Molecular Structure of Athabasca Asphaltene: Sulfide, Ether, and Ester

Energy & Fuels 1997, 11, 1171-1187

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Molecular Structure of Athabasca Asphaltene: Sulfide, Ether, and Ester Linkages Ping’an Peng, Angelina Morales-Izquierdo, Alan Hogg, and Otto P. Strausz* Department of Chemistry, University of Alberta, Edmonton, AB, Canada, T6G 2G2 Received February 10, 1997. Revised Manuscript Received July 15, 1997X

Athabasca n-C5-asphaltene was fractionated into occluded maltene, low and high molar mass (LMA, HMA) asphaltene, and the latter fractions were subjected to Ni2B reduction to cleave the sulfide C-S bonds, basic hydrolysis to cleave the ester C-O bonds, and BBr3 treatment to cleave the ether C-O bonds. Ni2B reduction of asphaltenes yielded 5-18% n-pentane solubles, which were separated into saturates, aromatics, and polars, and the saturates were analyzed for biomarkers. The residual asphaltene underwent 40% desulfurization and a greater than 4-fold drop in the MW of HMA but no change in the MW of LMA. The decrease in the MW is attributed to sulfide-bound core segments in the structure of the asphaltene: [core]-S-[core]-S-[core]-S-[core] + Ni2B f 4[core] + 3H2S. This is an important structural feature of Athabasca asphaltene and is responsible for its upgradability without excessive coke formation. The biomarkers of the asphaltene fractions were also characteristically different with regard to maturity status and composition. Both fractions yielded n-alkanes, cheilanthanes, regular steranes, hopanes, and gammacerane, and the LMA also contained dicyclic terpanes and C21-C25 steranes. Noteworthy was the absence of diasteranes, which are the only steranes in the maltene. In terms of the 20S/(S + R) steranes and 22S/(S + R) hopanes parameters the maturity varies as maltene > LMA > HMA. This difference is a manifestation of the thermocatalytic nature of the maturation process and the protection of the macromolecular nature of the asphaltene against contact with external reagents. Ni2B reduction indicates that (1) the n-alkane products arise from n-alkyl substituted thiolane/thiane and thiophene and (2) C27-C30 steranes are attached to the asphaltene by one S atom, and the C21-C25 steranes and terpanes by two S atoms. Basic and BBr3 hydrolyses of HMA showed that both ester and ether linkages of n-acids and n-alcohols are present and that the esters are of recent origin, whereas the ethers were derived from the original biotic source material of the bitumen.

The molecular structure of asphaltene has been a central problem in petroleum chemistry for many decades. Aside from its inherent scientific curiosity, interest in asphaltene has been driven by the influencesmostly adversesof asphaltene on the processes employed in the recovery, refining, and upgrading of petroleum. Also, the discovery of the presence of biological markers in asphaltene1-3 highlighted its relevance to the organic geochemistry of crude oil. The composition of Athabasca and related oil sand and heavy oil asphaltenes, along with their molecular structure, has been investigated extensively. By use of thermal degradation,4 ruthenium ions-catalyzed oxidation,5 naphthalene radical anion,6 and other reductions as well as various instrumental methods,4,7,8 a large number of major structural building blocks and struc* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Rubinstein, I.; Spyckerelle, C.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1-6. (2) Rubinstein, I.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1887-1893. (3) Ekweozor, C. M.; Strausz, O. P. In Advances in Organic Geochemistry 1981; Wiley-Heyden: London, 1983; pp 746-766. (4) Payzant, J. D.; Lown, E. M.; Strausz, O. P. Energy Fuels 1991, 5, 445-453. (5) Mojelsky, T. W.; Ignasiak, T. M.; Frakman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, O. P. Energy Fuels 1992, 6, 83-96. (6) Ignasiak, T.; Kemp-Jones, A. V.; Strausz, O. P. J. Org. Chem. 1977, 42, 312-320. (7) Cyr, N.; McIntyre, D. D.; Toth, G.; Strausz, O. P. Fuel 1987, 66, 1709-1714.

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tural elements of the molecular framework have been identified.9 Table 1 summarizes these compounds that, in total, are estimated to comprise over 50% of the asphaltene and are present in chemically bound form attached to the asphaltene core by one or two single covalent bonds. The alkyl carbons account for about 27% of the total carbon, and they have been shown to occur as side chains and bridges as well as ester-bound fatty acids connected to ring structures. The substitution pattern of the main building blocks identifiedsthe sulfur compounds, fluorenes, and alkylbenzenessbears the unmistakable hallmark of a common evolutionary pattern beginning with the cyclization and aromatization of normal alkanoic precursors, fatty acids, and perhaps alcohols. A large portion of the biomarkers in Athabasca asphaltene appeared to be concentrated in the low molar mass fraction of the n-pentane-precipitated asphaltene,10 and this low molar mass fraction could be removed by acetone or diethyl ether extraction. The distribution and nature of the biomarker hydrocarbons liberated from the asphaltene by mild thermolysis were similar to, yet characteristically different from, the biomarkers in the maltene fraction of the oil, and (8) Semple, K. M.; Cyr, N.; Fedorak, P. M.; Westlake, D. W. S. Can. J. Chem. 1990, 68, 1092-1099. (9) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 13551363. (10) McIntyre, D. D.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 2, 251-265.

© 1997 American Chemical Society

1172 Energy & Fuels, Vol. 11, No. 6, 1997

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Table 1. Structural Building Blocks Identified in Athabasca Oil Sand Asphaltene as Homologous Seriesa

Figure 1. Flow chart for the fractionation of crude Athabasca asphaltene sample A into high molar mass asphaltene (HMA), low molar mass asphaltene (LMA), occluded maltene, and occluded maltene saturates, aromatics, and polars.

a

R ) n-alkyl.

biodegradation of the oil did not affect the asphaltene or its biomarker content.1-3,11 These observations were explained in terms of the biomarker information content of the asphaltene representing an earlier stage in the maturation history of the crude oil. The slower maturation of the organic matter locked up in the asphaltene is then due to the protective environment provided by the large micellar structure of the asphaltene, preventing the potential reaction sites from getting into contact with extraneous reagents, catalysts, microbes, or surfacederived formation water flowing through the deposit. From studies of the naphthalene radical anion reduction6 of Athabasca asphaltene, it has been concluded that a prominent structural feature of the asphaltene molecule is the presence of molecular core segments that are held together by sulfide linkages: [core]-S-[core]-S[core]-S‚‚‚, etc. On electron transfer from the naphthalene radical anion these structures should fall apart to their core segments, resulting in a substantial decrease in molecular weight. Indeed, the observation of a drastic reduction of molecular weight was the basis of the above conclusion. Subsequently, the presence of aliphatic sulfides was confirmed by the results of ruthenium ions-catalyzed oxidation experiments,12 and later, the homologous series of 2-n-alkyl- and 2,5-di-nalkylthiolanes and 2-n-alkyl- and 2,6-di-n-alkylthianes were shown to be significant structural building blocks in the asphaltene molecule.13 The naphthalene radical anion reduction of n-pentane-precipitated asphaltene also produces some npentane soluble oil, the saturates portion of which has been shown to contain biological markers featuring a series of n-alkanes, tricyclic terpanes, and steranes, with (11) Rubinstein, I.; Strausz, O. P.; Spyckerelle, C.; Crawford, R. J.; Westlake, D. W. S. Geochim. Cosmochim. Acta 1977, 41, 1341-1353. (12) Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 3, 43-51. (13) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 117-131.

the conspicuous absence of the otherwise ubiquitous series of pentacyclic terpanessthe hopanes.14 The formation of this n-pentane soluble oil was explained in terms of the liberation of the remnants of the “original oil” that was entrapped “within the asphaltene matrix and protected from the effect of in-reservoir biodegradation”.14,15 In the present study, the asphaltene structure was further explored by means of detailed analyses of the chemolysis products from the nickel boride reduction of the carbon-sulfur bonds, the basic hydrolysis of the ester bonds, and the boron tribromide cleavage of the carbon-oxygen bonds in the molecule. Chemolyses in general are more selective and milder processes than thermolysis, and the structural integrities of the fragments produced can be expected more likely to be preserved. Experimental Section Samples. Two oil sand samples, A and B, both from the Syncrude quarry in the Lower Cretaceous Manville Group, were used. Their burial depths were somewhat less than 15 m. The samples contained about 10% bitumen, which was extracted with methylene chloride. Isolation of the Asphaltene. Crude asphaltene was precipitated by the addition of 40-fold volume/weight of n-pentane to the 1:1 volume/weight CH2Cl2 solution of bitumen. The precipitated asphaltene was filtered and washed with n-pentane three times. The asphaltene contents of the bitumens were 18.2 and 16.2% for samples A and B, respectively. The crude asphaltene (A: MW ) 2431 g mol-1, ash 3.52%) thus obtained was subjected to Soxhlet extraction with acetone for 1 week and fractionated according to the flow diagram depicted in Figure 1. The extraction was interrupted at two-day intervals, and the aggregated solid in the extraction thimble was pulverized, returned to the thimble and the extraction continued. Next, the acetone extract was dried in a rotary evaporator and then Soxhlet extracted with n-pentane for another week. The acetone-extracted asphaltene is labeled as high molar mass asphaltene (A-HMA, 4867 g mol-1), the n-pentane-extracted acetone extract as low molar mass asphaltene (A-LMA, 1213 g mol-1), and the n-pentane extract as maltene. The last was fractionated into saturates, aromatics, and polars by column chromatography on silica gel. Purification of Solvents and Chemicals. n-Pentane, dichloromethane, and toluene were distilled from CaH2. Tetrahydrofuran was dried and distilled from sodium. Methanol was distilled from Mg, and nickel chloride (50 g in 50 mL water) was extracted with 3 × 20 mL dichloromethane and (14) Ekweozor, C. M. In Advances in Organic Geochemistry 1985; Leythhaeuser, D., Rullko¨tter J., Eds.; Pergamon: Oxford, 1986; pp 1053-1058. (15) Ekweozor, C. M. Org. Geochem. 1986, 6, 51-61.

Structure of Athabasca Asphaltene

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Figure 2. Flow chart for the Ni2B desulfurization of asphaltene and chromatographic fractionation of products. dried under vacuum. NaBH4 and NaBD4 were washed with freshly distilled n-pentane prior to use. Ni2B Desulfurization (Figure 2). Desulfurization of HMA and LMA was performed using the method described by Back et al.16 and modified by Schouten et al.17 Typically, asphaltene (1 g), NiCl2 (3 g), THF (100 mL), and MeOH (50 mL) were stirred under an argon stream in an ice bath for 10 min. NaBH4 (3 g) was added slowly and the mixture refluxed for 16 h. After cooling, the mixture was centrifuged, the supernatant collected, and the residue extracted with 3 × 20 mL 2:1 CH2Cl2/n-C5H12. The extracts and supernatant were combined and washed with saturated aqueous NaCl, and the organic layer was collected and dried over Na2SO4. The solvent was removed in a rotary evaporator, and the residual asphaltene was precipitated with n-pentane. The maltene was separated into saturates, aromatics, and polars, and the saturates were further fractionated by urea adduction. To assess the efficiency of the desulfurization, dioctyl sulfide and 2-hexadecylthiolane were used as standards. The alkane yields were greater than 98% and in the GC-MS analysis of the products, alkenes were not detected. Urea Adduction. The saturates generated from HMA by desulfurization were further separated by urea adduction. Briefly, saturates (4 mg) in n-pentane (0.5 mL) were added to urea-saturated MeOH (5 mL). The mixture was kept at -10 °C for 24 h. Then the crystals were collected by filtration and dissolved in hot water (5 mL) to yield the urea adduct. The filtrate was extracted with n-pentane (3 × 5 mL), washed with distilled water (3 × 5 mL), and dried over Na2SO4. The solvent was removed in a rotary evaporator, leaving behind the urea nonadduct (Figure 2). Ester Cleavage (Figure 3). HMA (2.0 g) and NaOH (4 g) were dissolved in 1:1 toluene/MeOH and refluxed for 48 h. After the solution was cooled, 20 mL (0.5 M) of aqueous NaOH was added and the mixture extracted with 0.5 M aqueous NaOH (3 × 20 mL). The extracts were combined and acidified with 6 M HCl. Next, the organic acids were extracted with dichloromethane (3 × 20 mL), then methylated with BF3/ MeOH. The organic phase after NaOH extraction was concentrated, and the asphaltene was precipitated with n-pentane. The maltene products were separated on silica gel into saturate, aromatic, and polar fractions. The alcohols in the polar fraction were acetylated with acetic anhydride. Ether Cleavage (Figure 4). HMA (2.0 g) and 1.6 M BBr3 in dichloromethane (40 mL) were refluxed with magnetic stirring for 48 h. After the solution was cooled, 80 mL of ethyl ether was added and stirring continued. Then 40 mL of water was added, the ether layer was washed with water (3 × 40 mL), and the excess solvent was removed in a rotary evaporator. The residual asphaltene was precipitated with n-pentane and the maltene separated on a silica gel column into polar and nonpolar fractions. The latter, containing the bromides, was eluted with dichloromethane and the bromides reduced to alkanes or deuterated alkanes with LiAlH4 or LiAlD4. (16) Back, T. G.; Yang, K.; Krouse, H. R. J. Org. Chem. 1992, 57, 1986-1990. (17) Schouten, S.; Pavlovic, D.; Sinninghe Damste´, J. S.; de Leeuw, J. W. Org. Geochem. 1993, 20, 901-909.

Figure 3. Flow chart for the basic hydrolysis of asphaltene and chromatographic fractionation of the products. The numbers represent percentage yield values in terms of the starting asphaltene.

Figure 4. Flow chart for the BBr3 hydrolysis of B-HMA and chromatographic fractionation of the products. The numbers represent the percentage yield values in terms of the HMA. Gas Chromatography (GC), Gas ChromatographyMass Spectrometry (GC-MS), and Molecular Weight Determinations. Gas chromatographic analyses were performed on an HP-5830A apparatus with a flame ionization detector (GC-FID) and a split injector (split ratio of 15:1) using hydrogen as a carrier gas. Separation was done on a J&W fused silica (30 m × 0.22 mm) column coated with 0.25 nm DB-1. Samples were injected in dichloromethane solution, and the GC was temperature programmed from 50 to 300 °C at 10 °C/min and held at 300 °C for 20 min. GC-MS was performed on a Varian Vista gas chromatograph combined with a VG 7070E mass spectrometer. GC conditions were the same as with GC-FID. In typical experiments the temperature of the transfer line was 290 °C, that of the ion source was 250 °C, and an electron energy of 45 eV was applied. Data acquisition was done with VG 11-250 software using a PDP 11/24 computer. Vapor pressure osmometric (VPO) determinations of molecular weight were done in a Corona Westcan molecular weight apparatus Model 232A. Either benzene (at 40 °C), toluene (at 50 °C), or o-dichlorobenzene (at 112 °C) was used as solvent. With MW < 1000 g mol-1 two readings from a single dilution were done; with MW > 1000 g mol-1 readings were made at several dilutions and the values were extrapolated to zero concentration.

Results As found before, acetone extraction of Athabasca asphaltene removes 21.2% or about one-fifth of the asphaltene. The molar mass of the whole asphaltene A was 2431 g mol-1; that of the acetone-extracted residual asphaltene was 4867 and 5166 g mol-1 and that of the n-pentane extracted acetone extract 1213 and 1660 g mol-1 (VPO, 112 °C, o-dichlorobenzene) in two


Peng et al.

Table 2. Yieldsa of Saturate, Aromatic, and Polar Fractions from the Desulfurization of Athabasca Asphaltenes sample A saturate HMA protonated deuterated LMA protonated deuterated deuterated a

aromatic

sample B polar

0.050

1.928

1.866

0.047

2.614

2.038

0.319 0.359 0.321

10.852 12.500 12.209

5.904 7.047 10.201

HMA protonated protonated protonated

saturate

aromatic

polar

0.109 0.126 0.109

2.445 2.116 2.130

2.082 2.367 2.739

Wt % of the corresponding starting asphaltene. Total product recovery, including residual asphaltene, is about 95%.

parallel experiments. Thus, the extract contains low molar mass asphaltene fragments and resinous materials18-20 that had been complexed to the asphaltene. Also present was some of the maltene that had coprecipitated with the asphaltene and could not be removed from the crude asphaltene by washing with n-pentane. This occluded maltene has a composition that is different from that of the bulk maltene (Figure 1) in that it is significantly enriched in aromatics (41%) and polars (57%) and is correspondingly depleted in saturatessfrom about 20% in the bulk maltene to about 3%. This shift in class composition is a manifestation of the role of complexation caused by intermolecular forces arising from polar and aromatic functionalities in the maltene molecules. On the other hand, the saturated hydrocarbon molecules present in low concentrations appear to have been intercalated into the pores and internal cavities of the asphaltene molecules and micelles.21 From the coprecipitated maltene, only the saturate fraction was analyzed further by GC and GC-MS and compared with the saturates produced in the Ni2B desulfurization of LMA and HMA from samples A and B. Saturates from the Occluded Maltene. The composition of this fraction is similar to that of the saturates from the bulk maltene,3,22-25 and it is dominated by C13-C19 dicyclic terpanes, C19-C30 tricyclic terpanes with a maximum at C23, and C29-C35 hopanes. A trace amount of gammacerane was also noted. Also present were C21, C22 pregnanes, and C27-C29-rearranged steranes. The C-22 epimerization ratio of the C31 hopanes is at its terminal value, 61%, showing that sample A is mature. Again, as in the bulk maltene, n-alkanes, isoprenoids, and regular steranes were absent owing to the severely biodegraded state of the bitumen. Because of this loss of important biomarkers from the maltene and for other reasons as well (see below), the biomarker chemistry of the asphaltene in the bitumen and in other severely biodegraded crude oils is highly valuable. (18) Frakman, Z.; Ignasiak, T. M.; Lown, E. M.; Strausz, O. P. Energy Fuels 1990, 4, 263-270. (19) Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 171-179. (20) Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1987, 3, 131-138. (21) Strausz, O. P.; Ignasiak, T. M.; Kotlyar, L.; Montgomery, D. S. To be published. (22) Brooks, P. W.; Fowler, M. G.; Macqueen, R. W. Org. Geochem. 1988, 12, 519-538. (23) Dimmler, A.; Strausz, O. P. J. Chromatogr. 1983, 270, 219225. (24) Dimmler, A.; Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7, 231-238. (25) Hoffmann, C. F.; Strausz, O. P. Am. Assoc. Pet. Geol. Bull. 1986, 70, 1113-1128.

Figure 5. Gas chromatogram (top) and total ion current mass chromatogram (bottom) of the urea adduct from the Ni2B desulfurization of sample A-HMA showing the distribution of n-alkanes and midchain methylalkanes (*). The concentration distribution is represented by the gas chromatogram.

Saturates from the Desulfurization of the Asphaltenes. The products comprise complex mixtures of oily materials that can be fractionated into saturates, aromatics, and polars. These low molar mass materials were originally covalently bound to the asphaltene core by one or more sulfide linkages. Compound class yields from the desulfurization reactions are given in Table 2. In all cases the yields of the saturates are much lower than those of the aromatics or the commensurate yields of the polars. The reproducibility of the results was satisfactory except for the case of the LMA material where the results from the reduction experiments with deuterated solvents are somewhat scattered. Also, as seen from the data, the yields from the LMA are several-fold higher than those from the HMA. In the present study only the saturate fractions were further explored and prior to analysis they were fractionated by urea adduction into urea adduct and nonadduct fractions. (i) Urea Adducts of Saturates from HMA’s. This fraction contained n-alkanes and monomethyl branched n-alkanes, as shown for the case of A-HMA (Figure 5). The n-alkanes ranged from at least C12 to C35 with maxima at C18 and C20 and exhibited a distinct evento-odd dominance especially in the low molar mass region (Table 3). Some complex series of small peaks



Table 3. Geochemical Characterization of Saturates in the Maltene and Desulfurization Products of Asphaltene

sample A maltene LMA HMA sample B HMA a

alkane Cmax

OEPa

pristane/phytane

16, 18 18

0.59 0.82

0.51

20

0.92

sterane, %

sterane 20S/(S + R), %

27/total

28/total

29/total

H31,32,35b S/(R + S), %

36.6 22.1

44.8 36.5

21.0 22.0

34.3 41.6

61.0 37.0 35.7

23.3

43.0

16.9

40.1

∑(H32, H35)/ ∑(H33 + H34)

G30/H30 c

1.67

0.12 0.064 0.35

(n-C17 + 6 × n-C19 + n-C21)/(4 × n-C18 + 4 × n-C20), calculated from the m/z ) 85 fragments. b H ) hopanes. c G ) gammacerane.

Figure 6. Mass spectrum of deuterated n-C20H42 from the NiCl2/NaBD4/CH3OD desulfurization of sample A-HMA, revealing the presence of d0-d9 isotopomers.

elute between the dominant n-alkane peaks, and their mass spectra identify them as mid-methyl substituted alkanes. The position of the methyl group on the carbon skeleton varies between C-6 and C-16. For a given carbon number in the C12-C22 region either the 6-methyl or the 9-methyl substituent is the most abundant, whereas in the C23-C32 range it is the 12-methyl alkane that dominates. Desulfurization using deuterated solvents yielded multideuterated n-alkanes (Figure 6) as a result of deuterium uptake at the cleaved sulfur bond site(s). This deuteration pattern, in which the monodeuterated isotopomers are relatively minor, demonstrates that nalkyl sulfides, e.g.,

are relatively insignificant structural appendages in the asphaltene. The most abundant isotopomers are the d3 and d4 species, and the maximum number of deuterium atoms incorporated appears to be as high as eight or nine. An auxiliary study carried out on the NiCl2/NaBD4/ CH3OD desulfurization of the model compound 2-nhexadecyl thiolane revealed the occurrence of some D/H exchange during the desulfurization process, details of which will be published shortly, and therefore, the quantitative aspect of deuterium uptake is somewhat compromised. Nevertheless, the qualitative aspects appear to remain valid. The n-alkanes from B-HMA lie in a similar carbon range, C14-C35, as from A-HMA, but their distribution shows a less pronounced even-to-odd predominance. Deuterium incorporation during deuterium reduction experiments showed a distribution similar to that from A-HMA.

Figure 7. m/z ) 191 cross scan mass chromatogram displaying the distribution of C19-C29 tricyclic, C27 tetracyclic, C29C35 pentacyclic terpenoid hydrocarbons and gammacerane from the Ni2B desulfurization of sample A-HMA.

Figure 8. Plots of relative concentration versus number of deuterium atoms in C20-C23 tricyclic terpanes from the NiCl2/ NaBD4/CH3OD/THF desulfurization of sample A-HMA.

(ii) Urea Nonadducts of Saturates from HMA’s. This fraction of the saturates contained a series of alkylcy-


Peng et al.

Figure 9. Relative concentration distributions of hydrocarbon biomarkers from the Ni2B desulfurization of asphaltenes from samples A and B and those in the maltene occluded in asphaltene A, based on percentage areas from mass spectrometric total ion current measurements.

clohexanes, tricyclic terpanes, steranes, hopanes, and trace amounts of isoprenoids and gammacerane. (a) Alkylcyclohexanes. A short series of alkylcyclohexanes extending from C14 to C19 was detected in A-HMA. Alkylcyclohexanes have been reported to occur in several Ordovician samples26-28 where they show a regular distribution pattern characterized by an oddto-even preference in the C14-C24 domain. The present sample shows no such preference. (b) Tricyclic Terpanes. From A-HMA, this series extends from C19 to C30 and, as in the maltene, the C23 member is the most abundant (Figure 7). Deuterated desulfurization, Figure 8, leads to mainly mono- and dideuterated tricyclanes, revealing that these compounds were bound in the asphaltene through one and (mainly) two sulfur atoms positioned in the side chain. From B-HMA the yield of tricyclic terpenoids was much lower relative to the alkanes than from A-HMA. Relative concentration distributions for the various compound classes from samples A and B are illustrated diagramatically in Figure 9. (c) >C26 Steranes. Regular steranes appear in copious amounts in the products from both HMAs, while diasteranes, which are the only steranes in the occluded and bulk maltene, are absent in the A-LMA and A-HMA samples. This class of biomarkers is dominated by the C27 and C29 members (Figure 10). A C30 sterane in low concentration is also in evidence. The epimeric ratios at C-20 in both the 5R,14R,17R (Table 3) and 5R,14β,17β members of the series show that the steranes bound to the HMA of samples A and B are immature. Wellpreserved steranes along with the absence of diasteranes in the asphaltenes, and the presence of only (26) Fowler, M. G.; Abolins, P.; Douglas, A. G. Org. Geochem. 1986, 10, 815-823. (27) Hoffmann, C. F.; Foster, C. B.; Powell, T. G.; Summons, R. E. Geochim. Cosmochim. Acta 1987, 51, 2681-2697. (28) Reed, J. D.; Illich, H. A.; Horsfield, B. In Advances in Organic Geochemistry 1985; Leythaeuser, D., Rullko¨tter, J., Eds.; Pergamon Press: Oxford, 1986; pp 347-368.

Figure 10. m/z ) 217 cross scan mass chromatogram displaying the distribution of C27-C30 steranes from the Ni2B desulfurization of sample A-HMA.

diasteranes in the maltene (Figure 9), reflect the diagenetic and biodegradative changes the maltene had undergone during its burial history. When the desulfurization is carried out with deuterated solvents, the number of deuterium atoms incorporated into the product sterane hydrocarbon varies from zero to four, with a maximum at d1 incorporation (Figure 11). The position of the deuterium atom and thus the position of the sulfur bond in the sterane molecule can be deduced from the mass spectrum of the d1-sterane molecules, Figure 12. This suggests that the D atom is on the A,B-ring, probably in the C-2 or C-3 position as the incorporation and sulfur attachment site. (d) Pentacyclic Terpanes. C29-C35 hopanes were detected in all the samples studied (Figure 7). The C29C31 homologs were more abundant than the C32-C35 members. The distribution pattern of the C32-C35 members is C32 >C35 > C33, C34 (Table 3). The extent of deuterium incorporation in the hopane molecules



Figure 13. Plots of relative concentration versus number of deuterium atoms in the C30, C31, C32, and C35 hopanes from the NiCl2/NaBD4/CH3OD/THF desulfurization of sample AHMA. Figure 11. Plots of relative concentration versus number of deuterium atoms in the C27-C29 steranes from the NiCl2/ NaBD4/CH3OD/THF desulfurization of sample A-HMA.

Figure 12. Mass spectra of C27 and C27-d1 steranes.

during deuterium desulfurization is dependent on the length of the side chain, and the position of the maximum concentration versus the number of deuterium atoms incorporated in a given homologue (Figure 13) shifts from one in the C30 homologue to three in the C35 homologue. This trend can be attributed to an increase in the probability of sulfur incorporation with increasing length of the side chain in the hopane precursor molecules.

(iii) Saturates from the LMA’s. The yield of saturates from the LMA, as of the other two fractions, the aromatics and polars, is several times higher than from the HMA and amounts to nearly 0.3% of the LMA (Table 2). (a) n-Alkanes. n-Alkanes show an even-to-odd preference in the C15-C30 range exceeding that from the HMA's, as given in Table 3, and in deuterium desulfurization up to eight deuterium atoms are incorporated, as in the HMA’s. (b) Dicyclic Terpanes. Dicyclic terpanes were detected in the C15-C19 range, and as in the maltene, they are dominated by the C15 and C16 drimanes

Other, isomeric dicyclic terpanes could be present (as in the maltene) but were not detected. Deuterium desulfurization shows the uptake of two deuterium atoms in the B-ring or its alkyl substituents. (c) Tricyclic Terpanes. Tricyclic terpanes represent the most prevalent compound class hydrocarbons in the saturates. Their carbon range extends from C20 to C30. On deuteration, they, as their counterparts from the HMA, appear to take up mostly two deuterium atoms either in ring C or in the side chain. The linkage patterns to the asphaltene core are probably similar to those in the HMA. (d) Steranes. Steranes occur in the C27-C29 range as regular steranes, as in the HMA; diasteranes are absent. The distribution of individual compounds, as revealed by the m/z ) 217 mass chromatogram, is similar to that from the HMA (Figure 10). However, the relative concentration of the C-20 epimers is 36.6%, somewhat higher than that in HMA (Table 3). The mass spectra of the deuterated C27-C29 members also indicate that these molecules were bound to the asphaltene core via a single sulfur atom on the A,B-ring in the C-2 or C-3 position in the sterane. Figure 14 shows that along with


Peng et al.

Figure 14. m/z ) 217, 231, 245, and 259 cross scan mass chromatograms displaying the distribution of C21-C25 steranes from the Ni2B desulfurization of sample A-LMA.

the C27-C29 members there are also several C21-C25 steranes. Some of them, pregnanes and norcholanes, are present in the maltene, but none of them appear in the HMA. One of the series features a side chain on ring-D from C2 to C6, while other members have C1-C3 alkyl substituents in ring-A with each carbon skeleton containing up to three isomers. The most abundant

species are the C22 and A-ring monomethylated C22 steranes. Unlike the C27-C29 steranes, the C22-C25 homopregnanes display a distinct preference for d2 uptake on deuterated solvent reduction (Figure 15). A further distinction, as revealed by mass spectroscopic data, is the site of attachment: in the C27-C29 series it is the A,B-ring possibly at the C-2 or C-3 position, whereas in the C21-C25 series it is located in the side chain on the D ring. (e) Hopanes. C29-C35 hopanes were observed in low concentrations in the total ion current chromatogram.

Figure 15. Plots of relative concentration versus number of deuterium atoms in the C22, two C23 steranes, and the C26 steranes from the NiCl2/NaBD4/CH3OD desulfurization of sample A-HMA.

Their distribution patterns and relative isomer concentrations are identical with those in the hopanes from the HMA. Basic Hydrolysis of Ester Bonds. The flow diagram for the procedure employed and the products and their yields from the HMA of samples A and B were



Figure 16. m/z ) 74 cross scan mass chromatogram of the n-alkanoic acids liberated in the basic hydrolysis of sample B-HMA.

given in Figure 3. The total amounts of hydrolysis products liberated from the asphaltenes of the two samples (saturates, aromatics, and polars, including the acids as methyl esters and alcohols as acetates) were different, 6.7% from sample A and 3.8% from sample B. Also different were the amounts of acids and alcohols; the acids, measured as their methyl esters, amounted to 0.047 and 0.46% and the alcohols, as their acetates, to 2.43 and 0.89% from samples A and B, respectively. In addition to the acid methyl esters and alcohol acetates, small quantities of saturates and large amounts of polar materials, the compositions of which were not further explored, were isolated. The acid and alcohol compositions of the two samples were very similar. The acids comprised C14-C27 fatty acids in sample B (Figure 16) and C14-C21 fatty acids in sample A (Figure 17) and in both cases were dominated by the even-carbon-number members, in particular the C16 and C18 members. Other significant acid components detected were the C20 and C21 members of the tricyclic terpenoid acids previously identified in Alberta bitumens and the organic matter chemisorbed to the oil sand solids18,29,30 (Figure 17). The alcohol fraction from both samples (analyzed as the acetates) contained C12-C24 n-alcohols essentially free of the odd-carbon-number components (Figure 18). Cleavage of the Ether Bonds. Boron tribromide is a useful reagent for the cleavage of the ether and ester C-O bonds in macromolecules. The resultant bromide products can then be converted in a subsequent step to the corresponding alkanes by treatment with lithium aluminum hydride and analyzed by GC-MS. The flow diagram for the cleavage procedure of the ester and ether bonds in HMA sample B was given in Figure 4 together with the nature and yields of the products. The yield of saturates + aromatics + polars products here is 6%, considerably higher than in the basic hydrolysis (3.8%) as would be expected, since with BBr3 both the ester and ether bonds are cleaved, whereas with NaOH only the ester bond is supposed to be broken. Nevertheless, the yield of the CH2Cl2 fraction from the basic hydrolysis, containing the alcohol (29) Cyr, T. D.; Strausz, O. P. J. Chem. Soc., Chem. Commun. 1983, 1028-1030. (30) Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7, 127-140.

Figure 17. m/z ) 74 and 191 cross scan mass chromatograms showing the n-alkanoic acids (top) and the tricyclic terpenoid acids (bottom) liberated in the basic hydrolysis of sample A-HMA.

Figure 18. Total ion current mass chromatogram of the alcohols (analyzed as acetates) from the basic hydrolysis of sample B-HMA.

acetates (Figure 3), is several times higher than the yield of saturates in the BBr3 cleavage, which represents the combined yield of the alcohols from ester and ether cleavages. This apparent contradiction is really not surprising, since the crude chromatographic fractions cannot be expected to be pure compound classes, although in the BBr3 cleavage the saturates fraction is probably a good, clean, but not necessarily quantitative representation of the alcohols. The composition of the saturates displays an interesting distribution of n-alkanes (Figure 20), which are the sole major components, in the C11-C34 range. The characteristic feature of this distribution is the strong


Figure 19. Total ion current mass chromatogram of the hydrocarbons from the BBr3 cleavage of the C-O bonds in sample B-HMA.

Figure 20. Elemental data for the HMA and Ni2B-desulfurized HMA of sample A.

preference for the even-carbon-number members in the low molar mass (C12-C24) end of the spectrum where the n-alcohol products from the basic hydrolysis were essentially devoid of the odd-carbon-number members. Above C24 the preference for the even-carbon-number members is markedly reduced. This seems to suggest that the alcohols from the ether cleavage have an OEP (odd-even preference) of close to unity with a carbon range 11-31 and a maximum concentration at C26 and are mixed with the alcohols from the ester cleavage, which have an OEP of nearly zero and maximum concentrations around C14 and C16. The trace amounts of C29-C31 R,β-hopanes appearing at the tail end of the spectrum indicate the presence of ether-bound hopanols in the asphaltene. Discussion n-Pentane-precipitated Alberta oil sand asphaltenes have been shown to contain about one-fifth their weight in the form of acetone-extractable materials comprising some occluded maltene (4.6%), relatively high molar mass and highly polar resinous substances, and relatively low molar mass, degraded asphaltene fragments (16.7%, LMA). The two asphaltene fractions, the LMA and the acetone-extracted residual HMA, have somewhat different elemental and vastly different molecular compositions and molar masses. Therefore, it does not come as a surprise that the two fractions behave differently during chemical transformations, as seen

Peng et al.

Figure 21. Elemental data for the LMA and Ni2B-desulfurized LMA of sample A.

from the data presented above and in the graphical comparison of Figures 20 and 21. Here, we see that, for example, during Ni2B reduction LMA and HMA both undergo partial desulfurization and hydrogen uptake, but changes in O/C ratios and molar masses exhibit quite different trends: in LMA the O/C ratio decreases, while in HMA it increases (due to the high reactivity of reduced HMA with respect to oxygen and consequent uptake of aerial oxygen). Also, the molar mass remains unaltered within experimental error ((8%) in LMA, whereas it significantly decreases by about 4-fold in HMA. Ni2B is considered to specifically and selectively cleave the C-S bonds,16,17 and undoubtedly, the major chemical reaction taking place in the HMAs during Ni2B desulfurization is cleavage of the carbon-sulfur bonds, giving rise to the saturated, aromatic, and polar products in an aggregate amount close to 5% and causing the degradation of the asphaltene to its molecular core segments:

and

In the latter reaction, to estimate the number of sulfide bridges broken, it is necessary to take into account interfering side reactions that could affect the apparent molecular mass change. One such possible side reaction is related to the ester linkages, the presence of which has been firmly established in the present study. During NiCl2/NaBH4/THF/CH3OH reflux, transesterification with CH3OH may occur:

This reaction liberates alcoholic OH’s in the asphaltene, which through hydrogen bonding could cause an increase in the molecular aggregation. This hypothesis was tested in an auxiliary experiment in which a HMA sample was subjected to the procedure as applied in the Ni2B desulfurization experiments but without the NiCl2 reagent. It was indeed found that the molar mass of



the HMA had increased from its initial value (in this experiment) of 4630 to 5460 g mol-1. Moreover, it was also observed that after the basic hydrolysis with NaOH, the residual asphaltene partially lost its solubility presumably because of the molar mass increase due to the creation of alcoholic, carboxylic, and other highly polar functionalities, causing extensive aggregation of the asphaltene molecules. The small amount of residual asphaltene that could be solubilized had a molar mass of 5630 g mol-1sa comparable but larger increase than that with NaBH4 as the base. Thus, desulfurization breaks up the macromolecules by converting them to their covalent core segments:

resulting, after corrections for the release of low molar mass fragments and increased hydrogen bonding owing to transesterification, in a slightly larger than 4-fold drop in the molar mass. The true extent of molar mass reduction would probably be larger had desulfurization been complete. Desulfurization, however, is only 42% complete, and therefore, some sulfide bridges could have been left intact. Earlier studies by Strausz and co-workers6 on the naphthalene radical anion (NRA) desulfurization of Athabasca asphaltene

followed by stabilization of the resulting carbanions by alkylation with octyl iodide, yielded larger molar mass reductions probably because of the complete destruction of hydrogen bonding caused by carboxylic and alcoholic OH’s:

bond31

and Also, the NRA efficiently cleaves the ester any ester bridges that might have been present connecting core segments,

would be broken, contributing to the reduction of the molar mass. In contrast, the Ni2B reagent would not attack the ester bond. Among the causes responsible for the larger molar mass reduction in the NRA reaction, the less selective nature of this reagent and the complications arising from the need to stabilize the primary hydrocarbon anions (e.g., with octyl iodide) as well as to the occurrence of side reactions with the solvent THF can be mentioned. Also, the whole n-C5-asphaltene used in these experiments had a considerably higher molar mass, 5500 g mol-1 (VPO, CH2Br2, 50 °C), and therefore, the results are not directly comparable. In any event the results of the present study confirm the presence of core segments held together by sulfide linkages in the HMA with an average molar mass of (31) Mojelsky, T. W.; Peng, P.; Strausz, O. P. Unpublished results.

about 1200 g mol-1, which is about the same as the number average molar mass of the LMA that is not lowered by Ni2B desulfurization. This is a highly significant result in regard to our understanding of the molecular structure of asphaltene because, in accord with other results on Ni2B desulfurization presented in this paper, it clearly shows that, along with the cyclic sulfides30sthe n-alkanethiolanes and thianessacyclic sulfides are significant structural elements in Athabasca asphaltene. This structural feature is the one that is largely responsible for the desirable property possessed by all Alberta oil sand and heavy oil asphaltenes of facile degradability on coking and hydrocracking, thus precluding excessive coke formation. Also, as indicated by the Ni2B desulfurization experiments, acyclic sulfides bonding small structural units involving saturated, aromatic, and polar molecules to the asphaltene core are present as well. With regard to the evolution of the model of sulfidebridged core segments,

we reported6 their existence in Athabasca asphaltene based on the results from the NRA reduction in 1977. Later, in 1986, the presence of aliphatic sulfides in Athabasca asphaltene was confirmed14 by the appearance of sulfones in the aliphatic residue from the ruthenium ions-catalyzed oxidation. They accounted for about 25% of the sulfur in the asphaltene. Subsequently, in 1988, a homologous series of n-alkyl substituted thiolanes and thianes were isolated13 from the pyrolysis oil of Athabasca asphaltene. At this point it became problematic to distinguish between acyclic and cyclic sulfide linkages as bridging elements. The possibility of cyclic sulfide bridge breaking in naphthalene radical anion electron-transfer reactions

had to be taken into account, based on the following considerations. Ethers also undergo C-O bond cleavage in electron transfer but at a much slower rate. This reaction, in the case of the cyclic ether THF, is so slow that THF was used as the reaction medium in the C-S bond cleavage reactions. When asphaltene was desulfurized in the K/naphthalene/THF system and the resultant anions stabilized by octyl iodide, some octyl and decyl naphthalenes were identified among the products. The C2 unit in the latter product required to increase the C8-naphthalene to the C10-naphthalene had to come from THF. Thus, formally,

Therefore, with thiolanes the possibility of the analogous reaction being more facile and becoming the major reaction could not be discounted. This type of complication, however, does not apply to the Ni2B reduction, and the decrease in the molar mass must be due to cleavage of the C-S bonds alone, without any accompanying C-C bond cleavage. Therefore, the sulfides cleaved here must be noncyclic.


Also, in auxiliary experiments using the model compound n-hexadecylthiolane, it was shown that in the naphthalene radical anion reaction under the same conditions as with the asphaltene, only about 4% of the substrate decomposed, mainly via straightforward desulfurization.31 (This may explain why hopanes are

absent from the naphthalene radical anion reduction products.14,15 If the sulfur binding the hopane molecule to the asphaltene core is in a ring, then it may escape reduction.) Therefore, not even in the naphthalene radical anion reduction of the asphaltene could the decrease in molar mass be attributed to both C-S and C-C bond cleavages in cyclic sulfides. Highly activated C-C bonds such as in 1,2-polyarylalkanes,32 however, are known to undergo cleavage reactions, but such highly reactive molecules are not expected to be present in mature petroleum beyond trace quantities. That 1,2aryl- and polyarylethanes, for example, are present in trace quantities has been verified experimentally from the ruthenium ions-catalyzed oxidation of Athabasca (and also an immature Chinese crude oil) asphaltene where it was found that the yield of the expected acid, HO2C-CH2-CH2-CO2H, from the oxidation Ru(VIII)

aryl-CH2-CH2-aryl 98 HO2C-CH2-CO2H is only 0.24 diacid per molecule; i.e., there are only 0.24 1,2-dipolyaryl plus 1,2-diaryl structures per molecule. Diarylmethane, if at all, should be present at even lower concentration levels. Thus, such structures could have contributed to the molar mass reduction but only as relatively small correction factors. In 1992 Strausz et al.9 proposed a representative model for the molecular structure of Athabasca asphaltene, largely based on the experimentally detected structural building blocks. At that time the question of whether cyclic sulfides (experimentally proven to be present), acyclic sulfides, or both served as bridging units had not been resolved. From the present study it is now clear that the bridging units responsible for the degradation of asphaltene in the K/naphthalene/THF or NiCl2/NaBH4/CH3OH/THF reactions consist exclusively of acyclic sulfides. Thus, the proposed model will have to be modified accordingly33 and some of the

structural elements replaced by

The apparent constancy of the molar mass in the desulfurization of LMA may conceal a slight decrease corresponding to the increase caused by the increase in hydrogen bonding. This concealed drop, however, is quite small when compared to the drop observed with HMA, and this suggests that, in contrast to the HMA, structures in which large molar mass asphaltene core (32) Holy, N. L. Chem. Rev. 1974, 74, 243-277. (33) Strausz, O. P.; Peng, P.; Lown, E. M.; Mojelsky, T. W. To be published.

Peng et al.

segments are held together by a sulfide bridge are lacking in the LMA. The 4-fold reduction in molecular weight requires a minimum of about a 2.0% decrease in the sulfur content of the reduced asphaltene (if the core segments in the asphaltene form a linear array and assuming that on cleavage of the sulfur bond the sulfur is quantitatively removed from the core molecule):

The difference between the experimental value, 3.3%, and the 2.0% estimated value derived from the above scheme, 1.3%, then reflects the sulfur removed in the cleavage of the low molecular weight appendages and cyclic sulfides. Biomarkers in the Occluded Maltene. Standard n-pentane precipitation of asphaltene yields asphaltene with 4.6% occluded maltene, which differs in class composition from the bulk maltene in that it contains a higher percentage of aromatics and polars and correspondingly less saturates. The saturates contain essentially all the biomarkers detected in earlier studies on Athabasca maltene and in about the same distributions. Since the biomarkers in the asphaltene are characteristically different from those in the maltene, in the study of asphaltene biomarkers great care must be taken to remove all traces of maltene. Moreover, as demonstrated by the results of the present study, even this is not quite sufficient, and in order to bring to light the differences in the biomarkers contents of the LMA and HMA and to obtain meaningful results on asphaltene biomarkers, correctly reflecting the biomarker composition as it existed in the earlier diagenetic ages of the oil, it is also necessary to separate the LMA from the HMA. Biomarkers in the Saturates from the Ni2B Desulfurization. (i) HMA (78.7%). Treatment of HMA with Ni2B yielded 0.05-0.11% saturates, 2.02.6% aromatics, and 2.0-2.7% polar compounds, all of which were originally bound to the asphaltene molecules via sulfide linkages (Table 2). The saturates were then subjected to detailed biomarker analyses. (a) Alkanes. GC-MS analysis of the urea adduct fraction isolated yielded the total ion current mass chromatogram and gas chromatogram shown in Figure 5. These alkanes could have originally been present as alkyl side chains attached to the asphaltene via a sulfur atom or as a cyclic sulfide, a 2,5-dialkylthiolane, 2,6dialkylthiane, or 2,5-dialkylthiophene attached to the asphaltene via a sulfur atom at one end or both ends of the chain

Such alkyl and dialkyl cyclic sulfides and thiophenes are known thermolysis products of asphaltene. Differentiation among these possibilities can be accomplished by the use of deuterium labeling. Indeed, the results in Figure 6 show the incorporation of d1-d8 atoms into the resulting alkane products of which the


d2-d5 isotopomers, being the most abundant, suggest the prominence of the following structures:

The total amount of the n-alkane products from the Ni2B desulfurization is much less than the combined amount of cyclic sulfides and thiophenes produced in the thermolysis of asphaltene. This can be due partly to the incompleteness of the desulfurization (42%), but even if reasonable allowance is made for the incompleteness, the yield of alkanes is still 2 orders of magnitude lower than the yield of thianes + thiolanes + thiophenes in the thermolysis. The discrepancy shows that sulfur-attached n-alkyls, 2-n-alkyls, 2,5-din-alkylthiolanes, thiophenes, and 2,6-di-n-alkylthianes are much less abundant than their C-C bond-attached counterparts. Also, the sulfur-attached species, as indicated by their OEP (Table 3), were probably incorporated into the asphaltene core at an early stage of diagenesis. The irregular distribution of the n-alkanes apparent in the TIC chromatogram (Figure 5) is an instrumental artifact, and the correct concentration distribution is shown by the gas chromatogram. The small peaks appearing between the n-alkane peaks are due to branched alkanes, the mid-monomethyl branched series of which are designated with a cross. Sample B-HMA showed a similar alkane distribution but with a less pronounced even-to-odd preference. The origin of mid-methyl alkanes in crude oil is controversial, and it has been proposed that they originated from (i) diagenetic migration of a methyl group,34-36 (ii) specific biological sources,37,38 and (iii) the corresponding fatty acids.39 In the present instance the immediate sources of the alkanes are the alkylated cyclic sulfides and thiophenes. In the cyclic sulfides, sulfur migration along the chain in a single threecarbon-atom step has been shown to be facile.40

(34) Krasavchenko, M. I.; Zemskova, E. K.; Mikhnovskaya, A. A.; Pustil’nikova, S. D.; Petrov, Al. A. Neftekhimia 1971, 11, 803-809. (35) Hoering, T. C. Carnegie Inst. Washington, Year Book 1981, 80, 389-393. (36) Morozova, O. E.; Zemskova, Z. K; Petrov, Al. A. Neftekhimia 1972, 12, 635-645. (37) Fowler, M. G.; Douglas, A. G. Org. Geochem. 1984, 6, 105114. (38) Klomp, U. C. Org. Geochem. 1986, 10, 807-814. (39) Summons, R. E.; Powell, T. G.; Boreham, C. J. Geochim. Cosmochim. Acta 1988, 52, 1747-1763. (40) Payzant, J. D.; McIntyre, D. D.; Mojelsky, T. W.; Torres, M.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1989, 14, 461-473.


It is then conceivable that methyl migration may also be operative to some extent:

This would seem to tally with the observation that the most abundant positional isomers, the 6, 9, and 12 isomers, differ in the methyl position by three carbon units as in the case of sulfur migration in cyclic sulfides. Alternatively, the primary source of the mid-methyl carbon skeleton could have been mid-methyl bacterial fatty acids as suggested by Summons et al.39 or alcohols. In this connection, it is noteworthy that although there are reports of mid-methyl alkanes in modern sediments and organisms, most of the mid-methyl alkanes were found in early Paleozoic and Proterozoic oils and sediments. Hence, the presence of these compounds in the asphaltene may be an indication of a Paleozoic origin for the bitumen. (b) Alkylcyclohexanes. Alkylcyclohexanes have been reported in Ordovician samples26-28 where they all show a similar carbon number distribution with an odd-toeven preference in the C14-C24 range. In the short (C14-C20) series of alkylcyclohexanes detected in the urea nonadduct fraction, however, no such preference was evident. The appearance of alkylcyclohexanes would seem to support a Paleozoic origin for the bitumen, as indicated by the presence of the mid-methyl alkanes. (c) Tricyclic Terpanes. The distribution of the tricyclic terpanes is similar to that found before in the bulk maltene, with the most prominent member being the C23 homologue (Figure 8). The high relative concentration of the C23 homologue and of the C19-C25 portion compared to the C26-C30 portion of the series is an indication of a carbonate source rock for the bitumen.41 According to the deuteration experiments tricyclic terpanes are bound to the asphaltene core preferentially via two sulfide linkages positioned in the isoprenoid side chain. Cleavage of these sulfide bonds can give rise, at least conceptually, to the precursor of the ubiquitous tetracyclic terpenoid sulfides occurring in nearly all sulfur-containing crude oils,

(d) Steranes. Regular C27-C30 steranes with C27 and C29 as the predominant members are present in HMA’s (Figure 11). As the absence of diasteranes and the percentage distribution of the geologic (S) and biologic (R) epimers (around the C-20 chiral center) indicate (Table 3), the steranes in the asphaltene are immature. The absence of diasteranes is also consistent with anoxic, clay-poor carbonate source rock.41 The apparent relatively low maturity of these asphaltene steranes then renders their carbon number distribution a more reliable measure of their original input distribution than (41) Peters, K. E.; Moldowan, J. M. The Biomarker Guide; Prentice Hall: Englewood Cliffs, NJ, 1993.


that reflected by the steranes or diasteranes distribution in the maltene. According to Grantham and Wakefield,42 the C28/C29 sterane ratio for marine oils varies with age and has values in the 0.4-0.7 range for Upper Paleozoic and Lower Jurassic oils. The value of 0.420.53 found here would be consistent with an Upper Paleozoic age for the bitumen. As discussed above, tricyclic terpanes bound in the asphaltene by multiple sulfide linkages positioned on the side chain correlate with the tetracyclic terpenoid sulfides in the maltene. However, the analogous pentacyclic sulfidesscorresponding to the sulfide-bound steranes in the asphaltenesif at all present in the maltene, are present only at trace concentration levels. This may be in some way related to the nature of bonding of the steranes in the asphaltene, involving a single sulfide linkage positioned on the AB ring system rather than in the alkyl side chain. (e) Hopanes. This class of biomarkers is present in a lesser concentration than the steranes or tricyclanes (Figure 10). The concentration distribution of the members (Figure 8) characterized by a high C29 and C35 level resembles that reported in saline lacustrine sediments,43,44 in marine carbonates or evaporites, and in highly reducing marine conditions during deposition.41 The epimeric ratio at C-22 in C31, C32, C35 17R,21βhopanes from the occluded maltene (Table 3) is near the equilibrium value, indicating a thermally mature state, whereas from the asphaltenes the values are significantly below the equilibrium level, manifesting thermal immaturity. As the results in Figure 14 suggest, the main mode of bonding to the asphaltene core involves multiple linkages in the side chain, and the presence of the hexacyclic hopanoic sulfides in the maltene appears to validate the correlation between mode of bonding in the asphaltene and the occurrence of the corresponding cyclic sulfides in the maltene,

(f) Gammacerane. This compound, present in low concentrations in all three fractions studied (Table 3), is indicative of a hypersaline depositional environment or stratified water column.45 The gammacerane index (G/R,β-H30) is highest in the HMA, and the gammacerane concentration is highest in the occluded maltene. Gammacerane has been found to be present in petroleum corresponding to an age as early as the late Proterozoic.41 (ii) LMA (16.7%). In general, A-LMA afforded several times higher desulfurization product yields (Table 2) (42) Grantham, P. J.; Wakefield, L. L. Org. Geochem. 1988, 12, 6173. (43) ten Haven, H. L.; de Leeuw, J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1985, 49, 2181-2191. (44) Fu, J.; Sheng, G.; Peng, P.; Brassell, S. C.; Eglinton, G.; Jiang, J. Org. Geochem. 1986, 10, 119-126. (45) Sinningue Damste´, J. S.; Kenig, F.; Koopmans, M. P.; Ko¨ster, J.; Schouten, S.; Hayes, J. M.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1995, 59, 1895-1900.

Peng et al.

than A-HMA. Here again, only the saturates were investigated further. (a) n-Alkanes. The odd-to-even preference in the C15C30 range is even more pronounced here (Table 3). Otherwise, all product features are similar to the case of HMA. (b) Dicyclic Terpanes. The distribution of this class of biomarkers is similar to that in the maltene, and the two-deuterium-atom uptake reflects two sulfide linkages to the asphaltene core. Interestingly, dicyclic terpanes are present, if at all, only in trace concentration in the HMA, indicating a secondary origin for this series. Drimanes are ubiquitous in sediments and crude oils46 and are thought to have bacterial origin. Again, as with the multiple sulfide bond-linked hydrocarbons, their sulfidesswhich range from C17 to C31s are present in

the maltene fraction of the bitumen. (c) Tricyclic Terpanes. In addition to the main series

a number of isomers and stereoisomers not seen in the case of HMA are also present. Otherwise, all the product features are again similar to those from HMA. (d) Steranes. Major differences from the HMA are observed in this series with respect to maturity status, which is higher here (Table 3) (36.6% versus 22.1%), and in the appearance of a series of C21-C25 steranes (Figure 15). The mode of attachment of this series to the asphaltene core is distinctly different from that of the regular C27-C29 steranes in that it involves two sulfide linkages in the side chain as opposed to the single sulfide linkages in the AB-ring in the regular steranes. R,R,R- and R,β,β-pregnanes along with R,R,R- and R,β,β-bisnorcholanes are present in Alberta bitumen maltenes, but pregnanes and homopregnanes are not generally present in early Paleozoic or Proterozoic sediments. They are, however, common occurrences in relatively young oil shales, especially in hypersaline lacustrine sediments.43,47 Ten Haven et al.43 identified C21, C22, R,R,R- and R,β,β-pregnanes, along with C22-4methyl- and 4,4-dimethylpregnanes in gypsum from the Mesinian evaporite basin (Italy), and hypothesized that high concentration of pregnanes in the gypsum may be derived from unknown precursors that are indigenous to hypersaline environments. The entire C21-C25 series in the LMA is absent from the HMA, and the C23-C25 steranes are also absent from the maltene. Since they are absent from the HMA, they either originated from the steranes in the LMA during diagenesis of the oil or from secondary biotic source materials. Most pregnane (46) Alexander, R.; Kagi, R.; Noble, R. J. Chem. Soc., Chem. Commun. 1983, 226-228. (47) Restle, A. Ph.D. Dissertation, University of Strasbourg, 1983.


hormones found in nature are devoid of such complicating alkyl substitution and no organism is known to produce C23-C25 steranes. Nonetheless, a secondary origin appears most likely at this time. Their absence from the bitumen maltene suggests that they are readily biodegradable. Another significant variation in sterane distribution is the absence of diasteranes from the HMA and LMA and their presence in the maltene. Here again, the diasteranes were formed from regular steranes or from secondary biotic source materials in the maltene. They are more resistant against biodegradation than regular steranes41 and survived the biodegradation of the precursor oil to the bitumen. (e) Hopanes. The hopanes here are in every respect similar to those from the HMA except that they are richer in minor isomers; they show a slightly more mature character (Table 3). (f) Pristane/Phytane Ratio. The value of this ratio, 0.51, is indicative, in agreement with the other biomarker characteristics, of a reducing depositional environment for the bitumen source rock. Conclusions from the Ni2B Desulfurization Studies. It has been demonstrated that the three fractions isolated by solvent extraction from n-pentane-precipitated Athabasca asphaltene have different molecular weights and elemental and molecular compositions, and upon Ni2B desulfurization they release similar yet characteristically different biomarkers. The differences reflect not only variations in the apparent thermal maturity status of the three fractions, the HMA, LMA, and occluded maltene, but also variations in the kind of biomarkers. The least mature distribution is displayed by the HMA followed by the LMA. The occluded maltene is the most mature fraction, with the same distribution as the distribution of the bulk bitumen maltene. The gradation in apparent maturity is attributed to differences in the availability of external catalysts (clay, various light and heavy minerals). Asphaltene, by virtue of its high molar mass and tendency for aggregation, provides an environment that effectively prevents contact between its structural elements and external catalysts. Also, the biomarkers in the asphaltene are present in a chemically bound form, which in itself wouldsin most casessreduce somewhat the efficiency of the catalytic reactions. Thus, the slowness of the maturation process in asphaltene is a consequence of the macromolecular, micelle-like structure of asphaltene and at the same time points to the catalytic nature of the maturation processes. Upon Ni2B desulfurization, HMA releases about 5% n-pentane solubles whereas LMA releases, on the average, about 20%. This can be due to the cleavage of relatively more acyclic sulfide bonds even though desulfurization is lower in the LMA (38% vs 42% in HMA) and/or to the difference in the average mass of the sulfide-bound appendages. The HMA undergoes a large decrease in molar mass upon Ni2B desulfurization owing to the breakup of the sulfide linkages holding the molecular core segments together. The experimental value of the molar mass reduction was about 4-fold, but the true value (which should be somewhat larger) cannot be determined exactly because of the occurrence of counteracting


molecular aggregation side reactions resulting from reesterification of the ester linkages in the asphaltene with methanol. In sharp contrast, the LMA does not undergo significant molar mass reduction during Ni2B desulfurization, indicating that the LMA lacks structures in which asphaltene core segments are held together by sulfide linkages. The biomarkers liberated from the HMA are less mature than those from the other fractions, indicating that they have undergone fewer secondary chemical alterations, and therefore, they reflect more reliably the biomarker distribution as it existed in the young oil. The distribution of biomarkers unambiguously points to a carbonate source rock for the bitumen and may also indicate a Paleozoic age.41 Thus, the observed biomarker characteristics, namely, (i) the absence of diasteranes, (ii) low pristane/hopane ratio, (iii) high hopane 29 index, (iv) high hopane 32 and 35 indices, (v) presence of C30 sterane, (vi) high gammacerane index, and (vii) high C23 and high ∑C20-C25 tricyclic terpane index, when taken together with the high sulfur content and high V/Ni ratio, provide strong evidence for a marine, carbonate source rock of the precursor oil along with a strongly reducing, hypersaline depositional environment. Much weaker indications are for the age of the oil. Thus, the value of the C28/C29 sterane ratio and the presence of gammacerane, midchain monomethylalkanes, and the C14-C20 alkylcyclohexanes appear to point to a Paleozoic origin of the oil. The HMA underwent only relatively minor chemical alterations since the early age of the oil, and therefore the stronger even-carbon-number preference in the products of the thermally more mature LMA suggest secondary incorporation of thermally young biomolecules from multiple sources via sulfide linkages. This conclusion is further accentuated by the observation of even-carbon-number-dominated fatty acids5 from the thermolysis of the naphthenic residue from the ruthenium ions-catalyzed oxidation of Athabasca asphaltene; these were originally bound in ester form to the asphaltene. Further support for this conclusion is provided by the present hydrolysis studies, as will be discussed below. In the LMA the steranes, and to some extent the hopanes as well, show a higher state of maturity. Here, the percentage concentration of the steranes is reduced but diasteranes are still absent. Instead, C21-C25 steranes, A-ring C1-C3 alkylated pregnanes, and higher steranes appear. The diasteranes in the maltene could have formed from the isomerization of the regular steranes in the maltene, but it is not likely that the A-ring-alkylated pregnanes and steranes originated from the regular steranes. Instead, these biomarkers may be assumed to have incorporated into the LMA via multiple sulfide linkages from secondary biotic sources. Indeed, the marked differences between the biomarker profiles of the LMA and HMA, such as the presence of the mid-methyl alkanes and alkylcyclohexanes in the HMA and of the C21-C25 steranes and dicyclic terpanes in the LMA and their absence in the counterpart asphaltene, may suggest biotic sources additional to the original source material for Athabasca asphaltene, perhaps from secondary microbial degradation or through the mixing of different oils.


Biomarkers are also present in the asphaltene in C-C bound form, as was concluded from thermolysis1-3 and ruthenium ions-catalyzed oxidation studies.48 Also, some of the biomarkersse.g., the 2,5-di-n-alkylthiolanessare present in much greater concentrations in C-C than in C-S-C bound form. Basic Hydrolysis of A- and B-HMA. The HMA from samples A and B yielded a mixture of alcohols and acids upon basic hydrolysis, manifesting the presence of ester linkages in which either the alcohol or acid portion was bound to the asphaltene core by nonsaponifiable bonds and their counterparts were liberated by the hydrolysis, e.g.,

Sample B gave a higher yield of the alcohol concentrates and a lower yield of the acid concentrates (Figure 3). Otherwise, the products identified in the two samples were closely similar. The series of C14-C27 n-alkanoic acids detected in sample B (Figure 18) are dominated by the C16 and C18 members over an even-carbonnumber background distribution. Also detected in the acid fractions were C20 and C21 tricyclic terpenoid acids (Figure 18)

the carboxylic acid derivatives of the tricyclic terpanes present in HMA, LMA, and occluded maltene. The same acids, in different distributions, along with other members of the series, were identified as the principal carboxylic acid constituents in the maltene and as minor constituents of the carboxylic acids in the mineral matrix-bound organic matter of the Athabasca oil sand.29,30 In the alcohol fraction a short series of Ceven n-alcohols in the C12-C24 range (Figure 20) was detected. The strong dominance of the Ceven members in both the n-alcohol and n-acid series requires a recent origin and secondary incorporation into the asphaltene core. The origin of the n-alkanoic acid series, which is present in low concentration in the maltene and in the mineralmatrix-bound organic matter, has been related to the secondary microbial degradation of the precursor oil converting it into the present-day bitumen.28,49 The n-alcohols, which are also present in free form in the maltene, are probably from the same source. These results provide the first direct evidence for the presence of esters with both extraneous carboxylic acids and extraneous alcohols in a high molar mass asphaltene. The incorporation of extraneous carboxylic acids and alcohols into the asphaltene in the form of esters with internal alcohols and acids was probaly facile and efficient and favorably competed with incorporation of (48) Peng, P.; Strausz, O. P. To be published. (49) Mackenzie, A. S.; Wolff, G. A.; Maxwell, J. R. Advances in Organic Geochemistry 1981; Bjorøy, M., et al., Eds.; Wiley-Heyden: London, 1983; pp 637-649.

Peng et al.

the same compounds into the solvent-insoluble components of the oil sands.28 BBr3 Hydrolysis of B-HMA. Basic hydrolysis causes the cleavage of ester bonds but leaves the ether bond essentially unaffected. In boron tribromide hydrolysis, however, both the ester and ether bonds are cleaved. Consequently, the yield of saponified products from the boron tribromide hydrolysis of sample B-HMA is much higher, 6-7%, than that from the basic hydrolysis, 3.8%. Also, the alcohols (measured as alkanes (Figure 20)) extend to a higher carbon range, from C12 to C31 and beyond, because they represent the combined yield of n-alcohols bound to the asphaltene core by ester and ether linkages. In the distribution, the difference between the low mass members and high mass members is striking. In the low mass range, the Ceven members, which come mainly from ester cleavages, dominate, whereas in the high mass range, coming mainly from the cleavage of the ether bond, this even predominance disappears. Thus, it would appear that the n-alcohols in the ester bond are of relatively recent origin, while the n-alcohols bound in ether bonds to the asphaltene core may have been part of the original biotic source material for the bitumen. These results again represent the first clear evidence for the presence of ether bonds in a high molar mass asphaltene. Conclusions from the Ester and Ether Bond Cleavage Studies. A sizable portion of structural moieties, up to 6-7%, is bound to the asphaltene core by ether and ester bonds. From the hydrolysis products, a series of n-alcohols, traces of hopanols, n-alkanoic acids, and tricyclic terpenoid acids have been identified. The rest of the hydrolyzate, comprising an aromatic and a polar fraction, has not been investigated further at this time. The n-alcohols from the basic hydrolysis of the ester bond had carbon numbers in the C12-C24 range with maxima at C14 and C16 and with a low background showing a distinct Ceven preference. Both the n-alcohols and n-alkanoic acids are thought to have originated in the secondary biodegradation converting the precursor oil into present-day bitumen. The C20 and C21 tricyclic carboxylic acidssstructural derivatives of the tricyclic terpenoid hydrocarbons in the bitumensare members of the suite of tricyclic terpenoid acids comprising the bulk of the carboxylic acids in the maltene fraction of the bitumen and of the small amount of tricyclic terpenoid acids detected in the chemisorbed organic matter on the solvent-insoluble matrix of Alberta oil sands. From the above it follows that the following types of ester linkages exist in the HMAs:

BBr3 hydrolysis causes the cleavage of the ester and ether bonds and accordingly affords significantly higher product yields. The series of n-alcohols detected here had a wider carbon range, C12-C31 and beyond. After correction for the ester-bound n-alcohol products, the distribution of the residual n-alcohols arising from the cleavage of the ether bonds features a maximum around C26 and shows no even-to-odd preference. This suite of


n-alcohols, in contrast to its ester-bound counterpart, is thermally mature and probably originated from the primary biotic source material of the precursor oil. Thus, from the comparison of results obtained from the basic and boron tribromide hydrolysis, clear proof has been obtained for the presence of ether bonds, e.g.

in asphaltene. Considering the desulfurization and hydrolysis results together, it is concluded that up to about 11-12% of the high molar mass Athabasca asphaltene contains small n-pentane soluble molecular units bound to the asphaltene core by a sulfur or oxygen bond. Hydrolysis of the ester/ether bonds generates alcoholic and carboxylic hydroxyl groups in the asphaltene, leading to excessive hydrogen bonding, consequences of which are an increase in molar mass and partial loss of solubility in methylene chloride. It has been suggested by one of the referees that some aspects of the present results may be in conflict with the excellent work reported by and Altgelt and Boduszynski.50 However, it has to be kept in mind that the extensive studies published by these authors were done on distillation residues, which are materials vastly different from native asphaltenes, and therefore, the two sets of data are not correlatable. Overall Conclusions (1) HMA Athabasca asphaltene is characterized by a molecular structure in which core segments are bound together by acyclic sulfur linkages. (2) Such structural features are not present in LMA Athabasca asphaltene. (3) Both asphaltene fractions contain low molecular weight sulfide-bound appendages comprising saturates, aromatics, and resins. (50) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker, Inc.: New York, 1994 and references therein.


(4) The saturates contain a full complement of biomarkers. (5) The distribution of biomarkers reveals characteristic differences in the thermal maturity status of HMA, LMA, and occluded maltene, the apparent ages of which increase in the order of listing. (6) The results point to the importance of separating these fractions in meaningful biomarker studies of asphaltenes. The HMA underwent the least secondary chemical alteration, and its biomarkers reflect more reliably the biomarker distribution as it existed in the young oil. (7) All biomarker series are consistent with a carbonate source rock for Athabasca bitumen and may signal a Paleozoic age. (8) The sharp demarkation line in the biomarker distributions of the maltene, LMA, and HMA manifests major compositional differences among the fractions with solubility characteristics. (9) The differences in the apparent thermal maturity status of the fractions point to (a) the protective environment of the large micellar structure against the chemical effects of external catalysts, chemicals, and microbes and (b) the thermocatalytic nature of the “thermal” maturation process. (10) Asphaltene contains ester and ether bonds in which alcohols and carboxylic acids are bound to the core by C-O bonds and also C-C bonds. (11) The ester-bound acids and alcohols are of relatively recent origin, while the ether-bound alcohols are mature and appear to be ancient. Acknowledgment. We thank Alberta Energy Research and Technology Branch and the Natural Sciences and Engineering Research Council of Canada for financial support. P.P. thanks the National Natural Science Foundation of China for a grant allowing him to visit Canada. We also thank Drs. E. M. Lown and T. W. Mojelsky for helpful assistance. EF970027C

Molecular Structure of Athabasca Asphaltene: Sulfide, Ether, and Ester

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