Characterization of Nitrogen Bases in High-BoiIing Petroleum DistiI lates J. F. McKay," J. H. Weber, and D. R. Latham Laramie Energy Research Center, Energy Research and Development Administration, Laramie, Wyo. 82070
The nitrogen bases in eight high-bolllng crude oil distillates from four crude oils having different geological sources were characterized. Four basic compound types were found in all of the distillates-pyridine benzologs, dlaza compounds, amides, and carbazoles. A chromatographlcinfrared method was developed for the qualitative and quantitative determination of the major compound types. t h e data obtained using this method were compared with titration and total nitrogen data. This comparison demonstrated that the chromatographic-infrared method alone can be used to qualitatively and quantitatively analyze basic compound types in high-boiling petroleum distillates and that titration and total nitrogen data alone can be used to quickly estimate the kinds and amounts of major compound types in these distillates. The structures of the basic compounds were examined in detail using fluorescence, mass, and infrared spectrometry.
Crude oil distillates may be separated into seven fractions-acids, bases, neutral nitrogens, saturates, monoaromatics, diaromatics, and polyaromatics-using the separation scheme developed by the Bureau of Mines in American Petroleum Institute Research Project 60 (1, 2 ) . Further characterization of these fractions is desirable t o identify the major compound types. The polar compound types are important constituents of petroleum because, even in small amounts, they cause serious problems in processing and in the stability of the products. For example, the polar compounds cause catalyst poisoning and are involved with the formation of gums in products. Characterization of the acid fractions has been reported ( 3 ) .T h e analysis of base fractions according t o compound types is discussed in this paper. Early work on petroleum bases has been reviewed by Lochte and Littman ( 4 ) . Much credit is due t o t h e early workers who, without modern spectroscopic instrumentation, have identified many individual pyridine, quinoline, and benzoquinoline compounds. Recent research by Snyder (5-7) and Jewel1 and Hartung (8) has extended the compositional studies t o include the identification of amides, compounds containing both nitrogen and sulfur, and diaza compounds. Quantitative estimates of the different compound types have also been made using mass spectrometry (5-7) and potentiometric titration (9-21). This investigation extends previous work by developing a standardized scheme of analyzing basic compound types in 370-535 "C and 535-675 "C petroleum distillates. Bases in crude oils from four different geological sources were studied t o compare the composition of the base fractions. First described are titration data and elemental nitrogen and sulfur analyses of total base fractions. Second, a method is described involving adsorption chromatography and infrared spectrometry that was developed for the qualitative and quantitative determination of major compound types. T h e results obtained using this chromatographicinfrared method are then compared with the results of the
titration and total nitrogen determinations. Finally, the detailed analysis of the basic compound types using mass, fluorescence, and infrared spectrometry and nitrogen data is presented.
EXPERIMENTAL Apparatus. Infrared spectra were recorded using a PerkinElmer Model 521 infrared spectrophotometer; fluorescence emission and fluorescence excitation spectra were recorded using a Perkin-Elmer MPF-PA fluorescence spectrophotometer; high-resolution mass spectra were recorded on a Du Pont 21-110 double-focusing mass spectrometer; low resolution mass spectra were recorded on a Varian CH-5 single-focusing mass spectrometer. Basic nitrogen titrations were made using a Beckman Model 1063 titrimeter. The adsorption chromatographic column was a gravity flow column, 0.5-cm i.d. by 25 cm, packed with 5 g of adsorbent. Fractions of 15 to 30 ml were collected for infrared analyses. The gel permeation chromatographic (GPC) column was a water-jacketed glass tube, 1.3 cm-i.d. by 150 cm, packed with 80 g of gel. A constant-volume fraction collector (Model 270, Instrumentation Specialities Co.) was used to collect 3.4-ml fractions. Thin-layer separations were made with 20-cm plates covered with 0.5-mm thickness adsorbent. Reagents. Amberlyst-15 cation exchange resin (Rohm and Haas) was used to isolate the base fractions. Cellex-P cation exchange cellulose (Bio-Rad) was used to filter the base samples prior to adsorption Chromatography.Acid alumina (Bio-Rad, Ag-4, 100- to 200-mesh, factory activated) and basic alumina (Bio-Rad AG-10, 100- to 200-mesh, factory activated) were used for adsorption Chromatography. Cyclohexane, methylene chloride, and ethanol were the solvents used for adsorption chromatography. Poragel A-1, Waters Associates, was used with methylene chloride solvent for GPC. Silica gel G (E. Merck) was impregnated with 5% boric acid for thin-layer chromatography. Thin-layer plates were developed with chloroform. Solvents were flash distilled before use. Procedure. Sample Preparation. The crude oils were obtained through a sampling program conducted by API Research Project 60 and were selected for study partly because they represent crude oils having different geological classifications (3). The crude oils were stored in inert containers under a nitrogen atmosphere to inhibit the formation of artifacts from oxidation or photochemical reactions. Distillates having corrected boiling ranges of approximately 370-535 OC and 535-675 "C were obtained by distilling the crude oils in a Rota-film molecular still under reduced pressure (12). The distillates were subjected to a maximum temperature of 175-200 "C for 1 or 2 s. No evidence was found that chemical artifacts were formed during the distillation. Preparation of Base Fractions. The base fractions were obtained using a method described in Ref (1).Briefly, the high-boiling distillates from which acids had been removed were passed over Amberlyst 15 cation exchange resin, a macroreticular resin that can be used in nonaqueous systems. The materials retained by the resin were defined as the base fractions. The bases were removed from the resin using benzene/methanol/isopropylamine (55:37:8). Separation of Base Fractions. A diagram of the scheme for separation of the bases.is shown in Figure 1. The base fractions were dissolved in cyclohexane and passed through a column containing Cellex-P cation exchange cellulose. Less than 1%of each base fraction, a black material not definable by infrared, was retained on the column. This material was not identified. The eluate from the Cellex-P column was passed through an acidic alumina column (sample-to-adsorbent ratio 1 to 50) and eluted first with methylene chloride to remove the weakly held bases and then with absolute ethanol to remove the strongly held bases. Solvent was reANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
*
891
I Base fraction I
Table I. Percentage of Basic Compounds in HighBoiling Petroleum Distillates
I Cation resin
Crude oil Wilmington, Calif. Gach Saran, Iran bases
Acidic alumina
,
,
,
Basic alumina
,
,
,
fraction
Syfraction
Basic a c m i n a
1 11 )I 11 /I 1 I S;b-
fraction
-:S
fraction
Syfraction
-:S
Sr-
fraction
Figure 1. Separation of base fraction
moved from these two fractions, and the fractions were redissolved in cyclohexane. Each fraction was then placed on a basic alumina column, and the column was eluted with 1) cyclohexane 90%, methylene chloride lo%, 2) methylene chloride, and 3) absolute ethanol. Thus six subfractions were generated. Routine Methods of Nitrogen and Sulfur Analysis. Total nitrogen was determined using micro-Dumas, macro-Kjeldahl, and microcoulometric (Dohrmann) methods. Total sulfur was determined by combusting the sample and titrating the oxidation products with Ba(C104)2 to a colorimetric end point. Base fractions were titrated potentiometrically with perchloric acid dissolved in dioxane, using acetic anhydridebenzene (2:l) solvent. Bases having a half-neutralization potential (HNP) of 350 mV or less were classified as strong bases; those with an HNP greater than 350 mV were classified as weak bases. Infrared Spectral Method for Analyzing Base Fractions. Infrared spectra of the six base subfractions were recorded in methylene chloride solvent using 0.05-cm cells. Pyridine benzologs show characteristic absorption bands (doublet) at 1598 and 1567 cm-' (when these compounds have been separated from other aromatic compound types). Amide carbonyl absorption was observed at 1697, 1682, 1665, and 1638 cm-' with the absorption bands at 1697 and 1682 cm-' being especially well resolved due to efficient separation by adsorption chromatography. Additional amide N-H bands between 3450 and 3405 cm-' were observed but were not used for infrared analyses. Compounds having pyrrolic N-H absorption, such as carbazoles, were observed at 3460 cm-'. The absorption band at 1602 cm-' was assigned to the aromatic diaza compounds, aromatic compounds containing 2 nitrogen atoms per molecule. Quantitative infrared analysis required estimates of the average molecular weights of the total base fractions and estimates of the functional group absorptivities of the major compound types isolated by the separation scheme. The average molecular weights were estimated by low resolution mass spectrometry to be 400 for the 370-535 O C fractions and 500 for the 535-675 "C fractions. (Average molecular weights were difficult to estimate and may be in error by as much as 50 to 75 molecular weight units. Errors in average molecular weight represent a major source of error in the quantitative infrared analyses.) Infrared functional group absorptivities-the "apparent integrated absorption intensities, B" (I3)-were calculated from concentrates of petroleum samples using Beer's law according to the method previously described ( 3 ) . Values of B used for both the 370-535 "C and the 535-675 "C bases were as follows: pyridine benzologs, 0.31 X lo4; amides, 1.36 X lo4, carbazoles, 0.72 X lo4; diaza compounds, 0.72 X lo4. The units of B are 1 X mol-' X cm-*. Within the dilute concentrations used, lo-* to M, the infrared curves of the various functional groups showed areas that were directly proportional to concentration. 892
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
Recluse, Wyo. South Swan Hills, Alta.
Boiling range, OC 370-535 535-675 370-535 535-675 370-535 535-675 370-535 535-675
Wt % basic compounds 6.8 12.7 2.1 8.7 1.1 3.3 2.2 4.5
The following procedure was used for calculating the percentages of major compound types by the chromatographic-infrared method. Quantitative infrared spectra were recorded for each of the six chromatographic subfractions. The areas of the appropriate absorption bands were measured by planimetry, and the concentration of each compound type in each chromatographic subfraction was calculated. The weight in grams of each compound type in each subfraction was calculated, using the appropriate molecular weight. The weight of each compound type was then summed. The percent of each compound type in a total base fraction was then calculated from the combined weights of all of the compound types.
RESULTS AND DISCUSSION Three levels of information are produced by the separation scheme and the chromatographic-infrared analysis described in this paper. The discussion to follow is concerned first with the total base fractions-the percentage of a distillate they represent; their titration and total nitrogen and sulfur analyses. Second, the discussion is concerned with the analysis of major compound types. Finally, the detailed analysis of the compound types, is presented.
Percent of Basic Compounds in High-Boiling Distillates. Table I shows the percentages of bases obtained from eight high-boiling distillates. The Wilmington, Calif., distillates contain the highest percentages of basic compounds, and Recluse, Wyo., the lowest. In the oils studied, the higher boiling distillates contained a t least twice the weight percent of basic compounds as the lower boiling distillates. Thus, these results follow the generally observed trend that higher boiling distillates contain larger amounts of polar molecules than do lower boiling distillates. Titration and Total Nitrogen Data. The results of the potentiometric titrations and total nitrogen determinations of total base fractions from eight distillates are shown in Table 11. The first column contains the total nitrogen data. The weight percent total nitrogen ranges from 2.6 to 4.0 for the lower boiling fractions. If the average molecular weight of the lower boiling bases is 400 and each molecule contains 1 nitrogen atom, the percent of nitrogen would be 3.5%. The total nitrogen ranges from 1.8 to 2.9 for the higher boiling fractions. If the average molecular weight of the higher boiling bases is 500 and each molecule contains 1 nitrogen atom, the weight percent of nitrogen would be 2.8%. Thus, the nitrogen data alone indicate that the average structure is a molecule containing one nitrogen atom. The Recluse and Swan Hills fractions appear to contain some non-nitrogen compounds. However, the low nitrogen values for these fractions may represent experimental errors in the total nitrogen determinations. The second and third columns show the weight percent of nitrogen titrated as strong and weak bases, assuming that only nitrogen-containing molecules are titrated. From 80 to 90% of the total nitrogen is titratable, with the average being about 85%. The fourth column is the difference between the total nitrogen and the titrated nitrogen. The last three columns present these data as percentages of the
Table 11. Titration and Total Nitrogen Data of Base Fractions Weight uercent nitrogen
Titrated Fraction
Percent of nitrogen as
Total"
Strong base
Weak base
Not titratedc
Strong base
Weak base
Nontitrated base
3.87 2.90
2.17 1.89
1.04 0.46
0.66 0.55
56 65
27 16
17 19
4.01 2.34
3.17 0.98
0.24 0.89
0.60 0.47
79 42
6 38
15 20
2.62 1.80
1.65 1.37
0.60 0.36
0.37 0.38
63 64
23 22
14 14
2.92 2.11
1.81 1.37
0.79 0.36
0.32 0.38
62 65
Wilmington
370-535 "c 535-675 "C Gach Saran 370-535 "C 535-675 "C Recluse 370-535 "C 535-675 "C Swan Hills 370-535 "C 535-675 " c
27 11 17 18 Weight percent titrated nitroa Weight percent total nitrogen data are average values obtained by as many as three different methods. gen data are obtained from a single titration. By difference.
Table 111. Sulfur Analyses of Total Base Fractions Crude oil
Boiling range, "C
Wt % sulfur
370-535 1.24 535-675 1.79 (2.51Ia Gach Saran, Iran 370-535 2.27 535-675 2.89 (3.34) 370-535 2.16 Recluse, Wyo. 535-675 1.62 (1.55) South Swan Hills 370-535 2.52 (2.83) Alta. 535-675 1.49 (1.14) Numbers in parentheses are duplicate determinations Wilmington, Calif.
total nitrogen. In general, strong bases represent more than 60% of the nitrogen, weak bases represent about 20% of the nitrogen, and the nontitrated bases account for about 15% of the total nitrogen. Sulfur in the Base Fractions. Table I11 shows the sulfur data for the total base fractions. The sulfur ranges from approximately 1 to 3% for these samples, with the reproducibility being fairly poor. Based on molecular weights of 400 and 500 for the two distillate fractions, about one out of every four molecules in each fraction contains a sulfur atom.
Chromatographic-Infrared Determination of Major Compound Types. The key to the success of infrared analysis is the separation of the base fraction into six subfractions in which the various compound types are concentrated. Infrared analysis of the total base fraction is not possible because of the severe overlap of the definitive absorption bands. In the subfractions, spectral characteristics of the compound types can be recognized. The first subfraction contained the pyridine benzologs (strong bases); the middle subfractions were mixtures of diaza compounds, carbazoles, and amides; and the last subfraction contained amides. The results of the chromatographic-infrared analysis of the base fractions are shown in Table IV. All of the total base fractions contained the same compound types: pyridine benzologs, several different types of amides, carbazoles, and diaza compounds. Pyridine benzologs represent approximately half of the material; amides and diaza compounds each represent about 25% and carbazoles generally represent less than 5% of the bases. Amounts of different compound types are about the same in both fractions from the same oil; however, any trends would be difficult to ob-
serve with just four sets of data. Recovery data for the chromatographic separation step and accountability data for the infrared analysis are shown in Table V. Losses in the chromatographic separation are usually less than 10%. Quantitative infrared analysis of the low-boiling material collected after chromatographic separation usually results in 100 f 5 wt % of the material being accounted for, although extremes of f15% are occasionally experienced. Larger error was experienced in the higher boiling fractions, perhaps due to incorrect estimation of average molecular weights. Thus it appears that most of the major compound types are being analyzed by the method and that the infrared absorptivities used in the calculations are suitable.
Correlation of Titration and Total Nitrogen Data with Infrared Data. Gravimetric data alone cannot be used to measure the percentages of compound types in the base fractions because the chromatographic technique does not completely separate all of the compound types. Thus, a direct, independent method of checking the spectroscopically determined percentages of compound types is not available. One alternative is to compare titration and total nitrogen data with data obtained using the chromatographic-infrared method. This comparison is shown in Table VI. The titration data are in terms of percent of total nitrogen, as taken from Table 11. The infrared data are data from Table IV that have been recalculated in terms of percent of total nitrogen. The recalculations were made as shown in the following example using Wilmington data from Table IV. Assume a mixture of 100 molecules with the composition shown in line A. These 100 molecules would contain the number of nitrogen atoms shown on line B or a total of 117 atoms of nitrogen. A calculation of the percent of the nitrogen represented in each type is shown on line C. Pyridines
Line A, No. of molecules Line B, No. of N atoms Line C, Percent of
Diaza compounds
Amides
N-H
51
17
25
7
51
34
25
7
44
29
21
6
nitrogen The data in line C can now be used to calculate how the mixture would titrate on the basis of limited model compound data ( 1 1 ) . These data suggest 1) that pyridines and one nitrogen in diaza compounds titrate as strong bases, 2) that amides titrate as weak bases, and 3) that carbazoles ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
893
Table IV. Quantitative Determination of Compound Types by Chromatographic-Infrared Method of Analysis" Weight percent of base fraction
Base fraction
Pyridine benzologs, 1598, 1567 cm-'
Diaza compounds, 1602 cm-'
Wilmington 51 17 370-535 "C 57 535-675 "C 23 Gach Saran 370-535 "C 59 26 45 535-675 "C 30 Recluse 46 34 370-535 "C 57 26 535-675 "C Swan Hills 34 370-535 "C 26 49 29 535-675 "C a The weight percents have been normalized to equal 100%.
and one nitrogen in diaza compounds do not titrate. For the Wilmington bases the final calculation is: Strong bases = pyridines plus '12 diaza = 44 15 = 59% of the nitrogen Weak bases = amides = 21% of the nitrogen Nontitratable bases = carbazoles '/z diaza = 6 14 = 20% of the nitrogen Table VI shows t h a t the amounts of strong, weak, and nontitratable compounds determined from titration and total nitrogen data are approximately equal to those determined from infrared data by the chromatographic-infrared method. This general correlation of the two sets of data leads to several conclusions: I) specific compound types determined by infrared can be related quantitatively to the data provided by titration and total nitrogen determinations; 2) the chromatographic-infrared method of analysis alone can be used for the qualitative and quantitative determination of base compound types in high-boiling distillates; 3) titration and total nitrogen data alone can be used to estimate the kind and amounts of major compound types in the base fractions, provided the base fractions have been generated using the techniques described in the Experimental section. Detailed Analysis of Compound Types. This section of the paper describes the detailed analyses-the investigation of the structural features of the major compound types. The separation scheme shown in Figure 1 produced six subfractions from each base fraction. Thus, from eight base fractions, a total of 48 subfractions were generated. Quantitative infrared spectra were recorded for all 48 subfractions. Infrared spectra indicated that the same four compound types were present in the 370-535 "C subfractions from different oils and that the same compound types found in the 370-535 "C subfractions were also present in the corresponding 535-675 "C subfractions. Because of the apparent similarities of all of the base fractions, selected subfractions from two 370-535 "C base fractions were studied using titration, elemental analysis, high-resolution mass, and fluorescence spectrometry. The analysis of selected Wilmington and Gach Saran 370-535 "C subfractions is described below. Subfractions 1, 2, and 6 were examined because all of the major compound types in the base fractions could be identified by characterizing these subfractions. Together, these subfractions represent about 85% of each base fraction.
+
+
894
+
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
N-H compounds (carbazoles), 3460 cm-'
1638 cm-I
7 4
8
Amides
1
1665 cm-'
8 1
1682 cm-I
1697 cm-'
13 4
3 3
2 2
5
1
6
1
1
5
14
3
2 1
0 0
6
10
2
3
9
4
3
0 0
5
17 10
15
4
2
6
Table V. Recovery Data for ChromatographicInfrared Method Wt % material
Base fraction
Recovered after separation
Accounted for by infrared analyses
Wilmington 370-535 "C 99 (99)" 95 (85) 96 121 535-675 "C Gach Saran 370-535 "C 100 94 535-675 "C 91 116 Recluse 370-535 O C 91 (80) 94 (102) 535-675 "C 89 95 Swan Hills 370-535 "C 82 (86) 101 (95) 535-675 "C 99 116 " Numbers in parentheses are from duplicate runs.
Subfraction 1. Pyridine benzologs were identified as being the major components of subfraction 1. Duplicate total nitrogen determinations on the 370-535 "C subfractions from Wilmington and Gach Saran were 3.94, 3.63 wt % and 3.38, 3.52 wt 96, respectively, indicating that the average molecule in these subfractions contains one nitrogen atom. Titratable nitrogen in both subfractions was 100% strong base. The titration values were 3.18 wt % for the Wilmington subfraction and 3.37 wt % for the Gach Saran subfraction. The reason(s) for the difference in total nitrogen and titrated nitrogen values in the Wilmington subfraction is not known. The weight percent total sulfur in the Gach Saran subfraction is 1.64, indicating that about one of every five molecules contains a sulfur atom. Concentrations of sulfur compounds have not been detected in any of the six base subfractions. Therefore, it is assumed t h a t sulfur is randomly distributed in the nitrogen compound types as thiophenic or sulfide-type sulfur. Pyridine benzologs such as those in subfraction 1 show characteristic absorption in the infrared a t 1598 and 1567 cm-1 when the material has been isolated from other aromatic compound types. When other aromatic compound types are present in a mixture with pyridine benzologs, the infrared spectrum is not definitive and cannot be used for analytical purposes. A representative infrared spectrum from a Wilmington subfraction, recorded in methylene
Table VI. Comparison of Titration-Nitrogen Data and Infrared Data Percent of nitrogen as Strong bases From titrationnitrogen data
Base fraction Wilmington 370-535 "C 535-675 "C Gach Saran 370-535 "C 535-675 "C Recluse 370-535 "C 535-675 "C Swan Hills 370-535 "C 535-675 "C
Weak bases
Nontitrable bases
From IR data
From titration-
nitrogen data
From IR data
From titrationnitrogen data
59 65
27 16
21 13
17 19
20
65 79 42
68 58
6 38
10 18
15 20
22 24
63
60
66
23 22
13
64
13
14 14
27 21
62 65
48 61
27 17
29 14
11 18
22 25
56
From IR data
21
4, .1,598 1,598
1,567
1,697
.
3,500
3,400
1,800
1,703
1,600
1,500
1,400
WPVENUUBER, tn-'
t
Figure 2. Infrared spectrum of subfraction 1
* c_ v)
ZZ
c
Y
I +Model
compound
p
Petroleum sample
3'
WAVELENGTH, n m
Flgure 4. Fluorescence emission spectra
300
320
340
360
WAVELENGTH, nm
Figure 3. Fluorescence excitation spectra
chloride solvent, is shown in Figure 2. This spectrum is similar to that of many substituted pyridines and pyridine benzolog model compounds which show two absorption bands near 1600 cm-I with the higher wavenumber band having the stronger absorption intensity. Spectra of pyridine compounds showing two absorption bands have been previously observed in a Wilmington petroleum sample (14). Fluorescence spectra were obtained for several subfraction 1 samples that were further separated using gelpermeation chromatography and thin-layer chromatography. Although insufficient model compound spectra are available for an in-depth study, some comparisons are possible. Figures 3 and 4 show partial fluorescence excitation and fluorescence emission spectra of the model compound
t \ 280
300
320 340 360 WAVELENGTH, nm
380
Figure 5. Fluorescence excitation spectra
phenanthridine together with the corresponding spectra of a Wilmington petroleum sample. The similarity of the spectra suggest that condensed-ring systems such as phenanthridine are present in subfraction 1;however, t h e exact ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
895
U 4 -
5 -
Figure 7. Structures representative of compound types in subfraction 1 Wavtltnpth, nm 1.602
Flgure 6. Fluorescence emission spectra
Table VII. High-Resolution Mass Analysis of Subfraction 1 Observed average General formula
decimal mass valuea
Carbon Deviationb
Percent ionization
0.0028 7.2 15-30 Cn Hzn-7N 0.9405 0.0017 8.2 15-30 0.9271 CnHzn-gN 0.0016 10.9 15-34 CnH2n-11N 0.9137 0.0013 11.2 15-33 CnH2n-13N 0.9003 0.0010 10.4 15-35 0.8869 CnH~n-15N 0.0010 10.1 15-33 CnHzn-17N 0.8735 0.0008 8.7 15-34 CnH2n-19N 0.8601 0.0002 10.7 16-31 0.8467 CnH2n-21N CnH~n-23N 0.8324 0.0008 6.8 18-31 Cn H~n-25N 0.8065 0.0003 5.3 18-3 1 CnH2n-29N 0.7931 0.0015 2.4 19-31 Mass measurements were made using CH2 = 14.0000 amu. With this mass scale, decimal portion of mass to charge ratios are the same for homologous series of ions. Deviation is the absolute difference in mass between the formula mass and the observed mass.
,
position of the nitrogen atom in the ring' system is not known because other benzoquinolines have fluorescence spectra similar to that of phenanthridine. Figures 5 and 6 show partial fluorescence excitation and fluorescence emission spectra of 1,2-benzoacridine together with the corresponding spectra of a Wilmington petroleum sample. Comparison of the petroleum sample spectra with the spectra of 1,2-benzoacridine and 3,4-benzoacridine (not shown) suggests that the petroleum sample is a mixture of the two acridine benzologs. Results of the high-resolution mass spectral analyses of a Wilmington 370-535 OC subfraction 1 are shown in Table VII. The 2 series spread of -7 to -29 indicates that the sample is a complex mixture of compounds having various combinations of condensed aromatic and saturated acids. Cycloalkylpyridines would be found in the -7 to -9 series, and dibenzoacridines would be found in the -29 series. T h e percent ionization (assuming unit sensitivities) of the -11 to -17 2 series suggests that most of the compounds have two or three aromatic rings. The carbon ranges for the 2 series show that compounds in the -7 to -19 2 series have higher percentages of saturated carbon, due to more alkyl substitution, than the compounds in the -21 to -29 series. The combined chemical and spectroscopic data suggest that structures 1 through 5 shown in Figure 7 are 896
.
number range
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
3,500
3,400
1,600
1,700
1,600
1,500
1,400
WIVENUMBER, c i '
Flgure 8. Infrared spectrum of subfraction 2
Table VIII. High-Resolution Mass Analysis of Subfraction 2 Observed average General formula
decimal mass value"
Deviationb
Percent ionization
Carbon number range
0.0034 12.3 17-33 CnHzn-~lN 0.8432 0.0006 12.8 18-33 Cn H~n-23N 0.8327 CnHzn-z5N 0.8186 0.0013 5.1 20-33 CnHZn-17NS 0.8106 0.0007 7.5 14-29 C,Hzn-lgNS 0.7984 0.0019 4.0 17-31 CnHzn-12Nz 0.8913 0.0031 11.1 15-31 CnH~n-14N2 0.8802 0.0009 11.7 15-31 0.8705 0.0028 11.9 16-31 CnH~n-lfiN~ C, H ~ , - B N ~ S 0.8600 0.0024 11.9 12-29 CnH2n-10N2S 0.8478 0.0026 11.9 12-29 a Mass measurements were made using CH2 = 14.000 amu. With this mass scale, decimal portion of mass to charge ratios are the same for homologous series of ions. Deviation is the absolute difference in mass between the formula mass and the observed mass.
representative of the compounds in subfraction 1. Hundreds of individual compounds are present in the sample. The exact locations of the nitrogen and sulfur atoms are not known. All nitrogen is thought to be pyridine-type nitrogen, and sulfur is thought to be thiophenic as shown in structure 4, or sulfide-type sulfur. Except for compounds in the very low-numbered 2 series, such as the -7 series, most of the carbon not in aromatic ring systems is thought to be present in condensed, saturated ring systems, as shown in structure 5. Subfraction 2. The major compound types in subfraction 2 were identified as diaza compounds, amides, and carbazoles. Duplicate total nitrogen determinations on the 370535 "C subfractions from Wilmington and Gach Saran were 3.93, 3.76 wt % and 4.12, 4.08 wt %, respectively. The average molecule again contains one nitrogen atom, but the values higher than 3.5 wt % nitrogen suggest the presence
of diaza compounds. The Wilmington and Gach Saran subfractions titrated as mixtures of strong, weak, and nontitratable nitrogen, consistent with a mixture composed of diaza compounds, amides, and carbazoles. Wilmington subfraction 2 contains 1.20 wt % sulfur, indicating that one of every seven molecules contains a sulfur atom. An infrared spectrum representative of subfraction 2 is shown in Figure 8. Absorption due to pyrrolic N-H such as that found in carbazoles is seen at 3460 cm-l. The N-H absorption of certain diaza compounds, such as indazole, also occurs a t 3460 cm-' and may account for the small amounts of material associated with this band. Amide carbonyl absorption bands are seen a t 1697 cm-' and 1665 cm-l. Amide N-H absorption has never been observed together with these bands even a t high concentrations; thus, it is concluded that these amides are tertiary amides. Model compound studies suggest that the absorption a t 1697 cm-I is due to a carbonyl group in a five-membered ring. The material absorbing a t 1697 cm-l can be reduced with sodium borohydride, which suggests that the compound is not a true amide, but rather a compound containing a tertiary nitrogen atom and a ketone carbonyl group. The compounds absorbing a t 1697 cm-' and 1665 cm-I are probably the same amides observed by Snyder (6) in a California crude oil. Figure 8 does not show the amide absorption that is observed in some fractions a t 1638 cm-l. Amides having absorption a t this low wavenumber may be of the pyrid-4-one type. The symmetrical band a t 1602 cm-I results from the aromatic absorption of diaza compounds. The location of the two nitrogen atoms in the diaza molecules is not known: thus the specific type(s) of diaza compounds in subfraction 2 is unknown. Caution should be observed in using the 1602 cm-l absorption for identification purposes because most aromatic compound types show absorption in the 1600 cm-' region. The 1602 cm-l band can be used for the analysis of diaza compounds only in cases where the petroleum subfractions are generated according to the separation schemes previously described. Results of the high-resolution mass spectral analyses of a Wilmington 370-535 "C subfraction 2 are shown in Table VIII. The empirical formulas can be related to the compound types observed by infrared spectrometry: compounds in the -21, -23, and -25 Z series that contain one nitrogen atom are probably carbazoles. The compounds in the -12, -14, and -16 Z series containing two nitrogen atoms are probably the diaza compounds absorbing a t 1602 cm-I in the infrared. The amounts of diaza compounds determined by percent ionization and by measuring the infrared band of 1602 cm-' are about the same and represent about two thirds of the material in subfraction 2. The compounds containing both nitrogen and sulfur were not detectable as unique compound types in the infrared and, therefore, probably represent carbazoles, amides, or diaza compounds having sulfur randomly distributed in the molecular structures. For unknown reasons, compounds containing both nitrogen and oxygen were not observed in this sample by mass spectrometry. However, they were found in a comparable sample from another oil. The combined chemical and spectroscopic data suggest that structures 6 through 12 in Figure 9 are representative of the compounds in subfraction 2. Structures 6, 7, and 8 are examples of the carbazoles that may be present. Structures 9 and 10 are structures suggested t o be representative of the amides absorbing a t 1697 cm-1 and 1665 cm-1, respectively. Both of these amides are tertiary, cyclic amides, consistent with the infrared data and with the observation that they are easily eluted from a basic alumina column. Tertiary nitrogen structures cannot readily hydrogen bond to alumina and are eluted from an alumina column with
v: -7
6
6 -
N W
-9
10
Q&
&ot 12
11
Flgure 9. Structures representative of compound types in subfraction
2
3,500
3800
1,800
1,700
1,600
1,500
1.400
WLVENUMBER, cm-'
Figure 10. Infrared spectrum of subfraction 6
H 13 -
14 -
15 -
Figure 11. Structures representative of compound types in subfraction 6
relatively nonpolar solvents. Structures 11 and 12 represent the diaza compounds in subfraction 2. Infrared and titration data indicate that both of the nitrogen atoms in most diaza compounds are tertiary. When these compounds are titrated, only one nitrogen atom is reactive; apparently, formation of the salt causes the second nitrogen atom to be nonreactive. Trace amounts of porphyrins were observed by ultraviolet spectrometry in the 535-675 "C base subfractions 2. The ultraviolet spectra of these porphyrins were of the metalloporphyrin types. An ultraviolet spectrum of a Wilmington subfraction was very similar to that of nickel etioporphyrin. Subfraction 6. Amides were identified as being the major components of subfraction 6. A Gach Saran 370-535 "C subfraction contained 3.02%total nitrogen, and 3.38% nitrogen was titrated as weak base indicating all nitrogen compounds in the subfraction are weak bases. Sulfur analyses were not obtained. A representative infrared spectrum of subfraction 6 is shown in Figure 10. The amides in this subfraction are secondary amides, having infrared absorption a t 3410 cm-' due to the N-H stretching vibration and absorption a t 1682 cm-' due to the C=O stretching vibration. The position of these bands is similar to those of cyclic lactams such as dihydroquinol-2-ones (15). The secondary amides are apparently eluted from the alumina column later than the tertiary amides because of the ability of the secondary amANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
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Table IX. High Resolution Mass Analysis of Subfraction 6 General formula CnHzn-3NO cnH~n-5NO CnH2n-7NO Cn Hzn-gNO Cn HZn-11NO CnHzn-i3N0 CnH2,-15N0 CnH2n-17N0 CnHZn-1gNO CnHzn-gN02
Observed average decimal mass Deviatio- Percent nb ionization value" 0.9447 0.9312 0.9178 0.9030 0.8889 0.8766 0.8644 0.8508 0.8394 0.8783
0.0004 0.0002 0.0002 0.0012 0.0009
0.0008 0.0004 0.0002 0.0020 0.0021
6.1 11.3 11.3 13.4 12.6 14.2 13.8 7.3 2.8 7.3
Carbon number range 12-23 12-23 12-23 13-23 13-23
13-23 13-23 14-23 15-23 11-19
Mass measurements were made using CH2 = 14.0000 amu. With this mass scale, decimal portion o f mass t o charge ratios are the same for homologous series o f ions. Deviation is the absolute difference in mass between the formula mass and the observed mass.
ides to hydrogen bond with the alumina. The results of the high-resolution mass spectral analysis of subfraction 6 are shown in Table IX. The general formulas tend to be in lower numbered 2 series (-3, -5, - 7 ) than in previous subfractions, and the Z series stop a t -19. This indicates that the compounds in subfraction 6 tend to have a higher degree of saturation than do compounds in earlier subfractions. Structures suggested to be representative of compounds in subfraction 6 are shown in Figure 11. The secondary amides are thought to be predominately saturated compounds having fused ring systems substituted with alkyl groups. Structures 13, 14, and 15, in Figure 11 are similar to lactam model compounds that show absorption near 3410 cm-l and 1682 cm-l. Structures 13 and 15 resemble the P-quinolones found by Copelin ( 1 6 ) in a Wilmington, Calif., crude oil and may represent 2-quinolones that have been reduced during maturation.
CONCLUSIONS The techniques used in this investigation are suitable for the analyses of basic compounds in high-boiling petroleum distillates. The composition of basic compound types of different distillates may be directly compared, both qualitatively and quantitatively. Compositional data may be correlated with recovery, refining, and geological data as information from the analysis of a large number of crude
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
oils becomes available. The combination of this method and the one previously published for acids makes possible studies of the effects of specific polar compounds on the chemical and physical properties of the distillates. For example, the kinds and amounts of basic and acidic compound types may be directly related to the catalyst poisoning properties or the interfacial tension of a distillate. In addition, it should be possible to study the expected synergistic effects of the polar compounds on the properties of a distillate.
ACKNOWLEDGMENT The authors thank W. E. Haines for many useful discussions and for assistance in preparation of the manuscript. We also thank D. A. Nelson and Robert Mendoza of the University of Wyoming for recording the low voltage mass spectra.
LITERATURE CITED (1) D. M. Jewell, J. H. Weber, J. W. Bunger, H. Plancher, and D. R. Latham, Anal. Chem., 44, 1391 (1972). (2) D. E. Hirsch, R . L. Hopkins, H. J. Coleman, F. 0. Cotton, and C. J. Thompson, Anal. Chem., 44,915 (1972). (3)J. F. McKay, T. E. Cogswell, J. H. Weber, and D. R. Latham, "Analysis of Acids in High-Boiling Petroleum Distillates," Fuel, in press. (4) H. L. Lochte and E. R. Littman, "Petroleum Acids and Bases," Chemical Publishing Co., New York. 1955. (5) L. R. Snyder, Anal. Chem., 41, 314(1969). (6) L. R. Snyder, B. E. Buell, and H. E. Howard, Anal. Chem., 40 1303 (1968). (7) L. R. Snyder, Anal. Chem., 41, 1084 (1969). (8) D. M. Jewell and G. K. Hartung, J. Chem. Eng. Data, 9, 297 (1964). (9) F. P. Richter, P. D. Caesar, S. L. Meisel, and R. D. Offenhaufer, lnd. Eng. Chem., 44, 2601 (1952). (10) B. E. Buell. Anal. Chem., 39, 756 (1967). (1 1) I. Okuno, D. R. Latham, and W. E. Haines, Anal. Chem., 37 54 (1965). (12) H. J. Coleman, J. E. Dooley, D. E. Hirsch, and C. J. Thompson, Anal. Chem., 45, 1724 (1973). (13) R. N. Jones, D. A. Ramsay, D. S. Keir, and K. Dobriner, J. Am. Chem. SOC..74, 80 (1952). (14) C. F. Brandenburg and D. R. Latham, J. Chem. Eng. Data, 13, 391 (1968). (15) H. E. Baumgarten. P. L. Creger, and R. L. Zey, J. Am. Chem. SOC.,82, 3977 (1960). (16) E. C. Copelin. Anal. Chem., 36, 2274 (1964).
RECEIVEDfor review November 13, 1975. Accepted January 19, 1976. The research reported here was done under a cooperative agreement between the Energy Research and Development Administration, and the University of Wyoming. Mention of specific brand names or models of equipment is made for information only and does not imply endorsement by the Energy Research and Development Administration, Laramie Energy Research Center.
