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Identification of Acyclic Isoprenoid Hydrocarbons in Wax Derived from Tank Bottom Sludge Sanat Kumar,*,† K. M. Agrawal,† and Peter Fischer‡ Indian Institute of Petroleum, Dehradun-248005, India, and Institute of Organic Chemistry, University of Stuttgart, D 70569 Stuttgart, Germany Received July 4, 2003. Revised Manuscript Received April 30, 2004
The oil-soft wax fraction of the microcrystalline wax, extracted from the Bombay High crude tank bottom sludge (Indian origin), has been characterized using gas liquid chromatography (GLC), 1H and 13C nuclear magnetic resonance (NMR), and gas liquid chromatography-mass spectroscopy (GLC-MS). A series of saturated acyclic isoprenoid hydrocarbons have been identified in this wax. These structures have been confirmed by recording GLC-MS spectra of the oil-soft wax fraction at reduced ionization potentials of 30 and 20 eV and also recording the spectra of two known available standards, namely, pristane and phytane.
Introduction Waxes derived from crude oil tank bottom sludges are complex mixtures of high-molecular-weight saturated hydrocarbons containing predominantly branched chain alkanes. Alkylated aromatics may also be present in small quantities, depending on the degree of the refining of the wax.1-3 Although extensive studies have been reported in the literature on the structural composition of the paraffin and microcrystalline waxes, little work has been done on the composition of tank-bottomderived microcrystalline waxes, particularly on the type of branching present in them. This is due to their low volatility and high molecular weight, as well as the high complexity of the hydrocarbons present in them.4-6 In the present paper, an attempt has been made to determine the type of branched structures present in tank-bottom-derived wax, using gas liquid chromatography (GLC), nuclear magnetic resonance (NMR), and gas liquid chromatography-mass spectroscopy (GLCMS). Here, we report the presence of 2-methyl, as well as some typical polymethyl branches (isoprenoid structures), in the oil-soft wax fraction of microcrystalline wax derived from Bombay High crude oil. Isoprenoid compounds ranging from C13 to C20, with the exception of C17, have been identified in the wax. * Author to whom correspondence should be addressed. Fax: +91135-2660202. E-mail address:
[email protected]. † Indian Institute of Petroleum. ‡ University of Stuttgart. (1) Agrawal, K. M.; Suriyanarayan, M.; Joshi, G. C. Res. Ind. 1982, 32 (2), 103. (2) Thompson, J. S.; Grigsby, R. D.; Doughty, D. A.; Woodward, P. W. Characterization of High Boiling Sludge Waxes from Underground Crude Oil Tank Reservoirs. Presented at the Division of Petroleum Chemistry, American Chemical Society Meeting, Dallas, TX, April 9-14, 1989. (3) Agrawal, K. M.; Joshi, G. C. Fuel Sci. Technol. Int. 1994, 12 (7/ 8), 1105-1113. (4) Agrawal, K. M.; Joshi, G. C. Indian J. Technol. 1986, 24, 4344. (5) Musser, B. J.; Kilpatrick, P. K. Energy Fuels 1998, 12, 715725. (6) Kumar, S.; Gupta, A. K.; Agrawal, K. M. Pet. Sci. Technol. 2003, 21 (7/8), 1253-1263.
A fragmentation pattern has been proposed to assign the structures of methyl-branched alkanes. This has been confirmed by recording the GLC-MS spectra of the oil-soft wax at lower ionization potentials (30 and 20 eV) and also recording the structure of two known isoprenoids (pristane and phytane). Experimental Section Wax Sample Preparation. The soft wax component present in a crude microcrystalline wax derived from the tank bottom sludge of Bombay High crude oil was used for this study. This wax was obtained from the sludge by distilling it under vacuum, to remove the lighter fractions (naphtha, kerosene, gas oil, etc.). The residue, ∼65% of the sludge, boiling at ∼350 °C+, was treated with concentrated (98%) sulfuric acid in the molten state at 100-120 °C. The acid sludge formed in this manner was separated by decantation, and the acidified wax was neutralized with soda, followed by treatment with clay. This removed most of the unsaturated and heteroatom compounds present in it. Approximately 50% of the wax melting between 90 °C and 92 °C was obtained on the basis of the sludge. This wax was then de-oiled by percolating hexane in a percolation column packed with powdered microcrystalline wax at ∼20-25 °C. The hexane eluant was distilled, to recover the oil-soft wax fraction. The oil-soft wax portion constitutes ∼40% of the crude wax and was semisolid, having a melting point of 50-52 °C. The details of the processes have been reported elsewhere.7,8 Wax Analysis. 1H and 13C NMR spectra were recorded on a Bruker 500 MHz spectrometer at 70 °C in deuterated benzene solvent with tetramethylsilane (TMS) as the internal standard. GLC analysis was performed by recording the spectra on a Carlo Erba model HRGC 5300 gas chromatograph fitted with a 20 m × 0.3 mm capillary column having a polysiloxane stationary phase that had been synthesized in the laboratory (7) Agrawal, K. M.; Prakash Jai, M.; Gomkale, A. V.; Kumar, S.; Rawat, B. S. In Petrotech ‘97: Proceedings of the Second International Petroleum Conference and Exhibition, New Delhi, India, January 9-12, 1997, pp 59-62. (8) Kumar, S.; Agrawal, K. M.; Khan, H. U. Indian Patent Application No. 0406 DEL 2002, March 28, 2002.
10.1021/ef034027q CCC: $27.50 © 2004 American Chemical Society Published on Web 08/13/2004
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Figure 1. Gas liquid chromatography (GLC) chromatogram of the oil-soft wax fraction. at the University of Stuttgart.9 Hydrogen gas (H2) at 0.4 bar was used as an elutant, and the oven temperature was programmed as follows: 50 °C for 1 min, then heating at a rate of 10 °C per min, then 300 °C for 30 min. A mixture of n-alkanes (n-C10 + n-C17 + n-C26) was used to identify the compounds. GLC-MS analysis was performed on a Hewlett Packard model HP 5890 Series II gas chromatograph that was coupled to a Finnigan model MAT 95 mass spectrometer. A 30 m × 0.25 mm capillary column containing the HP-5 MS stationary phase was used. Helium was used as the carrier gas.
Results and Discussion Gas Liquid Chromatography (GLC) and Nuclear Magnetic Resonance (NMR) Analyses of Wax. The GLC chromatogram (Figure 1) shows the presence of n-alkanes that have carbon numbers of 12-35, with C17 having the maximum concentration. The distribution is Gaussian in nature. These alkanes are predominantly straight-chain alkanes, but there also is a significant presence of branched-chain alkanes. The 1H NMR spectrum (Figure 2a) shows prominent peaks in the region of 0.87-0.91 ppm, corresponding to the terminal methyl groups, and between 1.3 and 1.4 ppm, corresponding to the methylene groups (long carbon chain). The peak at 7.15 ppm is due to the traces of aromatic compounds present in this wax. There are also some absorption peaks in the range of 1.0-1.2 ppm, which are probably due to the protons attached to tertiary C atoms, indicating some branching in the molecules of this wax.10,11 The 13C NMR spectrum (Figure 2b) shows prominent absorption peaks at ∼14 ppm, corresponding to terminal (9) Markert, J. Diploma Thesis, Institute of Organic Chemistry, University of Stuttgart, Germany, 1996. (10) Clutter, D. R.; Pterakis, L.; Stenger, R. L.; Jensen, R. K. Anal.Chem. 1972, 44 (8), 1395-1495. (11) Cookson, D. J.; Smith, B. E. Org. Magn. Reson. 1981, 16 (2), 111.
methyl groups, and peaks in the range of 29-30 ppm, corresponding to the methylene groups in the long alkyl chain (backbone). The peaks located at ∼19.7 ppm are due to the methyl branches.10,11 Gas Liquid Chromatography-Mass Speectroscopy (GLC-MS) Analysis of Wax. The relative ion current chromatogram of the oil-soft wax is shown in Figure 3a. The mass fragments of peaks N1, N2,..., N21 are the typical mass fragment peaks observed in the mass spectra of n-alkanes.12 These show the expected intense molecular ion peak (intensity of >40%, relative to the base peak, m/e 57), an absence of peaks at m/e [M-15], peaks of weak intensity at positions m/e ) [M-29], [M-43],..., etc., and high-intensity peaks toward the lower end of the spectra (m/e ) 85, 71, 57). The relative intensities of these fragment ions are given in Table 1.These peaks can be taken as representatives of normal alkanes that have 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, and 22 C atoms. The other peakssP1, P2, P3, P4, P5, P6, and P7s show a relatively less-intense molecular peak (intensity of 15%, relative to the base peak) at one or more positions, corresponding to m/e ) [M-15], [M-29], [M-43], [M-71], [M-85], [M-99], ..., etc. and the highly intense peaks at positions m/e ) 85, 71, 57, indicating the presence of methyl-branched structures in them.13 The relative intensities of the mass fragments are shown in Table 2. The mass spectra of two of the compounds (P1 and P5) are shown in Figure 3b. Fragmentation Pattern of Methyl-Branched Alkanes. The fragmentation pattern of the branched alkanes has been studied by Albaiges.13 It has been reported that the isoalkane hydrocarbons (CnH2n+2) (12) Herban, A. Org. Mass Spectrosc. 1970, 4, 425-39, (13) Albaiges, J; Borbon, J.; Gassot, M. J. Chromatogr. 1981, 204, 491-498.
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Figure 2. NMR spectra of the oil-soft wax fraction: (a) 1H NMR spectrum and (b)
exhibit characteristic fragment ions at m/e CmH2m+1(m < n), because of the cleavage of the C-C bond next to the tertiary C atom. This ion loses a proton to form an alkene radical ion at m/e CmH2m. This creates a doublet of peaks with the CmH2m+1 ion giving the peak at odd m/e value and the CmH2m alkene radical ion at the even value (Figure 4).13-15 However, the relative intensity of the two peaks is dependent on the structure and the relative stability of the two fragments. It has been experimentally observed that, for an alkane that has methyl branching, when a fragment of 7, 8, or 9 C atoms is eliminated and if the fragment contains no other branching, the evennumbered peak due to the alkene ion has a greater intensity than the corresponding odd numbered peak due to the radical ion.15 Also, the fragments that are formed with the charge residing on the 2° C atom have a greater stability than the fragments that have a charge on the 1° C atom and, hence, give a more intense peak. The loss of a methyl and a propyl group from the molecular ion does not result in an intense peak, (14) Dean, R. A.; Whitehead, E. V. Tetrahedron Lett. 1961, 21, 768770. (15) McCarthy, E. D.; Han, J.; Calvin, M. Anal. Chem. 1968, 40 (10), 1475-1480.
13C
NMR spectrum.
because of the further cleavage of these fragments at an ionization potential of 70 eV. The peaks below m/e ) 85 are very intense and do not give any significant information, because these are formed invariably by the fragmentation of practically all alkanes.12 Interpretation of Fragmentation of Peak P1. Based on the aforementioned observations/hypotheses, we propose the head-to-tail structure (Figure 5) and fragmentation pattern to explain the molecular structure of the alkane corresponding to peak P1 (see Figure 5). The fragment peaks at positions 169, 141, 113/112, and 99/98 are due to the loss of -CH3, -C3H7, -C5H11, and -C6H13 fragments, respectively (see Figure 6). The ions labeled B, C, D, and E in Figure 6 correspond to the structures shown in Figure 7. Ion A is formed by the loss of a -CH3 group from the molecular ion and, hence, is a peak of weak intensity. Because the fragment contains methyl branches, the odd-numbered peak at 169 is dominant. Ion B is formed by the loss of a terminal propyl group and contains methyl branches. Thus, it gives peaks of weak intensity, with the odd peak dominating. Ion C is formed with the charge residing on a 2° C atom, and, hence, it gives a peak of strong intensity.
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Figure 3. (a) Relative ion current chromatograms of oil-soft wax. (b) Mass spectrum of P1. (c) Mass spectrum of P5.
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Table 1. Intensities of Mass Fragments of n-Alkanes at an Ionization Potential of 70 eV peak
molecular weight
molecular formula
[M]
[M-15]
[M-29]
N1 N2 N3 N4 N5 N6 N7 N8
170 184 198 212 226 240 254 268
C12H26 C13H28 C14H30 C15H32 C16H34 C17H36 C18H38 C19H40
24 25 27 30 31 33 36 38
0 0 0 0 0 0 0 0
5 4 2 4 4 4 4 3
Relative Intensities of Mass Fragments [M-43] [M-57] [M-71] [M-85] 8 8 6 6 6 6 6 6
10 10 9 7 9 7 9 8
10 10 8 11 7 10 9
12 11 13 9 10 10
99
85
71
57
11 13 14 19 19 22 24 24
42 47 48 52 56 58 60 60
65 70 73 75 78 79 80 83
100 100 100 100 100 100 100 100
Table 2. Relative Intensities of Typical Mass Fragments Actually Observed for Isoprenoid Compounds peak
molecular weight
mol ion
P1 P2 P3 P4 P5 P6 P7
184 198 212 226 254 268 282
23 12 20 20 16 22 20
99/98 9 1 2 12 9 12 12
25 0 2 8 17 8 5
113/112 18 24 20 20 13 26 17
17 34 15 16 14 25 5
Intensities of Mass Fragments at Various m/e Values 127/126 141/140 155/154 169/168 183/182 1 0 25 8 3 8 19
1 0 5 1 2 5 12
Because it contains no methyl branching, the even peak at m/e 98 has greater intensity. Ion D is formed with the charge residing at a 1° C atom and, hence, gives a weak peak. The methyl branch gives a more prominent peak at odd m/e value (i.e., 113). Also, because the probability of formation of ion D is much less compared to that of ion C, this peak has reduced intensity. Ion E is formed with the charge residing on a 2° C atom and contains a methyl branching. Thus, an intense peak is formed with the odd peak dominating at m/e 127. The mass fragments of the alkane corresponding to peak P1 are consistent with the proposed structure for
Figure 4. Fragmentation pattern of methyl-branched alkanes.
8 0 1 26 4 5 5
2 0 0 9 2 1 1
2 5 2 8 3 9 6
1 3 1 2 2 7 2
3 0 0 9 20 1 4
1 0 0 1 8 0 2
12 18 19 42 21
2 1 1 17 8
197/196
2 20
1
7
the alkane 2,6-dimethyl undecane and the proposed fragmentation pattern. Similarly, the intensities of mass fragment ions expected from the structures in Figure 8, corresponding to the peaks P2, P3, P4, P5, P6, and P7, are given in Table 3. The structural analysis has been further confirmed by recording the GLC-MS spectra of two known isoprenoid standards, namely, pristane (2,6,10,14-tetramethyl-pentadecane) and phytane (2,6,10,14-tetramethyl hexadecane), which were each procured from Promochem GmbH, Germany. The observed fragment peak pattern was consistent with those of the reference standards for pristane and phytane. Figure 9 shows the mass spectrum of pristane, recorded under identical conditions. The aforementioned structures have also been confirmed by recording the mass spectra at reduced potentials of 20 and 30 eV. A reduced ionization potential decreases the rate of secondary fragmentation, thereby enhancing the stability of fragment ions.12 Tables 4 and 5 give the intensities of the fragment ions of isoprenoid compounds obtained at these ionization potentials. The
Figure 5. Proposed structure of P1 (2,6-dimethyl undecane), molecular weight of 184.
Figure 6. Fragmentation pattern of P1. Legend for data on the right-hand side is as follows: W, weak intensity peak; S, strong intensity peak; O, odd value dominating peak; and E, even value dominating peak.
225/224
Figure 7. Structures of fragment ions of P1.
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Table 3. Expected Intensities of Fragment Ions of Isoprenoid Compoundsa Intensities of Mass Fragment at Various m/e Values
peak
molecular weight
P1 P2 P3 P4 P5 P6 P7
184 198 212 226 254 268 282
a
C7H15+/ C7H14+
C8H17+/ C8H16+
(99/98)
(113/112)
S*
S S* S S S* S S
W S*
C9H19/ C9H18+ (127/126)
C10H21+/ C10H20+ (141/140)
W
W
C11H23+/ C11H22+ (155/154)
C12H25+/ C12H24+ (169/168)
C13H27+/ C13H26+ (183/182)
C14H29+/ C14H28+ (197/196)
W S S S S
S
C16H33+/ C16H32+ (225/224)
W W W W W W
S S W
S
S
W
W
Values given in parentheses are m/e values. Abbreviations: S, strong intensity peak; W, weak intensity peak; *, even peak dominant. Table 4. Intensities of Mass Fragments of Isoprenoid Compounds at an Ionization Potential of 30 eV
peak
molecular weight
molecular formula
M
M-15
M-29
P1 P2 P3 P4 P5 P6 P7
184 198 212 226 254 268 282
C13H28 C14H30 C15H32 C16H34 C18H38 C19H40 C20H42
17 24 28 31 32 38 36
4 5 5 9 8 6 4
0 1 18 9 9 2 8
Relative Intensities of Mass Fragments M-43 M-57 M-71 M-85 113 99 8 8 3 24 9 4 2
intensities of the fragment peaks at these ionization potentials are greatly enhanced, confirming the presence of these fragments. The peaks I1, I2, I3, ..., I17 show moderately intense molecular ion peaks (20%, relative to the base beak at m/e 57, except for I1, where it is 8%), indicating that they are branched alkanes. The intensities of the fragment peaks are shown in Table 6. These data show a peak (intensity equal to that of molecular ion) at m/e [M-15] and a strong peak at [M-43] (intensity varying from 44% to 88% of that of the base peak), indicating the loss of a methyl radical and an isopropyl radical. The fragmentation in the lower mass range is analogous
0 1 6 13 0 4 2
3 3 13 36 4 2
36 44 42 76 37
22 52 30 34 35 56 38
26 7 10 23 29 27 28
85
71
57
43
10 16 48 58 24 49 64
51 100 100 100 100 100 100
100 100 87 84 100 97 90
18 18 15 24 11 13 12
to that observed in the case of normal alkanes. These represent the 2-methyl branched alkanes (see Figure 10). Our observation on the presence of isoprenoid structures in the wax is supported by earlier studies in which the presence of such isoprenoid structures in crude oil, gas oil, kerosene, and shale have also been reported earlier by many workers.14,16-24 The absence of C17
Figure 9. Mass spectrum of pristane.
Figure 8. Isoprenoid compounds identified in wax.
Figure 10. Some of the 2-methyl alkanes identified in the wax.
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Table 5. Intensities of Mass Fragments of Isoprenoid Compounds at an Ionization Potential of 20 eV peak
molecular weight
molecular formula
M
M-15
M-29
P1 P2 P3 P4 P5 P6 P7
184 198 212 226 254 268 282
C13H28 C14H30 C15H32 C16H34 C18H38 C19H40 C20H42
20 24 44 31 32 38 36
16 12 8 9 8 6 4
7 4 40 9 9 2 8
Relative Intensities of Mass Fragments M-43 M-57 M-71 M-85 113 99 42 34 10 24 9 4 2
6 12 24 13 0 4 2
13 13 36 4 2
90 44 42 76 37
92 100 58 34 35 56 38
69 14 21 23 29 27 28
85
71
57
43
50 28 50 58 24 49 64
81 94 100 100 100 100 100
100 74 47 84 100 97 90
10 3 2 24 11 13 12
Table 6. Intensities of Mass Fragments of 2-Methyl Alkanes at an Ionization Potential of 70 eV peak
molecular weight
molecular formula
[M]
[M-15]
[M-29]
I1 I3 I5 I6 I7 I8 I9 I10 I11 I12 I14 I15
198 226 254 268 282 296 310 324 338 352 380 394
C14H30 C16H34 C18H38 C19H40 C20H42 C21H44 C22H46 C23H48 C24H50 C25H52 C27H56 C28H58
8 16 18 17 16 18 22 19 20 15 14 15
9 17 18 14 15 16 19 17 19 14 14 15
2 0 0 0 0 0 0 0 0 0 0 0
isoprenoid has also been reported in some of the studies made earlier.14 The identification of these compounds in wax derived from tank bottom sludge can help in understanding the mechanism of formation of tank bottom sludge, as well the geochemical origin of crude oil. (16) Bendoriates, J. G; Brown, B. L.; Hepner, R. S. Anal. Chem. 1962, 34 (1), 49-53. (17) Oro, J.; Nooner, D. W. Nature 1962, 213, 1082. (18) Johns, R. B.; Belsky, T.; McCarthy, E. D.; Burlingame, A. L.; Houg, P.; Schnoes, H. K.; Richter, W.; Calvin, M. Geochim. Cosmochim. Acta 1966, 30, 1191. (19) Welte, D. Erdoel Kohle 1967, 20, 65. (20) McCarthy, E. D.; Calvin, M. Tetrahedron 1967, 24, 2109-2119. (21) Brooks, J. D.; Gould, K.; Smith, J. W. Nature 1969, 222, 257259. (22) Spyckerlle, C.; Arpino, P.; Ourisson, G. Tetrahedron 1972, 28, 5703-5713. (23) Albaiges, J.; Borbon, J.; Salagre, P. Tetrahedron Lett. 1978, 6, 595-598. (24) Albaiges, J. Phys. Chem. Earth 1980, 12, 19-28.
Intensities of Mass Fragments [M-43] [M-57] [M-71] 44 77 82 73 74 65 88 80 81 60 52 63
4 0 6 2 8 1 1 1 1 1 1 3
10 1 19 8 6 4 3 1 3 2 3 3
99
85
71
57
32 37 34 33 31 29 31 35 38 35 34 36
61 56 60 62 58 56 55 69 70 69 68 66
72 64 70 72 75 68 67 79 88 87 89 86
100 100 100 100 100 100 100 100 100 100 100 100
Conclusions The aforementioned study has thus demonstrated that the microcrystalline wax of tank sludge origin contains highly branched structures. A series of acyclic isoprenoid structures, ranging from C13 to C20, with the exception of C17, are present in the wax derived from Bombay High crude oil tank sludge of Indian origin. Furthermore, these isoprenoid structures are linked together head to tail. Acknowledgment. Two of the authors (S.K. and K.M.A.) are thankful to the German Academic Exchange Service (DAAD) for the financial support to them for their study and visit to the Institute of Organic Chemistry at the University of Stuttgart, Germany. EF034027Q
