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 1
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 2 Lab. 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, 13 C-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:
[email protected]. 631 DOI: 10.1081/LFT-120034205 Copyright & 2004 by Marcel Dekker, Inc.
1091-6466 (Print); 1532-2459 (Online) www.dekker.com
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Ali, Siddiqui, and Al-Hajji 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 the world petroleum refining industry is on the rise due to the increasing demands for transportation fuels. These heavy feedstocks contain significant amounts of asphaltenes, which are solubles, randomly polydispersed organic geomacromolecules. Asphaltenes creates many problems during crude oil production and refining operations. They are concentrated in petroleum residue and are responsible for high molecular weights, viscosity, density, heteroatoms, metals, and boiling points. They are the precursors for coking and hydrocracking of crude oils and residues. In order to minimize coke formation and develop new, more efficient technologies for heavy oil upgrading, a better and more comprehensive understanding of asphaltene chemistry is essential. Several studies for the elucidation of the molecular structure of petroleum asphaltene have been carried out and published (Seki and Kumata, 2000; Speight, 1994; Speight et al., 1985; Storm et al., 1993; Yen et al., 1984). At present time, a generalized picture of asphaltene molecules is that they have a very low hydrogen-to-carbon ratio and consists of condensed aromatic nuclei that carry alkyl and alicyclic systems held together by valence bonds between heteroatoms such as sulfur, oxygen, and metals (Speight, 1987, 1990). Support to this structure is also given by other workers (Ali and Saleem, 1994; Siddiqui and Ali, 1999a), who described asphaltene molecule be made up of three to five unit-sheets consisting of condensed aromatic and naphthenic rings with paraffinic side chains. These sheets are held together by heteroatoms such as sulfur or nitrogen and/or polymethylene bridges, thioether bonds, and vanadium and nickel complexes. RICO, first introduced by Djerassi and Engle (1953), is a chemolysis method that can be selectively applied to coal, petroleum residues,
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and asphaltenes. By this method aromatic carbons are converted preferentially to carbon dioxide and/or carboxylic acids while leaving aliphatic and alicyclic portions intact. This method was introduced to fossil fuel chemistry by Stock and Tse (1983) and followed by other researchers (Mallaya and Zingaro, 1984; Zijun et al., 1997). Strausz et al. (1992, 1998) were the first group applied the RICO reaction to asphaltenes to recognize aliphatic types. They processed the invaluable information from the RICO reaction along with those from NMR and pyrolysis studies to comprehend the structure of Alberta oil and asphaltenes. Nomura et al. (1998, 1999) have processed the information from the NMR work of the asphaltene sample together with data from the RICO reaction of the asphaltene to elucidate the distribution of the aliphatic carbons more precisely. Me´ndez et al. (2001) studied the suitability of catalytic oxidation with RuO4 for establishing structural differences between an air-blown and a thermally treated pitch. Murata et al. (2001) conducted the detailed analyses of RICO products of bituminous and brown coals using FD/MS and 13C-NMR. This work presents the results of a study in which the RICO method was used to probe the structural details of asphaltenes isolated from Saudi Arabian crude oils.
EXPERIMENTAL Arab heavy and Arab medium crude oils were collected from the production and storage facilities of Saudi Aramco through the courtesy of Laboratory R & D Center. Asphaltenes were removed as n-heptane insoluble material from the resid using the same method described earlier (Siddiqui et al., 1999b).
RICO Reaction One point zero gram of asphaltenes, 30.0 mL distilled water, 20.0 mL carbon tetrachloride, 20.0 mL acetonitrile, 15g sodium periodate, and 40 mg ruthenium trichloride hydrate were introduced in a 250-mL three-neck round bottom flask. The reaction was carried out in the flask by heating in an oil bath at ’40 C for 24 h with continuous magnetic stirring. N2 gas was passed through one inlet and the resulting CO2 gas was purged through drying tube containing anhydrous CaCl2. The amount of CO2 formed was absorbed in 300 mL of 0.1 M NaOH solution that was later titrated against 0.05 M H2SO4 solution.
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The further work-up of the reaction product was carried out by the following two different filtration methods.
Filtration Method #1 At the end of the reaction, the product was filtered under vacuum filtration to get first filtrate and the precipitate being washed with 50 mL of 5% NaOH solution to get the residue and another filtrate. Thirty milli liters of extra base solution of 5% NaOH was added to first filtrate. Both filtrates were combined and extracted two times with 50 mL of CH2Cl2 in a 250-mL separatory funnel. Organic phase and aqueous phase were separated. Aqueous phase was further diluted to 1000 mL with distilled water and sample was ready for lower molecular weight carboxylic acids determination by ion-chromatogram (IC) technique.
Filtration Method #2 Reaction started with again 1.0 g of asphaltenes and introduced all the chemicals as worked out in filtration 1. At the end of the reaction, the reaction product was filtered under vacuum filtration to get filtrate and the residue was washed with 50 mL CH2Cl2 and 50 mL H2O. It afforded insoluble residue that was further treated with 25 mL of 5 N HCl and filtered under vacuum to get the black colored residue. The CH2Cl2 and water solubles were separated into organic phase and aqueous phase. Aqueous phase was further extracted with another 50 mL of CH2Cl2 to get another organic phase and the aqueous phase was evaporated to dryness at 40 C. Both organic phases were combined and CH2Cl2 was evaporated using N2 gas that afforded organic residue. Aqueous phase and organic residue were methyl esterified by an ethereal solution of diazomethane.
Preparation of Diazomethane A solution was prepared by mixing 2.0 g KOH in 3 mL distilled water and 10 mL of 95% ethanol in a 250-mL side arm flask A. The solution was kept in an oil bath and the bath temperature was maintained between 62 and 65 C. Flask A was connected to another flask C and was kept in an 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 containing 20 mL diethyl ether was kept in an ice bath D with some salt. Eight point six grams of diazogen was dissolved in 40 mL of diethyl ether in a small separatory funnel B. Solution from B was poured slowly to flask A that liberated some gas and absorbed by 10 mL diethyl ether present in cylinder E. Complete transfer of solution from flask B to A was done in 20 min and the temperature of the bath was remained in the range of 65–70 C. At the end of the reaction diazomethane prepared in graduated cylinder E was later transferred into three small 50-mL flasks and covered with rubber septum.
Esterification of Aqueous Phase and Organic Phase of Filtration Two point zero grams of aqueous phase residue of each asphaltenes were weighed out into two small 25-mL flasks. The weighed residue was dissolved in the mixture of 10 mL CH2Cl2 and 10 mL ether and 7 mL of diazomethane was added in each flask. The contents of flask were filtered after 5 min. Zero point one gram of organic phase of each asphaltenes were also dissolved in 6 mL of CH2Cl2 and the solution was transferred equally into two small 25-mL flasks. In flask 1, 2 mL of diazomethane was added and evaporated it to dryness by blowing with N2 gas. Extra 2 mL CH2Cl2 and 2 mL diazomethane were further added. Similarly in flask 2, 4 mL of diazomethane was added, shaken, and dried with N2 gas. Another 2 mL of CH2Cl2 and 4 mL diazomethane were added and later on the solvents were evaporated.
Instrumentation Autospec-Q from Micromass, UK, interfaced to HP 5890 Series II GC-Mass Spectrometer was used in this study. The mass spectrometer was tuned using PFK (perforated kerosene) to a resolution of 1000 on 10% valley. A mass spectrometric experiment used the ‘‘Magnet’’ mode scanning from 600 to 20 amu, MS source temperature was set at 200 C, Electron Energy was set at 70 eV and trap, Current at 300 mA. The mass spectrometric was calibrated by PFK using the above experiment. For a satisfactory GC peak resolution, the following were employed: DB-1 column (60 m long, 0.25 ID, 0.25 mm), GC temperature program: 50 to 320 C at 10 C/min and then isothermal at 320 C for 20 min, GC/MS interface 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 flow rate of 1 mL/min. The use of IR, NMR, and HPGPC have been described earlier else where (Siddiqui et al., 1999b). Infrared spectra were recorded on a Perkin Elmer Model 1610 infrared spectrophotometer loaded with Infra Red Data Manager (IRDM) software. 13C and 1H-NMR spectra were recorded on a Varian XL-200 Pulse Fourier Transform (PFT) spectrometer operating at 200 MHz using 5 mm sample tubes. A Waters HPLC system 840 was used with a Model 501 pump, a 712 WISP auto injector and an R-401 refractometer as a detector. The molecular weight distribution was obtained using a Millennium 2010 Chromatography Manager program.
RESULTS AND DISCUSSION Table 1 shows the characteristic properties for Arab heavy and Arab medium asphaltenes. Arab heavy asphaltenes have higher GPC molecular weight distributions and Ni and V contents than Arab medium asphaltenes. Arab heavy asphaltenes have relatively more aliphatic hydrogen while Arab medium have a little more amount of aromatic hydrogen. The aliphatic carbon contents of Arab medium asphaltenes were higher and the aromatic carbon contents were lower than Arab heavy asphaltenes. The amounts of lower carboxylic acids obtained from filtration method 1 in RICO reaction are given in the following Table 2. Ion chromatogram analysis of both asphaltenes showed a large amount of oxalic acid followed by propionic acid. The amount of acetic acid in both asphaltenes found to be same and lowest. There are not any specific scientific reason for the production of higher amounts of oxalic acid but could be attributed to the structures and composition of both Arabian asphaltenes. These aliphatic monocarboxylic acids are stable at 40 C in the RICO procedure indicating that they are the product of
Table 1.
Asphaltenes Arab heavy Arab medium
Characteristic properties of asphaltenes.
C (%)
H (%)
GPC (MW)
Ni (ppm)
V (ppm)
Har (%)
Hal (%)
Car (%)
Cal (%)
83.22 83.71
8.25 8.33
1866 1611
19 15
60 45
8.1 10.2
91.9 89.9
36.0 30.8
64.0 69.2
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Yields of lower carboxylic acids from RICO reactions. Acetic acid (ppm)
Formic acid (ppm)
Oxalic acid (ppm)
Propionic acid (ppm)
10 10
10 19
39 62
23 25
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 cm1 covering the area 1625–1800 cm1 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 cm1 and centered at 3428 cm1. This is very typical and characteristic peak of OH hydrogen bond of carboxylic acids. A sharp and strong peak at 1258 cm1 was related to the antisymmetric 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 cm1 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 cm1 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 cm1 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
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Table 3. GPC molecular weight distribution of aqueous and organic phases of RICO reactions. Sample
Mn
Mw
Mp
Polydispersity
Arab Arab Arab Arab
290 163 102 151
365 454 233 373
275 276 266 253
1.26 2.79 2.28 2.48
heavy-aqueous heavy-organic medium-aqueous medium-organic
molecular weight distribution of aqueous and organic phases of Arab heavy and Arab medium asphaltenes is shown in the Table 3. GPC, with separation based on the size and shape of molecules, gives the number average molecular weight (Mn), the weight average molecular weight (Mw), and the peak molecular weight at the peak apex for each integrated peak (Mp). In the two phases, organic phases have the higher molecular weight distribution than aqueous phases; indicating that organic phases possess carboxylic acids with larger molecules. Arab heavy asphaltenes aqueous and organic phases have higher molecular weight distribution than Arab medium asphaltenes. The proton NMR spectra of organic phase of Arab medium and Arab heavy asphaltenes demonstrate the presence of oxygen functionality at 5.30 ppm. Proton NMR spectra can quantitatively illustrate the types of protons present in the carbon skeleton but difficult to quantify any protons attached to an electronegative atom like oxygen. The carboxylic protons appear down field after 10 ppm. The RICO reaction of Arab medium asphaltenes shows that organic phase contains approximately 97.0% aliphatic hydrogens, 2.1% hydrogens attached to oxygen functionality and merely 0.9% aromatic hydrogens. Thus, indicating that almost all-aromatic ring system is substituted. Whereas, in the case of Arab medium asphaltenes, aliphatic hydrogens were found to be 96%, aromatic hydrogens 3%, and oxygen functionalities containing hydrogens were 1%. The comparison of these two asphaltenes indicates that structure of asphaltenes play a vital role during the RICO reaction leading to different ratios of hydrogen atoms. Esterification of carboxylic acids produced in aqueous and organic phase following the RICO reaction of asphaltenes was performed in situ by freshly prepared diazomethane from diazogen. The ethereal solution of diazomethane was used immediately after preparation. The IR spectra of esterified aqueous phase of Arab medium asphaltenes gave a very sharp and intense absorption band at 1732 cm1 assigned to the C¼O
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stretching vibration of ester. Another sharp peak appeared in the esterified aqueous phase was at 1272 cm1 attributed to the C-O-C antisymmetric stretching vibration. A small unresolved hump, found at 3452 cm1, could be due to the hydrogen bonding among the un-esterified carboxylic acids left in smaller amounts. Some other sharp peaks observed at 1124, 1072, and 744 cm1. The C-H stretching vibrations of CH3 at 1380 cm1 and CH2 at 1460 cm1 were other prominent absorption bands observed. The infrared spectra of esterified organic phase produced similar pattern of functional groups frequencies in the fingerprinting region. The broader peak centered at 3464 cm1 was more pronounced. Carbonyl group stretching vibration at 1732 cm1, aromatic C¼C stretching vibration at 1600 cm1, aliphatic C-H stretching vibration of CH2 at 1462 cm1 and CH3 at 1378 cm1 were most resolved and strong peaks. Other absorption bands similar to aqueous phase were found at 1274, 1124, 1072, and 744 cm1. Combining the infrared information of aqueous and organic phases of Arab medium asphaltenes shows the formation of carboxylic acid and esters during the RICO reactions. The product types could well be identified using GC/MS techniques. The esterified aqueous and organic phases of Arab heavy asphaltenes produced almost similar trend of absorption peaks in the fingerprinting region of infrared spectra. The most prominent peaks found were carbonyl at 1730 cm1, methylene at 1462 cm1, methyl at 1378 cm1, ether linkage at 1274 cm1, and aromatic C¼C at 1600 cm1. Again a broad peak centered at 3446 cm1 was more intense in organic phase than aqueous. A point need to be emphasized here is the amount of diazomethane ethereal solution used. The same esterification reaction was carried out by three different amounts of diazomethane solution and the resulting products were found to be same without any visible changes in the infrared spectra of esterified product. Hence, it indicates that the esterification reaction was completed when first batch of ethereal diazomethane solution was added and there were no traces of unreacted carboxylic 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 determining oxygen types attached to carbon skeleton. The chemical shifts in 13C-NMR spectroscopy related to this study could be classified into following categories: 0–50 ppm aliphatic carbons; 50–65 ppm methoxy/alkoxy carbons; 110–160 ppm aromatic carbons. The region 160–185 ppm was assigned to carboxylic derivative
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Ali, Siddiqui, and Al-Hajji Table 4. Percent distribution of carbon types in Arab medium RICO reaction products. Chemical shifts ( ppm) 0–50 51–53 67–68 128–132 167 191
AM-ARE (%)
AM-OR (%)
40.7 24.8 6.6 19.3 6.4 2.2
52 15 4.6 12.4 5.5 9.2
Table 5. Percent distribution of carbon types in Arab heavy RICO reaction products. Chemical shifts ( ppm) 0–50 51–53 67–68 128–132 167 175
AM-ARE (%)
AH-OR (%)
49.6 13.4 7.3 22.5 7.3
73.5 9.6 2.9 8.8 2.2 2.9
carbons which was further specified as: 160–170 ppm carboxyl group connected with aromatic carbons (esters/anhydrides) and 170–185 ppm carboxylic groups connected with aliphatic carbons (acids). Percent distributions of carbon types in aqueous and organic phases obtained by 13C NMR are given in Tables 4 and 5. 13C NMR spectra of esterified aqueous phase of Arab medium asphaltenes shows sharp peaks 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 the spectra includes , , , and carbons in paraffinic straight chains which accounted 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 the alkoxy group attached directly to the aromatic rings (6.6%). There were three strong and sharp peaks in the 128–132 ppm region, which contributed 19.3% of the carbon. These peaks represents the combination
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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 [C6H6n (COOCH3)n], (n ¼ 3–6). A series of biphenyl polycarboxylic acid methyl esters [C12H10n(COOCH3)n], (n ¼ 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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Ali, Siddiqui, and Al-Hajji Table 6. Main peaks (m/z) in total ion chromatograms of aqueous and organic phase fractions of asphaltenes. Organic phases AH 51 88 132 198 253 299 338 372 402 429 534 571 609 639 698
AM
200 233 266 298 328 357 411 460 482 581 636 700 746 806
Aqueous phases AH
160 200 221 268 474 537 558 574 590
AM 139 161 195 263 295 326 382 432 568
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
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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. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Hexanoic acid, methyl ester Heptanoic acid, methyl ester Octanoic acid, methyl ester Nonanoic acid, methyl ester Decanoic acid, methyl ester Phthalic anhydride Undecanoic acid, methyl ester Dodecanoic acid, methyl ester Tridecanoic acid, methyl ester Tetradecanoic acid, methyl ester Pentadecanoic acid, methyl ester Hexadecanoic acid, methyl ester Heptadecanoic acid, methyl ester Octadecanoic acid, methyl ester Bis(2-ethylhexyl)phthalate
main components of RICO products appear to be methyl ester of both aliphatic and aromatic polycarboxylic acids.
CONCLUSIONS In the present research project, RICO method was applied to Arab heavy and Arab medium asphaltenes and products obtained were investigated using FT-IR, 13C NMR, IC, GPC, and GC/MS and the following conclusions were derived. The RICO reaction conditions, 40 C and 24 H, for the two asphaltenes were found to be appropriate with the completion of reaction. The oxidation reaction afforded carbon dioxide, lower molecular weight acids, water-soluble aqueous phase, dichloromethane soluble organic phase, and some insoluble materials. The appearance of sharp and intense peak centered at 1732 cm1 in the infrared spectra indicated the completion of RICO reaction. The addition of different amounts of ethereal solution of diazomethane made no impact on the esterification of acid products. The oxidation products of both Arab heavy and Arab medium asphaltenes were found to be dominated by a homologous series of
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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. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
2-Methyl-glutaric acid, dimethyl ester Dimethyl 2-methyl succinate Dimethyl adipate Dimethyl pimelate Dimethyl succinate Dimethylsuberate Hexamethyl millitate (hexamethyl benzenehexacarboxylate) Methyl ophthalate Pentamethyl millitate (pentamethyl benzenepentacarboxylate) Tetramethyl benzenetetracarboxylate Tetramethylpyrmillitate Triamethyl millitate (trimethyl benzenetricarboxylate) Trimethyl butane-1,2,4-tricarboxylate Trimethyl propane-1,2,3 tricarboxylate Phthalic anhydride Hexadecanoic acid, methyl ester Octadecanoicacid, methyl ester Bis(2-ethylhexyl)phthalate
straight chain monocarboxylic acids suggesting that the normal alkyl chains are major and very important constituents of the chemical structure of both asphaltenes. The presence of a dicarboxylic acid series in the RICO reaction products indicates the presence of polymethylene moieties bridging two aromatic units. The presence of benzene tetra-, penta-, and hexacarboxylic acids in the reaction products clearly suggests that both the asphaltenes comprised of relatively large but peri-condensed aromatic structures.
ACKNOWLEDGMENTS The authors wish to acknowledge the financial support provided by the University Research Committee of the King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia under the grant SABIC-2000/01. The facilities support provided by the Chemistry Department of the KFUPM is also gratefully acknowledged.
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