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Energy & Fuels 2001, 15, 444-448
The Role of Hydroaromatic Species in the Oxidation of Petroleum Bitumens P. R. Herrington* Opus International Consultants Ltd., Central Laboratories, P.O. Box 30-845, Lower Hutt, New Zealand Received August 23, 2000. Revised Manuscript Received November 29, 2000
Model dihydroaromatic compounds (0.021 M and 0.123 M) and Safaniya petroleum bitumen (ca. 0.021 M) in toluene and Decalin solution at 50.0 °C were oxidized with air for up to 500 h in the dark. The rate of oxygen uptake was measured by gas chromatography using a column packed with molecular sieve 5A at 70 °C. The model compounds showed a slow linear rate of oxidation with an induction period apparent in some cases. In contrast the bitumen showed no induction period and the typical rapid initial rate of reaction becoming linear at longer times (>100 h) was observed. Oxidation of the most reactive of the model compounds (9,10-dihydroanthracene) was inhibited by 2,4-di-tert-butyl-4-methylphenol whereas the reaction of bitumen was unaffected. Bitumen and DHA did not act synergistically to increase the overall rate of oxygen uptake (compared to the sum of the individual reactions) when co-oxidized in toluene. Bitumen (4 g, ca. 0.0064 mols) hydrogenation with o-chloranil (0.203 g, 0.00082 mol) in refluxing toluene and Decalin (110 °C and 160 °C, respectively) for up to 24 h indicated the presence of a dihydroaromatic group concentration of about 3 × 10-5 to 7 × 10-5 mol per gram of bitumen. However, removal of these groups through hydrogenation had no significant effect on the rate of oxygen uptake of bitumen solutions. Both model compound studies and hydrogenation experiments indicate that dihydroaromatic species may not be significant in the primary oxygen uptake process of bitumen oxidation.
Introduction Petroleum bitumens used in road construction are produced by the vacumn distillation of crude oils. Bitumens are a complex mixture of hydrocarbons (long chain alkanes, alkyl substituted aliphatic and polyaromatic ring systems) but with significant concentrations of heteroatomic (principally sulfur) species and also metals such as vanadium and nickel. Under ambient conditions bitumens react slowly with atmospheric oxygen and become hard and brittle at low temperatures leading to road failure. The mechanism of this reaction is still largely undetermined, in particular the initial oxygen uptake process. The oxidation reaction does not appear however to simply follow the classical autoxidation free radical chain mechanism observed in the case of pure hydrocarbons or light oils.1 The reasons for this are the lack of an observable induction period,2 the general ineffectiveness of chain breaking free radical inhibitors,3 and a reported kinetic chain length of zero.4 The absence of a free radical chain mechanism may be due to the presence of naturally * E-mail:
[email protected]. (1) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: Amsterdam, 1965. (2) (a)Blokker, P. C.; Van Hoorn, H. Proc. 5th World Pet. Cong. 1959, Section VI, paper 27, 417-432. (b) Dickinson, E. J.; Nicholas, J. H.; Boas-Traube, S. J. Appl. Chem. 1958, 8, 673-687. (c) Van Oort, W. P. Ind. Eng. Chem. 1956, 48, 1196-1201. (3) (a) Martin, K. G. J. Appl. Chem. 1966, 16, 197-202. (b) Martin, K. G. Proc. 4th Aust. Road Res. Board Conf. 1968, 4 (2), 1477-1494. (c) Januske, R. M. Ind. Eng. Chem. Prod. Res. Dev. 1971, 10 (2), 209214.
occurring inhibitors (probably phenols) in bitumen and heavy oils.5 A number of authors have suggested that compounds with hydroaromatic structures (AH2) may play a major role in the initial reaction of oxygen with bitumen6 through the uninitiated reactions shown in eqs 1 and 2. Subsequent reaction or decomposition of the hydroperoxides and peroxides formed with bitumen sulfides would account for the observed formation of sulfoxides and carbonyl containing species, respectively.
AH2 + O2 f HAOOH
(1)
AH2 + O2 f A + H2O2
(2)
Knotnerus7 studied the rate of oxygen absorption for 27 various model compounds in toluene solution (1.25 g in 50 mL) at 30 °C under 460-660 nm irradiation. Only benzo[b]fluorene, 5,12-dihydronaphthacene, and 9,10dihydroanthracene showed reactivies to oxygen of a (4) van Gooswillegen, E. H.; Berger, H.; Th de Bats, F. Proc. 3rd Eurobitume Symp., September, 11-13, 1985, 95-101. (5) (a) Mill, T.; Tse, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (3), 483-489. (b) Velikov, A. A.; Sizova, N. V.; Unger, F. G. Pet. Chem. 1996, 36 (5), 460-466. Translation from Neftekhimiya 1996, 36 (5), 458-463. (6) (a) Mill, T.; Su, M.; Yao, C. C. D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43 (4), 1053-1056. (b) Mill, T. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 1996, 41 (4), 1245-1249. (c) Petersen, J. C.; Harnsberger, P. M. Trans. Res. Rec. 1638, 1998, 47-55. (d) Petersen, J. C. Pet. Sci. Technol. Int. 1998, 16 (19 & 20), 1023-1059. (7) Knoterus, J. Ind. Eng Chem., Prod. Res. Dev. 1972, 11 (4), 411422.
10.1021/ef0001890 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
Hydroaromatics in Oxidation of Petroleum Bitumens
Energy & Fuels, Vol. 15, No. 2, 2001 445
Table 1. Safaniya Bitumen Properties penetration at 25 °C (ASTM D5) (dmm) softening point (ASTM D36) (°C) viscosity (AS 2341.3) at 60 °C (Pas) at 135 °C (Pas) elemental analysisa C (wt %) H (wt %) N (wt %) S (wt %) Ni (ppm) V (ppm) molecular weightb
186 40 58 0.25 83.7 10.8 0.5 5.5 18 65 630
a Ni and V by XRF; C, H, N, and S from ref 16c. b By VPO in toluene at 35 °C, ref 16c.
magnitude similar to that of bitumen. Another hydroaromatic, 1,2,3,4-tetrahydrophenanthrene, was unreactive under the conditions used. In contrast, none of the compounds was found to absorb oxygen when reacted in the dark. The direct bimolecular thermal reaction of triplet oxygen with ground state organic species is generally thermodynamically unfavorable. However, the uninitiated reactions of 4a,4b-dihydrophenanthrene and 9,10cyclopentano-4a,4b-dihydrophenanthrene with oxygen have been reported8 and are believed to proceed according to eqs 3:
AH2 + O2 f AH‚ + HOO‚ AH‚ + O2 f A + HOO‚ AH2 + HOO‚ f AH‚ + H2O2 2 HOO‚ f H2O2 + O2
}
(3)
A termolecular mechanism (eq 4) has been reported for the self-initiated autoxidation of tetralin9 and tetralin and indene with activation energies of 99.6 and 78.7 kJ/mol, respectively.10
2AH + O2 f 2A‚ + H2O2
(4)
In the present paper the results of experiments undertaken to provide more information on the possible role of hydroaromatic compounds in bitumen oxidation are reported. Studies were made on the rate of oxygen uptake of solutions of model compounds and on bitumen both before and after mild hydrogenation with ochloranil. Experimental Section Materials. The bitumen used was a straight-run, vacuum distilled, 180/200 penetration grade from Safaniya crude. Properties of the bitumen are given in Table 1. Solvents were 99% or better grade from Reidel de Hahn and Scharlau. Toluene and Decalin were dried by storage over molecular sieves 5A and 4A, respectively (previously extracted with solvent), and filtered immediately before use; 9,10dihydroanthracene (98%, ICN Biomedical), 9,10-dihydrophenanthrene (98%, Lancaster), 3,4-dihydro-1-phenylnaphthalene (97%, Lancaster), 1,2-dihydronaphthalene (96%, Acros), (8) (a) Bromberg, A.; Muszkat, K. A.; Fischer, E. Chem. Commun. 1968, 1352-1354. (b) Bromberg, A.; Muszkat, K. A. J. Am. Chem. Soc. 1969, 91 (11), 2860-2866. (9) Denisov, E. J. Russ. J. Phys. Chem. 1964, 38 (1), 1-8. (10) Carlsson, D. J.; Robb, J. C. Trans. Faraday Soc. 1966, 62, 34033415.
o-chloranil (>98%, Lancaster), tetrachlorocatechol (99%, Lancaster), and 2,4-di-tert-butyl-4-methylphenol (99%, Lancaster) were used as received. Oxygen Uptake Measurements. Rates of oxygen consumption were made using solutions in 25.0 cm3 toluene. Bitumen (2.000 g) or model compound (up to ca. 0.5 g) solutions were stirred at room temperature for 30 min before being transferred to 250 cm3 amber glass bottles. The bottles were sealed with Teflon “mininert” valves (Valco, Louisiana) and placed up to the neck into a water bath at 50 ( 0.05 °C. The valves were opened periodically to allow sampling of the reaction headspace through a rubber septum. Leak tightness of the valves over the experimental time frame was established by checking the headspace of a nitrogen-filled bottle for oxygen contamination from the air. Headspace sampling (200 ul) was carried out using a Hamilton gastight syringe fitted with a fine 28 gauge needle. Experiments using samples from a nitrogen-filled bottle showed that no significant diffusion occurred along the needle even after 1 min standing in the laboratory whereas sampling and injection was in all cases completed within 10 s. Oxygen concentrations were measured on a gas chromatograph (SRI Instruments,California ) equipped with a thermal conductivity detector and a 2 m stainless steel column packed with molecular sieve 5A at 70 °C. Under these conditions two peaks (oxygen plus argon and nitrogen) were observed. The oxygen consumed in the reactions was calculated assuming the concentration of nitrogen in the reaction headspace remained constant. Calibration curves were constructed using laboratory air (at 993.8 mbar and 20.9 °C) injections assuming the concentration of O2 and Ar totalled was 21.88% v/v;11 lower concentration gases were ignored. The curves were used to calculate a precise volume of headspace air (from the N2 peak area) and the volume of O2 + Ar injected in the sample injections. The calculated volumes are the equivalent air or O2 + Ar volumes (at 993.8 mbar and 20.9 °C) that would give the respective peak areas. As the headspace pressure was effectively constant and slightly higher than atmospheric a small, variable amount (depending on the day to day variation in atmospheric pressure) of sample would leak from the syringe immediately after sampling to equilibrate the pressure. Using N2 as an internal standard avoided this problem and additionally the much greater errors due to reading the syringe. The volume of O2 reacted (at 993.8 mbar and 20.9 °C) is given by the change in the (O2 + Ar)/air ratio from the initial value, multiplied by the total headspace volume (assuming that the argon concentration remains constant):
VO2 reacted ) (0.2188 - (VO2+Ar/Vair)) Vheadspace The number of moles of O2 reacted is given by
∆O2 ) VO2 reacted/VO2 molar where VO2 molar is the molar volume of O2 at 993.8 mbar and 20.9 °C assuming ideal behavior. Vheadspace was measured with distilled water of known density and corrected for the thermal expansion of toluene at 50 °C. The initial (O2 + Ar)/air ratio was confirmed by a blank experiment using 25.0 cm3 of toluene equilibrated at 50.0 °C for 24 h. Precision of Oxygen Uptake Measurements. Most experiments were carried out in duplicate and the average results reported. The precision (6%) of the method was determined as two standard deviations of the mean of 5 replicate measurements using 2.0 g solution of bitumen in 25.0 mL of toluene. (11) CRC Handbook of Chemistry and Physics, 65th ed.; CRC Press: New York, 1984.
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Table 2. Oxygen Uptake for Model Compound and Bitumen Solutions in the Dark oxygen uptake (mol O2 mol-1) ( 0.006 0.123 M
0.021 M
compound
150 h
250 h
150 h
250 h
9,10-dihydroanthracene 9,10-dihydrophenanthrene 1,2-dihydronaphthalene 3,4-dihydro-l-phenylnaphthalene bitumen 9,10-dihydroanthracenea +0.007 M bitumen 9,10-dihydrophenanthrenea +0.007 M bitumen 9,10-dihydroanthracenea +0.123 M bitumen 9,10-dihydroanthracene +1% w/w BHTb bitumen +1% w/w BHT
0.045 0.001
