Energy & Fuels 1992,6, 83-96
83
Structural Features of Alberta Oil Sand Bitumen and Heavy Oil Asphaltenes. T. W. Mojelsky, T. M. Ignasiak,t Z. Frakman,f D. D. McIntyre, E. M. Lown, D. S. Montgomery, and 0. P. Strausz* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2 Received March 18, 1991. Revised Manuscript Received September 18, 1991 Ru ion catalyzed oxidation (RICO) has been used to probe the structural details of asphaltene molecules from Athabasca bitumen. The ability of RICO to remove nearly quantitatively aromatic carbons while leaving aliphatic and naphthenic structures essentially unaffected has been exploited for the quantitative determination of the following structural features: (1)the distribution of n-alkyl chains attached to aromatic carbons with respect to chain length, total number of n-alkyl chains, and the number of carbon atoms in them; (2) the distribution of polymethylene bridges connecting two aromatic units with respect to bridge length, total number of bridges, and the number of carbon atoms in them; (3) the fractional amount of total sulfur present as saturated sulfides. Moreover, semiquantitative estimation could be made of the carbon in bridges connecting an aromatic to a naphthenic ring and of the aliphatic carbon attached to naphthenic rings. The various isomeric benzene di-, tri-, and tetracarboxylic acids produced from the condensed aromatic nuclei during RICO gives some insight into the mode of aromatic condensation in the asphaltene molecules, clearly establishing the absence of pericyclic aromatic Structures. RICO also made possible the detection of the presence, and estimation of the quantity of n-alkanoic acid esters anchored to the aliphatic/naphthenic cores of the asphaltene molecules. On pyrolysis these esters give rise to n-alkanoic acids which are characterized by a short carbon range up to -C22, high even carbon number preference, and the presence of small quantitites of C18unsaturated n-alkanoic acids, linking their origin to relatively recent microbial activities in the reservoir. The RICO of asphaltene also shows the existence of large aliphatic/naphthenic domains in the asphaltene, with MW up to -8700. RICO studies of asphaltenes from various Alberta oil sands and carbonate bitumens and a heavy oil reveal close structural similarities among these asphaltenes, which in turn point to a common origin for these huge oil accumulations. Significant progress has been made in recent years in the structural investigation of petroleum asphaltenes and high molecular weight petroleum fractions using the Ru ion catalyzed oxidation (RICO) of aromatic This method, which converts aromatic carbons in high yields to carbon dioxide without damaging the structural integrity of the aliphatic and naphthenic units, has been shown to be potentially suitable for the estimation of the concentration and chain length distribution of alkyl groups attached to aromatic carbons and of methylene bridges connecting two aromatic units in petroleum asphaltenes.4~~
The method is also promising for semiquantifying the concentration and chain length distribution of methylene bridges connecting one aromatic unit to one naphthenic unit. Lastly, an estimate of the concentration and chain length distribution of the total number of alkyl groups attached to naphthenic carbon and methylene bridges connecting two naphthenic units has also been shown to be feasible. In addition to analyzable products, the RICO of asphaltene yields a high molecular weight oxidized naphthenic/aliphatic residue manifesting the existence of sizeable naphthenic-aliphatic networks in the asphaltene structure. RICO was introduced into organic chemistry by Djerassi and Engle6in 1953. However, owing to the variability and unpredictability of the results the method did not become generally applicable until 1981 when Sharpless and coworkers7 suggested the use of acetonitrile as a cosolvent. Acetonitrile is thought to prevent the precipitation of lower-valent ruthenium carboxylate complexes, thereby improving the reproducibility, selectivity, and efficiency of the oxidation reactions. The Ru ion catalyzed oxidation
(1)Mojelsky, T.W.;Montgomery, D. S.; Strausz, 0. P. AOSTRA J . Res. 1985,2, 131-137. &re cdre (2) Mojelsky, T. W.; Montgomery, D. S.; Strausz, 0. F. AOSTRA J . Res. 1986, 3, 43-51. (3)Mojelsky, T.W.; Montgomery, D. S.; Strausz, 0. P. AOSTRA J. Ru ions Res. 1986, 2,177-184. NaIO. (4)c Strausz,a 0.P.Prepr.-Am. Chem. Soc., Diu. Pet. Chem.H 1989,34, HO O cO O * HOO$ $OOH 395-400. ( 5 ) Strausz, 0. P.; Lown, E. M. Fuel Sci. Technol. Int. 1991, 9, 269-281. * To whom correspondence should be addressed. (6)Djerassi, C.J.;Engle, R. R. J.Am. Chem. SOC.1953, 75, 3838-3840. ‘Present address: Coal Research Department, Alberta Research (7) Carlson, P. H.J.; Kalsuhi, T.;Martin, V. S.;Sharpless, K. B. J. Org. Chem. 1981, 46, 3936-3938. Department, 1 Oil Patch Drive, Devon, Alberta, Canada, TOC 1EO.
0887-0624/92/2506-0083$03.00/0
0 1992 American Chemical Society
Mojelsky et al.
84 Energy & Fuels, Vol. 6,No. 1, 1992
of organic compounds has been referred to in the literature as ”ruthenium tetroxide catalyzed” or “Ru(VIII) catalyzed” reactions. While these designations are unambiguous, they are technically incorrect because ruthenium tetroxide or Ru(VII1) represent only one oxidative state in the catalytic cycle:
agent
Ru(III)/Ru(II)
Ru(VII1)
For this reason the term “Ru ions catalyzed oxidation” (RICO) would be more preferable. RICO was first applied to fossil fuel chemistry by Stock and co-workers8in 1983 followed by Mallya and Zingarog in 1984 for the structural elucidation of coal, and to petroleum chemistry by Strausz and co-workers’ in 1985 for the study of asphaltene and other high molecular weight petroleum fractions. The present study was undertaken in order to gain quantitative data on some structural aspects of Athabasca asphaltene, to further explore the range of problems that can be pursued with the method, and to examine the variation in asphaltene structurefrom some of the major reservoirs of the Alberta oil sand bitumen and heavy oil deposits.
Experimental Section Materials. All solvents were distilled prior to we. Ruthenium
trichloride (as the trihydrate, RuCl3.3Hz0) was used directly as obtained. Due to the highly hygroscopic nature of this reagent, it was stored in a desiccator. Column chromatography was done on silica gel, particle size 0.063-0.200 mm, activity according to Brockman, 2-3. Diazomethane as an ethereal solution was prepared by addition of an ethereal solution of N-methyl-Nnitrosotoluenesulfonamideto an alcoholic solution of KOH at 60 “C and distilled as produced. The Athabasca bitumen sample was obtained from a location 18 m below the surface a t the Syncrude Canada Ltd. mine site near Fort McMurray, Alberta, coordinates 1-2-93-11W4. The Peace River sample was from a core, depth 578 m, obtained from Shell Oil Ltd., coordinates 4-21-85-18W5 and the steamflood sample from Shell Oil’sin situ Peace River pilot plant, coordinate 21-85-18W5. Lloydminster bitumen was obtained from Husky Oil Operations Ltd., depth 670 m, pumped from the Sparky Formation. Carbonate Triangle bitumen was obtained from the Grosmont Formation a t the site of the Union Oil steamflood operation, coordinates 55-88-19W4. The steamflood sample was supplied by Shell Oil from their in situ Peace River pilot plant, coordinate 21-85-18W5. The asphaltene samples used in this work were prepared by precipitation with n-pentane from a concentrated dichloromethane solution of bitumen and extracted with n-pentane in a Soxhlet thimble in order to remove complexed low molecular weight compounds. In the case of the oil sands samples (Athabasca and Peace River core), the bitumen was separated from the sand by Soxhlet extraction with dichloromethane. The other samples were used directly as received. After n-pentane extraction, the asphaltene samples were dried overnight in a vacuum oven at 60 OC.
Methods. (i) RZCO of asphaltene was carried out by stirring a mixture of asphaltene (300 mg), sodium metaperiodate (3.4 g), carbon tetrachloride (20 mL),water (30 mL),acetonitrile (20 mL), and ruthenium trichloride (10 mg)overnight at room temperature. The initial dark color of the solution changed to a pale tan during the course of the reaction. The products formed were partitioned between the two phases of the reaction mixture, the carbon tetrachloride/acetonitrile (organic) and the aqueous acetonitrile (water) phases. (8) Stock, L. M.; Tse, K. T. Fuel 1983, 62, 974-976.
(9)Mallya, N.;Zingaro, R. A. Fuel 1984, 63, 423-425.
(ii) Esterification. The two phases of the reaction were separated and the carboxylic acids formed were converted to their esters by treatment with excess diazomethane in ether solution. The organic phase, after drying over sodium sulfate, was treated with excess diazomethane for 0.5 h and then the ether and unreacted diazomethane were driven off with a gentle stream of nitrogen at icebath temperature. Methylation of the water phase was inefficient because the strongly acidic solution and the Ru ions had the combined effect of rapidly decomposing the diazomethane. Of the ultimately recovered ester fraction only 69% was obtained in the first methylation step even when a 3-fold volume exof the ethereal diazomethane was used. The second methylation step yielded 26% and the third step 5% of the recovered esters. After methylation the ethereal solutions were combined and the water phase was extracted with dichloromethane. The ether and dichloromethane solutions were combined and the solvents were driven off by a stream of nitrogen. The organic concentrate thus obtained was subjected to chromatographic fractionations prior to gas chromatographic (GC) analysis. (iii) Methylation Side Products. Gas chromatographic (GC)-mass spectrometric (MS) studies of the n-alkanoic acid methyl esters revealed the presence of a series of n-alkanes and terminal n-alkenes up to -CN in this fraction, the members of which eluted between the peaks of the n-alkanoic acid esters. Alkenes are known to readily react and become oxidized in RICO and therefore these n-alkenes were thought not to be genuine oxidation products but rather to originate from a polymerization side reaction of diazomethane during the methylation of the oxidation products. That indeed this was the case was proven in the following set of auxiliary experiments where OM is the oxidizing medium (NaI04,RuCl3.3Hz0,H20, CCh, CH,CN). Thus it was clearly established that the n-alkenes originated from the decomposition of diazomethane upon the catalytic effect of the oxidant dissolved in the CCl, and not from the oxidation of the asphaltene. OM
+ asphaltene OM
OM
CHbJz
alkanes
+ alkenes 5 C3,,
-
+ CCl,
no hydrocarbons
+ as
OM
-
haltene ,El4 layer
CCl, layer
+ CCl,
CH82
no hydrocarbons
alkanes
+ alkenes 5 Cm
(1)
(2) (3) (4)
(iu) Chromatography. (a) Gel Permeation Chromatography (GPC). After methylation the two phases of the reaction mixture were separately chromatographed, as indicated in the flow sheet in Figure 1,on a Bio-Beads SX-2 column according to the description given in a previous publication from this laboratory.1° Five fractions were taken, each with 40-mL methylene chloride as eluant and gravimetrically measured. The volatile portions of the products were lost in the process. ( b )Silica Gel Chromatography. The combined fractions 3-5 from the GPC fractionation with molecular weight (MW) 300-660 were applied to a 20-g silica gel column using the elution sequence given in Figure 1. Several fractions were collected. In shortcut analyses the GPC separation was omitted and the high molecular weight methylated products were removed from the solution by precipitation with an equal volume of n-pentane. The fdtrate was concentrated by evaporation and the residue was directly applied to the silica gel column. From the organic phase ester mixture, the hydrocarbons, if present, were removed with the n-pentane elution. The n-alkanoic acid esters eluted with methylene chloride and the a,w-di-n-alkanoic acid esters were eluted with 10% ethyl acetate in methylene chloride. The aqueous phase acids were esterified with excess diazomethane and thus became ether soluble. The dried ethereal solution was concentrated and the residue was chromatographed on silica gel. The shorter chain a,w-di-n-alkanoic acid esters (up to C,) were eluted together with the benzene carboxylic acid esters with 10% ethyl acetate in methylene chloride. (10)Ignasiak, T. M.; Kotlyar, L.; Samman, N.; Montgomery, D. S.; Strausz, 0.P.Fuel 1983, 62, 353-362.
Oil S a n d Bitumen
Structural Features of Alberta Sample
Reaction mixture
Organic layer
Water layer
-
40
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40
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I
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E y i Q i l s15% ! z hBJCs I 2 3 4 5
6 7
CH2Cl2 CHzCl2 CH2Cl2
$p$tYJwauml€3omL mmL 15 mL 45 mL
?2%c/CH2Cl2 65mL 209bMeOWCH202 -
-
0.04 0.03 0.28 1.37
benzene carboxylicacid esters benzene carboxylic acid esters
n-alkanoic acid esters
5.1 0.2 0.1 0.2 4.2 2.1
benzene carboxylic acid esters benzene carboxylicacid esters
u.wdicarboxylic acid esters
Organic phase 1
d
5 6 7
%EJC5 CH2Cl2 CH2C12 CH2Cl2 CH202 10% EtAdCHzCIz 20%McOWCH2Qz
%IF-u)mL 20 mL 15 mL 45 mL 65 mL
-
n-alkanoic acid esters n-alkanoic acid esters
u,wdicarhxylic acid esters
-
Figure 1. Experimental work-up procedure. (u) Quantification of the n-Alkanoic and a,w-di-n-Alkanoic Acids. The fractions isolated from the water and organic phases of the reaction mixture by silica gel chromatography were quantitatively analyzed by capillary GC. Careful handling of the monoacid ester fraction was required in order to minimize losses arising from volatility. Most of the n-alkanoic acid esters appeared in the organic phase of the reaction mixture and only small quantities of them with chain lengths of up to C7 were in the aqueous phase. The a,w-di-n-alkanoicacids were determined to be in the GPC-silica gel chromatographic fraction 6 concentrate. The C& members of this series appeared in the water phase and the CB-C26members in the organic phase of the reaction mixture. In the case of the monoacid esters where the losses in the lower MW range were high and the C& acid methyl esters coeluted with the solvent, it became necessary for the quantitative measurement of the n-alkanoic acids produced in the RICO reaction to employ a higher MW reagent for the esterification. Thus, phenacyl bromide, giving the less volatile and later eluting phenacyl ester 0
0
was used. (ui) Preparation of Phenacyl Esters. For the determination of the C3-C7acids the organic phase from the reaction mixture was extracted with 20 mL of 10% KOH solution and washed with water and the water extracts were combined. Next the water was removed in a rotary evaporator and the residue was then dissolved in 5 mL of H20 and made slightly acidic by the addition of dilute HCl. These acids were then reacted with 80 mg of phenacyl bromide in 10 mL of ethanol" under reflux for 3 h, following which the mixture was evaporated to dryness. The phenacyl esters were extracted with toluene and analyzed by GC. The aqueous phase from the reaction mixture was esterified separately after addition of KOH and removal of water. This esterification procedure was (11) Rather, J. B.; Reid, E. E. J. Am. Chem. SOC.1919, 41, 75-83.
Energy & Fuels, Vol. 6, No. 1, 1992 85 tested on acetic acid and high recoveries of phenacyl esters were achieved. (uii) Quantification of Acetic Acid. Since during the oxidation and 'subsequent work-up procedure the acetonitrile coeolvent was found to undergo partial hydrolysis to acetic acid, the procedure for the determination of the acetic acid produced in the RICO of asphaltene had to be modied. Thus,the acetonitrile cosolvent was replaced with propionitrile and the same procedures were applied as above. The quantitative data reported in the Results section on the n-alkanoic acid yields from C2 to C7 acids were obtained by this procedure. From heptanoic acid on, the measurements were done on the methyl esters. For heptanoic acid, the values obtained from the phenacyl ester and methyl ester measurements were identical. (uiii) Correlation Studies. The same experimental techniques were applied as above except that the GC analyses were done only on the organic phase of the reaction mixture and the fractionation of the esters was done by vacuum distillation at 230 OC and Torr. (ix) Pyrolysis of Nondistillable Residues. The nondistillable oxidized residues from the RICO after methylation were subjected to pyrolysis a t 375 OC as previously de~cribed.~ The pyrolysates thus obtained were chromatographed on 20 g of silica gel and eluted with succB88ive 100-mL portions of n-pentane, 20% benzene in n-pentane, dichloromethane, 10% ethyl acetate in dichloromethane, and 10% methanol in dichloromethane. ( x ) Solvent Fractionation of Asphultene. Asphaltenes obtained by n-pentane precipitation were fractionated by extraction with acetone into low molecular weight asphaltene (LMA, MW 890 (16%)) and high molecular weight asphaltene (HMA, MW 6800, (84%))(MW by vapor pressure osmometry in benzene). ( x i ) Determination of C02Produced in the RICO. The oxidation was carried out in a gas bubbler flask purged with flowing nitrogen. The exit gas passed through a drier tube and then through a tube fded with ascarite which was weighed before and after the experiment. Analyses. GC analyses were performed on a Hewlett-Packard HP 5730 or H P 5830 gas chromatograph in the flame ionization mode using a 30 m X 0.252 mm J&W Scientific fused capillary column coated with DB-1. Beginning at 50 "C, the GC oven was programmed to 290 OC at a rate which was typically 4 OC/min. The carrier gas used was hydrogen a t a flow rate of 40 mL/min, injections being made in the split mode. Hydrocarbons were used as internal standards when quantitative analyses on Athabasca asphaltene were performed. The response factors for the various esters relative to the internal standard were determined by injection of standard compounds and appropriate FID corrections.12 The gas chromatographiemass spectrometric (GC-MS) experiments were performed on a Vacuum Generators VG 7070E spectrometer equipped with a VG-11-250 data system and interfaced directly to a Varian 600 gas chromatograph. The GC was run in the splitless mode using helium as the carrier gas and the DB-1 column described above. The mass spectrometer was operated a t 70 eV in the electron impact mode and was scanned from 50 to 500 Da every 0.5 a. Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker WH-400 spectrometer operated in the FT mode, using CDCl, as a solvent. Proton spectra, obtained at 400 MHz, were acquired using a 90° pulse and 32K data points. For the 13C spectra, inverse gated decoupling with a 5-10-9 delay between the 4 5 O pulses was used in order to obtain quantitative spectra at 100.6 MHz using 16K data points. Preparation of Reference Compounds. The preparation of 1,2-, 1,3-,1,4-benzenedicarboxylic acids and 3-methyl-1,2benzenedicarboxylic acid as well as their corresponding esters has been previously described.' Tetramethyl 1,2,4,5-benzenetetracarboxylate was prepared by refluxing 1 g of commercially available 1,2,4,5-benzenetetracarboxylic anhydride (Aldrich) with 0.5 mL of concentrated sulfuric acid in 30 mL of methanol overnight and then isolating the product. 1,3,5-Benzenetricarboxylic, 1,2,3,4-benzenetetracarboxylic,and 1,2,3,5-benzenetetracarboxylic acids were prepared by the aqueous alkaline potassium permanganate oxidation13 of the corresponding tri- and tetra(12) McNair, H. M.; Bonelli, E. J. In Basic Gas Chromatography; Varian Aerograph Walnut Creek, CA, 1968; p 143.
86 Energy & Fuels, Vol. 6,No. 1, 1992
methylbenzene for 15 h. 1,2,4-Trimethylbenzenerequired a 48-h reflux to completely oxidize all methyl groups to 1,2,4-benzenetricarboxylic acid. In order to esterify all the acid groups, these acids appear to require both a period of reflux with concentrdted sulfuric acid in methanol as well as treatment with diazomethane. All of the esters gave reasonable proton NMR spectra. Benzenehexacarboxylicacid was prepared by the RICO of triphenylene. The acid, being water soluble, was esterified by treatment of the aqueous phase with excess ethereal diazomethane. The hexamethyl benzenecarboxylate product appeared as a single peak ( m / z 395) on the GC-MS fragmentogram.
Mojelsky et al.
5
10
15
20
25
30
Carbon number
Results and Discussion Most of the work in the present investigation was done on Athabasca oil sand asphaltene which is a dark brown, amorphous solid having a number average molecular weight of 3600 with a broad distribution ranging from a few hundreds to tens of thousands. I t is rich in heteroatoms, S 8.0 wt %, 0 3.0 wt %, N 1wt %, metals (Fe, V, Ni), and has an H/C atomic ratio of about 1.24. The chemical composition of asphaltenes has been the subject of many investigations. Recently some progress has been made in the identification of individual compounds present in Athabasca asphaltene. The compounds identified14-16 were all resinous substances which are incorporated into the asphaltene micelles and are partly responsible for the solubility of the asphaltene in the rest of the oil. Identified were series of carbazoles, porphyrins, cyclic terpenoid sulfides and sulfoxides, cyclic terpenoid carboxylic acids, naphthenic, aromatic and aliphatic carboxylic acids, alkyl fluorenones, and fluorenols. Valuable structural information has also been derived from thermal degradative studies on Athabasca asphaltene. Thus, it has been found that the pyrolysis oil from the mild thermolysis of HMA contains, in addition to homologous series of n-alkanesand terminal n-alkene~"-~Ohomologous series of n-alkyl-substituted thianes, thiolanes, thiophenes, benzothiophenes,21 and dibenzothiophenes22along with series of n-alkyl-substituted aromatic hydrocarbons, all bearing the same substitution pattern indicative of an n-alkanoic origin. Taken together, these compounds comprise a very substantial weight fraction of the asphaltene and they undoubtedly constitute major structural elemenh of the high molecular weight portion of the asphaltene. NMR studies on Athabasca a s ~ h a l t e n show e ~ ~ that ca. 43% of the carbon is aromatic, 31% is aliphatic, and ca. 26% is naphthenic. It has also been shown that the distribution of carbon among these structural environments is dependent on the molecular weight of the asphaltene fractions and the aromaticity tends to slightly decrease with increasing molecular weight. Assuming that all the aliphatic carbons are present as alkyl side chains, it was (13) Smith, M. E. J . Am. Chem. SOC.1921,43, 1920-1924.
(14)Frakman, 2.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, 0. P. AOSTRA J. Res. 1987,3, 131-138. (15) Frakman, 2.;Ienasiak. T. M.: Monteomerv. D. S.: Strausz. 0. P AOSTRA J. Res. 191
(17) Rubinstein, I.; Spyckerelle, C.; Strausz, 0. P. Geochim. Cosmochim. Acta 1979, 43, 1-6. (18) Rubinstein, I.; Strausz, 0. P. Geochim. Cosmochim. Acta 1979, 43, 1887-1893. (19) Ritchie, R. G. S.; Roche, R. S.; Steedman, W. Fuel 1985, 64, 391-399. (20) Speight, J. G. Fuel 1970, 49, 134-145. (21) Payzant, J. D.; Montgomery, D. S.; Strausz, 0. P. AOSTRA J. Res. 1988, 4 , 117-131. (22) Payzant, J. D.; Lown, E. M.; Strausz, 0. P. Energy Fuels 1991, 5,445-453. (23) Cyr, N.; McIntyre, D. D.; Toth, G.; Strausz, 0. P. Fuel 1987,66, 1709-1714.
Figure 2. Gas chromatogram of n-alkanoicacid methyl esters
after silica gel separation. The minor peaks appearing in the valleys of the main series are due to branched and terpenoid acid esters.
Carbon number
Figure 3. Distribution of n-alkyl groups attached to aromatic moieties in Athabasca asphaltene. Table I. Determination of n -Alkyl Groups Attached to the Aromatic Moieties of Athabasca Asphaltene w t % alkyl group no. of in asphaltene X alkyl n-alkyl 10 chainsd no. of Cd - -OOUDc1 4.87" 0.487 0.487 C, 7.5P 0.388 0.766 ~. 4.80b 0.167 0.502 5.79b 0.152 0.611 5.56b 0.118 0.588 6.51b 0.115 0.688 3.13c 0.047 0.332 3.0@ 0.041 0.326 3" 0.0354 0.319 3.07' 0.0325 0.325 3.14' 0.0304 0.334 3.2OC 0.0285 0.342 3.28c 0.0268 0.348 3.33c 0.0254 0.355 3.39c 0.0242 0.363 2-90' 0.0193 0.309 3.48c 0.0218 0.370 2.43c 0.0144 0.260 2.21' 0.0139 0.263 2.48' 0.0132 0.263 0.0055 0.115 0.6OC 0.0028 0.062 0.03 0.84 1.84 9.18 ~
"As phenacyl eater in both aqueous and organic layers. b A s phenacyl eater in organic layer. As methyl ester in organic layer. dPer 100 C in asphaltene.
possible to arrive at an estimate of the average length of about C9 for the alkyl chains in the asphaltene. Alkyl Side Chains Attached to Aromatic Carbon. In the present study, using RICO
Structural Features of Alberta Oil Sand Bitumen
as described in the Experimental Section, we have determined quantitatively the concentration distribution of alkyl chains according to chain length. The distribution of n-alkanoic acid methyl esters resulting from the RICO of Athabasca asphaltene follows a smooth pattern, extending to about C32 (Figure 2). Small quantities of iso, anteiso, and terpenoid monoacid esters eluting between the peaks of the n-alkanoic acid esters are also evident. The number of n-alkyl groups, R, per 100 C atoms in the asphaltene were calculated from the GC measurements and the data are tabulated in Table I and plotted as a function of carbon number in R in Figure 3. The most abundant alkyl group is methyl and the concentrations fall off, first rapidly and then more gradually with increasing chain length up to about CS1. The total number of alkyl groups is 1.8 per 100 C atoms and the total number of carbon atoms in the side chains is 9.2 per 100 C atoms. That, is 9.2% of the carbon in the Athabasca asphaltene sample under investigation is present as alkyl side chains attached to the aromatic carbons of mono- through polynuclear aromatic hydrocarbons or thiophenes, benzo-, dibenzo-, or higher aromatic thiophenes, etc. Homologous series of n-allryl-substitutedaromatic hydrocarbons and thiophenes, of the types
IR n-
2
Energy & Fuels, Vol. 6, No. 1, 1992 87
I
I
I
10
15
Figure 4. Gas chromatograph of a,w-dicarboxylic acid methyl esters from the Ru ions catalyzed oxidation of Athabasca asphaltene after GPC silica gel separation: (a) organic phase; (b) water phase.
+
8 D
-
kl
9 L
4
have been detected in the pyrolysis oil of Athabasca asphaltene.zlin Their substitution patterns provide evidence for their n-alkanoic origin. RICO completely destroys the thiophenes in asphaltenes, as was shown by NMR examination of the oxidized residue, which indicated the presence of only a few percent residual aromatic carbon. Therefore, a considerable portion of the 9.2% n-alkyl carbon must be attached to the thiophenic carbons in the asphaltene. The most abundant monocarboxylic acids detected had a straight-chain carbon skeleton and the alkanoic acids correspondingto the lesser quantities of homologous series of is0 and anteiso alkanes that have been noted in the pyrolysis products of Athabasca asphaltene beforez4could not have amounted to more than a few percent each of the n-alkanoic acids. From NMR measurement^^^ the total number of alkyl carbon is 31 per 100 C atoms. Of these, 4 are present in CH3 groups attached to naphthenic carbon and 9.2, that is 1/3 of the rest, in the form of C1-C30 n-alkyl groups attached to aromatic carbons. These 9.2 carbon atoms form on the average 1.8side chains (from the data in Table I) giving an average chain length of 4.7 carbons or, if the methyl group is exempted, 8.7C in 1.4 chains, giving an average chain length of 6.2, which may be compared with the NMR-derived value of 9 for all the alkyl chains in the asphaltene (not only those attached to aromatic rings). Alkyl Bridges between Aromatic Units. The oxidation of aromatic units connected by an alkyl bridge yields, according to the general mechanism alkanoic di(24) Rubinstein, I.; Strausz, 0.P. Geochim. Cosmochim. Acta 1979, 43, 1887-1893.
20
Carbon number
1
0061
!
1
3
I
5
2
i
0.02 O . O 4 _ i 0.00 0
4
8
12
16
20
24
28
Methylene chain length
Figure.5. Distribution of methylene chains attached to aromatic moieties in Athabasca asphaltene. C6 and C7 are interpolated values.
carboxylic acids. Normal-alkyl bridges give a,w-di-n-alkanoic acids.
Indeed, a homologous series of these acids in the C4-C26 range corresponding to Cz-Cz4polymethylene bridges has been detected in the oxidation products and quantified in the form of their methyl esters. A GC trace showing the concentration distribution of the methyl esters is given in Figure 4, and the quantitative results are presented in Table I1 and Figure 5. As with the n-alkyl side chains, the concentration distribution shows a monotonic decline with increasing chain length of the bridge from C3to Cz4. The concentration of the C2 bridge is lower than that of the C3bridge but this is more likely due to errors arising from increased volatility and differences in the elution behavior on the GPC and silica gel columns than to a diminished presence in the asphaltene. The shortest diacid detected was succinic acid, representing a dimethylene bridge in the asphaltene. Malonic acid, corresponding to a C1 bridge, is known to be oxidized' further in RICO and therefore it cannot be determined by this method. The concentration of C1 bridges, if at all present, would be expected to be low in asphaltene because of their increased reactivity owing to their double benzylic character and for this reason compounds containing C1 bridges would probably have been removed from or de-
Mojelsky et al.
88 Energy & Fuels, Vol. 6,No. 1, 1992 Table 11. Determination of Methylene Bridges Connected to Two Aromatic Moieties of Athabasca Asphaltene methvlene chain no. of methylene bridges" no. of C" 0.122 0.061 2 0.321 0.106 3 0.372 0.093 4 0.352 0.070 5 (0.311) (0.056)* 6 (0.272) (0.043) 7 0.245 0.031 8 0.250 0.028 9 0.245 0.024 10 0.220 0.020 11 0.206 0.017 12 0.154 0.013 13 0.142 0.010 14 0.130 0.009 15 0.082 0.006 16 0.107 0.006 17 0.070 0.004 18 0.060 0.003 19 0.051 0.003 20 0.042 0.002 21 0.033 0.001 22 0.016 0.001 23 0.016 0.001 24 3.82 0.608 Per 100 C in asphaltene. values.
Parentheses indicate interpolated
pleted in the deposit over geological times. In any event, even if C1 bridges are present the inability of the RICO method to measure them would not lead to any significant error in the measurement of aliphatic carbon. Oxalic acid, expected to arise from the RICO of the biphenyl linkage, is also oxidized in RICO to carbon dioxide. The presence of biphenyl linkages in Athabasca asphaltene has been shown by thermal degradation studies and also by the appearance of certain benzene tri- and tetracarboxylic acids in the RICO, as will be shown below. Because of the loss of the malonic acid product to RICO oxidation, the number of bridges calculated (Table II),0.61, and the number of carbon atoms in the bridges, 3.8, per 100 C in the asphaltene are slight underestimations. In any event the number of bridges and carbon atoms are both considerably lower than those for the side chains, 1.8 and 9.2, respectively. A more informative correlation between side chains and bridges is based on the comparison of their numbers in the same total carbon range in the molecules. Thus, if the aromatic carbon atoms are present in benzene rings a bridge length of three corresponds to a CI5molecule and a bridge length of (2%to a CS molecule. For alkylbenzenes these correspond to chain lengths of 9 and 30, respectively. Taking then the ratio (E number of bridges)/(C number of chains) in the C15-CNrange, Tables I and 11, we obtain 0.55/0.33 = 1.7. The meaning of this is that, for every n-alkyl side chain of sufficient length to form a second benzene ring, there are 1.7 structural units in the asphaltene molecules present where such a second benzene ring was formed. Since cyclization and aromatization of the n-alkanoic precursor most likely had taken place in a stepwise fashion in some unknown catalytic process during the thermal maturation of the asphaltene and its progenitors, n-alkaneC,
A
IX
5
12
7
130
Temperature ( " C )
200
Figure 6. Gas chromatograph of benzenepolycarboxylicacid methyl esters from the Ru ions catalyzed oxidation of Athabasca aaphaltene. Peak 5, phthalic acid; 6, 3-methylphthalicacid; 7, 4-methylphthalic acid; 8, 1,2,3-benzenetricarboxylic acid; 9, 1,2,4-benzenetricarbxylic acid; 10, methylbenzenetricarboxylic acid; 11, 1,2,3,4-benzenetetracarboxylicacid; 12, 1,2,4,5benzenetetracarboxylicacid; 13,1,2,3,5-benzenetetracarboxylic acid; x = unknown.
the ratio of aromatic side chains to aromatic bridges is an intrinsic property of the asphaltene reflecting ita maturation history (thermal stress, availability and nature of catalysts). Aromatic Units. The third group of acids found in the aqueous phase of the oxidation reaction mixture was a suite of benzene di-,tri-, and tetracarboxylic acids and their monomethyl derivatives. The GC trace of the methyl esters of these acids is shown in Figure 6. Altogether nine esters, 5-13, were identified The combined yield of these
acoZcH3 b"".'".
m c o Z c H
q C 0 Z c H 3
COzCH3
COzCH3
5
C02CH3
7
6
COzCH3
COzCH3
W O 2 C H 3
COZCHj 8
COZCHj
(C02CH3)3
9 q COzCH, o Z c H 3
COzCHj COZCH3 11
q
0 COzCH, Z C H
CH30zC
-
10
&02CH3 COzCH,
3
CH302C COzCH3 12
COzCH, 13
products was 1.4%, corresponding to about 0.6 C per 100 C atoms in the asphaltene. We have previously reported' the detection of the esters of phthalic acid, 3-methyl- and 4-methylphthalic acid in the organic phase following Ru ions catalyzed oxidations of high molecular weight Athabasca maltene fractions. Oxidation of a highly condensed system such as lignitez5 has been reported to give predominantly benzene di- to pentacarboxylic acids. Therefore, it would appear that the presence of two or more carboxylic acid groups on a benzene ring retards or prevents further oxidation of the ring, but one COzH may not provide sufficient deactivation. In fact, diphenic acids has been reported to be resistant to oxidation by Ru(VIII), and C02H, C02R, and m a t k Y 1 c X 7 m other electron-withdrawing group substituted aryl rings have been shown to resist ring oxidation.% Therefore, from the results in Figure 6, it can be concluded that Athabasca (25) Olson, E. S.; Diehl, J. W.Prepr. Pap.-Am. Chem. SOC.,Diu Fuel Chem. 1984, 29, 217-220.
Structural Features of Alberta Oil Sand Bitumen
Energy 62 Fuels, Vol. 6, No. 1, 1992 89
asphalkne contains condensed aromatic system covalently bonded to the naphthenicaliphatic framework. To account for the products formed, as examples, the following types of condensed aromatic nuclei may be considered.
XI + 12 anthracene
1-5
naphthalene
I
N+9
I11 + 11
phenylphenanthrene
phenanthrene
The bold-face aromatic rings would remain intact while the directly adjoining aromatic carbons would be oxidized to form the appropriate benzene polycarboxylic acid. The appearance of these products provides evidence for the presence of naphthalene-, anthracene-, and/or phenanthrene-type hydrocarbons and biaryl linkages in the asphaltene. On the contrary, the absence of 1,3,5-benzenetricarboxylic acid (verified by the co-injection of an authentic sample) from the oxidation products suggesta that in the condensed systems of asphaltene there are no bridging aromatic systems attached to the 1,3,5-positions of a central benzene ring. Biaryl linkages, while present, are relatively infrequent as judged from the relative concentrations of 9 13 and E5 - 8 + 10 - 12. Alkyl substitution, with the exception of the methyl group, markedly enhances the reactivity of the aromatic ring with regard to RIC026and therefore the yield of the benzene carboxylic acids will depend on the position of the alkyl substituent on the aromatic structure. This is illustrated by the following example:
+
CO2H
+
&-
RCO2H
+
COP
R
RICO
RCOzH
+
COP
For this reason the yields of the benzenepolycarboxylic acids provide only a lower limit for the estimation of the polyaromatic structures in asphaltene. Another important conclusion, based on the absence of benzenepenta- and benzenehexacarboxylicacids among the oxidation products, is that the extent of aromatic condensation-at least in Athabasca asphaltene-is less than was previously thought and pericyclic aromatic systems are not present in Athabasca asphaltene. The highest number of carbon atoms in an aromatic ring which are attached to other aromatic carbons outside the ring is four. That benzenehexacarboxylicacids form in the RICO of the appropriate structure was proven by the oxidation of triphenylene:
1
3
(26)Isley, W. H.; Zingaro, R. A.; Zoeller, J. H. Fuel 1986, 65, 1216-1220.
Of course, heteroatomatic systems would also undergo oxidative degradation. Thus, when benzothiophene was subjected to RICO most of it was completely oxidized. Dibenzothiophene was, however, quantitatively oxidized to its sulfone but alkyl-substituted dibenzothiophenes would be expected to be oxidized beyond the sulfone stage to yield alkanoic acids. Oxidation of Model Compounds. Table I11 lists the model compounds subjected to RICO as well as the product composition and product recovery (where available).6-8vn-31 It can be seen that in many cases the product recovery is less than quantitative and even with simple alkylbenzenes such as propylbenzene, tridecylbenzene, and pentadecylbenzene small quantities of products other than the expected alkanoic acid are formed. Isley et al.%studied the effect of varying the ratio of moles of NaI04 to moles of substrate and from the RICO of pentadecylbenzene using small molar ratios they isolated, in addition to the major product, hexadecanoic acid, small quantities of pentadecylphenone and shorter chain n-alkanoic acids, pentadecanoic and tetradecanoic acids. In our experimenta, the ratio of the oxidant to substrate was very high and we did not detect any phenones. Thus, it is possible that under our conditions the phenones were not formed or were further oxidized to the alkanoic acids. The formation of the lower n-alkanoic acids does not seriously interfere with the quantitative aspects of the oxidation reactions because the loss in the yield of the expected alkanoic acid due to the small quantities of the lower homologous acids produced is compensated for by the yield of side products from the reaction of the next higher homologue, etc. Although the quantitative aspects of the reaction of complex alkylarenes have not been tested, the NMR spectrum of the products indicates that the destruction of the aromatic structure of asphaltene in the RICO reaction is nearly complete, and therefore the numerical error in the data presented in Tables I and I1 cannot be excessive and probably would not exceed *lo%. As seen from Table 111,aryl, alkyl, and saturated cyclic sulfides are all oxidized to their sulfones which are stable against further attack by ruthenium ions. On the other hand, alkylthiophenes and condensed thiophenes must be oxidized to the alkanoic acids since the residual aromatic carbon content of the oxidized reaction mixture is very low (vide infra). Naphthenic Residue. The concentrations of carbon atoms in n-alkyl side chains, methylene bridges, and small aromatic clusters recovered from the RICO of Athabasca asphaltene are 9.2, 3.8, and 0.6 atoms per 100 C in the asphaltene, respectively. These results, however, were derived from analysis of only the low MW distillable fraction of the oxidized products, which was amenable to gas chromatographic analysis. The oxidation also affords a nondistillable residue in ca. 41% yield with elemental composition C 65%; H 7.6%; S 4.7%; N 0.8%; and 0 -22%. NMR analysis shows a very low aromatic carbon content (-3% ), suggesting that the residue consists essentially of oxidized naphthenic-aliphatic materials and the IR and NMR spectra reveal the presence of sulfones. (27)Schuda, P.F.; Cichowicz, M. B.; Heinmann, M. R. Tetrahedron Lett. 1983,3829-3830. (28)Olson, E.S.;Farnum, B. W. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1981,26,60-66. (29)Stock, L. M.; Wang, S. H. Fuel 1987,66, 921-924. (30)Olson,E.S.;Diehl, J. W.; Froehlich, M. L.; Miller, D. J. Fuel 1987, 66,968-972. (31)Olson, E.S.;Diehl, J. W.; Froehlich, M. L.; Miller, D. J. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986,31,97-101.
90 Energy & Fuels,
Vol. 6, No. 1, 1992
Mojelsky et al.
Table 111. Model Compounds Subjected to R u Ion Catalyzed Oxidation % product
substrate
product (s)O
benzene phenanthrene diphenyl sulfide methyl benzyl sulfide pyridylphen ylmethane cyclohexylbenzyl ether propylbenzene tridecylbenzene tetralin bibenzyl 4-pentylbiphenyl phenanthrene triphenylene 9,10-dihydrophenanthrene 1-decene phenylcyclohexane 1-methylnaphthalene isochroman 4-benzyloxobenzoic acid 4-nitroethoxybenzene 9,lO-dihydroanthracenec pentadecylbenzeneC 2,2-diphenylpropane 6-hydroxytetralin 6-methoxytetralin 2-tetralone lignite coals diheptyl sulfide dibenzyl sulfide diphenyl sulfide dibenzothiophene n-alkyl thiolanes and thianes benzothiophene 1- hexadecene 1,12-diphenyl dodecane
vigorous explosion phenanthraquinone diphenyl sulfone methyl benzyl sulfone 4'-pyridylphenone cyclohexyl benzoate butyric acid (95); propiophenone (5) tetradecanoic acid(91); tridecanophenone (9) adipic acid (75); 1-tetralone (8); glutaric acid (17) succinic acid (35); hydrocinnamic acid (63) hexanoic acid (51); benzoic acid (54); 4-pentylbenzoic acid (38) phthalic acid (5); diphenic acid (91); phenanthrenequinone (4) phthalic acid (75); benzenehexacarboxylic acid (25) succinic acid (2); phthalic acid (7); diphenic acid (55); 3-(2-carboxyphenyl)propionicacid (32)
recovery
nonanoic acid cyclohexanecarboxylic acid phthalic acid; 3-methylphthalic acid isochromone 4-hydroxybenzoic acid; benzoic acid ring attacked products anthrone; anthraquinone; anthracene; phthalic acid hexadecanoic acid (69); shorter homologous acids; pentadecylphenone dimethylmalonic acid glutaric acid; succinic acid glutaric acid; succinic acid succinic acid phthalic and other polybenzenecarboxylic acids diheptyl sulfone dibenzyl sulfone diphenyl sulfone dibenzothiophene sulfone thiolane and thiane sulfones completely oxidized pentadecanoic acid 1,12-tetradecanecarboxylicacid
28 31 58 67 90 76 70 100
74 100 55 81 89 94 good yield 95
65
--
99 88 85 100 100
93 100
ref 6b 6b 6* 6b 23 23 8 8 8 8 8 8 d
8 7 7 24 22 22 22 22 22 26 26 26 26 27 d d d d d d d d
Numbers in parentheses indicate % distribution. Acetonitrile as cosolvent was not used. Depends upon cooxidant to substrate ratio. This work.
The primary aim of the present work was to gain insight into the structural elements of asphaltene. Since the npentane-precipitated asphaltene contains significant amounts of complexed resins, the whole asphaltene was separated by acetone extraction into low molecular weight asphaltene (LMA) (MW 890, 16%, composed mainly of resinous substances) and high molecular weight (HMA) asphaltene (MW 6800,84%)as described before14and each of these fractions was subjected to RICO. The oxidation products were methylated with diazomethane and the nondistillable residues separated according to molecular weight into five fractions by GPC (Figure 1). The MW distribution of the fractions collected is reported in Table IV. The surprising feature of the data is the unveiling of the presence of sizable quantities of high molecular weight naphtheniealiphatic core fragments in the oxidation products with number average molecular weight of up to 5500-8700. Even if some degree of molecular association would be present, it could not be extensive in the esterified product and the existence of such large, nonaromatic domains in the asphaltene structure was not recognized before. It is also clear from the results that the oxidized residue from the HMA has a higher proportion of the higher MW fractions than the oxidized residue from the LMA, as expected. Carbon recovery in the oxidation producta of the native asphaltene showed 40% of the carbon appearing in the organic phase, 8% in the water phase, and 29% in the carbon dioxide evolved. The aromatic carbon content of native Athabasca asphaltene has been determined before
Table IV. GPC Separation of Methylated Products" fraction no. 1
MW range >5000
2 3 4 5
1200 660 450 300
organic layer water layer LMA HMA A A 19" (12) 35 (16) 38 (12.2) 43 (3.3) [55001 [8200] [76001 [8700] 19 (12) 22 (10) 23 (7.4) 19 (1.4) 24 (14) 16 (7) 18 (5.8) 16 (1.2) 16 (1.2) 22 (13) 16 (7) 13 (4.2) 6 (0.5) 16 (10) 11 (5) 8 (2.6)
" Weight percent of sample. In parentheses, weight percent of the asphaltene. In brackets, molecular weight. by NMR s p e c t r o ~ c o p yto~ ~be -43%. The difference between C,, (NMR) and C,, (CO,), 43-29 = 14%, is then partly due to the conversion of the aromatic carbon at the site of alkyl attachments to the carboxylic groups of the n-alkanoic acids (2% 1, the a,o-di-n-alkanoic acids (1.2%), and to conversion to benzene di- and polycarboxylic acids (-0.6%). The oxidized residue has an aromatic content amounting to 1.2% of the carbon in the asphaltene and its high oxygen content points to the presence of aromatic carbon-derived carboxylic groups. We will return to the question of carbon recovery after the presentation of additional pertinent results. Product yields for the oxidation of LMA and HMA along with relevant elemental analytical and GPC separation data are given in Table V. All the yield values are lower limits owing to losses due to volatility. Among the important conclusions which can be derived from the data are the following:
Structural Features of Alberta Oil Sand Bitumen
sample substrate acids Me esters
GPC fractions of esters
Energy & Fuels, Vol. 6, No. 1, 1992 91
Table V. Elemental Analysis of Substrates and Oxidized Products yield, wt % wt % (daf) source 1" 2a C H N 0 S ash% 100 16 78.32 8.37 0.96 4.39 7.96 0.00 LMA 100 84 79.29 7.84 1.09 3.83 9.95 2.03 HMA LMA 48.5 7.8 61.36 7.56 0.55 23.73 6.81 23.00 35.2 29.6 64.37 7.83 0.77 21.79 5.24 11.42 HMA LMA 61.5 9.9 66.73 8.73 0.92 18.10 5.52 0.00 46.5 39.1 70.49 7.68 0.69 17.41 3.72 4.25 HMA LMAwC 19.0 3.0 54.35 6.14 1.05 34.22 4.24 9.90 10.0 8.4 HMAwC 11.8 1.9 65.45 7.87 1.34 19.40 5.93 0.96 LMA-1 11.6 1.9 66.74 8.61 0.71 17.61 6.32 8.98 LMA-2 LMA-3 14.5 2.3 67.37 8.87 0.58 17.71 5.48 8.34 LMA-4 2.1 67.54 8.89 0.67 18.22 4.68 4.75 13.4 LMA-5 10.1 1.6 69.14 9.31 0.83 17.99 2.73 0.77 16.1 13.5 61.70 7.50 2.10 23.94 4.76 6.90 HMA-1 10.4 8.7 68.72 8.81 0.93 16.53 4.98 9.21 HMA-2 7.4 6.2 69.67 9.45 0.47 16.94 3.47 5.92 HMA-3 7.3 6.2 70.16 9.93 0.62 16.94 0.88 HMA-4 2.36 18.01 4.5 70.12 9.68 0.88 1.31 0.83 HMAd 5.4 1.34 26.47 3.39 3.3 60.42 6.32 5.45 3.3 AwC 1
1 = wt % of substrate; 2 = wt % of the native asphaltene.
In benzene. Water layer.
1. The molecular weight of the residue from the LMA is the same as that of the asphaltene but for the HMA the molecular weight of the residue is about one-fifth of that of the asphaltene. This is a clear sign that extensive degradation had taken place during RICO. On esterification, the acids from both the LMA and HMA show a drop of about 40% in molecular weight and this is attributable to the decrease in the carboxylic group associations. 2. The (H/C) atomic ratios in the acids are significantly higher than in the asphaltenes owing to the removal of the aromatic carbons and hydrogens with a low (Hmom/Cm0,,J atomic ratio of about 0.22.23 3. All fractions contain nitrogen, oxygen, and sulfur and their concentrations vary from fraction to fraction. 4. The LMA products contain more sulfur than the HMA produds even though the sulfur content of the LMA is lower than that of the HMA. Thus, the amount of sulfur remaining in the oxidation products is 52% in the case of LMA and 21% in the case of HMA, giving a value for the whole asphaltene of 25% which is the same as our previous e~timate.~ The sulfur is primarily present in the form of sulfones as indicated by the intense 1R absorption bands at 1300 and 1130 cm-', characteristic of sulfones, and the 33S NMR32spectrum. Also, since the NMR spectra of the oxidation products show the presence of only -3% aromatic carbon, the sulfones must have formed from the oxidation of saturated sulfides. These results are in agreement with earlier studies from this laboratory showing the presence of cyclic terpenoid sulfides in the LMA16and n-alkyl-substituted thianes and thiolanes as structural units in the HMAS2*That aliphatic and aromatic sulfides are converted to their sulfones under the conditions employed was shown in auxiliary studies (Table III). The rest of the sulfur in the asphaltene is then present as thiophenes, benzc-, dibenze, and higher condensed thiophene^:^ to the extent of 75% in the whole asphaltene and 79% in the HMA. The first unambiguous direct evidence for the existence of saturated sulfide linkages in an asphaltene was provided (32) McIntyre, D. D.; Strausz, 0. P. Mugn. Reson. Chem. 1987, 25, 36-38. (33) Payzant, J. D.; Rubinstein, I.; Hogg, A. M.; Strausz, 0. P. Geochim. Cosmochim. Acta 1979, 43, 1187-1193.
H/C 1.28 1.19 1.48 1.46 1.57 1.57d 1.36
MWb 890 6800 880 1400 530 780 390
1.44 1.55 1.58 1.58 1.62 1.46 1.54 1.63 1.70 1.66 1.26
5470 1200 660 430 300 8230 1200 660 450 300 8690
Calculated from GPC fractions.
in our preliminary studies on the RICO of Athabasca asphaltene in 19862and 198732and this was fully corroborated by the direct detection of n-alkylthianes and thiolanes in the pyrolysis of oil from the same asphalteneZ1and in other a s p h a l t e n e ~in~1988 ~ and 1989.35 The presence of sulfides in asphaltenes is important not only with respect to the molecular structure of asphaltenes but also with regard to the behavior of asphaltenes during thermal processing of the bitumen. The distribution of sulfur in the GPC-separated ester fractions (Table V) shows a definite trend. The fraction most abundant in sulfur is no. 2 for both the LMA and HMA, followed by fractions 1 3 4 5 in decreasing order of importance. The IR absorption intensities of the sulfones also vary with the MW of the fractions and fractions 1-3 feature the most intense absorption while fractions 4 and 5 barely show any sulfone absorption even though their sulfur content is still substantial. The mechanism and molecular form of sulfur removal in RICO are not known and are currently under investigation. The nitrogen content of asphaltene is also greatly reduced in the course of RICO. About 75% of the original nitrogen content, which represents the aromatic nitrogen, is removed and the remaining 25% may be present in nonaromatic forms such as amines, amides, intractable metal complexes, etc. The distribution of the residual nitrogen among the GPC fractions of the esters (Table V) shows the same trend for the LMA and HMA, namely, the highest MW fractions contain more than twice the amount of nitrogen as any of the lower MW fractions and the percent concentration goes through a minimum at fraction 3. The nitrogen content, like the sulfur content, should have decreased during methylation of the acids to produce the esters but the converse happened, the nitrogen content of the esters was slightly higher than that of the acids. This trend points to the incorporation of some nitrogen into the products during the diazomethane treatment of the acids. The fate of the nitrogen lost during RICO is not known and is currently under study.
---
(34) DamstC, J. S. S.; Dalen, A. C. K.-v.;Leeuw, J. W. d.; Schenck, P. A. J. Chromutogr. 1988,435, 435-452. (35) George,G. N.; Gobarty, M. L. In Geochemistry of Sulfur in Fossil Fuels; ACS Symposium Series 429; American Chemical Society: Washington, DC, 1990; pp 220-230.
Mojelsky et al.
92 Energy & Fuels, Vol. 6, No. 1, 1992
As expected, during the oxidation process, large quantities of oxygen are incorporated into all fractions of the products, mainly in the form of carboxylic and sulfone groups. The oxygen contents of the GPC fractions show little variation except that the highest MW fractions contain more oxygen than any of the remaining fractions and the water-soluble portions contain particularly high concentrations, 34% for LMA and 26% for the whole asphaltene. The (H/C) atomic ratios increase during esterification of the acids as a result of the replacement of hydrogen atoms with methyl groups and in the GPC fractions of the esters they exhibit a slightly increasing trend with decreasing MW of the fractions. Pyrolysis of the Naphthenic Residue. In order to further probe into the structure of the oxidized naphthenic-aliphatic core of the asphaltene, the oxidized residue after methylation was subjected to mild thermolysis. The following principal thermolysis products were identified: homologous series of n-alkanesand n-1-alkenes, homologous series of n-alkanoic acid and n-1-alkenoic acid methyl esters, and a short series of unesterified free nalkanoic acids. The n-alkanes and n-1-alkenes are formed by the cleavage of the n-alkyl side chains on the naphthenic core: m
e
r
R6 375 'c, c o r e D *
+
bR
+
+R
+
\R
carbon was originally the site of attachment of the alkyl bridge to the aromatic ring in the asphaltene:
m
r
e
p
i
,etc.
where R = n-alkyl, as well as by the cleavage of the polymethylene bridges between two naphthenic rings:
o
2
"
m
r
+ co,
methylalcon
W N 2
e
p
t
o
2
M
e
Finally, the n-alkanoic acids very likely originated from the thermal decomposition of n-alkanoic acid esters originally present in the asphaltene with their alcoholic portion covalently bonded to the naphthenic core: m e r o ' f i O0 R
*\R
H donor
R R -
Table VI. Yields of Products (wt 70 of Oxidized Residue) from the 375 "C Pyrolysis of the Oxidized Residues alkanes + asphaltene source alkenes esters carboxylic acids Athabasca 2.8 5.6 9.9 Lloydminster 1.1 7.8 3.7 Peace River 5.6 8.1 5.9 Ste~flood 3.6 5.7 5.7 Carbonate Triangle 8.0 6.7 5.0
L 375% c
o
r
e
r
+
HOCOR
This interpretation is supported by the observation that similar series of n-alkanoic acids are produced in the thermolysis and in the saponification of the asphaltene as well.36 Each of the product fractions, namely the n-alkanes + n-1-alkenes, the n-alkanoic + n-1-alkenoic acid methyl esters, and the free n-alkanoic acids, was measured gravimetrically and the results are summarized in Table VI. In every case the lower molecular weight C15portions of the fractions were lost and therefore the results seriously underestimate the true values. The loss, however, can be estimated if it is assumed that the chain length distribution of the aliphatics in the naphthenic systems is the same as in the aromatic systems. We first calculate the amount of carbon present in the naphthenic-attached side chains and bridges from the data in Table VI. This, representing the C15+ members only, is given by 2.8, the percentage of alkanes alkenes X (12/14)(C/CHJ X 0.4 (weight fraction of oxidized residue in terms of asphaltene) = 2.8 X 0.86 X 0.4 = 0.96% of the asphaltene or 0.96 X (1/0.8) = 1.2% of the carbon in asphaltene. In the case of the aromatic moiety of the asphaltene, using the data from Tables I and 11, the C15+portion of the n-alkyl side chains represents -2.9% and the C15+ portion of the polymethylene bridges comprises 0.6% of the total carbon. Therefore, the carbon in the naphthenic-attached chains and bridges is equal to 1.2 X (13/3.5) = 4.5% of the carbon in the asphaltene. The measured yield of the n-alkanoic + n-1-alkenoic acid methyl esters amounts to 2.2% of the asphaltene. Making the same assumption as above for the alkyl side chains and taking the average chain length of the esters as c16 and the correction factor for low MW losses as ((&,~/~c12+)&, in aromatic bridges, we calculate the amount of carbon present in bridges connecting an aromatic ring to a naphthenic ring as 2.2% esters X (MW CIB/MW C1,02H34) X (cbt/zclz+)a.b, = 2.2 x (180/270) x (3,8/1,1)
p* *acore +
core
+
I
A 375%
n-alkane + n-1-alkene
Similarly, the esters are produced by the cleavage of the n-alkanoic acid methyl esters attached to the naphthenic core at their alkyl end: 0
ii
C
'OMe 375 %
-
etc.
The carboxylic groups were formed during the RICO of the aromatic portion of the asphaltene and the carboxylic
(36) McIntyre, D. D.; Montgomery,D. S.; Strausz,0.P.AOSTRA J Res. 1986, 2, 251-265.
Energy & Fuels, Vol. 6, No. 1, 1992 93
Structural Features of Alberta Oil Sand Bitumen = 5.1% of total carbon in asphaltene. If the average lengths of the bridges between two aromatic rings and between an aromatic and a naphthenic ring are the same, then the number of bridges between an aromatic and a naphthenic ring is 0.61 X (5.1/3.8) = 0.82. From the above data it is possible to estimate the degree of alkyl substitution in the aromatic and naphthenic systems in the asphaltene. Thus (Cdkyl/Carom) = (13 2.6)/(43) = 0.36, (C&i/Cm hth)23 = (4.5 + 2.6)/(26) = 0.27 and their ratio, ( C ~ , / C ~ ~ m ~ / ( C & ~ = /1.3, Cm showing ~~~) that the aromatic rings are slightly more alkylated than the naphthenic rings. Lastly, from the measured yield of the n-alkanoic acids, 4.1% of the asphaltene, and with the assumption of an average chain length of CI4,the minimum value for the percentage carbon present in these acids is 3.9% of the total. From these data the number of n-alkanoic acids can be calculated as 0.3 per 100 C atoms. And now, returning again to the question of aromatic carbon balance, the number of aromatic carbons from RICO can be counted as follows: aromatic-attached n-alkyl 1.8 per 100 C 1.2 per 100 C aromatic-attached bridge benzenecarboxylic acids 0.6 per 100 C 1.2 per 100 C aromatic carbon in oxidized residue
+
aromatic/naphthenic-attached bridge COP total from RICO total from NMR
Table VII. Distribution of the Aliphatic Carbon in Athabasca Asohaltene no. of chains C.M in structure no. of Ca or bridees"
o"^^o
r
9.2
1.8
3.8
0.61
3.2
0.63
1.3
0.21
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0.82
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0.3
0 o"^^o 0 -0-C-
II
total from RICO total from NMR
-
26.5 27
Per 100 C atoms in asphaltene. Scheme I
0.8 per 100 C 29.0 per 100 C 34.6 per 100 C 43.0 per 100 C
Thus, the products of the RICO account for about 80% of the aromatic carbon originally present in the asphaltene. The missing 20% may then be attributable to losses due to volatility, occurrence of side reactions, appearance of unidentified products, inherent errors, and decarboxylation of the carboxylic acid products on thermolysis of the residue converting aromatic-derived products to apparent aliphatic-derived products.36 Assuming that in the naphthenic systems the distribution of chains and bridges and the number of carbon atoms in them is the same as in the aromatic systems, their values can be estimated: no. of chains = 3.2 X (1.8/9.2) = 0.63/100 C in asphaltene no. of bridges = 1.3 X (0.16/3.8) = 0.21/100 C in asphaltene no. of carbon atoms in chains = 4.5 X (9.2/13) = 3.2/100 C in asphaltene no. of carbon atoms in bridges = 4.5 - 3.2 = 1.3/100 C in asphaltene The results on aliphatic carbon content are summarized in Table W from which it is seen that the sum of aliphatic carbons estimated in the present study is the same, -27%, as obtained from previous NMR measurement^.^^ The total number of aliphatic chains can also be estimated. Not counting directly ring-attached methyls, the number of aromatic-attached n-alkyl side chains is 1.84-0.49 = 1.35. The number of naphthenic-attached n-alkyl chains is 0.63 and the number of n-alkanoic acids present in ester form is 0.3, all per 100 C atoms in the asphaltene. Thus,the sum, 1.35 + 0.63 + 0.3 = 2.3, is equal to the number of terminal methyl groups that can be determined by NMR spectroscopy. The value reported was 3 but the actual value should have been less, about 2.3, and thus the agreement between the two measurements is again good.
A
-2H
A
-6H
Geochemical Aspects of Asphaltene Structure. As was pointed out above, both the aromatic rings and the naphthenic rings of Athabasca asphaltene contain sulfur. This is known from thermolysis studies on Athabasca asphaltene in which, in addition to the thiophene structures mentioned before, significant quantities of homologous series of 2-n-alkyl- and 2,5-di-n-alkyl-substituted thiolanes and thianes (Srmalkyl e m a l k y l ~ m a l k y ~l
m
a
W etc.
14
15
16
17
have been identifiedq2I From the sulfur content of the HMA oxidation products it was estimated that about 21% of the sulfur is present in these types of naphthenic structures. Therefore, a significant fraction of the aliphatic carbons is associated with naphthenic sulfur compounds. Moreover, it has also been shown that all of the sulfur heterocycles, aromatic and naphthenic, possess a unique substitution pattern clearly indicative of their n-alkanerelated origin. Examples of these are the above thiane and thiolane structures and thiophene structures 2-4. An identical substitution pattern was recently found for the n-alkyl-substituted fluorene hydrocarbons 1 in the pyrolysis oil of Athabasca asphaltene as In fact, all homologous series of compounds from the pyrolysis oil identified thus far conform to this substitution pattern showing that the asphaltene or a significant portion of it originated by the catalytic thermal cyclization of n-alkanes or n-alkanoic precursors (Scheme I).
Mojelsky et al.
94 Energy & Fuels, Vol. 6, No. 1, 1992 1
,
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Figure 7. Gas chromatograms of n-alkanoic acid methyl esters from the Ru ions catalyzed oxidation of various asphaltenes.
Ii I I
Carbonate Triangle ?9
Athabasca ,--__ U 130
200
Temperature ("C) I
I
I
8
10
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1
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Figure 9. Gas chromatograms of the benzenepolycarboxylic acid methyl esters from the RICO of various asphaltenes. Peak labels as in Figure 6.
1
% I
18
Peace River
Figure 8. Gas chromatograms of a,w-dicarboxylic acid methyl esters from t h e Ru ions catalyzed oxidation of various asphaltenes.
n-alkanes. Eglinton and c o - w ~ r k e r shave ~ ~ , subjected ~~ a variety of type I, 11, and I11 kerogens to RICO and found that, in all cases, straight-chain acids were the predominant products. It follows, then, that the n-alkanoic framework so predominant in Athabasca asphaltene is probably a major structural feature of other asphaltenes as well. Microbiologicalremoval of the n-alkane complement from the original oil that was the precursor to the Athabasca bitumen then produced the bitumen as it exists today. The n-alkyl moieties of the asphaltene and other high molecular weight fractions of the bitumen, however, were protected from microbial attack by the micellar structures of these materials. Consequently, asphaltene may be viewed as a most valuable biomarker in petroleums and bitumens in particular, possessing a direct information content with regard to the nature of the precursor oil. Returning to the question of the origin of the n-alkanoic acids in the pyrolysis products of the oxidized residue, it should be noted that their carbon number ranges from ca. C, to 422 and the series is dominated by the even carbon number members, and in particular, the CI6 member.
If the asphaltene is an intermediate stage in the decomposition of kerogen to oil, as is generally assumed, then the kerogen and its derived oil must have been rich in
(37) Boucher, R.J.;Standen, G.; Eglinton, G. Fuel 1991, 70,695-702. (38) Boucher, R. J.; Standen, G.; Patience, R. L.; Eglinton, G. Org. Geochem. 1990,16, 951-958.
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Energy & Fuels, Vol. 6, No. I, 1992 95
Structural Features of Alberta Oil Sand Bitumen Athabasca
14
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Figure 10. Gas chromatograms of the n-alkanes (2) and 1-alkenes (1)from the 375 O C pyrolysis of the (methylated) oxidized residues of various asphaltenes.
Thus, they are clearly of recent origin and must have been produced in the course of the secondary microbiological degradation of the precursor oiF9and became chemically bound to the asphaltene by an ester linkage. Small quantities of n-alkanoic acids dominated by the c16 and C18 members are present in the a ~ p h a l t e n e ,in~ ~the maltene fraction of the bitumen,@and in chemically bound form on the sa11d.~' Comparative Studies. The focus of interest in the present work was the structural details of the Athabasca asphaltene. However, in order to obtain an insight into the structural variability of asphaltenes, some preliminary studies were also carried out on a few asphaltene samples from the Western Canadian sedimentary basin. Investigated were asphaltene samples from Peace River bitumen, Peace River steam-produced bitumen, Carbonate Triangle bitumen, and Lloydminster heavy oil asphaltene. On RICO each of these asphaltenes yielded a series of n - W i o i c acids (Figure 7), a,w-di-n-alkanoicacids (Figure 8),and benzenecarboxylic acids (Figure 9), with closely similar distributions in approximately the same carbon number range. The gravimetric results of these experi(39)Mackenzie, A. S.; Wolff, G. A.; Maxwell, J. R. In Adoances in Organic Geochemistry, 1981; Bjplroy, M., Albrecht, C., Cornford, C., de Groot, K., Eglinton, G., Galimor, E., Leythaeuser, D., Pelet, R., Rullkatter, J., Speers, G., Eds.; Wiley-Heyden: Chichester, U.K., 1983; pp 637-649. (40)Cyr, T.D.;Strausz, 0. P. J. Chem. Soc., Chem. Commun. 1983, 1028-1030. (41)C y , T.D.;Strausz, 0. P. Org. Geochem. 1984,7,127-140.
14
16
18
20
Carbon Number Figure 11. Gas chromatograms of the carboxylic acids from the 375 O C pyrolysis of the (methylated)oxidized residues of various
asphaltenes.
ments are listed in Table VI. A close inspection of the GC traces of the n-alkanoic acids in Figure 7 reveals that the smooth distribution of the acids is perturbed by the appearance of a slight excess of the (216 and c18 members. This excess is probably due to the liberation of the small quantities of the c16 and c18 acids that remained from the microbiological degradation of the precursor oil and were trapped in the asphaltene micelles by the destruction of the asphaltene molecules during the oxidation process, or alternatively, to their formation by the slight hydrolysis of n-alkanoic acid esters under the acidic conditions of the oxidation. I t is remarkable that even this slight irregularity in the concentration distribution of the acids is so clearly recognizable in each case, with the exception of the steam-treated Peace River sample where the increased thermal stress and water washing appear to have reduced the excess amounts of the C16and c18 members. The types of benzenecarboxylic acids produced in the oxidation are also similar throughout the asphaltenes, with the exception of the steam-treated Peace River sample where the concentrations of peaks 6-13 relative to peak 5 (the 1,2-benzenedicarboxylic acid, which is the most abundant in all the samples) are greatly reduced. The similarity in the nature of the products, their carbon range, and concentration distribution from the five asphaltene samples studied further extends to the pyrolysis products of the oxidized residues as well. This can be seen from the GC traces reproduced in Figure 10 for the n-alkanes + n-1-alkenes, and in Figure 11 for the n-alkanoic acids.
96 Energy & Fuels, Vol. 6, No. 1, 1992
The GC traces of the n-alkanoic acids deserve some comment in that all five samples show a distinct preference for the even carbon number members in the C7-Czzrange dominated by the CI6and c18 members, and the presence of two or three smaller peaks at in addition to the n-octadecanoic acid. These small peaks are probably due to the monounsaturated n-C18-9-cisand n-C18-ll-cis acids and the diunsaturated n-C18-9,12-cis,cisacid, as have been found in previous studies on Athabasca oil sand^^^^"' and they provide additional evidence for a not-too-distant microbiological degradation of these oils. From the results of these comparative studies, one can clearly conclude that all five asphaltene samples investigated have closely similar molecular structures and the small variations apparent may be due to slight differences in reservoir conditions (temperature, chemically active minerals, water washing, activity of sulfate-reducing bacteria and other microbiological processes, etc.). Consequently all five asphaltenes must have originated from n-alkane-derived kerogens. These findings on asphaltene structure corroborate earlier observations on the similarities between various Alberta oil sand maltene components and asphaltenes and are also in line with the suggestion of V i g r a s ~that ~ ~ the bitumens in the various Alberta ac(42)Vigrass, L.W.AAPG Bull. 1968,52, 1984-1999.
Mojelsky et al.
cumulations belonged to a single oil system. The results of the present study on asphaltenes are similar to those from earlier studies from this laboratory3 on the RICO of the nondistillable (230 OC, Torr) portions of the aromatic and polar fractions of Athabasca bitumen. These findings, taken together, constitute clear evidence that the high molecular weight portions of the deasphaltened bitumen are composed of incompletely degraded asphaltene fragments in which the n-alkyl moieties were also at least partially protected from microbial attack and that these high molecular weight portions were derived from asphaltene. This is in sharp contrast to the composition of the lower molecular weight distillable portions of Athabama bitumen which are devoid of n-alkanoic structural moieties and are mainly composed of terpenoid structures. Further studies on the ruthenium ion catalyzed osidation of various asphaltene and maltene fractions are currently in progress.
Acknowledgment. We thank the Alberta Oil Sands Technology and Research Authority and Natural Sciences and Engineering Research Council for financial support, Dr. Theodore J. Cyr for many helpful comments and discussions, and Syncrude Canada Ltd., Husky Oil,Shell Oil, and Union Oil for kindly providing samples. Registry No. (CH,),(SRU),25038-57-7.
