol9Methylindene10Naphthalene11C2-Alkyltetralins122-Methylnaphthalene13a1-Methylnaphthalene13b2-Ethylnaphthalene14c1-Ethylnaphthalene14d2,6- and 2,7-Dimethylnaphthalene14e1,3- and 1,7-Dimethylnaphthalene14f

14g 1,6-Dimethylnaphthalene1,4- and 2,3-Dimethylnaphthalene14h1,5-Dimethylnaphthalene14i1,2-Dimethylnaphthalene14j

15 AcenaphthaleneC3-Alkyltetralins16C4-Alkyltetralins171,3,7-Trimethylnaphthalene18k1,3,6-Trimethylnaphthalene1811,3,5- and 1,4,6-Trimethylnaphthalene18m2,3,6-Trimethylnaphthalene18n1,2,7-, 1,6,7- and 1,2,6-Trimethylnaphthalene18o1,2,5-Trimethylnaphthalene18p1,2,5,6- + 1,2,3,5-Tetramethylnaphthalene19Phenanthrene20Methylphenanthrenes21

proportions of these products in the resinite and vitrinite are compared in Fig. 3,which shows the summed peak area integration values from the representative ionchromatograms shown in Fig. 2. All peak areas were first normalised to the m/z 128area of naphthalene, which was consistently detected in very high abundance.

Clearly, almost identical distributions (Fig. 2(a) and (b)) and similar relativeamounts (Fig. 3) of alkylnaphthalenes are present in both macerals. The mostabundant alkylnaphthalene isomers are 1,6-dimethylnaphthalene, 1,2,5-trimethyl-naphthalene and 1,2,5,6-tetramethylnapthalene (which may be co-eluting with1,2,3,5-TeMN) [29]. 1,6-dimethylnaphthalene is a characteristic monomeric isomer


formed during the catagenetic breakdown of polycadinene, which is present indammar resins derived from angiosperm trees [24]. 1,2,5-trimethylnaphthalene and1,2,5,6-tetramethylnaphthalene have a more ubiquitous occurrence (e.g. Ref. [30]),although they have been detected in high abundance in resinite-rich coals [31].

Fig. 2. Partial mass chromatograms from the in situ laser micropyrolysis GC–MS analysis of Fushuncoal (conditions as in Fig. 1), showing (a)–(b) alkylnaphthalenes (summed m/z 142+156+170+184);(c)–(d) alkyltetralins (summed m/z 160+174+188); and (e)–(f) n-alkanes (m/z 57) of resinite andvitrinite macerals. Numbered peak assignments are listed in Table 1, Cn=n-alkane, Tn=Cn-alkylte-tralins.


Fig. 3. Bar graph showing �(m/z 160+174+188), �(m/z 142+156+170+184) and �(m/z 57) fromthe resinite and vitrinite maceral analysis of Fushun coal shown in Figs. 1 and 2 to compare the relativeamounts of alkylnaphthalenes, alkyltetralins and n-alkanes in these macerals. All product abundanceswere determined by peak area integration and normalised to the abundance of naphthalene.

The similar alkylnaphthalene signatures detected in the vitrinite and resinitemacerals indicates resinous input into the vitrinite. This could be due to extensiveassimilation by the vitrinite of free hydrocarbons generated from the resinitemaceral or from an indigenous resin content of the precursor wood. Resins havebeen shown to generate liquid hydrocarbons at relatively low maturity levels, suchas that of the Fushun coal (R0=0.55%) [16].

In contrast to the alkylnaphthalene distributions, however, the distributions (Fig.2(c) and (d)) and relative quantity (Fig. 3) of the alkyltetralins detected from theresinite and vitrinite macerals are noticeably different. A considerably largerproportion of alkyltetralins was detected in the resinite. The assignment of individ-ual isomers was not attempted for these products, nevertheless, qualitative differ-ences are readily evident amongst the C2–C4 alkyltetralin distributions (Fig. 2(c)and (d)) in the two macerals. The resinite shows a major C3 alkyltetralin, and amore extensive array of C2 and C4 alkyltetralins. A major C4 alkyltetralin (assignedas 1,1,5,6-tetramethyl-1,2,3,4-tetrahydronaphthalene) has previously been reportedfrom the analysis of resin-rich coals [31].

The reduced concentration of alkyltetralins in vitrinite suggests these compoundsare not liberated as freely from the resinite as other hydrocarbons such as thealkylnaphthalenes. The alkyltetralins may be predominantly kerogen bound andnot as mobile as free hydrocarbons, which may readily assimilate with the vitrinite


and other coal moieties. It may be that complete aromatisation of bound alkylte-tralins to form alkylnaphthalenes occurs simultaneous to their cleavage from thekerogen macromolecule.

A prominent series of n-alkanes was detected in the vitrinite pyrolysate (Fig.2(f)), indicating significant aliphatic input. In comparison, a significantly lowerconcentration of n-alkanes (Fig. 2(e) and Fig. 3) was detected in the resiniteanalysis. It has been previously demonstrated by artificial maturation experimentsthat resins produce a light, non-paraffinic oil [16]. The n-alkanes detected in theresinite analysis may have originated from extraneous organic material. Freealiphatic hydrocarbons generated from the vitrinite and possibly other maceralswithin the coal may have adsorbed onto the surface of the resinite maceral.

A similar qualitative similarity was previously observed in the pyrolysates fromseveral different maceral concentrates isolated from a bituminous coal, with somehydrocarbon families being consistently detected with very similar distributionpatterns [10]. Whilst the similarity of this data may in part have been due to theimpurity of the isolation procedure, it was also suggested that the maceralsexperienced some mixing during diagenesis. It was proposed that free hydrocarbonsgenerated from certain macromolecules may have become intimately associatedwith each maceral, and/or low molecular weight hydrocarbons may have reacted toform microscopically unidentifiable geopolymers throughout the coals [10]. Intra-seam homogenisation of mobile organic compounds has also been demonstrated inCarboniferous coals [32].

An alternative explanation for the extraneous n-alkanes in the resinite analysis isthat a small proportion of vitrinite (or another coal maceral) may have also beenhit by the laser and included in the analysis. A photomicrograph with UVexcitation of the resinite after pyrolysis is shown in Fig. 4. The laser crater isobserved to be constrained within the two-dimensional surface boundaries of thefluorescing resinite maceral; however, penetration of the laser crater through theresinite maceral into other organic moieties below cannot be completely ruled out.

3.2. In situ analysis of the cutinite and 6itrinite macerals in the Hequ coal

The TICs from the in situ laser micropyrolysis GC–MS analysis (16 W s−1 for5 s, 20× objective) of cutinite and vitrinite macerals in the Hequ coal are shownin Fig. 5. The respective maceral data again reflects a high qualitative similarity.Both analyses show high abundances of alkylbenzenes and alkylnapthalenes, withlesser amounts of alkylphenols and a homologous series of n-alk-1-enes/n-alkanes.The n-aliphatics have a bimodal distribution centred on �C13 and �C26, the latterof which is more pronounced in the cutinite analysis (Fig. 5(a)).

Whilst the same types of aromatic and aliphatic products have been previouslydetected in extant cuticular material [2,33–37], fossilised cutinite typically has asignificantly more aliphatic-rich composition [2,33,34,37–39]. The aliphatic natureof cutinites increases with maturation and this was proposed to be due to theselective preservation of a highly resistant non-saponifiable polymethylenic biopoly-mer. Such a biopolymer is thought to be an indigenous constituent of cuticular


plants [2,33,34,39], which was more diagenetically stable than intercellular mem-brane moieties such as cutin (insoluble polyester) and wax (soluble lipids). How-ever, there is now some conjecture about the presence of a non-saponifiablepolymethylenic moiety in all plant cuticles after little evidence for such a biopoly-mer was detected in cuticles separated from recent conifers and Ginko leaf[36,37,40]. As an alternative explanation to account for the increased aliphaticity offossilised cutinite it was proposed that cutin monomers (e.g. waxes) may beincorporated into the kerogen macromolecule during coalification [36,37].

The relative proportion of the major pyrolysate classes detected in the cutiniteand vitrinite macerals of the Hequ coal are compared in Fig. 6. The abundance ofthe C2 alkylbenzenes was calculated by integration of m/z 91, the C1–C2 alkylphe-nols by integration of m/z 94+107, the C1–C2 alkylnaphthalenes by integration ofm/z 142+156 and the C9–C29 n-alkanes by integration of m/z 57. All productabundances were again first normalised to the m/z 128 abundance of naphthalene.

The abundance of n-alkanes in the cutinite is almost twice that in the vitrinite,indicating a greater aliphatic nature. This is consistent with previous studies ofcutinite which have shown this maceral to be highly aliphatic [2,33,34,37–39].However, in consideration of these previous studies on fossilised cutinite a morehighly aliphatic dominated distribution was anticipated from the analysis of theHequ coal cutinite. The relatively high proportion of aromatic pyrolysates in thecutinite analysis (Fig. 5(a)) suggests significant input of organic material foreign to

Fig. 4. A photomicrograph under UV flourescence excitation of a large resinite maceral in Fushun coalafter pyrolysis with a laser energy of 25 Ws−1×5 s, 20× objective. Photo width =400 mm.


Fig. 5. TICs from the (16 W 5 s; 20× objective) in situ laser micropyrolysis GC–MS analysis of (a)cutinite and (b) vitrinite macerals of the Hequ coal. Numbered peak assignments are listed in Table 1,Cn=n-alkane. * = contaminant.

the cutinite macromolecule. The Hequ coal is moderately mature (Rf=0.73%), sosome hydrocarbon generation from other coal moieties would be expected; thesefree hydrocarbons may have adsorbed onto the surface of the cutinite maceral.Alternatively, inclusion in the laser analysis of vitrinite or another coal macerallocated below the surface of the cutinite may be a more reasonable assumption.

The product comparison in Fig. 6 also suggests a slightly higher alkylphenol andalkylnaphthalene content in the cutinite. In consideration of the results from mostprevious cutinite studies this is more difficult to rationalise than the typicallyobserved aliphatic enrichment. However, intact cuticles isolated from spruce needleswere shown to contain a lignin-type biopolymer [41] and alkylphenols can result


from the coalification of lignin [7,42]. Alternatively, alkylphenol and alkylnaph-thalene pyrolysates may have been contributed to by pyrolysis of an additionalmaceral to the cutinite targeted in this analysis. In this case, the other maceralcannot be vitrinite, which contains a lower abundance of these products (Fig. 6).

3.3. In situ analysis of a solid organic bitumen and the carbonate matrix of thePingdingshan limestone

The TIC from the laser micropyrolysis GC–MS analysis of both a large solidorganic bitumen in the Pingdingshan limestone and the surrounding carbonatematrix is shown in Fig. 7. The solid bitumen was irradiated with an energy of 14Ws−1 whilst the limestone analysis combined the products from three separate lasershots at 20Ws−1. A photomicrograph (in white light illumination) of the solidbitumen before and after pyrolysis is shown in Fig. 8. The second laser crater in thetop right-hand corner of the photomicrograph corresponds to the analytical datashown in Fig. 7(a). Interestingly, the laser craters in the solid bitumens areoblong-shaped. The laser obviously pyrolyses the solid bitumen more efficientlythan the surrounding carbonate matrix, indicating better heat propagation throughand/or greater volatility of the organic material.

Fig. 6. Bar graph showing �(m/z 91), �(m/z 94+107), �(m/z 142+156) and �(m/z 57) from thecutinite and vitrinite maceral analysis of the Hequ coal shown in Fig. 5 to compare the relative amountsof alkylnaphthalenes, alkyltetralins and n-alkanes in these macerals. All product abundances weredetermined by peak area integration and normalised to the abundance of naphthalene, which wasconsistently detected in very high abundance.


Fig. 7. TICs from the in situ laser micropyrolysis of Pingdingshan limestone. (a) A solid bitumen witha laser energy of 14 Ws−1 and (b) the homogenous carbonate matrix, by three laser shots at 20 Ws−1.Numbered peak assignments are listed in Table 1, Cn=n-alkane, Cn-DBF=Cn-alkyldibenzofurans. *= contaminant.

The TIC of the solid bitumen shows a simple distribution of low molecular weightalkylbenzenes, alkylnapthalenes and n-alkanes (Fig. 7(a)). A distinct distribution ofalkyldibenzofurans are observed at higher retention times. The alkyldibenzofurandistribution is better observed from the summed mass chromatogram shown in Fig.9. The identification of these aromatic products was made from accurate massmeasurement (at mass resolution of 1000) and GC retention times which alloweddistinction from alkylbiphenyl compounds with the same nominal mass. Alkyldiben-zofurans are not as widespread in the sedimentary record as the other aromaticproducts, and their detection in the solid bitumen in relatively high abundances issuggestive of a strongly aromatic matrix with oxygen functionality and alkyl linkages.It has been proposed that alkyldibenzofurans usually prevail over alkyldibenzothio-phenes in source rocks deposited in a freshwater environment [43,44]. Further, the


Fig. 8. A photomicrograph of the large solid bitumen in the Pingdingshan limestone (a) before and (b)after pyrolysis with a second laser event (14 W s−1×5 s, 20× objective). The crater in the bottom leftcorner was from a previous analysis. Photo width= l mm.


oxygen in alkyldibenzofurans is likely derived from terrestrial source material ratherthan being incorporated upon deposition in an oxic environment [45].

The same product classes are also observed in the analysis of the carbonate matrixof the limestone (Fig. 7(b)). However, the limestone experienced less intense pyrolysis(i.e., smaller craters and less visually observed luminescence) so the analysis wasobtained by combining the products from three separate laser craters generated withrelatively severe laser conditions (20 Ws−1 for 10 s with the 10× objective, comparedto 14 Ws−1 for 5 s with the 20× objective used to analyse the solid bitumen). Thelimestone may not adsorb the infrared laser radiation as efficiently as the organic solidbitumen. The detection in the carbonate matrix of the same hydrocarbon productsdetected in the solid bitumen suggests that sub-microscopic solid bitumens are presentwithin the carbonate matrix.

A second solid bitumen in a Pingdingshan limestone was also analysed with similarlaser conditions, but despite bright luminescence during the laser event (usually typicalof significant pyrolysis of organic material), no pyrolysates were detected from thismaterial. Such a result may be indicative of overmature organic material with greateraromaticity. Interestingly, the surrounding carbonate matrix also contained relativelyhigh proportions of alkylnaphthalenes, n-alk-1-enes/n-alkanes and alkyldibenzo-furans, again indicating the presence of sub-microscopic organic material throughoutthe bulk carbonate.

4. Conclusions

The in situ laser micropyrolysis GC–MS analyses of the resinite and cutinitemacerals in two Chinese coals (Fushan and Hequ) showed a variety of alkylaro-

Fig. 9. Partial summed m/z 160+174+188 mass chromatogram showing the alkyldibenzofurandistribution of the solid bitumen analysis of the Pingdingshan limestone in Fig. 7(a). MBp=methyl-biphenyl, DBF=dibenzofuran, Cn-DBF=Cn-alkyldibenzofurans.


matic and n-aliphatic hydrocarbon pyrolysates. The data from these macerals weremostly consistent with previously reported data obtained on maceral concentratesand maceral-enriched coals, reflecting the relatively successful in situ analysis ofthese macerals. However, the analyses also included hydrocarbon products thoughtto be alien to these macerals (e.g. the low concentration of n-paraffins in the resiniteanalysis, and the relatively high abundance of aromatic hydrocarbons in the cutiniteanalysis). Further, a resin-derived alkylnaphthalene signature was detected invitrinite macerals of the resin-rich Fushun coal. These results may be due toextensive assimilation of the free hydrocarbon fraction of the coal, or alternativelymay reflect the non-exclusive targeting of the selected macerals by the laser. Furtherresearch is required to distinguish these two scenarios. One solution to surmountthe hindrance of free hydrocarbon assimilation would be to first remove the freehydrocarbon fraction by solvent extraction or some other technique, whilst beingcareful to maintain the morphology of the macerals. Alternatively, low maturitycoal macerals in which significant amounts of free hydrocarbons have not beengenerated could be studied.

A solid bitumen present in the Pingdingshan limestone was shown to include adistinctive distribution of alkyldibenzofurans. The relatively high abundace of thesehydrocarbons may be indicative of a fresh water environment with terrestrialorganic matter input. Analysis of the morphologically homogeneous carbonateshowed that sub-microscopic material was widely dispersed throughout this Chineselimestone.

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In situ analytical pyrolysis of coal macerals and solid bitumens by laser micropyrolysis GC–MS

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Transcript of In situ analytical pyrolysis of coal macerals and solid bitumens by laser micropyrolysis GC–MS