Organically bound sulphur in coal: A molecular approach

Fuel Processing Tech.noiogy, 30 (1992) 169-176 Elsevier Science Publishers B.V., Amsterdam

109

Et’eview _

Organically bound sulphur in coal: A molecular approach*

Jaap S. Sinninghe Damste and Jan W. de Leeuw De& Uniuersity of Technology, Fadty of Chenid Eflgimering ad M~th7k S&WZ, Or&!anic Geochemistry Unit, De Vries uan Heystplantsoen 2,2628 RZ Delft (Nethedands)

(Received July 4th, 1991; accepted in revised form November 1st.. 1991)

Abstract

A critical review of literature concerning the molecular characterization of low and high molecular weight organosulphur constituents pre.sent in roar a ____ _a wel! as a detailed analvsis of organic crlphur ccmpounds present in flash evaporates and pyrolysates of a suite oi coals ranging in sulphur content and degree of coalification reveals that the structure of organosulphur constituents in coal is very similar to that of organosulphm constituents present in bitumen and kerogen of sediments and in petroleums. Based on extensive knowledge with respect to the origin, nature and fate of organosulphur constituents in sediments and oils it is concluded that organosuiphur constituents in coal are generated in the early stages of diagenesis (i.e. peat stage) by reaction of H,S or polysulphides (HS; ) with functionalized precursors yielding organic mono- and polysulphide moieties and thiophenes and that by increasing coalification alky!ated thiophenes are converted to alkyiated benzo[b]thiophenes and dibenzothiophenes via cyclisation and subsequent aromatization reactions.

1. INTRODUCTION

Although coals are estimated to represent only 1% of the organic matter in the subsurface of the Earth [ l] detailed qualitative and quantitative studies of organically bound sulphur in coals are highly justified because of the world- wide environmental problems due to the high SO2 emissions caused by the utilization of coals as a major fossil fuel. Knowledge of the molecular structures of organic sulphur compounds (OSC) and sulphur-containing moieties present in high molecular weight fractions of coals is also of fundamental interest. Recent investigationa have shown that carbon skeletons of suiphur-containing substances carry valuable and subtle information about the degree of thermal maturation (coalifieation) and about the depositional palaeoenvironment

Correspondence to: Dr. J.S. Sinninghe Dam&& Delft University of Technology, Faculty of Chem- ical Engineering and Materials Science, Organic Geochemistry Unit, De Vries van Heystplant- soen 2,2628 RZ Delft (Netherlands)

*Delft Organic Geochemistry Unit Contribution No. 227.

037%3820/92/$35.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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12-71. In this respect sulphur-containing substances present in the geosphere are considered biomarkers.

Apart from organically bound sulphur coals can also contain pyrite, elemental sulphur, inorganic sulphates and minor amounts of inorganic sulphides other than pyrite. In an excellent review dealing with the sulphur distribution in American bituminous coals Stock et al. [8] make clear that elemental sulphur and inorganic sulphates are the result of weathering processes upon stor- age and are not present in pristine coals. This means that in essence we have to deal with only two modes of occurrence of sulphur in coal and probably also in non-coaly sedimentary organic substances, inorganic sulphides (mainly pyrite) and orgamcally bound sulphur, assuming that the sedimentary organic matter to be analysed has not undergone natural weathering. Most of the pyrite and other inorganic sulphur-containing substances can be removed relatively easily from the coals. This is not the case for sulphur-containing organic substances. Therefore a better understanding of the molecular structures of OSC in coals as well as in other fossil fuels such as petroleum as a function of depositional environment and degree of coalification is a requirement in attempts to develop strategies to minimize SO2 emissions resulting from utilis- ation of fossil fuels, in particular coals. The depositional environmcmt in terms of standing crop and physicochemical conditions (e.g. salinity, oxygen level, temperature, rate of sedimentation, availability of iron) is crucial for deter- mining the type and quantity of organic sulphur-containing substances present in coals because it has become clear recently that these substances are formed in the top layers of sediments or even in stratified water columns during the very early stages of diagenesis [ 2,3,5,6,9-211. This early generated suite of OSC is subjected to transformation reactions induced by increasing tempera- turpQ ~lnon bnrisl over geological times. The spectrum of OSC encountered in -WY -_r..-- I_____ coals and in other sedimentary organic matter thus reflects depositional environment and degree of maturation.

In coal science many attempts have been made to discriminate between organic and inorganic sulphur-containing substances on the one hand and between different families of organic sulphur-containing substances (e.g. thiols vs. sulphides vs. thiophenes) on the other hand using a variety of techniques and methods. Stock et al. [ 81 critically reviewed the most important metho- dologies in this respect and came to the conclusion that some ofthem are prom- ising though the limitations are numerous, not only in a quantitative sense but also qualitatively. These observations underline the need to analyze sulphur- containing organic substanpes in coals and other sedimentary organic matter on an individual basis in a molecular fashion. This review will focus on these aspects of organically bound sulphur in coal.

Over the last five years our knowledge of the molecular structure of organically bound sulphur in sediments and fossil fuels has increased dramatically; literally thousands of structures of OSC have been firmly identified especially

111

in crude oils and in extracts of recent and ancient sediments ranging from very immature to mature (for a recent review, see Sinninghe Damste and De Leeuw [4] ). A priori, there is no reason to believe that sulphur-containing organic substances occurring in lignites, brown coals and bituminous coals should be different from those in sediment extracts, kerogens and crude oils, particularly if we compare the definitions of coal and the so called type III kerogen. Ac- cording to Francis [22 ] “coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natura! agencies, interspersed with smaller amounts of inorganic matter. The plant debris was derived mainly, though not exclusively, from vegetation growing on land”. Tissot and Welte 1231 state that type III kerogen “is mostly derived from terrestrial higher plants.” Hence, these authors refer to exactly the same originally contributing organic matter. The differences between the two ad- dress the relative amounts of inorganic vs. organic matter in the sample (kerogen is the insoluble organic matter concentrated and isolated from a mineral matrix) and to the solubility of the organc matter in organic solvents (coal consists of a soluble and an insoluble fraction). We think that these differences have hardly any significance for the structures of OSC and sulphur-containing moieties present in high molecular weight organic matter in both of them.

For this reason and because the far greater part of rigorous structural iden- tificatiens of OSC is based on analyses of non-coaly samples it is logical to start this review by summarizing the molecular structures of sulphur-containing substances present in low and high molecular weight fractions of recent and ancient sediments as well as in crude oils. In the second part of our review we compare these substances with those encountered in coal samples. Because of the scarcity of literature rigorous structural identifications of OSC in coats and coal pyrolysates we have analyzed a variety of coal samples for sulphur- containing organic substances and we report the results in the third part of this paper. The final part of the review is dedicated to a discussion of OSC in coals with respect to their origin, the mechanism ofsulphur incorporation and characteristic changes of distribution patterns of OSC as a function of coalification. Throughcut this paper comparisons will be made between sulphur- containing organic substances present in coals and non-coaly sediments. The review ends with a number of conclusions and recommendations for future studies.

The compounds mentioned in this review are named according to the IIJPAC nomenclature for polycyclic aromatic compounds, summarized by Lee et al. [ 241. For clarity and convenience, names, structures and numbering of several poiycyclic aromatic thiophenes are summarized in Fig. 1.

2. ORlGlN AND DIAGENETIC PATHWK:S OF ORGANICALLY BOUND SULPHUR IN THE GEOSPHERR

In this section an overview of the recent developments in the characterisation and geochemistry of organically bound su.lphur in fossil organic matter

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Fig. 1. Structures, numbering and nomenrlatur~~ of some selected aromatic OX.

wiil be given. Most of these studies are not concerned with organically bound sulphur in coal, but we think that these studies can be of great value in our understanding of the origin and diagenetic pathways of organically bound sulphur in coal. For more detailed reviews we refer to Sinninghe Damstd and De Leeuw [ 4] and Qrr and Sinninghe Dam& [ 251.

This section is divided into three parts: one will deal with low molecular weight OSC, the second part deals with organically bound sulphur present in macromolecules (e.g. kerogen, coal, asphaltenes) and the last part will sum- marize the effects of maturation on both forms of organically bound sulphur. The division between individual OSC and sulpbur-containing macromolecules is of course arbitrary; we s7ill adhere to a separation at a molecular weight of ca. 800 daltons.

2.1 Low mobcular weight 0.X

The identification of low molecular weight OSC in fossil organic matter started with the identification of ten alkyl sulphides in Ohio crude oil 1261.

,ss ; 086 C

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and synthetic compounds. This new group of OSC can be c!e.ssified according to their carbon skeletons and their sulphur-containing functional groups (Fig. 3). In total, they represent more than 1500 individual OSC. Some examples of the structures involved are given in Fig. 4. For a detailed overview of all the identified structures see Sinninghe Damste and De Leeuw [ 4 1 and reference 3 cited therein.

All these OSC can be related to biochemical precursors and testify, at tb. molecular level, to the occurrence of sulphur incorporation into sedimentar organic matter. Figure 5 gives three examples of presumed precursors and their corresponding OSC. These examples show that the original sites of the func- tionalities determine the position of sulphur incorporation into the molecule. Since these OSC are found in thermally immature sediments (mean vitrinite reflectance $ c 0.3% ), e.g. oceanic sediments sampled during DSDP and ODP sampling cruises [10,21], their presence is additional evidence for the very early incorporatioa of sulphur into sedimentary organic matter. The identification of Cz6 and C,, -unsaturated highly branched isoprenoid thiolanes in a 5000 year old sediment from the Black Sea [19] strongly supports this theory.

The actual mechanism of sulphur incorporation is still not completely understood. The work of Vairaivamurthy and Mopper [ 38,391 has shown that. H.$S can react with activated double bonds (e.g. the double bond in acrylic acid) by a nucleophilic addition reaction in ._ Faoent sediments. However, there

S-COiJTAlNING FUNCTIONAL GROUP

number of isomers: a i-10

@ 11-100

Q >lOO

Fig. 3. Major groups of 0% with carbon skeletons identical with those of geologically occurring hydrocarbons identified in the 1960’s. They are grouped according to their carbon skeleton and sulphur-containing functional group. Number of isomers identified are indicated. These data are compiled from Refs. [2,3,5,6.9-21,80,83,90,121-124,156-1701.

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RI and P.2 = n-w XII XIII

Fig. 4. Examples of structures of OSC of the group? indicated in Table 2. Carbon skeletons are indicated with bold lines. (Reprinted with permission from Geochemistry of Sulfur in Fossil Fuels (W.L. Orr and CM. White, Eds.), ACS Symp. Vol. 429, W.L. Orr and J.S. Sinninghe DamstB, Geochemistry of sulfur in petroleum systems, 0 199b! American Chemical Society. F

is also evidence for the fact that sulphur incorporation reactions occur with non-activated double bonds [ 2,191. It is possible that an as yet unknown nat- urally occurring catalyst plays a role in this respect. Furthermore, incorporation of polysulphides into sedimentary organic matter has to be considered as an alternative for reaction with hydrogen sulphide. Kohnen et al. [ 17,201 have identified a number of QSC in immature sediments which contain a six- or seven-membered ring with two and three adjacent sulphur atoms, respectively, and with linear and isoprenoid carbon skeletons. These OSC probably origi- nate from incorporation of polysulphides into specific lipids.

2.2 Organically bound sulphur in fossil macromolecules

A significant part of the organically bound sulphur in fossil organic matter (i.e. kerogen, coal, oil, bitumen) is present in a macromolecular form. Kerogen, which normally represents at least 90% of the organic matter in immature

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precursor organic sulphur compound

b) - /, ..____a>

Fig. 5. Three examples of presumed precursors and their corresponding OSC. Bacteriohopanete- trol (a) is biosynthesised by bacteria [ 171,172], phyta-1,3-diene (b) results from the ubiquitous geologically occurring phytol [ 173 ] and the C2:, highly branched isoprenoid alkadiene (c) is found in sea ice diatoms [ 1741. (Reprinted with permission from Geochemistry of Sulfur in Fossil Fuels (W.L. Orr and CM. White, Eds.), ACS Symp. Vol. 429, W.L. Orr and J.S. Sinninghe Damste, Geoc%emistry of sulfur in petroleum systems, 0 1990 American Chemical Society.)

sediments, ‘nay contain up to 14% (by weight) organic sulphur 1401. Coal may contain up to 10% organic sulphur [ 411. In crude oils and bitumens the polar (often referred to as resin or NSG fraction) and asphaltene fractions may also contain a large fraction of the total organic sulphur [ 311.

Characterisation of the organically bound sulphur in these fractions at a molecular level has been mainly obtained so far by two degradative methods: thermal and chemical degradation. Solid state spectroscopic techniques cur- rently lack the necessary resolution to provide detailed information on the form and the molecular arrangement in which t.he organic sulphur is bound. The X-ray technique (X-ray absorption near edge structure spectroscopy; XANES) recently reported by George and Gorbaty [42] is an exception in this respect; it can provide a determination of the form and amount of organically bound sulphur in high molecular weight organic material. However, it does not give information on moieties bound by sulphur in the macromolecules.

2.21 Thermal degradation Thermal degradation of macromolecules affords smaller fragments, which

can subsequently be characterised. The structure and abundance of the generated products will provide information with respect to the macromolecular structure. Ideally, the thermal degradation products are smaller, but still in- formative, parts of the macromolecules which have not undergone any structural modification. This latter aspect is the vulnerable part of the method. Therefore, thermal degradation is often performed in an inert atmosphere (pyrolysis) and under conditions which minimize the formation of secondary

ll?

pyrolysis products. By definition, thermal degradation is not suitable for the characterisation of thermally labile moieties.

Flash pyrolysis in combination with gas chromatography and/or mass spectrometry is a thermal degradation method which fulfills the conditions for a proper characterisation of macromolecules as described above. Curie point flash pyrolysis is mostly used. With this method the sample is applied to a ferromagnetic wire which is subsequently rapidly inductively heated to its Curie temperature due to the interaction of the wire with a high frequency electro- magnetic fie!d. It is performed in an open system and the pyrolysis products are rapidly removed from the heated zone by a gas flow. This technique has been used for the characterisation of bio- and geopolymers such as lignins (e.g. [43,44] ), humic substances (e.g. [45,46] ), coal and its maceral fractions (e.g. [ 47-49]), asphaltenes (e.g. [ 50-521) and kerogens (e.g. [ 51-591). A limita- tion of this technique is that no fractionation of the pyrolysate into compound classes can be performed. Therefore, the gas chromatograms and total ion current traces resulting from this technique may be extremely complex, which may prevent proper compound identification and quantitation. These problems can be overcome by performing a much more elaborate off-line pyroiysis on a selected number of samples, which enables fractionation of the pyrolysate although losses of volatile pyrolysis products occur. Furthermore, the less con- trolled pyrolysis conditions during off-line pyrolysis promote secondary reactions of primary pyrolysis products.

Recently, flash pyrolysis techniques have also been applied to the characterisation of organically bound sulphur in fossil macromolecules [60-761. Or- ganically boond sulphur in kerogen and other sedimentary macromolecules is though to be present in sulphicle, di- and/or polysulphide moieties and as a constituent of aromatic heterocycles [ 23 1. (Poly)sulphide bonds are not very stable thermally and will decompose into the uninformative pyrolysis product hydrogen sulphide [ 77,783. Therefore, thermal degradation is not an effective method to study the nature ofthese moieties. Wowever, aromatic sulphur compounds (e.g. thiophenes and benzo [ b] thiophene) are thermally stable up to temperatures greater than 800” C i 79 ] . Therefore, flash pyrolysis is useful to study these kind of moieties in fossil macromolecules. Flash pyrolysis of organic sulphur-containing fossil macromolecules will generate low molecular weight CSC which can be separated from each other and other, non-sulphur- containing pyrolysis products by gas chromatography. Since extremely complex pyrolysates are generated, sulphur-selective detection is a prerequisite. The flame photometric detector (FPD) is such a detector and can be used in parallel with the FID. An example is given in Fig. 6 which shows the FID and FPD chromatograms of the pyrolysate ofthe immature (RO =0.37% ), sulphur- rich kerogen of the Triassic Serpiano shale (Switzerland). Peak identifications refer to Tables 1 and 2. The FPD chromatogram reveals that hydrogen sulphide (from thermal degradation of cyclic and acyclic sulphide and poly-

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119

TABLE 1

Non-sulphur compounds identified in flash pyrolysates of kerogens and coals

Peak” Compound

A B C D E F G H I J K L M N 0

: R S T U v W X Y 2

Benzene Methylcyclohexane Toluene 2-Methylheptane 2,6-Dimethylheptane Etbylbenzene m- andp-Xylene ff -Xylene 2,6-Dimethyloctane 4-Methylnonane Phenol 2,&Dimethylnonane g-&~~vlnhmol 4-r------” m- andp-Methylphenol C,-phenols iu’apbtbaiene 2.6-Dimethylundecane 2-Methylnaphthalene I-Methylnaphthalene Co-Naphthalenes Cadaiene ( 1 -isopropyl-4,7-dimethylnaphthalene ) Norpristane (2,6,10-trimethylpentadecane) Pristane (2 6,10,14-tetramethylpentadecane) Prist-I-ene Prist-2-ene Phytane (2,$,10,14-tetramethylbexadecane)

“Letters refer to peaks in Figs. 6,12,18-24, and 26-28.

Fig. 6. Par&jai (O-40 min) FID (A) and FPD (B) chromatogram of the pyrolysis products generated upon flash pyrolysis (Curie temperature 61O”C, 10 3) of the extracted whole rock (TQC 37.8%; McEvoy and Giger [81]) of the Serpiano oil shale. E 25 mx0.32 mm i.d. iused :;ilics capdlary column co&d *with CP Z-5 (film thicknessz0.45 pm j was used with helium as carrier gas. The temperature was programmed from 0°C (5 mii), by using cryogenic cooling, to 300°C (15 min) at 3°C min-‘. The effluent of the capillary column was split to the FID and FPD using a stream splitter at the end of the capi!lary column. The FID chromatogram is normaiised on the most abundant, “non-gas” peak. The FPD chromatogram is normalised on the second most abundant aikylthiophene. Letters and numbers refer to Tables 1 and 2, respectively. Black dote and triangles indicate n-alk-1-enes and n-alkanes, respectively. Italic numbers indicate the number of carbon atoms of these compounds.

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TABLE 2

Sulphur compound identified 1.1 flash pyrolysates of kerogens and coals

Peak” Compound I_-

1 Hydrogen sulphide 2 Thinphene 3 2-&%,hy!thiophene 4 R-Metbylthiophene 5 2-Ethyhhiophene 6 2,5.Dimethy!tbiophene 7 Z&Dimethylthiophene 8 2,3-Dimethylthiophene 9 $4Dimethylthiophene

10 2-Propylthiophene II 2-Ethyl-5-methylthiophene 12 2-Ethyl-4-methylthiophene 13 2,3,5-Trimethylthiophene 14 2-MetnyL5vinylthiophene 15 2,3,4-Trimethyltbiophene 16 3-Isopropyl-2-methylthiophene 17 2-Methyl-5-propylthiophene and 2,5-drethylthiophene 18 Unknown suiphur compound 19 2-Butylthiophene 20 2-Ethyl-3,5_dimethylthiophene 21 Ethyldimethylthiophene 22 5-Ethyl-2,3-dimethylthiophene 23 Ethyldimethylthiophene 24 2,3,4,5-Tetramethylthiophene 25 2-Ethyl-5 q~~pylthiophene 26 3,5-Dime:‘?yl-2-propylthiophene and a C,-thiophene 27 2-Butyl-5.methylthiophene 28 2-Pentylthiophene and 2,3-dimethyl-5-propylthiophene 29 Renzo [blthiophene 30 2,3-Dimethyl-5-(2-methylpropyl)thiopheneand a C,-thiophene 31 2-Butyl-5-methylthiophene 32 5-P*utyl-2,Sdimethylthiophene 33 2-Methyl-5-pentylthiophene 34 2-Hexyltniophene and 2-butyl-3,5_dimethylthiophene 35 C,-Benzo[b]thiophenes 36 ‘I-Methylbenzo [blthiophene 37 P-Methylbenzo [ b] thiophene 38 5- and S-Methylbenzo[b]thiophene 39 4- and 3-Methylbenzo [ b ] thiophene 40 C,-Benzo [ bl thiophenes 41 2- and 4-Ethylbenzo[b]thiophene 42 2,6-Dimethylbenzo[b]thiophene 43 2,4-Dimeth.ylbenzo[b]thiophene 44 2,3-Dimethylbenzo [bltbiophene 45 C3-benzo[bjthioynenes

TABLE 2 (continued)

46 C,-benzo[b]thiophenes 47 Dibenzothiophene 48 C,-dibenzothiophenes 49 4-Methyldibenzothiophene 50 2- and 3-Methyldibenzothiophene 51 1-Methyldibenzothiophene 5.2 C,-dibenzothiophenes 53 Benzonapbthothiophene 54 C!-benzona~nthor‘nirl?~~~~~

^Numbers refer to peaks in Figs. 6,12-14,18-24, and 26-28.

sulphide moieties), thiophene and C,-C, alkylated thiophenes are the major sulphur-containing pyrolysis products. The alkylthiophenes have been ide,n- tified by detailed mass spectrometric studies combined with the synthesis of a number of authentic standards [ 64 ], They are dominated by a limited number of the theoretically possible isomers; e.g. (i) in the &-cluster Z-methylthiophene is much more abundant than 3-methylthiophene; (ii) of the six Cz-thiophenes 3-ethylthiophene and 3,4-dimethyithiophene are very minor and (iii) the C,-thiophenes are dominated by 2-ethy!-5-methylthiophene and 2,3,5-trimethylthiophene. The alkyl substitution patterns of the dominant alkylthiophenes bear a strong similarity to those of OSC recently identified in immature bitumens and crude oils 1651. Therefore, the C,-C, alkylthiophenes are thought to be formed by P-cleavage of specific thiophene units in the fossil macromolecules with linear, isoprenoid, branched and steroidal side-chain carbon skeletons. Examples of such thiophenic moieties and their corresponding pyrolysis products are shown in Fig. 7. These thiophenic moieties in fossil macromolecules are believed to have a similar origin as the low molecular weight OSC in bitumens and crude oils; they are probably also formed by incorporation of inorganic sulphur species into functionalized precursor lipids [65,66,80] during early diagenesis.

Sinninghe DamstQ et al. [64,65] and Eglinton et al. [72-741 have shown that alkylthiophene compounds are always present in flash pyrolysates of immature kerogens, petroleum and bitumen asphaltenes and coals, even if the amount of organically bound sulphur is low as in Type I kerogens. In organic sulphur-rich samples the alkylthiophenes are often major pyrolysis products. For example, in the flash pyrolysate of the Serpiano oil shale (the kerogen has a total sulphur content of 9.0 wt.% [81] ) the alkylthiophenes are major peaks in the FID chromatogram (Fig. 6A). Although the sulphur in the alkylthiophenes represent only ca. 6% of the total organically bound sulphur, it was shown that there is a distinct correlation (?=0.89) between the thiophene ratio: (2,3-dimethyylthiophene)/(n-non-i-ene+ 1,2dimethylbenzene), measured in flash pyrolysates, and chemically determined atomic S,,/C ratios [ 721.

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alkylthiophene

moieties in kerogen fl2sh pyrolysis products

Fig. 7. Examples of thiophenic units with linear, branched, isoprenoid and steroidal side-chain carbon skeletons present in kerogen and their presumed flash pyrolysis products. (Reprinted with permission from Geochemistry of Sulfur in Fossil Fuels (W.L. Orr and CM. White, Eds.), ACS Symp. Vol. 429, J.S. Sinninghe Dam&, T.I. Eglintcn, W.L.C. Rijpstra and J.W. de Leeuw, Char- acterization of organically bound sulfur in high molecular weight, sedimentary organic matter using flash pyrolysis and Raney Ni desulfurisation, 0 1990 American Chemical Society.)

Furthermore, a trivariate plot was used to depict the relative abundances of each of the three individual components used to derive the thiophene ratio. This plot is reproduced in Fig. 8. It shows the position of 68 immature (& < 0.5% ) kerogens of four different types [ 231. Type II-S kerogens are defined as kerogens with an atomic S,,./C r 0.04 6401. These kerogens are clearly different from the other samples and are found in the lower right-hand corner of the diagram. Their thiophene ratios are B 0.6-0.7 [ 721. Hydrogen-rich, Type I kerogens plot in close proximity to the n-non-l-ene apex whilst hydrogen- poor Type III kerogens and coals are located towards the 1,2-dimethylbenzene apex. Type II kerogens are seen in the central part of the diiagram. Although the amounts of 2,3-dimethylthiophene relative to the total C,-C, alkylated thiophenes varies from sample to sample, this potential source of error does not affect the overall bulk variation in the thiophene content between samples. Therefore, use of this simple ternary diagram to obtain an idea about the hydrogen and sulphur content and, thus, about the quality of the fossil organic matter seems justified. This is particularly useful because information on the abundance of organically bound sulphur in fossil organic matter is difficult to obtain due to its intimate association with pyrite [40,82].

22.2 Chemical degradation Sulphur-selective chemical degradation of high molecular weight fossil sub-

stances has only been successfully applied with soluble macromolecules (i.e.

123

c, 0 Fig. 8. Ternary plot showing the reiative abundances of 2,3-dimeth_ylthiophene, n-non-1-ene and 1,2-dimethylbenzene in flash pyrolysates (610°C) of immature (R, ~0.5%) kerogens and coals. (Reproduced by permission of the Publishers, Butterworth-Heinemann Ltd. from Eglinton et al. [72] 0 1990.)

present in the polar and asphaltene fractions of sediment extracts and crude oils). Raney Ni reductively cleaves carbon-sulphur bonds selectively at temperatures well below the temperature at which C-S bonds cleave thermally, and, therefore, desulphurixation of polar fractions releases molecules attached to the macromolecular tte*rix by one or more (poly)sulphide linkages [ 14,66,86,83 1. Raney Ni desulphurization was already being used in the early sixties to characterize OSC (e.g. Thompson et al. [84] ). The compounds generated by Raney Ni desulphurization can be characterised by W-MS and provide information on the structures of the sulphur-linked moieties of the macromolecules. An example is given in Fig. 9 which shows a gas chromatogram of the apolar compounds generated upon Raney Ni desulphurixation of the polar fraction of a sediment extract of a Miocene marl layer from the northern Apennines (Italy) [ 85 1. Apart from a series of n-alkanes, isoprenoid alkanes (e.g. phytane), steranes and hopanes are present. These hydrocarbons were part of macromolecules in which these moieties were bound to each other by intermolecular sulphur cross-links. Flash pyrolysis experiments have shown that some of these moieties may also contain additional intramolecular sul-

124

NORTHERN APENNINES MARL

. n-alkanes

Fig. 9. Total ion current of the apolar products obtained after Ranejr Ni dcsulphurization (yield 11 wt.%) of the polar fraction (sac Ref. [66] for experimental details) of the extract of a Miocene marl layer from the Perticara Basin, Northern Apennines, Italy. Italic numbers indicate total numbers of carbon atoms of n-alkanes. (Reprinted with permission from (Geochemistry of Sulfur in Fossil Fuels (W.L. Orr and CM. White, Eds. ), ACS Symp. Vol. 429, J.S. Sinninghe Dam&, T.I. Eglinton, W.I.C. Rijpstra and J.W. de Leeuw, Characterization of organically bound sulfur in high molecular weight, sedimentary organic matter using flash pyrolysis and Raney Ni desulfurization, 0 1990 American Chemical Society.)

phnr cross-links since series of alkylthiolanes and alkylthianes were formed ~GUJ B macromolecular fraction of an extract of a Jordan oil shale [67]. Al- though it has been previously stated (see Section 2.2.1) that (poly)sulphide moieties generate hydrogen sulphide upon pyrolysis this can be explained by the rapid removal of flash pyrolysis products from the heated zone.

Another recently developed sulphur-selective chemicai degradation method is treatment with MeLi/MeI [86 ]i. This method selectively and quantitatively cleaves di- and polysulphide bonds and methylthioethers are formed. Appii- cation of this technique to different macromolecular fractions from a solvent extract of an immature bituminous shale from the Vena de1 Gesso basin (Italy) revealed that a major portion of the total amount of sulphur-linked biomarkers in macromolecules is linked via di- or poiysulphide moi&es to the macromolecular network [86]. Since the di- or polysulphide linkages are at*t,ached at specific positions of the bound biomarkers, it was proposed that they were formed by intramolecular incorporation reactions of inorganic polysulphides (HS; ) into low molecular weight functionaliaed lipids during early diagenesis.

125

Application of this chemical degradation method on a suite of oils revealed that di- or polysulphide moieties can still be present in petroleum [87].

On the basis of the results from thermal and chemical degradation it can be concluded that the structure of these macromolecules is comparable to t,hose of low molecular weight OSC described earlier. Therefore, the timing and the mechanism of formation of these sulphur-rich macromolecules is likely comparable to that of the low molecular weight QSC, although sulphur ixcorpo- rated in an intermolecular fashion (i.e. cross-linking) occurs as well. It is thought that in certain types of depositional environments the sulphur :incorporation process acts as a significant pathway in the formation of macromolecular material since it transforms low molecular weight lipid precursors into high molecular weight organic matter (“natural vulcanisation”).

2.3 Effect of thermal maturation on organically Sound su1ph.w in fossil organic matter

Relative low degrees of thermal stress have a significant effect on the organically bound sulphur in fossil organic matter. Gransch and Posthuma [88] noted that the early generation products from artificial maturation (330°C for a few to 500 h) of a sulphur-rich kerogen (La Luna, Venezuela) contained much more organic sulphur tha~l did the products obtained later. This means that the organically bound sulphur is preferentially lost from the kerogen during the early stages of catagenesis. This has been confirmed by more recent. studies of Eglinton and co-workers [ 72,73 1. Thiophene moieties were observed to be removed from kerogen during both natural and artificial (ZOO-360°C for 72 h) maturation more rapidly than both aliphatic and aromatic hydrocarbon moieties. The thiophene ratio (see above) decreased with increasing maturation for three natural maturity sequences and an artificial maturity set f 731, indicating a decrease in the atomic S,,/C ratio of the kerogen with increasing maturation. The decrease in the thiophene ratio was demonstrated to be due to an absolute decrease in the abl.mdance of the thiophene precursors. Kohnen et al. [89] have recently shown that upon artificial maturation (ZOO-330°C for 24 to 2064 h) of a sulphur-rich kerogen (Jurf ed Daramish oil shale, Jordan) the major sulphur-containing products are C,-C, alkylated thiophenes, the same products as generated upo~ flash pyrolysis [ 64 1. Furthermore, these alkylthiophenes were more abundant relative to n-alkanes at lower artificial maturation tempei&ares. Similar findings were reported by Ishiwatari et al. f 751. These results confirm those of Eglinton et al. [73] but a proper explanation at the molecular levei for the reason why the alkylthiophenes are generated at lower levels of thermal exposure than hydrocarbons is still lacking.

In addition to the rapid loss of organically bound sulphur from kerogen upon natural and artificial maturation processes specific changes were observed in the distributions of sulphur-containing flash pyrolysis products [69,71,73].

126

2

I ? i -l_L_.l_

L

Artificial Maturation Kimmeridge kerogen, Dorset, U.K.)

I I

[25o”c[

16 ‘3 I

I I I1 I I

127

Fig. 11. Presumed diagenetic pathways for low molecular weight OSC and suiphilr-containing moieties in high molecular weight substances starting from a hypothetical iinear precursor. (Re- produced with permission from Organic Geoc;iemistry, 16, J.S. Sinninghe Dam&e and J.W. de Leeuw, Analysis, structlne and gaochemical significance of organically bound sulphur in the geosphere: State of the art and future research), 0 1990 Pergamon Press pk. f

Fig. 10. Partial FPD chromatograms (O-60 min) of flash pyrolysate (610°C) of the isolated kerogen of the black stone band from the Kimmeridge (Dorset, U.K.) and the extracted residues of this kerogen after artificial maturation (hydrous pyrolysis, 3 days) at different temperatures, Experimental conditions are as indicated in the caption of Fig. 5. Peak identification: (1) thiophene, (2) P-metbylthiophene, (3) 3-methylthiophene, (4) 2-ethylthiophene, (5) 2,5-dimethylthiophene, (6) 2,4-dimethylthiophene, (7) 2,3-dimethylthiophene, (6) 3,4-dimethylthiophene, (9) 2-propylthiophene, (10) 2-ethyl-5_methylthiophene, (11) 2-ethyl+methylthiopbene, (12) 2,3,5trimethyIthiophene, (13) 2-methyl-5-vinylthiophene, (14) 2,3,4-trimethyithiophene, (Xi! benso[b]thiophene, (16) C,-benzo[b]thiophenes, (17) Ce-benzo[b]thiophenes, (18) C,- benzo[b]thiophenes. (Reprinted with permission from Geochemistry of Sulfur in Fossil Fuels (W.L. Drr and C.M. White, Eds.), ACS Symp. Vol. 429, T.I. Eglinton, J.S. Sinninghe Dam&, M.E.L. Kohnen, J.W. de Leeuw, S.R. Larter and R.L. Patience, Analy;is ofmaturity-relatedchanges in the organic sulfur composition of kerogens by flash pyrolysis-gas chromatography, 0 1990 American Chemical Society. )

128

The most important change is the increase of alkylbenzo [blthiophenes and alkyldibenzothiophenes relative to alkylthiophenes. This is illustrated by the FPD chromatograms from flash pyrolysis of Kimmeridge kerogen and residues from artificial maturation (72 h) of this kerogen at different temperatures (Fig. 10). These changes were also noted in natural maturity sequences [ 69,731. An explanation for the observed differences is cyclisation and subsequent aromatization of alkylthiophene precursors in the kerogen. Tllese reactions have also been proposed to explain the compositional differences of immature bitumens and crude oils and more mature samples with respect to sulphur compounds [ 3,4,13,65,90]. The latter contain QSC with thiophene only as part of more complex ring systems (i.e. benzo [ b] thiophenas, dibenzothiophenes and benzonaphthothiophenes) with very short alkyl side chains (e.g. [91-951) whilst less mature samples contain mainly alkylthianes, -thiolanes and -thiophenes. The disproportionately large amounts of 2,4-dialkylbenzo [b] thiophenes relative to other alkylated benzo [ b] thiophenes found in sediments and oils [ 13,901 testifies to the occurrence of these reactions in nature since alkylbenzo[b]thiophenes with this specific substitution pattern are likely to be formed by cyclisation and subsequent aromatization of 2,5-dialkylthiophenes. White et al. (961 proposed similar reactions to explain the presence of a number of OSC families in an extract of the Rasa coal. In Fig. 11 some proposed diagenetic pathways of sulphur-containing units in both iow and high niolec- ular weight organic matter are illustrated with a linear precursor moiety as an example. Methyl transfer reactions are proposed by Radke [ 97,98] to explain iscmcrisation reactions of both eromatic hydrocarbons and OSC and can be used to assess the maturity level.

3. ORGANIC SULPHUR COMPOUNDS AND SULPHUR-CONTAINING MOIETIES IN HIGH MOLECULAR WEIGHT SUBSTANCES IN COAL

In this section an overview is given of literature data dealing with the identifications or tentative identifications of extractable and (X-amenable OSC and of sulphur-containing pyrolysis products generated from high molecular weight substances via flash-pyrolysis. Data concerning QSC released from coals via selective chemical reagents are not considered here because this aspect has been reviewed recently by Stock et al. [8].

3.1 Organic sulphur compounds in coal

A large variety of solvents has been used to extract coals. Relatively widely used are pyridine, ethylenediamine and tetrahydrofuran because the yields of extracted compounds are high (in the range of about 25 wt.% ). A slight dis- advantage of the use of these solvents is their relatively high boiling points and polarities in comparison with other solvents such as pentane or dichlorome-

129

thane; during the concentration of extracts, volatile components are removed together with these high-boiling solvents and a.re not analysed. Complete removal of pyridine is difficult, probably because it associates or bonds (hydrogen bonds) to the extracts.

Organic sulphur compounds in coal extracts have been analysed in different ways. A few investigators have applied high-resolution mass spectrometry (HRMS) on total extracts [ 96 ,9S- 1021. Fractionation of total extracts to con- centrate OSC have been perfo:med by iiquid-exchange chromatography [ 103- 1051 and by means of column chromatography followed by oxidation, column chromatography and reduction [106]. In most cases the OSC-enriched fractions have been anaiysed further by gas chromatography combined with flame ionisation and flame photometric detection (FID and FPD, respectively) and/ or mass spectrometry (GC-MS) [ 100,102,103,107-1091.

The different modes of fractionation, separation and identification represent different scopes and limitations; GC-MS is a powerful technique for identifying individual compounds but only the volatile part (i.e. which is GC-amenable j of the extract can be analysed. The application of low-voltage high- resolution mass spectrometry (LVHRMS) is to some extent limited by the high complexity of coal extracts whilst isomer distributions cannot be determined at all. No real identifications can be made by LVHRMS because in

I -n,..^+,.^C..-^^----^^^:~l^ &%._,......,.....A o ~GXZEh umuy ulr~u~cu~c3 cut2 pu3nlu~ XVI ~vu3pvu~~uo v-i~ui~ u*kl uy..Iv ~._._._.____ ..Ath the ~nrn~ nlmmontsl

composition. Furthermore, one cannot be certain that t,he mass spectra are representative of the entire sample. However, LVHRMS is useful for the determination of molecular formulae of compounds (especially aromatics) in complex fuel mixtures. Only by means of GC-MS analysis using standard OSC structural identifications can be confirmed.

Despite the excellent and very extensive work of several investigators (e.g. [ llO-1141) over the years it is almost impossible to unequivocally identify polycyclic aromatic thiophenes present in sedimentary samples such as coals. The vast numbers of the monomethyl derivatives of bicyclic, tricyclic and tetracyclic aromatic thiophenes (see Table 3) illustrates this point. In many cases the alkylsubstituents extend up to Cl,, and hexa- or heptacyclic aromatic thiophenes are not uncommon. As a consequence many thousands of polycyclic aromatic OSC may be present in a coal extract sample and rigid structural identifications based on comparison with standards is a very difficult task. This can be illustrated further by the following; The relatively simple family of monomethyl tetracyclic aromatic thiophenes (molecular weight 248 g/mol) consists of ca. 180 components, if the five monomethyl phenanthro[4,5&d]thiophenes (molecu!ar weight 222 g/mol) and eight methylphenaleno[6,7&]thiophenes are excluded. “Only” 33 of these components are available as standards [113]. Although the mass spectra and the relative GC-retention times of these standards are known, the identification of these components in coal extracts must still be considered tentative because other,

TABLE 3

Number of isomers of methyl substituted po!ycyciic aromatic thiophenes with 2,3 and 4 rings -- -

Compound class Number of isomers

Met.hylbenzo[b Jthiophenes 6 Metbylbenzo[c]thiophenes 3

Total bicyclic aromatic thiophenes with one methylsubstituent

9

Methyldibenzothiophenes Methylnaphtho[ 1,2-bjthiophenes Methylnaphtho[l,Z-c]thiophenes Methylnaphtho[2.1-bjthiophenes Methylnaphtho[2,3-b]thiophenes Methylnaphtho[2,3-clthiophenes

Total tricyclic aromatic thiophenes with one methylsubstituent

40

Methylphenanthro[4,5-bcdlthiophene Methylanthra [ 1,2-b] thiophene Methylanthra! 1,2-c]thiophene Methylanthra[2,1-blthiophene Metbylanthra[2,3-blthiophene Methylanthra[2,3-clthiophene Methylbenzo[b]naphtho[ 1,2-dlthiophene Methylbenzo[b]naphtho[2,l-dlthiophene Methylbenzo[b]naphtho[2,3-dlthiophene Methylphenanthro[ 1,2-blthiophene Methy$henanthro[l,2-clthiophene Methylphenanthro[2,1&]thiophene Methylphenanthro [2,3-blthiophene Methylphenanthro[2,3-clthiophene Methylphenantbro[3,2-blthiophene Methylphenantbro[3,4-blthiophene Methylphenanthro [ 3.4-c j thiophene Methylphenanthro[4,3-blthiophene Methylphenanthro[S,lO-b]thiophene Methylphenanthro[9,10-cjthiophene Methylphenaleno[6,7-Gclthiophenes

8 10 10 10 10 IO 10 19 10 10 10 10 10 IO 10 10 10 10 ;- IV

10 6

TOM tetracyclic aromatic thiophenes with one methylsuhstituent

206

as yet unknown, components of the “248” series will have similar or identical mass spectra and relative retention times. Consequently almost all identifications of polycyclic aromatic thiophenes present in extracts or pyrolysates of

coals or coal-derived products must be considered tentative. Throughout this review it is, however, assumed that polycyclic aromatic thiophenes analysed by GC-MS and by comparisons of mass spectra and re!ative retention times with those of available standards are the components reported, uniess there is circumstantial evidence to doubt the reported structures.

The combined application of GC-FID, GC-FPD, GC-MS and LVHRMS has led to identifications of a number of OSC in coals. Thiophene, benzo[b]thiophene, dibenzothiophene, benzo[b]naphtho[&l-dlthiophene and their alkylated derivatives, with up to 10 carbon atoms in the alkyl substituents have been reported. It must be said that much more work has been done on structural identifications of OSC present in coal-derived products, such as solvent refined coals (SRC’s), distillates, etc. According to Stock eb al. [ 8] the knowledge of structures of OSC in coal-derived products is of limited use with respect to OSC in pristine coals since “it must be stressed that the information that has been derived so far by the study of thermal!y -transformed coals cannot be extrapolated to generate a trustworthy picture of coal’s sulphur compounds”.

3.1.1 Alkylthiophenes The occurrence of alkylthiophenes in coal extracts has been reported only

twice [ 101,108]. This limited number of reports may be the consequence of the conditions used during extraction of the coals. It is indeed noteworthy that in the reports cited, the extractions were performed at ambient temperature [101] or by Soxhlet extraction at a relatively low temperature using the meth- anol/benzene azeotrope [lOS]. Hence, although it is likely, it is dificult to conclude with certainty that the volatile alkylthiophenes are generally present in coal extracts. Although a detailed study of the distribution patterns of al: Ci- and Cz-alkyltbiophenes has been performed in coal products [ 112 ] in both papers dealing with coal extracts no detailed analysis of the alkylthiophenes is reported. Hence, nothing is known about the isomeric composition of alkylthiophenes in coal extracts. It is also not clear whether the presence of alkylthiophenes is restricted to relatively low rank coal samples as is the case in immature crude oils and sediment extracts 131. Alkylthiophenes with more than nine carbon atoms have not been encountered in coal extracts, whilst they have been identified in sediment extracts and crude oils [2,3,11]. This may reflect the terrestrial origin of coal precursors reiative to the algal/bacterial origin of petroleum.

3.12 Alkylbenzolb]thiophepzes Alkylbenzo [blthiophenes are frequently observed in coal extracts

~96,100,102,103,107]. Components up to C,, have been noticed. In only one case were full structures of four alkylbenzo[b] thioghenes (see Table 4) assigned [lOO]. Due to the very limited knowledge of isomer distribution pat-

132

terns of alkylbenzo [t ] thiophenes it is impossible at this stage to verify whether 2,4-dialkylbenzo [b] thiophenes are the most abundant ones. This abundance wou!d be expected if the pathways of formation of alkylbenzo [ b ] thlophenes in coals are similar to those postulated for these compounds in sediments and crude oils (see Section 2 ).

3.1.3 Alkyldibenzothiophenes Alkyldibenzothiophenes have been reported in many coal extracts

[96,100,102,103,107,108]. Alkyl derivatives have been encountered with a total number of carbon atoms up to 29 [96].

The structures of eight alkyldibenzothiophenes have ‘been positively identified in coal extracts (see Table 4) [ 100,103]. These rather limited data concerning the alkyl substitution patterns prevent conclusions about the forma- t,ioil of t,hese compounds in the subsurface. It is speculated that alkvldibenzothiophenes are formed from 2-alkyl substituted benzo[b] thiophenes via cyclisation and subsequent aromatization as shown in Scheme 1 [ 41. The reported occurrence of 1,2,3,4,4a,46-hexahydrodibenzothiophene in the extract of the Bevier seam coal [ 1001 to some extent supports the proposed pathway because it demonstrates the occurrence of intermediate hexahydro- components in coals. Via a similar pathway alkylnaphtho [2,1-b] thiophenes can be formed from I-alkylbenzo [b] thiophenes (see Scheme 1).

The tentative assignments of aikgi naphthy! sulphides and phenlrlthio- phenes 11031 in an extract of PSOC-521 coal are noteworthy within this con- text.. White et al. [96] observed molecular formulae consistent with the alkyl naphtyl sulphide family during the LVHRMS analysis of a Rasa coal extract. The elemental compositions of the alkyl naphthyl sulphides and alkylphenyl- thiophenes determined by LVHRMS as well as their reported nomina! mass spectra allows for an alternative tentative assignment as tetrahydroalkylben- zothiophenes and/or tetrahydroalkylnaphtho [ 2,1-b] thiophenes, such as the intermediates shown in Scheme 1. Furthermore, the mass spectrum shown by

Scheme 1

133

Nishioka [103] .!ny reflect one of the tetrahydromethyl compounds instead of propyl naphthyi suiphide. Th ..e intensity of the ion at m/z 159 (M+-43) is probably too low in the case of the latter compound. The relatively high number of homologies consistent with the molecular formulae of alkyl naphthyl sulphides by White et al. [96] are also consistent with the extensive series of a?b$benzo [blthiophenes with a similar internal distribution pattern if they represent one or both the tetrahydrobenzo [ b] thiophene intermediates depicted in Scheme 1. However, at this stage only speculation or at the best working hypotheses can be put forward. Solid stn;ctr;ra! identifications of these OSC are essential to understand their pathways of genesis in coals.

3.1.4 Benzo[b]naphtho[Z,l-d]-, benzo[b]naphtho[l,2-d]- and phenanthro[4,3-blthiophenes

These compounds (see Pig. ! ) as well as s, 1 I* , -w4 ctf their methy! derivatives have been reported in coal extracts [ 1001. White et al. [ 961 reported by means of LVHRMS analysis of a solvent extract of the Rasa coal molecular formulae which are consistent with these compounds. By means of CC-MS and LVHRMS and comparisons with standards a few components (Table 4) of these series were positively identified [ 1001. In the paper by White et al. [ 100 ] peaks 27 and 28 are mislabelled [ 115], Peak 27 should be labelled as ll-meth-

TABLE 4

Standard confirmed identified OSC in coal extracts

Compound Reftxence

3,5-c,imethylbenzo [blthiophene 3.6 Dimethylbenzo [ b] thiophene 2-Ethyl-5,7-dimethylbenzo [ b] thiophene !&;I-Propyl-7-ethylbenzo[b]thiophene 1,2,3,4,4a,40-hexahydrodibenzothiophene Dibenzothiophene 4-Methyldibenzothiophene 2- and/or 3-Methyldibmzothiophene 1-Methyldibenzothiophene 4,6-Dimethyldibenzothiophene 2,6-Dimethyldibenzothiophene 2,6-,3,7- and/or 3&Dimethyldibenzothiophene 1,7-Dimethyldibenzothiophene Benzo[b]naphtho[2,1-dlthiophene lo-Methylbenzo[b]naphtho[2,1-d]thiopheneand/or 11-Methylbenzo[b]naphtho[l,2-dlthiophene Phenaleno[6,7-bclthiophene Phenanthro[4,3 blthiophene Phenanthro[4,5-bcdlthiophene

White et al. [ 100 ! W.&e et al. [ 1001 Whltr et ai. [ 1001 White et al. [ 1001 White et al. [ 1001 White et at. [ IOO]; Nishioka f 103 j White et al. [ 1001; Nishioka [ 1031 White et al. [ 1001; Nishioka [ 1031 White et al. [ 1001; Nishioka [ 1031 White et al. [lciO] White et al. [ 1001 White et al. [ 1001 White et al. [ 1001 White et al. [ IOO] White et al. [ 1001

White et al. [ 1001 White et al. [ 1001 White and Lee [ 107 ]

134

ylbenzo[b]naphtho[1,2-dfthiophene and/or IO-methylbenzo[b]naphtho [2,1- dlthiophene. Peak 28 is unknown. The formation of ll-methylbenzo- fb]naphtho[ P,2-dlthiophene can be rationalized via the postulated ring clo- sure/aromatization mechanism of 1,9-dialkyldibenzothiophenes (see Scheme 1). The alkyl phenanthryl sulphides postulated to be present in the Rasa coal extract on the basis of LVHRMS data can also represent the tetra- hydrobenzonaphthothiophene intermediates 1961.

The presence of these compounds has been reported several times in coal extracts [?03,107]. Compounds having molecular formulae consistent with alkylphenanthro [4,5&d]thiopheneF up to C,, were observed in the extract of the Rasa coal by means of LVHRMS analysis [96]. It has been suggested by White et al. [ 107,116] and by Nishioka [ 1031 that these compounds could be formed through incorporation of sdIphur derived from either elemental sulphur or pyrite during coalification. -;;fiis is supported by the fact that (i) relatively high concentrations of elemental sulphur were observed in the extract of Homestead, Kentucky coal in conjunction with phenanthro[4,5- bcdlthiophene [ 1071 and (ii) the relative abundance of the latter compound and two methy!at.ed derivatives in SRC fractions of the PSOC 521 coal and their absence in the extract of PSOC 521 coal [ 1031. Simulation experiments also indicated the possibility of incorporation of elemental sulphur in polycyclic aromatic hydrocarbons to generate qolycyclic thiophenes [ 1161. How- ever, the presence of elemental sulphur in t.fTe Homestead, Kentucky coal is most likely due to weathering of this coal sample: [8,107]. Hence, incorporation of elemental sulphur in nature is rather unlikely because of the absence of elemental sulphur in pristine coa! samples. An origin from l-alkyldibenzothiophenes via ring ciosure and subsequent aromatization as a result of increasing coalification or under the conditions of the coal derivatisation processes may serve as an alternative (Scheme 2). The intermediate 8,9dihydro derivatives proposed in this hypothetical reaction sequence match the HRi’vIS data observed in the Rasa-coal extract (White et al. [96]; C14+,,H10+znS, n=O-lo), although they were tentatively ascribed to alkyl phenanthryl sulphides. The relatively extended series up to C,, seems to be in agreement with those found for the alkylbenzo [ blthiophenes (up to Czs) and the alkylphenanthro [ 4,5- bed 1 thiophenes (up to CZo ) in the same Rasa coal extract [ 961. The distribution patterns of these series indicate that the most abundant eomponents have

Scheme 2.

alkyl side-chains containing 6 to 10 carbon atoms. Such distribution patterns are not expected from incorporation of sulphur into alkylphenanthrenes, because in coal extracts phenanthrene (C,,) and methylphenanthrenes (C,,) are usually abundant alkylphenanthrenes. More rigid identifications are a preteq- uisite to better understand the origin and formation of the alkylphenanthro [ 4,5&d] thiophenes.

3.1.6 Tiziols and disulphides Although the presence of thiols or disulphides in coal extracts is expected on

the basis of functional group analyses of OSC by reductive or oxidative methods (e.g. [117]) to the best of our knowledge individual thiols or disulphides have never been positively reported by GC-MS analyses. It has been speculated that thiols are rather unstable during isolation and work-up procedures (White et al. [ 100 ] and references cited therein) or even during GC-MS analysis [ 1181 and are converted to disulphides. However, recent work with standard n-al- kylthiols, dialkyl disulphides and dialkyl tetrasulphides and the relative abundance of di- and trisulphides in extracts of very immature sediments [ 17,201 indicate that thiols and disulphides are not as unstable as assumed previously, provided that samples are not exposed to air for long periods and that samples are not heated too high during @C-MS analyses.

Analysis of coal extracts or OSC enriched fractions of coal extracts by LVHRMS are not conclusive concerning the presence or absence of thiols and/ or disulphides. As indicated by White et al. [ 1001 the presence of C,,H,S and C,,H,,S components in Bevier seam coal may represent naphthyl thiols but other compounds are equally possible. The same holds for the compounds with molecular formulae C12+nH 10+2nS2 (n=O-3) which can be ascribed to di- phenyl disulphides [96,100]. Treatment of three high sulphur coals with meth- yliodide failed to produce HI, indicating that thioi groups are absent [ 1191.

Misinterpretations of mass spectra obtained via ‘J-C-MS analysis have also contributed to the confusion concerning the presence or absence of disulphides in mature samples of the subsurface [ 1031. A series of alkyl. phenyl disulphides was thought to be present in Wilmington crude oil. The mass spectrum shown was ascribed to pentyl phenyl disulphide. It lacks, however, the expected major fragment ion due to S-S cleavage (nt/z 109) [ 118,120]. The mass spectrum shown is virtually identical to the mass spectrum of a well characterized C,, bicyclic terpenoid sulphide [ 121-1241 present in a variety of petroleums. Based on a comparison of the data presented by Nishioka [103] and by Payzant et al. [121-1241 it is almost certain that the reported series of alkyl phenyl di- . . ..1..%..1.5. :r w,:,...;,, mq.l~UU~U 1“ I. ~‘~~~1~U”” _I”%.” “1. UA.. uuYY”.-J rr+nn m-sw?o nil a-a ~rtnslix; ;P&C nf hirvriir nnd fvinvc!ir: - ____” __ ___J ___+ -__- ____~ terpenoid sulphides.

This example, though referring to an analysis of OSC in a crude oil, once more illustrates the need to identify individual components unequivocally before firm conclusions on their origin can be drawn. Based on the data reported

136

we have to conclude that at the present stage no rigid evidence exists for the presence of either thiols or disuiphides in coal extracts.

3.2 Sulphur-containing moieties in high-molecular-weight substances present in coal

Analytical pyrolysis in combination with gas chromatography (Py-GC ) or gas chromatography-mass spectrometry (Py-GC-MS) or directly coupled with mass spectrometry (Py-MS) is regularly used to identify sulphur-containing moieties in non-extractable substances in coals. (See for a discussion of the scope and limit,ations of flash pyrolysis Section 2). It must be noted that in several flash-pyrolysis studies the coals were not extracted C99,125,126]. Con- sequently, a part of the “pyrolysis products” may represent OSC which are thermally extracted. In the following it is, however, assumed that the products reported are true pyrolysis products and are representative of the sulphur- containing moieties of the high molecular weight fractions of the coals. The OSC generated by analytical pyrolysis in principle can result from bond cleavage of high molecular weight sulphur-containing substances and from reactions of organic compounds with elemental sulphur or pyrite.

Calkins [ 1251 has shown elegantly that only the highly volatile pyrolysis products SO, and CSp decrease in pyrolysates of a high-sulphur Pittsburgh coal when the pyrite was substantially removed. No significant increase or decrease of ot.her sulphur-containing pyrolysis products (mainly mixtures of alkjrlated thiophenes, benzo[b]thiophenes and dibenzothiophenes) was observed. This observation is supported by flash-pyrolysis data obtained for low-sulphur coals to which relatively high amounts of elemental sulphur or pyrite were added; no increase of sulphur-containing pyrolysis products was observed 11271. However, Winans and Neil1 [99] report that the presence of pyrite during pyrolysis enhances the amounts of aliphatic sulphides considerably. Their data are based on investigations of pyrolysates with high-resolution mass spectrometry. A more detailed analysis of these pyrolysates is required to understand precisely the possible role of pyrite during pyrolysis. Different pyrolysis conditions (pyrolysis temperature and heating rate, open vs. semi-closed type pyrolysis) may also contribute to confusion in the literature concerning the possibie role of elemental sulphur or pyrite during pyrolysis experiments. Based upon the experiments by Calkins [ 1251 and our own observations we assume that during open-system flash pyrolysis experiments performed with untreated or extracted coal samples neither pyrite nor elemental sulphur can generate sulphur-containing pyrolysis products apart from CS, and SOZ.

The pyrolysis products observed in flash-pyrolysates of coals, macerals and extracted coals or macerals analysed by GC-MS and/or GC-FPD are alkylthiophenes (up to C&), alkylbenzo[b]thiophenes (up to C,,), alkyldibenzothiophenes (up to C15) and a benzonaphthonthiophene [60,62,65,73,125,

126,128-1311. Only in two cases were detailed structures of the C, to C,-aikyl- thiophenes reported [ 65,731. These pyrolysis compounds indicate that alkylthiophene and polycyclic aromatic alkyithiophene moieties are present in the macromolecular matrix of coal. It is assumed that these moieties are, at least in part, the precursors of their extractable counterparts described in the previous section. The formation of these sulphur-containing moieties is not completely clear. It is suggested that the same sequence of reactions starting from long-chain alkylthiopbenes as depicted in the previous paragraph is operating within t,he macromolecular matrix as well. Such a mechanism is recently discussed for non-coaly sediment samples (see Fig. 11). A more detailed knowledge of the substitution patterns of the alkyl derivatives of the polyeyciic thiophenic pyrolysis products will certainly help to unravel mechanisms of the formation from their precursors in nature and mechanisms of the genesis of polycyclic aromatic thiophenes from these precursors during the coalification process.

High molecular weight sulphur-containing pyrolysis products have also been observed indirectly by flash pyrolysis-mass spectrometry [60,99,129,132,133 1. Application of flash pyrolysis-low resolution mass spectrometry only allows the tentative identification of highly volatile pyrolysis products such as H&S, CQS, CH,SH, CS, and SC&. Flash-pyrolysis combined with LVHRMS is a much more powerful method of analysis [99 1. Such Py-MS studies indicate that apart from the above described polycyclic aromatic thiophenes sulphur- containing pyrolysis products with additional heteroatoms (Q,N) are present in relatively high amounts in a high sulphur Illinois No. 6 coal [99]. Until recently such pyrolysis products have not been detected in the GC-amenable part of coal pyrolysates. More accurately, Py-GC-MS analysis of the Illinois No. 6 coal did not revpa! sulphur-containingpyrolysis products with additional heteroatoms (see Section 4). Therefore, it is concluded that these products are either too polar to elute from GC columns or that the accurate masses observed represent fragment ions of high molecular weight pyrolysis products, which are, though for another reason, also not GC-amenable.

4. ANALY§:S OF ORGANICALLY BOUND SULPKUR IN SOME SELECTED COALS BY FLASK PYROLYSIS/THERMAL EXTRACTION

In this section we will describe some results on organic sulphur in coals which have been obtained in our laboratory. These results will be discussed in light of the concepts of origin and diagenetic pathways of organically bound sulphur in the geosphere as described in the second part of this paper. In a fevw cases (Rasa, Illinois # 6, Pittsburgh # 8) direct comparisons can be made between previously published data concerning the organic sulphur and data of the samples described here.

A number of coals have been studied by thermal extraction/flash pyrolysis

in combination with GC with dual flame ionisation and sulphur-selective flame photometric detection and GC-MS. Some bulk data of the studied coals are presented in Table 5. These data reveal that this suite of samples consists of sulphur-rich (Gardanne, Illinois # 6; ca. 5% total S), very sulphur-rich (Cher- okee, Rasa; ca. 12% total S) and “normal” to sulphur-lean coals (others). Mean vitrinite reflectance data indicates that this suite of samples also covers a maturity range from & = 0.38-0.68%.

4.1 Gardanne coal

The Gardanne coal is from the Houilleres de Province Mine (France) and was obtained from the Dutch (SBN) coal bank. It is of high volatile bituminous rank and contains high amounts of organic sulphur (4.94%, Table 5). Figure 12A shows the FID chromatogra.,. y - -f the flash 2yrolysate using a ferromagnetic wire with a Curie temperature of 610°C. Letters and numbers in Figs. 12A and 1.2B refer to compounds listed in Tables 1 and 2, respectively. The major groups of compounds present in the pyrolysate are: acyclic hydrocarbons (n-alkanes, n-alk-l-enes, prist-1-ene), aromatic hydrocarbons (mainly alkylbenzenes) , phenols and sulphur compounds (mainly thiophenes ) . Except for the phenols, these compounds are common pyrolysis products of kerogens [ 134,135 I. By using a ferromagnetic wire with a Curie temperature of 358°C instead of 610°C it was shown that all these compounds are for the far greater part (estimated to be at least > 95% ) pyrolysis products. At this lower Curie point temperature the energy released is not sufficient to cleave carbon-carbon bonds and only low molecular weight compounds entrapped in the coal matrix are evaporated (“thermai extraction”) and subsequently anaiysed [ 136 1.

The distribution of the sulphur-containing pyrolysis products can be revealed by using the sulphur-selective FPD. Figure 12B shows the FPD chromatogram of the pyrolysate, Apart from very volatile sulphur compou\ Ids (e.g. I&S, SO,, CS,), low molecular weight alkylated thiophenes are the major sulphur-containing compounds in the pyrolysate of the Gardanne coal. These compounds have been identified by flash Py-@C-MS and by comparison with published retention time data [64]. Mass chromatography of m/z 97,98,111, 112,125,126,139,140,153,154,167 and 168 fragments (Fig. 13) also reveals the distribution of the C,-C, alkylthiophenes. Accurate mass chromatography (mass window 0.02 dalton) was used to eliminate contributions from alkenes; their mass spectra also contain ions with these m/z values. The distribution of the alkylthiophenes as shown by these two techniques is comparable (cf. Figs. 12B and 13). Differences are due to (i) the quadratic response of the FPD, (ii) differences in ion yields of different alkylthiophenes and (iii) co-elutions of alkylthiopbnes with other suiphur compounds (e.g. unsaturated alkylthiophenes).

These al!ryithiophenes are major components in the pyrolysate: e.g. apart

TAB

LE 5

Ele

men

tal a

nd g

eoch

emic

al d

ata

of t

he s

elec

ted

co-!

sam

ples

Coa

: A

sh*

VM

” SW

B

Pyri

te’

Sulp

b.”

(%)

(%)

@73

) W

) (%

I

Car

dann

ea

18.9

45

.0

5.05

1.

13

0.06

Il

linoi

s #6

’ 15

.5

40.1

4.

83

2.81

0.

01

Pitt

sbur

gh #

t3’

9.

3 37

.8

2.19

1.

37

0.01

B

e&h-

Zap

Lign

iW

9.7

44.9

0.

80

0.14

0.

03

Ras

a~

8.0

48.3

10

.84

0.30

0.

02

Che

roke

e (P

SOC

667

F 21

.2

35.2

13

.61

6.01

1.

69

Mah

akam

372

45h

5.2

n.d.

2.

25

1.04

n.

d.

Mah

akam

372

4?’

5.0

n.d.

2.

10

0.63

n.

d.

Mah

akam

372

4ah

3.0

n.d.

2.

13

0.59

n.

d.

Mah

akam

372

4gh

4.4

n.d.

1.

46

0.42

n.

d.

Mah

akam

372

50h

4.7

n.d.

1.

87

0.51

n.

d.

Mah

skam

372

53h

4.1

n.d.

0.

85

0.21

n.

d.

Mah

akam

372

54h

n.d.

n.

d.

n.d.

n.

d.

n.d.

“Dry

wei

ght b

asis

. “D

ry a

nd m

iner

al m

atte

r fr

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6 0.

48

424’

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80

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1.

43

2.48

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99

0.47

41

7k

$1

84.9

8 5.

44

1.68

0.

91

6.99

0.

65

439k

;3

74

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4.90

1.

17

0.71

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40

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1.24

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1.29

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420

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73.4

2 5.

77

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182j

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27

5 27

8 29

1 27

4 27

7 26

4 26

1

a LA -

25

B FPD

12

j’6 20 I?‘= 2a i-3 Lkb, *LYLE-._

100 150 ZOO 250

tempera:“re (“C) --

Fig. 12. Partial (O-90 min) FID (A) and FPD (B) chromatogram of the flash pyrolysate (610°C) of the Gardanne coal. Exper!mental conditions, normalisation of the chromatograms and peak identification as indicated in the caption of Fig. 6.

6

I I 600 600

I 3 I ioo[; 1200 1400 scan number ----+-

Fig. 13. Partial summed, accurate mass chromatogram of m/z 96.99,97.99,111.00,112.01,125.02, 126.02, i39.03,140.03,153.05,I54.05, i67.06 and 168.06 fragments (mass window 0.02 dahon) of the flash pyrolysate (610°C) of the Gardanne coal, showing the distributions of the C,-C, alkylthiopbenes. Chromatographic conditions as indicated in the caption of Fig. 6. Mass spectrometry was performed with a VG-70s instrument operated at 70 eV with a mass range m/z 40-500, a cycle time of 1.8 sand a resolution of 1000. Peak numbers refer to compounds listed in Table 2.

from the gaseous compounds, 2-methylthiophene corresponds to the second most abundant peak in the FID chromatogram (peak 3). The other major alkylthiophenes (compounds 2,4-g, 11,13) are also clearly reflected in the FID chromatogram. Absolute quantification of these alkylthiophenes was done by co-pyrolysis of a known amount of the polymer standard poly(4+butylstyrene). This polymer degrades quantitatively and reproducibly to its monomer [55 1. The results are listed in Table 6. At first sight these results are rather disappointing; on a weight basis the alkylthiophenes in the pyrolysate represent only ca. 0.25% of the total coal matrix and the sulphur in these thiophenes only ca. 2% of the total organically bound sulphur. These relatively low yields have also been reported for other compound c!asses (e.g. n-alkanes, isoprenoid alkenes, aromatic hydrocarbons) in kerogen pyrolysates. In a recent review on analytical pyrolysis of kerogen Larter and Horsfield [ 1351 extensively discuss this problem. They show a number of examples in which a very good correlation was shown between bulk parameters of kerogen and correspondingparam- eters obtained from kerogen pyro!ysates. They conclude that “Py-GC provides quant.it.atively re!evant structural information at the molecular level”. As discussed in Section 2 of this paper we have observed a close relationship between

142

TABLE 6

Yield of a number of tbiophenes and thiolanes on flash pyrolysis (610°C) of two coais

Organic sulphur compound Yield (mg/g coal)”

Gardanneb Mahakam 37245

Thiophenes Thiophene 0.16 0.07 Z-Methylthiophene 0.48 0.17 3-Methylthiophene 0.17 0.17 2-Ethylthiophene 0.14 0.05 2,5-Dimethylthiophene 0.29 0.11 2,4-Dimethylthiophene 0.25 0.13 2,3-Dimethylthiophene 0.26 0.14 3,4-Dimethylthiophene 0.06 0.04 P-Propylthiophene 0.05 0.03 2-Ethyl-5-methylthiophene 0.23 n.09 2-Ethyl-4-methylthiophene 0.14 0.07 2,3,5-Trimethylthiophene 0.23 0.12 2.3,4-Trimethylthiophene 0.09 0.06

I’hiolanes Thiolane 0.05 0.03 2-Methylthioiane 0.06 0.02 2-Ethylthiolane 0.03 <O.Ol

“Measured by quantitation of 2,3_dimethylthiophene in the FID chromatograan [ 721. The concentration of the other OSC was determined by integat,ion of the FPD chromatogram followed by a correction for the quadratic response of the FPD and the percentage of sulphur in the molecules using 2,3-dimethylthiophene as a reference. “Average of two determinations. Reproducibility is > 90% for yields > 0.10 mg/g coal.

atomic S/C! ratios in kerogens and asphaltenes, and the thiophene ratio (2,3- dimethylthiophene/ (1,2_dimethylbenzene Cn-non-1-ene) ) derived from Py- GC data. A quantitative dependence of absolute pyrolysate alkylthiophene yield on the kerogen S/C atomic ra5o also suggests that pyrolytic sulphur species are proportionally representati: 2 of organically bound sulphur species in the kerogen.

Another major group of sulphur compounds are the alkylbenzo- [b j thiophenea, although they are much less dominant as the alkylthiophenes (Fig. 12B). Their distribution pattern i& exemplified by a summed mass chromatogram of m/z 134,147,148,l61,3L62,175,176,189 and 190 fragments (Fig. 14; numbers refer to compounds listed in Table 2). These ions are the major ions in the mass spectra of benzo [ b] thiophene and its C&C, alikyiderivatives. Again accurate mass chromatography was used to eliminate contributions from other compounds than alkylioenzo [b jthiophenes as much as possible. This figure reveals how complicated the distributions of alkylbenzo [ blthiophenes in

39

L 38

(I~

41

L -

1400 1600 1800 scan number ----w

2000

Fig. 14. Partial summed, accurate mass chromatogram of m/z 133.99,147.00,148.00,161.01,162.02, 175.03, 176.03,189.03 and 190.04 (mass window 0.02 datton) of the flash pyroiysate (610°C) of the Gardanne coal, showing the distribution of benzo [blthiophene and its C,-C, alkylated derivatives. Experimental conditions are indicated in the caption of Fig. 13. Peak numbers refer to compounds listed in Table 2.

coal pyrolysates can be. Some of these alkylbenzo [ b] thiophenes have been identified recently bq’ mass spectral cha:acterisation and relative retention time data of standards [ 1371 f The relative abundance of 2- and4-methyl- and ethylbenzo[b]thiophenes (peaks 31, 39 and 41) and 2,4dimethylbenzo- [blthiophene (peak 43) is in agreement with the proposed origin of benzo [ b] thiophenes units in fossil macromolecules (i.e. cyclisation and subsequent aromatization of alkylthiophene moieties) as discussed earlier (see Fig. 11) . The mentioned products are derived from benzo [b ] thiophene units with linear carbon skeletons [65,137]. More condensed polycyclic aromatic thiophenes (dibenzothiophene, benzo [b ]naphthothiophenes) and. their alkylated derivatives were not detected.

Saturated sulphur heterocycles (e.g. thiolanes) are only identified as minor compounds in the flash pyrolysate of the Gardanne coal. This is due to the fact that relatively weak, non-thiophenic S-C bonds are broken during pyrolysis. Accurate mass chromatography of m/z 8’7 reveals, however, an homologous series of 2-alkylthiolanes (Fig. 15A) These thiolanes are probably formed by C-C bond cleavage within the macromolecular structure before cleavage of the C-S bond in the thiolane ring. Thiophenes with long alkyl side-chains also occur in the pyrolysate; accurate mass chromatography of m/z 97 and 111 shows the presence of homologous series of 2-alkylthiophenes and 2-methyl-5-alkyl-

144

t

z E

i,i

11 500

i

( 2500 3000 3500 scan number --m---c

Fig. 15. Partial, accurate mass cbromatogram of m/z 87.00 (mass window 0.02 dalton) of the flash pyrolysate (610°C) of the Gardanne coal, showing the distribution of C,-Cs, 2-alkylthioianer which are indicated with black dots. Italic numbers indicate number of carbon atoms of these compounds. Thesecompounds were identifiedusingdata from Sinninghe Dam& et al. [64]. Note that these compounds are only minor OSC in the pryofysate since they are only very minor peaks in the FPD chromatogram (Fig. 12B).

thiophenes (Figs. 16A and EB, respectively) extending up to Cna. These three homologous series have been previously identified in kerogen pyrolysates [64,65].

The dominance of sulphur compounds in the flash pyrolysate is also revealed by its Py-MS spectrum (Fig. 17A). The most abundant peak at m/z 34 is ascribed to hydrogen sulphide and the peaks at m/z 84,98,112,126,140 and 154 are ascribed, in part, to alkylthiophenes. The ions at m/z 48 (CH,SH), 60 (COS) and 64 ( SO2 or S,) are also due to sulphur compounds. The Iarge peaks at m/z 160 and 145 are due to 4-t-butylstyrene which is formed by co-pyrolysis of the internal standard, poly-4-t-butylstyrene. The ions at m/z 94, 108, 122 and 138 reflect phenol and its Ci-Cs alkylderivatives and the ions at m/z 42, 43,56,57, 70, 71,84 and 85 alkanes and alkenes. Aromatic hydrocarbons in the pyrolysate give ions at m/z 78,92,106,120. The Py-MS results are in good agreement with those of the Py-GC analysis (cf. Fig. 12A and 17A).

4.2 Illinois # 6 coal

The Illinois # 6 coal from the Argonne Premium coal collection [ 1381 was immediately analysed after opening the sample ampoule. It is a high volatile

145

m/z111.00+002

B 15 *

/

1500 2000 2500 3000 3500 scan number -----lp

Fig. 16. Partial, accurate mass chromatograms of m/r 96.99 (A) and m/r 111.00 (B) (mass window 0.02 dalton) 0; the flash pyrolysate of (610°C) of the Gardanne coal, showing the distributions of C,,-C,, 2-alkylthiophenes (black dr)t~) and 2-alkyl-5.methylthiophenes (black triangles), respectively. Italic numbers indicate number of carbon atoms of these compounds. These alkylthiophenes were identified using data from Sinninghe Dam& et al. (64 j.

bituminous coal and has a relatively high organic sulphur content (2.48%; Ta- ble 5). Figure 18A shows the FID chromatogram of the flash pyrolysate obtained with a ferromagnetic wire with a Curie temperature of 610°C. The chro- mstogram is dominated by peaks reflecting phenol and alkylated phenols (peaks K, M-O) but n-alkanes and n-alk-1-enes, aromatic hydrocarbons and pristane and prist-1-ene are also present in the pyrolysate. Sulphur compounds are not abundantly reflected in the FID chromatogram but their distribution pattern is clearly revealed by the FPD chromatogram (Fig. 18B). Alkylthiophenes dominate over alkylbenzo[b]thiophenes but the latter compounds are some- what more dominant than in the Gardanne y/rolysate (cf. Figs. 12B and 18B).

Figure 19A shows a FID chromatogram of the thermal extract of the Illinois # 6 coal. This chromatogram was obtained by using a ferromagnetic wire with a Curie temperature of 358°C instead of 610°C. The major thermal extraction products are normal and branched saturated hydrocarbons, the isoprenoid hydrocarbon pristane (peak W) and naphthalene and alkylated naphthalenes. The corresponding FPD chromatogram (Fig. 19B) reveals that the alkylated thiophenes and benzo [ b] thiophenes present in the “610’ C pyrolysate” are for the far greater part pyrolysis products and not initially present in the coal.

150 200 160

MAHAKAM 37245

145

108

200

Fig. 17. Py-MS spectra of (A) the Gardanne coal, (B) the Mahakam 37247 coal and (C) the Cherokee coal. Samples were pyrolysed at 770°C and the formed pyrolysis products were ionised at 14 eV. Scan ranges were m/z 25-250 for (A) and (B) and m/z 25-200 for (C). Poly(4-t- butylstyrene) was co-pyrolysed in case of the Gardanne and Mahakam coal, which gives rise to ib monomer, 4-t-butylstyrene (m/r 160,145). Spectra (A) and (B) were kindly provided by Dr. Eglinton. For full experimental details we refer to Eglinton et ai. [ 1331.

Only hydrogen sulphide is detected in the FPD chromatogram. It may result from (poly)sulphide moieties in the macromolecular matrix which start to decompose at lower temperatures since carbon-sulphur bonds are weaker than carbon-carbon bonds and can, thus, be cleaved at lower temperatures.

These results are in good agreement with the flash pyrolysis-GC-MS results reported by Hughes et al. [ 1251 for an Illinois # 6 coal in that these authors also found thiophene and its Cr-C3 alkylated derivatives, benzo[b]thiophene and its C,-C, alkylated derivatives and dibenzothiophene. A more detailed comparison is not possible because Hughes and co-workers did not identify individual isomers. A direct comparison of our results with those reported by

J+K

FPO

ci 6 i0 lb0 160 2Lio 250 temperature (“C) -----a-

Fig. 18. Partial (O-90 min) FID (A) andFPD (B) chromategram ofthe flash pyrolysate (610°C) of the Illinois # 6 coal. Experimental conditions, norma!isation of the chromatograms and peak identifications as indicated in the caption of Fig. 6.

148

a FID

5 FPD

0 0 50 100 150 200 --z temperature (“C) -

Fig. 19. Partial (O-90 min) FID (A) and FPD (B j chromatogram of the thermal extract (356°C) of the Illinois #6 coal. Experimental conditions, normalisation (except for the FPD chromatogram j of the chromatw-nms *ad peak idcctifications as indicated in the caption of Fig. 6. “._____ -.

149

Winans and Neil1 [ 99 ] for exactly the same coal sample is not possible since these authors report primarily on the 450°C vacuum tar obtained by a batch- wise, closed-system pyrolysis. Organic sulphur compounds with additional heteroatoms as reported to be present in the tar of this coal [99] were not detected in the “610°C” flash pyrolysate. This may imply that OSC with additional heteroatoms are secondary pyrolysis products.

4.3 Cherokee coal

The Cretaceous Cherokee coal was obtained from the Penn State coal bank (PSOC 667) and originates from Iowa, U.S.A. It has a subbituminous rank and its elemental analysis data (Table 5) reveals that this coal contains a very high amount of sulphur (13.6%). However, the sulphate content is also high, which indicates that the coal sample is highly weathered [8]. Figure 20A shows the FID chromatogram of the flash pyrolysate obtained using a ferromagnetic wire with a Curie temperature of 610°C. It consists mainly of n-alkanes and n-alk-1-enes, alkylbenzenes, phenols and sulphur compounds. The latter compounds are clearly visible in the FID chromatogram. The FPD chromatogram (Fig. 20B) shows that the alky!thiophenes dominate over alkylbenzo [b] thiophenes and are quite comparable in relative amounts to that of the Illinois # 6 coal (cf. Figs. 18I3 and 20B). However, there is one major difference; the very broad, fronting peak indicated in the FPD chromatogram of the Cherokee coal. This peak is due to the presence of a large amount of elemental sulphur in the coal which simply evaporates during analysis as confirmed by a thermal extraction experiment. Elemental sulphur is formed during weathering of coal from pyrite (e.g. IS] ) and its presence corroborates the conclusion from the elemental analysis that this coal is weathered. The presence of elemental sulphur in the flash pyrolysate of the Cherokee coal is confirmed by its Py-MS spectrum (Fig. 17C ) . The large peak at m/z 64 (S, ) is indicative of elemental sulphur, although a contribution of SO2 cannot be excluded.

4.4 Ra.sa coal

The Rasa coal is from Yugoslavia and was obtained from Dr. White (Pitts- burgh Energy Technology Center, U.S.A. ). It is +-an o~,.,melysulphur rich (11.6%, Table 5) despite its relatively high level of thermal maturity as indicated by its relatively high mean vitrinite reflectance ( I?0 = 0.68% ) . Figure 21A shows the complex FID chromatogram of its “610°C flash pyrolysate” in which linear, branched and isoprenoid saturated hydrocarbons, alkylbenzenes, alkylnaphthalenes, phenols and sulphur compounds dominate. The FPD chromatogram (Fig. 21B) reveals the distribution pattern of the sulphur compounds; alkylbenzo[b]thiophenes are more abundant than alkylthiophenes and some alkyldibenzothiophenes are also encountered. This distribution pattern is dif=

+-y 0 sb

13 11 i

FPD

Id0 160 260 260 temper&we (“C) -----a-

Fig. 20. Partial (O-90 min) FID (A) andFPD (B) chromatogram of the flashpyrolysate (61O”C! of the Cheroktee coal. Experimental conditions. normalisation of the chromatograms and peak identif%ntionn as indicated in the caption of Fig. 6.

151

ferent from those of the previous described coal samples. The FID and FPD chromatograms (Figs. 22A and 22I3, respectively) obtained by thermal extrae- tion of this coal reveal that a significant portion of the compounds in the “610” C pyrolysate” is, at least partly, present as low molecular weight compounds entrapped in the coal matrix. This holds for most of the compounds apart from the n-alk-1-enes, the phenolic components and the alkylthiophenes. The relatively large amounts of cadalene (l-isopropyl-4,7dimethylnaphthalene; peak V) indicate a contribution of resinous material to the coal. This observation is consistent with the results obtained by a petrographic examination of the Rasa coal which showed it to be high in resinous material [96]. A comparison of the two FPD chromatograms indicates that the major part of benzo [blthiophene and its C,-alkylated derivates in the “610°C pyrolysate” are indeed formed by pyrolysis. Changes in the distribution patterns of the C,- and C&-cluster indicate that some isomers are also mainly formed by pyrolysis. It is quite surpris- ing that the alkylthiophenes are all macromolecularly bound, whilst only a fraction of the alkylbenzo [b] thiophenes is part of the macromolecular matrix. It is not likely that this relates the pretreatment (e.g. drying) of this sample since low molecular weight hydrocarbons are abundant in the thermal extract (Fig. 22A).

Recently published LVMRMS data obtained for the uafractionated solvent extract of the same Rasa coal [96] enabled us to corndare these data with thermal extraction data obtained by thermal extraction-GC-LRMS of the unextracted Rasa coal. These results will be described in detail elsewhere [ 1391. Selected characteristic ions for different families of OK!, as observed in the LVI-IRMS analysis [96 ] , were used to generate mass chromatograms in order to determine if the same ions of the C9SC families in the solvent extract of the Rasa coal. Table 7 summarises the results obtained in this way. The presence alkylbenzo [b Jthiophenes, alkyldibenzothiophenes, alkylbenzonaphthothio- phenes (and/or alkylphenanthrothiophenes with molecular weight, MW = 234+n.14) and the alkylphenanthro[4,5&d]thiophenes (MW= 208fn.14) in the extract is confirmed by our results. For all the alkyi aryl sulphides given by White et al. [96] alternative, though still tentative, structures are proposed which are in accordance with both the LVI-IRMS and GC-LRMS data. The alternative structures indicate that the presence of alkyl aryl sulphides in this coal extract is unlikely. It should be noted that the structures given by White et al. [96] were described as only possible compound types, and that other structures were also possible. The assigned molecular formulae are thought to be correct [96]. All the proposed structures belong to the different families of alkylated polycyclic aromatic thiophenes and their partly hydrogenated counterparts in agreement with their proposed pathways of formation starting from thiophenes with long alkyl side-chains (see Fig. 11). Even the compounds with two sulphur atoms detected by LVI-IRMS fit in this working hypothesis if an

152

2

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Fig. 21. Partial (O-90 min) FID (A) and FPD (13) chromatogram of the flash pyrolysate (610°C) of the Rasa coal. Experimental conditions, normalisation of the chromatograms and peak identifications as indicated in the caption of Fig. 6. The FPD chromatagram is normalised on peak 37.

153

c I G

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?_,_J_J i 0 0 50 100 150 PLQ 250

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Fig. 22. Partial (O-90 min) FID (A) and FPD (B) chromatogram of the thermal extract (358°C) of the Rosa coal. Experimental conditions, normalisation of the chromatograms and peak identifications as indicated in the caption of Fig. 6. The FPD cbromatogram is normalised on the most abundant C,-benzo[b]thiophene.

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A quantitative comparison of our results with those reported by White et al. [96] reveals that alkylbenzo [ b] thiophenes are the most abundant OSC in the thermal extract whilst the LVHRMS data seem to indicate that alkylbenzo [b] thiophenes are minor OSC relative to more condensed polycyclic aromatic thiophenes. This may be explained by (i) losses of low molecular weight compounds during the preparation of the pyridine extract by evaporation at elevated temperatures, (ii) relatively large differences in ionisation yields at low voltage for the different families of polycyclic aromatic thiophenes, (iii) discrimination of relatively low-volatile polycyclic aromatic thiophenes in the interface between the flash pyrolysis unit and the GC capillary in the Py-GC mode.

4.5 Mahakam coals

The Mahakam coals come from the Mahakam delta (Kalimantan, Indone- sia). Seven samples (see Table 5) were taken from a borehole (Handil627) penetrating this Miocene deltaic sequence. The Mahakam delta has been extensively studied (e.g. [ 140-1441) and it is thought that the coals represent a relatively uniform sequence containing organic matter varying in properties due mainly to different levels of thermal maturity. This set of samples varies in rank from subbituminous to high volatiie bituminous. Figure 23A shows the FID chromatogram of the “610°C” flash pyrolysate of the least mature sample (Mahakam 37245) which is from a depth of 1390 m. The low level of thermal maturity of this coal as evident from its mean vitrinite reflectance & =0.38% is also revealed by the molecular data; the relatively high amounts of phenols (peaks K,M,N,O) 1145 1, the low pristane formation index (PI?T= [pristane] / [pristane+prist-t-ene+prist-2-ene] = [WI/[ W+X+ Y] =0.13 [146] ) and the low amounts of free n-alkanea. Despite the low organic sulphur content (1.29%, Table 5) and, consequently, the low thiophene ratio (0.17; Eglinton

et al. [ 731) the FPD chromatogram (Fig. 23B ) reveals the distribution of alkylthiophenes and other sulphur compounds quite nicely. The distribution is dominated by alkylthiophenes and alkylbenzo [b] thiophenes are only minor constituents. Absolute quantitation of the alkylthiophenes and alkylthiolanes by co-pyrolysis with a known amount of a polymer standard (Table 6) revealed that the absolute amounts of alkylthiophenes generated from this coal are indeed lower than from the Gardanne coal. This is in accordance with the lower organic sulphur content of this coal. This is also revealed by a comparison of the Py-MS spectra of these two coals (cf. Figs. 17A and 17B). In the mass spectrum of the pyrolysate of the Gardanne coal the ions at m/z 98! 112 and 126 (alkylthiophenes) are indeed more abundant relative to m/z 16C and 145 (which are from the monomer 4-t-butylstyrene formed from the polymer standard) than these ions in the Py-MS spectrum of the Mahakam 37245 ( oal. The latter spectrum also shows a lower abundance of m/z 34 which is due to hydrogen sulphide. The large peak at m/z 198 is probably due to cadalene, which is also quite abundant in the FID chromatogram of the flash pyrolysate (peak V in Fig. 23A).

From the more mature sample (4 = 0.59% ) from a depth of 2950 m (close to the threshold of oil generation at ea. 3000 m) a flash pyrolysate was generated dominated by C,-C,, n-alkanes (Fig. 24). Although no thermal extraction experiment was performed with this coal sample, it is presumed that these n- alkanes occur as free compounds entrapped in the coal matrix since the corresponding n-alk-1-enes are relatively low. Figure 24A also reveals the lower relative abundance of the phenols and a higher PFI of 0.84 [ 1461 as compared with the pyrolysate of Mahakam 37245. These three phenomena eonfirm its higher level of thermal maturity. The corresponding FPD chromatogram (Fig. 24B ) reveals the higher amounts of alkylbenzo [ b ] thiophenes relative to the alkylthiophenes in the pyrolysate of the more mature coal (cf. Figs. 23B and 24B). This change in distribution of the sulphur-containing pyrolysis products is gradually occurring with increasing levels of thermal maturity. This is exemplified by the FPD chromatograms of the seven samples from this borehole (Fig. 25).

4.6 Pittsburgh # 8 coal

The Penn-ylvanian Pittsburgh # 8 coal is from the Argonne Premium Coal Collection [ 1381 and was immediately analysed after opening of the sample ampoule. It is of high volatile bituminous rank (RO =0.65% ) and contains a low amount of organic sulphur (0.91%, Table 5). A comparison of the FID chromatograms of the “610°C” flash pyrolysate (Fig. 26A) and the product mixture obtained by thermal extraction (Fig. 27A) indicates that most of the products occur mainly as free compounds entrapped in the coal matrix. This holds for the linear, branched and isoprenoid alkanes, methylcyclohexane (peak

159

FID

25

B FPD

Pig.23. Partial (O-90 min) FID (A) and FPD (B) chromat.ogmm of the flashpy?eWte (610°C) of the Mahakam 37245 coal. Experimental conditions, normalisation of the chromatowms and peak identifications as indicated in the caption of Fig. 6.

3

Q, 98

FPD

/ 11 0- 0 50 100 150 200 250

temperature (“C) -+

Fig.24. Partial (O-90min) FID (A) andFPD (B) chromatogramofthe flashpyrolysate 1610°C) of the Mahakam 37253 coal. Experimental conditions, norm&s&ion of the chromatograms and peak identifications as indicated in the caption of Fig. 6.

161

Fig. 25. Partial (O-60 min) FPD chromatograms of the flash pyrolysates (610°C) of the Mahekam coals. Experimental details as indicated in the caption of Fig. 6. For peak identifications see the caption of Fig. 10. (Reprinted with permission from Geochemistry of Sulfur in Fossil Fuels ( W.L. Orr and C.M. White, Eds.), ACS Symp. Vol. 429, T.I. Eghnton, J.S. Sinninghe Dam&, M.E.L. Khonen, J.W. de Leeuw, S.R. Larter and R.L. Patience, Analysis of maturity-related changes in the organ? sulfur composition of kerogens by flash pyrolysis-gas chromatography, 0 1990 Amer- ican Chemical Society.)

162

C), the alkylbenzenes and alkylnaphthalenes. Surprisingly, this does not hold at ali for the sulphur compounds. At 358°C (thermal extraction) no sulphur compounds are generated from the coal as shown by the very noisy FPD chromatogram (Fig. 27B). At 610°C dibenzothiophene and its alkylated derivatives are the major suiphur compounds (Fig. 26B ) . These compounds are reflected as major peaks in the FID chromatogram (Fig. 26A). Alkylated thiophenes and benzo jb] thiophenes, common sulphur compounds in the pyrolysates of the other coals, are completely absent in the Pittsburgh # 8pyrolysate.

It is worthy of note that the results reported by Calkins [ 125 ]i osi the flash pyrolysis using a Pyroprobe system in combination with GC-MS of a high- sulphur (S,,, = 4.65 wt.% ) Pittsburgh # 8 R&F coal are quite comparable to those reported here for the flash pyrolysates of the Gardanne, Illinois # 6 and iow-rank Mahakam coals. Surprisingly enough, the relatively high amounts of alkylthiophenes and alkylbenzo [ b] thiophenes in the flash pyrolysate of the Pittsburgh # 8 R&F coal [ 125 ] are not at all encountered in the flash pyrolysate of the Pittsburgh # 8 from the Argonne Premium set. This m;;y be related to either differences in rank and sampling locations between the two coals (as also indicated by the large differences in sulphur content) or by the experimental GC conditions used by Calkins [ 1251 which probably prevented the elution of the less volatile alkyldibenzothiophenes or a combination of these reasons.

4.7 Beulah-Zap coal

The Beulah-Zap lignite is also from the Argonne Premium Coal Collection [ 138 ] and contains a very low amount of organic sulphur (0.71%) Table 5 ) for a coal of such low rank. Its flash pyrolysate (610°C ) is dominated by phenol and alkylphenols (Fig. 28A). The corresponding FPD chromatogram (Fig. 28B) shows, as expected, a very low abundance of sulphur compounds in its pyrolysate; apart from hydrogen sulphide some alkylthiophenes are just detectable.

4.8 Specific conclusions

(1) Flash pyrolysis/thermal extraction-gas chromatography of coals is a rapid technique to get an idea about the low molecular weight OSC entrapped in the coal matrix, the organically bound sulphur in high molecular weight substances and the abundance of organic sulphur. However, (poly)sulphide moieties are not properly characterised since they mainly generate hydrogen sulphide, which may also be derived from pyrite.

(2) Weathered coals are easily recognised by this method due to the relatively high amounts of elemental sulphur in the thermal extract/pyrolysate. Elemental sulphur is absent in pristine coals.

(3) Alkylthiophenes and alkylbenzo [blthiophenes are, apart from very vol-

163

/

B FPD

52

0’ 0 50 700 150 200 250

temperature (“C) -

Fig. 26. Partial (O-90 min) FID (A) and FPD (B) chromatogram of the flashpyrolysate (610°C) of the Pittsburgh # 8 coal. Experimental conditions, normalisation of the chromatograms and peak identifications as indicated in the caption of Fig. 6. The FPD ehromatogram is normalised to peak 50.

F .

10 .

A FID

FPD

0 0 50 100 150 200 250 tevperature (“C) ___)

Fig. 27. Partial (O-90 min) FID (A) and FPD (B) chromatogram of the thermal extract of the Pittsburgh #8 coal. Experimental conditions, normalization of the chromatogranu and peak identifications as indicated in the caption of Fig. 6. The FPD chromatogram is normalised on the hydrogen sulphide peak.

165

B FPD

0 0 50 100 150 zc!rJ temperature (“C) -

250

Fig. 28. Partial (O-90 min) FID (A) andFPD (B) chromatogram of the flashpyrolysate (610°C) of the Beulah-Zap lignite. Experimental conditions, normalisation of the chromatngrams and peak identifications a~ indicated in the caption of Fig, 6,

166

atiie products, the major sulphur-containing flash ryrolysis products of most of the coals studied. The isomer distributions of the alkylthiophenes and, probably, also tbose of the alkylbenzo[b Jthiophenes are dominated by a limited number of the theoretically possible isomers. These findings are highly comparable with those for kerogens of different types [65]. Therefore, it is proposed that the organic sulphur in coal has a similar origin as the organic sulphur in kerogen, i.e. sulphur enrichment of the organic matter during very early diagenesis. This suggests that the major part of the organic sulphur is formed during or shortly after deposition of the organic matter (i.e. in the peat stage) by reaction of hydrogen sulphide or inorganic polysulphides (HS; ) and not during the latter stages of diagenesis or the early stages of catagenesis by, for example, reactions of the coal organic matter with elemental sulphur or pyrite at elevated temperatures.

(4) Thermal evaporation experiments with four coals (Gardanne, Illinois # 6, Pittsburgh # 8 and Rasa) have indicated that only the Rasa coal contains low molecular weight OS% entrapped in the coal matrix in concentrations comparable to those of sulphur-containing products in the flash pyrolysates (610°C) of these samples. Alkylthiophenes were not present in this thermal extract and are, thus, only found as pyrolysis products. The presence of low molecular weight OSC is not, as expected, related to the maturity level (rank j of the coal since the Pittsburgh #8 coal has a similar vitrinite reflectance as the Rasa coai (Table 5) but does not contain low molecular weight OSC.

(5) A significant change with rank in the composition of the sulphur-containing pyrolysis/thermal evaporation products of the coals is noted with increased relative amounts of OSC with thiophene rings as part of the polycyclic ring systems (i.e. benzo[b]thiophenes, dibenzothiophenes) with increasing maturity. This is most evident for the samples from the core from the Ma- hakam delta (Fig. 25) but can also be recognised in the pyrolysates of the other coais. For example, the more mature Rasa and Pittsburgh # 8 coals contain relatively more benzo [b] - and dibenzothiophenes in their pyrolysates. This is explained by cychsation and aromatization reactions of macromolecularly L. ..-.J rl:--L--- .-:+n :- il... -n-z. T.V.,., 00 I.,, h-c.- ,bscribed Fe,. nth,nv f,,l.a.c UOUllU ruLvp”“x~~ Llllru ‘1‘ VllV UPl‘lr “UJ U” “C.&U UbU.1 ..w a a ““...,I -“I-” of fossil organic matter (see Section 2). However, it does not seem that these molecular transformations can be related to “absolute maturity levels”. For example, the Pittsburgh # 8 coal has a mean vitrinite reflectance comparable to that of the Rasa coal, but in its pyrolysate only dibenzothiophene and its E._ . _.a__ ..-. ..-. _.._n m-_^ &,,,.4 . ..&l.? in +h_ . IL.I^c A ..I^-:..-‘:. ^ U”.XJ lU”IU U”ll.LlV‘ir” ._%I%. I”UI.U wi~*,“u ll. S.._ - pyrolysate of the Rasa coal alkylthiophenes are still abundant sulphur-containing pyrolysis products. A reason for this anomaly may be that the degree of possible cyclisation/aromatization depends on the length of the alkyl side-chains of the initial thiophene units. Further work is required to resolve these remaining questions.

167

5. GENERAL CONCLUSIONS AND RECOPvlIvIENDATIONS

5.1 Conclusions

(1) In contrast with coal-derived products only a limited number of alkylthiophenes and polycyclic aromatic thiophenes and their alky!derivatives have been tentatively identified by GC-MS and LVHRMS methods in coal extracts. At the molecular level no evidence exists for the presence of thiols and sulphides in coal extracts. Molecular formulae have been observed by LVHRMS that are consistent with those of thiols and disu’rphides from samples obtained by solvent extraction of coal. However, as emphasized here and by White et al. ]96,100] LVHRMS data alone can only provide precise mass information on the species present and can not be used alone to identify compounds. Although quantitative information of QSC is scant the available results indicate that a very minor part of the organically bound sulphur is represented by these compounds [ 1031.

(2) Sulphur-containing moieties in high molecular weight substances present in coals have mainly been characterised by pyrolysis methods combined with GC-FID/FPD, GC-MS and LVHRMS. The most abundant sulphur-containing pyrolysis product is HzS, probably derived from thermolysis of (poly)sulphide moieties. Although minor amounts of alkylthiolanes are observed in coal pyrolysates information concerning the (poly)sulphide-linked carbon skeletons is minimal, due to their thermal lability: The other major QSC released during pyrolysis are alkylthiophenes and poiycyclic aromatic thiophenes and their alkyl derivatives. A substantial number of alkylthiophene isomers and some alkylbenzo [b ] thiophenes have been identified rigorously in only a few coal pyrolysates. The quantities of some major alkylthiophenes released upon pyrolysis represent a small part of the total organic sulphur. How- ever, their amounts are several orders of magnitude higher than those of aromatic thiophenes in coal extracts.

(3) The similarities of types and distribution patterns of organosulphur constituents in the high molecular weight substances of coals on the one hand and keragens on the other hand indicate that their structures and their pathways of formation are highly comparable. This implies that the far greater part of organically bound sulphur in coals, like in sediments, is derived from &S and/or polysulphide incorporation into organic matter at very early st.ages of diagenesis. Therefore, reactions of elemental sulphur and/or pyrite with organic substances at elevated temperatures during the coalification process is considered to be an unlikely substantial source for organic sulphur.

(4 i The increasing relative amounts of more and more condensed polycyclic aromatic thiophenes in coals with increasing coalification are thought to be the sesuit of cyclisation and subsequent aromatization reactions initially starting from thiophene units with long alkyl chains. Most of the data reported for

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OSC in coal extracts seem to fit into this general framework. The degree c’ condensation of polycyclic aromatic thiophenes in coal extracts seems to .&e higher than that observed in sediment extracts and crude oils. This may relate to the lower expulsion efficiency of coals causing a longer residence time of low molecular weight (sulphur) compounds at elevated temperatures which results in substantially longer reaction times.

5.2 Recommendations for future research

(1) Much more work has to be devoted to the unequivocal quantitative iden - tification of individual 0% in extracts and pyrolysates of coals to fully understand their origin and fate.

(2) To better understand the origin and mechanism of formatien of organically bound sulphur in coals more molecularly oriented studies should be dedicated to assess the types and distributions of organosulphur constituents in marine-influenced coal depositional environments, (e.g. salt marshes, swamps and peats ) .

(3) Roth coal geochemistry and petroleum geochemistry would benefit from a better and more frequent exchange of information concerning the isolation, separation, identification, quantitation and interpretation of organically bound sulphur in fossil fuels and related materials.

6. GLOSSARY

DSDP FID FPD GC GC-FID GC-FPD GC-LRMS GC-MS HRMS IUPAC LVIiRMS MS ODP osc Py-GC Py-GC-MS Py-MS SRC XANES

Deep sea drilling project Flame ionization detector Flame photometric detector Gas chromatography Gas chromatography with flame ionization detection Gas chromatography with flame photometric detection Gas chromatography low resolution mass spectrometry Gas chromatography mass spectrometry High resolution mass spectrometry International Union for Pure and Applied Chemistry Low voltage high resolution mass spectrometry Mass spectrometry Ocean drilling project Organic sulphur compounds Pyrolysis gas chromatography Pyrolysis gas chromatography mass spectrometry Pyrolysis mass spectrometry Solvent refined coal (s ) X-ray absorption near edge structure spectroscopy

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Organically bound sulphur in coal: A molecular approach

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