Energy & Fuels 1994,8, 455-462
455
Kinetic Study of the Oxygenation of 2,B-Dimethylpyrrole. A Model Compound Study Designed To Probe Initiation of the Oxidative Degradation of Petroleum Products Bruce D. Beaver,. Eric Treaster, Joanne D. Kehlbeck, and Gregory S. Martin Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania 15282
Bruce H. Black CEO-Centers, Inc., 10903 Indian Head Highway, Fort Washington, Maryland 20744 Received March 11, 1993. Revised Manuscript Received December 6, 199P
Initial rate studies of the oxygenation (50-70 O C ) of 2,5-dimethylpyrrole (DMP), in nitrobenzene, reveal that this reaction is second order overall and first order in both DMP and 0 2 . The presence of an equimolar concentration (based on DMP) of the phenolic antioxidant BHT does not inhibit the initial rate of DMP oxygenation. With dodecane or toluene, as solvent, DMP oxygenation (70120 "C) is approximately first order in both DMP and 0 2 . DMP oxygenation in dodecane in the presence of 1 or 3 equiv of BHT results in an approximate 52% reduction in the rate of DMP oxygenation. In addition, the oxygenation of 1-deuterated (ND) DMP (in deuterated methanol) does not exhibit a primary kinetic isotope effect. In light of these experimental observations, both the mechanism of DMP oxygenation and its potential relevance with respect to the oxidative degradation of petroleum products are discussed. Finally, experimental evidence from simulated oxidative degradation (LPR 100 O C ) of a JP-5jet fuel and a JP-5jet fuel blending stock are presented to illustrate the relevancy of the DMP model compound study to actual fuel oxidative degradation.
Introduction The oxidative degradation of petroleum products is a complex yet fascinating problem. It is becoming clear that the most simple description of the oxidative degradation of petroleum products involves the operation of two distinct processes (Scheme l).QThe first process encompasses the reaction of trace indigenous reactive molecules with dissolved molecular oxygen to form the initial oxidation products (usually an organic hydroperoxide). The conversion of these initial products into secondary oxidation products (i.e., alcohols,acids, carbonyl compounds, etc.) is dependent both upon hydroperoxide structure and environmental conditions. The second phase of the petroleum degradation process involves the expression of the presence of these recently oxidized fuel molecules through the operation of routine organic chemistry. Most thorough literature reports of the oxidative degradation of petroleum products are consistent with the operation of the paradigm depicted in Scheme 1.Some good examples of this are reflected in the investigations of the ambient degradation of diesel fuels reported by Pedley et al.? and most recently by Tort et alS4 Our current work is focused extensively upon enhancing the understanding of the first phase of the petroleum degradation process in which oxygen is incorporated into reactive indigenous fuel molecules. Previously we have proposed that a reaction can occur between electron-rich molecules (i.e., molecules with low oxidation potentials) *Abstract published in Advance ACS Abstracts, January 16, 1994. (1) Beaver, B. D. Fuel Sci. Technol. Znt. 1991,9(10), 1287-1336. (2) Batte, B. D.; Fathoni, A. Z. Energy h e l a 1991,5,2-21. (3) Pedley, J. F.; Hiley, R. W.; Hancok,R. A.Fuel1988,67,1124-1130. (4) Tort,F.; Waegell,B.; Bernaeconi, C.; Germanaud,L. 4th Int. Conf. Stability Handling Liq. Fuels; DOE/CONF-911102; 1992, 662-666.
Scheme 1 Trace indigenous react*c fuel molcculcs
I
0 Petroleum
Dcgrad;luon
and oxygen via a mechanism which is not consistent with operation of the well-known peroxyl radical-chain procesa.6 We designate this reaction as electron-transfer-initiated oxygenation (ETIO) and define the most simple example of this process aa any oxygenation reaction in which the rate-limiting step involves an electron transfer from the substrate to molecular oxygen. (6) Beaver, B. D.; Gilmore, C. Fuel Sci. Technol. Znt. 1991, "7),811-
823.
0881-0624194/2508-0455$04.50/0 0 1994 American Chemical Society
456 Energy & Fuels, Vol. 8, No. 2, 1994
As articulated in recent publications:16 the ET10 concept provides a rational explanation for two important experimental observationsnoted in many literature reports examining the oxidative degradation of various petroleum products, namely, the appearance of an oxygen order dependency (first-order oxygen dependency in the most simple case) in the rate law describing the degradation and the inability of hindered phenol antioxidants to significantly limit the rate of oxygenation. It is important to note that we do not view ET10 as a single mechanism but as a family of reactions in which electron transfer is an integral part of the mechanism. Delineation of the different mechanistic possibilities for ET10 is beyond the scope of this paper. The important point at this time is that the ET10 hypothesis postulates the existence of reactions that are mechanistically distinct from the conventional peroxyl radical-chain mechanism which is routinely invoked to account for the oxidative degradation of petroleum products. We believe that additional studies designed to confirm the existence, ubiquitousness, and the nature of ET10 will significantly enhance our understanding of the oxidative degradation of petroleum products. We postulate that the following experimental observations (examining jet fuel thermal instability) when interpreted in terms of the ET10 concept provide a rational account of the nature of the chemical reactions between oxygen and trace fuel components at higher temperatures: (i) Jones et al. have recently reported how different concentrations of dissolved oxygen affect the formation of insolubles in jet fuels a t 185 “C. One of the two Jet As examined, one was found to produce insolubles at a rate which exhibited a zero-order dependency in oxygen, while the other fuel produced insolubles at a rate that exhibits a first order dependence in oxygen.7 (ii) Heneghan et al. have recently argued that thermally stressing several jet fuels in the “Phoenix Rig” results in oxygen consumption curves that are consistent with a firstorder dependency in oxygen.s (iii) Classical chain-breaking donor antioxidants (i.e., substituted phenols and phenylenediamines) usually do not inhibit deposit formation (sometimes they actually increase deposit formation) when a fuel is stressed at temperatures of 150-300 oC.eJo (iv) Black and Hardy” have recently reported a very interesting study which examines the affect of oxygen overpressure upon hydroperoxide formation in the simulated aging of several different jet fuels. Six different jet fuels were examined by measuring peroxide formation (at 100 “C) as a function of time, with different overpressures of oxygen (a modification of the LPR method developed for accessing diesel fuel stability12). Of the six fuels examined, four exhibited little or no relationship between oxygen overpressure and peroxide formation. This ob1991,6, 274-280. (7) Jonee, E. G.;Baletar, W. J.; Anderson, S. D. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1992,37(2), 393-409. (8)Henenhan. S. P.: Martel. C. R.:. Williams..T. F.:. Ballal. D. R. Trans. ASME 1995, paper 3 92-GT-106. (9) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fueb. ASTM Publication Code No. 31-001092-12, Chapter E,1991. (10)Sulaymon, A. H.; Mohammed, A. K.;Al-Rawi, Y. M. Fuel Sci. Technol. Int. 1989, 7,123-141. (11)Black, B. H.; Hardy, D. R.4thZnt. Conj. Stability Handling Liq. Fuels. DOE/CONF-911102, 1992,678-690. (12) Hardy, D.R.;Hazlett, R. N.; Beal, E. J.; Burnett, J. C. Energy Fuels 1989,3, 2&24.
Beaver et al.
servation is consistent with operation of a classicalperoxyl radical chain for hydroperoxide formation.lS When such a mechanism is operative, its existence can be experimentally detected since the rate law for peroxide formation will exhibit both a zero-order dependency in oxygen14and exhibit a rate of oxidation which is significantlysuppressed by the presence of hindered phenol antioxidants (i.e., chainbreaking donor antioxidantsl6). Two of the six jet fuels studied exhibit an order dependency in oxygen for hydroperoxide formation. The first, designated Kuwait No. 1,is afield sample JP-5 refined from Kuwait Export crude that had been both hydrotreated and hydrocracked. The second, coded Fuel No. 2, is a hydrocracked, caustic-washed additive-freeJP-5 blending stock refined from an Alaskan North Slope crude. Fuel No. 2 was doped with 22.6 mg/L of 2,6-di-tert-4methylphenol (26dtb4mp), also commonly referred to as BHT, prior to oxygen overpressure experiments. The results of oxygen overpressure studies on these two fuels will be presented and rationalized. Previous Investigations of DMP Oxygenation. The ET10 hypothesis was originally formulated to account for certain “seemingly unusual” experimental observations for the oxygenation of DMPS6 Most startling was the apparent inability of phenolic antioxidants to inhibit DMP oxygenation in a base diesel fuel. This observation was striking since phenolic antioxidants are also known to be generally ineffective in limiting deposit formation during the oxidative degradation of diesel fuels. The reason for this situation is not clear and we speculated that there might be a relationship between the mechanism(s) for both DMP and diesel sediment precursor Oxygenation. Consequently, DMP oxygenation in organic solvents is considered an appropriate model for experimental testing of the ET10 hypothesis. DMP is considered to be representative of electron rich molecules which are frequently components of higher boiling fuel fractions. Use of this simple system, instead of actual fuel systems, facilitates a more detailed study of the fundamental nature of the reaction of molecular oxygen with electron rich molecules. The Nature of the DMP Oxygenation Product(s). The final DMP oxygenation product is a complex intractable material whose structure has been previously investigated.16J7 Briefly, the low-temperature oxygenation (43-80 OC) of DMP, in both diesel fuel and dodecane, produces an insoluble sediment with the approximate elemental composition of C ~ H ~ N O IVarious . ~ . ~ ~methods of mass spectrometricanalysis16-18reveal a sediment whose structure is best described as an oxygen-containing DMP oligomer with a molecular weight of less than 400. Frankenfeld et al.16 have found that sediments of similar elemental composition are produced by both photochemical and thermally initiated DMP oxygenation. However, sedimentation versus time curves suggest that different mechanisms for DMP oxygenation are operative in each case. Most interestingly, the elemental composition of the sediment produced by DMP oxygenation is remarkably (13) Fordor, G.E.; Naegeli, D. W.; Kohl,K. B. Energy Fuels 1988,2, 729-734. (14) Betta, J. Q. Rev. 1971,25,266-288. (16)Scott, G. Bull. Chem. SOC.Jpn. 1988, 61, 166-170. (16) Frankenfeld, J. W.; Taylor, W. F.; Brinkman, D. W. Znd. Eng. Chem. Prod. Res. Deu. 1983,22,808-614. (17) Malhotra, R.;St. John, G. A. 2nd Znt. Conj. Stability Handling Liq. Fuels, Southweet Ree. h t . 1988,327-336. (18)Beaver, B.D.; Hazlett, R. N.; Cooney,J. V.; Watkine, J. M.Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32(1), 638-646.
Oxygenation of 2,5-Dimethylpyrrole
Energy & Fuels, Vol. 8, No. 2,1994 467
insensitive to the nature of the solvent. Although, the rate of autoxidative DMP sediment formation increases from n-dodecane < j e t fuel < No. 2 diesel, the sediment elemental composition is quite similar. Frankenfeld et al. proposed that the presence of acidic catalysis alters the rate but not the mechanism of the oxidative sedimentation process. We believe these results suggest that, at the temperatures examined, DMP is more reactive with dissolved oxygen than other indigenous fuel molecules found in typical middle distillates. Unfortunately, a more detailed understanding of the structural features of the sediment derived from DMP autoxidation is only partially available. On the basis of MS, IR, and solid-state 13C NMR, Frankenfeld et al.16 have proposed partial sediment structures, a few of which are shown below (1-111). These authors have suggested
I
II C
H\ ~ I
~
C
\O I
C
~
C
H
~
-
m that the DMP sediment "most likely arises from a free radical autoxidative oligomerization"in which the pyrrole ring remains mostly intact. However, Hoft et ale" have isolated and characterized by IR, UV, and lH NMR a single oxidation product (yield not mentioned) for the autoxidation of 2,4-DMP in refluxing benzene. The hydroxypyrrolinone structure (IV) was proposed. It is interesting to note that such structures are known to be the major product of the photochemical oxidation of alkylpyrroles. In addition,hydroxypyrroliionesare known to be reactive molecules which could conceivably be an important intermediate in DMP oxidative oligomer formation.20 Previous Articulation of the ET10 Concept. The ET10 concept is envisioned as an entire family of oxygenation mechanisms (not just one mechanism) in which the common feature is electron transfer from the oxidation substrate to molecular oxygen. We have previously proposed21*22the most simple depiction of the electron-transfer mechanism shown in Scheme 2 for the initial stages of the oxygenation of C-alkylpyrroles. The first step of the oxygenation involves the reversible formation of a molecular association complex (Le., chargetransfer complex) between the pyrrole and dissolved oxygen. This hypothesis is supported by previously published UV spectroscopic data;28 in addition, such associationcomplexesbetween triplet oxygen and electron(1s) Hoft, E.; Katritzky, A. R.; Nerbit, M. R. Tetrahedron Lett. 1967, 32,3041-3044; Tetrahedron Lett. 1968,16, 2028. (20)Irvine, D.G. Znt. Reu. Neurobiol. 1974, 16, 145-172. (21) Beaver, B. D.; Cooney,J. V.; Watkinr, J. M. J. Heterocycl. Chem. 1986,23,1096-1097. See a l m Fuel Sci. Technol. Znt, 1988,6(2), 131160. (22) Beaver, B. D.; Cooney, J. V.; Watkine, J. M. Heterocyclee 1986, 23,2848-2861. (23) Cooney, J. V.; Hazlett, R. N. Heterocycles 1984,22, 1613-1618.
Scheme 2
Molecular Association Complex
L
J
Electron Transfer Complex
Y
02''
Endoperoxide
4
7
I.I Final Producl(s)
Solvent Separated Ions
rich molecules are well-known.24 The rate-determining step depicted in Scheme 2 utilizes the alkylpyrrole as an electron donor and oxygen as an electron acceptor. Such a mechanistic sequence qualitatively explains why electron-rich pyrroles react more rapidly with oxygen compared with pyrroles that contain electron-withdrawing substituents.26 Additionally,this scheme accounts for the observation that there exists a correlation between the pyrrole anodic oxidation potential and the pseudo-firstorder rate constant for oxygenationeZ2As shown in Scheme 2, the electron-transfer complex is postulated to be in equilibrium with an endoperoxide intermediate. Such endoperoxides are believed to be intermediates in the photochemical dye-sensitizedoxidationof pyrroles.28Such a common intermediate for both sensitized photoxygenation and thermal oxygenation could account for the observation that many photochemical and thermal oxygenationsproduce similar types of oxidation producta.n-2e In light of the increase in the number of experimental reports that can be interpreted as being consistent with operation of the ET10 concept, we have extended our previous studies on the oxygenation of 2,bdimethylpyrrole.21*22The present work representa a more detailed investigationof the experimentalnature of the oxygenation of 2,5-dimethylpyrroleover the temperature range of 60120 OC. On the basis of this experimental work, we have modified the ET10 concept and discuss its potential relevance to the oxidative degradation of petroleum products.
Experimental Section Gas chromatography spectra were obtained with a Varian Model 3700 gas chromatograph(column30 m; 100% methylpolysiloxane) interfaced with a HP-3396A integrator. All eolventa were purchased from Aldrich and were ACS reagent grade or better and were used without further purification. 2,s-Dimethylpyrrole was purchased from Aldrich and was distilled prior to use. BHT, NP-dimethylcyclohexylamine and tris(4-bro(24) Buchachenko, A. L. R w s . Chem. Reo. 1986,64,117-128. (26) Campaigne, E.; Shutske, G. M.J. Heterocycl. Chem. 1974,11, 929-936. (26) Lightner, D.A.; Bieacchi, G.S.; No&, R. D. J. Am. Chem. SOC. 1976,!38,802-807. (27) Landen, 0. L.; Park, Y. T.;Lightner, D. A. Tetrahedron 19SS,39, 1893-1907. (28) Foote, C. 9. In Singlet Olcygen;W a m w " , H. H., Murray,R. W., W.; Academic Prees: New York, 1979; Chapter 6.
(29) The formation of both a dioxetane and/or an unrtable allylic hydroperoxide k also theoretically pomible.
Beaver et al.
458 Energy & Fuels, Vol. 8, No. 2, 1994
mopheny1)aminiumhexachloroantimonate were purchased from Aldrich and were used without further purification. General Procedure for Kinetic Runs. A 100-mL threenecked 14/20 round-bottomed flask was fitted with two rubber septa (on the outer necks). The inner neck of the flask was then fitted with a condenser which was equipped with a gas adapter and connected to an oil bubbler. A disposable pipet was connected to a regulated gas supply for use as a gas delivery tube. The pipet was inserted through one of the septa forming a seal which can withstand a slight positive pressure. The flask was immersed up to its neck in a Precision Instruments Model 260 constanttemperature water bath equipped with a recirculation pump capable of maintaining the temperature within 0.1 OC. A 50-mL solution of DMP (usually 0.02 M), any additives if desired, and tridecane (internal standard) were added to the three necked flask already immersed in the water bath. A 1-mL aliquot of the solution was removed and repeatedly subjected to quantitative GC analysis. The initial relative concentration of DMP was determined by the ratio of peak areas for DMP and tridecane. The reaction was started by slowly bubbling gas (usually 02)through the reaction solution. After the appropriate amount of time a 1-mL aliquot was removed from the flask and subjected to quantitative GC analysis in order to determine the final relative DMP concentration. Calculation of Reaction Rates. From the known amount of DMP dissolved, the molarity of the solution was calculated. This molarity was then assumed to equal the mean of the initial GC analyses. After time t (min), typically between 5 and 10% reaction, the peak area ratios were determined and assumed to be the fraction of the DMP still present. A percentage of the amount of DMP still remaining was calculated (based on the peak area ratios) and this percent was multiplied by the original molarity of the solution. Calculation of the initialrate then merely involved subtracting the two molarities and dividing by the time t (min). Confidence limits were calculated for most of the sets of initial rate measurements by using the expression
where X is the mean of a number of measurements, n, s is the standard deviation, and t is the student t variate for the 95% limit of confidence. Determination of Order of Reactants. Measurement of initial reaction rate at various concentrations of reactants allows the order in each reactant to be calculated. The initial rate at a given concentration would have the form rate, = k(DMP1''[O2]*
At a different [DMP] concentration, the reaction rate would be rate, = k[DMPl"[O,lb Solving for the order of the reactant ( a ) (DMP) yields the expression 0'
log[(ratel)/(rate2)l log[(DMP1)/(DMP,)I
This was the method used to also calculate the order for oxygen. Preparation of Deuterated DMP. Intoa 30-mL separatory funnel was added 2 mL of DMP and 2 mL of D2O. Vigorous shaking waa administered for approximately 1h, with the D2O being replaced with fresh D2O at the 30-min mark. Infrared spectroscopy revealed that the N-H peak (3410 cm-l) had been replaced by the N-D peak (2534 cm-l) to an extent of greater than 95%. The 1-D-DMP was utilized immediately. Jet Fuel Oxygen Overpressure Studies. All oxygen overpressure experiments were performed in a Model 6255 Precision Scientific Oven. The temperature was monitored with a Type K thermocouple connected to a Model KM1202 KaneMay Data Logger. The KM1202 waa programmed to record the temperature at 15-min intervals. The pressure in the LPR was set using a Model FA233 Wallace and Tiernan Series 1500
absolute pressure gauge. A 10-L LPR, whose design has been previously described,'2 was used in all experiments. A Mettler DL-21 automatic titrator was used for peroxide determinations. Analyses were performed according to ASTM D3703-86: the Standard Test Method for Peroxide Number in AviationTurbine Fuels.11 The method was modifiedto determine the end points potentiometrically rather than colorimetrically. A Mettler DM 140-SCcombined platinum ring electrodefor redox reactions was used in the potentiometric titrations. The two fuels in which an oxygen order dependency was observed have been previously described." Initial experiments with the Kuwaiti fuel were conducted at five oxygen partial pressures. The lowest oxygen partial pressure tested was that of atmospheric air pressure, i.e., assuming 21% oxygen at 14.7 psia, approximately 21 kPa of oxygen. Additional experimentswere performed at 50,75,and 100psiaof air pressure, Le., approximately 72,109, and 145 kPa of oxygen. The highest pressure tested, approximately 241 kPa (35 psia), was added as pure oxygen. The recorded pressure was that measured at the start of each test. In subsequent experiments with Fuel No. 2 the 109 kPa of oxygen (75 psia of air) test was omitted. The various pressures were tested in random order and not from lowest pressure to highest. This was to avoid the possibility that the observed increase in hydroperoxide formation rate with oxygen partial pressure was due to changes in the fuel during storage as the studies progressed. The fuel was not stored under an inert atmosphere between testa but was kept in cold storage continuously. A sample set consisted of five 100-mL aliquots of fuel in five separate 125-mL brown borosilicate glass bottles. Four vented bottles were placed in the LPR which was subsequently pressurized to the pressure of interest. The fifth bottle, used as a measure of the fuel's initial hydroperoxide concentration, was wrapped with aluminum foil and titrated at the same time as the 24-h sample. At 24h intervals the LPR pressure was measured andrecorded. The LPR was then depressurized and the sample was removed. The sample was allowed to cool at room temperature for 25 min followed by an &min ice bath quench. The sample was then titrated for hydroperoxide concentration by the previously described potentiometric method. Samples were also removed and analyzed at 48,72, and 96 h. The experiments using pure oxygen employed a slightly different preseurization/depreurization/repressurizationmethod. During initial pressurization at the start of the test and repreesurization after removal of samples at intermediate intervals, the reactor was purged with 10 reactor volumes (100 L) of oxygen followed by three 500 kPa (75 psia) pressurization/ depressurizationcycles. On the third cycle the pressure was slowly bled down to the set pressure of 241 kPa (35 psia). Prior to sample removal of intermediate intervals, the reactor was depressurizedand then purged with 10reactor volumes of nitrogen prior to to removal of the reactor lid. This was to avoid the hazard of releasing an enriched oxygen/fuel vapor mixture into the oven when the reactor lid waa removed.
Results and Discussion Nature of the Reaction of D M P with M o l e c u l a r Oxygen. Our interest in the mechanism of DMP oxygenation was piqued when preliminary studies suggested operation of a mechanism other than a generic peroxyl radical chain. For instance, although Cooney and Wechtelsohave noted the presence of an induction period:l the presence or absence of a hindered phenol antioxidant did not effect t h e initial rate of DMP oxygenation in diesel fuel. (30) Cooney, J. V.; Wechter, M.A. Fuel 1986,66,433-436. (31)We have occasionally observe sluggish DMP consumption which
can be due to an induction period. However, we cannot rule out that this observation ia merely the result of inherent experimental error in the
respective methods of rate determination.
Energy & Fuels, Vol. 8, No. 2, 1994 459
Oxygenation of 2,5-Dimethylpyrrole Table 1. Initial Rate (M/min) for 2,s-Dimethylpyrrole Oxygenation. exDt no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
initial rate (hconf limit) M/min 5.42 (h0.36) X 1.60 (t0.14) X 1.58 (h0.12) X 4.56 (h0.21) X 4.75 (h0.18) X 1.35 (ic0.13) X 9.75 (k0.49) X 2.89 (h0.21) X 1.07 (h0.41) X 2.30 (ic0.38) X no reaction 3.64 (t0.38) X 8.30 (h0.35) X 5.5 x 10-6 0.9 x 10-6 1.1 x lo" 2.0 x 10-6 5.30 (ic0.59) X 6.56 (h0.50) X 6.62 (h0.29) X 5.03 (t0.94) X 5.00 (ic0.93) X
10-6 10-6 lo" 10-6 10-6 10-6
lo" lo" lo" 10-6 10-6 le
10-6 10-6 10-6
lo" lo"
0 Experimental conditions: Exp. no. 10.02M DMP (nitrobenzene) at 50 OC, 100% 02; exp. no. 2 same as no.1 with 20% 0 2 ; exp. no. 3 same as no. 1 at 70 "C;exp. no. 4 same as no. 2 at 70 "C;exp. no. 5 same as no. 3 with toluene as solvent; exp no. 6 same as no. 4 with toluene as solvent; exp. no. 7 0.02 M DMP (dodecane) at 120 "Cwith 0.02 M NJV-dimethylcyclohexylamine,100% 02;exp. no. 8 same as no. 7 with 20% 0 2 ; exp. no. 9 same as no. 7 with 0.002 M DMP; exp. no. 10 same as no. 8 with 0.002 M DMP; exp. no. 11 0.02 M DMP (nitrobenzene) at 50 "C 100% nitrogen; exp. no. 12 same as no.11 with 0.3 mL of acetic acid added; exp. no. 13 same as no. 12 with 0.9 mL of acetic acid added; exp. no. 14 same as no. 1 with 0.02 M NJVdimethylcyclohexylamine;exp. no. 15 same as no. 14 with 20% 02; exp. no. 16 same as no. 14 with 0.04 M DMP; exp. no. 17 same as no. 15 with 0.04 M DMP; exp. no. 18 same as no. 1 with0.02 M BHT and 0.02 M NJV-dimethylcyclohexylamine;exp. no. 19 0.02 M DMP (methanol) at 60 "C, 100% 02; exp. no. 20 0.02 M 1-D-DMP (deuterated methanol) at 60 "C,100% 0 2 ; exp. no. 21 same as no. 7 with 0.02 M BHT added;exp. no. 22 same as no. 7 with 0.06 M BHT added.
In the most simple scenario for the initial stages of DMP oxygenation, there are many distinctly different mechanisms possible. This reaction could be initiated by (a) one electron transfer from DMP to oxygen followed by standard radical cation chemistry, vide supra. Alternatively, (b) an indigenous initiator radical could abstract a hydrogen atom from the NH and/or the methyl CH. A further possibility is (c) bimolecular initiation by DMP and 0 2 followed by radical addition to the unsubstituted end of a DMP double bonds to yield the known a-amino radi~al.3~ This reaction would be analogous to the classical mechanism for the uncatalyzed autoxidation of styrene33 and indene."' In each of the above cases, subsequent addition of the pyrrole radical to molecular oxygen would yield a pyrrole peroxyl radical. We herein report a more detailed study designed to clarify some of the mechanistic ambiguities for the initial stages of the reaction of molecular oxygen with 2,5dimethylpyrrole. Since Cooney and Wechter30 have reported that DMP consumption correlates well with oxygen uptake, we have monitored the DMP oxygenation by measuring DMP consumption with quantitative GC. In Table 1, experiments 1-8, is reported an initial rate (32) Beckwith, A. L.; Eichinger, P. H.; Mooney, B. A.; Prager, R. H. A u t . J. Chem. 1983,36, 719-739. (33) Miller, A. A,; Mayo, F. R. J. Am. Chem. SOC.1956,78,1017-1023. (34) (a) Russell, G. A. J. Am. Chem. SOC.1956, 78, 1041-1046. (b) Zhang, X.; Bordwell, F. G. J. Org. Chem. 1992,57,4163-4168.
Table 2. Summary of Reaction Order Data for DMP Oxygenation exp.
amine
solvent
temp,%
land2 3and4 5and6 14-17 14-17 7-10 7-10
no no no yes yes yes yes
nitrobenzene nitrobenzene toluene nitrobenzene nitrobenzene dodecane dodecane
50 70 70 50 50
120 120
DMPorder 02order
-
-
-
1.1 1.2 1.10 0.96
0.76 0.77 0.78 1.1 1.1 0.95 0.75
study for the oxygenation of DMP in nitrobenzene, dodecane, and toluene. These results can be interpreted as being consistent with a rate law which is approximately first order in both oxygen and DMP under the experimental conditions employed (see Table 2). During the course of this study we hypothesized that traces of acid might contribute to the consumptionof DMP via a nonoxygenation pathway. Radical cations are known to be acidicMband it is anticipated that loss of a proton from this putative intermediate in mechanism (a) would yield a stable radical species.
-
DMP" DMP' + H+ In light of this possibility, an acid-catalyzedDMP coupling reaction35 might be competitive with DMP oxygenation. This hypothesis was tested by measuring the initial rate for DMP consumption, in the absence of oxygen and in the presence of traces of added acetic acid. The results reported in Table 1, experiments 11-13, are consistent with the above hypothesis. In light of these results, the initial rate for DMP oxygenation was measured, in the presence of an acid scavenger (1equiv of a tertiary amine, NJV-dimethylcyclohexylamine). As summarized in Table 2, the presence of the amine additive does presumably inhibit the acid-catalyzed loss of DMP with a concomitant increase in the accuracy for measuring the oxygen order dependency. It is apparent from Table 2 that, with the solvents examined,the DMP oxygenationexhibits a rate law which is approximately second order overall, first order in DMP, and first order in oxygen. This experimental observation is totally consistent with the operation of the ET10 mechanism, scenario a, and is inconsistent with operation of a generic peroxyl radical-chain mechanism, scenario b, vide infra. In addition, this result is inconsistent with the operation of mechanistic scenario c, which would exhibit a rate la+4a with [021°.6and [DMP13/2. The rate law for a peroxyl radical-chain mechanism depends critically on the nature and the relative rates of the initiation and termination steps. Consequently,there is a circumstance where mechanistic scenario b could still be viable. In most peroxyl radical-chain mechanisms (i.e., AIBN initiation, air-saturated solutions) termination is described by reaction 1. If such a peroxyl radical-chain
ROO* R*
+ R*
+R*
-1
molecular produds
(1a) (1b)
mechanism were operative, a zero-order dependency in 02 and first-order dependency for DMP would be expected? However, some peroxyl radical-chain mechanisms (35) Joule, J. A.; Smith, G. F. Heterocyclic Chemistry, 2nd ed.; Van Noetrand Reinhold Wokingham, Berkshire, UK, 1978; pp 202.
460 Energy &Fuels, Vol. 8,No. 2, 1994
Beaver et al.
do exhibit an oxygen order d e p e n d e n ~ y .For ~ ~ instance, if R' is a particularly stable radical, termination steps (la and lb) would be operative, and the overall rate law becomes3'
Scheme 3
-d[OJ/dt = k2kp[RH][O~lR~"2/(2k2k,[0~12 + 4k2k,k4,[RHl [021+ 2kp2klb[RH12)'/2 In order to test the possibility of operation of such a mechanism for DMP oxygenation the order in both DMP and 02 was determined in dodecane at 120 "C. If the above rate law described the DMP oxygenation, the observed order in 02 would approach zero as the DMP concentration is successively decreased, since the latter two terms in the denominator become less important. These experiments were performed at DMP concentrations of and M. Table 1,experiments 7-10, reveal that at both of these DMP concentrations the reaction is approximately first order in both DMP and 02.This result suggests that the operation of the mechanism described by the above rate law is not likely under our experimental conditions. In order to additionally test the possibility that DMP oxygenation occurs via a peroxyl radical-chain mechanism, oxygenation's were performed in the presence of an equimolar concentration (based on DMP) of a hindered phenol antioxidant. Interestingly, with dodecane as solvent the presence of BHT inhibits DMP oxygenation. In Table 1, experiment 7 vs. 21, a reduction in rate of ' is noted. This observation is consistent with about 52% the presence of a peroxyl radical in DMP oxygenation in dodecane. However, when the BHT concentration is increased to 3 equiv, no additional decrease in DMP loss is noted (Table 1, experiment 22). This observation suggests that a portion of the DMP oxygenation is occurring by a mechanism which does not have a peroxyl free-radical intermediate. With nitrobenzene as solvent, Table 1,experiment 1vs. 18, reveals that the presence of BHT/amine only minimally effectsthe initial rate for DMP oxygenation. This result also is inconsistent with operation of a peroxyl radical-chain mechanism for DMP oxygenation, i.e., scenario b. The nonviability of operation of mechanistic scenario b (case of NH abstraction) was additionally confirmed by a simple experiment. It was anticipated that deuteration of the DMP nitrogen would not affect the initial rate of DMP oxygenation (i.e., absence of a primary kinetic isotope effect). This hypothesis was tested and found to be valid when deuterated DMP oxygenation is performed in deuterated methanol (Table 1, experiments 19 and 20). The last 15 years has resulted in the recognition of a novel oxygenation reaction for certain types of dienes38 and olefins.39 For instance, many workers40 have been examining the oxygenation of tetraalkyl olefinscontaining alpha-branched alkyl groups which hold their alpha C-H bonds near the nodal plane of the olefin system. When such olefins are subjected to one-electron oxidation, the (36) We are grateful to a referee for this observation. (37) Howard,J. A. HomogeneousLiquid Phase Autoxidations. InFree Radicals, Kochi, J. K., Ed.;Wiley-Interscience: New York, 1973;Vol. 2, PP 32. (38) Barton, D. H. R.; Haynes, R. K.; Leclerc, G.; Magnus, P. D.; Menzies, D. J. Chem. SOC.,Perkin 1 1975, 2056-2065. (39) Nelson, S. F.; Akaba, R. J.Am. Chem. SOC. 1981,103,2096-2097. See also: Clennan, E. L.; Simmons, W.; Almrren, C. W. J.Am. Chem. Soc. 1981,103, 2098-2099. (40) For instance see, Neson, S.F.; Teasley, M. F. J. Org. Chem. 1986, 51, 3474-3479; Akaba, R.; Sagkuragi, H.; Tokumaru, K. Tetrahedron Lett. 1984,25, 665-668.
. . .
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resulting cation radicals are unusually stable, which facilitates their spectroscopic investigation. In the presence of molecular oxygen, it has been shown40that these cation radicals add molecular oxygen, which concomitantly results in the initiation of the chain oxygenation mechanism shown in generic form (with DMP as the substrate) in Scheme 3. Since the slow step in Scheme 3 is still proposed to be one-electron transfer from DMP to oxygen, this mechanism is kinetically indistinguishable from Scheme 2. The intriguing feature of this mechanism is that it utilizes an intermediate [closed oxygenated radical cation (C)] which is postulated to be a more potent oneelectron oxidant than molecular oxygen (for the case of olefinic oxygenated cation radicals). Among the oneelectron oxidants which have been shown to initiate the thermal oxygenation of the above-mentioned dienes and olefins is tris(4-bromopheny1)aminium hexachloroantimonate. Preliminary data (not shown) indicate the presence of catalytic amounts of this salt result in a significant enhancement of the observed initial rate for DMP oxygenation (in nitrobenzene, 50 "C). In light of this preliminary experimentalobservation, and in addition to the previously noted effect@) of BHT on DMP oxygenation, we have modified the original ET10 mechanism21.22to include the solvent-separated DMP radical cation and the superoxide anion being in equilibrium with the electron-transfer complex (Scheme 2). In this manner the above-mentioned catalytic effect of the antimonate salt can be readily accommodated. Presumably, the antimonate salt increases the concentration of the solvent separated DMP radical cation (by direct one-electron oxidation of DMP) which subsequently can react with molecular oxygen to form the oxygenated DMP radical cation (as shown in Scheme 3). It also is assumed the resultant oxygenated intermediate can initiate a radical cation chain oxygenation as depicted in Scheme 3. In addition, the presence of the solvent-separated DMP radical cation would be expected to ultimately initiate a peroxyl radical chain which is consistent with our results in dodecane, vide infra. This mechanistic proposal is significant since it allows a rational explanation for the observed DMP oxygenation even though the postulated rate-limitingstep is most likely not thermodynamically feasible.41 In addition, this mechanistic modification allows for the potential of proton-
Oxygenation of 2,5-Dimethylpyrrole 50.00
Energy & Fuels, Vol. 8, No. 2, 1994 461
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Figure 1. (a, top) Oxygen overpressure studies at 100 "C of Kuwait No. 1JP-5. (b,bottom) Same experimentswith ordinate expanded.
Figure 2. (a,top) Oxygen overpressure studiesat 100 O C of JP-5 blending stock (Fuel No. 2) doped with 22.6 mg/L BHT. (b, bottom) Same experiments with ordinate expanded.
induced disproportionation of superoxide as an additional driving force.42 Jet Fuel Oxygen Overpressure Studies. The relevance of our model DMP oxygenation study to actual fuel degradation is supported by the study summarized in Figures 1and 2. In Figure l a is revealed the effect of oxygen overpressure on peroxide formation in Kuwait No. 1JP-5. Figure l b depicts the same data with an expanded ordinate. It can be seen that there is a considerable effect of oxygen overpressure on peroxide formation. Similar results were obtained with Fuel No. 2 which contains approximately 22.6 mg/L of a phenolic antioxidant. Figure 2areveals that increase in oxygen partial pressure increases the rate of peroxide formation. Figure 2b is the same data shown with an expanded ordinate. Much previous work has shown that, when jet fuel oxidatively degrades via a peroxyl radical-chain mechanism, the rate of hydroperoxide formation, i.e., the rate of oxidation, is independent of oxygen partial pressure above approximately 10 kPa." The data in Figures 1and 2, however, reveal that the oxidation of these fuels does have an order dependence in oxygen. The most logical interpretation of this data is that hydroperoxide formation occurs via a peroxyl radical-chain mechanism in which the initiation step is an ET10 reaction. In addition, this initiation step is not inhibited by phenolic antioxidants.
Such a mechanism would have an order dependency in oxygen for peroxide formation and could account for the results observed in Figures 1and 2. This proposal is also consistent with our results for DMP oxygenation in dodecane.
(41) Andrieur, C. P.; Gareil, M.; Pinson, J.; Martin,D.Fuel Sci. Technol. Int. 1993, II(1), 29-55. 142) Chin, D. H.; Chiericato, G.; Nanni Jr., E. J., Sawyer, D. T. J . Am. Chem. SOC.1982,104,1296-1299.
Conclusion We have provided experimental evidence that reveals that the oxygenation of DMP, in nitrobenzene, has a rate law which is second order overall and first order in both DMP and 0 2 . The presence of an equimolar concentration of BHT was found to not effect DMP oxygenation. These experimental observations, along with other observations cited earlier, are consistent with the mechanism@)depicted in Schemes 2 and 3. The lack of inhibition by BHT could be due to the ability of nitro compounds to act as radical traps, thus suppressing peroxyl radical-chain formation. In addition, nitrobenzene would be expected to significantly stabilize the polar transition required by Scheme 2.
In dodecane and toluene, and presumably other nonpolar solvents, DMP oxygenation is approximately first order in both DMP and oxygen. In dodecane the presence of BHT significantly inhibits DMP oxidation which is consistent with the intermediacy of a peroxyl radical in nonpolar solvents. Chain initiation by electron transfer (or hydrogen atom transfer) from DMP to oxygen could yield active oxygen and the radical cation (or free radical) of DMP. Pyrrole radical cations are known to be generally
Beaver et al.
462 Energy & Fuels, Vol. 8, No. 2, 1994
unstable43144and could fragment into radical species,which in turn, could initiate a peroxyl radical-chain mechanism. Significance of the ET10 Concept for Enhancing Our Understanding of Oxidative Degradation of Petroleum Products. We propose that the occurrence of ET10 during the oxidative degradation of a petroleum product would be indicated by an oxygen order dependency in the rate law for degradation and (usually) a significant degree of unresponsiveness to the presence of standard peroxyl-chain-breaking antioxidants. There are two distinctly different ways in which the ET10 hypothesis can account for unresponsiveness to peroxyl-chain-inhibiting antioxidants. The first possibility is that the ET10 mechanism could generate free radicals which only partially diffuse away from the electron-transfer solvent cage. If this were the case, phenolic antioxidants would inhibit the ET10 initiated peroxyl-chain mechanism but not necessarilythe ET10 pathway. A second possibilitywould be a direct ET10 reaction of indigenous fuel molecules with dissolved oxygen to directly form insolubles or their precursors. Two literature examples which can be viewed as consistent with the latter possibility are the simulated oxidativedegradation of both coal-derivedliquids reported by Brown and Karns45 and bitumens reported by Knoter~s.~6
Although experimental evidence consistent with the ET10 concept has only been provided for 2,5-dimethylpyrrole, it is likelythat other structural classesof molecules can undergo this type of oxygenation chemistry. Molecules that have oxidation potentials in the same range as DMP include, alkyl polycyclic aromatics, alkylphenols, and arylamines (very preliminary evidence suggests that formation of the oxygen charge-transfer complex is also very important). Consistent with this hypothesis is the observation that use of LCO blending stocks,which contain indigenous alkyl phenols, is usually deleterious to the storage stability of the resultant diesel/LCO fuels.47 In addition, the presence of various arylamines that “survive” catalytic hydrotreatment of heavier feed stocks& might produce “ET10 active molecules” which could degrade the storage stability of subsequent finished products.
(43) Tabba, H. D.; Smith, K. M. J. Org. Chem. 1984,49,1870-1875. Andrieux, C . P.; Audebert, P.; Hapiot, P.; Saveant, J. M. J. Am. Chem. SOC.1990, 112, 2440-2442. (44)Diaz, A. F.; Martinez, A.; Kanazawa, K. K.; Salmon, M. J . Electroanal. Chem. 1981, 130, 188-187.
(45) Brown, F. R.; Karn, F. S . Fuel 1980,59,431-435. (46)Knoterus, J. Znd. Eng. Chem. Prod. Rea. Deu. 1972,11,411-422. (47) Power, A. J.; Mathys, G. 1. Fuel 1992, 71,903-908. (48)Schmitter, J-M.; Ignatiadis, I.; Dorbon, M.;Arpino, P.; Toulhoat, H.; Huc, A. Fuel 1984,63, 557-664.
Acknowledgment. We are grateful to the Office of Naval Research, the Naval Research Laboratory, and the Air Force Office of Scientific Research for their generous financial support. In addition, we thank the R. K. Mellon Foundation for a summer fellowship to J.D.K., and the ALCOA Foundation for a summer fellowship to G.S.M. ~
