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# ^ :CHEMICAL CHARACTERIZATION OF COALS, COAL MACERALS
AND THEIR PRECURSORS
A study by analytical pyrolysis
CHEMICAL CHARACTERIZATION OF COALS, COAL MACERALS AND THEIR PRECURSORS
A study by analytical pyrolysis
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus, prof. dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van
een commissie door het College van Dekanen daartoe aangewezen, op donderdag 3 december 1987 te 16.00 uur door
Margriet Nip
geboren te Schiedam, geologisch doctoranda
TR diss]
1592
Dit proefschrift is goedgekeurd door de promotor Prof. Drs. P.A. Schenck
Dr. J.W. de Leeuw en Dr. J.J. Boon hebben als begeleiders in hoge mate bijgedragen aan het totstandkomen van het proefschrift. Het College van Dekanen heeft hen als zodanig aangewezen
STELLINGEN
behorend bij het proefschrift
CHEMICAL CHARACTERIZATION
OF COALS, COAL MACERALS AND
THEIR PRECURSORS
A study by analytical pyrolysis
door
Margriet Nip
CONTENTS page SUMMARY 15 SAMENVATTING 17 CHAPTER 1: 19 INTRODUCTION
1:1 Coal, coal macerals and their precursors 19
1:2 Curie point pyrolysis 23 1:3 Framework of the thesis 25
CHAPTER 2: 27 THE CHEMICAL STRUCTURE OF A BITUMINOUS COAL AND
ITS CONSTITUTING MACERAL FRACTIONS AS REVEALED BY FLASH PYROLYSIS
2:1 Introduction 28 2:2 Experimental 29
2:2:1 Samples 29 2:2:2 Curie point pyrolysis 33
2:3 Results and discussion 34 2:3:1 Py-GC and Py-GC-MS of the coal and the density
fractions 34 2:3:2 The internal distribution patterns of the
various groups of pyrolysis products 35 2:3:3 Schematic representation of the pyrolysates
of the density fractions 43 2:3:4 Comparison between the pyrolysis data and the
ultimate analysis data of the density fractions 52
2:3:5 The purity of the density fractions 52 2:3:6 The pyrolysate of coal SIU 647 J in relation
to the pyrolysates of its density fractions 54
CHAPTER 3: 57 THE CHARACTERIZATION OF EIGHT MACERAL
CONCENTRATES BY MEANS OF CURIE POINT GAS CHROMATOGRAPHY AND CURIE POINT PYROLYSIS-GAS CHROMATOGRAPHY-MASS SPECTROMETRY
3:1 Introduction 58 3:2 Experimental 59
3:2:1 Samples 59 3:2:2 Sample preparation 60
3:2:3 Curie point evaporation-gas chromatography (Py-GC) 60 3:2:4 Curie point pyrolysis-gas
chromatography-mass spectrometry (Py-GC-MS) 61 3:2:5 Multivariate treatment of the data by factor
analysis 61 3:3 Results and discussion 61
3:4 Acknowledgement 77
CHAPTER 4: 79 A NEW NON-SAPONIFIABLE HIGHLY ALIPHATIC AND
RESISTANT BIOPOLYMER IN PLANT CUTICLES: EVIDENCE
FROM PYROLYSIS AND 13C-NMR ANALYSIS OF
PRESENT-DAY AND FOSSIL PLANTS
4:1 Introduction 79 4:2 Results and discussion 80
4:2:1 Fossil plant cuticles 80 4:2:2 Recent plant cuticles 81 4:2:3 Chemical characterization of the new,
non-saponifiable highly aliphatic
biopolymer 86 4:2:4 Occurrence of the new, non-saponifiable
highly aliphatic biopolymer 88
CHAPTER 5: 89 ANALYSIS OF MODERN AND FOSSIL PLANT CUTICLES BY
CURIE POINT PY-GC AND CURIE POINT PY-GC-MS: RECOGNITION OF A NEW, HIGHLY ALIPHATIC AND RESISTANT BIOPOLYMER
5:2 Experimental 90 5:2:1 Samples and sample treatment 90
5:2:2 Curie point pyrolysis-gas chromatography
(Py-GC) 92 5:2:3 Curie point pyrolysis-gas
chromatography-mass spectrometry (Py-GC-MS) and gas
chromatograpy-mass spectrometry (GC-MS) 93
5:2:4 Gas-chromatography (GC) 93
5:3 Results and discussion 95 5:3:1 Recognition of the new, highly aliphatic
biopolymer 95 5:3:2 The fossilization potential of the new
biopolymer 97 5:3:3 The absence of cutin in fossil plant
cuticles 99 5:3:4 The presence of the new biopolymer in
a sediment, a lignite and a coal 101 5:3:5 A possible oil-precursor function of the new
biopolymer 101 5:3:6 The occurrence of the highly aliphatic biopolymer 103
CHAPTER 6: 105 A FLASH PYROLYSIS AND PETROGRAPHIC STUDY OF
CUTINITE FROM THE INDIANA PAPER COAL
6:1 Introduction 105 6:2 Experimental 106
6:2:1 Samples 106 6:2:2 Sample analysis 111 6:3 Results and discussion 116
6:4 Conclusions 129 6:5 Acknowledgement 130
CHAPTER 7: 131 CURIE POINT PYROLYSIS-MASS SPECTROMETRY, CURIE
POINT PYROLYSIS-GAS CHROMATOGRAPHY-MASS SPECTROMETRY AND FLUORESCENCE MICROSCOPY AS ANALYTICAL TOOLS FOR THE CHARACTERIZATION OF TWO UNCOMMON LIGNITES
7:2 Experimental 132 7:2:1 Samples 132 7:2:2 Sample preparation 133
7:2:3 Curie point pyrolysis-mass spectrometry
of the lignites 133 7:2:4 Curie point evaporation-mass spectrometry
of the standard mixture 134 7:2:5 Curie point pyrolysis-gas chromatography 134
7:2:6 Curie point pyrolysis-gas
chromatography-mass spectrometry 135 7:3 Results and discussion 135
7:3:1 Py-MS of the lignites 135 7:3:2 Py-GC and Py-GC-MS 136 7:3:3 Fluorescence microscopy 138 7:3:4 Evaporation-GC and extraction of PSOC-975 141
7:3:5 Comparison of the Py-MS and Py-GC
and Py-GC-MS results 143 7:3:6 Mass chromatography of important Py-MS
peaks 143 7:3:7 Low and high eV conditions in the mass
spectrometer 146 7:4 Conclusions 148 7:5 Acknowledgement 148
CHAPTER 8: 149 CHEMICAL CHARACTERIZATION OF HUNGARIAN BROWN
COALS BY CURIE POINT PYROLYSIS-LOW ENERGY ELECTRON IMPACT MASS SPECTROMETRY AND MULTIVARIATE ANALYSIS AND BY CURIE POINT PYROLYSIS-GAS CHROMATOGRAPHY-PHOTOIONIZATION MASS SPECTROMETRY
8:1 Introduction 149 8:2 Experimental 150
8:2:1 Samples 150 8:2:2 Curie point pyrolysis-low energy
electron impact mass spectrometry (Py-MS) 151 8:2:3 Numerical analysis of the Py-MS data 152 8:2:4 Curie point pyrolysis-gas
chromatography-photo-ionization mass spectrometry (Py-GC-PIMS) 153
8:3 Results and discussion 153 8:3:1 Py-MS fingerprinting of the brown coals 153
8:3:2 Deconvolution of the Py-MS mass peaks by
Py-GC-PIMS 158 8:3:3 Chemical and geological interpretation
of the Py-MS data 167
8:4 Conclusions 169 8:5 Acknowledgement 170
CHAPTER 9: 171 COMPARISON OF FLASH PYROLYSIS, DIFFERENTIAL
SCANNING CALORIMETRY, 13C-NMR AND IR
SPECTROSCOPY IN THE ANALYSIS OF A HIGHLY ALIPHATIC BIOPOLYMER FROM PLANT CUTICLES
9:1 Introduction 172 9:2 Experimental 172
9:2:1 13C cross polarization magic angle
spinning NMR spectroscopy 172 9:2:2 IR spectroscopy 173 9:2:3 Differential Scanning Calorimetry 173
9:2:4 Acid treatment 173 9:2:5 Curie point pyrolysis-gas chromatography 174
9:2:6 Curie point pyrolysis-gas
chromatography-mass spectrometry 174
9:3 Results and discussion 174
REFERENCES 181
This thesis is based on the following publications:
Chapter 2: M. Nip, J.W. de Leeuw, P.A. Schenck and J.C. Crelling.
Energy and Fuels (submitted).
Chapter 3: M. Nip, J.W. de Leeuw and P.A. Schenck.
Geochimica Cosmochimica Acta (accepted in revised form).
Chapter 4: M. Nip, E.W. Tegelaar, J.W. de Leeuw, P.A. Schenck and P.J. Holloway.
Naturwissenschaften 73 (1986), 579-585.
Chapter 5: M. Nip, E.W. Tegelaar, H. Brinkhuis, J.W. de Leeuw, P.A. Schenck and P.J. Holloway.
In: Advances in Organic Geochemistry-1985 (eds. D. Leythaeuser and J. Rullkötter), Org. Geochemistry 10 (1986), 769-778.
Chapter 6: M. Nip, J.W. de Leeuw, P.A. Schenck, W. Windig, H.L.C. Kleuzelaar and J.C. Crelling.
Geochimica Cosmochimica Acta (accepted in revised form).
Chapter 7: M. Nip, J.W. De Leeuw, P.A. Schenck, H.L.C. Meuzelaar, S.A. Stout, P.H. Given and J.J. Boon.
J. Anal. Appl. Pyrol. 8 (1985), 221-239.
Chapter 8: M. Nip, W. Genuit, J.J. Boon, J.W. de Leeuw, P.A. Schenck, M. Blazsó and T. Szekély.
J. Anal. Appl. Pyrol., in press.
Chapter 9: M. Nip, J.W. de Leeuw, P.J. Holloway, J.P.T. Jensen, J.C.M. Sprenkels, M. de Pooter and J.J.M. Sleeckx.
J. Anal. Appl. Pyrol., in press.
The publishers of Energy and Fuels, Geochimica Cosmochimica Acta, Naturwissenschaften, Organic Geochemistry and the Journal of Analytical and Applied Pyrolysis are gratefully acknowledged for their permission to use the already published papers in this thesis.
Summary
This thesis describes investigations into the chemical structure of coal by means of analytical pyrolysis. In the first part of the thesis the relationship between the structural elements present in coals, coal macerals and their precursors (plant tissues) is described (chapters 2 to 6). In the second part experiments are described which focus on the nature of the apparent discrepancies which may occur with respect to the identification and interpretation of pyrolysis products, in case data obtained by pyrolysis-mass spectrometry are compared with data obtained by pyrolysis-gas chromatography-mass spectrometry (chapters 7 and 8). Data obtained for macromolecular materials by pyrolysis-gas chromatography-mass spectrometry do not always appear to give a complete description of the chemical structure of these materials (chapter 9).
Coals are stratified accumulations of organic matter which is mainly derived from organisms of the plant kingdom. Coals consist of microscopically recognizable constituents, socalled macerals, which are often assumed to be chemically and physically homogeneous. Macerals are fossil plant tissues which, dependent on the chemical resistance of their constituting biopolymers towards coalification, mainly determine the chemical nature of coals. Attention is paid exclusively to bituminous coals and their macerals. The macerals were isolated from the coals by means of density gradient centrifugation, an isolation method which exploits the variation of density between the individual macerals.
In chapter 2 it is demonstrated that individual macerals which are of the same coal rank have different chemical natures. Density gradient centrifugation is an isolation method which does not yield pure macerals. Each density fraction is relatively enriched, however, in a specific maceral, which facilitates the recognition of macromolecular structures dominantly present in specific macerals.
The chemistry of macerals changes as a function of increasing coalification stage of the coal from which they originate (chapter 3). The chemical nature of vitrinites of higher coal rank is similar to that of inertinites, indicating that fusinitization and coalification both have ultimately the same impact on the chemistry of these two maceral groups. Direct correlations exist between some of the proximate and ultimate analysis data of the macerals and the chemical nature of their pyrolysates.
The diagenetic evolution of one single type of plant tissue (plant cuticles) into its corresponding maceral (cutinite) is described in chapters 4, 5 and 6. Modern plant cuticles consist of soluble waxes, the polyester cutin and in some cases of a polymethylenic biopolymer (chapter 4). Upon increasing diagenesis, this polymethylenic biopolymer is relatively enriched as revealed upon comparison of modern and fossil plant cuticles (chapter 5). The maceral cutinite consists almost exclusively of this biopolymer (chapter 6). This biopolymer, which is the most
resistant constituent of plant cuticles, explains the highly aliphatic nature of the maceral cutinite.
In chapter 7 it is described that apparent discrepancies between pyrolysis-mass spectrometric and pyrolysis-gas chromatographic-mass spectrometric data, obtained for two uncommon lignites, are of a twofold nature. On the one hand these discrepancies arise because of the highly tentative identification of individual components present in the lignite pyrolysates based on the presence or absence of nominal masses in their pyrolysis-mass spectra. On the other hand these discrepancies exist because of the different response factors of the detection systems used to monitor the pyrolysates: low voltage electron impact mass spectrometry in the case of pyrolysis-mass spectrometry and high voltage electron impact mass spectrometry in the case of pyrolysis-gas chromatography-mass spectrometry.
These discrepancies are further evaluated and partly overcome in chapter 8 where pyrolysis-low voltage electron impact mass spectrometric data are compared with pyrolysis-gas chromatographic-photoionization mass spectrometric data of a series of Hungarian brown coals.
In an attempt to elucidate the exact chemical structure of the polymethylenic biopolymer, encountered in some modern and fossil plant cuticles, a direct
comparison was made between pyrolysis data and solid state 13C-NMR and IR data
of the biopolymer (chapter 9). The solid state 13C-NMR and IR data show that the
biopolymer does consist partly of polymethylene chains and partly of a polysaccharide moiety. The two moieties do not form a simple mixture but are probably covalently bound. Surprisingly enough, the polysaccharide fraction is not monitored upon pyrolysis-gas chromatography and pyrolysis-gas chromatography-mass spectrometry.
These results indicate that in case a complete elucidation of the structure of macromolecular materials is required, data obtained by analytical pyrolysis should be compared with data obtained by other analytical methods.
Samenvatting
Dit proefschrift beschrijft de resultaten van een onderzoek naar de chemische structuur van steenkool gebaseerd op gegevens, verkregen met analytische pyrolyse. Het eerste gedeelte van het proefschrift handelt over moleculaire structuurelementen in steenkool, steenkool maceralen en hun uitgangsproducten, plantenweefsels (hoofdstuk 2 tot en met 6). Het tweede gedeelte van het proefschrift beschrijft een aantal experimenten die gericht zijn op het onderzoek naar de aard van de (schijnbare) verschillen die kunnen ontstaan wat betreft de identificatie en interpretatie van pyrolyse producten wanneer analytische pyrolyse gebruikt wordt in combinatie met massaspectrometrie enerzijds en met gas chromatografie-massa spectrometrie anderzijds (hoofdstuk 7 en 8). De chemische structuur van macromoleculaire materialen blijkt niet altijd volledig beschreven te worden met behulp van pyrolyse-gas chromatografie-massaspectrometrie (hoofdstuk 9).
Steenkool is een gestratificeerde accumulatie van organisch materiaal dat hoofdzakelijk afkomstig is van organismen uit het plantenrijk. Steenkool wordt gedacht te zijn opgebouwd uit microscopisch onderscheidbare delen, de maceralen, waarvan men vaak aanneemt dat ze ook in chemische en fysische zin specifiek zijn. Maceralen zijn gefossiliseerde plantenweefsels die, afhankelijk van de biologische en chemische resistentie van de erin voorkomende biopolymeren, in hoge mate de chemische structuur van steenkool bepalen. Aandacht is uitsluitend besteed aan bitumineuze steenkolen en maceralen daaruit. De maceralen zijn uit de steenkolen geisoleerd door middel van dichtheidscentrifugering, een isolatie methode waarbij gebruik gemaakt wordt van de verschillen in dichtheid die bestaan tussen individuele maceralen.
In hoofdstuk 2 wordt aangetoond dat individuele maceralen met dezelfde inkolingsgraad verschillende chemische structuren bezitten. Dichtheidscentrifuge ring blijkt een methode te zijn die niet leidt tot de isolatie van zuivere maceralen. De dichtheidsfracties bevatten echter wel zeer duidelijk hogere concentraties aan individuele maceralen zodat de macromoleculaire structuur van specifieke maceralen herkend kan worden.
De chemische structuur van maceralen verandert met het voortschrijden van het inkolingsproces (hoofdstuk 3). De chemische structuur van vitrinieten van hogere inkolingsgraad lijkt veel op die van inertinieten. Blijkbaar hebben 'fusinitisatie' en inkoling een zelfde invloed op de chemische structuur van deze twee maceraal-groepen. Duidelijke correlaties zijn aangetoond tussen de gehaltes aan vluchtige bestanddelen en de percentages koolstof, waterstof, zuurstof, stikstof en zwavel van de maceralen enerzijds en chemische karakteristieken van hun pyrolysaten anderzijds.
corresponderende maceraal (cutiniet) wordt beschreven in de hoofdstukken 4, 5 en 6. Recente plantencuticulae bestaan uit een oplosbare wasfractie, het polyester cutine en in de meeste onderzochte gevallen uit een biopolymeer met een polymethyleen structuurelement (hoofdstuk 4). Tijdens het diagenese proces wordt het polymethylene biopolymeer relatief geconcentreerd zoals blijkt uit de vergelijking tussen recente en fossiele plantencuticulae (hoofdstuk 5). Het maceraal cutiniet bestaat vrijwel uitsluitend uit dit biopolymeer (hoofdstuk 6). Dit biopolymeer, dat het meest resistente onderdeel van plantencuticulae vormt, bepaalt het sterke alifatisch karakter van het maceraal cutiniet.
In hoofdstuk 7 wordt beschreven dat de waargenomen verschillen wat betreft de identificatie en interpretatie van pyrolyseproducten verkregen van twee 'ongewone' lignieten met behulp van pyrolyse-massaspectrometrie enerzijds en pyrolyse-gas chromatografie-massaspectrometrie anderzijds, tweeërlei oorzaak kunnen hebben. Deze verschillen kunnen ofwel ontstaan door de voorlopige, doch niet volledig gerechtvaardigde identificatie van pyrolyseproducten, gebaseerd op de aan- of afwezigheid van nominale massa's in hun pyrolyse-massaspectra, ofwel zij worden veroorzaakt door de verschillende responsiefactoren der detectie systemen die gebruikt zijn om de pyrolysaten te registreren: lage energie elektronenstoot ionisatie condities in de massaspectrometer in het geval van pyrolyse-massaspectrometrie en hoge energie elektronenstoot ionisatie condities in de massaspectrometer in het geval van pyrolyse-gas chromatografie-massaspectrometrie.
Deze verschillen worden nader besproken in hoofdstuk 8. Een serie Hongaarse bruinkolen werd zowel met pyrolyse-'lage elektronenstoot ionisatie' massaspectro-metrie als met pyrolyse-gas chromatografie-fotoionisatie massaspectromassaspectro-metrie geanalyseerd.
In een poging om tot een meer gedetailleerde structuuropheldering te komen van het biopolymeer met een polymethyleen structuurelement, aangetroffen in sommige recente en fossiele plantencuticulae, werden gegevens van dit biopolymeer verkregen met behulp van pyrolyse, vergeleken met gegevens verkregen met vaste
stof 13C -NMR en IR spectroscopie (hoofdstuk 9). De gegevens verkregen met vaste
stof 13C-NMR and IR spectroscopie toonden aan dat het biopolymeer gedeeltelijk
bestaat uit polymethyleen ketens en gedeeltelijk uit een polysaccharide fractie. Beide fracties vormen geen mengsel maar lijken onderling covalent gebonden te zijn. De polysaccharide fractie bleek, in tegenstelling tot wat in het algemeen het geval is, echter in het geheel geen detecteerbare pyrolyseproducten op te leveren.
Deze resultaten geven aan dat, om tot een volledige beschrijving van de chemische structuur van macromolekulaire structuren te komen, gegevens verkregen met behulp van analytische pyrolyse vergeleken moeten worden met gegevens verkregen met behulp van andere analytische methoden.
chapter 1
INTRODUCTION
1:1 COAL, COAL MACERALS AND THEIR PRECURSORS
Coals are stratified accumulations of organic substances which are mainly derived from a large variety of organisms of the plant kingdom. The chemical composition of coals is in the first instance determined by the chemical nature of the original plant material, the chemical characteristics of which depend on several factors.
Firstly, the evolutionary stage of the plants and hence the geological era in which they evolved and initially accumulated. Plants are very heterogeneous in that they consist of a great variety of biopolymers. Although evolution greatly altered plant anatomy, it is thought that the nature of these biopolymers did not change much. Their relative abundance and distribution in plant tissues however changed considerably (GIVEN et al., 1980).
Secondly, the geological and ecological setting where the accumulation of plant material occurred. Only in ecosystems, where geological and climatic conditions, water level fluctuations and nutrient supply are optimum, specific types of plant communities will flourish to such an extent that they eventually will give rise to massive accumulations of .plant debris. These conditions only prevailed during specific periods in the geological past in specific parts of the world.
Thirdly, the biochemical and geochemical processes which transformed the plant material originally present. The extent to which the plant material will be chemically altered depends on the chemical resistance of its constituting biopolymers against these diagenetic processes.
In case plant debris are accumulated, the majority of their constituting biopolymers will initially be degraded by micro-organisms. Only a small fraction (5-10% , GIVEN and DICKINSON, 1975), will escape this mineralization process but will be chemically altered to some extent to form peat. Further exposure to elevated temperatures with increasing burial will cause progressive chemical changes of the peat material.
Peats will be converted into lignites initially and eventually into bituminous coals and anthracites.
Ever since coal petrographers started to examine coals under the microscope it was acknowledged that coals are of a very heterogeneous nature. It was observed that coal consists of a large variety of optically homogeneous aggregates. STOPES (1935) was the first to name these aggregates macerals, a term which is derived from the latin verb 'macerare', which means 'to form out of a weak mass'. By using this term she described the process by which the various plant tissues after deposition are decomposed and incorporated in the coal structure in the form of their macerated counterparts. Such an approach implicates that each type of plant tissue, dependent on its chemical behaviour during biochemical and geochemical diagenetic processes, will be expressed in the structure of coals in the form of a single maceral.
It is assumed that these homogeneous aggregates are characterized by distinct physical and chemical properties.
The chemical and optical properties of macerals depend on the chemical nature of their original plant tissues and the rank of the coal from which they originate. In lignites three groups of macerals are petrographically distinguished: the huminite, liptinite (exinite) and inertinite maceral groups (STACH et al., 1982).
Whereas the chemical and optical properties of the macerals which belong to the liptinite (exinite) and inertinite maceral groups show relatively slight changes, the macerals which belong to the huminite maceral group do exhibit significant optical and chemical changes once they have reached the bituminous coal stage. In bituminous coals this group of macerals is commonly referred to as the (pseudo)vitrinite maceral group.
Figure 1:1 shows the schematic representation of the relation between the most significant maceral forming parts of plants and their corresponding macerals in the lignite and bituminous coal stages.
The macerals which belong to the huminite and (pseudo)vitrinite maceral groups are all derived from woody tissue. Their main biochemical precursors are lignin and cellulose. Dependent on the biochemical and geochemical degradation pathways of these biopolymers, six macerals are distinguished within the huminite maceral group, whereas three macerals are distinguished within the vitrinite maceral group (STACH et al, 1982).
It must be noted that recently coal petrographers started to devide the vitrinite macerals into two general subgroups. Based on microscopic observations, they recognize vitrinite on the one hand and pseudovitrinite on the other hand (WINANS and CRELLING, 1984). Pseudovitrinite has a higher reflectance and exhibits greater homogeneity. Therefore, the material often collected from vitreous layers in coals which was commonly referred to as vitrinite in the past, actually is pseudovitrinite.
The most important individual macerals which belong to the liptinite (exinite) maceral group are sporinite, cutinite, suberinite, resinite and alginite. The biochemical precursors of these macerals exhibit a lipid-like chemical nature, as
MACERAL PRECURSORS BITUMINOUS COALS Woody tissue (lt9n1n/ce))u!ose) TentlnUe Ulnlnite Gelinite etc. t Telinite Colllnite Vitrodetrlnlte I (PSEUDO)VITBIWITES -HLMINITES
Spores/Pollen Cullies. Roots/Stems/ Resins Mg— (sporopoUentn) (waxes/cutin/polyraethylenic blopolytner) Corklfled cell walls (mono-, sesqul-, d l - , ("resistant algal
(suberln) and trlterpenolds) blopolyner")
Fossil spores/pollen Fossil cuticles
Fuslnite Seml-fusitilW Inertodetrlnite
Fossil roots/stems/ corklfled cell walls
Fossil restnl Fossil algae
Alglnlte
MACERALS
Fig. 1.1. Schematic representation of the relation between the most significant maceral forming plant tissues and their corresponding maceralsin the peat, lignite and bituminous coal stages.
indicated by the term liptinite. The term exinite in this respect refers to the location of these biopolymers in the outer cell walls (exines) of various plant organs, which does not hold for these biopolymers in general.
The biochemical precursor of sporinite is sporopollenin, a biopolymer which occurs in the outer cell wall of spores and pollen.
Cutinite originates from plant cuticles, which are thin continuous layers which cover the outer surface of the aerial parts of plants where no secondary growth occurs. Suberin is the precursor of suberinite. This biopolymer is known to occur in corkified cell walls in barks, but also at the surface of roots, stems and fruits. Resinite originates from plant resins and waxes. The precursors of alginite are biopolymers which occur in the outer cell walls of algae.
The most significant macerals which belong to the inertinite group are semi-fusinite, semi-fusinite, micrinite, macrinite, inertodetrinite and sclerotinite. Sclerotinite is thought to be derived from dark fungal tissues and sclerotia. The other inertinite macerals are derived basically from the same precursor as the huminite and (pseudo)vitrinite macerals, woody tissue, except for micrinite which is thought to be partly a coalification product of liptinite constituents. Whereas huminite, vitrinite and pseudovitrinite are formed from woody tissue which has undergone rather 'gentle' biochemical and geochemical degradation processes, semi-fusinite, fusinite and inertodetrinite have arisen from woody tissue which has endured 'fusinitization', caused by e.g. charring, oxidation, mouldering, etc.
The exact origin of macrinite is not well understood at present.
Fusinite and semi-fusinite form distinct particles in coals which are distinguished based on their different degree of fusinitization. Macrinite and inertodetrinite both occur in a dispersed, finely grained form in coals. Inertodetrinite is considered to be a mixture of mainly fusinite, semi-fusinite and macrinite.
Until the nineteen twenties, only the chemistry of integral coals was studied (for a review, see VAN KREVELEN, 1961). However, after coal petrographers had discovered that coals consist of macerals, increasing attention was paid to the chemistry of macerals and their chemical modification upon increasing coalification. A famous example of such a study is that of VAN KREVELEN (1961), who studied the changes in H/C and O/C ratios of different maceral groups upon increasing coal rank. The main aim of these maceral studies was to predict the reactivity of different maceral types upon various technological processes such as carbonization (BROWN
etal., 1964; MACKOWSKY and SIMONIS, 1969), combustion (KRÖGER etal.,
1957) and liquefaction (MITCHELL et al, 1977; SHIBAOKA et al, 1978). The introduction of advanced analytical techniques such as analytical pyrolysis in combination with gas chromatography, mass spectrometry and gas
chromatography-mass spectrometry, Fourier transform-infrared and solid state 13C-NMR
spectroscopy facilitated the elucidation of structures of macerals in rather detail (MEUZELAAR et al, 1984a/1984b; VAN GRAAS et al, 1980a/1980b;
BRENNER, 1984; KUEHN et al., 1984; PUGMIRE et al., 1984). Considerable differences between the chemical natures of the macerals were found by application of these methods. However, only in a few cases it was attempted to relate these differences to the different chemical structures of integral coals on the one hand and to those of the maceral precursors on the other hand.
The relation between the structural elements present in coals, coal macerals and their precursors, i.e. plant tissues, is one of the subjects studied by organic geochemists. Part of these studies focus on the soluble organic material of coals, which only comprises a very small fraction by weight of the total organic matter present in coals. The results of these studies are reviewed by GIVEN (1984).
The advanced analytical techniques previously mentioned, however, allow the structural elucidation of the insoluble organic matter in coals. Hence, these analytical methods facilitate a more detailed investigation of the chemical changes of the various biopolymers present in plant tissues during increasing coalification, leading to their incorporation into the coal structure.
This thesis deals with the study of the chemical relationships between coals, coal macerals and their precursors. Special attention has been focussed on the chemical characterization of macerals which occur in the bituminous coal stage. The separation of single macerals from a coal usually is achieved in two ways, either by hand-picking or by techniques which exploit the variation of density between the individual macerals. Both methods are reviewed by WINANS and CRELLING (1984). The maceral concentrates used in the studies presented in this thesis were all isolated by means of density gradient centrifugation.
Curie point pyrolysis combined with gas chromatography, mass spectrometry and gas chromatography-mass spectrometry were the most important analytical methods used. A general introduction into these analytical methods will be presented in the next part of this chapter.
1:2 CURIE POINT PYROLYSIS
Pyrolysis is the thermal degradation of materials in an inert atmosphere. Generally, two applications of pyrolysis are distinguished. On the one hand, pyrolysis is applied in the industrial field, in which case it is generally mentioned as 'applied pyrolysis'. On the other hand, pyrolysis is used analytically. In that case it is referred to as 'analytical pyrolysis'.
The main objective of applied pyrolysis, which is a large-scale operation, is the production of pyrolysis products.
very useful for the chemical characterization of materials by means of their pyrolysis products (IRWIN, 1979). Upon pyrolysis of these materials, such as present in bacterial cell walls, soils, tissues and sediments, thermally induced bond cleavages occur, which result in the conversion of a part of these macromolecules into a complex mixture of volatile pyrolysis products.
The first scientific study of analytical pyrolysis was reported by WILLIAMS (1862). Since then, it lasted more than eighty years before new developments were reported in the analytical pyrolysis field. BACHMAN et al. (1947) combined analytical pyrolysis with mass spectrometry which facilitates the identification of the pyrolysis products. Many studies have been performed combining these two analytical methods in the nineteen forties and early fifties. These studies are reviewed by IRWIN (1982).
With the introduction of pyrolysis-gas chromatographic systems it became possible to separate the complex mixture of pyrolysis products into the individual components (RADELL and STRUTZ, 1959; LEHRLE and ROBB, 1959; MARTIN, 1959). The combination of pyrolysis-gas chromatography with mass spectrometry was acknowledged to be a valuable analytical method (GIBSON, 1964).
Since then, analytical pyrolysis rapidly developed, mainly due to improvements of the instrumentation used (MEUZELAAR and IN'T VELD, 1972; MEUZELAAR and KISTEMAKER, 1973; MEUZELAAR et al., 1973; SCHULTEN et al., 1973; OERTLI et al., 1973; TYDEN-ERICSSON, 1973; LÜDERWALD and RINGSDORF, 1973; MEUZELAAR etal., 1975).
Many types of pyrolysis units have been developed. LEVY (1966) classifies them into two groups, based on the type of heating device, as furnace pyrolyzers and 'pulse-mode' pyrolyzers. Within the group of pulse-mode pyrolyzers, two types of pyrolysis units are distinguished: heated filament and Curie point pyrolyzers.
In the case of heated filament pyrolyzers, the filament is resistively heated by means of an electric current. In the case of Curie point pyrolyzers, the ferromagnetic filament is inductively heated due to its interaction with a high frequency electromagnetic field.
Curie point pyrolysis was first described by GIACOBBO and SIMON (1964). A ferromagnetic wire, on which a sample is mounted, is inductively heated by a high frequency induction coil until its Curie temperature is reached.
Curie point pyrolysis usually is performed in an open system, using fast heating rates. Subsequently, the pyrolysis products are rapidly removed in order to avoid the formation of secondary pyrolysis products. Therefore, the most important advantage of Curie point pyrolysis is that it produces mainly primary pyrolysis products which are highly characteristic for the chemical nature of the materials they are derived from.
virtually impossible to quantify the amount of sample which is applied to a ferromagnetic wire. Moreover, it is almost impossible to quantify the absolute amounts of pyrolysis products generated. The advantages and disadvantages of several pulse-mode pyrolysis methods are discussed by IRWIN (1982) and MEUZELAAR et al. (1982).
In this thesis, mass spectrometry, gas chromatography and gas chromatography-mass spectrometry were used to structurally identify the pyrolysis products of coals, coal macerals and their precursors.
In the case of Curie point pyrolysis-mass spectrometry the pyrolysis reactor is enclosed in a vacuum system and is connected to the electron impact ion source of a quadrupole mass spectrometer via a heated expansion chamber. The pyrolysis products are ionized under low voltage electron impact ionization conditions (13-17 eV) in order to limit fragmentation of the pyrolysis products. A 'fingerprint' of the total pyrolysate is obtained in the form of a mass spectrum.
In the case of Curie point pyrolysis-gas chromatography, the mixture of pyrolysis products is swept into a capillary gas chromatographic column by means of a helium flow. The pyrolysis products are separated gas chromatographically and are recorded by a flame ionization detector. A preliminary, but not firm, identification of the pyrolysis products can be made on the basis of their relative retention times. Curie point pyrolysis-gas chromatography-mass spectrometry allows the firm identification of the individual pyrolysis products on the basis of their relative retention times and on comparison of their mass spectra with those of standard compounds. Usually, high voltage electron impact ionization conditions (80 eV) prevail in the ion source of the quadrupole mass spectrometer. Under these conditions not only ionization but also fragmentation of the pyrolysis products occurs.
1:3 FRAMEWORK OF THE THESIS
In chapter 2, the chemical differences between macerals of the same coal rank are discussed. These macerals (cutinite, resinite, sporinite, vitrinite, pseudovitinite, semi-fusinite and fusinite) were isolated from an Upper-Carboniferous high volatile bituminous coal, obtained from the Roaring Creek area in Indiana, U.S.A.
It was attempted to relate various pyrolysis products of the macerals to the chemical structure of their likely precursors. Moreover, the chemical relationship between the individual macerals and the coal which they are derived from, is evaluated.
In chapter 3, the results are presented of a study of eight maceral concentrates, isolated from British Upper-Carboniferous medium to high volatile bituminous
coals. Differences between the pyrolysates of the macerals sporinite, vitrinite, semi-fusinite and semi-fusinite of the same coal rank are discussed, as well as differences between the pyrolysates of these individual macerals as a function of increasing coal rank. Moreover, the pyrolysis data of the individual macerals are compared with the data of petrographic, proximate and ultimate analyses using multivariate analysis. Chapters 4, 5 and 6 deal with the diagenetic evolution of one single type of plant tissue (modern plant cuticles) into its corresponding maceral (cutinite). Chapter 4 describes the chemical nature of the structural components of modern plant cuticles. In chapter 5 the chemical characteristics of fossil plant cuticles are evaluated as well as the differences between fossil and modern plant cuticles. Chapter 6 describes the chemical nature of the maceral cutinite, isolated from the Upper-Carboniferous high volatile bituminous Indiana paper coal, Roaring Creek area, Indiana, U.S.A.
In chapters 7, 8 and 9, critical comments are presented concerning the interpretation and comparison of the results obtained by the various analytical techniques used in this thesis.
Chapter 7 deals with the discrepancies which occur upon comparison of data from two uncommon lignites obtained by Curie point pyrolysis-low voltage electron impact mass spectrometry on the one hand and Curie point pyrolysis-gas chromatography-high voltage electron impact mass spectrometry on the other hand.
In chapter 8 the evaluation of a study of Hungarian brown coals is presented in which the discrepancies described in chapter 7 are partly overcome by a direct comparison of Curie point-low voltage electron impact mass spectrometric and Curie point-gas chromatographic-photoionization mass spectrometric data.
In chapter 9 results of a study are presented which indicate that Curie point pyrolysis data do not always give a complete description of the chemical nature of insoluble materials. In case a complete structure elucidation of macromolecular material is required, data obtained by analytical pyrolysis should be compared with data obtained from other analytical techniques.
Chapter 2
THE CHEMICAL STRUCTURE OF A BITUMINOUS COAL AND ITS CONSTITUTING
MACERAL FRACTIONS AS REVEALED BY FLASH PYROLYSIS
ABSTRACT
In order to study the relationships between the chemical structures of coals, coal macerals and their precursors (plant tissues), a high-volatile bituminous Upper-Carboniferous coal and its constituting maceral fractions cutinite, resinite, sporinite, vitrinite, semi-fusinite and fusinite were investigated by Curie point pyrolysis-gas chromatography and Curie point pyrolysis-gas chromatography-mass spectrometry. The maceral fractions were isolated from the coal by density gradient centrifugation. By means of this isolation method density fractions are obtained which yield relatively pure macerals. From a chemical point of view however, these density fractions consist of complex mixtures of macerals. A pseudovitrinite from the same seam was also studied.
The pyrolysate of the coal mainly consists of alkylbenzenes, alkylphenols, alkylnaphthalenes, alkylindenes, alkylnaphthols, alkylphenanthrenes, alkylanthracenes, alkylfluoranthenes, alkylpyrenes and homologous series of
n-alkanes and n-alk-1-enes with carbon numbers ranging from C7 to C30. The same
families of pyrolysis products are also present in the pyrolysates of the maceral fractions although their relative contributions vary considerably.
By studying the internal distribution patterns of the alkylderivatives of the above mentioned groups of pyrolysis products it was attempted to relate structural elements of the maceral fractions, as reflected by their pyrolysis products, to structural moieties of the precursors of the macerals which are most dominantly present in these fractions.
The internal distribution patterns of some of these families of pyrolysis products are similar for all maceral fractions. This similarity suggests that these families of pyrolysis products are derived from sources which are shared between the maceral fractions. These sources are thus not representative for the structures of the most dominantly present macerals.
each maceral fraction were compared. The most abundantly present groups of pyrolysis products in the pyrolysate of each maceral fraction were discussed in terms of the chemical nature and diagenetic evolution of the precursors of the most dominantly present maceral. Moreover, a comparison was made between these pyrolysis data and the H/C and O/C ratios of the density fractions.
The observations that all groups of pyrolysis products mentioned above occur in the pyrolysates of all maceral fractions and that the internal distribution patterns of some of them are similar for these fractions may be explained in two ways. Under certain diagenetic conditions, either some macerals become fluidized and intermingle with each other and/or other macerals, or lower molecular weight compounds react with each other forming products which occur through all macerals in such a way that entirely pure macerals cannot be separated from each other by means of density gradient centrifugation.
This process may explain the observation that the density values of macerals which are isolated from coals with carbon percentages between 50 and 87 % C, do not change uniformly with increasing coal rank.
2:1 INTRODUCTION
Coals are extremely complex, heterogeneous materials which mainly consist of a large variety of organic materials derived from plant tissues. The biopolymers present in plant tissues and their chemical behaviour upon coalification determine for the greater part the chemical structure of a coal.
One way to study the chemistry of coals is to analyze the fossil counterparts of the coal-forming plant tissues, macerals. In this way, it is attempted to relate structural elements present in coals to structures present in macerals and their precursors.
Since the application of more advanced analytical techniques, such as FT-IR and
solid state 13C-NMR spectroscopy, X-ray analysis and analytical pyrolysis in
combination with gas chromatography, mass spectrometry and gas chromatography-mass spectrometry, a more detailed knowledge has been obtained of the chemical relationships between coals, coal macerals and their precursors. Moreover, the development of more advanced isolation methods for the separation of macerals from a coal, such as density gradient centrifugation, has significantly enhanced the possibilities to investigate relatively pure maceral fractions. It must be emphasized however that maceral fractions isolated from a coal by whatever technique will never yield the pure maceral in the strict sense.
Studies in which FT-IR and 13C-NMR spectroscopy and analytical pyrolysis are
used for the analysis of maceral fractions isolated by density gradient centrifugation were reported by BRENNER (1984); KUEHN et al. (1984); PUGMIRE et al. (1984) and NIP et al. (1987a).
In this paper, the above mentioned approach was used to analyze an Upper-Carboniferous coal and its constituting maceral fractions. The coal (SIU 647J) originates from the Roaring Creek area, Indiana, U.S.A. (EGGERT and PHILLIPS, 1979/1982). Its constituting maceral fractions studied here (cutinite, resinite, sporinite, vitrinite, two semi-fusinites and fusinite), isolated from coal SIU 647J by density gradient centrifugation, together account for approx. 80 % by weight of the total organic matter in the coal.
The maceral fraction pseudovitrinite was studied too. This maceral fraction was isolated by density gradient centrifugation from the same seam as coal SIU 647J.
Curie point pyrolysis in combination with gas chromatography and gas chromatography-mass spectrometry were used to analyze the coal and the maceral fractions. Curie point pyrolysis has proved to be a very useful technique for the chemical characterization of insoluble organic matter present in coals and sediments and its precursors (LARTER et al., 1979; VAN GRAAS et al., 1980a/1980b; MEUZELAAR et al., 1984a/1984b; SAIZ-JIMENEZ et al., 1987).
This article is devided into two parts. In the first part, attention is paid to the internal composition of the families of pyrolysis products which are most abundantly present in the pyrolysates of the coal and the maceral fractions.
In the second part of the article, the differences between the chemical natures of the pyrolysates of the maceral fractions are discussed based on histograms, in which the relative contributions of these families of pyrolysis products to the 'total pyrolysate' are shown. Moreover, the chemical relation between the pyrolysate of the coal on the one hand and the pyrolysates of the maceral fractions on the other hand is discussed.
2:2 EXPERIMENTAL
2:2:1 Samples
From now on, the maceral fractions obtained by density gradient centrifugation will be called density fractions. This name was chosen in order to avoid confusion with the term maceral which in this paper refers to the pure fossil counterpart of a
Density fractionation
The density fractions cutinite, resinite, sporinite, vitrinite, semi-fusinite and fusinite were separated from the Upper-Carboniferous Indiana cuticular coal SIU 647J by means of density gradient centrifugation. For a more detailed description of coal SIU 647J and the density gradient centrifugation method, see NIP et al. (1987a) and references therein.
Figure 2:1 shows the density profile of sample SIU 647J. The most dominant peak represents the density fraction vitrinite. From this fraction, a sample was collected with a density of 1.273 gm/ml (VIT).
100' 90 80-70 > QC Ui > 60-O iu 50-ae Ui > 40 < 20- 1 0-1.2 1.0 1.1 1.2 1.3 DENSITY (gm/ml)
Fig.2:l. Density profile of coal SIU 647J.
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1.4
—I
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The shoulder which represents the liptinite density fractions (with density values ranging from 1.0 to 1.27 gm/ml) was reseparated into narrower density ranges (fig.2:2). Based on fluorescence microscopy of the density fractions which represent individual peaks in this density profile, the density fractions cutinite (d = 1.089 gm/ ml, CUT), resinite (d = 1.116 gm/ml, RES) and sporinite (d = 1.158 gm/ml, SPOR) were isolated.
1.1 1.2
DENSITY (gm/ml)
Fig.2:2. Density profile of the concentrated liptinite density fractions of coal SIU 647J. The same procedure was followed for the inertinite density fractions (with density values ranging from 1.35 to 1.49 gm/ml). The density profile of the inertinite density fraction of sample SIU 647J shows a broad peak on the low density and a definite shoulder on the higher density side (fig.2:3). The broad peak represents the semi-fusinite density fraction of which two samples were collected (semi-semi-fusinite A, d = 1.362 gm/ml SFUSA, and semi-fusinite B, d = 1.413 gm/ml, SFUSB). The shoulder represents the fusinite density fraction a sample of which was collected with a density value of 1.471 gm/ml (FUS). <—V
£
>• tc Ui > O KJ UJ ec U l > i n Q£ 1UW8 0 - 60-" 402 0 0 -1 3 r ' N > - ^ B i 1.4 \ . C 1 1.5 DENSITY (gm/ml)Fig.2:3. Density profile of the inertinite density fractions of coal SIU 647J (A = SFUSA, B = SFUSB, C = FUS).
The density fraction pseudovitrinite (d = 1.306 gm/ml, PSEUDOVIT) was separated by density gradient centrifugation from a hand-picked vitrain band from the same seam as coal SIU 647J.
Petrographic analysis
Petrographic analysis data of sample SIU 647J, obtained by combination of white and blue light microscopy on the one hand (CRELLING and BENSLEY, 1980) and by the standard petrographic method ASTM D-2799 on the other hand (CRELLING, 1987) are presented in Table 2:1.
SIU 647] Vitrinite Pseudo-vitrinite Sporinite Resinite Cutinite Fluorinite Bituminite Liptodetrinite Fusinite Semi-fusinite Macrinite Semi-macrinite Micrinite 51.6# 14.1# 6.3# 0.5# 4.1# - # 0.3# 0.4# 3.1# 16.0# 0.1# 0.5# 3.0# 44.9* 6.6* 13.8* 1.1* 7.3* 0.2* 3.0* 2.3* 8.3* 4.8* _ * _ * 8.0*
Table 2:1. Petrographic analysis data of coal SIU 647J (# = data obtained by the standard petrographic method ASTM D-2799; * = data obtained by combination of white and blue light analysis).
Ultimate analysis
The ultimate analysis values, H/C and O/C ratios of sample SIU 647J and of the individual density fractions, except for PSEUDOVIT, are shown in Table 2:2. The samples were analyzed in duplicate by standard procedures. These data were kindly provided by the Wyoming Analytical Laboratories, Inc., Laramie Wyoming 82070, U.S.A.
Sample density C (%) H (%) N (%) S (%) lot Ó (%) H/C 0 / C SIU 647J -79.25# 79.04# 7.71# 8.07# 1.51# 1.49# 2.41# 2.38 # 9.12 # 9.02# 1.17 1.22 0.09 0.08 CUT 1.089 79.45* 78.61* 9.01* 8.59* 1.04* 0.88* 0.44* 0.40* 10.06* 10.44* 1.36 1.29 0.10 0.10 RES 1.116 77.82* 77.84* 8.29* 8.38* 0.76* 0.89* 0.50* 0.41* 11.91* 11.81* 1.27 1.26 0.11 0.11 SPOR 1.158 74.44* 74.64* 7.59* 7.62* 0.76* 0.89* 1.33* 1.55* 12.50* 12.10* 1.17 1.17 0.12 0.12 Sample density C (%) H (%) N ( % ) S (%) lot
ó (%)
H/C O/C VIT 1.273 74.44* 74.64* 5.08* 5.33* 1.51* 1.74* 0.64* 0.49* 18.33* 17.80* 0.82 0.86 0.18 0.18 SFUSA 1.362 78.76* 78.87* 4.03* 4.10* 1.63* 1.51* 0.38* 0.28* 15.20* 15.24* 0.61 0.62 0.14 0.14 SFUSB 1.413 79.69* 79.89* 3.63* 3.60* 1.83* 1.60* 0.47* 0.34* 14.38* 14.57* 0.55 0.54 0.13 0.14 FUS 1.471 78.19* 78.36* 2.89* 2.91* 1.32* 1.36* _ * _ * 17.60* 17.37* 0.44 0.44 0.17 0.17 Table 2:2. Ultimate analysis values, H/C and O/C ratios of coal SIU 647J and its individualdensity fractions. The samples were analyzed in duplicate. These data are not known for PSEUDOVIT. # = dry, ash free; * = dry, not ash corrected (ash content less than 1% except for SFUSA, SFUSB and FUS); " = by difference. Density in gm/ml.
2:2:2 Curie point pyrolysis
Prior to Curie point pyrolysis, approximately 10 ug of a crushed coal or density fraction (particle size about 40 microns) was pressed onto the surface of the ferromagnetic wire according to a method described by VENEMA and VEURINK (1985).
Curie point pyrolysis-gas chromatography (Py-GC)
Py-GC was carried out using an instrument as described by VAN DE MEENT et
al. (1980). The Curie temperatures of the wires used were 770°C and 358°C. The
wires were kept at the final temperature for 10 s. The gas chromatograph (Packard Becker, Model 419), equipped with a cryogenic unit (Packard, Model 799), was programmed from 0°C (5 min) to 300°C (25 min) at a rate of 3°C/min. Separation was achieved using a fused silica capillary column (25m x 0.32 mm i.d.), coated with CP-Sil 5 (film thickness 0.42 um, C.B.). Helium was used-as the carrier gas.
Curie point pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS)
The same type of pyrolysis unit and capillary column as mentioned before were also used in the Py-GC-MS mode. GC-MS was performed on a Varian 3700 gas chromatograph connected to a Varian MAT 44 quadrupole mass spectrometer.
Electron impact mass spectra were obtained at 80 eV under the following conditions: cycle time, 2 s; mass range, m/z 20-450 up to scan 250 and m/z 50-450 after scan 250; m/z 28, 32, 40 and 44 were omitted from the reconstructed total ion currents, because an open atmospheric split was used as the interface.
2:3 RESULTS AND DISCUSSION
2:3:1 Py-GC and Py-GC-MS of the coal and the density fractions
Figure 2:4 shows the Py-GC trace of coal sample SIU 647J. The identification of the pyrolysis products is based upon comparison of their mass spectra and relative retention times with those of standard compounds (SVOB and DEUR-SIFTAR, 1974; LEE et al., 1979, SCHRODER, 1980, RADKE et al., 1982/1986, ROWLAND et.al., 1986).
The pyrolysate of sample SIU 647J is mainly characterized by the presence of alkylbenzenes, alkylphenols, alkylnaphthalenes, alkylindenes, alkylnaphthols, alkylphenanthrenes, alkylanthracenes, alkylfluoranthenes, alkylpyrenes and homologous series of n-alkanes and n-alk-1-enes (fig.2:4).
The chemistry of coals is for the greater part determined by the chemical nature of their constituting macerals. A detailed knowledge of the chemistry of macerals, which is related to the chemical structure of their precursors, plant tissues, provides a
j « l [6 7
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9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2627 28 tenFig.2:4. Py-GC trace of coal SIU 647J. For analytical conditions, see Experimental. The numbers refer to the chain lengths of the n-alk-1-enes and n-alkanes. i = indene (* = contamination).
clue to the understanding of the origin and chemical nature of coals. Therefore, it was decided to analyze the density fractions CUT, RES, SPOR, VIT, SFUSA, SFUSB and FUS by Py-GC and Py-GC-MS in order to define their chemical relationships with coal SIU 647J. It must be emphasized that based on petrographic analysis, these density fractions consist of relatively pure macerals. Whether this holds also on a chemical basis is one of the objectives of this study.
Figure 2:5 shows the Py-GC traces of CUT, VIT and FUS. The pyrolysates of these density fractions are presented here because they represent the three maceral groups which are distinguished petrographically in bituminous coals (STACH et al., 1982).
The pyrolysates of the three density fractions are mainly characterized by the same pyrolysis products as those encountered in the pyrolysate of coal SIU 647J. It is clear however, that the relative contributions of these pyrolysis products to the pyrolysates of these density fractions vary very significantly.
CUT is mainly characterized by the homologous series of n-alkanes and n-alk-1-enes (fig.2:5a), VIT contains relatively more phenolic and other aromatic pyrolysis products (fig.2:5b) and in the pyrolysate of FUS, a relatively high contribution of polycyclic aromatic hydrocarbons is observed (fig.2:5c).
2:3:2 The internal distribution patterns of the various groups of pyrolysis products
The relative abundances of the individual components within the classes of pyrolysis products mentioned above were investigated to see whether systematic changes occur within the different density fractions, thus leading to a better
ik
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&§ &'' AIU-'J è- *4 ^ 4/ .£ m' FUS /VvVvWv 9 10 11 12 13 U 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 H°[)Fig.2:5. Py-GC traces of the density fractions CUT (a), VIT (b) and FUS (c). For analytical conditions, see Experimental. The numbers refer to the chainlengths of the n-alk-1-enes and n-alkanes. i = indene (* = contamination).
understanding of the chemical nature and the diagenetic evolution of the higher plant precursors of the most dominantly present macerals.
To this end, the relative abundances of the alkylderivatives within these groups of pyrolysis products were calculated, based on data obtained by mass chromatography. Characteristic m/z values were chosen for each group of pyrolysis products (Table 2:3). The identifications of the peaks in the mass chromatograms were checked by inspection of the full mass spectra.
groups of pyrolysis products alkylbenzenes alkylphenols alkylindenes alkylfluoranthenes/alkylpyrenes C7 to C12 n-alkanes C7 to C!2 n-alk-1-enes alkylnaphthalenes alkylnaphthols alkylphenanthrenes/ alkylanthracenes p * f * /-< * / - > * ' - O ^ l * ^ 2 ^ 3 78 92 106 120 94 108 122 136 116 130 144 202 216 m/z 57 and 71 m/z 55 and 69 128 142 156 170 144 158 172 178 192 206 220 Table 2:3. Characteristic m/z values, used for mass chromatography in order to calculate the
internal distribution patterns for various groups of pyrolysis products of the density fractions (* = alkylderivatives).
The internal distribution patterns of the C0- to C3-alkylbenzenes, C0- to C3
-alkylphenols, C0- to C2-alkylindenes, C0- and Q-alkylfluoranthenes and
alkylpyrenes and C7 to C,2 n-alkanes and C7 to C12 n-alk-1-enes are very similar for all
density fractions. This similarity suggests a common origin of these groups of pyrolysis products in all density fractions. Because of this similarity, the distribution patterns of these groups of pyrolysis products will be discussed for only one density fraction (VIT).
The internal distribution patterns of the C0- to C3-alkylnaphthalenes, C0- to
other hand are not similar for all density fractions. Hence, it is concluded that these pyrolysis products reflect the presence of characteristic structural elements of the macerals which are most dominantly present in the density fractions.
Figure 2:6 shows the distribution pattern of the C0- to C3-alkylbenzenes in VIT
(Table 2:3). Seven C3 -alkylderivatives, represented by six GC peaks, are shown in
the distribution pattern. Isopropyl benzene was recorded as well, but only in very low quantities.
E.B. M / P O -alkylbenzenes
C3
Fig.2:6. Distribution pattern of the alkylbenzenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. B = benzene; T = toluene; E.B. = ethylbenzene; M-/P- = meta- and paraxylene; O- = ortho-xylene; C3= C3-alkylbenzenes: A = n-propylbenzene; B = l-methyl-3-ethyl- +
l-methyl-4-ethylbenzene; C = 1,3,5-trimethylbenzene; D = 1-methyl-2-ethylbenzene; E = 1,2,4-trimethylbenzene; F = 1,2,3-trimethylbenzene (PÜTTMANN, pers. comm.).
The origin of alkylbenzenes in the pyrolysates of coals and density fractions is not fully understood. They may originate from lignin, an important constituent of woody tissue. The phenolic moieties present in lignin may have been chemically altered due to the coalification and/or fusinitization process. It is thought that upon increasing
influence of these diagenetic processes, all -OCH3 groups are finally removed from
the lignin originally present (VAN KREVELEN, 1961; SENFTLE etal., 1986). The remaining structural elements may give rise to toluene and ethylbenzene upon pyrolysis (fig.2:6). Probably only a selective part of the lignin biopolymer is diagenetically altered in such a way that it gives rise to alkylbenzenes upon pyrolysis, since phenolic pyrolysis products, which may be indicative also for the presence of diagenetically altered lignin, are encountered as well in the pyrolysates of the density fractions (vide infra).
The alkylbenzenes may also originate from cyclic structures of isoprenoid moieties such as present in sporopollenin and higher plant resins. Several structures are proposed for sporopollenin (LIBERT, 1974; MARCHAND etal., 1974; BROOKS and SHAW, 1978). Generally, sporopollenin is thought to be an oxidative copolymer of (3-carotene and the carotenoid ester of antheraxanthin (BROOKS and SHAW, 1978). This source could explain the presence of o-xylene, 1,2,4-trimethyl-and 1,2,3-trimethylbenzene (fig.2:6). In case these pyrolysis products would be derived from resins, they may as well originate from monocyclic monoterpenoid hydrocarbons, although a relatively higher abundance of isopropylbenzene could be expected in that case.
Figure 2:7 shows the internal distribution pattern of the alkylphenols in VIT
(Table 2:3). Five C2-alkylphenols are represented by only two GC peaks. They were
tentatively identified as 2,4-dimethylphenol and 2,5-dimethylphenol (peak A, fig.2:7) and 3-ethylphenol, 4-ethylphenoI and 3,5-dimethylphenol (peak B, fig.2:7). 2,6-dimethylphenol, 2-ethylphenol, 2,3-dimethylphenol and 3,4-dimethylphenol
were present as well, but in very small amounts. Eight C3-alkylderivatives were
recorded by mass chromatography (fig.2:7).
The phenolic pyrolysis products probably are derived from diagenetically altered lignin present in woody tissue. Apart from lignin, modern woody tissue consists of a cellulose and hemicellulose fraction (TIMELL, 1957; ASPINALL, 1970). After deposition, the cellulose and hemicellulose portions are decomposed by micro organisms (HATCHER et al., 1981/1982/1983a/1983b; HEDGES et ai, 1985, SAIZ-JIMENEZ et al., 1987), although lignin is known to decelerate the decay of associated polysaccharides (BARGHOORN, 1952; SEN and BASAK, 1957; NICHOLAS, 1973).
The rate of lignin decomposition highly depends on the burial conditions, which can be either aerobical or anaerobical (HEDGES et al., 1985 and references therein). Compared with its associated polysaccharide fraction however, lignin is
only slightly (bio-)chemically modified viz. by the demethylation of the -OCH3
1
!1
A1
B'/V7\V7\VA^W\^r^
PH M + P C2 CJ alkylphenolsFig.2:7. Distribution pattern of the alkylphenols in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. PH = phenol; O = orthocresol; M+P = meta- and paracresol; C2= Q-alkylphenols, tentatively
identified as A: 2,4- + 2,5-dimethylphenol and B: 3- + 4-ethylphenol + 3,5-dimethylphenol; C3= C3-alkylphenols.
demethoxylation of the lignin biopolymer (CHAFFEE et al., 1984). However, only p-cresol and 4-ethylphenol may be expected to arise from diagenetically altered lignin upon pyrolysis. Another possible source of phenolic pyrolysis products may be diagenetically altered sporopollenin. Phenolic pyrolysis products are encountered in the pyrolysates of modern and thermally treated sporopollenin of L. clavatum (SCHENCK et al., 1981a). GIVEN et al. (1960) noted the presence of phenolic groups in sporinites, which they thought to originate from the cyclic end groups of antheraxanthin. Their formation may have resulted also from thermally induced cleavage of ether bonds which are proposed by LIBERT (1974) to occur in the structure of at least modern sporopollenin. If alkylphenols are formed from sporopollenin upon pyrolysis in this way, typical C3-alkylphenols, like
3,4,5-trimethylphenol, are expected to occur in the pyrolysates of the density fractions. It was however not possible to assign structures to each C3-alkylderivative.
The high similarity of the alkylphenol distribution patterns in all pyrolysates may point towards a common origin of these pyrolysis products in the density fractions. GIVEN (1984) suggests a possible contribution of condensed tannins which occur ubiquitously in modern plants. In view of their chemical structure, they may give rise to phenolic pyrolysis products. It is worthwhile mentioning that the possibility of a source of alkylphenols other than lignin is subject of increasing debate (SENFTLE et
al., 1986). The chemical nature of this source and its mode of occurrence in specific
higher plant tissues has not been traced yet but deserves more attention.
Figures 2:8 and 2:9 show the distribution patterns of the C0- to C2-alkylindenes and
C0- and C,-alkylfluoranthenes and alkylpyrenes in VIT respectively (Table 2:3).
Although more C,- and C2-isomers are known for indene, only two C,- and four C2
-alkylderivatives were found in the pyrolysates of the density fractions. The C3
-alkylderivatives were not recorded by mass chromatography (fig.2:8).
alkylindenes
Fig.2:8. Distributionpattern of the alkylindenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. I = indene;
With respect to the alkylfluoranthenes and alkylpyrenes, only four C,-alkylderivatives were recorded in the pyrolysates of the density fractions. The C2
-and C3-alkylderivatives were not encountered at all (fig.2:9).
The origin of these two latter groups of pyrolysis products is not known at present.
alkylfluoranthenes/pyrenes
Fig.2:9. Distribution pattern of the alkylfluoranthenes and alkylpyrenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. F = fluoranthene; P = pyrene; C,= Q-alkylfluoranthenes/pyrenes. The similar distribution patterns of the families of pyrolysis products discussed above suggest that diagenetically altered biopolymers originating from different plant tissues are - at least in part - present in all density fractions. As a consequence these fractions, although petrographically defined as relatively pure macerals, are chemically spoken no pure macerals at all.
The internal distribution patterns of the C0- to C3-alkylnaphthalenes, C0- to C2
-alkylnaphthols and C0- to C3-alkylphenanthrenes and alkylanthracenes for all
With respect to the alkylnaphthalenes, ten C2-alkylnaphthalenes were tentatively
identified to be represented by six GC peaks (fig.2:10). Although more C3
-alkylderivatives are known for this type of compounds, only ten were revealed upon mass chromatography (fig.2:10).
The origin of these pyrolysis products in the density fractions is virtually unknown. They may be formed upon coalification and/or charring from other, probably less aromatic, structural moieties originally present in the structures of the macerals. In the case of the density fraction RES, these types of pyrolysis products may be partly derived from higher plant resins, in which case they are maturation products of bicyclic sesquiterpenoid hydrocarbons (PHILP et al., 1981; MUKHOPADHYAY and GORMLY, 1984; SIMONEIT et al., 1986).
The pyrolysates of all density fractions except PSEUDOVIT reveal seven C,- and
five C2- alkylnaphthols. Only four C2-alkylderivatives were recorded in the
pyrolysate of PSEUDOVIT (fig.2:11). The origin of these pyrolysis products is as yet unknown.
With respect to the internal distribution patterns of the alkylphenanthrenes and alkylanthracenes, five C,-alkylphenanthrenes and alkylanthracenes, tentatively identified as 3-methylphenanthrene (peak A), 2-methylphenanthrene (peak B), 2-methylanthracene (peak C), 9-methylphenanthrene and 1-methylanthracene
(peak D) and 1-methylphenanthrene (peak E, fig. 2:12), eight C2- and four C3
-alkylderivatives were recorded in the pyrolysates of the density fractions (fig.2:12). Phenanthrenes and anthracenes are known to be maturation products of tricyclic diterpenoid hydrocarbons present in plant resins (PHILP et al., 1981; SIMONEIT et
al., 1986; VENKATESAN et al., 1986). The distibution pattern of these classes of
compounds in the density fraction RES might reflect mostly such an origin. The dissimilar distribution patterns observed in the other density fractions point, at least partly, to other sources of these pyrolysis products.
2:3:3 Schematic representation of the pyrolysates of the density fractions
The internal distribution patterns of the alkylderivatives of the various families of pyrolysis products in the pyrolysates of the density fractions did not clearly show differences that could be related to differences in chemical structures of the macerals, which are on a petrographical basis most dominantly present in the density fractions. Therefore, it was decided to investigate whether the distribution patterns of the groups of pyrolysis products were characteristic for the chemical nature of the density fractions.
To this end, the complex pyrolysates of the density fractions were represented by
normalized histograms of the summed peak intensities of the C7 to C[ 2 n-alkanes, C7
to C, 2 n-alk-1-enes, C0 - to C3- alkylbenzenes, C0 - to C3 -alkylphenols, C0 - to C3
-alkylnaphthalenes, C0 - to C2 -alkylindenes, C0 - to C2 -alkylnaphthols, C0 - to C3
PSEUDOVIT jzozizii RES W ntAnrAnrAÜrMn b SFUSA
h HnaÉI
SFUSB FUS U\mm[7i\nvm&
Fig.2:10. Distribution patterns of the alkylnaphthalenes in the pyrolysate of CUT (a), RES (b), SPOR (c), VIT (d), PSEUDOVIT (e), SFUSA (f), SFUSB (g) and FUS f/ij. Peak intensities were calculated relative to the highest peak in each distribution pattern. N = naphthalene; 2 = 2-methylnaphthalene; 1 = 1-methylnaphthalene; C2= Q-alkylnaphthalenes, tentatively identified as A: 2-ethyl- + 1-ethylnaphthalene; B: 2,6- + 2,7-dimethylnaphthaiene; C: 1,3-+ 1,7-diiÜ'ethylnaphthalene; D: 1,6-dimethylnaphthalene; E: 2,3-+ 1,5-dimethylnaphthalene; F: 1,2-1,5-dimethylnaphthalene; C3= C3-alkylnaphthalenes
