Nitrogen and Oxygen Compound Types in Petroleum Total Analysis of a 700-850
O F
Distillate from a California Crude Oil
L. R. Snyder, B. E. Buell, and H. E. Howard Union Oil Co. of California, Union Reseach Center, Brea, Calif. The oxygen and nitrogen compound types present in the 700-850 “F fraction of a California crude oil were analyzed by means of a standard separation scheme, high resolution mass spectrometry, and other techniques. All compound types present in other than trace amounts were accounted for; their molecular structures were determined and their concentrations wt) were measured. Major compound types (>O.l% included the indoles, carbazoles, benzcarbazoles, pyridines, quinolines, phenanthridines, hydroxypyridines, hydroxyquinolines, dibenzofuranes, naphthobenzofuranes, phenols, aliphatic ketones, carboxylic acids, and sulfoxides. Lesser amounts of azaindoles, azacarbazoles, phenyl ketones, 2-hydroxybiphenyls, benzofuranes, aliphatic esters and ethers, and aliphatic di-carbonyl compounds (diketones, diesters, etc.) were also present. However, several of the structural assignments for these minor compound types are tentative.
I CATION EXCHANGE
SILICA
THENITROGEN AND OXYGEN compounds present in the higher boiling fractions of petroleum are of interest for a number of important reasons, and past studies of these petroleum heterocompounds now account for a large part of the literature. Previous workers have reported the presence in petroleum of a wide variety of compound types [see ( I ) for a partial review] but have not attempted the complete, detailed analysis of all the nitrogen and oxygen compound types in any given sample. In only a few cases [e.g., (2)] have the relative amounts of particular heterocompound types been reported. Consequently it is still unclear which heterocompound types in petroleum distillates are major components, which are minor components, and which are present in only trace quantities (or not at all). In the present study a 700-850 O F distillate from a California crude oil was subjected to a previously described (3) standard separation procedure, and the resulting fractions were analyzed in detail by a combination of high resolution mass spectrometry, infrared and ultraviolet spectrophotometry, elemental analysis, and acid and base titration. The structures of the various nitrogen and oxygen compound types present in the original sample in significant amounts were determined, and their concentrations were measured. This represents the first reasonably complete analysis of the nitrogen and oxygen compound types in a high boiling petroleum distillate.
A 5 s23
A 4 5 S4
Figure 1. Initial sample separation scheme
Separation Scheme. A composite crude oil from the Wilmington (California) field was separated by the standard scheme of the preceding paper (3). Ten grams of a 7000 850 O F distillate fraction were first separated in a trial run, in order to determine the best grouping of standard fractions.
The separation was then repeated on 50 grams of sample distillate according to the modified scheme of Figure 1, yielding the major fractions of Table I. For a discussion of fraction labels (AISo,A S z 4 ,etc.) see the preceding paper (3). Because we did not plan a detailed analysis of the carboxylic acid fraction, anion exchange was omitted in the scheme of Figure 1, and the carboxylic acids in the original sample were intentionally sacrificed in the alumina separation [see (3)]. Charcoal chromatography was used to concentrate aromatic heterocompound types from the A1S24,A”S24,A3S34,and A d 4 fractions for subsequent analysis by mass spectrometry. Prior to this concentration step, the low voltage mass spectra of these fractions showed extensive fragmentation from aliphatics and poorly defined parent peak regions. As indicated in Table I, several fractions were analyzed approximately ( 55 %) for total aliphatic heterocompounds by means of charcoal chromatography (3). Additional separations were carried out on some of the fractions of Figure 1, using standard techniques described previously (3). All standard fractions-i.e. those of Figure 1-were dissolved in nitrogen-flushed benzene and stored at -20 “C in nitrogen-topped glass containers. Chemical Determinations. Various fractions were analyzed for nitrogen by a variation of the method of Martin (4, for oxygen by a variation of the method of Meade et al. (5), and for sulfur by the X-ray fluorescence method of Fog0 (6). All three determinations can be made with about 10 mg of sample. Sulfide sulfur was determined by a variation of the method of
(1) L. R. Snyder and B. E. Buell, J. Chem. Eng. Data, 11, 545 (1966). (2) L. R. Snyder and B. E. Buell, Anal. Chim. Acta, 33,285 (1965). (3) L. R. Snyder and B. E. Buell, ANAL.CHEM., 40, 1295 (1968).
(4) R. L. Martin, ANALCHEM., 38, 1209 (1966). (5) C.F. Meade, D.A. Keyworth, V. T. Brand, and J. R. Deering, ibid., 39, 512 (1967). (6) J. K. Fogo, unpublished data, 1967.
EXPERIMENTAL
VOL. 40, NO. 8, JULY 1968
1303
Table I.
Chemical and Infrared Analysis of Standard Fractions of Figure 1
Fraction
%w
AQ
79.6
ASQ AiSi ASzr AtSo AS1 AS14 AaSe A a& A aSi
3.09 0.29 0.15 6.16 0.57 0.54 0.22 0.26 0.40 0.23 I .34 0.15 0.09 0.39 0.56 1.05 1.98 2.90
Ads4
Adz A Sa A4SQl Ais4 ASts CEi CEi CEa
Original 700-850 OF distillate Composite of above fractions
-
T S I O
%N 0.033
0.032 0.45 0.17 0.24 2.19 0.73 0.06 4.2 5.5 1.12 5.6 3.7 0.17 1.15
0.05 4.2 4.0 4.5
Z0
Total
Yield
SUl- Sulffides oxideso
.-
Verv weak
% N as N-Hb
% wt
Aliphaticso
mequivlg 0.015
0.024 0.43 1.41 2.44 0.25 0.70 2.56 0.5 1.3 0.70 3.84 0.51 2.59 1.63 4.32 3.09 2.33 2.04 0.44
2.89 6.67 8.76 2.09 4.82 5.72 0.75 0.31 0.30 2.45 0.06 0.96 0.76 3.33 0.44 0.73 2.20 0.57
0.18 5.21 6.39 0.33 3.43 5.23 0.07 0.35 0.20 0.34
0.2 0.7 1.7 0.6 0.6 0.6 0.4 0.5 0.2 0.2 0.2 1 .o 0.2 3.2 0.4 0.7 2.4 0.5
1
0.0
0.0 0.0 0.0 0.8 0.3 0.0 3.5 5.4
0.3 6.9 2.9 0.0 0.4 0.0 4.1 1.2 0.0
54 78
2 12 68 2 39 61 0 -7 -25 -5
Very weak bases
0.08 0.3 0.67 0.07 0.62 0.50 0.57 2.7 2.8 0.70 3.3 2.3 1.4 1.0 2.9 -
(0.013)d
0.0438
0.70 2.9 0.33 (2.5)d
0.4551
0.34
0.437
0.21 (0.39)f
0.1
0.1
0,088 0.092
By infrared; Equation 2. Equation 1. c Charcoal chromatography (approximate values). d Basic nitrogen (titration in glacial acetic acid). 8 0.38% N by Kjeldahl. Plus oxygen from carboxylic acids lost during separation (calculated from strong acid titration), 0 Value for combined fractions A 1 - 8 ~ 4 0
* By infrared;
Hastings (7), by using 2.3 gramsfliter iodine solution [see (S)]. Weak and very weak acids and bases were determined by the procedures of Buell(9,IO). Instrumental Techniques. Infrared spectra were recorded on a Perkin-Elmer Model 621 grating spectrophotometer. Five per cent solutions of sample in cyclohexane were run in 0.2-mm cells with a volume of -0.1 ml (unless otherwise noted). Ultraviolet spectra were obtained on a Cary Model 11 spectrophotometer in cyclohexane solution. NMR spectra were obtained on a Varian Model A-60 spectrometer at a constant oscillator frequency of 60 Mc and a magnetic field of 14,600gauss. Low ionizing voltage mass spectra were obtained by using a Consolidated Electrodynamics Corp. Model 110 high resolution mass spectrometer. Instrumental conditions were 50 HA anode current, 8 kV ion accelerating voltage, 270 "C source temperature, and 9 V (nominal) ionizing voltage. In most cases, resolution was 1/10,000 (base line), which permitted easy differentiation of peaks whose masses differed by as little as 1/16,000. Final mass spectra were displayed as strip chart recordings of galvanometer deflections. Exact mass measurements on individual peaks were carried out by peak matching, by using the circuitry provided by the manufacturer. The accuracy of peak matching is claimed by the manufacturer to be rt5 ppm; we found that major peaks in the mass spectrum gave measured masses which agreed with calculated masses within =k3 ppm std dev, and rt6 ppm maximum. Sample
(7) S. H.Hastings, ANAL.CHEM., 25,420 (1953). (8) L. R. Snyder, ibid.,33, 1538 (1961). (9) B. E. Buell, ibid., 39, 756,(1967). (10)Ibid.,p 762. 1304
ANALYTICAL CHEMISTRY
fragmentation under these conditions was generally unimportant, as shown by well defined parent peak regions in every case. Sample sensitivity was adequate for the convenient mass measurement and digitization of peaks falling on major compound type series. In many cases, this was also true for minor compound types (peak heights less than 10% of major compound types). MASS SPECTRAL INTERPRETATION
The major analytical technique used in the present study was high resolution mass spectrometry at low ionizing voltagei.e., parent peak analysis. This is now a standard procedure for the analysis of petroleum fractions [e.g., 11-13] and needs only a few additional comments. Equal parent peak sensitivities were assumed in the present mass spectral analyses. Exact mass measurements were not obtained for every peak in the parent peak region, because this is not necessary for petroleum fractions of the type under study. Exact mass measurements were made instead on every peak falling within a 14 integral-mass-unit range taken from the center of the parent peak distribution (measured peaks generally fell within the range 270 < m/e < 300). Examination of an integral mass peak-cluster-e.g., m/e 266.2 f0.2-removed by 14 mass units from a measured cluster-e.g., 280.2 rt 0.2-then permitted
(11)E.G.Carlson, G . T. Paulissen, R. H. Hunt, and M. J. O'Neal, Jr., ANAL. CHEM.,32, 1489 (1960). (12)H. E. Lumpkin, ibid., 36, 2399 (1964). (13)T. Aczel and B. H. Johnson, 153rd National Meeting of the American Chemical Society, Miami Beach, FIa., April, 1967.
m/e
-
Figure 2. The resolution of nitrogen-rich from corresponding nitrogen-poor peaks in the mass spectra of fractions A& and CE,
the assignment of empirical formulas to peaks in the unmeasured cluster by the principle of homology. This process was repeated to eventually cover the entire parent peak distribution. The assignment of formulas in this fashion was confirmed in several cases by peak matching outside the main range of exact mass measurement. An accuracy of 1 5 ppm in the exact mass measurements permits an unambiguous assignment of empirical formula to a particular peak in the mass spectrum, as long as a limited total number of N, S, and/or 0 atoms are contained within the corresponding molecule. Elemental analyses for the various fractions, in conjunction with other fraction properties, confirmed that this was true for all major sample components. In the case of major compound types within a particular fraction, there was never any question of the formula of a particular peak, and these assignments were later confirmed by a variety of other analyses on the fraction (see later discussion). In the case of minor components in a given fraction, some ambiguity in the formula assignments occasionally arose as a result of two factors. The exact mass measurement of very small peaks is generally less reliable than for major peaks, and in many cases these small peaks appeared to be unresolved doublets. In some cases the presence of an unresolved doublet was obvious from the greater width of the peak or from peak asymmetry (corresponding to two unresolved peaks of different heights). In these cases, the measured exact mass of the peak often did not correspond to any reasonable empirical formula but instead fell between the masses of two structures which were expected to be poorly resolved under the conditions used. In the latter case, examination of adjacent peaks (m/e equal m 2, m 4, etc.) corresponding to the cycloalkyl derivatives of the peak in question (m/e equal m) often solved this problem; these peaks would have exact masses which approached that of one or the other of the two unresolved compound types. Empirical formulas were never assigned to minor sample components unless measured masses confirmed the formula over several adjacent peaks-Le., for mje equal to rn, m 2, etc. For major compound types it was observed that the parent compound type-P.g., carbazoles, C,H2,-1,N is generally ac-
+
+
+
companied by cycloalkyl derivatives-e.g., cycloalkyl carbazoles, C,H2,-,,N-and/or benzologs-e.g., indoles C,H2n--9N,benzcarbazoles C,H2,-nlN-and this was assumed to be true for minor components as well. The difference in molecular weight (m/e) between a compound containing one or more nitrogen atoms C,H,N, and the molecular weight of a fragment peak (CmilHn+~Np--l) or an isotope peak (C,H,+1Np-113C) which corresponds to substitution of one nitrogen atom by CH2 or I3CH is only 0.008-0.013 mass units. For sample components in the 250300 molecular weight range, the differentiation of such species requires a resolution of 1/20,000 to 1/40,000. The mass spectrometer used is capable of this resolution under optimum conditions and for selected samples. In the case of the low ionizing voltage analysis of high boiling petroleum fractions, however, the best base line resolution we have attained so far is about 1/16,000. At higher resolution the peaks observed are barely distinguishable from background noise. At a resolution of 1/16,000, it was just possible to distinguish nitrogen containing compounds (C,H,N) from corresponding hydrocarbon peaks in fraction A2Sa, and dinitrogen compounds (C,H,N2) from mononitrogen compounds (C,tH,?N) in fraction CE,. The remaining fractions of Figure 1 did not contain significant amounts of two partially resolved species of the above type. Figure 2 illustrates the interpretation of these partially resolved peaks in fractions A2So and CE3. In each fraction, the nitrogen rich component was present in small concentration, and it was possible to recognize its presence as an asymmetric bulge on the low molecular weight side of the associated nitrogen poor peak ( A ) . The center of the nitrogen rich peak (C,H,N,) can be obtained by interpolation between the centers of peak A and any adjacent peak B, because the exact masses of A and B can be measured experimentally. If we mark an equal distance x from the center of A on each side (see Figure 2), the height of the nitrogen rich peak is given as h - h’. When the nitrogen rich component is present in large concentration relative to corresponding fragment and isotope peaks (generally the case in the remaining fractions of Figure I), there is no problem in measuring either its exact mass or its relative peak height. VOL 40, NO. 8, JULY 1968
1305
Table 11. Aromatic Nitrogen Compounds in the Standard Fractions of Figure 1; Determined by High Resolution Mass Spectrometry. of 7OC-850 O F distillate as indicated compound type CnH2n+sN Total Fraction -5.7 -7.7 -9.7 -112 -132 -15.7 -172 -192 -21.7 - 2 3 ~ -252 wt 0.56 A0 0.02 Also 0.03 AS,
Average mol wt
z
(300)b
(300)* (300)h
0.00
AS24 AzSo AzSi AZS24 AaSo A3Si A 3s2 Ass34 A Ysz A4S3 A4801 A4sS4 Ass23 CEi CEe CEa
0.02
0.01
0.02
0.13 0.11 0.01
0.05 0.05 0.01
0.04 0.01 0.01
0.03 0.01
0.25 0.24 0.04
0.08
0.02 0.01 0.06
0.01
0.24
0.01 0.02 0.01
0.34
0.33 0.02
0.02 0.06 0.04 0.10 0.02
0.01
0.01
0.04
305 305 317
0.00
0.11 0.12 0.04 0.75 0.03
0.14 0.05
0.02
0.05
294 288 289 252 242
0.21 1.25
0.01
0.08 0.00
0.01
0.09
301
0.00
0.03 0.06 Composite analysis Indoles ( CE1,CE2) Carbazoles ( Aa- CE1,CE2) Benzcarbazoles ( A e 5 , CE2,CE,) Pyridines and higher Benzologs(CE3) 0.03 0.06
0.01 0.03 0.13
0.03 0.10 0.23
0.03 0.14 0.30
0.34 0.13 0.03 0.25 0 . 1 8 ~ 0 . 1 3 ~ 0.07< 0.13 0.43 0.40 0.28 0.29
0.03 0.02 0.16
0.04
0.13
0.15
0.11 1.47
0.01 0.15 0.41
0.05
0.06
0.01
0.29
0.16
0.09
0.13
0.23
0.30
0.43
0.09
0.05
0.80
0.30
0.40
0.28
0.01
0.86
268 291 300
0.80
0.09
2.47
Parent peak analysis; results assume equal sensitivities and are normalized to total aromatics in given sample. * Estimated values; corresponding nitrogen compound concentrations are calculated from N (elemental analysis) of fraction, c Indoles and carbazoles.
z
(1
Table 111. Analysis of Alumina Subfractions from CE2
Compound type concentrations wt). SulfN N-0 oxides compds compds
(z
Fraction
el Range
CEA CE2AiGb CEJz CEzA3 CEzA 4 C&Alob CEA
60.17 0.17-0.20 0.20-0.24 0.24-0.30 0.30-0.45 0.45-0.54 0.54+
Yielda ( % wt) 0.08
0.16 0.24 0.14 0.63 0.57 0.14
zNn-h
gram)
0.2 0.7 3.1 3.7 0.7(2.2)~ 0.0 0.0
0 0 0 0
10 59 74
zS,, zN n + / z N, 0.3
5
0.00
0.08
0.00
0.4 0.3 0.4 1.6 5.4
21 87 128
0.01 0.01 0.01 0.11
0.11 0.18 0.09 0.29
0.33 0.05
0.00 0.00
0.04 0.04 0.03 0.20 0.24 0.09
3.8
27(85)c -
zwt Nitrogen compounds C,H2,+,Ne - 9z CEzAi CE2Aln CEz-4 CEzAi C E d4 CE2A4o CEzA 5
0.009 0.003
0.011 0.005
-1 3 ~
0.023 0.034 0.033 0.027
-1 5 ~ 0.011 0.026 0.040 0.014 0.085
- 52
C 4 A CEzAle CEPAO CEzA3 CEzA 4 CEiAaad CE2A5
-1lz 0.022 0.016 0.035 0.027
0.020 0.002
- 72
0.027 0.002
- 172 0.007 0.012 0.043 0.010 0.060
-1 9 ~ 0.004 0.013 0.019 0.007 0.025
-212
-232
0.010 0.002 0,001 0.115
0.016
% wt Nitrogen-oxygen compounds C,H2,+,NOe -1lz -132 -152 -1 7 ~ 0.001 0.002 0.001 0.009 0,009 0.016 0.006 0.003 0.007 0.009 0.012 0.010 0.002 0.006 0.008 0.007 0.004 0.021 0.019 0.074 0.053 0.022 0.025 0.050 0.045 0.028 0.025 0.004 0.029 0.021 0.012 0.013
- 92
-1 9 ~
-21z
0.003 0.004 0.002 0.014 0.011
0.008
0,005
0.001
Concentrations are % wt of 700-850 "Fdistillate. this fraction would be part of CE2A2 in the standard alumina separation. c Calculated from CHC13 solution data; see text. d CE2A4= eluted with 43 vol acetonitrile-benzene; this fraction would be part of CEtAs in the standard alumina separation. Results by high resolution mass spectrometry; concentrations are % wt of 700-850 O F distillate.
* CE2Alo eluted with 2 0 z vol benzene-pentane;
1306
ANALYTICAL CHEMISTRY
Av mol wt 314 300 300 314 253
I
250
300
350
250
300
350
250
300
350
mF Figure 3. Ultraviolet spectra of various fractions (a) A&, (b) CE1, 0.34 & e o & 0.36 fraction from alumina, (c) CE?A1-3,( d ) CE,, pentane eluate from charcoal, ( e ) CE3,20 vol benzene-pentane eluate from charcoal, (f)CE3, hot toluene eluate from charcoal, (g) pure nitrogen compounds, ( h ) CE2AsaC1,(i) A&
DETERMINATION OF INDIVIDUAL HETERCOMPOUND TYPES
Aromatic Nitrogen Compounds: CnH2,,,N. Table I1 provides a summary of the aromatic nitrogen compound types in the standard fractions of Figure 1, apart from N-0 N-N, and N-S compounds. These latter compound types are discussed separately (see below). INDOLES, CARBAZOLES, AND BENZCARBAZOLES (A3-5, CEi-2). The ultraviolet (UV) spectra of fractions ABS1,A&, A&, and A 4 S 3 show the characteristic carbazole pattern--e.g., Figure 3a-and the mass analyses of these fractions (Table 11) indicate major amounts of nitrogen compounds having the composition of the alkyl carbazoles (C,H2n-ISN). Lesser amounts of aromatic nitrogen compounds fall on series corresponding to cycloalkyl (C,H2n-17 N) and polycycloalkyl substituted carbazoles. The nitrogen contents of these fractions and their very weak acid contents (Table I) support the presence of carbazoles in these fractions. Further separation of fraction CEl on alumina and CE, on both alumina (see Table 111) and silica showed the presence of carbazoles and benzcarbazoles in these latter fractions. Figure 3b shows the UV spectrum of one of these benzcarbazole concentrates; the arrows indicate band maxima for 1,2-benzcarbazole [see (14)]. The additional small bands at 245 and 270 mp can be attributed to minor amounts of carbazoles and 3,4-benzcarbazoles, respectively, in this fraction. The mass analyses of fractions CE1 and CE, (see Table 11) confirm the presence of benzcarbazoles by a jump in compound type concentration at CnH2n-21N(alkyl benzcarbazoles). Further separation of fractions A3S34and A484 on charcoal gave aromatic fractions
(14) L. R. Snyder and B. E. Buell, ANAL.CHEM., 36,767 (1964).
whose UV spectra suggested mixtures of carbazoles plus benzcarbazoles. The identification of carbazoles and benzcarbazoles in the fractions of Figure 1 is consistent with the known separation characteristics of these compound types [see (31. The presence of carbazoles and benzcarbazoles in the various fractions and CEIp2is also supported by their infrared (IR) spectra. These fractions show the presence of a sharp band at 2.87 p (3490 cm-I) (see Figure 4a) which is also found in the IR spectra of carbazole and the isomeric benzcarbazoles, and is characteristic of the N-H group in these compounds. Carbazoles and benzcarbazoles have been found previously in a number of different crude oils--e.g., (2). Indoles were observed in minor amounts in fraction CE, and in larger concentrations in CE,. These two standard fractions- were first separated by alumina chromatography into subfractions, and these were in turn separated on charcoal into Coand C1subfractions. The UV spectra of the individual A1--3C0 subfractions from both CE1 and CE2 were essentially identical, and the composited CE2A1-3C0 fraction is shown in Figure 3c. The characteristic pattern of the alkyl indoles is apparent [cy. (14)]; there is a major maximum at 225 mp, a smaller maximum at 270 mp, and a shoulder at 280 mp. The molar absorptivities of these indoles subfractions ( A I - ~ C ,from , CE1 and CE,) at 275 mp averaged 0.60 i 0.05 X lo4 cm2/mole, us. a corresponding value of 0.48 X l o 4for indoles isolated from a cracked distillate (14). The composite CE2A1-aC1 fraction has a UV spectrum which is similar to that of the CE2A1-3CO fraction; it is shown in Figure 3c as a dashed curve. The maximum at 270 mp in the COfraction is shifted down to 260 mp in the C1 fraction, and the spectrum of the C1 fraction shows a more pronounced tail in the 300-350 mp region. The arrows show VOL 40, NO, 8, JULY 1968
1307
A
I d 2.6
3.0 '5.0
6.0
2.6
3.0 5.0
P
6.0
,
I*
2.6
& 6.0
3.0 5,O
Figure 4. Infrared spectra of various fractions Cyclohexane solvent unless otherwise noted. ( a ) A&, ( b ) C R in CHCl3,( c ) CEZA2in CHC13, ( d ) CEZA,in CHCb, C E A , in CHCh, (f)C E Z Ain~ CHCb, (g) CE,, (4 (i) A3S34
(e)
the positions of the corresponding maxima and shoulder in an indoles concentrate from a cracked gas oil (14). It seems likely that these differences in the UV spectra of the Co and C1 fractions arise largely from differences in the alkyl substitution pattern on the indole nucleus, plus possibly the presence of small amounts of other compound types. The mass spectra of these two fractions are essentially the same, showing the presence of alkyl indoles (C,HP,-gN) and higher cycloalkyl derivatives, with minor amounts of N-0 compounds. Table I11 summarizes the mass analyses of the alumina fractions from CE2. The mass analysis of CE1 (Table 11) confirms the presence of indoles in this fraction as well. We conclude that the aromatic nitrogen compounds in the AI- 3 subfractions from CE, and the CnHzn-gN,C,H2,-nN, and CnH2,--13N compounds in CE1 (Table 11) are largely indoles. Previous workers--e.g., (15, 16)--have reported the presence of indoles in straight run petroleum distillates, but the present study represents the first conclusive identification of this compound type in petroleum. The mass analyses of fractions A3-81-4 and CE1 indicate that the carbazoles and benzcarbazoles account for essentially all of the aromatic nitrogen compounds in these fractions (exclusive of N-0 compounds). These carbazoles and benzcarbazoles are expected to be the N-H derivatives (rather than N-alkyl derivatives) on the basis of the known separabilities of these various compound types [see (3)]. We can
therefore expect an approximate proportionality between the IR absorbance A2.g7(cm2/gram) at 2.87 for the N-H group of these fractions and the percent nitrogen in aromatic nitrogen compounds, % N,. The quantity N, is calculable from the mass data of Table I1 and the fraction yield data of Table I. In two cases the % N, value for a fraction was slightly higher than N by elemental analysis. Because elemental analysis (for nitrogen) was considered more reliable than mass N was taken as an upper limit on % N,. Figure analysis, 5 illustrates the proportionality of and % N, for the above fractions. Considering the semiquantitative nature of the mass analysis data of Table 11, and the variability of I R
(15) R. W. Sauer, F. W. Melpolder, and R. A. Brown, Ind. Eng. Chem., 44, 2606 (1952). (16) D. K. Albert, ANAL..CHEM., 39, 1113 (1967).
Figure 5. Correlation of N-H absorbance at 2.87 p (Az.87)us. per cent nitrogen as aromatic compounds N, from mass spectral analysis) in fractions As- SI4
1308
ANALYTICAL CHEMISTRY
1
2
3 % Nn
4
5
(z
!
7
I
I
8
9
I
l- SHIELDING NUMBE,R Figure 6 . NMR spectra of fractions CE2Aloand CE,A, absorptivities (see below), the linearity and scatter of the plot of Figure 5 seem reasonable. Figure 5 thus further confirms that the aromatic nitrogen compounds in these fractions are almost entirely carbazoles and benzcarbazoles. The slope of the plot in Figure 5 also permits us to estimate the percent nitrogen as N-H groups Nn-J in the remaining fractions of Table I1 from the relationship:
(z
% Nn-h
=
0.0187 A?.si
z
(1)
Tables I and I11 summarize values of Nn-h determined in this way. N-ALKYL INDOLES AND CARBAZOLES (A2, CEJ. Table I11 lists values of NnPh(Equation 1) for the various alumina fractions from CE,. We can calculate the per cent nitrogen as aromatic nitrogen compounds (excluding N-0 compounds) N, for each fraction from the mass spectral data of Table 111. The ratio N,-a/z N, then represents the fraction of nitrogen compounds which contain an N-H group. We see that the nitrogen compounds of fractions CE2A1and CELAlaare predominantly free of N-H groups, while most of the nitrogen compounds of fractions CEzAz and CE2A3contain a n N-H group. Fraction CE?A4gives value determined in a low value of N n P h / ZN, for an cyclohexane (the usual technique), but comparison of the A2.87values of fractions A 3 and A , in chloroform solution suggests a much higher value of % Nn-h for fraction CE2A4. Fraction CErA4 contains a large fraction of 1,2-benzcarbazoles (shown by mass and UV spectra), and in equimolar benzene solution for carbazole is twice as great as that for 1,2-benzcarbazole. On the basis of these various observations, we believe that the nitrogen compounds in CE2A4 are largely N-H rather than N-alkyl. If we accept the latter interpretation, it appears that the nitrogen compounds in the various fractions of Table 111 are predominantly Nalkyl substituted indoles in the case of CE2A1and CE2A1,, N-H substituted indoles in the case of CE2A, and CE2AP, and N-H carbazoles and benzcarbazoles in the case of CE2A , . This interpretation agrees reasonably well with the adsorptivity values (el) of these various compound types ( I , 3). Thus the N-alkyl indoles are calculated to have el values of 0.16-0.17 and should be concentrated into fractions CE2A1 and CE2A1,. The data of Table 111 indicate that 39% of the and 1 2 z N-alkyl indoles are in CE2A1,49% are in CEPAIQ, are in CErA2. Similarly the N-H indoles are calculated to have E: values of 0.28-0.33 and should be concentrated into
z
z
fractions CE2A3--4.Almost all of these compounds are found in fractions Az and A 3 . The N-H carbazoles and benzcarbazoles have el values of 0.34-0.41 and should be concentrated in fraction CE2A4. Essentially all of these compounds are found in that fraction (mass and UV data). The proton N M R spectra of fractions CE.'A1-3were obtained in an attempt to confirm the N-alkyl indole structure for the nitrogen compounds in fractions CE2A1and C R A , , . Methyl groups substituted on the indole nitrogen atom give rise to a sharp band at 6.63 7 and methylene substituents on the nitrogen atom are expected to give a corresponding band at 6.2-6.3 7. These bands are sufficiently displaced from other proton bands to be easily observed in fractions C E J - 3 of Table 111. Contrary to expectation, none of the CE2A1--3fractions showed N-alkyl proton bands in the 6.2-6.7 T region. Figure 6 shows the NMR spectra of fractions CE2A1, and CE2A2 as a n example. The arrows refer to the 7 values of various protons substituted on, or adjacent to, the indole nucleus, (the N-H band is quite broad and cannot be distinguished from other protons on the aromatic ring). The only structure which is fully consistent with all of the data we have for the nitrogen compounds in fractions CE2A:and CE2AlQis an indole substituted on the nitrogen atom by a quaternary alkyl group, as in 1 or 11:
m m N
R-7-R I
N
L_I(
R I II It seems improbable that all of the N-alkyl indoles in the original sample are of this type, because tertiary alkyl groups are not common in other petroleum compounds-e.g., hydrocarbons. However there seems to be n o alternative structure which will accommodate all of the experimental evidence. We conclude that the various nitrogen compounds in fractions CE2A1--1of Table 111 are indoles, carbazoles, and benzcarbazoles, and that fractions CE2A1and CE2AI,probably consist of N-alkyl indoles. Table I1 shows the presence in the various A? fractions of a series of nitrogen compounds beginning either with CnH2n--11N or CnH2n-liN. From the distribution of compound type concentrations in these fractions it is reasonable to conclude that we are dealing with two benzologous series which begin with one or the other of the above two formulas. The VOL 40, NO. 8, JULY 1968
0
1309
Nn-h concentrations of these fractions (Equation 1) suggests that 25% of these nitrogen compounds contain the N-H group and are, hence, indoles or carbazoles. Separation of fraction A S l by charcoal chromatography and examination of the UV spectra of the resulting subfractions show the presence of some carbazoles in ASIC1. On the basis of the adsorptivity values (€1) of the N-alkyl indoles and carbazoles (0.16-0.22), we believe that the various nitrogen compounds in the A? fractions of Table I are N-cycloalkyl indoles and carbazoles-e.g., 11. Some of the indoles of the type found in subfractions CEzAluand CEzA2(Table 111) and in corresponding subfractions from CE, are expected to be only partially adsorbed during cation exchange-cf. N-0 compounds below. These compounds should, therefore, be found in the ASo-r fractions of Table 11. Because most of the compounds in the A , and A2 subfractions from CE, fall on the CnH2n-11N and higher 2 series, it is reasonable to identify these compound types with the corresponding CnHBn--llN(and higher 2) compound types found in fractions A2S0-4 of Table 11. Actually the cycloalkyl indoles and carbazoles are the only reasonable structures which we can propose for the nitrogen compounds in the A ? S O - fractions. ~ Aromatic nitriles and N-phenyl pyrroles are the only other simple nitrogen compound types which are weakly enough adsorbing on alumina to fall in these fractions (3). Aromatic nitriles can be ruled out, because fractions A2S0-4 show no IR bands in the 4.3-4.5 p region. Similarly the N-phenyl pyrroles are not expected in these fractions, because the preceding cation exchange separation would have destroyed any pyrroles present in the original sample (3). We tentatively conclude, therefore, that the nitrogen compounds of fractions are a mixture of N-H and N-alkyl cycloalkyl indoles and carbazoles. PYRIDINES, QUINOLINES, AND PHENANTHRIDINES (CE3). The basic nitrogen compounds of petroleum-[.e., fraction CE3are generally assumed to consist of pyridines, quinolines, and higher benzologs, because previous studies-e.g., (]’/)-have shown the presence in petroleum of numerous compounds of this type. Titration of CE3shows that this fraction does not contain any anilines or aliphatic nitrogen bases. Anilines acetylate in acetic anhydride and titrate as very weak bases, while in acetonitrile aliphatic amines appear as stronger bases than the pyridines and quinolines [see (9, IO)]. The mass analysis of CE3(Table 11) is consistent with a mixture of substituted pyridines and higher benzologs, showing compounds beginning with the alkyl pyridines CnH2n-5N and extending through the alkyl quinolines CnH2n-11N and higher benzologs. Further separation of fraction CE, on charcoal, by using pentane, various mixtures of benzene-pentane, and finally hot toluene, gave partial separation of CE? according to aromatic ring number. Figures 3 d-f show the UV spectra of three of these fractions. Figure 3e confirms the presence of quinolines in an intermediate fraction. The other two fractions (Figures 3 4 3f) were contaminated by small amounts of quinolines, and the contribution of these quinolines to the UV spectra was partially backed out by using dilute solutions of the quinoline fraction (Figure 3e) as reference solvent. The first-eluted fraction (Figure 3d) shows a UV spectrum which resembles that of highly substituted pyridines; broad maximum at 275 mp, and a continual rise in absorbance at lower wavelengths (there is also a small remaining quinoline contribution in the 300-350 mp region). The last-eluted fraction (Figure 3f) has a UV spectrum which is quite similar to that of
phenanthridine. Because the UV spectra of other three-ring 5,6- and 7,8-benzoaromatic nitrogen compounds-e.g., quinoline, acridine; Figure 3g-are significantly different, we conclude that phenanthridines are the principal three-ring aromatics present in the original crude distillate. Phenanthridines have been observed previously (18) in crude oil. Examination of the UV spectra of the various charcoal fractions from CE3 permits an approximate breakdown of the various aromatic nitrogen compounds according to ring number: 25 pyridines, 65 quinolines, 10% phenanthridines. This analysis is consistent with the mass data of Table I1 for CE,. Aromatic Di-Nitrogen Compounds: CnH,,+,N2. Aromatic compounds containing two nitrogen atoms per molecule were observed in only one of the standard fractions of Figure 1 : CE3. The quantitative determination of these di-nitrogen compound types was made difficult by their low concentrations and poor resolution from associated mono-nitrogen peaks (see previous discussion). These di-nitrogen compounds account for approximately 4 x of fraction CE3, and are distributed by z number as follows:
(17) H. L. Lochte and E. R. Littman, “The Petroleum Acids and Bases,” Chemical Publishing Co., New York, 1955.
(18) D. hl. Jewel1 and G. J. Hartung, J . Chem. Eng. Data, 9, 297
1310
0
ANALYTICAL CHEMISTRY
z
z
CnHzn-zzN~0 . 2 Z wt CnH2n-lezN2 0.4% wt CnHgn-lOzNz 0.4 % Wt CnHZn-18zN2 0.6 wt CnH~n-12zN20.6 % Wt CnH2n-20zN2 0.7 % CnHzn-i4ZNz 1.1 Wt These structural formulas are consistent with the azaindoles (CnH2n-sN2)-e.g., 111-the azacarbazoles (CnH2n-14N2)e.g., IV-and the isomeric benzimidazoles (V) and dibenzimidazoles (VI) :
%
N\
N
m
H I11
H
Iv
V
VI
1-Azacarbazole and the imidazoles are sufficiently basic to be concentrated into fraction CE,, while 7-azaindole is a weaker base and might be expected in fraction CE, [see (1) and (3)]. However other azaindole isomers are expected to be more basic. Derivatives of azapyridine-e.g., VI1 ; CnHnz-rN,and its benzologs-e.g., phenazines-are generally weaker
ON w
bases, and they should be concentrated into CE2 and/or CE3 (3). A careful examination of CE, was made, but its mass spectrum does not indicate the presence of any di-nitrogen compounds. We conclude that the only di-nitrogen compounds present in the original crude distillate in significant amounts are found in CE3,and are probably azaindoles and azacarbazoles. Aromatic Nitrogen-Oxygen Compounds: CnHzn+sNO. Aromatic compounds containing one nitrogen and one oxygen atom per molecule were found in several of the standard fractions. Table IV summarizes these analyses by high resolution mass spectrometry, These N-0 compounds are believed to
( 19 64).
Table IV. Aromatic N,O Compounds in the Standard Fractions of Figure 1; Determined by High Resolution Mass Spectrornetrya % of 700-850 ”F distillate as indicated compound type C,Hz,-,NO Total Av mol wt Fractionb -5z - 7 ~ - 9 ~ -112 - 1 3 ~ -15z - 1 7 ~ - 1 9 ~ - 2 1 ~ -232 Wt 0.02 286 A 3.52 0,001 0,002 0.004 0.005 0,004 0.001 0.03 297 A S 3 4 0.002 0.011 0.003 0.004 0.003 0.001 0.01 258 A4S3 0,001 0,004 0.002 0,004 0.003 0.006 0.002 0.06 238 A464 0.034 0.011 0.005 0.04 308 CEi 0.002 0.003 0,003 0,004 0.006 0.007 0,006 0.005 0.005 0.72 288 CEz 0.02 0.03 0.06 0.12 0.17 0.19 0.08 0.04 0.01 0.17 300 CE3 0.031 0.034 0.031 0.034 0.024 0.017 1.14 0.00 Composite 0.02 0.03 0.06 0.16 0.22 0.28 0.14 0.08 0.04 a Parent peak analysis; results assume equal sensitivities and are normalized to aromatics in given sample. * Other fractions did not contain measureable amounts of aromatic N/O compounds.
z
be principally 2- and/or 4- hydroxypyridines, hydroxyquinolines, and possibly higher benzologs. We will refer to this class of compounds as pyridones and quinolones. The mass spectral data show compounds beginning with the alkyl pyridones (C,Hr,-5NO), with a large increase in the concentration of these compounds at the alkyl quinolones ((2,H2,-llNO). The pyridones and quinolones are very weakly basic and should therefore concentrate into fraction CE2,with slight overlap into fractions CE1and CE,. This is observed; 90% of these N-0 compounds are found in fractions cE1-3, and the bulk of them are in CE,. The pyridones and their benzologs should be concentrated into fractions A 4 and A6 on alumina (see discussion below), and this is observed for the N-0 compounds of Tables I11 and IV. The quinolones give strong amide bands at 6.04-6.07~(1655-1647 cm-’) in the IR [see (19) and Table V), and as seen in Figure 4b fraction CE, has a pronounced band maximum at 6.05 p (1647 cm-’) [CHC13 solvent here and in both (19) and Table VI. The CE2A4,CE2A4,,and CE2A5subfractions of Table I11 were further separated on charcoal, in order to concentrate any quinolones away from pyridones, sulfoxides, and some of the nitrogen compounds. The U V spectrum of a CE2AjCisubfraction is shown in Figure 3h, and it is essentially the same as that of 2-quinolone-cf., (19)-arrows in Figure 3g indicate band maxima for 2-quinolone. The U V spectra of the CELA4Cland CE2A4aC1subfractions also resemble the spectrum of Figure 3b, although the band maxima are less pronounced. The IR spectra of the alumina fractions of Table 111 show the presence of several distinct absorption bands in the amide region (5.8-6.1 p ; cf, Table V). Several of these are shown in Figures 4 c--f’ (CHCI, solvent). The early fractions (CE2A1-3), which contain only minor amounts of N-0 compounds, show weak bands at 5.81 (1720 cm-l), 5.95 (1680 cm-l), and 6.04 (1655 cm-l) p,-e.g., Figure 4c. Fraction CE?A4 shows a single, strong band at 6.04 I.( (Figure 4d), fraction CELA4,shows a major band at 5.95 P and a minor band at 6.08 p (1645 cm-l) (Figure 4e), and fraction CE2Aj shows a strong band at 6.08 /I with a shoulder at 5.95 p (Figure 4f). These latter data suggest that different N-0 compound types are being concentrated into fractions CErA4, CE2A4a,CE2Aj. The N-H grouping in 2-quinolone gives a small absorption band at 2.94 p (3400 cm-l), and this band is observed in fractions CE2A4,and CE2A5 It is much reduced in intensity in fraction CELA4,however (see Table 111). These observations suggest that the N-alkyl quinolones (and associated benzologs) are concentrated into fraction CE2A4,and the N-H quinolones are the main components of fraction (19) E. C. Copelin, ANAL.CHEM.,36, 2274 (1964).
Table V. Position of Carbonyl Infrared Band for Various Amides. (dilute solution in CHC13) Compound Band maximum, p N-Propionyl indole 5.88 Acetanilide 5.93 5.97 1-Naphthyl acetamide Benzamide 5.98 2- and 4-Quinolone 6.04 N-Methylacetanilide 6.08 3-Acetylindole 6.08 a The term “amide” is customarily restricted to compounds with the -C=O structure. Here we broaden the definition to include
I
N
/\
compounds such as 4-pyridone and 3-acetyl indole. These compounds exhibit many of the physical properties of amides (weak basicity, IR absorption at 5.9-6.1 k ) by virtue of the conjugation of -N
/ \
and
\
C=O groups through a linking vinyl group
/
CE2Aj. In support of this, the adsorptivity on alumina (el) of the N-alkyl quinolones and benzologs is 0.44-0.58, which should place these compound types in fractions CE2Al and CE,A4, ( I , 3). The adsorptivity of the N-H quinolones is 0.6+, which means these compound types should be found in CE2A5. The cycloalkyl quinolones (C,H13NO) clearly pre, suggests that these comdominate in fraction C E ~ A Iwhich pounds are N-cycloalkyl derivatives--e.g., VIII. The mass and U V data for fraction CE2A4a
wo VI11
indicates that these N-0 compounds are also derivatives of the quinolones and pyridones, but it is difficult to explain the displacement of the amide absorption band to 5.95 p. However, it was observed that these amide absorption bands are subject to pronounced solvent effects, as illustrated in Figure 4g for fraction CE, in cyclohexane--cf. Figure 4b for CHC13 as solvent. Fractions CEzA4 and CEzA4, each develop a new band at 5.90 p in cyclohexane. The intensity of the amide bands in the various fractions of Tables 111 and IV was observed to correlate approximately VOL. 40, NO. 8, JULY 1968
0
131 1
%, Nno Figure 7. Correlation of per cent nitrogen as aromatic N-0 compounds (% Nn0) us. infrared absorbance in amide region (Ano) Standard fractions of Figure 1. 0 Fractions of Table 111 (from CEJ. 0 Fractions of Table IV (not including CE,)
with the concentration of N-0 compounds in these fractions. This is illustrated in Figure 7 where the amide absorbance of these fractions A,, is plotted against the N as N-0 compounds from the data of Tables I11 and IV. A,, is essentially an integrated absorbance value. It is the sum of absorbance values at the four wavelengths associated with band maxima in the various fractions of Table 111 (5.90, 5.95, 6.02, and 6.06 p in cyclohexane solution), corrected for interference from carbonyl absorbance (- 1.2 for the various fractions of Table IV also contain aliphatic ketones (see later discussion). The correlation of Figure 7 confirms that the various N-0 compounds of Table IV are predominantly amides. The 2-quinolones have been found previously in crude oil (19). Aromatic Nitrogen-Dioxygen Compounds: C,HYn+zN02, Subfractions CE2Asand CE2A3(Table 111) were found to contain compounds having the empirical formulas C,H,,+,N02, with zequal to -11, -13, -15, and -17. The total concentration of these compounds was only 0.02% of the crude distillate. The empirical formulas support the dihydroxyquinolines and their cycloalkyl analogs. However such compounds should be concentrated into later fractions--i.P., Other Nitrogen Compound Types. Nitrogen containing compounds, other than those described previously, were not found in any of the fractions of Figure 1. Fractions A. and were not mass analyzed for nitrogen compounds because of the small nitrogen concentration of A . (0.03 %) and the small amount of total nitrogen compounds in fraction AISO+(see Tables I and 11). Forty per cent of the nitrogen in A . is basic, and it is reasonable to assume that these basic nitrogen compounds are highly hindered pyridines and quinolines [see ( I ) ] . The remainder of the nitrogen compounds in fractions A. and AlS1-4 (which amount to only 4 % of the total nitrogen containing compounds in the original crude distillate) are probably the same nitrogen compound types found in the various A z fractions (N-alkyl indoles and carbazoles). Aliphatic compounds which contain nitrogen appear to be essentially absent from the crude distillate. Figure 8 shows a plot of total N as aromatic nitrogen compounds (determined by high resolution mass spectrometry) 6s. the 13 12
ANALYTICAL CHEMISTRY
o/o
N ( E L E M E N T A L ANALYSIS)
Figure 8. Correlation of per cent nitrogen as aromatic compounds (% N calculated from mass analysis) L‘S. per cent nitrogen by elemental analysis Standard fractions of Figure 1
% N by elemental analysis for each of the fractions of Table I (except the Ao-1 fractions). With only one exception (A3S2) the points of Figure 8 lie close to the line of unit slope or above it, showing that all of the nitrogen compounds in these fractions have been accounted for. The calculated % N value for fraction A3S2is significantly lower than the elemental N value, but this seems to represent an error in the calculated N value. The Nn--h value of A3S2(Equation 1) falls quite close to the unit slope curve (dark circle of Figure 8). Consequently it appears that all of the nitrogen in the original crude distillate is present as aromatic nitrogen compounds. Aromatic compounds containing both a nitrogen and a sulfur atom in the same molecule were not found in any of the fractions of Table I. While N-S compounds are not resolved from certain N compounds by present mass spectrometers-e.g., thiazoles, C,H2,-3NS, from cycloalkyl quinolines, C,H?n-13N---exact mass measurement can be used to determine which compound type (N or N-S) is the major contributor to a given parent peak. If exact mass measurement is sufficiently precise, the relative amounts of the N and N-S compounds contributing to a given peak can be estimated by comparison of the measured mle value with the values for the corresponding N and N-S compounds. The accuracy of the present mass measurements could not rule out the presence of as much as 30 of an N-S compound for any given N compound peak. However, if N-S compounds were present in significant amounts in any fraction, some peaks in that fraction should show the N-S compound predominating over the corresponding N compound. This was never observed. Furthermore, there should have been a significant bias in the measured mje values of the various nitrogen compounds toward higher values than calculated for a nitrogen compound. The weighted average difference between experimental and calculated m/e values for all the nitrogen compounds of Table I was 0.0 ppm. If as much as 10% of these compounds were N-S compounds, the average difference between experimental and calculated mje values should have been f1.2 ppm. Finally it was oband CE1--3 served that all of the sulfur in fractions of Table I can be accounted for as sulfoxides (see below). This means that only insignificant amounts of the nitrogen
Aromatic Oxygen Compounds in the Standard Fractions of Figure 1 Determined by High Resolution Mass Spectrometry" wt of 700-850 "F distillate as indicated compound type C,Hz,+,O -10~ -12~ -14~ -16~ -18~ -20~ -22~ -24~ -26~ 0.03 0.03 0.03 0.15 0.10 0.01 0.14 0.15 0.05
Table VI.
Fraction
- 6~
-82
Ao ASo
0. o o c
0.026 0.003 0,001
0,002 0,001
0.005 0,001
A4S3 A45S01 A4S4 AS23 CEi CEz CE3
0.002
0.003
0,004
0.004
0.008
0.262 0.005 0.003
0.001
0.065 0.004 0.002 0.001
0.017 0.001 0.004
0.43 0.05
0.02 0.01
0,001
0,001
0,001
0.002
0.014
0.006
0.003
0,002
0.01 0.07
0.001
0.008
0.017
0,017
0,013
0.008
0.008
0,011
0,008
0,003
0.011
0.075
0.108
0,119
0.004
0.006
0.008
0.081 0,007
0.006
0.065 0.006
0.032 0.003
0.03
0.03
0.03
0.32
0.28
0.09
0.26
0.06
0.02
1.12
0.003
0,006
0.008
0,011
0,011
0.012
0.008
0.006
0.001
0.07
Composite analysis Furanes (A0-3SO) Phenyl ketones (AZSI-4)
0.008
0,034 0.007
0.026
0.006
260 313 308
0.00 0.00
A62
Adz
Av mol wt 294 328
0.OOC
AiSi AIS24 AzSo A2Si A2S24 A3SO A3S1 AS34
Total 0.35b 0.34
0.004
350 350
0.00
0.07
347
0.00
0.059
0.54 0.04 0.00 0.00
321 294
Phenols
0.73 0.15 0.15 0.10 0.08 0.08 0.04 0.01 (AB--s,CEI)0.09 Parent peak analysis; results assume equal sensitivities and are normalized to total aromatics in given fraction. b Calculated from elemental oxygen analysis of fraction (see text). c These fractions were not mass analyzed, because only small amounts of heterocompounds are involved and because these compounds were believed to be largely aliphatic. a
compounds in these fractions (which account for 7 8 x of the nitrogen compounds in the total sample) could be in the form of N-S compounds. A few workers have reported the presence in petroleum of compounds which contain both nitrogen and sulfure.g., (20, 2I)-but our data suggest that such compounds are at most minor components of the present crude distillate. Aromatic Oxygen Compounds: C,H2,+,0. Table VI summarizes the aromatic oxygen compound types found in the standard fractions of Figure 1, as determined by high resolution mass spectrometry. No aromatic compound types on the C,H2,,+,0S or C,,H?,,+,02series were observed. BENZOFURANES, DIBENZOFURANES, AND NAPHTHOBENZOFURANES (Ao+So). The only oxygen compounds which should be present in the various Ao-sSo fractions of Table VI are the furanes and higher benzologs [see ( I , S)]. The furanes (beginning with the alkyl furanes, CnH2n--IO)and benzofuranes (C,,H2,-100) should be contained entirely in Ao. The dibenzofuranes (C,,H2,-lsO) should be distributed between A. and Also, and the naphthobenzofuranes (C,,H2,,-?2)0 should be in fraction A2So. This is essentially what is observed in Table VI. Furanes are absent from the original crude distillate, small amounts of benzofuranes are found in Ao, dibenzofurane; are present in fractions A0 and AI-~SO, and naphthobenzofuranes are found in fraction A S o (with a small amount of naphthobenzofuranes in A S o ) . These furane benzologs account for all of the oxygen present in the A o J o fractions. In the case of the A. fractions, the furane benzologs (as determined by mass spectral analysis) appeared (20) C . La Lau, Anal. Chim. Acta, 22, 239 (1960). (21) S . L. Gusinskaya, V. Yu. Telly, and A. Aidogdyev, Uzb. Khim. Zh., 11, 21 (1967).
to contain three times as much oxygen as indicated by elemental analysis of the fraction. This is probably caused by the lower mass spectral sensitivities of benzene hydrocarbons (C,,H2,,-6plus cycloalkyl derivatives), which are concentrated in this fraction. The data for fraction A0 in Table VI have been normalized to the concentration of total oxygen compounds as calculated from the elemental oxygen analysis for the fraction. Dibenzofuranes have been found in other crude oils-e.g., (22). PHENYLKETONES(A2S1-J. Fractions A2Sl and AuS24 show a series of aromatic oxygen compounds beginning with the C,,H2,,-s0series. These compounds are believed to be phenyl ketones, such as acetophenones or benzyl alkyl ketones, along with corresponding cycloalkyl derivatives and higher benzologs. Three pieces of evidence support this structural assignment. The phenyl ketones begin with the C,H2,,-s0 series, they are predicted to concentrate into fractions A2-3Sl--1, and the IR spectra of fractions A S l and A2S24 indicate that these aromatic oxygen compounds are carbonyl derivatives (see discussion below). PHENOLS AND HYDROXYBIPHENYLS (A3--5,C E J . Fraction A S z 3 is clearly a concentrate of alkyl phenols and corresponding cycloalkyl derivatives, plus possibly a small amount of higher benzologs. The UV spectrum of this fraction (Figure 3i) closely resembles that of alkyl substituted phenols (with the fine structure at 280 mp smoothed out, as is typical of petroleum compound types). The I R spectrum of this fraction, and of pure phenol, shows a narrow band at 2.76 p (3620 cm-l) which is characteristic of the 0-H group (see Figure 4h). The calculated weak acid content of A &
(22) F. F. Yew and B. J. Mair, ANAL.CHEM., 38, 231 (1966). VOL. 40, NO, 8, JULY 1968
1313
I
9.8
9
IO
P
I1
12
Figure 9. Infrared spectrum of CE, in the sulfoxide absorption region, showing base line subtraction technique in calculation of A9.*
v. s
is 2.8 mequiv/gram, us. an experimental value of 2.9 mequiv/ gram. The elemental analysis of this fraction shows a high oxygen content (but see below), and only minor amounts of nitrogen and sulfur. Finally, the alkyl phenols begin with the series C,Hzn-eO, the first series observed in A S 2 3 . Phenols also occur in fractions A3S34, A&, A4jSO1,and CE1, as shown by the 2.76-11 band and the mass spectral analyses of these fractions. A second, smaller band a t 2.82 p (3550 cm-l) is also observed in the IR spectrum of A S Z 3(Figure 4h). This is a narrow band, and its intensity relative to the 2 . 7 6 - ~ band does not change as the concentration of A s z 3(in cyclohexane) is varied from 2.0-0.1% wt. This shows that the 2.82-p band arises from intramolecular (not intermolecular) hydrogen bonding. The wavelength of the 2.82-p band suggests that the phenol 0-H group is hydrogen bonded to an olefinic or aromatic group within the same molecule [see (2311, and this is consistent with the molecular formulas of these compounds (which contain only carbon and hydrogen, apart from the 0-H group). The 2.82-p band is enhanced in fraction A4jS01 to the point where it and the 2.76-1 band are of equal intensity, and the aromatic oxygen compounds in this fraction (see Table VI) show a marked jump in concentration at CnH2n--140.This suggests that the 2-hydroxybiphenyls (CnH2,-140) and their cycloalkyl derivatives are responsible for the 2.82-p band. In the compound 2-hydroxybiphenyl, the 0-H group on one ring can hydrogen bond to the adjacent ring, as shown by the single (narrow) 2.80-p band observed in the IR spectrum of 2-hydroxybiphenyl itself. Because there is no reasonable alternative to the 2-hydroxybiphenyls as a source of the 2.82-p band in fractions At&Yol and A S z 3 , we tentatively conclude that these compounds are present in the crude distillate in minor amounts. The combined relative intensity of these two bands (2.76 and 2.82 p ) parallels the concentrations of aromatic oxygen compounds in the above five fractions, and we can assume that these aromatic oxygen compounds are largely phenol derivatives. The mass analyses of Table VI are in agreement with this conclusion, because each series of oxygen compounds begins either with CnH2n--60or CnH2,,--140. Elemental analysis of fractions A4801 and AS23 (the major phenol fractions) shows only half as much oxygen as is calculated from the mass spectral analyses of these fractions. This cannot be explained in terms of likely errors in the mass spectral analysis, and the presence of nonoxygen-containing
compounds in these fractions, which may have escaped detection, seems quite improbable. We conclude that there has been a consistent error in the analysis of these fractions for oxygen, which may be attributable to the fact that these were among the first samples we analyzed for oxygen by the present method. Phenols have been found previously (17) in low boiling petroleum distillates. Aliphatic Compounds Containing Oxygen. CARBOXYLIC ACIDS. Carboxylic acids are assumed to be the sole weak acid constituents of the present crude distillate, The concentration of these compounds in the distillate was calculated from the concentration of weak acids (0.056 mequiv/gram) and an estimate of their molecular weight (310). Other studies have shown that these acids are predominantly nonaromatic. Carboxylic acids are not present in any of the standard fractions of Figure 1, for strong acids are lost during the initial alumina separation [see (3)]. SULFOXIDES (ALL FRACTIONS).Sulfoxides have been observed as artifacts in a number of different crude oils (24). Their presence in several of the standard fractions of Figure 1 could be shown by a characteristic IR band at 9.8 p (1030 cm-l), as in Figure 9. Aromatic compounds containing both sulfur and oxygen were not observed in any of the standard fractions, so these sulfoxides can be assumed t o be entirely aliphatic. Sulfoxides are the only monofunctional sulfur compound expected to concentrate in fractions A 4 - - S i 4 and CE1+ and it was observed that the absorbance of the 9.8-p band A9.8(measured above background; see Figure 9) for these fractions correlates well with their sulfur contents (Figure 10). This suggests that virtually all of the sulfur in these fractions is in the form of sulfoxides. This was confirmed by further separation of the CE, fraction (which contains the largest amount of sulfur of any of these latter fractions, and has the largest absorbance a t 9.8 p). CE2 was separated on both alumina and charcoal, and all of the sulfur was found in the CE2A45Cofraction. Aliphatic sulfoxides are predicted to concentrate into this fraction (3). The molar absorptivity of petroleum sulfoxides at 9.8 p (A9.8) can be calculated from the correlation of Figure 10, equal
(23) K. Nakanishi, “Infrared Absorption Spectroscopy-Practica1,” Holden-Day, San Francisco, 1962, p 30.
(24) I. Okuno, D. R. Latham, and W. E. Haines, ANAL.CHEM., 39, 1830 (1967).
1314
ANALYTICAL CHEMISTRY
Figure 10. Correlation of sulfoxide absorption band a t 9.8 ,U with per cent sulfur in fractions Aa-sS1-r and CE1-3
Table VII. Aliphatic Oxygen Compounds in the Standard Fractions of Figure 1
Fraction ASid ASzad AzSi A2S21
A3S1 A $2 A834
AaSz A4S3 Adot Ada Ass23
Total 0.29 0.15 0.31 0.42 0.02 0.05 0.16 0.03 0.06
% wt Aliphaticsa Sulfides Sulfoxides 0.13 0.02 0.10 0.03 0.19 0.03 0.03 0.31 0.01 0.01 0.01 0.01 0.01 0 .OOe
0.00
0.00e
0.03 0.01
0.00
0.00’
0.00
0.24
0. o o e 0.00c
0.13 0.02
0.00
Elemental analysis of other aliphaticsb Other 0.14
%O
%S
0.02
2.2 12
0.09 0.07
0.0
1.6 5 5.0
16.0
0.0
0.00
0.03 0.15
4 4.4
0
0.00 0.05 0.00
4.2
0.0
0.11
11.7
0.5
0.00
2.9
A,,
40
210 125 330 75 125 765 35 585 85 680 25 Total
7 wt Aliphatic oxygen compoundsc 0.06 0.02 0.00
0.07 0.00
0.03 0.13 0.00
0.04 0.00
0.11 0.00
0.46 Of 7W850 “F distillate. b Calculated by difference, using data of Tables I, IV, V, and VI. oxygen compounds; assume 100% oxygen compounds have oxygen content of 5 % wt. c Other aliphatics times estimated d These fractions were not mass analyzed, but are assumed 100% aliphatics. e Estimated values. 4
to 137 liters moles-’ cm-l. Previous workers (24) have reported an average value of A 9 , 8equal to 300 liters moles-’ cm-1. The difference between these two values can be explained in large part by our use of a background subtraction technique in measuring A9.8(Figure 9), as well as by our use of a high resolution spectrophotometer. The correlation of Figure 10 permits us to determine the per cent sulfur as sulfoxides S, in other fractions of Table I by the relationship
% Sa, = 0.023As.s.
(2)
Values of % S,, for the various standard fractions are reported in Table I. With an estimate of the molecular weight of these sulfoxides (330), we can calculate the concentration of sulfoxides in each of these fractions. As predicted ( I , 3), the sulfoxides are concentrated in the and CE24 fractions (total of 1.00% wt sulfoxides). Only small amounts of sulfoxides occur in the A 3 fractions (total 0.03% wt), but larger amounts (0.54% wt) are found in the fractions. This unusual distribution pattern, coupled with the fact that the fractions contain large amounts of sulfides, suggests that the sulfoxides in the Ai-* fractions were formed by oxidation of sulfides during the silica separation of these fractions. If so, we can estimate that about 0.5 sulfoxides were produced during the entire separation, because the original crude distillate appears to contain about 1 % wt sulfoxides (see Table I). Previous workers (24) have reported that sulfoxides are not present in fresh crude oils, but are formed by air oxidation of sulfides during handling. The oxygen and weak base contents of our starting crude oil did not change significantly during distillation, which suggests that the 1 % wt sulfoxides found in the crude distillate were present in the original crude oil, or else were formed during handling of the crude oil before distillation. OTHERALIPHATIC OXYGEN COMPOUNDS (Al--6S1--4). Table VI1 summarizes the concentrations of total aliphatics in each of the standard fractions of Figure 1, along with corresponding concentrations of sulfoxides and sulfides (see data of Table I for these calculations). Significant amounts of aliphatics (other than sulfides) are not expected in fractions A1--3S0, and the only aliphatics present in fractions CE1-3 are sulfoxides.
Thus, the difference between total aliphatics and the sum of sulfides plus sulfoxides in the fractions of Table VI1 (other aliphatics) should equal the remaining aliphatic oxygen compounds (recalling that sulfides and sulfoxides are largely concentrated into the COfractions, and are hence classified as aliphatics). Saturated hydrocarbons are precluded from the fractions of Table VII, and aliphatic nitrogen compounds are not present in the crude distillate--i.e., correlation of Figure 8. We can calculate the elemental composition of these “other” aliphatics by difference,and as seen in Table VI1 these compounds have generally large oxygen contents and considerably smaller sulfur contents. It should be emphasized that these latter elemental analysis data are necessarily quite approximate. If we assume an oxygen content of at least 5 for pure aliphatic oxygen compounds, we can estimate the aliphatic oxygen compound content of each of the fractions of Table VII. These values are generally close to the concentrations of other aliphatics in Table VII, as expected. The above aliphatic oxygen compounds appear to be largely carbonyl derivatives, because the concentrations of these compounds in the fractions of Table VI1 correlate roughly with the IR absorption of these fractions in the carbonyl region (5.6-5.9 p). Figure 4i shows the IR spectrum of sample A3S34, the fraction containing the largest concentration of aliphatic oxygen compounds. A major band at 5.88 p (1703 cm-’) is observed, a smaller band at 5.76 p (1735 cm-l), and a minor band at 5.68 p (1760 cm-l). The 5.88-p band can be attributed to aliphatic ketones. It is the major carbonyl band in each of the fractions of Table VI, and we conclude that aliphatic ketones are major components of the aliphatic oxygen compounds of Table VI. Aliphatic ketones have been found previously in petroleum distillates-e.g. (25). Jenkins (26) has shown that petroleum esters account for a band at 5.75 p and are minor constituents of petroleum fractions boiling in the range of the present crude distillate (700-850 O F ) . The 5.76-p band was present in all of the fractions which contain aliphatic oxygen compounds, and we conclude that it represents small amounts of aliphatic esters in (25) C. F. Brandenberg, G. L. Cook, W. E. Haines, and D. R. Latham, J. Chem. Eng. Data, 9, 563 (1964). (26) G. I. Jenkins, J. Inst. Petrol., 51, 313 (1965). VOL 40, NO, 8, JULY 1968
e
1315
Table VIII. Summary of Nitrogen and Oxygen Containing Compound Types Present in the 700-850 OF Crude Distillate
zwt
Aromatic compounds CnJ&n+zN Indoles Carbazoles Benzcarbazoles Pyridines Quinolines Phenanthridines
0.590 3 . 62b 0.48 0.65 1.71 0.26
CnHzn+zNz Azaindolesc Azacar bazolesc
0.03 0.09
CnHzn+sNO Pyridones Quinolones
0.11+ 1.03-
CnHzfi+sN02 CnHzn+zO Benzofuranes Dibenzofuranes Benzonaphthofuranes Phenyl ketones Phenols
7.3
0.1
1.1
0.02
= 0.3
hydes, or ketones) should be concentrated into the A2--3 fractions [see ( I , 31, and these fractions contain the bulk of the aliphatic oxygen compounds of Table VI (0.23 wt). Overlap of these same mono-carbonyl compounds into AISz4 (0.02%) and A S 3 (0.04x) is to be expected. The 0.11 wt of aliphatic oxygen compounds in AlySl appears anomalous, if we assume these are mono-carbonyl derivatives. The high A,, value of this fraction (Figure 11) and the high oxygen content of these aliphatic oxygen compounds (Table VII) suggests instead that at least some of these compounds are di-carbonyl derivatives (diesters, diketones, etc.). Alcohols d o not appear to constitute a large part of these aliphatic oxygen compounds, for the 2.76- and 2.82-,LL bands appear only in fractions which contain phenols. No other sharp bands in the 2.7-2.85 p region were observed, but a broad, low-intensity band occurs between 2.8 and 3.0 p in fraction A&. Possibly this arises from an internally hydrogenbonded hydroxyl group-e.g., -OH ' ' ' O=. We conclude that the aliphatic oxygen compounds of Table VI1 are principally monofunctional ketones, with lesser amounts of ethers, esters, and difunctional compounds such as diketones. The calculated sulfur contents of the "other aliphatics" of Table VI1 suggest that about 2 0 z of these aliphatic oxygen compounds contain a sulfur atom.
z
2.5 1.09 1.00d 0.46 12.9
wt of these are believed to be N-alkyl substituted.
These structures are highly tentative. Does not include sulfoxides believed formed during silica separation. e Principally aliphatic ketones, with some ethers, esters, and difunctional carbonyl compounds. c
d
-
these fractions. The band at 5.68 p may arise from cyclobutanone derivatives, 7-lactones, or vinyl or phenyl esters. The sum of absorbances (cm2/gram) at 5.68, 5.76, and 5.88 p (Aco) for each of the fractions of Table VI1 is plotted cs. the per cent aliphatic oxygen compounds in each fraction in Figure 11 (open circles and triangles). There is a rough correlation between these two quantities (solid line), suggesting that these aliphatic oxygen compounds are largely carbonyl derivatives. The deviation of these points from the solid line can be partly rationalized in terms of the composition of each fraction. The A1 fractions (open triangles) can contain aliphatic ethers [e1 = 0.07; see ( I , 3)] as well as carbonyl compounds, if ethers are present in the crude distillate. This offers a n explanation for the low value of A,, for fraction AIS1. If both ethers and carbonyls are present in the A I fractions, the ethers should concentrate into A S l and the carbonyls into A1&. This appears t o be the case, and we conclude that the aliphatic oxygen compounds in AISl are largely aliphatic ethers. We have previously postulated that fractions A S l and A S z 4 contain aromatic ketones. If so the concentrations of these compounds should be added to the per cent aliphatic oxygen compound values of Figure 1 1 . The resulting values (dark circles connected t o original open circles) are seen to give a better correlation, supporting our original conclusion that the aromatic oxygen compounds in these fractions are phenyl ketones. Aliphatic compounds containing one carbonyl group (esters, aldeANALYTICAL CHEMISTRY
Figure 11. Infrared carbonyl absorbance (Aco)of fractions of Table VI us. per cent aliphatic oxygen compounds in each fraction
z
* 0.7%wt of these are believed to be N-alkyl substituted.
13 16
% A L I P H A T I C OXYGEN COMPOUNDS
1.9 0.09 0.69 0.34 0.07 0.73
Aliphatic compounds Carboxylicacids CnHln+lCOOH Sulfoxides CnH2,+,S0 Other aliphatic oxygen compounds$ Total
A co
DISCUSSION
The various oxygen and nitrogen containing heterocompounds present in the 700-850 O F crude distillate are summarized in Table VIII. No other heterocompound types are believed to be present in this sample in significant amounts (>0.1% wt), and most of the various structural assignments of Table VI11 seem reasonably firm. The same analytical approach is applicable to both higher and lower boiling distillate fractions, and future papers will deal with these results. We will defer discussion of the significance of the data of Table VI11 until that time. Because the various hydrocarbon and sulfur compound types can be separated into narrow compound type fractions (27), and because there is n o reason to doubt that high resolution mass spectrometry can provide detailed analyses of these fractions as well, it is now possible to completely analyze any crude distillate for all compound types present in significant amounts. Hopefully, the time and expense required for this analysis can be (27) L. R. Snyder, ANAL.CHEM., 37,713 (1965).
reduced to the point where such data will be available routinely. As indicated in the preceding section, some of the structural assignments of Table VI11 are tentative. More work is required to determine the structure of the CnH2n+zN2aromatics, and to confirm the structure of the phenyl ketones. Similarly a more detailed analysis and confirmation of the other aliphatic oxygen compounds of Table VI1 seems desirable. Certain ambiguities in the analytical data for the N-alkyl indoles and carbazoles, and the pyridone derivatives, should be resolved. For the most part, however, these are minor points which cannot seriously affect the conclusions of Table VIII. Hopefully, our continuing study of the higher and lower boiling fractions from the present crude oil will answer some of these questions.
ACKNOWLEDGMENT
We are grateful for the advice and/or assistance of several people at the Union Research Center in connection with the present project: F. 0. Wood for experimental work in connection with separations and sample handling, A. E. Youngman for determining the UV spectra, R. F. Buhl and R. J. Kinsella for obtaining the IR spectra, G. C. Graham for the NMR spectra and help in their interpretation, U. Niwa for the nitrogen determinations, E. C. Schluter for the oxygen determinations, J. K. Fog0 for the sulfur determinations, and J. R. Fox and E. C. Copelin for valuable discussions and for editing the original manuscript. RECEIVED for review February 6, 1968. Accepted May 2, 1968.
Nuclear Magnetic Resonance Chemical Shifts of Oxygenated Unsaturated AI iphatics Nugent F. Chamberlain Esso Research & Engineering Co., Baytown Research and Development Dicision, Box 4255, Baytown, Texas
The NMR chemical shifts for unsaturated aliphatic acids, esters, aldehydes, ketones, alcohols, and ethers are correlated with their corresponding molecular structures in a series of charts. The marked effectsof conjugation on the chemical shifts are further correlated with simple resonance hybrids and steric conformations which may simplify remembering and roughly predicting these effects. These correlations should make possible faster and more convincing structural determinations of unsaturated oxygenated aliphatics, revealing in many cases the presence or absence of conjugation and the cis-trans isomeric structure associated with it.
THE REMARKABLE effectiveness of NMR spectrometry in revealing molecular structure depends, at present, to a large extent on empirical correlations of structure with chemical shifts, coupling constants, and band shapes. Fast and convincing analyses require rapid access to precise and detailed correlations. NMR data for derivatives of aliphatic olefins have been scarce and scattered. The importance of their oxygenated derivatives in polymerization, surface activity, and natural products, however, makes it desirable to gather together the available data for this class of compounds. An extensive literature search has provided enough information for effective detailed correlations of chemical shift with structure for the unsaturated oxygenated aliphatics. The complete correlations are presented in this paper to promote greater speed and assurance in making structural analyses of compounds in this class. These correlations are presented in chart form to provide rapid access to the data and show graphically the effects of substituent position and of conjugation on the chemical shifts. This graphic presentation also facilitates the prediction of data which are not directly available. This combina-
77520
tion of measured and predicted data provides a more complete and more useful correlation for each compound type. DATA SELECTION AND EXAMINATION
The accuracy of correlations of chemical shift with molecular structure is greatest when all nonstructural effects (solvent, concentration, temperature, hydrogen bonding, etc.) on chemical shift are eliminated or at least held constant. On the other hand, the general utility of such correlations is enhanced if they include the shifts due to the uncontrollable nonstructural effects normally encountered in analytical work. The data used in this study were selected to provide a workable compromise between these opposing characteristics. Useful- accuracy was achieved by selecting only those data measured on proton signal stabilized spectrometers or measured by the sideband technique, at room temperature, and referenced to internal tetramethylsilane. Necessary exceptions were the data for acrylic acid, for which the shift measurements were less precise, and acrolein, which was referenced directly to cyclohexane and converted to TMS reference by addition of a constant. Utility was enhanced by including data for neat liquids and for solutions in CDCla and CC14. Data for acids and esters in DzO, acetone, and tetrahydrofuran were also included because they agreed within acceptable limits with data for these compounds in CDC18 and CCI4. The data selected by the foregoing criteria were examined critically to reduce errors and inconsistencies. For published spectra, all interpretations were checked and incomplete ones completed, assuming the compounds to be as claimed. For each compound, the shift of each functional group identified by NMR was plotted separately. Data for similar compounds were plotted on the same chart, permitting rapid VOL 40, NO. 8, JULY 1968
0
1317
