Influence of Sulphur Cross-linking on the Molecular-Size Distribution

Chapter 5

Influence of Sulphur Cross-linking on the Molecular-Size Distribution of Sulphur-Rich Macromolecules in Bitumen 1

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1

Stefan Schouten , Timothy I. Eglinton , Jaap S. Sinninghe Damste , and Jan W. de Leeuw 1

Downloaded by FUDAN UNIV on April 22, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch005

1

Division of Marine Biogeochemistry, Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 2

A sulphur-rich bitumen was separated by gel permeation chromatography (GPC) and the fractions obtained were analyzed for free and sulphur-bound carbon skeletons. Free and low-molecular-weight sulphur compounds are mainly present in GPC-fractions representing M < 800. Desulphurization with deuteriated nickel boride of the polar fractions of the GPC-fractions showed that fractions of increasing molecular weight have increasing amounts of deuterium incorporated into the released carbon skeletons. This indicates that sulphur-rich geomacromolecules were separated on basis of molecular weight through gel permeation chromatography. These results further support the idea that sulphur-crosslinking is an important factor in the formation of highmolecular-weight substances present in bitumens. w

The incorporation of sulphur into organic matter in the subsurface is believed to occur during very early diagenesis (e.g. 1-3). Hydrogen sulphide (e.g. 2,4), polysulphides (e.g.5-7) and elemental sulphur (e.g. 8) are suggested as inorganic sulphur species which react with functionalized lipids. Depending on the number and positions of the functionalities in the organic substrate the sulphur may react either intermolecularly or intramolecularly. In the case of the latter reaction, cyclic (poly)sulphides are formed which are subsequently converted to thiophenes during increased diagenesis. Intermolecular reactions of sulphur with functionalised lipids results in the formation of dimers or, when multiple functionalities are present, in a complex mixture of oligoand polymers. Several studies (e.g. 8-14) have shown that this extensive crosslinking can lead to high-molecular-weight compounds present in polar and asphaltene fractions and in kerogens. Sinninghe Damste* et al. (10) and Kohnen et al. (11) suggested that with increasing molecular size (i.e. with increasing crosslinking) the solubility of the macromolecular aggregates decreases. Thus, kerogens may contain sulphur-rich macromolecules with a high degree of crosslinking whilst asphaltenes and polar 0097-6156/95/0612-0080$12.00/0 © 1995 American Chemical Society Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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fractions may contain less cross-linked macromolecules which are still soluble in organic solvents. To test this hypothesis, a sulphur-rich bitumen from the Monterey Formation was subjected to gel permeation chromatography (GPC), a technique commonly used in polymer sciences to separate polymers into molecular size fractions. Eglinton et al (15) already reported on the X-ray absorption and pyrolysis-mass spectrometry analyses of these fractions. The data indicated that the thiophene/sulphide ratio, a parameter thought to be related to the degree of polymerization, increases with GPC-fractions of decreasing molecular weight. Here we report on the presence of free and sulphur-bound hydrocarbons in the total bitumen and its GPC-fractions.


Experimental Sample Preparation. An immature sediment sample from the Monterey Formation at Naples Beach (California) was taken from a 10 cm interval at the base of Unit 315 approximately 9 m below the lowest phosphorite horizon (15) and was comprised of a lenticularly laminated claystone. It was ultrasonically extracted with CH 0H/CHC1 and the resulting extract was washed with distilled water and dried using sodium carbonate. 3

3

Gel Permeation Chromatography. The gel permeation chromatography (GPC) of the Monterey bitumen has been described by Eglinton et al (15). Briefly, the bitumen was separated into nine fractions by GPC using three Shodex styrene-divinylbenzene columns (KF-801, KF-802 and KF-802.5; column dimensions 300 mm x 8 mm) and CHC1 as eluent (0.5 ml/min). The column effluent was monitored by a UV/VIS (254 nm) detector (Hitachi L-4200) and a refractive index detector (Shodex RI-71). Approximate molecular sizes of GPC fractions were determined by comparison with retention times for polystyrene standards (Polymer Labs Inc.) 3

Analysis of Bitumen and GPC-fractions. The bitumen and its GPC-fractions were analyzed as shown in Figure 1. Apolar fractions were isolated by column chromatography using A1 0 as stationary phase and a mixture of hexane/dichloromethane (9:1 v/v) as eluent. The residual polar material left on the column was ultrasonically extracted from the stationary phase using a CH 0H/CH C1 (1:1 v/v) mixture. The polar material was desulphurized using deuteriated nickel boride (16) and the released hydrocarbons were isolated using column chromatography (A1 0 ; eluent hexane/dichloromethane 9:1 v/v). A known amount of an internal standard [2,3dimethyl-5-(r,r-d -hexadecyl)thiophene] was added prior to isolation of the apolar fraction and the desulphurization of the residue. Before quantification of the hydrocarbons released by desulphurization of the residue the mixture was hydrogenated using H /Pt0 . 2

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2

2

3

2

2

2

Gas Chromatography. GC was performed using a Carlo Erba 5300 instrument, equipped with an on-column injector. A fused silica capillary column (25 m x 0.32 mm) coated with CP Sil-5 (film thickness 0.12 pm) was used with helium as carrier gas. For detection a flame ionization detector (FID) was used. The samples (dissolved in ethyl acetate) were injected at 75 °C and subsequently the oven was programmed to 130 °C at 20 °C/min and then at 6 °C/min to 320 °C at which it was held for 10 min.

Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

82

GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR

bitumen gel permeation chromatography


column chromatography

apolar fraction

addition of standard

residual polar fraction

addition of standard

Deuteriated nickel boride

desulphurized polar fraction column chromatography

apolar compounds

residue

H2/Pt02

hydrogenated apolar compounds

Figure 1. Analytical

flow diagram.

Gas Chromatography-Mass Spectrometry. GC-MS was performed on a HewlettPackard 5890 gas chromatograph interfaced to a V G Autospec Ultima mass spectrometer operated at 70 eV with a mass range m/z 40-800 and a cycle time 1.8 s (resolution 1000). The gas chromatograph was equipped with a fused silica capillary column (25 m x 0.32 mm) coated with CP Sil-5 (film thickness = 0.2 /*m). The carrier gas was helium. The samples were on-column injected at 50 °C and subsequently the oven was programmed to 130 °C at 20 °C/min and then at 4 °C/min to 300 °C at which it was held for 10 min. Results A sulphur-rich bitumen, obtained by extraction of a sediment sample from the Monterey Formation, was subjected to gel permeation chromatography to separate sulphur-rich macromolecules on bases of molecular-weight (Figure 2). Nine fractions were obtained (15) of which the two highest molecular weight fractions (1 and 2) were combined since they represented very low amounts of material. The total bitumen and the GPCfractions were subjected to column chromatography to obtain the apolar fraction. In this


5. SCHOUTEN ET AL.


83

way apolar low-molecular-weight compounds were removed from the more polar highmolecular-weight compounds. The residual polar fraction was desulphurized using deuteriated nickel boride to asses the amount of sulphur-linkages of the sulphur-rich geomacromolecules (Figure 1). Free Hydrocarbons. The gas chromatogram of the apolar fraction of the total bitumen revealed that it predominantly consists of a complex mixture of sterenes, aromatic steroids, phytane, lycopane, hopanoids, thiophenes and a C alkene (Figure 3). GC-analysis of the apolar fractions of the 8 GPC-fractions revealed that only GPC-fractions 7 to 9 contained significant amounts of GC-amenable material. Other apolar fractions only contained internal standards and contaminants as GC-amenable components. Eglinton et al (15) determined, using polystyrene standards, that fractions 7 to 9 mainly contain compounds with a molecular weight lower than 800. This is in agreement with the presence of low-molecular-weight apolar compounds in these fractions since apolar fractions are generally assumed to contain compounds with a molecular weight smaller than 800 (77). Most of the sterenes, aromatic steroids and thiophenes are present in GPC-fraction 8 whilst in fraction 9 naphthalenes, phenanthrenes and dibenzothiophenes are dominant. Fraction 9 thus contains compounds with a similar or even higher polarity as those in GPC-fraction 8 (dibenzothiophenes versus thiophenes and sterenes) but generally with lower molecular weights. This suggests, at least for the compounds in the apolar fractions, that the GPC-separation is based on molecular-weight rather than polarity.


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Sulphur-Bound Hydrocarbons. Desulphurization with deuteriated nickel boride of the residual polar material (Figure 1) of the total bitumen resulted in the formation of several deuteriated carbon skeletons (Figure 4). Phytane is present in significant amounts and possesses 0-2 deuterium atoms. Steranes range in carbon number from to C and have predominantly 0-3 deuterium atoms incorporated. The dominant hopane is the C 17B,21B(H)-hopane which possesses 0 to 8 deuterium atoms. N-alkanes are present in relatively high amounts but, due to the low intensity of molecular ions in their mass spectra, the extent of deuterium labelling could not be determined. Desulphurization of the residual polar fraction of the GPC-fractions only yielded deuteriated hydrocarbons for fractions 4 to 9. The GPC-fractions 1+2 and 3 may nevertheless contain sulphur-bound compounds but due to the high-molecular-weight nature of the fractions the nickel boride reagent may be less effective because of steric hindrance. Quantification of the cholestane and C hopane carbon skeletons released upon desulphurization show that the low-molecular-weight fraction 8 contains the highest amounts (Figure 5). Importantly, the pattern of deuteriation changes with GPC-fraction. Cholestane possesses 0 to 4 deuterium atoms in GPC-fraction 5 (1 deuterium atom dominant) but this changes to 0-1 deuterium atoms in GPC-9 (0 deuterium atom dominant; Figure 6). The C hopane has incorporated 0-9 deuterium atoms in GPC-fraction 5 (7 deuterium atoms dominant) but 0-2 deuterium atoms in GPC-fraction 9 (0 deuterium atom dominant; Figure 7). Using these data in combination with the quantitative data presented in Figure 5, it is possible to calculate the average deuteriation of cholestane and the C hopane in the desulphurized bitumen: 0.7 deuterium atoms for cholestane and 2.4 deuterium atoms for the C hopane. These average numbers of deuterium atoms compare very well with those obtained by desulphurizing the total bitumen: 0.8 29

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Figure 2. GPC chromatogram of Monterey Formation bitumen. F1-F9 denote the time zones used to collect molecular size-fractions. Approximate molecular size ranges for each fraction based on calibration with polystyrene standards.


Mw


-

hop-17(21)-ene

Figure 3. Gas chromatogram of free hydrocarbons present in the bitumen.

rentention time

internal standard



0 - 2 D

rentention time

j.s.

squalane

2

Figure 4. Gas chromatogram of hydrocarbons released upon desulphurization of the residual polar fraction of bitumen using deuteriated nickel boride. Numbers indicate total number of carbon atoms of n-alkanes. C $ HBI= highly branched isoprenoid.

phytane


0 - 8 D

C35 hopane


5.

SCHOUTEN ET AL.


Figure 5. Amount of cholestane and C of the GPC-fractions.

3 5

hopane released upon desulphurization


87

88


for cholestane and 2.3 for the C hopane. This indicates that the sulphur-rich geomacromolecules present in the GPC-fractions are representative of the bitumen as a whole. The occurrence of carbon skeletons without deuterium atoms seem rather peculiar but has been noted before using other deuteriated desulphurization reagents like Nickelocene (12) and Raney Nickel (75) and is attributed to impurities in the reagents and the faster reaction rate of hydrogen compared to deuterium and exchange reactions. It must also be noted that the high amounts of cholestane and C hopane without deuterium atoms in GPC-fraction 9 may have been due to an incomplete removal of the apolar fraction due to some problems with the column chromatography at that time. Using the results from desulphurization of model compounds by deuteriated nickel boride (16) it is possible to reconstruct the type of sulphur compound from the deuteration pattern. Compounds with one (poly)-sulphide bond yield compounds with predominantly one deuterium atom. Desulphurization of thiolanes yield compounds with two deuterium atoms although small amounts of 1 and 3 deuterium atoms can be detected. Desulphurization of a thiophene yields a more complex pattern which is dominated by 6 deuterium atoms, but also with substantial amounts of 7 and 8 deuterium atoms probably due to exchange with the allylic hydrogen atoms. Released cholestane possesses up to 4 deuterium atoms. This deuterium pattern can be obtained by desulphurizing steroidal thiolanes and/or thiols. Such compounds, however, can be excluded in the residual polar material since steroidal thiolanes and thiols elute in the apolar fraction (8-9). The deuterium pattern of cholestane is therefore explained by the presence of cholestane with predominantly 1 and 2 intermolecular sulphur-linkages. The small amounts of cholestane with three and 4 deuterium atoms may be derived from desulphurisation of steroidal thiolanes linked by a sulphur-bond to the macromolecular network or, less likely, from cholestane with 3 intermolecular sulphur-linkages. The deuteriation pattern of the C hopane is dominated by 2 and 7 deuterium atoms. The hopane with mainly 2 deuterium atoms may result from desulphurization of a hopanoid thiolane. As in previous example, this is considered unlikely, since hopanoid thiolanes are eluting in the apolar fraction. Thus the hopane carbon skeletons with mainly two deuterium atoms are derived from hopanes attached to the macromolecular network by two sulphur atoms. The hopane carbon skeletons with mainly 7 deuterium atoms are probably derived from a hopanoid thiophene linked to the macromolecular network by one sulphur atom. A similar complex deuteration pattern is visible for the C hopane released by desulphurization of GPC-fractions 4, 5 and 6 as for standard hopanoid thiophenes, indicating that it is indeed derived from a hopanoid thiophene linked by one sulphur. The question remains why the intermolecularly linked hopanoid thiophene, present in small amounts (Figure 5), is predominant in the high-molecular-weight fractions. A separation based on polarity instead of molecular weight can be excluded since hopanoid thiophenes linked by one sulphur atom are less polar then the C hopanes linked by two sulphur atoms which are present in the other fractions. One might reason that this C hopanoid thiophene moiety is only linked to the macromolecular network by one sulphur-atom, so that it can not act as a cross-linking agent but is only attached at the fringes of the crosslinked macromolecule. The nickel boride reagent may still be capable of desulphurizing peripheral parts of the macromolecular network but fails to desulphurize the core of the network due to steric hindrance. 35


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5.

SCHOUTEN ET AL.

C D O c ca ~o c nca


avg = 0.8 D

Total extract

avg = 0.2 D

GPC-9

avg = 0.5 D

GPC-8

avg = 0.8 D

GPC-7

avg = 1.2 D

GPC-6

avg = 1.5 D

| GPC-5

89

D

mm

F —

avg = 1.2 D

| GPC-4

-!

N o . deuteriums

Figure 6. Extent of deuterium labelling of cholestane released desulphurization of the bitumen and its GPC-fractions.


upon

90


avg = 2.3 D

1 Total extract

HI 0

111 H I

1

2

3

4

5

_

8

7

*

8

10


avg = 0.5 D

0

..

1

2

3

4

5

GPC-9

e

7

8

»

10

avg = 2.1 D

| GPC-8

avg = 2.3 D

GPC-7

avg = 2.9 D

GPC-4

_

F I

No. deuteriums

Figure 7. Extent of deuterium labelling of pentakishomohopane released upon desulphurization of the residual polar fraction of the bitumen and its G P C fractions.


5.

SCHOUTEN ET AL.


91

Excluding this peripheral moiety, the overall pattern is thus characterized by an increasing average amount of deuteriation for GPC-fractions of increasing molecular weight. This supports the concept that geomacromolecules in fractions with increasing higher molecular weight are more crosslinked and thus have more C-S bonds per carbon skeleton. XANES- and pyrolysis-data indicated this already for the Monterey bitumen (Eglinton et al, 1994) but these analysis were performed on the whole fractions without prior removal of low-molecular-weight apolar compounds. In addition these prior studies did not yield direct information on the type and number of sulphur linkages for discrete compounds that the chemolysis results have provided.


Conclusions Free and low-molecular-weight sulphur compounds are mainly present in GPC-fractions representing M < 800. Desulphurization of the polar material of the GPC-fractions of increasing molecular weight showed that carbon skeletons released have an increasing amount of deuterium labelling. This indicates that sulphur-rich geomacromolecules in fractions of higher molecular weight are indeed more crosslinked and thus have more C-S bonds per carbon skeleton. w

Acknowledgements This study was partly supported by a PIONIER-grant to JSSD from the Netherlands Organization for Scientific Research (NWO) and a grant to TIE from the Basic Energy Science division of the US Department of Energy (DE-FG02-92ER14232). Dr. M . Lewan (USGS, Denver) is thanked for provision of the Monterey sediment sample and Ms. J.E. Irvine is thanked for technical assistance. This is Division of Marine Biogeochemistry Contribution no. 178. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Vairavamurthy, A.; Mopper, K. Nature 1987, 329, pp. 623-625. SinningheDamsté,J.S.; Rijpstra, W.I.C.; Kock-van Dalen, A.C.; de Leeuw, J.W.; Schenck, P.A. Geochim. Cosmochim. Acta 1989, 53, pp. 1443-1455. Kohnen, M.E.L.; Sinninghe Damsté, J.S.; Kock-van Dalen, A.C.; ten Haven, H.L.; Rullkötter, J.; de Leeuw J.W. Geochim. Cosmochim. Acta 1990, 54, pp. 3053-3063. Fukushima, K.; Yasukawa, M.; Muto, N . ; Uemura, H . ; Ishiwatari, R. Org. Geochem. 1992, 18, pp. 83-91. Kohnen, M.E.L.; Sinninghe Damsté, J.S.; ten Haven, H.L.; de Leeuw, J.W. Nature 1989, 341, pp. 640-641. de Graaf, W.; Sinninghe Damsté, J.S; de Leeuw, J.W. Geochim. Cosmochim. Acta 1992, 56, pp. 4321-4328. Krein, E.B.; Aizenshtat Z. Org. Geochem. 1994, 21, pp. 1015-1025. Schmid, J . C . Ph.D. dissertation 1986, University of Strasbourg, pp. 263. Sinninghe Damsté, J.S.; Rijpstra, W.I.C.; de Leeuw, J.W.; Schenck, P.A.; In Advances in Organic Geochemistry 1987 (Matevelli, L . ; Novelli,M., Eds.), Org. Geochem. 1988, 13, pp. 593-606.


92 10. 11. 12. 13.


14. 15. 16.


SinningheDamsté,J.S.; Eglinton, T.I.; Rijpstra, W.I.C.; de Leeuw, J.W. In: Geochemistry of Sulphur Compounds in Fossil Fuels (Orr, W.L.;White, C . M . , Eds.), ACS Symp. Ser. 1989, Washington DC, pp. 486-528. Kohnen, M.E.L.; Sinninghe Damsté,J.S.; Kock-van Dalen, A.C.; de Leeuw, J.W. Geochim. Cosmochim. Acta 1991, 55, pp. 1375-1394. Richnow, H.H.; Jenisch, A . ; Michaelis, W. In: Advances in Organic Geochemistry 1991 (Eckhardt, C.B. et al., Eds.), Org. Geochem. 1992, 19, pp. 351-370. Adam, P.; Schmid, J.C.; Mycke, B.; Strazielle, C.; Connan, J.; Huc, A.; Riva, A.; Albrecht, P. Geochim. Cosmochim. Acta 1993, 57, pp. 3395-3419. Hoffman, I.C.; Hutchison, J.; Robson, J.N.; Chicarelli, M.I.; Maxwell, J.R. In: Advances in Organic Geochemistry 1991 (Eckhardt, C.B. et al., Eds.), Org. Geochem. 1992, 19, pp. 371-388. Eglinton, T.I; Irvine, J.E.; Vairavamurthy, A.; Zhou, W.; Manowitz, B. In: Advances in Organic Geochemistry 1993 (Telnaes, N.; van Graas, G.; Øygard, K., Eds.), Org. Geochem. 1994, 21, pp 781-800. Schouten, S.; Pavlovic, D.; Sinninghe Damsté, J.S.; de Leeuw, J.W. Org. Geochem. 1993, 20, pp. 901-909.

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Influence of Sulphur Cross-linking on the Molecular-Size Distribution

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