PETROLEUM SCIENCE AND TECHNOLOGY

Vol. 22, Nos. 5 & 6, pp. 631–645, 2004

Structural Studies on Residual Fuel Oil

Asphaltenes by RICO Method

Mohammad Farhat Ali,1,* Mohammad Nahid Siddiqui,1 and

Adnan Ahmed Al-Hajji2

1Department of Chemistry, King Fahd University of

Petroleum and Minerals, Dhahran, Saudi Arabia2Lab. R&D Center, Saudi Aramco, Dhahran

ABSTRACT

Structural characterization of asphaltenes isolated from Saudi

Arabian heavy and medium crude oils was undertaken by using

ruthenium ion catalyzed oxidation (RICO) method. The RICO

method was capable to convert aromatic carbons selectively into

carbon dioxide and carboxylic acids and esters group while leaving

aliphatic and naphthenic structures of asphaltenes essentially

unaffected. Detailed analyses of RICO products of both Arab

heavy and Arab medium asphaltenes were conducted using FT-IR,13C-NMR, IC, GPC, and GC-MS techniques. These analyses

indicate that the aqueous phase fraction (water-soluble products)

obtained from RICO reaction of asphaltenes consists of aliphatic

*Correspondence: Mohammad Farhat Ali, Department of Chemistry, King

Fahd University of Petroleum and Minerals, Dhahran 31261, , Saudi Arabia;

Fax: +9663 860 4277; E-mail: mfali@kfupm.edu.sa.

631

DOI: 10.1081/LFT-120034205 1091-6466 (Print); 1532-2459 (Online)

Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com

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dicarboxylic acids and aromatic poly carboxylic acids with longer

alkyl chains. The 13C-NMR and GC-MS analyses of organic phase

products of asphaltenes indicate that this fraction contains large

amount of aliphatic carboxylic acids with longer alkyl groups. The

oxidation products of both Arab heavy and Arab medium

asphaltenes were found to be dominated by a homologous series of

straight chain monocarboxylic acids suggesting that the normal alkyl

chains are major and important constituents of the chemical

structure of both asphaltenes.

Key Words: Asphaltenes; RICO reaction; IR; NMR; HP-GPC.

INTRODUCTION

The contribution of heavier crudes and petroleum residues to theworld petroleum refining industry is on the rise due to the increasingdemands for transportation fuels. These heavy feedstocks containsignificant amounts of asphaltenes, which are solubles, randomlypolydispersed organic geomacromolecules. Asphaltenes creates manyproblems during crude oil production and refining operations. They areconcentrated in petroleum residue and are responsible for high molecularweights, viscosity, density, heteroatoms, metals, and boiling points. Theyare the precursors for coking and hydrocracking of crude oils andresidues. In order to minimize coke formation and develop new, moreefficient technologies for heavy oil upgrading, a better and morecomprehensive understanding of asphaltene chemistry is essential.Several studies for the elucidation of the molecular structure of petroleumasphaltene have been carried out and published (Seki and Kumata, 2000;Speight, 1994; Speight et al., 1985; Storm et al., 1993; Yen et al., 1984). Atpresent time, a generalized picture of asphaltene molecules is that theyhave a very low hydrogen-to-carbon ratio and consists of condensedaromatic nuclei that carry alkyl and alicyclic systems held together byvalence bonds between heteroatoms such as sulfur, oxygen, and metals(Speight, 1987, 1990). Support to this structure is also given by otherworkers (Ali and Saleem, 1994; Siddiqui and Ali, 1999a), who describedasphaltene molecule be made up of three to five unit-sheets consisting ofcondensed aromatic and naphthenic rings with paraffinic side chains.These sheets are held together by heteroatoms such as sulfur or nitrogenand/or polymethylene bridges, thioether bonds, and vanadium and nickelcomplexes.

RICO, first introduced by Djerassi and Engle (1953), is a chemolysismethod that can be selectively applied to coal, petroleum residues,

632 Ali, Siddiqui, and Al-Hajji

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and asphaltenes. By this method aromatic carbons are convertedpreferentially to carbon dioxide and/or carboxylic acids while leavingaliphatic and alicyclic portions intact. This method was introduced tofossil fuel chemistry by Stock and Tse (1983) and followed by otherresearchers (Mallaya and Zingaro, 1984; Zijun et al., 1997).

Strausz et al. (1992, 1998) were the first group applied the RICOreaction to asphaltenes to recognize aliphatic types. They processed theinvaluable information from the RICO reaction along with thosefrom NMR and pyrolysis studies to comprehend the structure ofAlberta oil and asphaltenes. Nomura et al. (1998, 1999) have processedthe information from the NMR work of the asphaltene sample togetherwith data from the RICO reaction of the asphaltene to elucidate thedistribution of the aliphatic carbons more precisely. Mendez et al. (2001)studied the suitability of catalytic oxidation with RuO4 for establishingstructural differences between an air-blown and a thermally treated pitch.Murata et al. (2001) conducted the detailed analyses of RICO products ofbituminous and brown coals using FD/MS and 13C-NMR.

This work presents the results of a study in which the RICO methodwas used to probe the structural details of asphaltenes isolated fromSaudi Arabian crude oils.

EXPERIMENTAL

Arab heavy and Arab medium crude oils were collected from theproduction and storage facilities of Saudi Aramco through the courtesyof Laboratory R & D Center. Asphaltenes were removed as n-heptaneinsoluble material from the resid using the same method described earlier(Siddiqui et al., 1999b).

RICO Reaction

One point zero gram of asphaltenes, 30.0mL distilled water, 20.0mLcarbon tetrachloride, 20.0mL acetonitrile, 15g sodium periodate, and40mg ruthenium trichloride hydrate were introduced in a 250-mLthree-neck round bottom flask. The reaction was carried out in the flaskby heating in an oil bath at ’40�C for 24 h with continuous magneticstirring. N2 gas was passed through one inlet and the resulting CO2 gaswas purged through drying tube containing anhydrous CaCl2. Theamount of CO2 formed was absorbed in 300mL of 0.1M NaOH solutionthat was later titrated against 0.05M H2SO4 solution.

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The further work-up of the reaction product was carried out by thefollowing two different filtration methods.

Filtration Method #1

At the end of the reaction, the product was filtered under vacuumfiltration to get first filtrate and the precipitate being washed with 50mLof 5% NaOH solution to get the residue and another filtrate. Thirty milliliters of extra base solution of 5% NaOH was added to first filtrate. Bothfiltrates were combined and extracted two times with 50mL of CH2Cl2 ina 250-mL separatory funnel. Organic phase and aqueous phase wereseparated. Aqueous phase was further diluted to 1000mL with distilledwater and sample was ready for lower molecular weight carboxylic acidsdetermination by ion-chromatogram (IC) technique.

Filtration Method #2

Reaction started with again 1.0 g of asphaltenes and introduced allthe chemicals as worked out in filtration 1. At the end of the reaction, thereaction product was filtered under vacuum filtration to get filtrate andthe residue was washed with 50mL CH2Cl2 and 50mL H2O. It affordedinsoluble residue that was further treated with 25mL of 5N HCl andfiltered under vacuum to get the black colored residue.

The CH2Cl2 and water solubles were separated into organic phaseand aqueous phase. Aqueous phase was further extracted with another50mL of CH2Cl2 to get another organic phase and the aqueous phasewas evaporated to dryness at 40�C. Both organic phases were combinedand CH2Cl2 was evaporated using N2 gas that afforded organic residue.Aqueous phase and organic residue were methyl esterified by an etherealsolution of diazomethane.

Preparation of Diazomethane

A solution was prepared by mixing 2.0 g KOH in 3mL distilled waterand 10mL of 95% ethanol in a 250-mL side arm flask A. The solutionwas kept in an oil bath and the bath temperature was maintained between62 and 65�C. Flask A was connected to another flask C and was kept inan ice bath with some salt. This side arm flask was connected to a

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graduated cylinder E by a delivery tube. Graduated cylinder E containing20mL diethyl ether was kept in an ice bath D with some salt.

Eight point six grams of diazogen was dissolved in 40mL of diethylether in a small separatory funnel B. Solution from B was poured slowlyto flask A that liberated some gas and absorbed by 10mL diethyl etherpresent in cylinder E. Complete transfer of solution from flask B to A wasdone in 20min and the temperature of the bath was remained in the rangeof 65–70�C. At the end of the reaction diazomethane prepared ingraduated cylinder E was later transferred into three small 50-mL flasksand covered with rubber septum.

Esterification of Aqueous Phase and

Organic Phase of Filtration

Two point zero grams of aqueous phase residue of each asphalteneswere weighed out into two small 25-mL flasks. The weighed residue wasdissolved in the mixture of 10mL CH2Cl2 and 10mL ether and 7mL ofdiazomethane was added in each flask. The contents of flask were filteredafter 5min. Zero point one gram of organic phase of each asphalteneswere also dissolved in 6mL of CH2Cl2 and the solution was transferredequally into two small 25-mL flasks. In flask 1, 2mL of diazomethanewas added and evaporated it to dryness by blowing with N2 gas. Extra2mL CH2Cl2 and 2mL diazomethane were further added. Similarly inflask 2, 4mL of diazomethane was added, shaken, and dried with N2 gas.Another 2mL of CH2Cl2 and 4mL diazomethane were added and lateron the solvents were evaporated.

Instrumentation

Autospec-Q from Micromass, UK, interfaced to HP 5890 Series IIGC-Mass Spectrometer was used in this study. The mass spectrometerwas tuned using PFK (perforated kerosene) to a resolution of 1000 on10% valley. A mass spectrometric experiment used the ‘‘Magnet’’ modescanning from 600 to 20 amu, MS source temperature was set at 200�C,Electron Energy was set at 70 eV and trap, Current at 300mA. The massspectrometric was calibrated by PFK using the above experiment. For asatisfactory GC peak resolution, the following were employed: DB-1column (60m long, 0.25 ID, 0.25 mm), GC temperature program: 50 to320�C at 10�C/min and then isothermal at 320�C for 20min, GC/MSinterface temperature was set at 310�C, GC injector temperature was set

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at 320�C with a split ratio of 1:150 using high purity helium at a flowrate of 1mL/min.

The use of IR, NMR, and HPGPC have been described earlier elsewhere (Siddiqui et al., 1999b). Infrared spectra were recorded on a PerkinElmer Model 1610 infrared spectrophotometer loaded with Infra RedData Manager (IRDM) software. 13C and 1H-NMR spectra wererecorded on a Varian XL-200 Pulse Fourier Transform (PFT) spectrom-eter operating at 200MHz using 5mm sample tubes. A Waters HPLCsystem 840 was used with a Model 501 pump, a 712 WISP auto injectorand an R-401 refractometer as a detector. The molecular weightdistribution was obtained using a Millennium 2010 ChromatographyManager program.

RESULTS AND DISCUSSION

Table 1 shows the characteristic properties for Arab heavy and Arabmedium asphaltenes. Arab heavy asphaltenes have higher GPCmolecular weight distributions and Ni and V contents than Arabmedium asphaltenes. Arab heavy asphaltenes have relatively morealiphatic hydrogen while Arab medium have a little more amount ofaromatic hydrogen. The aliphatic carbon contents of Arab mediumasphaltenes were higher and the aromatic carbon contents were lowerthan Arab heavy asphaltenes.

The amounts of lower carboxylic acids obtained from filtrationmethod 1 in RICO reaction are given in the following Table 2.

Ion chromatogram analysis of both asphaltenes showed a largeamount of oxalic acid followed by propionic acid. The amount of aceticacid in both asphaltenes found to be same and lowest. There are not anyspecific scientific reason for the production of higher amounts of oxalicacid but could be attributed to the structures and composition of bothArabian asphaltenes. These aliphatic monocarboxylic acids are stable at40�C in the RICO procedure indicating that they are the product of

Table 1. Characteristic properties of asphaltenes.

Asphaltenes

C

(%)

H

(%)

GPC

(MW)

Ni

(ppm)

V

(ppm)

Har

(%)

Hal

(%)

Car

(%)

Cal

(%)

Arab heavy 83.22 8.25 1866 19 60 8.1 91.9 36.0 64.0

Arab medium 83.71 8.33 1611 15 45 10.2 89.9 30.8 69.2

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asphaltenes oxidation only. In the filtration 2 procedure, reaction

mixture was separated into three portions: (i) solid precipitate, (ii)

aqueous phase, and (iii) organic phase. The solid precipitate was found

to be an inorganic material, insoluble in CDCl3 and CH2Cl2. An IR

spectrum of residue was not very informative as there were no absorption

peaks in the fingerprinting region. No infrared and NMR spectra could

be recorded for the aqueous phase solid of the reaction. Since it was

water-soluble material hence remain insoluble in organic solvents for

NMR studies. IR spectroscopy was again not much helpful as peaks were

not sharp and resolved and no significant peaks were noted.The organic phase was studied by NMR and IR spectroscopy. The

infrared spectra of both Arab heavy and Arab medium asphaltenes

displayed a distinct and very important C¼O stretch absorption at

around 1718 cm�1 covering the area 1625–1800 cm�1 due to carbonyl

and/or carboxyl groups. The carbonyl group absorption peaks cover the

region containing the absorption bands for carboxylic acids, ketones, and

anhydrides. Another very strong and broad peak was found between

2700 and 3600 cm�1 and centered at 3428 cm�1. This is very typical and

characteristic peak of OH hydrogen bond of carboxylic acids.A sharp and strong peak at 1258 cm�1 was related to the anti-

symmetric stretching vibration of C-O-C group suggesting the formation

of either in the form of ether or esters along with carboxylic acids. A

small and visible absorption band at 936 cm�1 was attributed to the

presence of C-OH deformation vibration. There were some more well

resolved and strong peaks at around 790, 730, and 578 cm�1 which could

be associated to the O-C¼O and C-C¼O bending vibrations of

carboxylic acids and esters. The absence of another important absorption

band at 1032 cm�1 indicates the removal of S atoms otherwise formation

of S¼O groups due to oxidation was well expected. All absorption bands

were well resolved, strong and clear, thus suggesting the completion of

oxidation reaction and product obtained was a carboxylic acid. The GPC

Table 2. Yields of lower carboxylic acids from RICO reactions.

Asphaltenes

Acetic

acid

(ppm)

Formic

acid

(ppm)

Oxalic

acid

(ppm)

Propionic

acid

(ppm)

Arab heavy 10 10 39 23

Arab medium 10 19 62 25

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molecular weight distribution of aqueous and organic phases of Arabheavy and Arab medium asphaltenes is shown in the Table 3.

GPC, with separation based on the size and shape of molecules, givesthe number average molecular weight (Mn), the weight average molecularweight (Mw), and the peak molecular weight at the peak apex for eachintegrated peak (Mp). In the two phases, organic phases have the highermolecular weight distribution than aqueous phases; indicating thatorganic phases possess carboxylic acids with larger molecules. Arabheavy asphaltenes aqueous and organic phases have higher molecularweight distribution than Arab medium asphaltenes.

The proton NMR spectra of organic phase of Arab medium andArab heavy asphaltenes demonstrate the presence of oxygen functionalityat � 5.30 ppm. Proton NMR spectra can quantitatively illustrate the typesof protons present in the carbon skeleton but difficult to quantify anyprotons attached to an electronegative atom like oxygen. The carboxylicprotons appear down field after � 10 ppm. The RICO reaction of Arabmedium asphaltenes shows that organic phase contains approximately97.0% aliphatic hydrogens, 2.1% hydrogens attached to oxygenfunctionality and merely 0.9% aromatic hydrogens. Thus, indicatingthat almost all-aromatic ring system is substituted. Whereas, in the caseof Arab medium asphaltenes, aliphatic hydrogens were found to be 96%,aromatic hydrogens 3%, and oxygen functionalities containing hydro-gens were 1%. The comparison of these two asphaltenes indicates thatstructure of asphaltenes play a vital role during the RICO reactionleading to different ratios of hydrogen atoms.

Esterification of carboxylic acids produced in aqueous and organicphase following the RICO reaction of asphaltenes was performed in situby freshly prepared diazomethane from diazogen. The ethereal solutionof diazomethane was used immediately after preparation. The IR spectraof esterified aqueous phase of Arab medium asphaltenes gave a verysharp and intense absorption band at 1732 cm�1 assigned to the C¼O

Table 3. GPC molecular weight distribution of aqueous and organic phases of

RICO reactions.

Sample Mn Mw Mp Polydispersity

Arab heavy-aqueous 290 365 275 1.26

Arab heavy-organic 163 454 276 2.79

Arab medium-aqueous 102 233 266 2.28

Arab medium-organic 151 373 253 2.48

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stretching vibration of ester. Another sharp peak appeared in theesterified aqueous phase was at 1272 cm�1 attributed to the C-O-C anti-symmetric stretching vibration. A small unresolved hump, found at3452 cm�1, could be due to the hydrogen bonding among theun-esterified carboxylic acids left in smaller amounts. Some other sharppeaks observed at 1124, 1072, and 744 cm�1. The C-H stretchingvibrations of CH3 at 1380 cm�1 and CH2 at 1460 cm�1 were otherprominent absorption bands observed. The infrared spectra of esterifiedorganic phase produced similar pattern of functional groups frequenciesin the fingerprinting region. The broader peak centered at 3464 cm�1 wasmore pronounced. Carbonyl group stretching vibration at 1732 cm�1,aromatic C¼C stretching vibration at 1600 cm�1, aliphatic C-H stretch-ing vibration of CH2 at 1462 cm�1 and CH3 at 1378 cm�1 were mostresolved and strong peaks. Other absorption bands similar to aqueousphase were found at 1274, 1124, 1072, and 744 cm�1. Combining theinfrared information of aqueous and organic phases of Arab mediumasphaltenes shows the formation of carboxylic acid and esters during theRICO reactions. The product types could well be identified using GC/MStechniques.

The esterified aqueous and organic phases of Arab heavy asphaltenesproduced almost similar trend of absorption peaks in the fingerprintingregion of infrared spectra. The most prominent peaks found werecarbonyl at 1730 cm�1, methylene at 1462 cm�1, methyl at 1378 cm�1,ether linkage at 1274 cm�1, and aromatic C¼C at 1600 cm�1. Again abroad peak centered at 3446 cm�1 was more intense in organic phase thanaqueous.

A point need to be emphasized here is the amount of diazomethaneethereal solution used. The same esterification reaction was carried outby three different amounts of diazomethane solution and the resultingproducts were found to be same without any visible changes in theinfrared spectra of esterified product. Hence, it indicates that theesterification reaction was completed when first batch of etherealdiazomethane solution was added and there were no traces of unreactedcarboxylic acids in either aqueous phase or organic phase of products.Carbonyl groups have no direct representation in proton NMR spectra,so 13C-NMR spectroscopy provides unique information of their analysis.It emphasizes the importance of 13C-NMR spectroscopy in determiningoxygen types attached to carbon skeleton.

The chemical shifts in 13C-NMR spectroscopy related to this studycould be classified into following categories: � 0–50 ppm aliphaticcarbons; � 50–65 ppm methoxy/alkoxy carbons; � 110–160 ppm aromaticcarbons. The region � 160–185 ppm was assigned to carboxylic derivative

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carbons which was further specified as: � 160–170 ppm carboxyl groupconnected with aromatic carbons (esters/anhydrides) and � 170–185 ppmcarboxylic groups connected with aliphatic carbons (acids).

Percent distributions of carbon types in aqueous and organic phasesobtained by 13CNMR are given in Tables 4 and 5. 13CNMR spectraof esterified aqueous phase of Arab medium asphaltenes shows sharppeaks in the aliphatic region (0–50 ppm) at 10.70, 13.79, 22.17, 23.50,24.60, 28.67, 30.11, and 38.47 ppm. These integrated peak areas of thespectra includes �, �, �, and � carbons in paraffinic straight chains whichaccounted to be 40.7%. The methoxy or alkoxy carbons (� 51–53 ppm)were found to be 24.8%. Another peak at 67.68 ppm was assigned to thealkoxy group attached directly to the aromatic rings (6.6%). There werethree strong and sharp peaks in the � 128–132 ppm region, which con-tributed 19.3% of the carbon. These peaks represents the combination

Table 4. Percent distribution of carbon types in Arab

medium RICO reaction products.

Chemical

shifts (� ppm)

AM-ARE

(%)

AM-OR

(%)

0–50 40.7 52

51–53 24.8 15

67–68 6.6 4.6

128–132 19.3 12.4

167 6.4 5.5

191 2.2 9.2

Table 5. Percent distribution of carbon types in Arab heavy

RICO reaction products.

Chemical

shifts (� ppm)

AM-ARE

(%)

AH-OR

(%)

0–50 49.6 73.5

51–53 13.4 9.6

67–68 7.3 2.9

128–132 22.5 8.8

167 7.3 2.2

175 2.9

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of ortho, meta, and para substituted aromatic carboxylic acids. Another

peak at 167 ppm was attributed to the carboxyl carbon attached to the

aromatic ring system (6.4%). The remaining peak at 191 ppm could not

be assigned, however, it accounted for 2.2% of the total carbons.The organic phase of Arab medium asphaltenes shows same peaks

with different intensities and one additional peak at 174 ppm. This

additional peak (9.2%) was assigned to the carboxyl carbons connected

to the alkyl groups. However, unassigned peak in Arab medium aqueous

phase at 191 ppm was missing. The organic phase was more populated in

the aliphatic carbon region indicating the presence of open chain esters in

substantial amount. Similar trend was observed in the distribution of

carbons in the aqueous and organic phases of Arab heavy aphaltenes.

The aliphatic carbons (� 0–50 ppm); alkoxy/methoxy carbons (� 51–

53 ppm); and aromatic acid carbons (� 128–132 ppm) were found to

constitute higher number of carbons in aqueous phase than organic phase

of Arab heavy asphaltenes. There was an additional peak at 175 ppm in

the organic phase while missing in aqueous phase.On comparing the aqueous and organic phases of Arab medium and

Arab heavy asphaltenes, it was observed that aqueous phase of Arab

heavy asphaltenes possess higher amounts of aliphatic, alkoxy/methoxy,

and aromatic acid carbons than Arab medium aqueous phase. The

organic phase of Arab heavy has higher contents of aliphatic carbons

only and rest all types of carbons were lower in amounts than Arab

medium organic phase asphaltenes. The identification of these carbons

can best be achieved by using GC/MS spectrometry.The mass spectrum of each component was recorded for the

identification of acids and esters both in organic and aqueous phases.

Table 6 shows some of the main peaks in total ion chromatogram (TIC)

of Arab heavy and Arab medium organic and aqueous phases. Some of

these peaks correspond to a series of benzene polycarboxylic acid methyl

esters [C6H6�n (COOCH3)n], (n¼ 3–6). A series of biphenyl polycar-

boxylic acid methyl esters [C12H10�n(COOCH3)n], (n¼ 6–10) have also

been identified. GC/MS analysis results agree with GC analysis of same

fractions. Carboxylic acid yields identified from organic and aqueous

phases of RICO of Arab heavy and Arab medium asphaltenes are

recorded in Tables 7 and 8.The EI mass spectra of esterified RICO fractions showed an

abundance of low molecular weight esters as well as medium to high

molecular weight esters. Ions corresponding to COOHþ (m/z 45) or the

loss of the radical COOH� are observed for short-chain aliphatic acids.

The McLafferty rearrangement leads to the most characteristics ions at

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m/z 60, 74, 88, etc., and these ions, formally RCO2Hþ, can be used to

characterize the position and extent of branching along with alkyl chain.Fragmentation at the ether oxygen gives acylium ion (RCOþ)

formation, resulting in the loss of methoxyl radical in the case of methyl

esters. The product ion subsequently undergoes CO loss to form a

carbonium ion (Rþ) especially when Rþ is secondary or tertiary.

Fragmentation via the alkyl substituent on the carbonyl group also

occurs by McLafferty rearrangement (�-cleavages, with H transfer), to

give m/z 74 in methyl esters. �-Cleavages yields m/z 87 in methyl esters,

which is accompanied by ions at m/z 143, 199, 255, and so on. All these

fragments are present in the mass spectra of RICO organic phase

fractions (Lambert et al., 1998; McLafferty, 1980). Aromatic esters

identified from their mass spectra show distinctive molecular ions that

increase in relative abundance with increasing molecular weight. If the

aromatic ring contains alkyl groups, these will undergo the typical

fragmentations of arylalkanes. The molecular ion peak at m/z 178 and

fragment ions at m/z 56, 77, 105, 123, and so on are distinctly visible. The

Table 6. Main peaks (m/z) in total ion chromatograms

of aqueous and organic phase fractions of asphaltenes.

Organic phases Aqueous phases

AH AM AH AM

51 139

88 161

132 160 195

198 200 200

253 233 221 263

299 266 268 295

338 298 474 326

372 328 537 382

402 357 558 432

429 411 574 568

534 460 590

571 482

609 581

639 636

698 700

746

806

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main components of RICO products appear to be methyl ester of bothaliphatic and aromatic polycarboxylic acids.

CONCLUSIONS

In the present research project, RICO method was applied to Arabheavy and Arab medium asphaltenes and products obtained wereinvestigated using FT-IR, 13CNMR, IC, GPC, and GC/MS and thefollowing conclusions were derived.

The RICO reaction conditions, 40�C and 24 H, for the twoasphaltenes were found to be appropriate with the completion ofreaction. The oxidation reaction afforded carbon dioxide, lowermolecular weight acids, water-soluble aqueous phase, dichloromethanesoluble organic phase, and some insoluble materials. The appearance ofsharp and intense peak centered at 1732 cm�1 in the infrared spectraindicated the completion of RICO reaction. The addition of differentamounts of ethereal solution of diazomethane made no impact on theesterification of acid products.

The oxidation products of both Arab heavy and Arab mediumasphaltenes were found to be dominated by a homologous series of

Table 7. Carboxylic acids and their methyl esters

identified from the organic phase of the RICO

reaction of Arab heavy and Arab medium asphaltenes.

No. Acid molecule

1. Hexanoic acid, methyl ester

2. Heptanoic acid, methyl ester

3. Octanoic acid, methyl ester

4. Nonanoic acid, methyl ester

5. Decanoic acid, methyl ester

6. Phthalic anhydride

7. Undecanoic acid, methyl ester

8. Dodecanoic acid, methyl ester

9. Tridecanoic acid, methyl ester

10. Tetradecanoic acid, methyl ester

11. Pentadecanoic acid, methyl ester

12. Hexadecanoic acid, methyl ester

13. Heptadecanoic acid, methyl ester

14. Octadecanoic acid, methyl ester

15. Bis(2-ethylhexyl)phthalate

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straight chain monocarboxylic acids suggesting that the normal alkylchains are major and very important constituents of the chemicalstructure of both asphaltenes. The presence of a dicarboxylic acid seriesin the RICO reaction products indicates the presence of polymethylenemoieties bridging two aromatic units. The presence of benzene tetra-,penta-, and hexacarboxylic acids in the reaction products clearlysuggests that both the asphaltenes comprised of relatively large butperi-condensed aromatic structures.

ACKNOWLEDGMENTS

The authors wish to acknowledge the financial support provided bythe University Research Committee of the King Fahd University ofPetroleum and Minerals (KFUPM), Dhahran, Saudi Arabia under thegrant SABIC-2000/01. The facilities support provided by the ChemistryDepartment of the KFUPM is also gratefully acknowledged.

Table 8. Carboxylic acids and their methyl esters identified from the aqueous

phase of the RICO reaction of Arab heavy and Arab medium asphaltenes.

No. Acid molecule

1. 2-Methyl-glutaric acid, dimethyl ester

2. Dimethyl 2-methyl succinate

3. Dimethyl adipate

4. Dimethyl pimelate

5. Dimethyl succinate

6. Dimethylsuberate

7. Hexamethyl millitate (hexamethyl benzenehexacarboxylate)

8. Methyl ophthalate

9. Pentamethyl millitate (pentamethyl benzenepentacarboxylate)

10. Tetramethyl benzenetetracarboxylate

11. Tetramethylpyrmillitate

12. Triamethyl millitate (trimethyl benzenetricarboxylate)

13. Trimethyl butane-1,2,4-tricarboxylate

14. Trimethyl propane-1,2,3 tricarboxylate

15. Phthalic anhydride

16. Hexadecanoic acid, methyl ester

17. Octadecanoicacid, methyl ester

18. Bis(2-ethylhexyl)phthalate

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Received March 6, 2003Accepted April 7, 2003

Structural Studies of Asphaltenes by RICO Method 645

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