Energy & Fuels 1991,5,869-874
869
Characterization of Unresolved Complex Mixtures of Hydrocarbons from Lubricating Oil Feedstocks Mark Gought and Steven Rowland* Department of Environmental Sciences, Polytechnic South West, Drake Circus, , Plymouth PL4 8AA, U.K. Received May 28, 1991. Revised Manuscript Received August 2, 1991 Lubricating oils, particularly those blended from hydrocarbon feedstocks for use as automobile lubricants, are economically important petroleum products. The hydrocarbons of waste lubricating oils are also quantitatively important sources of environmental pollution. However, both the feedstocks and the hydrocarbons found in sedimenh polluted with waste lube oils are very difficult to characterize by existing analjrtical methods and little is known, in detail, of their chemical composition. For example, the hydrocarbons often appear as unresolved complex mixtures (UCMs) when examined by techniques which are routinely successfully used to characterize other hydrocarbon mixtures such as crude oils (e.g., gas chromatography (GC) or GC-mass spectroscopy (GC-MS)). We report here our attempts to characterize UCMs from three lubricating oil hydrocarbon feedstocks by oxidative degradation followed by GC and GC-MS of the functionalized products. Oxidation with chromium trioxide produced reasonable yields (ca. 70430%) of total recoverable products, of which ca. 90% was functionalized. Furthermore, and perhaps most importantly, in each case about 15% of the oxidation products were resolved by GC and GC-MS and were identified as n-carboxylic acids, ketones, and lactones. On the basis of these identifications and from the reported mechanism of Cr03 oxidations, potential precursor compounds for some of the hydrocarbon components of the lubricating oil UCMs are proposed. These include simple monoalkyl acyclic and monocyclic alkanes. Examples of these alkanes were synthesized and the synthetic alkanes also oxidized by the Cr03method. Many of the products were again n-acids, ketones, and lactones which confirms the proposed UCM alkane structures. I t remains to be seen whether the remaining functionalized UCM can be more easily characterized than the parent alkane UCM.
Introduction The term "lubricating oil" is used to describe a diverse range of products, most of which are blended from hydrocarbon feedstocks produced from the dewaxing and dearomatization of the vacuum distillates of crude oils.' Blending to achieve the desired physical properties (e.g., viscosity) is usually made in the absence of a detailed knowledge of the chemical compositions of the feedstocks since most analytical techniques provide only limited information about these complex hydrocarbon mixturesa2 When they are discarded, lubricating oils are a major source of environmental hydrocarbon contamination and since the lube oils themselves are poorly characterized, analysis of the lube hydrocarbons in environmental samples is also difficult. Techniques such as GC and GC-MS leave substantial proportions of the oils unresolved as sc~calledunresolved complex mixtures (UCMs)?l4 Similar UCMs are found in crude oils after they have been subjected to weathering and biodegradation, whether in the environment or in reservoir.6-6 We recently reported a summary of a series of oxidative studies that we conducted to try to further elucidate UCM composition in both crude oils and lube oils.' We now report the details of the lube oil analyses.
Experimental Section Aliphatic hydrocarbon UCMs were isolated from the hydrocarbon base stocks of three lubricating oils (Silkolene 150; Shell Paraffinic; Shell Naphthenic). The detailed refining histories of these oils are not known, although in general lube oils are prepared 'Southern Region Laboratory, National Rivers Authority, 4 The Meadows, Waterberry Drive, Waterlooville, Portsmouth, Hants,
U.K.
0887-0624/91/2505-0869$02.50/0
from high-vacuum distillates of crude oils by solvent extraction with furfural and solvent dewaxing to remove n-alkanes.' The Silkolene oil was most likely derived from a paraffinic crude of Middle Eastern or North Sea origin (A. Walker, Silkolene Lubricants, UK), the Shell Paraffinic lube base oil was derived from Brent, Murchison, Dunlin, and Cormorant North Sea crudes and the Shell naphthenic from Tia Juana heavy Venezuelan crude (M. Day, Shell Lubricants, UK). UCMs were isolated by published methods6r9which involved column chromatographic separation (40 g of 60-120 mesh silica; 20 g of grade I neutral alumina) of whole oils (2 g) into aliphatic fractions by elution with hexane (ca. 180 cm3). Aliquots of the aliphatic UCM (ca. 100 mg) were further fractionated by silver ion thin layer chromatography @ioz gel, 0.5 mm thickness, hexane Rf 0.95-0.99) followed by triplicate urea adductions to remove normal and simply branched alkanes.'O These UCM isolates were characterized by a variety of chromatographic and spectroscopic methods including GC, GC-MS, probe ELMS, CI-MS, elemental analysis, and high-temperature gas Chromatography (HT-GC). Oxidation of the urea nonadduct UCM isolates of all three oils was performed at 70 OC for 1 h in a (1) Klamann, D. Lubricants and Related Products; Verlag-Chemie: Weinheim, 1984. (2) Hala, S.; Kuras, M.; Popl, M. Analysis of complex hydrocarbon mixtures, part B, group analysis and detailed anulysis; Comprehensive analytical chemistry, Vol. 13; Svehla, G., Ed.; Elsevier: Amsterdam, 1981. (3) Thompson, S.; Eglinton, G. Mar. Poll. Bull. 1978,9, 133-136. (4) Voudrias. E. A.: Smith. C. L. Estuarine Coastal Mar. Sci. 1986.22
(4); 271-284. ( 5 ) Connan, J. In Adoances in Petroleum Geochemistry; Volume I, Brooks. J.. Welte.. D... Eds.:. Academic Press: New York.. 1 9 M. Vol. I.. DD __ 299-336. '
'
(6) Jones, D. M. Ph.D. Thesis, University of Newcastle upon Tyne,
UK, 1986. (7) Gough, M. A.; Rowland, S. J. Nature 1990,344,648-650. (8) Gough, M. A. Ph.D. Thesis, Polytechnic South West, Plymouth, UK. ~~~.1989. ~~-~ (9) Douglas, A. G.; Hall,P. B.; Bowler, B.; Williams, P. F. V. Proc. R.
SOC.Edin. 1981,80B, 113-134. (10) Murphy, M. T. J.; McCormick, A.; Eglinton, G. Science 1967,157, 1040-1042.
0 1991 American Chemical Society
Gough a n d Rowland
870 Energy & Fuels, Vol. 5, No. 6,1991 171
\-
1 I O
2
\
141
151
111
Ill
111
Figure 1. Synthetic alkanes for oxidation experiments. Figures in brackets indicate observed order of reactivity to oxidation (1 = most, 7 = least reactive). Cr03/glacial acetic acid mixture." Thus, UCM (ca. 10-50 mg) was added to glacial acetic acid (10 cm3) in a two-necked round-bottomed flask (25 cm3) equipped with a reflux condenser. The solution was heated (70 2 OC, water bath) with stirring (5 min) followed by addition of the oxidant (1O:l molar ratio Cr03:UCM, assuming 352 g mol-' for UCM; Le., molecular weight of CZ saturated alkane). The solution was maintained at 70 "C with stirring for 1 h, cooled (ice water bath), and transferred to a separating funnel with water (10 cm3) and dichloromethane (3 x 10 cm3). The use of dichloromethane a t this stage contrasts with the use of hexane in the published method" and significantly improves recovery of oxidized products. The extracts were combined and washed (water, 2 X 10 cm3), concentrated to near dryness (Buchi, 40 "C), and the products and unreacted hydrocarbons hydrolyzed (5% methanolic KOH) under reflux before acidification to pH 1 (concentrated HCl), addition of water (5 cm3),and reextraction (dichloromethane 1 X 10 cm3, 2 X 5 cm3). Extracts were water-washed (2 x 10 cm3) and dried (anhydrous sodium sulfate) and the solvent was removed (Buchi, nitrogen stream). Total oxidation products (ca. 30 mg typically) were subjected to methylation (BFJmethanol complex) prior to separation from unoxidized UCM by column chromatography (10% deactivated silica, 1 X 20 cm columns, 15 cm3of hexane gave UCM, 20 cm3 of dichloromethane, followed by 20 cm3 dichloromethane:methanol 1:l gave oxidized products). Removal of solvent gave the total oxidation products which were weighed then examined by GC and GC-MS. Resolved oxidation products were identified by cochromatography with authentic compounds on GC columns of differing polarity, and by comparison of GC retention indexes with literature values. Additional identifications were made by E1 and CI GC-MS by a comparison with the mass spectra of authentic compounds and published spectra, by spectral interpretation, or by comparisons with the oxidation products of synthesized model hydrocarbons.8 Synthetic model UCM alkanes (see below) were oxidized and treated in the same way. Water-soluble oxidation products of the Silkolene oil UCM were also examined.12
*
(11) Brooks,P. W.; Eglinton, G.; Gaskell, S. J.; McHugh, D. J.; Maxwell, J. R.; Philp, R. P. Chem. Ceol. 1977,20, 189-204. (12)Eglinton, T. I.; Curtis,C. D.; Rowland, S. J. Mineral. Mag. 1987,
51,495-503.
Syntheses. Czs model UCM alkanes (Figure 1; 1-7) were synthesized according to two general reaction schemes:8 the simple methyl-branched alkanes 2-methyltetracosane and 9-methyltetracosane (2,3) and the "T-branched" 7-hexylnonadecane (4) were prepared from alcohols resulting from Grignard couplings of the respective ketones and alkyl magnesium bromides. The cycloalkane 9-(2-cyclohexylethyl)heptadecane (5) was similarly prepared from, the product of Grignard coupling of methyl 3cyclohexylpropanoatewith a C8 bromide. All intermediates were purified and characterized as detailed elsewhere.8 The acyclic isoprenoids (6, 7) were synthesized as described p r e v i o ~ s l y . ~ ~ * ~ ~ All compounds were at least 96% pure as determined by GC. Apparatus. GC used a Carlo Erba 5300 Mega gas chromatograph fitted with a fused silica capillary column (25 m X 0.32 mm i.d.) coated with DB-5 (J & W Scientific), typically programmed from 50 to 300 OC at 5 "C/min, and held at 300 "C for 20 min. Hydrogen was the carrier at a flow rate of 2 cm3/min. High-temperature gas chromatography used the same instrument fitted with an aluminum-clad fused silica capillary column (25 m X 0.25 mm i.d.) coated with methyl silicone '400" (Quadrex Corp.). On-column injection was used with flame ionization detection, but the flame tip was ceramic and the detector base set at 420 "C. The oven was temperature programmed from 100 to 400 OC at 5 "C/min, isothermal at 400 "C for 10 min. GC-MS utilized a Carlo Erba 5160 Mega Series gas chromatograph coupled to a Kratos MS25 double focusing magnetic sector mass spectrometer. This was equipped with a 30-m fused silica capillary column coated with DB-5 (J&W). The column oven was typically programmed from 50 to 300 "C at 5 "C/min, and held at 300 "C for 20 min. The ion source temperature was 250 "C, the ionizing voltage 40 eV, and the filament emission current set at 400 pA. CI GC-MS utilized the same system; however, isobutane was fed into the ion source at a pressure of 0.67 bar, the ionizing voltage was 58 eV, and the filament emission current was 500 pA. Probe ELMS was carried out on the same MS system, but samples (ca. 5 pg) were introduced via a direct insertion probe and data processed according to published procedure^.'^
Results and Discussion GC-MS of the isolated lube oil alkanes showed that they comprised almost entirely UCMs of hydrocarbons with single maxima centered at o r around t h e GC retention position of n-pentacosane (n-Cw; Figure 2a,b) or n-tricosane (n-C23; Figure 2c). High-temperature GC did not reveal any later eluting high molecular weight compounds. Therefore, a number of Cw alkanes comprising n-Cw (I), two methyl-branched analogues (2,3), a T-branched alkane (4), a cyclic alkane (5),and two highly branched acyclic isoprenoids (6,7)were synthesized as "model" or reference compounds so that their oxidation behavior in Cr03/HAC could be compared to that of the lube oil UCMs. Oxidation of Model Alkanes. Alkanes 1-5 were oxidized individually. T h e low synthetic yields of 6 and 7 prevented oxidation of these separately but a mixture of small amounts of 1-7 was oxidized. The results are summarized in Table I. Oxidation of n-pentacosane (1) produced, in moderate n-acids and in lesser yield (51% ), homologous series of C, abundance C7-20 n-a,w-diacids. A high recovery of unreacted alkane was consistent with the reported reactivity of primary and secondary C-H bonds.Ie T h e formation of a homologous series of n-acids is consistent with previous reports that all methylene groups in a linear alkyl chain a r e equivalent a n d oxidized at t h e same rate.17 T h e series of n-a,w-diacids presumably or(13)Spykerelle, C.; Arpino, P.; Ourisson, G. Tetrahedron 1972, 28, 5703-5713. (14)Robson, J. N.;Rowland, S. J. Nature 1986,324, 561-563. (15)Hood, A.; O'Neal, M. J. In Aduances in Mass Spectrometry; Waldron, J. D., Ed.; Academic Press: London, 1959. (16)Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry, Springer-Verlag: Berlin, 1984.
Energy & Fuels, Vol. 5,No. 6, 1991 871
Unresolved Complex Mixtures of Hydrocarbons
Table I. Summary of Oxidation Products of Lube Oil UCMs and Synthetic Alkanes substrate lube oil n-CZ5 2-MT 9-MT 7-n-HN 9-(2-CHE)H oxidation Droduct UCMs (1) (2) (3) (4) (5) n-monocarboxylic acidsb
mix (1-7)
n-a,w-dicarboxylicacids0Vb n-alkan-2-0nes~*~ isoalkan-2-0nes~*~ y-methyl-y-lactoneso4 methyl-branchedo4 y-methyl-y-lactones w-oxo-y-methyl-y-lactonesa4 methyl-branched w-oxo-y-methyl-y-lactonesa4
isoprenoid acidsaSbsd isoprenoid ketonesbsd methyl-branched acidsavb keto acidsbVd cyclohexanecarboxylic acidsavbvd midchain alkylosb*d ketones 'GC equivalent chain length value compared to refs 24 and 26. bEI mass spectrum compared to library (NBS) and ref 25. cCI mass spectrum (NH,).dComparisonof mass spectra (EI)and ECL with oxidation products of model compounds.
a
la
1)
al
40 mln
Figure 2. Gas chromatograms of aliphatic UCM hydrocarbons isolated from (a, top) Silkolene 150 lubricating oil feedstock, (b, middle) Shell paraffinic lubricating oil feedstock, and (c, bottom) Shell naphthenic lubricating oil feedstock. For GC conditions see text.
iginates from secondary oxidation of the n-monoacids. In contrast to the oxidation of n-pentacosane the major oxidation products of 2-methyltetracosane (2) were c6-18 n-a,w-diacids. This is consistent with a relatively rapid formation of n-Cz2acid (derived from oxidative cleavage of the C 4 bond adjacent to the branch position) followed by oxidation of the alkyl chain. The lower abundance of the residual alkane is also consistent with the known (17) Roceck, J.; Mares, F. Collect. Czech. Chem. Communy959,24, 2741-2747.
greater reactivity of tertiary C-H bonds in comparison to secondary and primary C-H bonds. The oxidation of n-monoacids is known to preferentially take place at positions C-6 and above,17and this may account for the absence of diacids below c6. Oxidation of 9-methyltetracosane(3) produced a mixture of compounds in which residual alkane was identified as a minor component. The major compounds identified were n-a,o-diacids (c6-cll) and a series of compounds with abundant m/z 99 ions in their E1 mass spectra. The latter ion is a distinguishing feature of C5-substituted dialkyly-lactones.18 CI GC-MS provided molecular weight data consistent with a series of w-carboxy-y-methyl-y-lactones. The precise origin of these two series of major oxidation products of 9-methyltetracosane is unclear, since neither are primary oxidation products (which are n-C15 acid and n-heptadecan-2-one).18 The diacids must originate from secondary oxidation of the ketone or monoacid. Secondary oxidation is commonly observed in chromic acid oxidations of saturated hydrocarbons and both ketones and acids are susceptible to degradation by the oxidant. Ketones, in particular, appear to be degraded quite rapidly to acids via C-C bond cleavage adjacent to the carbonyl and as a result are rarely observed as major products in CrOs oxidations. In contrast, the presence of a carboxylic acid functionality in a linear alkyl chain appears to exert a stabilizing effect at the C-1 to C-5 position-oxidation preferentidy takes place at positions C-6 and above. For pentadecanoic acid this would result in the production of diacids in the range CG15which is close to that observed (C6-11). The origin of the carboxylactones is more difficult to explain, as they cannot be related to any of the primary or indeed secondary oxidation products observed. Their derivation as lactones would require the intramolecular esterification of a precursor y-hydroxy-y-methyl substituted diacid (Figure 3). This itself must originate from a primary intermediate in the oxidation sequence, Le., the tertiary alcohol 9methyltetracosan-9-01. However, under the acidic conditions of the experiment the labile alcohol would be expected to dehydrate. It must in some way be protected against dehydration, possibly as a chromate ester, the rate (18) Porter, Q. N.; Baldas, J. Mass Spectrometry of Heterocyclic Compounds; Wiley: New York, 1971. (19) Cason, J.; Searing Fressenden, J.; Ape, C. L. Tetrahedron 1959,
7, 289-298.
Cough and Rowland
872 Energy & Fuels, Vol. 5, No. 6,1991
*OH
- 3-'
U-CARIOXY-~-UETHVL-~-LACTON~S
0 H O+OH
3
1
H+ \ C r 0,
v 0 0
lqd
-AOH
Figure 4. Mechanism of the Cr03oxidation of branched alkanes (ref 16). Figure 3. Nature and origin of y-methyl-y-lactones.
of hydrolysis of which is sufficiently low to allow oxidation of the adjacent alkyl side chains. Subsequent hydrolysis would yield the hydroxy diacid which under the conditions of recovery (i.e., acidification with HCl) may undergo intramolecular cyclization to form the lactone. Thus, it appears that the oxidation of 9-methyltetracosaneproceeds via a complex series of competitive reactions involving oxidation of substrate, intermediates, and products. The ultimate nature of the end product is governed by the lability of each molecular type involved. Whatever the precise mechanism, it is of interest that the series of carboxylactones were also later observed as oxidation products of the UCM. Oxidation of 7-n-hexylnonadecane (4) produced a mixture of compounds in moderate yield (54%). The major component was residual unoxidized alkane. Oxidation products included dodecanoic acid, nonadecan-7-one, and tridecan-7-one, each of which represents a predicted product derived from oxidative cleavage adjacent to the tertiary C atom (Figure 4). The predicted C6 monocarboxylic acid was not recovered, presumably due to its volatility or preferential solubility in the aqueous layer. However, although most of the predicted oxidation products were observed, it is interesting to note the abundance of the C12monoacid relative to the alkyl ketones derived from cleavage of the same C-C bond the proportion of acid is almost twice that of the ketone. This possibly represents the greater susceptibility of the ketone to further oxidation in the reaction mixture and which is a feature observed in the CrOs oxidation of saturated hydrocarbons.20 Other components observed as minor products of the oxidation of 7-n-hexylnonadecanewere a homologous series of C7-13 n-monoacids. These must be produced from secondary oxidation of the ketone. The C13 acid can be ex(20) Wiberg, K. B. In Oxidation in Organic Chemistry, part A Wibert, K. B., Ed.; Academic Press: New York, 1965.
plained by assuming acid-catalyzed enolization of the higher ketone followed by oxidative cleavage of the double bond. This reaction sequence has been proposed previously for the Cr03 oxidation of aliphatic ketones in acidic media.20 Oxidation of 9-(2-cyclohexylethyl)heptadecane(5) produced mainly n-octanoic acid, i.e., the predicted product of oxidation at the tertiary position of the alkyl side chain. Other n-acids identified were heptanoic acid and nonanoic acid. These presumably originate from secondary oxidation of heptadecan-9-one. The 2-cyclohexylethanoicacid is also a product predicted from oxidation at the tertiary position of the alkyl side chain, whereas 3-cyclohexylpropanoic acid could be derived from secondary oxidation of 1-cyclohexylundecan-3-one.3-Octylundecanoicacid was tentatively identified and possibly arises from oxidation at the ring-alkyl junction, which is also a tertiary center. Alkyl-substituted cyclohexanes are known to produce alkylmonocarboxylic acids derived from oxidation at this position.20 Other compounds identified included the residual alkane (5) and two additional compounds for which the electron impact mass spectra were weak and did not provide significant diagnostic ions. However, CI data suggested molecular weights of 384 and 410 respectively. One compound is possibly a CZ5keto acid which could conceivably be produced from oxidation at the second tertiary center. The corresponding C7keto acid, 6-oxoheptanoic acid, is reported to be a major product in the Cr03 oxidation of methylcyclohexane.20 Oxidation of a mixture of all the model alkanes produced a moderate yield of functionalized products (33% recovery). Many of these were observed when the individual hydrocarbons were oxidized, but some were not (Table I). The principal products were homologous series of C6-23 n-monocarboxylicacids. Acids expected from the primary and secondary oxidation of tertiary C atoms were dominant. Other compounds identified included n-C10,1317,19 alkyl ketones and C6,7,10,21 isoprenoid-derived acids, a 6, ketone,
Unresolved Complex Mixtures of Hydrocarbons
Energy & Fuels, Vol. 5, No. 6,1991 873
Table 11. Mass Spectral and Gas Chromatographic Data for Representative Lactone Types no. of mol wta E1 mass spectrum comoound carbons (ECL)* 114 (6.61) 114 (M', l%), 87 (2%), 70 (20%), 59 (25%), 55 (30%),43 (50%), 99 (100%) y -methyl- y -lactone 6 methyl-branched 11 184 169 (M+- CH3, l%), 151 (4%), 109 (4%), 99 (100%) y-methyl-y-lactone w-carboxy-y -methyl-y-lactone methvl-branched w-carboxy-y-methyl-y-lactone
9 11
200 (12.85) 185 (M'+ -CH3,5%), 169 (M" - OCH3, lo%), 113 (lo%), 99 (100%) 213 (M'+ - CH,, lo%), 197 (Me+- OCH3, 15%), 88 (lo%), 99 (100%) 228
*From NH, CIMS and/or EIMS. Cf ref 24.
a Clo oxoacid, and various y-methyl-y-lactones. The isoprenoids were expected products of oxidation of 6 and 7. Only Cg and Clo diacids were present and w-oxo-ymethyl-y-lactones were not observed. This contrasts with the oxidation of some of the individual hydrocarbons (e.g., 3) and indicates that the mechanisms by which mixtures of hydrocarbons are oxidized are still incompletely known. An attempt was made to determine the relative reactivity of each alkane (1-7) to oxidation by normalizing integrated GC peak areas of the oxidation products of 2-7 to those of n-pentacosane, which was least reactive. The order broadly correlated with the number of tertiary C atoms available for oxidation (Figure 1). The reactivity of the alkanes has important implications for the structural characterization of UCMs by chemical oxidation since overoxidation to nondetected products would mean that the proportion of some components may be incorrectly estimated by the procedure. Prior to oxidation, 6 and 7 accounted for 29% of the total. After oxidation, products identified as having derived from the isoprenoids (including the residual alkanes) comprised 13% of the total. Thus ca. 16% of the expected isoprenoid-derivedproducts were not observed. I t appears that oxidation underestimates the proportion of acyclic isoprenoid structures originally present in a complex hydrocarbon mixture, perhaps because the alkanes are oxidized to carbon dioxide or to more water-soluble shorter chain acids. For all other alkanes in the mixture the estimated contribution to the total integrated area varied by only f l % before and after oxidation. Lube Oil UCM Oxidation. Gas chromatograms of the products of Cr03 oxidation of the UCMs are shown in Figure 5. GC of the water-soluble products (Silkolene only) did not reveal any additional compounds, though formic and acetic acid would not have been detectable by the methods medal2 Oxidation produced reasonable yields of total recoverable products (7340%; ref 7) and chromatographic fractionation of the oxidation products showed that most of this (89-92% of products) was functionalized. The most surprising result of the oxidation of the UCMs was the apparently high proportion of resolved oxidation products in the chromatograms (Figure 5). However, the appearance of the chromatograms is misleading. Measurement of the proportion of resolved oxidation products relative to oxidized UCM showed that only about 15% of the oxidation products were resolved (Revill, 1991,personal communication). This has to be taken into consideration when conclusions are made about UCM composition from the resolved oxidation products. Nonetheless, this is a much greater proportion of resolved material than in the original unoxidized UCM (cf. Figures 2 and 5) and since these compounds could be identified by E1 and CI GC-MS (Table 11), oxidative degradation does provide some useful additional information about UCM composition. Additionally, the distributions have potential use or 'fingerprinting" different lube oils.' The most abundant resolved products (Table I) were
m
I
3l
40
m,n
Figure 5. Gas chromatograms of oxidation products of lube oils (from top to bottom) a-c as in Figure 2. c6-20 n-monocarboxylic acids with lesser amounts of nala-dicarboxylic acids (c&& max. Cg),n-alkan-2-ones (C8-C15, max. C12),"ison-methyl branched alkan-2-ones (C8-CI2,max. C8), and a quantitatively important series of y-methyl-y-lactones. Carboxylactones and methylbranched carboxylactones were also identified (Table 11). More minor products included monomethyl branched monocarboxylic (C&o) and dicarboxylic acids (C8, C9, C:J. For the former, the positions of the methyl substituent were located at the is0 and anteiso positions of the alkyl chain. More highly methyl-substituted monocarboxylic acids, ketones, and keto acids included Cll-C16 acyclic isoprenoid-derived monoacids, a C13 isoprenoid and a Clo keto acid ketone (6,10-dimethylundecan-2-one), (8-oxo-4-methylnonanoic acid). Initially the most surprising result of the chemical oxidation of the UCM was the high proportion of aliphatic straight-chain products in the resolved oxidation products of all three lubricating oils. However, the distributions can be rationalized in part by reference to the mechanism of the oxidation16v20p21 and by a comparison with the results
874 Energy & Fuels, Vol. 5, No. 6, 1991
of oxidation of the reference alkanes, particularly 3,4, and 5.
We assume that, in the absence of n-alkanes, which were removed by urea adduction, the n-acids originate from oxidation at the tertiary centers of monoalkyl-substituted, acyclic, or possibly monocyclic alkanes (e.g., 4 and 5). Such compounds are only slightly adducted by urea, as was shown by the behavior of synthetic 4 and 5. The proposed mechanism proceeds via incorporation of a hydroxyl group at the branch position (Figure 4). In an acidic medium the resultant alcohol would rapidly dehydrate to a mixture of alkene isomers. Oxidative cleavage of the double bonds would result in the formation of a mixture of n-acids and n-alkanones. The maximum chain length observed for the n-acids derived from the UCM oxidation suggests that terminal alkyl chains on precursor compounds do not greatly exceed Czo. For Cz(t30acyclic alkanes, >500 compounds similar to 7-hexylnonadecane(4) are possible. This mixture alone would probably be difficult to resolve by GC. Thus it appears that up to about 15% of the UCM may comprise simple acyclic alkanes. Interestingly, biological oxidation of the Silkolene oil UCM by Pseudomonas aeruginosa also resulted in removal of ca. 15% of the UCM in 14 dayseZ2After this the rate of biodegradation of the UCM was no longer comparable to that of 7-hexylnonadecane (4). This is also consistent with our hypothesis that the UCM comprises about 15% of alkanes similar to 4. The n-w-diacids probably arise through oxidative cleavage of dialkyl-substituted alkyl chains or secondary terminal oxidation of n-monocarboxylic acids as observed during oxidation of some of the model compounds. The methyl ketones identified presumably arise through oxidation at the tertiary centers of simple methyl-branched alkyl chain linkages. The observed carbon number range would require the presence of methyl groups at the C-7 to C-14 positions along the n-alkyl chain. The methylbranched alkan-2-ones can be specifically ascribed to dimethyl-substituted alkyl linkages, with at least one methyl group adjacent to the terminal position of the alkyl chain. In an acidic medium, y-methyl-y-lactones could result from the intramolecular esterification of 4-hydroxy-4methyl carboxylic acid precursors (Figure 3). Possible precursor alkanes could therefore comprise dialkyl-substituted alkyl chains, with at least one methyl substituent. In order to produce the range of compounds observed (C6-10),the methyl group would have to be at the C-2 (i.e., iso), C-3 &e., anteiso), or C-4 to C-6 positions. The series of w-carboxy-y-methyl-y-lactoneswould originate by a similar mechanism from the internal esterification of wcarboxy-4-hydroxy-4-methylcarboxylic acid precursors (Figure 3). Isoprenoid acids and ketones identified as aliphatic UCM oxidation products may arise through oxidation of the resolved series of acyclic isoprenoid alkanes observed by GC-MS in the two paraffinic oils. However, their very low abundance in the UCMs prior to oxidation argues against this. They are more likely to be produced through oxidation of acyclic isoprenoid alkyl moieties on precursor UCM hydrocarbons. This is supported by the identification of isoprenoid oxidation products in the naphthenic lube oil UCM, and in a biodegraded crude oil in which (21) Freeman, F. In Organic Synthesis by Oxidation with Metal Compounds; Mijs, W . J., De Jonge, C., Eds.; Plenum Press: New York, 1986. (22) Gough, M. A.; Rhead, M. M.; Rowland, S. J. Org. Ceochem., in press.
Gough and Rowland resolved acyclic isoprenoids were absenta8 One unexplained anomaly is the absence, as UCM oxidation products, of many of the midchain alkyl ketones predicted from the oxidation mechanism (Figure 4) and as observed in the oxidation of the model compounds. A possible explanation is their susceptibility to further oxidation as noted in the oxidation of 7-n-hexylnonadecane. Another surprise was the identification of only small amounts of the cyclic compound 2-cyclohexylethanoicacid in the UCM oxidation products. Presumably this originates from oxidative cleavage about a tertiary position on a branched alkyl-substituted monocyclohexyl moiety, as was observed for the synthetic cyclic alkane (5). However, probe EI-MS2J5and elemental analysisa (average formula C,H,+,,,) of the Silkolene oil and NMR of the Shell oilss suggested that we should expect 3&46% monocyclic oxidation products. There are several possible explanations for this paucity. One possibility is that ring opening of cyclic alkanes occurred during oxidation, resulting in the formation of acyclic ketones, acids, and keto acids. Indeed this was observed to some extent, when the synthetic cyclic alkane (5) was oxidized (Table I). I t is also possible that the majority of cyclic oxidation products might still be unresolved by GC (i.e., be part of the 85% unresolved, functionalized products). Other methods (e.g., FAB-MS) will be needed to investigate this.
Conclusions 1. Chemical oxidative degradation of lube oil hydrocarbon feedstocks consisting of UCMs produced reasonable yields of resolved oxidized products (ca. 15%). 2. The resolved products were mainly n-alkanoic acids. The low yields of cyclic products may be partly caused by ring opening during oxidation or they may still be unresolved. 3. Resolved alkyl-substituted products were essentially derived from monomethyl- or dimethyl-branched alkyl linkages, and included homologous series of n-a.lkan-2-ones, isoalkan-2-ones, y-methyllactones, and monomethyl-substituted mono- and dicarboxylic acids. 4. Multiply branched acids and ketones were also identified, although in lesser quantities. These may have arisen through oxidation of acyclic isoprenoid-type alkyl linkages on unresolved precursor alkanes. 5. The observation of straight-chain compounds as the major resolved products of UCM oxidation suggests that the UCM comprises, in part, simple mondkyl-substituted acyclic and monocyclic alkanes. The synthesis and oxidation of representative hydrocarbons supports these suggestions. Acknowledgment. We gratefully acknowledge the help given by Dr. M. Rhead, Mr. S. Hird, and Mr. A. Revill (Polytechnic South West) during the study period. We thank Mr. A. Aldridge (Database, Stroud, Glos.) for assistance with computing, and Mr. A. Walker (SilkoleneUK Ltd) and Mr. A. Day (Shell Lubricants UK Ltd.) for the provision of the lubricating oils. Registry No. 1, 629-99-2;2, 1560-78-7; 3, 65820-49-7;4, 135943-74-7; 5, 25446-35-9; 6,51794-16-2; 7,89208-99-1. (23) Rossini, F. D.; Mair, B. J.; Streiff, A. J. Hydrocarbons from Petroleum (API Research Project 6). ACS Monograph Series; Reinhold: New York, 1953. (24) Rostad, C. E.; Pereira, W. L. J . High Res. Chromatogr. Chromatogr. Commun. 1986, 360,79-88. (25) Hermann, F.; Oldrich, D.; Churacek, J. J. Chromatogr. 1986,360, 79-88.
(26) Body, D. R. In CRC Handbook of Lipids, Mangold, H. K., Zweig, G., Sherma, J., Eds.; CRC Press: Boca Raton, FL, 1984; Vol. 1.
