Additional Structural Details on Athabasca Asphaltene and Their

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Additional Structural Details on Athabasca Asphaltene and Their Ramifications Otto P. Strausz,* Thomas W. Mojelsky, Farhad Faraji, and Elizabeth M. Lown Department of Chemistry, University of Alberta, Edmonton, AB Canada T6G 2G2

Ping’an Peng Guangzhou Institute of Geochemistry, Guangzhou 510640, People’s Republic of China Received December 28, 1998. Revised Manuscript Received January 16, 1999

After some general comments on the concept of asphaltene, outstanding problems relating to the molecular structure of Athabasca asphaltene are discussed in light of new results on aromaticattached appendages derived from ruthenium-ions-catalyzed oxidation (RICO). Detected were homologous series of R-branched C1-C4 n-alkyl side chains up to C30-C40 in an aggregate amount of ∼10% of the n-alkyl side chains, C15-C20 regular isoprenoids, C20-C28 cheilanthanes, C27-C32 hopanes, C27-C29 steranes, C21-C24 pregnanes, and a number of branched hydrocarbons giving hydroxy carboxylic acids. The nature and distribution of these aromatic-attached biomarkers are similar but not identical to those reported to be attached to the asphaltene via a sulfide bridge. They may have originated from secondary biotic sources and became incorporated into the asphaltene via a Friedel-Crafts-type reaction. Additional, previously not considered reactions in the RICO of asphaltene are described, and aspects of the analytical procedures are reviewed. Also, a new protocol minimizing losses due to separations and volatility is discussed. Further structural elements of the asphaltene molecule were identified in the polar fraction of the asphaltene pyrolysis oil, including alkylpyridines and -quinolines, n-alkanoic/alkenoic acids, n-alkylamides (tentative), and n-alcohols. All straight-chain species were dominated by even carbon members. It is shown that contrary to recent erroneous suggestions in the literature, pericondensed aromatic units play a very minor role in the molecular structure of petroleum asphaltene.

Asphaltene, usually the highest molecular weight (MW) component of crude oil, is a friable, amorphous dark solid which is colloidally dispersed in the oily portion of the crude. When it is separated from the crude by solvent precipitation, which today is the generally accepted method for the isolation of this material, asphaltene has a dark brown color. After removal of the chemisorbed resins and low-MW asphaltene fragments, the residual asphaltene left behind after evaporation of the solvent is jet black and the chemisorbed material is deep reddish-brown. In solution, asphaltene has a dark color, the intensity of which varies with concentration, but it does not turn red or any other spectrally clean color at any concentration. It is, however, possible to isolate small quantities of individual colored components of the chemisorbed material from the crude asphaltene (e.g., fluorenones (bright red), polycondensed aromatics and condensed thiophenes (yellow/orange), various vanadyl porphyrins (violet, green, yellow, etc.)). Asphaltene is thermolabile and upon heating decomposes with intumescence without a sharp melting point while giving off gases and vapors. For petroleum chemists, the term “asphaltene” is generally applied in the narrow sense of petroleum asphaltene. There are, however, other asphaltenes * To whom correspondence should be addressed.

which are not related to petroleum, such as coal, shale, etc., asphaltenes. These are chemically distinctly different materials and, as has been aptly remarked, “the similarity between coal- and petroleum-derived asphaltenes begins and ends at the definition of the separation procedure”.1 Coal asphaltenes, for example, have lower MWs and hydrogen and sulfur contents and higher aromaticity and oxygen contentssmainly in phenolic OH and aromatic carbonyl formssthan petroleum asphaltenes. Yet another type of asphaltene is the one that occurs in the bitumen which is a constant companion of kerogen in sedimentary rocks. After kerogen, this rock asphaltene is probably the most abundant form of organically bound carbon on earth, and in this context, asphaltene chemistry is of epistemological significance. Petroleum asphaltene falls into different groups, according to molecular structure as well as chemical and physical properties. The “definition” of asphaltene can be formulated on two levels: on the level of operational formalism and the conceptual level of molecular structural compound class type. In operational terms, asphaltene was originally defined by Boussingault2 in (1) Bockrath, B. C.; Schweighardt, F. K. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981; pp 29-38. (2) Boussingault, J. B. Ann. Chim. Phys. 1837, 64, 141-151.

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208 Energy & Fuels, Vol. 13, No. 2, 1999

1837 as the distillation residue of a bitumen. Much later, in the early part of this century, Marcusson3 introduced a solvent precipitation method using petroleum naphtha. The use of a single-component precipitant was suggested by Strieter4 in 1941, and the use of n-pentane for the isolation of Athabasca asphaltene was first reported by Pasternak and Clark5 in 1951. In current practice, the choice of precipitant is usually n-heptane and asphaltene is defined as that portion of the crude oil that is soluble in toluene (or organic solvents having solubility parameter values in the 17.521.6 MPa1/2 range) and insoluble in n-heptane. The difference in yield (and composition) between n-pentane and n-heptane asphaltene varies from sample to sample (between the extremes of 10-98%), the n-heptane precipitate always being smaller. But even the nheptane precipitate always contains chemisorbed and n-heptane-insoluble resinous and other maltene materials (carboxylic acids, fluorenones, fluorenols, polycyclic terpenoids, thiolane- and thiane-derived and acyclic sulfoxides, carbazoles, quinolines, vanadyl porphyrins, etc.) as well as low-MW asphaltene fragments. Thus, the solubility-based definition encompasses not only all the various groups of asphaltene, but all those materials which are foreign to the bulk of the asphaltene in size, chemical composition, and properties; in the conceptual “definition” of asphaltene, they are not considered to belong to asphaltene. From the standpoint of industrial recovery and transportation of crude oil and crude oil products, it is the operational definition that is paramount in most instances whereas in the upgrading of bitumen and refining of heavy crudes the molecular structural property-based definition is equally important. Conceptually, petroleum asphaltenes are soluble, polydisperse, random organic geomacromolecules. Small molecules such as, for example, a dicyclic terpenoid sulfoxide

even though tending to come down with asphaltene on precipitation, do not belong to asphaltene from a compositional point of view: they are merely coprecipitated or chemisorbed foreign substances, the bulk of which can be readily removed from the asphaltene. In scientific studies conducted for the elucidation of the covalent molecular structure or other fundamental chemical or physical properties of asphaltene, it is imperative that all foreign substances be removed from the asphaltene proper as completely as possible without significant loss of the asphaltene itself. The task of structural elucidation of the polydispersed, random asphaltene macromolecule is arduous enough in itself without having to deal with the presence of interfering foreign substances. Regrettably, this self-evident principle is often overlooked in practice. The fundamental properties of asphaltene, as of other substances, are determined by chemical composition, (3) Marcusson, J. Z. Angew. Chem. 1916, 29, 346. (4) Strieter, O. G. Nat. Bureau of Standards J. Res. 1941, 26, 415. (5) Pasternak, D. S.; Clark, K. A. Research Council of Alberta Report No. 58, 1951; pp 1-14.

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molecular structure, molecular conformation, colloidal structure, and size. Recent studies6-11 have revealed an unexpected close compositional and structural similarity among native petroleum asphaltenes from a variety of sources with relatively small and characteristic variations, reflecting differences in origin (biotic source materials), depositional environment and conditions, and diagenetic, migrational, and thermal history of their formation. Because of its thermal reactivity during industrial manipulationsscoking, cracking, distillations asphaltene may undergo profound chemical alterations. The sight of this simple and well-known fact is again too often lost in practice, and this can become the source of considerable frustration and confusion in the literature. Until not long ago asphaltene was considered by many to have an intractable and undecipherable molecular structure. True, the asphaltene molecule presents a most formidable challenge to the structural chemist, but progress is being made, and ultimately it will lead to a satisfactory understanding of the complex and intricate structure of this molecule. Presently, asphaltene may be thought of as not-so-random assemblages of small to mid-size alkyl- and naphthenoaromatic hydrocarbons and their sulfur and, to a lesser extent, nitrogen derivatives. Some of these structural units are linked together by, in addition to C-C linkages which can be quite long, C-S and C-O heterolinkages. The not-so-randomness requires comments. This term simply reflects the recognition that hidden under the apparent randomness, some clear molecular structural principles prevail and, aside from some relatively minor characteristic variances, apply to most of the asphaltenes studied regardless of age, origin, burial depth (within limits), or geographical and even continental location. Among the physical properties of asphaltene from a technological point of view, the most important one is its solubility because the undesired spontaneous precipitation of asphaltene can cause severe operational difficulties in every stage of the processing and handling of petroleum. Heavy oils and bitumens have high asphaltene contents, and as a first guess one may conclude that these oils are more prone to precipitate their asphaltenes than are lighter oils with lower or very low asphaltene contents. However, one has to remember that asphaltene is thermolabile and light oils with low asphaltene contents often come from deep reservoirs with high geothermal temperatures. Owing to their extensive geothermal history, the asphaltene fractions of these oils are cracked in reactions not unlike those in upgrading operations. As a result, the alkyl side chains, bridges, and other appendages are lost, and this leads to an increase in aromatic condensation and an increase in the alkane and aliphatic content of the (6) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 13551363. (7) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171-1187. (8) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M.; Kowalewski, I.; Behar, F. Energy Fuels 1999, 13, 228. (9) Peng, P.; Morales-Izquierdo, A.; Lown, E. M.; Strausz, O. P. Energy Fuels 1999, 13, 248. (10) Peng, P.; Fu, J.; Sheng, G.; Morales-Izquierdo, A.; Lown, E. M.; Strausz, O. P. Energy Fuels 1999, 13, 266. (11) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278.

Structural Details on Athabasca Asphaltene

maltene fraction: each of these changes negatively affects solubility, and together they may effect a strong reduction in the solubility of the residual asphaltene. Also, it is possible that asphaltene precipitation takes place in the reservoir and that the supernatant oil becomes saturated with asphaltene. In a deep reservoir the temperature and pressure are both high, and during the passage of the oil to the surface as the values of these parameters decline, the solubility of the asphaltene will decrease and may cause further precipitation. Heavy oils, on the other hand, have experienced, as a rule, only mild exposure to geothermal heat; their asphaltene’s alkyl complement is intact and the high aromatic and resin contents of their maltene fraction makes this maltene an excellent solvent for asphaltene, capable of peptizing large amounts of it. As the preceding discussions demonstrate, solubilitys as are other propertiessis determined by molecular composition, structure, size, conformation, and colloidal structure. The same parameters determine chemical reactivity as well. Asphaltene has the least favorable composition of the crude oil fractions: it has the highest NOS, metal, and ash contents, the lowest H/C ratios, and the highest MW. Also, asphaltene is known to be the most important source of coke during the cracking and refining operations. The residual asphaltenes from these operations, aside from their solubility-based operational definition, have little in common with their predecessor, the native asphaltene. Their MW is much reduced, aliphatic and sulfide bridges broken, and appendages largely removed, and at the same time their aromaticity increased. Athabasca tar sand asphaltene is one of the most extensively investigated asphaltenes. Instrumental and, in particular, chemical studies we have developed have brought to light many structural features of this material and led to the detection and identification of a host of constituent molecules. Some of the architectural principles of this asphaltene have also been elucidated and the roles of C-C, C-S, and C-O bridges and sidechain appendages demonstrated.6,7,12-16 Chemical studies on Athabasca asphaltene involved thermolysis,12-15 ruthenium-ions-catalyzed oxidation (RICO),16 nickel boride cleavage of the C-S bonds, basic hydrolytic cleavage of the C-O bond in esters, and boron tribromide cleavage of the C-O bonds in ethers and esters.7 In the present paper, we report new results on the RICO reactions involving the release of biological markers and other products, some modifications in the procedures, and supplementary results on the thermal decomposition products, reflecting the chemical structure of the asphaltene molecule. Studies were carried out on asphaltenes from which low-MW materials were removed by a simple procedure involving Soxhlet extraction of the n-C5-asphaltene with acetone for 1 week.7 The extracted asphaltenes were (12) Payzant, J. D.; Lown, E. M.; Strausz, O. P. Energy Fuels 1991, 5, 445-453. (13) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 117-131. (14) McIntyre, D. D.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 2, 251-265. (15) Strausz, O. P.; Lown, E. M.; Mojelsky, T. W.; Peng, P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43, 917-923.

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Figure 1. Separation scheme for analysis of the polar compounds present in Athabasca asphaltene pyrolysis oil.

black and their MW effectively doubled. The acetonesoluble materials were extracted with n-pentane in the Soxhlet apparatus for 1 week, yielding small quantities of chemisorbed maltenes as the n-pentane-soluble fraction and the deep reddish-brown acetone-soluble asphaltene with a MW about one-half that of the initial crude asphaltene. In some cases, these low-MW asphaltenes (LMA) were studied in parallel with the highMW asphaltenes (HMA). Experimental Section The same experimental procedures for pyrolysis were used as those described before12-15 and for RICO as described before6,16 or in parallel publications.8-10 In the pyrolysis study, a 10% toluene solution of the n-C5asphaltene (acetone extracted to remove coprecipitated resinous substances for 68 h) was introduced dropwise (20 drops per min) into a glass bulb kept at 430 °C, and the vapors and gases evolved were swept with fast-flowing nitrogen (40 mL per min) into a cold trap.12 At the conclusion of the pyrolysis, the trap was removed and the entire distillate subjected to column chromatographic separation on a silica-gel column. The saturates were eluted with n-pentane, the aromatics with 50% toluene/n-pentane, and the polars with 10%MeOH/toluene, yielding 0.17 g (7.0%) of polars. The composition of the saturates and aromatics was reported before,12 and in the present work the polar material was analyzed following the scheme outlined in Figure 1. The alkanoic acids were converted to their methyl esters by treatment with diazomethane, generated by distillation at 65 °C. Through a dropping funnel, Diazald (Aldrich, 2.0 g in (16) 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. Strausz, O. P.; Lown, E. M.; Mojelsky, T. W. Proceedings of the 5th UNITAR/UNDP International Conference on Heavy Crudes and Tar Sands; Meyer, R. F., Ed.; UNITAR: New York, 1991; Vol. 1, pp 13-25. Strausz, O. P.; Lown, E. M. Fuel Sci. Technol. Int. 1991, 9, 269-281. Strausz, O. P. In Fundamentals of Resid Upgrading; Heck, R. H., Degnan, T. F., Eds.; AIChE Symposium Series 1989; American Institute of Chemical Engineers: New York, 1989; Vol. 85, pp 1-6. Strausz, O. P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1989, 34, 395400. Strausz, O. P. Proceedings of the 4th UNITAR/UNDP International Conference on Heavy Crudes and Tar Sands; Meyer, R. F., Wiggins, E. J., Eds.; Alberta Oil Sands Technology and Research Authority: Edmonton, AB, 1989; Vol. 2, pp 607-628. Strausz, O. P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1988, 33, 264-268. Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 3, 43-51. Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 2, 177-184.

210 Energy & Fuels, Vol. 13, No. 2, 1999 100 mL of ether) was slowly added to a solution of 5 g of KOH in a mixture of 8 mL of water and 25 mL of ethanol. The resulting distillate was collected in an ice-water-cooled receiver. This distillate was added to the polar fraction dissolved in about 20 mL of CH2Cl2. The combined solution was gently swirled and then set aside at ambient temperature for 3 h before being concentrated by rotary evaporation. The concentrate was chromatographed on a column of 30 g of silica gel 60, prepared as a slurry in n-pentane. Elution with 100 mL of n-pentane gave only a trace of material. Elution with 100 mL of toluene gave 0.041 g of the methyl ester fraction, Figure 1. Finally, elution with 10 mL of CH3OH and 100 mL of 10% CH3OH/toluene brought down the rest of the polar fraction (0.11 g) off the column. The basic nitrogen compounds were isolated as their copper complexes. The polar residue from the diazomethane reaction was dissolved in 50 mL of toluene. To this solution was added 1.5 g of CuCl2‚2 H2O, and the suspension was heated to reflux for 3.5 h. After cooling, the suspension was filtered and the residue washed with toluene (10 × ∼10 mL) and with pentane (5 × ∼10 mL) until the filtrates were colorless. The black solid was decomposed with water (2 × 50 mL), and the resulting solution was extracted with toluene (∼50 mL) and CH2Cl2 (∼15 mL). The latter two solutions were combined, dried over anhydrous Na2SO4, and concentrated to give 10 mg of the pyridines and quinolines fraction. The original filtrates from the complex separation and the toluene and pentane washings were combined and concentrated to give a black residue, 77 mg. Bound acids were liberated by basic hydrolysis and esterified with ethylbromoacetate. Into a 100-mL round-bottom flask equipped with a magnetic stirrer and reflux condenser were added the polar residue, following basic nitrogen compound removal, 100 mg of powdered KOH, 113 mg of K2CO3, and 36 mg of 18-crown-6-ether. The reagents were dissolved in 25 mL of toluene and stirred for 10 min in a nitrogen atmosphere. Ethyl bromoacetate (0.167 mL) in 10 mL of toluene was added via a syringe, and the mixture was kept at reflux for 22 h. Upon cooling, the solution was extracted with 50 mL of water. The organic solution was dried over anhydrous Na2SO4 and concentrated, and the concentrate was chromatographed on 10 g of silica gel 60, prepared as a slurry in n-pentane. Elution with 100 mL of toluene gave 17 mg. Elution with 5 mL of CH3OH and then with 100 mL of 10% CH3OH/toluene left a residue of 103 mg after concentration. Alcohols in the polar residue were derivatized using modifications of reported procedures.17 To a 25-mL round-bottom flask equipped with a magnetic stirrer were added about 50 mg of polar residue, 43 mg of tert-butyldimethylsilyl chloride, 34 mg of imidazole, and 1.0 mL of acetonitrile. To bring the polar material into solution, CH2Cl2 was also added dropwise until a homogeneous solution resulted. Stirring was continued for 20 h at room temperature. Then ether (10 mL) was added, as was CH2Cl2 dropwise, to maintain a homogeneous solution. This was then washed with water (2 × 10 mL), dried over anhydrous Na2SO4, and concentrated on the rotary evaporator. The concentrate was chromatographed on 10 g of silica gel 60, prepared as a slurry in n-pentane. Elution with 100 mL of toluene gave 3 mg of silyl ethers. Further elution with 5 mL of CH3OH and 100 mL of 10% CH3OH/toluene gave 36 mg of a polar residue. This was then added to a 100-mL round-bottomed flask equipped with a magnetic stirrer and reflux condenser. To the flask was added 25 mL of distilled dioxane and ∼200 mg of LiAlH4. The suspension was heated to reflux for 2.5 h. Upon cooling, a few drops H2O were added. When the reaction subsided, several drops of aqueous KOH were added, followed by a few more (17) Detty, M. R.; Seidler, M. D. J. Org. Chem. 1981, 46, 12831292. Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190-6191.

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Figure 2. Gas chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the LMA. The numbers refer to the carbon numbers of the n-alkanoic acid moieties.

Figure 3. Gas chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers of the n-alkanoic acid moieties.

Figure 4. Total ion current mass chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the LMA. The numbers refer to the carbon numbers of the n-alkanoic acid moieties. drops of H2O. The reaction mixture was transferred to a separatory funnel with the aid of 10 mL of water and 50 mL of toluene. After the resulting emulsion broke, the aqueous phase was re-extracted with a further 15 mL of toluene and 10 mL of pentane. The combined organic extracts were dried (anhydrous Na2SO4), filtered, and concentrated to leave 11 mg of residue. The residue was silylated as described above. The resulting product from the reaction was chromatographed on a column


Figure 5. Total ion current mass chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers of the n-alkanoic acid moieties.

Figure 6. m/z ) 88 cross-scan chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the LMA. The numbers refer to the carbon numbers in the acids. of 10 g of silica gel 60, prepared as a slurry in n-pentane. The material eluting with 100 mL of toluene gave the silyl ether fraction.

Results Minor Products from the RICO of Athabasca Asphaltene. Both the Athabasca LMA and HMA were subjected to RICO using the same procedure as before,16 and they both yielded a series of alkanoic acids which were methylated with diazomethane. The resulting methyl esters were then separated by adduction chromatography with urea into an adduct and a nonadduct fraction. GC-MS total ion current mass chromatograms and gas chromatograms of the urea adduct fractions from the RICO of the HMA and LMA are shown in Figures 2-5. As seen, the chromatograms of the HMA


Figure 7. m/z ) 102 cross-scan chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the LMA. The numbers refer to the carbon numbers in the acids.

Figure 8. m/z ) 88 cross-scan chromatogram of the urea adduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers in the acids.

indicate a fairly clean series of n-alkanoic acids up to about C40 with only minor amounts of other series appearing mainly at the high-MW end. It is necessary to point out that the low-MW end of this and other alkanoic acid series was lost by evaporation, and therefore, their gas chromatograms do not correctly represent the distribution of the low-MW products. In contrast, the LMA features significant concentrations of long homologous series of R-methyl, R-ethyl, some β-methyl n-alkanoic acid methyl esters, and perhaps some other moderately branched n-alkanoic acid methyl esters along with some ketones and other compounds eluting between the n-alkanoic acid ester peaks, Figures 6 and 7, in approximate relative concentrations R-H:R-methyl:R-ethyl ) 785:69:1. These branched esters are usually found in the urea nonadduct fraction, which indeed contains copious amounts of these esters, however, the separation in the case of the LMA is incom-


Figure 9. Total ion current mass chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers in the acids. The peaks labeled “i” correspond to regular isoprenoids: i-C20 is pristane carboxylic acid and i-C21 is phytane carboxylic acid.

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Figure 11. m/z ) 88 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers in the acid moieties.

Figure 12. m/z ) 102 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA. The carbon numbers refer to the carbon numbers in the acid moieties.

Figure 10. m/z ) 74 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA. The numbers refer to the carbon numbers of the n-alkanoic acids.

plete. The HMA urea adduct fraction also contains some of these esters but only in trace amounts. Clearly detectable are only the R-methyl esters, Figure 8. The removal of the n-alkanoic esters by urea adduction is incomplete, and the major series in the urea

nonadduct fractions is still the n-alkanoic ester series followed by the R-methyl, R-ethyl, R-n-propyl, and R-nbutyl n-alkanoic ester series, Figures 9-17. The carbon range of the series for both the HMA and LMA nonadducts extends up to C26-C28 with approximate total ion current relative ion intensities for the HMA R-H:R-Me: R-Et:R-n-Pr:R-n-Bu ) 93.0:17.4:2.01:1.04:1.00 and for the LMA nonadducts R-Me:R-Et:R-n-Pr ) 19.6:3.0:1.0. In addition to the normal and R-alkyl-branched alkanoic acids, some β-methyl and regular isoprenoid acids, including pristane and phytane carboxylic acids (as methyl esters), have also been detected. Moreover, in the case of the HMAsbut not the LMAs the urea nonadduct fraction also contained series of




Figure 16. m/z ) 102 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the LMA. The carbon numbers refer to the carbon numbers in the acids.


Figure 17. m/z ) 116 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the LMA. The carbon numbers refer to the carbon numbers in the acids. Scheme 1

Figure 15. m/z ) 88 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the LMA. The numbers refer to the carbon numbers in the acids.

tricyclic terpenoid (cheilanthanoic) acids, pentacyclic terpenoid (hopanoic) acids, steranoic and pregnanoic acids.

The GC-MS m/z ) 191 cross-scan mass chromatogram showing the tri- and pentacyclic terpenoid acids is shown in Figure 18. The carbon range for the tricyclic acids extends from C21 to C29 and for the pentacyclic series from C28 to C33. In each case, one member of the respective series, corresponding to methyl branching in the side chain, is missing, Scheme 1. The formation of the missing members would have required the removal of two alkyl groups from the same carbon by the


Figure 18. m/z ) 191 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA showing the tricyclic and pentacyclic terpenoid members.

cleavage of two C-C bonds, a low-probability process. For the same reason, the C28 member of the cheilanthanoic acid series should have a low probability of forming as well, because the C-27 position is a methyl branching site:

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Figure 19. m/z ) 275 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA showing the sterane members.

As seen from Figure 18, the C28 member is present but in a somewhat diminished concentration relative to its neighbors, the C27 and C29 members. Methyl branching in the side chain introduces a chiral center at C-22 in the cheilanthanoic acid and at C-28 in the hopanoic acid series, and each subsequent member is split into an R and S enantiomer. As noted from the above structures and from Figure 18, all these cyclic terpenoid acids have their carboxylic group at the terminal position in their side chains, as indicated by their mass spectral cracking patterns: Figure 20. m/z ) 217 cross-scan chromatogram of the urea nonadduct fraction of alkanoic acid esters from the RICO of the HMA showing the pregnane members.

Steranes are present in the C28-C30 range, Figure 19, and as indicated by their mass spectrometric cracking patterns, the carboxylic groups here are positioned not on the side chains but on the A rings, probably on the

C-3 atoms. There are several isomers apparent at each carbon number. PregnanessC21 steranessand homopregnanes up to C24 are present as their carboxylic acid derivatives, Figure 20. At each carbon number they occur in at least two isomeric forms. All the pregnanes, in contrast to the steranes, carry their carboxylic groups in their side chains and not on their ring skeletons. The same functionalization was found for the sulfur-attached pregnanes and steranes in Athabasca asphaltene7 and in the Carom-C and C-S cleavage products of some Chinese asphaltenes and kerogens.9 Other Types of Reactions Occurring in RICO Experiments. Thus far we have considered only the oxidation of aromatic carbon to carbon dioxide and



Figure 21. FTIR spectrum of the organic phase of the RICO reaction mixture.

Figure 22. FTIR spectrum of the polar fraction of the organic phase of the RICO reaction mixture.

carbonyl functionalities in the RICO reactions. However, other reactions may also take place parallel to the oxidation of aromatic carbon but at a slower rate. These include transformation of primary alcohols to carboxylic acids, secondary alcohols to ketones, and ethers to esters. In addition, aliphatic C-H bonds may also be attacked. Thus, reactive tertiary C-H bonds at the bridgehead positions and even secondary C-H bonds in dicyclic hydrocarbons may be oxidized to alcohols or ketones.18 The reactivity is influenced by the substituents on the carbon atom being attacked and steric crowding. Some examples are

up of aliphatic hydroxy esters and perhaps some alcohols. That the hydroxyl groups are mainly tertiary was proven in acetylation experiments. Tertiary alcohols are difficult to acetylate, and acetylation requires the presence of a catalyst (e.g., 4-methylaminopyridine). But the FTIR spectrum of the acetylated hydroxy esters showss even when a catalyst was usedsthat the hydroxyl absorption still persists, and therefore, the acetylation was quite incomplete. This low reactivity proves that the hydroxyl groups are tertiary. Indeed, the RICO of asphaltene is expected to produce some tertiary alcohols from the oxidation of alkylnaphthenoaromatic, various branched alkyl aromatic and alkyl naphthenic structural elements, e.g.

and

The question then arises whether alcohol formation plays any role in the RICO reaction of asphaltenes. As seen from Figure 21, showing the FTIR spectrum of the reaction mixture from the organic phase obtained in the RICO of Athabasca asphaltene, the answer is affirmative. The two dominant functional groups present are carbomethoxy (1739 cm-1) and alcoholic hydroxy (3338 cm-1; dimeric, polymeric, carbonyl-bound). This reaction mixture was then separated by column chromatography into an alkanoic acid methyl ester fraction, an alkanoic diacid methyl ester fraction, and a polar fraction by successive elutions using n-pentane, CH2Cl2, and 20% CH3CO2Et/CH2Cl2:10% CH3OH/CH2Cl2. The FTIR spectrum of the monoacid esters showed a clean fatty acid ester fraction with only a trace of hydroxyl present, but that of the polar fraction, Figure 22, displays strong ester and hydroxyl absorptions. This fraction is made (18) Bakke, J. M.; Lundquist, M. Acta Chem. Scand. 1986, B40, 430-433.

The acetylated portion of the tertiary alcohol/ester fraction was separated chromatographically and analyzed by GC-MS. Of the numerous acetylated alcohols appearing in the total ion current chromatogram of the fraction, at present only a few have been identified because their acetylated derivatives do not seem to give molecular ions upon electron impact in the mass spectrometer and standards were not available. Nonetheless, some of the major peaks in the total ion current chromatograms could be tentatively assigned to acetylated tert-hydroxy alkanoic acid methyl esters, Figure 23

representing aromatic-attached branched alkyl and naphthenoaromatic moieties, which on RICO are oxi-


Figure 23. Mass spectral data for the acetylated tert-hydroxy alkanoic acid methyl esters from the RICO of Athabasca asphaltene.

Strausz et al.

formation of the corresponding diacids from the oxidation of the alkenyl bridges between aromatic carbons and the second one to the formation of carboxylic acids from the thermolysis of the asphaltene. RICO Reaction Methodology. In the initial stages of our studies16 on developing the RICO methodology for the structural study of asphaltene, we processed the aqueous and organic phases of the reaction mixture separately. Some of the products were equipartitioned between the two phases and measured separately in the two phases and their amounts summed. Moreover, to reduce losses due to volatility, the low-MW portion of the monoacids was esterified not with diazomethane, as were the rest of the monoacids, but with phenacyl bromide. This esterification was carried out under reflux conditions to attain speed and to drive off as much of the HBr formed as possible, which led to some loss of the acids and esters. Thus, it appeared to be desirable to avoid the need for handling the two phases separately and reflux conditions as well as to reduce losses due to volatility in general. For these reasons, we recently modified the esterification procedure and carried out the esterification on the acid salts rather than the free acids using boron tribromide as a reaction promoter,10 e.g.

dized to the hydroxy acids

The acids were analyzed after CH2N2 and Ac2O/catalyst treatment. Other reactions that might have relevance are the oxidation of primary n-alcohols attached to aromatic carbons to R,ω-di-n-alkanoic acids

(the analogous reactions of aromatic-attached fatty acids would lead to diacids as well), the oxidation of ethers to esters,

After completion of the oxidation reaction, the acidic mixture was neutralized with sodium hydroxide, the solvents removed, and the esterification carried out in three steps taking three aliquots of the neutralized reaction mixture: (1) in aliquot 1, esterification was done with octadecyl alcohol for the measurement of the octadecyl esters of the eC12 monoacids; (2) in aliquot 2, esterification was performed with octyl alcohol for the measurement of the octyl esters of the eC12 diacids (and, if desired, the >C12 monoacids); and (3) in aliquot 3, esterification was carried out with diazomethane or 12% boron trifluoride in methanol for the measurement of the >C12 mono- and diacids and diazomethane for the measurement of the benzenepolycarboxylic acids. The average esterification yields determined using standard acids were as follows: BF3/MeOH CH2N2 BBr3/C18H37OH BBr3/C8H17OH

and the oxidation of sulfides to sulfones (sulfinic, sulfonic esters)

The first of these reactions may contribute to the

C10-C30 aliphatic mono- and diacids benzenedi- through hexaacids C2-C9 monoacids C4 diacid

97.5% 96% 94% 90%

Details of the experimental procedures and the results obtained using this method are presented in two accompanying articles9,10 in this issue. The one disadvantage of this method is that excess BBr3 can cause the decomposition of the esters, once formed. Therefore, the optimum amount of BBr3 has to be determined in a separate experiment. Yet another alternative method that has been examined was one in which the low-MW fatty acids (C2-C9) were measured directly using a capillary GC column of nitroterophthalic acid, designed for the analysis of fatty acids. Once the oxidation reaction was complete, the pH of the reaction mixture was adjusted to 8-9 and the solvents and most of the water were removed by



Figure 24. Gas chromatogram of the low-MW n-alkanoic acids from the RICO of Athabasca asphaltene. Table 1. C2-C9 Side Chains in Athabasca Asphaltenea carbon no. of acid 2 3 4 5 6 7 8 9 i-4 i-5 total a

no. of C per 100 C in asphaltene

no. of alkyl groups per 100 C in asphaltene

1.09 0.91 0.72 0.61 0.49 0.42 0.36 0.33 0.20 0.29 5.44

1.09 0.46 0.24 0.15 0.10 0.07 0.05 0.04 0.07 0.07 2.39

Pyrolysis of Athabasca Asphaltene. In previous studies, a multitude of homologous series of compounds have been isolated and identified from the saturate and aromatic fractions of the pyrolysis oil of Athabasca asphaltene.12-15 In the present study, the polar fraction of the pyrolysis oil was separated as described in ref 12 and examined. Analysis of the reaction products followed the scheme outlined in Figure 1. The methyl esters consisted of a short series of n-alkanoic acid esters dominated by the C16, C18, and a C18 monounsaturated members. CuCl2‚2H2O complexation of the basic nitrogen compounds produced a complex mixture containing small quantities of a number of alkylpyridines and -quinolines. The former have C7-C9 side chains, and in the latter series, the side chains extend from C2 to C10, each with a number of isomers. Some illustrative mass spectra obtained in GC-MS analyses showing the isomeric alkylpyridine and -quinoline species are reproduced in Figure 25. At this point the polar residue was assumed to contain some amides since its FTIR spectrum featured a strong, broad carbonyl absorption centered around 1645 cm-1. To liberate their acid functionalities, the amides were hydrolyzed and the liberated acids esterified by KOH/ 18-crown-6 ether/ethyl bromoacetate treatment. This step was then followed by silylating any alcohols present in the polar residue to form silyl ethers and the chromatographic removal of the ethers. GC-MS analysis of the silyl ether fraction, Figure 26, revealed the presence of n-alcohols from at least C10 to C18 as well as

From direct measurement of the n-alkanoic acids.

distillation, and after acidifying the solution, the reaction mixture was distilled under vacuum. The distillate was then analyzed by GC. A gas chromatogram of the C2-C9 acids is shown in Figure 24, and the quantitative data obtained on an Athabasca sample are given in Table 1. This procedure minimizes volatility losses and obviates the need for esterification. As shown before,16 hydrolysis and/or oxidation of the acetonitrile cosolvent in the course of the RICO reaction does not produce inconsequential quantities of acetic acid, relative to the quantities produced from the oxidation of methyl arenes in the asphaltene sample. As seen from the data obtained from a mixture of 3.6 g of NaIO4/10 mg of RuCl3‚3H2O/30 mL of H2O/20 mL of CCl4 and 20 mL of CH3CN at different temperaturesand reaction times, 23 °C 23 °C 23 °C

8 h RICO 16 h RICO 64 h RICO

0.030 mmol of acetic acid 0.044 mmol of acetic acid 0.055 mmol of acetic acid

42 °C 42 °C

8 h RICO 16 h RICO

0.078 mmol of acetic acid 0.090 mmol of acetic acid

the quantity of acetic acid produced in the RICO reaction from the acetonitrile cosolvent increases with reaction time and rising temperature. The rate of acetonitrile decomposition would probably be accelerated at the low pHs caused by the carboxylic acids produced in the asphaltene oxidation reaction.

In the n-alcohol series, the even-carbon-number members are somewhat more abundant than the odd-carbonnumber members except, strangely enough, the C17 member. After removal of the silyl ethers, the residue gave a fairly clean FTIR spectrum of alkanoic acid esters (1750 cm-1). Therefore, the esters were reduced to the alcohols with LiAlH4 and the alcohols silylated to the silyl ethers. The GC of the ethers, Figure 27, shows n-alcohol ethers from C8 to C22 with a strong preference for the evencarbon-number members dominated by the C16 and C18 homologues. In previous studies it has been noted that n-alcohols and n-alkanoic acids in a free state show up in most compound-class fractions of Athabasca maltene. For this reason, we have examined the LMA as well for the presence of these compound classes. The LMA was subjected, without any prior heat treatment, to a simplified version of the procedure outlined in Figure 1. As the GCs show, Figure 28, indeed both alcohols and alkanoic acids are present in a free state in the LMA in distributions very similar to those found in chemically bound form in the HMA. Discussion RICO Reactions. As shown before,8-10,16 the major products of the RICO reactions of Athabasca asphaltene,


Strausz et al.

Figure 27. Gas chromatogram of the silylated alcohols derived from complexed acids in the pyrolysis oil of Athabasca asphaltene. The numbers refer to the carbon numbers of the alcohols (acids).

Figure 28. Gas chromatograms of the free n-alcohols and n-alkanoic acids present in Athabasca LMA. In the alcohols, chromatogram a ) an alcohol of tetrahydronaphthane, b ) C7 benzyl alcohol, and c ) benzyl alcohol. The numbers refer to the carbon numbers of the alcohols (acids).

Figure 25. Mass spectra of some of the alkylpyridines and alkylquinolines present in the pyrolysis oil of Athabasca asphaltene.

Figure 26. Gas chromatogram of the silyl ethers of the alcohols present in the pyrolysis oil of Athabasca asphaltene. a ) an alcohol of tetrahydronaphthalene, b ) C7 benzyl alcohol, c ) benzyl alcohol. The numbers refer to the carbon numbers of the n-alcohols.

as of all asphaltenes studied to date, are n-alkanoic acids (C1-C35), R,ω-di-n-alkanoic acids (C4-C35), smaller

amounts of benzenedi- through hexacarboxylic acids, and a nondistillable oxidized residue. Mild pyrolysis of the latter, after methylation with diazomethane, produces series of n-alkanes/n-alkenes, n-alkanoic/n-alkenoic acid methyl esters, and free n-alkanoic acids. The FTIR spectrum of the oxidized residue also indicates the presence of aliphatic sulfones and possibly sulfinic and sulfonic acid esters. In the present study, the minor RICO products were explored. These include R-methyl, R-ethyl, R-n-propyl, and R-n-butyl n-alkanoic acids. Additional species detected were regular isoprenoid acids including pristane and phytane carboxylic acids and in the HMA but not in the LMA C21-C29 tricyclic terpenoid (cheilanthanoic) acids, C28-C33 pentacyclic terpenoid (hopanoic) acids, C28-C30 steranoic acids, C21-C24 pregnanoic acids, and possibly some dicyclic terpenoid acids. Alkanoic Acids. As seen from Figures 2-19 both the HMA and the LMA yield, in addition to the major n-alkanoic acid series, R-alkyl (Me, Et, n-Pr, n-Bu) branched n-alkanoic acids. The carbon range from the HMA is in the C1-C40 region and from the LMA up to C28. In both cases the concentration drops off monotonically with increasing chain length. The normal and branched acids were fractionated as their methyl esters by urea adduction chromatography. The separation, however, is incomplete, and some of the branched esters


are adducted while some of the normal esters are carried over into the nonadduct fraction. The relative concentration distribution of the series follows the order R-H . R-Me . R-Et > R-n-Pr > R-n-Bu. The total relative amount of branched acids is somewhat higher in the LMA than in the HMA. In addition to the above series, small quantities of regular isoprenoid acid esters (including pristane and phytane) and some β-methyl alkanoic acid esters and, in the case of LMA, perhaps some ketones

and other branched acid esters are also produced. The combined yield of the branched acid esters is estimated to be on the order of about 10% of the n-alkanoic acid esters. Originally, these acids were alkyl side chains attached to aromatic rings in the asphaltene molecule:

The origin of these R-branched alkyl groups is not known, but it may be surmised that, like the isoprenoids, they were incorporated into the aromatic moieties of the asphaltene by some Friedel-Crafts-type reactions of n-alkyl precursors with suitable functional group(s) in the right (R-δ) positions. Cyclic Terpenoid Acids. Tricyclic terpenoid (cheilanthanoic) acids (C21-C29) and pentacyclic (hopanoic) acids (C28-C33), and perhaps some dicyclic terpenoid acids as well, are produced in the reaction of the HMA, Figure 18. Starting with the C24 member, the tricyclic acids are split into R and S epimeric pairs, indicating that the C-22 atom became a chiral center. As the mass spectra of the series show, the carboxylic group is in the terminal position on the side chain, corresponding to the site of attachment of the cheilanthane molecule to the aromatic carbon in the asphaltene. The concentrations of the C27-C29 members are markedly lower than those of the C22-C26 members; in the case of free cheilanthane molecules in the maltene (with each of the corresponding members containing one less carbon atom), this has been suggested to be a characteristic feature of carbonate source rocks19 for the oil or bitumen. This distribution of the Carom-C-bound cheilanthanes in the asphaltene is in agreement with the concentration distribution of the C-S-C-bound cheilanthanes from the nickel boride desulfurization of asphaltene7 and with that of the free cheilanthanes in the saturate fraction of the bitumen.20 The one difference between the Carom-C- and C-S-C-bound cheilanthanes is that while in both cases the attachment is through (19) Peters, K. E.; Moldowan, J. M. The Biomarker Guide; Prentice Hall: Englewood Cliffs, NJ, 1993. (20) Ekweozor, C. M.; Strausz, O. P. In Advances in Organic Geochemistry 1981; Bjoroy, M., et al., Eds.; Wiley Heyden: London, England, 1983; pp 746-766. Ekweozor, C. M.; Strausz, O. P. Tetrahedron Lett. 1982, 23, 2711-2714.


the side chain, in the former it is via a single bond whereas in the latter case it is mainly via two C-S-C bonds. Pentacyclic terpenoid acids, the hopanoic acid series, also have their carboxylic groups in the terminal position in the side chain. The C-22 carbon, as pointed out before, is a chiral center, and therefore, the C30 and higher members appear as R and S epimeric doublet pairs. The biologic epimer is the S, and the R epimer is the geologic product of the thermal isomerization process. The S/(S + R) ratio is a thermal maturity indicator.19 At full maturity, the epimeric distribution in the parent hydrocarbon reaches equilibrium with values of 0.55 (C31) and 0.58 (C32). From Figure 18 the approximate values for the corresponding acids are 0.59 and 0.61, implying full maturity. Hopanes are also present in the asphaltene as hydrocarbons attached to the asphaltene core via one, two, or three C-S-C bonds in the side chain.7 The S/(S + R) epimeric ratios have decidedly lower values, 0.45 (C31) and 0.44 (C32), indicating significant immaturity. The discrepancy is resolved by assuming that the Carom-C-bound hopanes were incorporated into the asphaltene from secondary sources rather than directly from the original biotic source material of the bitumen. In effect, the secondary source could have been the hopane complement of the saturate fraction of the bitumen because in the saturates the free hopane hydrocarbons have S/(S + R) ratios of about 0.61. Alternatively, the secondary source could have been some of the hopane precursors, e.g., bacteriohopanetetrol. Indeed, this proposition gains support from the carbon-number distribution of CaromC-bound hopanes, which displays abrupt truncation at C32 not only in Athabasca but also in Jinghan asphaltenes9 as well as in the free hopanoic acid series found in Athabasca resins.21 This appears to be consistent with an oxidative cleavage of the C-32-C-33 bond in bacteriohopanetetrol:

It should be added here that the hopane S/(R + S) ratios reach their equilibrium values (∼0.61) at the onset of the early stages of oil generation, and therefore, the equilibrium ratio does not imply a high degree of maturity for the oil.19 Steranoic Acids. These acids are present in the C28C30 range with a number of isomers at each carbon number, Figure 19. The relative concentration distribution according to carbon number is similar but not identical to that of the sterane hydrocarbons from the nickel boride reduction of the asphaltene,7 i.e., C28 > C30 > C29 (corresponding to the C27 > C29 > C28 hydrocarbons). As indicated by the molecular structure (21) Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7, 127-140. Cyr, T. D. AOSTRA Fellowship Report; Department of Chemistry: University of Alberta, 1983.


Strausz et al. Scheme 2

in Figure 19, the steranoic acids are attached to the aromatic carbon in the asphaltene in the A ring, probably at the C-3 position, and not in the side chain as found to be the case with the tricyclic and pentacyclic terpenoid acids. It is probably also worth pointing out that regular steranes are practically absent from the maltene because, being readily biodegradable, they were metabolized by the organisms that caused the degradation of the precursor oil of the bitumen. Diasteranes, which are much more resistant against biodegradation, have been shown to be present in the maltene but are absent in the asphaltene. Pregnanoic Acids. These acids are present in the C22C25 range and their site of attachmentsunlike that of the C28-C30 steranessis at the side chain. Pregnanes were not detected in the nickel boride desulfurization products of the HMA but were present in the nickel boride desulfurization products of the LMA where the site of attachment was also in the side chain.7 Moreover, there are mainly two C-S-C bonds between the pregnane and the asphaltene core, but there is only one Carom-C bond linking the pregnane and the asphaltene core. Binding Sites. As we have seen, each hydrocarbon biomarker molecule is attached to the asphaltene core at a specific site which is independent of the nature of the binding linkage, i.e., C-S-C or Carom-C. This fact provides conclusive evidence for the role that the functional group in the free biomarker plays in the mechanism of the incorporation process. The most probable functional group is the hydroxyl group, which is known to be present in the side chains of the homohopane precursors, the bacteriohopane tetrols, and at the C-3 position in the A ring of the sterane precursors, the sterols. Indeed, small quantities of C28-C31 hopanols have been detected in the hydrolysis products of Athabasca HMA7 and C30 hopanol in the hydrolysis products of Jinghan HMA.9 In the latter case, significant amounts of regular C16-C20 isoprenoid and steranoid alcohols have been identified, and from both Jinghan and Athabasca HMA, copious quantities of n-alcohols have been identified. An alternative functional group for incorporation could be the carboxyl group: both cheilanthanoic20 and hopanoic21 acids have been shown to be present in Athabasca maltene. In summary, tricyclic triterpanes, pentacyclic triterpanes, and pregnanes are attached to the asphaltene core at their side chain and regular steranes (and 4-methyl steranes) at their A ring, regardless of the nature of the bond (C-S-C or Carom-C) in all three

asphaltenes studied this way to date: Athabasca,7 Jinghan,9 and another asphaltene from China.10 Therefore, we conclude that this selectivity in the incorporation of biomarkers into the asphaltene (and in kerogen, as the results of Jinghan kerogen9 demonstrate) is of general validity and that it reflects the functional-group position of the incorporating biomarker molecule in the incorporating reaction. Reversing this argument, it also follows that regular steranes and their shorter side chain relatives, the pregnanes, have their functional groups in different locations within the molecule and therefore the pregnanes are not the derivatives of the regular steranes but rather originate from different biotic source material. Led by these considerations, in a retrospective review we think it likely that the R-branched alkyl side chains on the aromatic carbons originate from a FriedelCrafts substitution reaction of the appropriate alcohol with the aromatic structures in the asphaltene. The relative importance of these aromatic-bound biomarker hydrocabons varies. In Jinghan asphaltene,9 they followed the order hopanes > steranes > isoprenoids in a total amount of about 5% of the n-alkyl side chains; in the immature Chinese asphaltene referred to before,10 the order was isoprenoids > (0.08 C per 100 C in the asphaltene) > hopanes (0.05 C) > cheilanthanes > steranes, and in Athabasca asphaltene, cheilanthanes > hopanes . steranes. In this latter case, the distribution is similarsbut not identicalsto that of the C-S-C-bound biomarkers and on the whole is in agreement with, and lends support to, the organic geochemical conclusions arrived at before regarding the origin, depositional environment, and diagenetic history of the Athabasca tar sand formation. As stated above, the total amount of Carom-C-bound hydrocarbon biomarkers in the asphaltene is low; nevertheless, in combination with their C-S-, C-O-, and presumably Caliph-C-bound analogues, they may play a significant role in solubilizing the asphaltene owing to their high solubility in the maltene. Other Types of Reactions Occurring in the RICO System. As discussed in the Results section, there are a number of significant reactions occurring in the RICO system which have not been considered before. The most important is the oxidation of tertiary C-H bonds at alkyl branchings and naphthenic ring condensations. Some hypothetical examples are as shown in Scheme 2. GC-MS analysis of the methylated and acetylated polar portion of the oxidized residue



indeed reveals the presence of a host of compounds whose mass spectra are consistent with a tert-hydroxy acid structure. These mass spectra are currently under study, and at present, only a few have been assigned, indicating the following structural elements:

According to the 13C NMR spectrum of the oxidized residue, some ketones are also formed; the weak resonance at ∼212 ppm suggests that they are probably cyclic. The presence of free carboxylic acids and alcohols in the asphaltene was demonstrated by the effects of methylation and acetylation on the MW (VPO) of asphaltene:

The maximum contribution to the R,ω-di-n-alkanoic acid series that could arise from the RICO reaction of alcohols (assuming that all the alcohols were gC3 n-alcohols anchored to aromatic carbons in the asphaltene) can be estimated to be about 8% of the diacids. Ethers are oxidized to esters in RICO, and ethers have been shown to be present in Athabasca asphaltene.7 The oxidation may take place on either side of the oxygen atom:

In other words, approximately 2.7 H-bonds per 10 molecules are operative, one of which involves the carboxylic hydrogens and 1.27 involves the alcoholic hydrogens: If the ether was originally attached to aromatic carbons, then the end product will be a diacid monoester, e.g.

The alcohols, mainly n-alcohols, would be oxidized in RICO to carboxylic acids and the secondary alcohols to ketones:

If R is an aromatic moiety, then a primary alcohol would give a dicarboxylic acid and the secondary alcohol a keto carboxylic acid. Thus, a long-chain terminal n-alcohol attached to an aromatic ring would lead to the formation of R,ω-di-n-carboxylic acids, increasing the yield of this series arising from the oxidation of n-alkenyl bridges connecting two aromatic moieties:

A keto acid could conceivably also be formed from the oxidation of fluorenes or other hydroaromatics:

According to the results of hydrolysis studies, n-alkyl ethers form a homologous series but the dominant ethers are those which are formed from more complex aromatic and polar alcohols. Sulfides are oxidized to sulfones and perhaps sulfinic and sulfonic acid esters. These compounds remain in the oxidized residue, which exhibits a complex IR absorption spectrum in the 1100-1350 cm-1 region where these compounds absorb, and therefore, the spectrum does not lend itself to easy interpretation. RICO Reaction Methodology. In the first phase of our studies on the RICO of asphaltene during the period 1984-1986, we focused our attention on developing suitable laboratory protocols and delineating the main features of the reactions involved and the nature of the reaction products. Since the reaction is nonhomogeneous, the products partition between the aqueous and organic phases. This, in conjunction with the necessary phase separation, solvent evaporations, esterifications under reflux conditions, and diazomethane treatment of the aqueous phase for esterification, led to smalls but not negligiblesselective losses in product recoveries. To remedy these shortcomings, in 1996 we developed different approaches to the handling and recovery of the reaction products. In this new method, the acid products were converted to their sodium salts at the conclusion of the oxidation reaction. The solution was filtered, and the solvent was evaporated in a rotary evaporator, and the salts were dissolved in water and divided into three


Strausz et al.

aliquots. The esterification was then done directly on the sodium salts with octadecyl alcohol for the most volatile acids and octyl alcohol for the less volatile acids with the aid of boron tribromide as a reaction promoter

RCO2Na + C18H37OH + BBr3 f RCO2C18H37 + NaBr + HOBBr2 The salts in the least volatile portion were either converted back to their acid forms and then methylated with diazomethane or the methylation was done on the dry salts with an excess of BF3/MeOH (12%, 4 mL) to give their methyl esters. The benzenepolycarboxylic acids were isolated from the aqueous phase by evaporation of the water followed by extraction of the acids with acetone. The acetone solution was then methylated with diazomethane. The method outlined above and described in more detail in accompanying articles in this issue9,10 is considered to be superior to the one we developed earlier and expected to yield more accurate quantitative data. In 1997 Wang et al.22 reported improvements to our earlier method regarding the esterification of the aliphatic and benzenepolycarboxylic acids. Their benzenepolycarboxylic acid esterification step is quite similar to the one described above except that the dry acids were methylated directly with ethereal diazomethane without any prior acetone extraction. (At this time it is not clear whether the acetone extraction step offers any advantage.) This method, as shown by Wang et al.,22 overcomes the progressively selective loss of the higher benzenepolycarboxylic acids. For example, the ratio of the hexa- to diacid yield is about 6-fold higher than that obtained by our earlier method in which the ethereal diazomethane treatment was done on the aqueous solution of the benzenecarboxylic acids. The diazomethane decomposition is accelerated in the aqueous acids, and as shown years ago,16 the esterification had to be carried out 3 times with at least a 3-fold excess of fresh diazomethane solutions (yielding 69%, 26%, and 5% of the total products) to achieve a high ester recovery. However, it is evident that beyond the triacids, the recovery is still incomplete. Undoubtedly, it is better to achieve as high a product recovery as possible; on the other hand, the overall significance of the benzenetetra through -hexaacid recoveries was exaggerated and completely misinterpreted by Wang et al.22 To understand why this is so, we need to briefly review the basic principles involved: a pericondensed aromatic structure is defined as ring systems in which three rings share one common aromatic carbon. This organic chemical definition of pericondensation bears only a remote relevance to the structural chemistry of asphaltene. In asphaltene chemistry, pericondensation is meant to denote a condensed aromatic sheet in which at least one of the rings is completely surrounded by aromatic rings. Thus, the simplest pericondensed system according to this definition is coronene and the complex ones include ovalene, circumanthracene, dodecabenzocoronene, etc. (22) Wang, Z.; Liang, W.; Que, G.; Qian, J. Pet. Sci. Technol. 1997, 15, 559-577.

Such systems are thought to yield benzenecarboxylic acids upon RICO. One example of a pericondensed aromatic structure suggested by Yen23 is the hypothetical structure for a Laquinillas asphaltene

It is important to keep in mind that on RICO, the yields of benzenepolycarboxylic acids are strongly dependent on the mode of aliphatic substitution of the aromatic core, e.g.

This simple example illustrates the important feature of the RICO reactions of how aliphatic substitution governs the yields of the benzenepolycarboxylic acids. Considering a more common example, let us take coronene. Statistically, we should get

But upon aliphatic substitution

Thus, it is seen that a few simple strategically placed aliphatic substituents can wipe out the major product, (23) Yen, T. F. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981; pp 39-51.


the tetraacid, but would not suppress the formation of the hexaacid. It can also be readily shown that strategically located aliphatic substituents can affect the ratios of the benzenepolycarboxylic acids produced or even wipe out all the di- through pentaacidssbut not the hexaacid. Consider, as an example, the above structure suggested for Laquinillas asphaltene.23 Here, the aliphatic substituents would wipe out the benzene acids from all the peripheral benzene rings, but the two central rings would still form benzenehexacarboxylic acids. Therefore, the general conclusions we have arrived at are aliphatic substituents can drastically affect the yields and ratios of the benzenedi- through -pentacarboxylic acids and in the extreme can entirely suppress their formation; in contrast, aliphatic substituents cannot prevent or suppress the formation of benzenehexaacids (because in this case the six carboxylic carbons form from ring junction carbons which do not carry substituents); as a consequence, the ratios of the various benzenecarboxylic acid products do not represent the ratios of the expected parent benzene ring positions in the asphaltene substrate molecules; if benzenehexaacids are not formed in the RICO reaction, pericondensed structures in the parent asphaltene molecule are absent; the yield of the benzene hexaacid, in sharp contrast to those of the lower benzene acids, is directly related to the concentration of the pericondensed structures in the asphaltene and as an upper limit for each hexaacid formed, seven pericondensed aromatic ring systems may be present in the asphaltene. In reality, owing to the excess aliphatic carbons present and their efficiency in suppressing di- through pentaacid formation (one CH3 can suppress the oxidation of two rings to benzene acids), the expected ratio of hexaacid formed to pericondensed structure present is much higher and, in fact, can exceed unity because hexaacid may also be formed from small, not fully condensed structures, e.g.

Energy & Fuels, Vol. 13, No. 2, 1999 223 Table 2. Carbon Balance from the RICO of Athabasca Asphaltene oxidation product in ether phase in aqueous phase in CO2 low-MW asphaltenes total in initial asphaltenes C recovery Carom recovery

asphaltene (wt %)

C in product (wt %)

C in asphaltene (wt %)

53.25 9.2 85.1 24.8

59.43 54.35 27.3 52.82

31.7 5.0 23.2 13.1 73.0 79.1 92.3 29.0 1.8 1.2 0.6 1.2 0.8

CO2 n-alkanoic acids n-alkanoic diacids benzene acids Carom in oxidized residue n-alkanoic acid methyl esters from pyrolysis

(34.6 C)/(100 C in asphaltene)

their derivatives, which could further reduce the concentration of large, structurally significant pericyclic systems in the asphaltene. Therefore, in closing this topic, Wang et al.’s22 measurement on Gudao asphaltene is in agreement with our conclusions on Athabasca, Lloydminster, Peace River, Carbonate Triangle, a Chinese lacustrine, Boscan and Duri asphaltenes and further demonstrates the minor significance of large pericyclic aromatic systems in the molecular structure of petroleum asphaltene. In the same article Wang et al.22 also reported the carbon dioxide yield from Gudao asphaltene as being (15.5 CO2)/(100 C) in the asphaltene and commented that our measured value of (29 C)/(100 C) is probably wrong. This is not the case because if it were, then our carbon balance would be too high, above 100%. As seen from Table 2, the actual carbon balance is quite good, amounting to 92.3%. The missing 7.7% is consistent with expected volatility losses. From 13C NMR, Carom ) 43% and the difference, (43 - 34.6 ) 8.4 C)/(100 C) in asphaltene can be attributed to hydroxy acids, naphthenic acids,

branched alkanoic acids, and

Returning now to the work of Wang et al.22 who reported that the relative yield of benzenehexacarboxylic acid from the RICO of Gudao asphaltene is much higher than that obtained from Athabasca asphaltene, evidently these authors were mistakenly led to the conclusion “that the aromatic structures of the asphaltene molecule are mainly pericondensed” in the Gudao asphaltene. The measured value of the carbon-atom yield in benzenehexaacid was (0.56 C)/(100 C) in the asphaltene. This translates into a yield for C6(CO2H)6 of 12/0.56 × 100 ) 2143 C, giving one C6(CO2H)6 or 1/0.8517 (%C in asphaltene) × 2143 × 12 ) 30 194 g asphaltene having one pericyclic system. In terms of molecular entities, since the MW was 3960 g mol-1 one asphaltene molecule out of every 30194:3960 ) 7.62 molecules has one pericondensed ring system. In all probability, a portion of the hexaacid comes from small aromatic molecules such as triphenylene, perylene, and

etc., and volatility losses. Thus, we can conclude that contrary to Wang et al.’s suggestion, the value of 29% for CO2 formation is correct and the difference between the two measured values reflects the difference in the molecular structure of the native Athabasca asphaltene and the asphaltene from Gudao vacuum residue (temperature (350 °C?) and heating times were not given). Asphaltene decomposes above 200 °C, and the structure of the residual asphaltene is altered in proportion to the thermal stress experienced, consequently thermally processed asphaltene from one source should not be compared to native asphaltene from another source. Pyrolysis of Athabasca Asphaltene. The present study represents the continuation and extension of earlier studies on the pyrolysis products of Athabasca asphaltene.12-14 Previously, a host of molecules com-


prising homologous series of compounds have been identified, bringing to light important details of the covalent structure of the asphaltene molecule. In these studies, the pyrolysis products were separated into saturate, aromatic, and polar compound class fractions but only the compositions of the saturate and aromatic fractions were reported;12 here, we describe the chemical composition of the polar fraction, which corresponds to 7.0% of the pyrolysis oil. Following the separation scheme shown in Figure 1 and the same experimental procedure described in ref 12, the first series of compounds identified was a short suite of alkanoic acids dominated by the C16 and C18 members in the form of their methyl esters. The only significant difference from the suite of n-alkanoic acids detected before in the basic hydrolysis products of the native asphaltene and in this study of the asphaltene pyrolysis oil products (vide infra) is the appearance of an intense peak of octadecanoic (probably oleic, cis-9octadecenoic) acid. These acids could have been originally bound to the asphaltene core either at their terminal position, e.g., core)-(CH2)15CO2H, or at their carboxylic group in ester bonds, e.g.

We will return to these carboxylic acids shortly. In the basic nitrogen compound fraction separated by CuCl2‚2H2O complexation, only a few alkylpyridines and a series of alkylquinolines could be identified. The mass spectra in Figure 25 show the presence of ethyl alkylpyridines:

Only the C7-C9 derivatives were detected. The alkylquinolines ranged from C2 to C10 side chains with several isomers at each carbon number. In many cases the mass spectrum of these basic nitrogen compounds consists only of the molecular ion. In a few cases it is possible to see some fragmentation and tentatively assign a structure to one component of the substituent. Three mass fragmentograms for isomeric C5-quinolines (MW 199) are shown in Figure 25 and have probable structures:

Fragmentation of B and C suggests that there is an n-propyl at the 8 position because alkyl groups adjacent

Strausz et al.

to heterocyclic nitrogen tend to lose a hydrogen atom with the cyclic stabilization of the resultant m/z ) MW - 1 ion or in the present case the 198 ion. CuCl2‚2H2O complexation was also carried out on the LMA (acetone extract). No basic nitrogen compounds were isolated. Hence, the pyridines and quinolines detected in the HMA originated only from the thermal decomposition of the HMA. At this point the FTIR spectrum (1645, 3000 cm-1) of the residual polar fraction suggested the possibility that aliphatic amides might be present. Therefore, the residue was treated with KOH/18-crown-6-ether/ethyl bromoacetate in order to hydrolyze the amides and esterify the resulting acids. After chromatographic removal of some nonpolar material, the polar residue was silylated to convert any alcohols present to silyl ethers, which were then separated chromatographically. The short series of n-alcohols detected by GC, Figure 26, and GC-MS revealed a somewhat erratic distribution which, however, is not necessarily inconsistent with the distribution of n-alcohols from the BBr3/LiAlH4 cleavage of the ester and ether bonds in the native HMA.7 An alcohol of tetrahydronaphthalene, benzyl alcohol, and C7-benzyl alcohols were also found. As seen by these results and in line with expectation, thermolysis is not as clean a process as chemolysis in liberating bound alcohols in their original form. After removal of the silyl ethers, the ester functions were reduced to alcohols with LiAlH4 and the resulting alcohols silylated as before to give the suite of n-alcohol silyl ethers depicted in Figure 27. The distribution pattern of the silyl ethers (originally corresponding to the acids liberated from hydrolysis of amides in the pyrolysis oil) is nearly identical to that found before in the basic hydrolysis of the native HMA. The carbon range extends from C8 to C22 with C16 and C18 as the predominant members and a strong even-to-odd preference. The LMA was also examined for alcohols and acids but without any heat treatment. The gas chromatograms shown in Figure 28 reveal the presence of alcohols and acids in distributions closely similar to those formed in the pyrolysis of the HMA. In summary, the pyrolysis oil of Athabasca asphaltene was found to contain small quantities of alkylpyridines and alkylquinolines. These have also been detected in the maltene fraction of the bitumen but are not present in the free state in the asphaltene. It also contains a homologous series of n-alcohols and n-alkanoic acids. Their distribution (∼C8-C22) is characterized by an even-to-odd carbon-number preference, and the acids distribution, which is dominated by the C16 and C18 members, closely resembles the distribution of the n-alkanoic acids from the basic hydrolysis of the native HMA. The LMA was also found to yield similar series of alcohols and acids without any heat treatment. The strong dominance of the even-carbon-number members of the series, as stated before, is a manifestation of a recent origin and secondary incorporation into the asphaltene. The origin of the n-alkanoic acid series has been related to the secondary microbial degradation of the precursor oil,21,24 converting it to the present-day bitumen. The n-alcohols are probably from the same sources as well.


Important information was also obtained on the molecular environment of the bound acids in the asphaltene. Previously, it was thought that all the bound acids are present in ester forms. The present work suggests that carboxylic acids may also be present in amide linkages. Covalent Structure of Athabasca Asphaltene: Current Status. Athabasca asphaltene is one of the most extensively investigated petroleum asphaltenes.6,7,12-16,20,21 Most of the molecular structural details were derived from studies employing a combination of basic instrumental and chemical methods. The latter included pyrolysis and series of chemolyses involving oxidative, reductive, hydrolytic and hydrogen-bonding cleavages. Table 3 presents a summary of the structural units and structural elements identified, along with an indication of their relative importance. A striking feature of the data in Table 3 is the role that sulfur in sulfide linkages, oxygen to a lesser extent, in ether and ester, and possibly nitrogen in amide linkages play in holding together the molecular core segments and attachments. About 40% of the sulfur is in sulfide bonds, about 60% of which is in positions holding together core segments. The rest is in bonds linking low-MW compounds to the core and in cyclic sulfides and thiophenes. Oxygen-held attachments amount to about 6.4% and sulfides-held attachments to about 4.3% of the asphaltene. In each case, approximately one-half of these attachments are aromatic and one-half polar. The small amount of saturates present consists entirely of biomarkersscheilanthanes, hopanes, steranes, etc. These biomarkers are of great importance in establishing the origin and history of the bitumen and its precursor oil. Biomarkers, locked up in the asphaltene geomacromolecule, are effectively protected from chemical alterations effected by catalysts, microbes, and other reagents by the size and micellar colloidal structure of the asphaltene in the oil. Consequently, they undergo much fewer alterations with time than the free biomarkers in the oil, and therefore, give a more authentic representation of the composition and organic geochemical parameters of the young oil. The molecular architecture described above then endows the asphaltene molecule with a high degree of reactivity because of the low value of the C-S bond dissociation energy in sulfides and the inherently high reactivity of sulfur, due to its high polarizability and tendency for valence-shell expansion with respect to free radicals. The ramification of this architecture vis-a`-vis heavy oil technology then lies in the ease with which the asphaltene falls apart at the sulfide C-S bonds upon cracking or hydrocracking, effectively counteracting any tendency for growth in molecular size and accompanying extensive coke formation. Of the C-O bond-held attachments, interesting aspects are the fairly large concentrations of n-alkanoic acid and n-alcohol components present in ester (probably amide) and ether functionalities. The n-alkanoic chains in the acid and alcohol series from the esters (24) Mackenzie, A. S.; Wolff, G. A.; Maxwell, J. R. In Advances in Organic Geochemistry 1981; Bjoroy, M., et al., Eds.; Wiley Heyden: London, England 1983; pp 637-649.


display strong even-to-odd carbon-number preference and are dominated by the C14, C16, and C18 members, clearly indicating a fairly recent microbiological origin most likely related to the microbial activities which were responsible for the degradation of the precursor oil to bitumen. The even-to-odd carbon preference in the alcohols from the ethers is less pronounced, and the ethers appear to have originated from the primary biotic source material of the oil. In addition to the chemically bound acid and alcohol functionalities, free acids and alcohols anchored at their terminal alkyl positions to the asphaltene core are present as well. These functionalities can bring about molecular aggregation via hydrogen bonding (which can be broken by blocking the functional group, e.g., methylation of the carboxyl group or acetylation of the alcohol) and thus affect the solubility of asphaltene. Other important structural elements holding together molecular segments are the alkenyl bridges between two aromatic, one aromatic and one naphthenic, and two naphthenic ring structures:

The first of these bridges can be quantitatively determined by the RICO method, the second one can be semiquantitatively estimated by a combination of RICO and pyrolysis, and the third can only be guesstimated. Their relative importance is in the order arom-arom > arom-naphth > naphth-naphth. The reactivity of these structural elements arises from the weakness of the benzylic C-C bond and its tendency to cleave into a benzyl and an alkyl radical:

All appendagessaromatics, polars, alkyl/ether/ester/ amide side chains, alkenyl bridges, etc., whether sulfuror oxygen-boundspositively influence the solubility of asphaltene in the maltene. Removal of these groups by cracking, hydrocracking, high-temperature “distillation”, etc., will adversely affect solubility. At the same time the removal of these appendages initiates condensation and aromatization in the residual asphaltene, e.g., Scheme 3. The decrease in the alkyl content of the asphaltene and the reciprocal increase in the alkyl content in the maltene, along with the increased aromaticity and size of the aromatic sheets of the residual asphaltene, all tend to decrease the solubility of the asphaltene in the maltene. The polar side chains, especially long-chain fatty acids and alcohols anchored to the asphaltene at their terminal alkyl position, may also endow the asphaltene with surfactant properties, causing a reduction in the interfacial tension at the water-oil interface, especially at elevated pHs. The effect can be magnified upon prolonged exposure to basic conditions when hydrolysis produces free long-chain acids and alcohols and more


Strausz et al.

Table 3. Structural Building Blocks Identified in Athabasca Oil Sand Asphaltene as Homologous Seriesa

a

R ) n-alkyl. b /// ) highest abundance.

acids and alcohols anchored to the asphaltene at their terminal positions. This structural feature provides the

foundation for explaining the experimentally observed surfactant/interfacial properties of asphaltene.


Energy & Fuels, Vol. 13, No. 2, 1999 227 Scheme 3

Naphthenic carbon comprises approximately 30% of Athabasca asphaltene. Several series of aromatic hydrocarbons and thiophenes having one or more condensed naphthenic rings have been detected, as of course thiolanes, thianes, smaller amounts of cyclic terpenoid sulfides and the naphthenic-type hydrocarbon biomarkers (cheilanthanes, steranes, hopanes, dicyclic terpenoids, and other 1-3 ring naphthenes) attached to the asphaltene via C-S, C-O, or Carom-C bonds, along with small amounts of their alcohol and acid derivatives; these represent most of the naphthenic carbons. All the six-membered naphthenic rings condensed to aromatic rings are substituted in Athabasca asphaltene, but in the immature Jinghan9 and other saline lacustrine crude asphaltenes,10 detectable amounts of unsubstituted condensed cyclohexane rings have been found. The aromatic fraction of the pyrolysis oil contains alkylaromatics with 1-3 rings along with condensed naphthenic derivatives, sulfur aromatics with up to 5 rings, and nitrogen aromatics, as illustrated in Table 3. The liquid product yield from the pyrolysis experiment was 34% and that of the gaseous and volatile products was 13%, for a total of 47%. This represents the minimum fractional amount of the asphaltene molecule which lies outside a highly condensed aromatic ring nucleus, because upon mild pyrolysis (430 °C) a highly condensed aromatic structure would not be expected to fragment to low-MW aromatic

molecules.

Instead, it would undergo condensation to form charlike material. However, structures bound to the core would give the observed products

This conclusion is further supported by the low total yield of benzenepolycarboxylic acids from the RICO reactions of the various asphaltenes which have been studied to dates(1.7 ( 1.0 C)/(100 C) in the asphaltenes and the relatively small fractional amount (

Additional Structural Details on Athabasca Asphaltene and Their

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