Energy & Fuels 1998, 12, 1369-1374
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Spectroscopic Studies of Oxidative Degradation of Base Oils A. Adhvaryu,*,† J. M. Perez,† I. D. Singh,‡ and O. S. Tyagi‡ Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, and Indian Institute of Petroleum, Dehradun 248 005, India Received June 15, 1998. Revised Manuscript Received August 20, 1998
NMR and FTIR spectroscopies have been used for obtaining component-level information on the thermo-oxidation of some base oils. Base oils were first separated by column chromatography into their saturates, aromatics, and polars. These fractions as well as the neat base oil were oxidized by a modified IP 306 benchtop (simulated) method. A 1H and 13C NMR-derived structural model was utilized to monitor the structural changes and rearrangements occurring in the carbon skeleton of the hydrocarbons during oxidation of base oils/fractions. FTIR was used to measure variations in the concentration profiles in the carbonyl absorption region (1649-1815 cm-1). Difference FTIR spectra and deconvolution techniques were employed to detect new compounds formed as well as minor concentration changes in oil components. NMR and FTIR results were utilized to study the role of different hydrocarbon types in the oxidation of base oil and to establish the reaction mechanism responsible for the degradation of base oils.
Introduction NMR (1H and 13C) and FTIR spectroscopic analysis methods are fast becoming workhorse techniques for providing analytical data on degradation of lube base oils. These data are essential in the formulation of longlife lubricating oils and tribological investigations. The current understanding of lubricant oxidation is still not fully developed. This is due to extremely complex processes1 involving primary oxidation product formation and subsequent conversion to high molecular weight oxidation products.2,3 Very little information is available on the role of different components of base oils in the degradation process. This is of much greater importance than normally perceived. Base oils, by virtue of differences in the nature of their sources (crude oils) and production methodologies, are highly complex mixtures of a large number of hydrocarbons having wide variations in their chemical structure and composition. The saturate fractions are usually a complex mixture of normal and isoparaffin chains and cycloalkanes, while the aromatic concentrate is a mixture of mono-, di-, and polyaromatic compounds having high naphthenic and alkyl substitution on peripheral aromatic carbons. Infrared spectroscopy is a potential analytical tool for the study of lubricant degradation through monitoring additive depletion, degradation products, and fuel contaminants. The formation of oxidation products is further complicated by the nature of the base oil as well as the presence and influence of blended additives. Additives can change the specific compounds produced and their rates of formation.4 Earlier investigations in †
Pennsylvania State University. Indian Institute of Petroleum. (1) Emanuel, N. M.; Denisov, E. T.; Maizns, Z. K. Liquid phase oxidation of hydrocarbons; Plenum Press: New York, 1967; p 191. (2) Kreuz, K. L. Lubr. Eng. 1970, 56 (6), 77. (3) Hercamp, R. D. SAE Paper No. 831720, 1983. ‡
this area were mainly directed toward additive depletion and gross formation of oxidation products,5-7 where data generation was limited to manual measurements from chart recordings.7 In most cases, measurement of a carbonyl oxidation product was confined to a single frequency. Of late, with the application of computerassisted IR spectroscopy, spectral data can be mathematically processed to derive more meaningful information from the spectra of degraded base oils.8 Various investigators9,10 have reported the measurement of the total oxidation product by integral area calculation, where oxidation has been measured as the gross change in composition. Moreover, computer-assisted FTIR spectroscopy can give significantly better sensitivity to small changes in spectral data, which is essential in obtaining detailed information about the chemical processes involved in oxidative degradation and its inhibition by additives. Some very important investigations in this direction are those of Coates et al.11 and Hsu et al.12 Nevertheless, a wide gap exists in the knowledge of the chemistry of the transformation of various chemical species (hydrocarbon group types) in the degrading process of lube oils. In the present paper, some of the findings on the oxidative behavior of selected base oils using 1H and 13C NMR-derived structural parameters are reported. This (4) Coates, J. P.; Setti, L. C. ASLE Trans. 1985, 29 (3), 394. (5) Barcello, J. R.; Otero, C. J. Inst. Pet. 1964, 50 (481), 15. (6) Stavinoha, L. L.; Wright, B. R. SAE Paper No. 690776, 1969. (7) Willermet, P. A.; Mahoney, L. R.; Bishop, C. M. ASLE Trans. 1979, 23 (3), 217. (8) Coates, J. P.; Setti, L. C. SAE Paper No. 831681, SP-558, 1983, 37. (9) Wooton, D. L.; Lawrance, B. J.; Damrath, J. G. SAE Paper No. 841372, SP-589, 1984, 71. (10) Coates, J. P.; Setti, L. C.; McCaa, B. B. SAE Paper No. 841373, SP-589, 1984, 81. (11) Coates, J. P.; Setti, L. C. ASTM, STP 916, 1987, 57. (12) Hsu, S. M.; Ku, C. S.; Pei, P. T. ASTM, STP 916, 1986, 27.
10.1021/ef980134m CCC: $15.00 © 1998 American Chemical Society Published on Web 10/08/1998
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Table 1. Physiochemical Characterization of Base Oils Used in the Study API gravity, deg kinetic viscosity (cSt) at 100 °C kinetic viscosity (cSt) at 40 °C viscosity index aniline point, °C sulfur, wt % nitrogen, ppm mol wt (VPO) % volatilitya % depositsa
AA-10
AA-11
AA-12
AA-13
AA-14
28.4 4.85
27.1 9.62
26.5 14.02
24.4 33.48
26.5 10.13
27.66
82.68
150.03
544.03
89.53
94 93.8 1.01 331 360
93 102.4 0.95 389 455
89 106.8 1.34 330 515
94 119.2 1.29 375 735
92 102.2 1.20 324 440
80.17 2.62
78.79 4.15
73.19 12.23
70.28 15.17
75.23 10.47
a At 225 °C for 30 min using PSMO [PMSO ) 13.8 + 2.92NOACK].
procedure gave a fair understanding of the various carbon types Car, Car,alk, Cnp, Cip, CN, etc., and their relative concentrations in the base stocks both before and after thermo-oxidation. The variation in the carbon skeleton due to structural rearrangements and molecular conversions were monitored in terms of changes occurring in the average structural parameters of various chromatographic fractions and neat base oils. Both FTIR and (1H, 13C) NMR spectroscopies were found to be useful in understanding the molecular chemistry involved in the oxidative degradation of base oils. Experimental Section Base oils AA-10, AA-11, AA-12, AA-13, and AA-14 were selected for the present study. AA-15-S is a synthetic base oil containing 77% saturates and 23% aromatics mixed in weight proportion and contains no polars. These base oils were separated into saturates, aromatics, and polars using a modified ASTM D 2549 method based on column chromatography.13 Table 1 presents the physical properties of the base stocks used in this study. Oxidation Study. The base oils and their chromatographic fractions (saturates and aromatics) were oxidized using a modified IP 306 benchtop method.14 In this method, the sample (25 g) was placed in a glass reactor and maintained (in static mode) at 120 °C for 48 h in the presence of activated Cu wire catalyst and dry O2 was passed through it at the rate of 1 L/h. Spectroscopic Studies. Infrared spectra were recorded on a Perkin-Elmer 1760X FTIR system equipped with a KBr beam splitter. The regular scanning range used for the samples was 400-4000 cm-1. The signalto-noise ratio was 3000:1, and a spectral resolution of 4 cm-1 was used. The scan speed optimally used was 1 cm/s OPD velocity. The system has a temperaturestabilized detector having FR-DTGS coating. In all cases, neat samples were analyzed in a standard cell having a fixed path length of 1 mm in order to observe small changes occurring in the spectra outside the regions of the main hydrocarbon absorption. The spectra were recorded under interleaved mode. The same cell was used for both neat and oxidized samples in order to cancel any effect of the cell characteristics, (13) ASTM-2549, ASTM manual on Hydrocarbon Analysis, 4th ed.; Drews, A. W., Ed.; Anual Book of ASTM Standards, Section 5, 1995. (14) IP 306. Standard methods for Analysis and Testing of Petroleum and related Products and British Standards 2000 parts; Institute of Petroleum Handbook, 1997.
Figure 1. Percent increase in the area of the carbonyl region (1650-1815 cm-1) on oxidation of saturates, aromatics, and synthetic blend.
path length, and sample thickness on the difference spectra. The data processing involved transmittanceabsorbance conversion, absorbance subtraction, overlay, difference spectra, and spectral deconvolution. The 1H and 13C NMR spectra were recorded quantitatively in the Fourier transform mode on a JEOL FX 100 pulse spectrometer equipped with a computer having 24K memory. Observing frequencies of 99.5 and 25.0 MHz were used for the 1H and 13C experiments, respectively. A pulse length of 12 µs corresponding to a 90° flip angle and a pulse delay of 4 s were used for the 13C measurements. Cr(acac)3 was used as a relaxation reagent, and a gated proton decoupling sequence was used to further reduce the NOE. The samples were dissolved in CDCl3 in 50% w/v concentration with TMS as the internal standard. A spectral width of 5000 Hz was used in all cases. For 1H experiments, the samples were dissolved in CDCl3 at 15% w/v concentration and scanned over a spectral width of 1000 Hz with a flip angle of 90° corresponding to a pulse width of 15 µs; a delay of 10 s was used. Results and Discussion The average structural parameters computed from the (1H and 13C) NMR measurements on neat saturated concentrate do not show significant changes in the percentage composition of normal, iso-, and cycloparaffinic carbons following its oxidation. However, the average chain length (ACL) was found to decrease compared to the unoxidized saturated concentrate. This fact suggests that saturates (as a bulk) are relatively thermally stable. The long aliphatic side chains on condensed naphthenic rings were possibly cleaved through a radical-induced path, which could account for the decrease in the ACL observed in such systems. Figure 1 shows the change in the area of the carbonyl absorption (1650-1815 cm-1) peak in the IR spectra due to the formation of certain oxygenated compounds during oxidative degradation. Peripheral naphthenic rings of condensed saturated structures having aliphatic side chains are potential candidates for such reactions. A similar observation has also been reported by Murray et al.15 The partial overlayed FTIR spectra of the oxidized saturated concentrate (Figure 2) show, besides C-H stretching band, a broadening in the 2500-3500 cm-1 region, which indicates the formation of carboxylic (15) Murray, D. W.; MacDonald, J. M.; White, A. M.; Wright, P. G. Proc. 11th World Pet. Congr. 1984, 4, 447.
Oxidative Degradation of Base Oils
Energy & Fuels, Vol. 12, No. 6, 1998 1371
Table 2. Variation of the NMR-Derived Parameters of Aromatic Concentrate upon Oxidation unoxidized aromatics oxidized aromatics
%Car
%Car,Me
%Car,alk
%Car,j
%Car,b
%Csat
%Cnp
%CN
%Cip
fc
ACL
C/H
19.6
1.6
6.4
16.4
3.2
80.4
30.7
17.5
33.2
0.2
8.1
0.5
21.6
0.0
7.2
17.2
4.3
78.4
32.9
19.5
26.0
0.2
7.0
0.6
Figure 2. Partial FTIR spectra of oxidized saturated concentrate.
acid. The IR peaks at 1175 and 1027 cm-1 are likely associated with low-medium molecular weight aliphatic esters produced during oxidation reaction. The 1H and 13C NMR results (Table 2) show that the aromatic concentrate has a high percentage of naphthenic carbons (%CN) attached to aromatic structures (%Car). These rings are highly substituted by normal (%Cnp) and isoparaffinic groups (%Cip). This fact is further supported by the high %Car,Me and %Car,alk value, indicating high aromatic substitution through methyl and alkyl (n- and iso-) groups. The value of %Car,b (3.2) also indicates a substantial amount of carbons at condensed points. The above structural details at the molecular level suggest that aromatics are relatively more prone to oxidative degradation. An extensive NMR study on the oxidized fraction of neat aromatics (Table 2) show that the %Car increases while the %Csat decreases during the oxidation of the oils. Thermal cleavage and evaporative loss of the small fragment (C2-C3) (Table 1) from long-chain paraffinic (n- and iso-) compounds as well as long-chain alkyl groups substituted on aromatics could be the most likely reason for such an increase in the concentration of %Car as a percentage of total aromatic concentrate. Similar observations have also been reported earlier by Naidu et al.16 The data also show an increase in the concentration of alkyl-substituted aromatic carbons (%Car,alk) upon oxidation, associated with a corresponding decrease in %Car,Me. This suggests an alkylation reaction through a radical pathwaysa phenomenon that is more likely in such oxidation reactions.15 Highly reactive alkyl radicals, generated from long alkyl side chains, end up getting attached to radical centers on the aromatic rings, as these sites are more stable through resonance. A (16) Naidu, S. K.; Klaus, E. E.; Duda, J. L. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 613.
Figure 3. Partial FTIR spectra of aromatic concentrate and its oxidized product: unoxidized aromatics (s); oxidized aromatics (- - -).
detailed MS study by Murray et al.15 has also revealed a relative increase in the concentration of alkyl benzenes during oxidation of lube base oils. The present experimental data on carbons at condensed points (Car,b), compactness factor (fc), and C/H ratio show an increase in their values upon oxidation. Physical observations have shown an increase in the viscosity of the system as the oxidation proceeds, suggesting formation of high molecular weight, condensed polymeric oxygenated compounds which are essentially sludge precursors. An overlay of the FTIR spectra of the oxidized and unoxidized neat aromatic fraction, Figure 3, shows three major peaks at 1715, 1775, and 1840 cm-1. A peak at 1740 cm-1, observed earlier as a shoulder, was also resolved through spectral deconvolution. As oxidation progressed, it was observed that the absorption of the 1715 and 1775 cm-1 peaks increased significantly. The 1740 cm-1 band arises mainly from ester-type compounds, while the 1715 cm-1 band is from ketonic species. The 1775 cm-1 band is expected mainly from five-membered ring lactones with a possible contribution from peroxy ester type compounds.4 It is inferred from these results that the aromatics (mainly the naphtheno aromatics and polyaromatics) are more thermally unstable and readily undergo thermo-oxidation to form oxygenates (polar
1372 Energy & Fuels, Vol. 12, No. 6, 1998
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Table 3. Variation of NMR-Derived Structural Parameters of Synthetic Blend upon Polar Doped Oxidation AA-15-Sa + 0% polarb AA-15-Sa + 1.2% polarb
%Car
%Car,H
%Car,alk
%Car,j
%Car,b
%Csat
%Cnp
%CN
%Cip
fc
σ
ACL
3.5 4.4
2.7 2.3
1.2 1.7
3.9 4.0
0.0 0.3
96.5 95.6
37.4 39.4
34.7 23.7
24.4 32.5
0.0 0.1
0.3 0.4
25.4 16.2
a Synthetic base oil containing 77% saturates and 23% aromatics. b Column chromatographycally separated (mainly sulfur and nitrogen heterocyclics).
Table 4. Decrease in the Average Chain Length of Aromatic and Saturated Concentrate of Base Oils upon Oxidation AA-10
AA-11
AA-12
AA-13
AA-14
sat arom sat arom sat arom sat arom sat arom unoxidized 24.9 7.9 23.9 9.0 24.5 11.5 33.5 11.7 32.8 12.2 oxidized 19.8 5.8 21.9 6.4 23.2 9.7 32.1 10.2 31.0 9.1 % decrease 20.7 26.5 8.3 29.1 5.3 16.0 4.3 12.9 5.6 24.7
Table 5. Variation of NMR-Derived Parameters on Oxidation of Different Base Oils %Car %Car,alk %Car,b %Csat %CN %Cp
Figure 4. Partial overlayed FTIR spectra of polar doped oxidized synthetic blend.
compounds), which then undergo a condensation polymerization reaction and increase the viscosity of the oil. The oxidation studies on the synthetic blends (of saturates and aromatics mixed in their yield proportions) have shown that they follow an oxidation profile that is intermediate to its constituent hydrocarbons (Figure 1). This explains why the presence of saturates in the synthetic blend significantly lowers the carbonyl absorption to less than one-half compared to the neat aromatic oxidation. Table 3 presents the NMR-derived parameters on a polar doped synthetic blend. Their detailed analysis in terms of structural changes in the carbon skeleton shows that, in the polar doped fraction, Car (as a percentage of the total stock) has increased (4.4%) while the unsubstituted aromatic carbons (%Car,H) have decreased, with a significant increase in Car,alk (1.7%). Polars present in the base oils are mainly sulfur, nitrogen, and oxygen heterocyclics. Though sulfur compounds are believed to have some antioxidant character, basic nitrogenous compounds function as a pro-oxidant. The NMR data also indicates that polars play a role in the radical-initiated alkylation reaction that is closely associated with lube oxidation. A significant decrease in ACL (16.2) with a corresponding increase in both %Car,alk and %Car,b values was observed. These results suggest that condensation of the aromatic ring has taken place and that the resulting condensed structures have an alkyl substitution with a much shorter chain length, leading to an increase in the viscosity. An overlay of the difference FTIR spectra of the synthetic blend in the carbonyl region (Figure 4) show two major peaks at 1715 and 1775 cm-1 assigned, respectively, to ketonic and lactone/ester species in the oxidized oil. The peaks at 1688 and 1675 cm-1 that occur as broad shoulders and could be resolved through spectral deconvolution are assigned to R,β unsaturated carbonyl compounds and benzoic acid type moieties.
unoxidized AA-10 oxidized AA-10 unoxidized AA-11 oxidized AA-11 unoxidized AA-12 oxidized AA-12 unoxidized AA-13 oxidized AA-13 unoxidized AA-14 oxidized AA-14
5.3 9.1 5.6 8.3 3.9 9.9 4.3 6.6 3.3 7.5
1.6 2.6 2.1 3.1 1.5 3.6 1.8 2.5 0.7 2.2
0.9 1.6 0.1 1.6 2.3 1.3 0.2
94.7 90.9 94.4 91.7 96.0 90.2 95.7 93.4 96.7 92.5
19.1 20.9 22.4 25.4 22.9 27.4 30.7 29.5 23.0 27.9
75.6 70.0 71.9 66.4 73.1 62.7 65.0 63.9 73.7 64.6
σ
ACL
0.3 0.4 0.4 0.6 0.5 0.4 0.4 0.5 0.6 0.4
16.4 15.0 17.6 15.9 18.2 16.4 18.5 17.0 17.2 14.6
Figure 5. Variation of %Car,alk on oxidation of base oils.
A detailed 13C NMR study on the chromatographically separated fractions of oxidized base oil have shown that ACL decreases both in the saturated and aromatic concentrate upon oxidation (Table 4). The decrease in the aromatic concentrate was from 7.9 to 5.9 (26.5% reduction), while in the saturate the decrease in ACL was from 24.9 to 19.8 (20.7% reduction). These data suggest that the alkyl part can originate from the normal and/or isoparaffinic structures of the saturated concentrate as well as from the long alkyl side chains attached to aromatic ring structures in aromatic concentrates. The migration of the broken alkyl group from the aromatic side chain (through radical path) accounts for maximum alkylation on the unsubstituted aromatic carbons. On the basis of the inferences drawn from the study of simplified saturated and aromatic concentrates as well as their synthetic blends, the structural changes during oxidation of neat base oils are explained. Table 5 presents the NMR data on neat and oxidized base oils. In general, the %Car,H values have decreased with an increase in the %Car,alk (Figure 5). This is further substantiated from the decrease in the ACL. The increase in the percentage of aromatic carbons at
Oxidative Degradation of Base Oils
Figure 6. Partial 13C NMR spectra of a typical unoxidized (A) and oxidized (B) base oil.
Figure 7. Variation of the polar (wt %) content with the saturated and aromatic content of base oil on oxidation (P1 ) unoxidized base oil; P2 ) oxidized base oil).
condensed points (%Car,b) also supports the view on the formation of condensed aromatic structures. The formation of polycondensed aromatic structures is further confirmed from the appearance of sharp peaks in the 128-130 ppm range in the 13C NMR spectra. These are mainly from oxygenated products (polars) formed during thermo-oxidation of base oils and finally end up as oilinsoluble deposits (Table 1). Similar observations of the formation of polyaromatic condensed structures on zeolite catalysts have also been reported by Fonseca et al.17 Figures 7 and 8 illustrate the effect of different carbon types on the yield of polar fractions separated chromatographically from oxidized base oils. It may be seen that the polar yield on neat oxidation decreases with an increase in the saturated carbons of the base oil. It is also observed that normal and isoparaffinic type hydrocarbons of the saturated fraction behave almost in a similar manner during thermal oxidation (Figure 8). The larger slope observed in case of isoparaffins is most likely due to the higher reactivity of these compounds in the radical-initiated oxidation reaction. The presence of naphthenic carbons in oil decreases its oxidation stability.15 The aromatics, on the other hand, display an opposing trend (Figure 9). The changes in %Car and %Car,alk values with the polar-yield-percent indicate that the carbons in the aromatic rings are stable and do not take part in the oxidative degradation of oil, but those to which alkyl groups are attached do take part in it. This effect is more pronounced when (17) Fonseca, A.; Zeuthen, P.; Nagy, J. B. Fuel 1996, 75 (12), 1363.
Energy & Fuels, Vol. 12, No. 6, 1998 1373
Figure 8. Variation of the polar (wt %) content with %CN, %Cnp, and %Cip of base oils on oxidation.
Figure 9. Variation of the polar (wt %) content with %Car and %Car,alk of base oils on oxidation.
Figure 10. Partial FTIR spectra of a typical base oil on oxidation overlayed with its deconvoluted spectra.
the ACL value is large. Hoo et al.18 have also reported similar observations. Thermal oxidation of base oil results in the formation of a complex carbonyl envelope in FTIR spectra from an array of oxygenated carbonyl compounds, which made it difficult to obtain quantitative information on individual compound types. However, the deconvolution of different spectra showed two major components centered around 1715 and 1775 cm-1 (Figure 10). The former was broadened due to the bands at 1700 and (18) Hoo, G. H.; Lewis, E. Proceedings of the International Symposium on Production and Application of Lube Base Stocks; Singh, H.; Rao, T. S. R., Eds.; TATA MacGraw Hill Publishing: New Delhi, 1994; p 326.
1374 Energy & Fuels, Vol. 12, No. 6, 1998 Table 6. Infrared Group Frequencies of Common Carbonyl-Containing Oxidized Compounds group frequency cm-1
1842 1823 cm-1 1777 cm-1 1742 cm-1 1735 cm-1 1715 cm-1 1685 cm-1 1673 cm-1 1601 cm-1
Adhvaryu et al. Scheme 2. Ether Formation during Base Oil Oxidation
functional group four-membered lactone anhydride five-membered ring lactone/peroxy ester peroxy-acid/ester six-membered lactone ketone substituted benzoic acid conjugated ketone aromatic ring vibrations associated with aromatic carboxylic acids
Scheme 1. Typical Oxidation Scheme for Base Oils
bine via an acid- or base-catalyzed Aldol condensation to form high molecular weight intermediates that are sludge precursors. The appearance of some ethercontaining compounds (Scheme 2) may be explained by the fact that when the rate of oxidation becomes limited by O2 diffusion due to an increase in the viscosity of the reaction medium, ether is formed as a side product of the oxidation. Conclusions The mechanism of oxidative degradation of base oils studied by NMR and FTIR techniques reveal that saturates, which include normal and isoparaffins and mono- and multiring naphthenes are thermally stable and degrade at similar rates. However, long alkyl substitution on naphthenic rings make the side chain susceptible to oxidation. Aromatic compounds on the other hand show a wide spectrum of oxidation stability. Long alkyl-substituted aromatic rings are highly susceptible to oxidation. Similarly, naphtheno aromatics and polyaromatics are thermally unstable. Oxidative degradation also results in a higher percentage increase of Car,alk compared to Car of the base oils. Aromatic rings are mainly alkylated by alkyl radicals generated from long alkyl chains attached to aromatic structures.
1740 cm-1. Spectral deconvolution shows the presence of several other bands of lower intensity associated with base oil oxidation. The bands resolved in the carbonyl region are at 1842, 1823, 1742, 1735, 1685, and 1673 cm-1. These absorption bands have been assigned to different functionalities (Table 6). These assignments reveal the formation of five-membered lactone, peroxy ester, peroxy acid, ester, ketone, and substituted benzoic acid type compounds during the oxidation of base oils. The formation of such compounds is also reported by Coates et al.4 In view of the above discussion, a mechanistic pathway is drawn for the lube oil oxidation. Figures 1 and 2 explain, to a fair extent, the formation of various carbonyl compounds upon oxidation of oils. The aromatic carboxylic acid formed through oxidation at the R-carbon of the ring substituents (Scheme 1) was confirmed from the appearance of an absorption peak at 1685 cm-1 upon deconvolution of the FTIR spectra. The formation of ketone species was identified from their characteristic absorption bands at 1715 and 1673 cm-1. It is reported16 that when aldehydes and ketones are formed in the primary oxidation phase, they com-
Acknowledgment. One of the authors (A.A) thanks C.S.I.R., New Delhi, for a research fellowship during the course of this work. Nomenclature %Car ) percent aromatic carbon %Car,H ) % protonated aromatic carbon [Har/C/H] %Car,Me ) % methyl-substituted aromatic carbons [HR,Me/C/H × 1/3] %Car,alk ) % alkyl-substituted (not methyl) aromatic carbons %Car,J ) % peripheral aromatic carbons [Car,H + Car,Me + Car,alk] %Car,b ) % aromatic carbon at condensed points [Car - Car,J] %Csat ) % saturated carbons %Cnp ) % normal paraffinic carbons %CN ) % naphthenic carbons %Cip ) % isoparaffinic carbons fc ) compactness index of the aromatic part of the sample [Car,b/ Car] σ ) ratio of substituted aromatic to total aromatic carbons [Car,Me + Car,alk/Car] ACL ) average chain length of the aliphatic hydrocarbons present in the system (both in saturates and attached to aromatic and naphthenic rings) EF980134M
